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F10 / F11 (2011 - Current)
The new chapter in the highly successful story of the BMW 5 Series Sedan (F10) and wagon (F11)

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  #51  
Old 07-15-2013, 02:23 PM
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Originally Posted by DavidNJ View Post
I didn't know the answers...but I learn quickly. "Unsprung weight" is one of the popular misunderstandings, I challenge you to find a wheel only change that had a discernable difference. On same race cars they go crazy with this stuff. Magnesium hubs. Titanium brake discs. Aluminum lug nuts. But it is all about rotational inertia; how quickly the car accelerates or stops.

Yes there are numerous benefits, which is why it is an obvious benefit

If you think there is a post anywhere where it was unsprung weight only (a wheel change without a wheel width, offset, tire, or suspension change) that had an affect on ride or handling, post it. If you have a serious technical article where someone did the test, post it.

Consider it posted below.

Yes...I know more than a casual amount about automotive suspensions. However, the whole issue of RFTs in the BMW was a void to me. I now can wind my way through the posts, fit them into context.

Maybe some of us know a thing or two about suspension too.
Tell that to Dunderhi or the countless others - including me - who have switched stock wheels for lighter rims and have noticed a significant improvement in ride and handling.

If you refuse to accept that reducing unsprung and rotational mass can positively benefit ride and handling that's your call. Wiser heads know appreciate and notice the difference.
You rang...

As an engineer myself, I have worked out the force equations and conducted testing on my own cars. My conclusion is that the benefits are real with cost being the only negative. In my previous post, I overlooked the fact that for my 335d I did do the intermediate step of swapping the tires first. Why did I feel I needed to swap out the wheels after I made the miraculous change to a nonRFT, especially since the tire alone stopped the pothole explosions? Well, the heavy OEM wheel exacerbated the softness of the nonRFT where the tire would bottom out on potholes. Also, the heavier wheels were more resistant to changes in direction, which also worked against the inherent flexibility of the nonRFT in terms of turning crispness. Installing lighter wheels with the same tires reduced the forces acting upon the shocks and tires to minimize the impacts and sharpened the turning response. In my experience losing 12-18lbs of unsprung rotating mass per corner is tremendous. In the world of suspension dynamics, one-dimensional spring models fall into the same category as assuming the car is in a vacuum when calculating acceleration.

BTW, have you ever noticed my MPG numbers in my signature? Lower unsprung rotational mass is worth a couple of those MPGs.

Finally, if you always drive your car at no more than 5/10ths all of this discussion is meaningless and I would think you would be better off buying a car with nonRFTs and full size spare - you can always add the inflator and plug kit as back-up at a later time.
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  #52  
Old 07-15-2013, 03:22 PM
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BTW, have you ever noticed my MPG numbers in my signature? Lower unsprung rotational mass is worth a couple of those MPGs.
Yes, I've wondered, because the MPG I'm getting in my 550, although mostly city lead foot driving, is disappointing. You're probably right about the reduced rotational mass improving MPG.

Nice posts about unsprung weight. Reducing it does make a difference. I wish I had the $ right now to reduce mine
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  #53  
Old 07-15-2013, 03:45 PM
The X Men The X Men is offline
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You rang...

As an engineer myself, I have worked out the force equations and conducted testing on my own cars. My conclusion is that the benefits are real with cost being the only negative. In my previous post, I overlooked the fact that for my 335d I did do the intermediate step of swapping the tires first. Why did I feel I needed to swap out the wheels after I made the miraculous change to a nonRFT, especially since the tire alone stopped the pothole explosions? Well, the heavy OEM wheel exacerbated the softness of the nonRFT where the tire would bottom out on potholes. Also, the heavier wheels were more resistant to changes in direction, which also worked against the inherent flexibility of the nonRFT in terms of turning crispness. Installing lighter wheels with the same tires reduced the forces acting upon the shocks and tires to minimize the impacts and sharpened the turning response. In my experience losing 12-18lbs of unsprung rotating mass per corner is tremendous. In the world of suspension dynamics, one-dimensional spring models fall into the same category as assuming the car is in a vacuum when calculating acceleration.

BTW, have you ever noticed my MPG numbers in my signature? Lower unsprung rotational mass is worth a couple of those MPGs.

Finally, if you always drive your car at no more than 5/10ths all of this discussion is meaningless and I would think you would be better off buying a car with nonRFTs and full size spare - you can always add the inflator and plug kit as back-up at a later time.
I guess a lot of us drives a train in this forum I happen to drive the electrical part of the train myself. As an fellow engineer, it sure would be interesting to see some of those force equation or simulations you did. I am not a mechanical engineer, but we are talking about 4000 pounds of down force on those wheels, reducing the weight of the wheels by a few pounds will not make a tremendous difference in my opinion. If the wheels were suspended in mid air, that would be a different story. Of course the down force will decrease as the velocity of the vehicle increase and the centrifugal force of the wheel will also increase with speed.
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  #54  
Old 07-15-2013, 04:10 PM
DavidNJ DavidNJ is offline
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Originally Posted by cordoor View Post
Yes, I've wondered, because the MPG I'm getting in my 550, although mostly city lead foot driving, is disappointing. You're probably right about the reduced rotational mass improving MPG.

Nice posts about unsprung weight. Reducing it does make a difference. I wish I had the $ right now to reduce mine
First, I wouldn't be proud of 14.9mpg.

Second, unless all you did was change the wheels, you have no evidence to that effect.

Third, it is possible that it improves mileage since it reduce rotational inertia. Fuel economy, acceleration, braking are all affected. The effect could be measurable, if it was 1mpg that would be surprising.

But it wouldn't change the ride/handling in any measurable way.
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  #55  
Old 07-15-2013, 05:28 PM
DavidNJ DavidNJ is offline
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Originally Posted by cordoor View Post
Yes, I've wondered, because the MPG I'm getting in my 550, although mostly city lead foot driving, is disappointing. You're probably right about the reduced rotational mass improving MPG.

Nice posts about unsprung weight. Reducing it does make a difference. I wish I had the $ right now to reduce mine
The highest mileage on the F10 would probably come with the Continental PureContact with EcoPlus Technology available in standard 535 (245/45-18) and 528 (225/55-17) sizes. It has a wear rating of 700.

The stickier performance tires tend not to have the highest mileage and sometimes may be more prone to punctures (because they pick up the debris which is later pushed through the tire).
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  #56  
Old 07-15-2013, 06:32 PM
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Originally Posted by cordoor View Post
Yes, I've wondered, because the MPG I'm getting in my 550, although mostly city lead foot driving, is disappointing. You're probably right about the reduced rotational mass improving MPG.

Nice posts about unsprung weight. Reducing it does make a difference. I wish I had the $ right now to reduce mine
Thanks.

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Originally Posted by The X Men View Post
I guess a lot of us drives a train in this forum I happen to drive the electrical part of the train myself. As an fellow engineer, it sure would be interesting to see some of those force equation or simulations you did. I am not a mechanical engineer, but we are talking about 4000 pounds of down force on those wheels, reducing the weight of the wheels by a few pounds will not make a tremendous difference in my opinion. If the wheels were suspended in mid air, that would be a different story. Of course the down force will decrease as the velocity of the vehicle increase and the centrifugal force of the wheel will also increase with speed.
Although my true calling is post hole digging, I try to make sure the cattle pusher on the train moves through the air as cleanly as possible. If not, I'll mount a rocket launcher on the engine to take car of business.

From a comfort perspective, it's not the 4000lbs pushing down that is the real issue, it's the force of the wheel pushing upward that hurts the ride. If the tire hits a bump or the edge of a pothole, it already has a thousand pound load pushing down on it, but the wheel and tire will have no choice but respond to the change in elevation. So, the bump pushes the 90lbs of wheel/tire/brake upward with a high instantaneous acceleration (many Gees) which results in a sufficient force to compress the suspension completely and deflect the car upward, which means the force is in excess of the 1,000lbs of car normally sitting over that corner. So now let's reduce our weight by 18lbs or by 1/5th. Now when we hit the same pothole, the wheel only pushes upward with 80% of the force and the suspension isn't compressed as hard and car doesn't lift as much, or maybe not at all. A small change in overall weight has a large impact if it is in a crucial location.

So that was phase one. What about phase two? Huh there's a phase two? Sure there is.

Even after the wheel has reached the peak of deflection at the top of the bump, the 90lbs of unsprung weight still has large amount of momentum and this momentum acts to continue lift the tire off of the ground, while the suspension attempts to push it back down. Airborne tires can lead to a dangerous situation. In the case of the lighter setup, there's 20% less momentum and the tire is more likely to retain contact with the road surface. Remember kids, safety first.

Now the explanation above wa an unsprung weight discussion which assumed the identical tire and the only difference was the unsprung weight. In the same scenario, the nonRFT will deflect more than an RFT to absorb more of the impact, which why nonRFTs are praised for their comfort. If the impact is sufficiently large enough, the nonRFT may deflect too much and cause damage to the rim, which is the danger with heavy rims and soft tires; insufficient deflection of the wheel andtoo much deflection of the tire. RFTs tend to cause less rim damage, unless the impact was sifficiently large enough to destroy the RFT sidewall which is what happened on many GY Eagle LS II. A bent rim on an RFT usually means the tire is probably toast. I guess I should go back and add that to mu plusses and minuses.

Anyway, as posted comfort is only one of the benefits from the improved physics associated with decreased unsprung mass. Acceleration, braking, balance, crispness, and fuel economy all benefit from decreased mass - assuming the rest of the suspension system compatible with such an improvement. No myths, just physics.

BTW, I'll take a look to see if I can find some of my old notes. IIRC, the hardest part was estimating the segmentation of the wheel construction to integrate the mass properly rather than assuming a solid disk which what most people do. I always hated cylindrical coordinates, but wheels are much more dynamic than dead weight.

Below is an interesting article, that isn't necessarily an apples-to-apples comparison for our discussion, but if one looks at how each width, profile, and weight affects the physics of the situation a lot of information can be gleened form this chart below.


Effects-of-upsized-wheels-and-tires-tested
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  #57  
Old 07-15-2013, 07:31 PM
DavidNJ DavidNJ is offline
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Thanks.



Although my true calling is post hole digging, I try to make sure the cattle pusher on the train moves through the air as cleanly as possible. If not, I'll mount a rocket launcher on the engine to take car of business.

From a comfort perspective, it's not the 4000lbs pushing down that is the real issue, it's the force of the wheel pushing upward that hurts the ride. If the tire hits a bump or the edge of a pothole, it already has a thousand pound load pushing down on it, but the wheel and tire will have no choice but respond to the change in elevation. So, the bump pushes the 90lbs of wheel/tire/brake upward with a high instantaneous acceleration (many Gees) which results in a sufficient force to compress the suspension completely and deflect the car upward, which means the force is in excess of the 1,000lbs of car normally sitting over that corner. So now let's reduce our weight by 18lbs or by 1/5th. Now when we hit the same pothole, the wheel only pushes upward with 80% of the force and the suspension isn't compressed as hard and car doesn't lift as much, or maybe not at all. A small change in overall weight has a large impact if it is in a crucial location.

So that was phase one. What about phase two? Huh there's a phase two? Sure there is.

Even after the wheel has reached the peak of deflection at the top of the bump, the 90lbs of unsprung weight still has large amount of momentum and this momentum acts to continue lift the tire off of the ground, while the suspension attempts to push it back down. Airborne tires can lead to a dangerous situation. In the case of the lighter setup, there's 20% less momentum and the tire is more likely to retain contact with the road surface. Remember kids, safety first.

Now the explanation above wa an unsprung weight discussion which assumed the identical tire and the only difference was the unsprung weight. In the same scenario, the nonRFT will deflect more than an RFT to absorb more of the impact, which why nonRFTs are praised for their comfort. If the impact is sufficiently large enough, the nonRFT may deflect too much and cause damage to the rim, which is the danger with heavy rims and soft tires; insufficient deflection of the wheel andtoo much deflection of the tire. RFTs tend to cause less rim damage, unless the impact was sifficiently large enough to destroy the RFT sidewall which is what happened on many GY Eagle LS II. A bent rim on an RFT usually means the tire is probably toast. I guess I should go back and add that to mu plusses and minuses.

Anyway, as posted comfort is only one of the benefits from the improved physics associated with decreased unsprung mass. Acceleration, braking, balance, crispness, and fuel economy all benefit from decreased mass - assuming the rest of the suspension system compatible with such an improvement. No myths, just physics.

BTW, I'll take a look to see if I can find some of my old notes. IIRC, the hardest part was estimating the segmentation of the wheel construction to integrate the mass properly rather than assuming a solid disk which what most people do. I always hated cylindrical coordinates, but wheels are much more dynamic than dead weight.

Below is an interesting article, that isn't necessarily an apples-to-apples comparison for our discussion, but if one looks at how each width, profile, and weight affects the physics of the situation a lot of information can be gleened form this chart below.


Effects-of-upsized-wheels-and-tires-tested
Ok, I get it, you're not an engineer. However, I strongly believe differential equations followed by system dynamics should be required for all college graduates.

First, yes, rotational inertia affects performance. The heavier sticker tires accelerated slower on a relatively low powered car (86-88mph in the 1/4 mile). Notice, that it wasn't enough to effect braking. The wider tires had a bigger impact when the performance the inverse of a 911 Turbo S (2.9sec 60-0). That is similar to the question which accelerates faster, RWD or xDrive? It depends how much power you are trying to put down.

However, nothing there about ride or handling. When you suddenly unload a wheel (a step function) it doesn't accelerate with all the energy stored in the spring. First, any left/right coupling (the anti roll bar) reduces the effective force from the spring. Second, it is directly opposed by the shock. Then, as the displacement increases, there is less force in the spring...until it is unloaded. There is minimal underdamping. When you you hit a bump your car doesn't oscillate up and down. Dropping off a 2"-3" curb (unloading one wheel without hitting the tire edge) probably wouldn't hurt anything. Running into a 2"-3" curb, where the upward motion of the tire was opposed by the shock (stiffer than the spring at those frequencies), the spring, the anti-rollbar, and the mass of the car (as the wheel goes up it has a greater percentage of the weight on it). That causes a huge load on the tire (which is also a spring damper). When the tire bottoms (bent wheel anyone?) the forces on the wheel or much higher.

Does it have to be that way? No. Off road racing trucks handle bumps much bigger than that without a problem. I've seen mountain bikes, especially downhill models with huge suspension travel, run into curbs at speed and just smoothly go on the sidewalk.

How? Lots of wheel travel. Large wheel diameter. Lots of tire sidewall height to deflect. Off road trucks have multiple shocks to increase the force there. The shocks have two sets of valving depending on the wheel displacement. They have hydraulic bump stops. To get the huge wheel travel you need dual rate springs. At ride height the softest springs are collapsed. This is also done with stock car springs, especially on the rear, where they would unload at full extension.

Diameter counts. A larger diameter can drive over obstacles. Here in the Northeast mountain bike tires have gone from 27" to 29" with a significant increase in the number of hardtail models.

If you remember the old days BMWs used accelerate much more quickly than you'd expect based on their horsepower ratings. One of the reasons is the engine had less rotational inertial Small rod and crank journals. etc. Earlier this year Matt Kenseth was penalized heavily in NASCAR because one connecting rods was underweight.

This also affects valve trains. Not from overall vehicle inertia, but because inertia in the valve train requires stiffer valve springs which soak up power (turning it into heat), put a load on the camshaft, and are harder to control. To control valve spring heat race cars have oil sprays on the springs, and the springs themselves get special coatings.

While we have gotten this far afield (those new to this thread can go back a few posts to find non-runflat recommendations) we can talk about Mercedes Active Body Control. The only active suspension on the market, it uses hydraulic cylinders to move the wheel up and down. However, limited to 100Hz and being reactive, it really could only respond to driver input: body roll, dive, lift.

For 2014 they have one of the most innovative solutions I've seen in automotive use. They use the stereo camera used for lane departure detection and adaptive high beams to read the road surface. It then can detect the pothole or curb in the above example. With that knowledge it can push the wheel down or pull it up just like you would move your foot to a stair you same...or to the same curb. It can't read everything, but would handle many of the issues discussed.

Here unsprung weight differences of the magnitude being discussed here still aren't a major issue.
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  #58  
Old 07-15-2013, 10:00 PM
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Ok, I get it, you're not an engineer. However, I strongly believe differential equations followed by system dynamics should be required for all college graduates.
Dude, I have a Ph.D. in Engineering with over 100 hours of graduate course work in AE, ME, EE, ESM, ChemE, Math, Physics, CompSci, and a few electives in Mgt & Acct for fun. Tack on another 48hrs of dissertation and then we have an engineering degree worth talking about. So yes, in my undergraduate days I certainly found the time to take the basics like ODEs & Dynamics, but as you may now be realizing, you aren't really "the" engineer around here.

Anyway please go buy a car, or better yet, use your existing car and buy multiple varieties of tires, wheels, springs, shocks, and develop and execute a comprehensive test plan to collect significant amounts quantitative data, then analyze that data and share your thoughtful insights with the rest of us. Just please stop guessing our empirical results based on a bicycle you saw going down a hill. Can you do that for us? Hopefully, there won't be an obstinant person that refuses to believe your real world experience because he has a simplistic theory based on what he learned in a Dynamics class 30 years ago.


BTW, mountain bike wheels actually went from 26" to 29", which is basically the 700c standard which has been on hardtail cyclocross bikes for about a hundred years. So today's hot trend in mountain bikes is basically a cyclocross bike with fatter tires. Guess what? I saw Greg Lemond in a high wheel bike race last year, so I know what's next big thing in mountain biking.
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  #59  
Old 07-15-2013, 11:14 PM
evanswan evanswan is offline
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swap

Just put on Michelin PSS non run flat 550i. much nicer ride. better grip.
It's just more enjoyable. Is a pinch softer when getting aggressive. But nothing that is significant to detour form the other benefits.

I love um
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  #60  
Old 07-15-2013, 11:18 PM
DavidNJ DavidNJ is offline
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Dude, I have a Ph.D. in Engineering with over 100 hours of graduate course work in AE, ME, EE, ESM, ChemE, Math, Physics, CompSci, and a few electives in Mgt & Acct for fun. Tack on another 48hrs of dissertation and then we have an engineering degree worth talking about. So yes, in my undergraduate days I certainly found the time to take the basics like ODEs & Dynamics, but as you may now be realizing, you aren't really "the" engineer around here.

Anyway please go buy a car, or better yet, use your existing car and buy multiple varieties of tires, wheels, springs, shocks, and develop and execute a comprehensive test plan to collect significant amounts quantitative data, then analyze that data and share your thoughtful insights with the rest of us. Just please stop guessing our empirical results based on a bicycle you saw going down a hill. Can you do that for us? Hopefully, there won't be an obstinant person that refuses to believe your real world experience because he has a simplistic theory based on what he learned in a Dynamics class 30 years ago.

BTW, mountain bike wheels actually went from 26" to 29", which is basically the 700c standard which has been on hardtail cyclocross bikes for about a hundred years. So today's hot trend in mountain bikes is basically a cyclocross bike with fatter tires. Guess what? I saw Greg Lemond in a high wheel bike race last year, so I know what's next big thing in mountain biking.
These forum posts are the ways to gain empirical data. If you notice, it takes a while for people browsing the forum to post. You see issues that weren't expected (the RFT tire failures). You see different people's solutions. Sometimes I argue with misinformation, sometimes I let it pass. Depends on if it is clouding the data.

Oh...I have a bit of more recent experience.
My mountain bike is about 15 years old (Ventana Marble Peak)...and yes I've played with the rear shock pressure and front shock springs and oil. You did catch a typo though. However, the wheel size think is a topic these days: Shootout: Wheel Wars 29 vs 27.5 vs 26

You are the one who posted that rather strange description of how a suspension responds. I can say one thing, little changes are hard to measure. If the laps are in range of 15 sec +/- 0.05 sec a little rotational inertia is not individually measurable. From a handling standpoint, springs are usually in about 10% increments. You can feel that. The spring rubbers can be a little harder to determine, they typically come in three stiffnesses

Net, we never break stuff from rebound on the street. It is when the tire bottoms against the rim (a sidewall bubble and probable bent rim). That may occur when the entire suspension bottom or not. It depends on the frequency of the input.

I'm guessing the RFT sidewalls are less tolerant. However, a while ago I tried 18" rims with 245/40 tires on our MB. I bought 6 rims figuring I'd have a rim that couldn't be repaired sooner or later. I went through 3 non-repairable rims within 6 months. My S2000 with similar sidewall height has had one catastrophic blowout. The rim was repairable. My Supra TT with just a bit taller tire (255/40) hasn't had any failures. Net: I'm guessing the 245/40 sidewall height is just a little to small for some tires in combination with some suspensions (the S2000 is 1500# lighter than some F10s and 1100# lighter than the MB).

My guess is we are arguing over nothing...
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  #61  
Old 07-15-2013, 11:44 PM
DavidNJ DavidNJ is offline
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Just put on Michelin PSS non run flat 550i. much nicer ride. better grip.
It's just more enjoyable. Is a pinch softer when getting aggressive. But nothing that is significant to detour form the other benefits.

I love um
Could you elaborate on your definition of "softer"? Is it in steering response? Ride firmness?

BTW, which tire are you comparing it to?
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Old 07-15-2013, 11:58 PM
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Re: How Much Did Non-Runflats Help Your Handling

I changed out my rft's eagle rsa's on me e60 immediately after buying it. I can't quite put my finger on it... But my non rft continental dws are just better. Grip, wear, noise, price. Just better. You won't regret getting rid of rft trust me.

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Old 07-16-2013, 05:55 AM
radarguy radarguy is offline
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dunderhi
I've been sitting back and listening to the misinformation in this discussion and was about to correct some of it, but your sprung to unsprung weight explanation is exactly correct. The referenced Car and Driver article is very good and appropriate for this discussion. It brings a lot of the theoretical stuff into the real world.
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Old 07-16-2013, 06:35 AM
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demas, what are the roads like around Tokyo and in Japan in general? Smooth or US-like potholed and heaved? Do you have IAS?

The Primacy HP is a low performance (in the US "grand touring summer") tire that was in some 3-series sizes that BMW got Michelin to also make in a 5-series 19" sizes, including the M-Sport staggered 19" sizes. My guess is they did that because the softer low performance passenger car tire was less stiff in RFT low-profile size. That accounts for your ok ride. My guess is the PSS tires would be night and day with a similar ride.
The roads here are generally very good. I do have IAS. I know that the Primacy HP's are grand touring hence my initial desire to replace them but like I mentioned, they haven't really disappointed - to my surprise.

we'll see, I will run this setup for a while.
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Old 07-16-2013, 10:25 AM
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dunderhi
I've been sitting back and listening to the misinformation in this discussion and was about to correct some of it, but your sprung to unsprung weight explanation is exactly correct. The referenced Car and Driver article is very good and appropriate for this discussion. It brings a lot of the theoretical stuff into the real world.
Thanks!
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Old 07-16-2013, 10:47 AM
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Although my true calling is post hole digging, I try to make sure the cattle pusher on the train moves through the air as cleanly as possible. If not, I'll mount a rocket launcher on the engine to take car of business.

From a comfort perspective, it's not the 4000lbs pushing down that is the real issue, it's the force of the wheel pushing upward that hurts the ride. So that was phase one. What about phase two? Huh there's a phase two? Sure there is.

Anyway, as posted comfort is only one of the benefits from the improved physics associated with decreased unsprung mass. Acceleration, braking, balance, crispness, and fuel economy all benefit from decreased mass - assuming the rest of the suspension system compatible with such an improvement. No myths, just physics.
Doctor dunderhi, I too have a true calling, but it is not in post hole digging As far as comfort, the spring rate and the shocks on the F10 is design to handle the weight of the heavier runflats, changing the weight of the tires may result in the ride being too soft, I think you have mention that already, which might necessitate the need to stiffen up the springs and the shocks. The delta in accelaration time from that C&D article seems a bit high but it certainly look close enough that it might be within the margin of error of that test. Braking distance seem a bit odd, I would think the heavier wheel should take longer to brake due to the added inertia. but like the editor said, that could be due to the wider tires. Handling wise, which was the original topic of this thread seem to favor the heavier wheel as well, again that might be due to the wider tires too. As the speed of the car increase, it take less boost from the power steering to mover the front wheels, Knowing that, I am not sure how much a role a heavier wheel will play in high speed handling.
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Old 07-16-2013, 12:38 PM
DavidNJ DavidNJ is offline
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The roads here are generally very good. I do have IAS. I know that the Primacy HP's are grand touring hence my initial desire to replace them but like I mentioned, they haven't really disappointed - to my surprise.

we'll see, I will run this setup for a while.
In all the tests the difference is only a small amount in absolute grip. Few people drive hard enough to deal with the slip angle differences at the limit. On the street, we see initial response and a progressive increase in effort. The runflats usually excel at the first and fall down a bit on the latter.

To soft for the suspension is a bit more complicated. Ideally, we want the the suspension to work with the tire to maintain an even load during undulations. That allows the tire to maintain a constant slip angle and grip so the driver isn't sawing at the wheel. It gives a sense of control. The other aspect, roll angle, which affects how the tire is aligned with the ground, is generally unaffected by the tire itself. However, BMW may have softened roll stiffness on some models to compensate for the stiff tire sidewalls.

While I couldn't find a tire test with graphs, this Car and Driver best handling car test has graphs. The Best-Handling Car in America for Less Than $100,000 - Comparison Tests . Of specific interest are the graphs of steering angle and steering effort. When driving hard, does the steering feel linear with increasing effort until a gradual roll off near the limit (which you will never see on the street). Does the tire maintain constant angle with minute effort fluctuations over road imperfections?

The runflats should give sharp or sharper initial turn in (the Z06 and GTR in the Car and Driver test are on runflats) but then respond a little less predictably.

This is a back to back track test of RE70A RFTs (not an F10 tire) with PSS tires on a GTR: http://topspeedtuned.com/official-gt...-review-by-tsm. This is the video to go with it:

There could have been a bit different front/rear roll stiffness built in for the different tires. However, doesn't both xDrive and IAS have active yaw rate management? In that case, the effect should be negligible.

BTW, how do you like IAS? Most owners love it. The US magazine panned it initially so BMW doesn't provide them with it on cars anymore. I would imagine it is quite useful in Tokyo. The HK and Singapore forum posters seem to think it is an important feature to have in those cities.
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Old 07-16-2013, 12:51 PM
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Doctor dunderhi, I too have a true calling, but it is not in Post hole Digging As far as comfort, the spring rate and the shocks on the F10 is design to handle the weight of the heavier runflats, changing the weight of the tires may result in the ride being too soft, I think you have mention that already, which might necessitate the need to stiffen up the springs and the shocks. The delta in accelaration time from that C&D article seems a bit high but it certainly look close enough that it might be within the margin of error of that test. Braking distance seem a bit odd, I would think the heavier wheel should take longer to brake due to the added inertia. but like the editor said, that could be due to the wider tires. Handling wise, which was the original topic of this thread seem to favor the heavier wheel as well, again that might be due to the wider tires too. As the speed of the car increase, it take less boost from the power steering to mover the front wheels, Knowing that, I am not sure how much a role a heavier wheel will play in high speed handling.
Yes, less sprung weight allows you to add stiffer suspension components to return to the original comfort level, yet still benefit from the stiffer suspension's handling characteristics. Dinan's Shockware might be the cheapest route. Wow, did I just say Dinan was the cheapest at something?

I think the C&D article made a pretty good attempt at capturing the diffences and anomalies. The tires did widen and stifffen the tires while the weight was added, so despite the weight addition, things like braking performance were offset by tire width. The same could be said about the width and lower profile in skidpad performance. The change in the tire compound could have also been an influence. Finding apples-to-apples test are dificult. The key reason for linking this article was to show that they are significant changes when changing the wheels and tires. Anyway, the tire and wheel are major parts of the suspension system. Given that the F10 has been out for a couple of years now, there a few options to modify the car's performance or dare I say it - leave it the was it was the day it left the factory.
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Old 07-16-2013, 01:30 PM
DavidNJ DavidNJ is offline
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This thread is beginning to sound like one on global warming.

Two spreadsheets people put online to calculate wheel/tire rotational inertia. Have fun:

http://www.mazdas247.com/forum/attac...1&d=1173530905

http://the-welters.com/racing/rotational.xls

For those who think unsprung weight affects ride/handling, what do you think of solid axles? Or the solid axle Mustang finishing 3rd in Car and Driver's best handling car under $40k?
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Old 07-16-2013, 02:08 PM
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For those who think unsprung weight affects ride/handling, what do you think of solid axles? Or the solid axle Mustang finishing 3rd in Car and Driver's best handling car under $40k?
While we're leaping all over the net, what do you think of this clear statement on BMW's own website that reducing unsprung mass benefits ride and handling ?

http://www.bmw.com/com/en/owners/acc...oy_wheels.html

"Original BMW light alloy wheels emphasise the dynamic character and exclusive style of your BMW. Not only that, these lightweight cast aluminium wheels also reduce unsprung masses, help the suspension to smother surface imperfections and improve handling and braking"
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Old 07-16-2013, 02:48 PM
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This thread is beginning to sound like one on global warming.

Two spreadsheets people put online to calculate wheel/tire rotational inertia. Have fun:

http://www.mazdas247.com/forum/attac...1&d=1173530905

http://the-welters.com/racing/rotational.xls
Using your inertia spread sheets, my 72lb unsprung rotating mass reduction is equivalent to 344lbs of chassis weight reduction, resulting in a 2.4% improvement in acceleration/braking while freeing up 77ftlbs of torque to improve my gas mileage. So you are okay with all of this, but you still think unsprung weight is a myth?

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Originally Posted by DavidNJ View Post
For those who think unsprung weight affects ride/handling, what do you think of solid axles? Or the solid axle Mustang finishing 3rd in Car and Driver's best handling car under $40k?
I thought you didn't give any credence to magazines in your early posts. Anyway a solid axle adds more weight and they are rarely used outside of pick-up trucks today. Was Ford able to put a heavy solid axle in the front? No, because that blunder alone would have been enough for Ford to lose all engineering credibility and signal their ultimate demise.

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Originally Posted by morellomax View Post
While we're leaping all over the net, what do you think of this clear statement on BMW's own website that reducing unsprung mass benefits ride and handling ?

http://www.bmw.com/com/en/owners/acc...oy_wheels.html

"Original BMW light alloy wheels emphasise the dynamic character and exclusive style of your BMW. Not only that, these lightweight cast aluminium wheels also reduce unsprung masses, help the suspension to smother surface imperfections and improve handling and braking"
You mean the German Engineers at BMW now use the myth of unsprung weight to improve the performance of their cars. Man, I bet those M5 guys are really upset they have forged wheels and nonRFTs based upon a myth.
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Old 07-16-2013, 05:05 PM
DavidNJ DavidNJ is offline
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Are those analyses dead accurate? No, but close enough for government work.

2%...as it affects power limited acceleration or braking. However, braking is usually always traction limited and acceleration is traction limited off the line (or longer if you are trying to put down 600hp as many current sedans do.)

On fuel economy it is a bit different. The energy isn't being turned into heat as it is under braking. It is being turned into kinetic energy. If you minimize your braking, most of that energy is returned. Csaba Csere wrote an editorial about that.

2% on acceleration is maybe 1/10th of a second 0-60. It could be 2mph in the 1/4 mile. However, it is more likely to show up in lightweight engine parts rotating at 7000rpm than in wheels rotating at 600-1000 rpm. If it added 2% to fuel economy, that would be a couple of 10ths of a mpg, not something you can reliably repeat.

Different magazines at different times have different qualities to their tests. Back in the 1960s and a couple of years into the 1970s there was Sports Car Graphic. Even in the days of limited instrumentation, they had numerically intense road tests including understeer angle left/right and roll angle. They also published Carroll Smith's series of 7 articles are race car preparation that later became his book Prepare to Win.

More recently, occasionally, Car and Driver leverages some of the advanced measurement tools available. This was one of the cases and it provides some insights into what we consider 'good' handling and 'bad' handling.

But this was about lightweight and handling. Solid rear axles handle without issue. Ford used its dual I-beam semi-solid axle Front axle on even small trucks until recently.



Really, this isn't a big deal. The differences between tires for exceed an mass differences among the alternatives. Most people here seem to be sticking with the stock wheels. The inertia differences being considered are barely measurable with a stopwatch.

Even if the stock suspension may have non-optimal damping for a non-runflat (the GTR test I posted found the PSS didn't quite have the balance in stock sizes), the dynamic dampers, either standalone or part of Adaptive Drive, probably cover the range adequately. Most people here are leasing, so making substantial suspension mods is probably not a good option.

For the longest time I couldn't get the super late model to handle. Everyone had recommendations. Nothing worked. Then I talked to the manufacturer, set it back to stock settings, and worked from there. I ended up making the front softer and softer. The front bar a tiny 1". Stiffening the back. Raising the panhard bar. Lowering the front roll center (longer upper control arms). Changing the kingpin angle (different front spindles) and slowly but surely it started to work. Twitchy exits needed a rear roll steer adjustment (trailing arm angle). Then I swapped the whole thing out for a soft spring/big bar setup. But did it from a known baseline.

In this case, everything is much simpler. It is really just tires.

But just to throw a wrench in the works, the 2014 Corvette is rolling on Michelin Pilot Super Sport ZPs made in North Carolina . Not listed on the Michelin website yet or at Tire Rack, and the Corvette size isn't an F10 size. However, this tire may come in F10 sizes and would represent the latest runflat technology applied to the latest high performance technology.
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Old 07-16-2013, 06:27 PM
The X Men The X Men is offline
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This thread is like watching a fight between two heavy weight, so far, too close to call. I really think the truth is somewhere in the middle. Tire weight does make a difference in MPG but not as much as people think. Acceleration wise, at 45 MPH, a 20 inch wheel will spin at 2,376 RPM, given those numbers, the weight will make a difference in acceleration, just not as much as most people think. Regarding the topic of the thread, handling, it seems to be the consensus that runflats have a sharper turn in, but the conventional tires excel in handling thereafter, I will call it a wash.
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Old 07-16-2013, 07:04 PM
DavidNJ DavidNJ is offline
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Originally Posted by The X Men View Post
This thread is like watching a fight between two heavy weight, so far, too close to call. I really think the truth is somewhere in the middle. Tire weight does make a difference in MPG but not as much as people think. Acceleration wise, at 45 MPH, a 20 inch wheel will spin at 2,376 RPM, given those numbers, the weight will make a difference in acceleration, just not as much as most people think. Regarding the topic of the thread, handling, it seems to be the consensus that runflats have a sharper turn in, but the conventional tires excel in handling thereafter, I will call it a wash.
Lets go the booth to look at the video on this referee call.

A F10 tire typically is around 780 rev/mi. at 45mph (3/4 mi/min) it is turning around 585 rpm. 2376rpm would be around 180mph. In most parts of the country that is a very serious speeding ticket.

I don't think there is any disagreement that that weight does affect acceleration. Just that the 8 lb/wheel difference between an Michelin PSS and Bridgestone RE50A RFT is not very substantial. About the same as 10 gallons of gas. Or about the weight of a 9 year old.
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Old 07-16-2013, 09:25 PM
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2%...as it affects power limited acceleration or braking. However, braking is usually always traction limited and acceleration is traction limited off the line (or longer if you are trying to put down 600hp as many current sedans do.)

On fuel economy it is a bit different. The energy isn't being turned into heat as it is under braking. It is being turned into kinetic energy. If you minimize your braking, most of that energy is returned. Csaba Csere wrote an editorial about that.

2% on acceleration is maybe 1/10th of a second 0-60. It could be 2mph in the 1/4 mile. However, it is more likely to show up in lightweight engine parts rotating at 7000rpm than in wheels rotating at 600-1000 rpm. If it added 2% to fuel economy, that would be a couple of 10ths of a mpg, not something you can reliably repeat.
At this point you are clearly making up numbers without a single calculation or measurement to back up your 0.1s, 2mphs, or 10ths of an mpg. I owned a 2011 BMW 550xi for 38kmi. I swapped out the wheels, tires, springs, exhaust, and had the engine tuned. I measured my acceleration times both via GPS and at the dragstrip. I repeated high Gee maneuvers in the same locations and noted the improvements. I conducted slalom like maneuvers to assess control in emergency situations. I recorded every drop of fuel for 69 fill-ups. No I am not your typical F10 owner, but I know these cars shine their best when they are pushed hard. I have collected plenty of empirical data on the F10. You have collected none yourself, but you continue to tell me what I observed firsthand is a lie. I've been a part of this forum since the beginning when we hoping for an aluminum chassis on the F10, or at least an aluminum subframe. So over the years, I have sparred with the internet engineers and the old-timey racers who were adamant that a RWD F10 would always be faster than an AWD F10. I certainly proved them wrong, time and time again. They would argue with me that I would never crack 4 seconds in an F10 even with Dinan. They treated my theories as sheer blasphemy. Of course they were wrong and they stopped arguing with me about things they didn't fully understand. I assume you will continue down your path until you garner firsthand experience or grow weary of this board. You are not the first and unfortunately you won't be the last. The world is bigger than your own past experiences. Try to learn from others and not dismiss them because they have different than your expectations.

As a result of sharing my firsthand experiences here on this board, performance oriented buyers & owners that followed my recommendations and made similar purchases & modifications have been very happy with their cars. I am probably responsible for an unusually high percentage of F10 550 xDrivers here at the Fest. Heck, a guy in Louisiana bought and modded a 550 xDrive - Louisiana! Now, don't forget "true enthusiasts" only drive RWD drives as any former E60 owner will tell you. Anyway, not one of these 550xi owners has come back and called me a liar. So from my firsthand experiences I stand by my post #32 and my other posts whether you choose believe them or not. If you want to keep sparring you can, but what's the point?
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