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E90/E91/E92/E93 (2006 - 2013)
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  #26  
Old 10-24-2011, 05:15 PM
Squidget Squidget is offline
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Originally Posted by dtc100 View Post
But what if one does occasionally auotx (not on the tracks) the car? Square or staggered? Wide or very wide?
Whenever I asked that kind of question, the "real" drivers always said it was the wrong question. If you have to ask, you couldn't use the answer.

In other words, until you can consistently run constant laps, hot or cold tires, over and over, then worrying about tire tuning is rather pointless. Once your skill has exceeded the limit of the setup, then you can make better judgments about race setups.

Which is why no one wants to give you advice on tires. It depends on so many things that depend on so many things. One of those things being driving style. And without being next to you in the seat to see how you drive, we couldn't possibly tell you the right tires. Even professionals never stop playing with setups based on hundreds of factors.

Even if you have hundreds of hours of seat time, it's difficult to really judge your car setup. Hence the extensive use of telemetry to gather and analyze track data. Motec, etc. First time I saw a traction circle plot, I was blown away.

So I think the only answer is "try different setups, and see which one gives you the best time." It's impossible to know ahead of time what will work for you.

Yeah, that's a sucky answer. But it's the only honest one I have.
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  #27  
Old 10-24-2011, 05:15 PM
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Somebody needs to define under- over- and neutral steering.

I learned neutral is where the front and rear slip angles are the same, under steer is where the front slip angle is larger than the rear, and over steer where the rear slip angle is larger.

Correct?
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  #28  
Old 10-24-2011, 05:34 PM
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Quote:
Originally Posted by Llando88 View Post
Somebody needs to define under- over- and neutral steering.

I learned neutral is where the front and rear slip angles are the same, under steer is where the front slip angle is larger than the rear, and over steer where the rear slip angle is larger.

Correct?
Under- and over-steer are usually much more generalized terms. Under-steer most commonly means that the path of the front end of the vehicle is a circle of larger radius than the intended turning circle. Over-steer usually means that the path of the rear end of the vehicle is a circle of larger radius than the intended turning circle. For most intents and purposes, 'intended turning circle' may be replaced with 'the other end of the vehicle.'

Slip angle and whatnot play a major role in whether over- or under-steer are induced, but I'm not sure it can be summarized quite that simply.

Edit for clarity:

Slip angle is not, as many believe, the amount of traction loss a given tire experiences under lateral load. It is, instead, a measure of the lateral deformation of the tire and contact patch. Slip angle, specifically, is the angular measurement between the vertical flat-plane center-line of the tire and the vertical flat-plane center-line of the wheel. It's not sufficient to declare that zero slip angle is good, nor is it correct to indicate that large slip angle is best - a very exacting range of slip angles are required for various scenarios and each car (notably in F1 racing) has its suspension tuned to this specific range of slip angles.
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Last edited by CT; 10-24-2011 at 05:46 PM.
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  #29  
Old 10-24-2011, 05:37 PM
dtc100 dtc100 is offline
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My understanding in the most elementary form is, during a hard turn, if the front tires lose traction, not the rear, you have understeer.

If the rear tires lose traction but not the front, you have oversteer.

My experience matched the book, understeer is easy to correct, just left off the throttle a little your front tires will regain traction.

Oversteer is difficult to correct, frankly in my very limited autox experience I had not tried to correct them
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  #30  
Old 10-24-2011, 05:40 PM
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Originally Posted by Squidget View Post
Since you have the experience tracking a 335 with square setup, then you can answer my bet! With square setup, what's the bias? Not sharp turning, but when the body is relatively settled. Say, a high-speed sweeper turn?

Over, under, or neutral steer?
With Kumho Ecsta or Dunlop Sports, neutral, nothing but neutral. It's a beautiful thing. Take a look at a 'map' of NHMS, specifically the 'esses' coming onto the main straight. In the middle of the last 's' you are accelerating balls to the wall in third gear with a left hand sweeper onto the track. That sure meets your definition. My car is rock solid in an area where the 'line' extends to within feet of an 8' concrete wall. No margin for error and none required.

Last edited by DSXMachina; 10-24-2011 at 05:42 PM.
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  #31  
Old 10-24-2011, 05:45 PM
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Originally Posted by DSXMachina View Post
With Kumho Ecsta or Dunlop Sports, neutral, nothing but neutral. It's a beautiful thing. Take a look at a 'map' of NHMS, specifically the 'esses' coming onto the main straight. In the middle of the last 's' you are accelerating balls to the wall in third gear with a left hand sweeper onto the track. That sure meets your definition. My car is rock solid in an area where the 'line' extends to within feet of an 8' concrete wall. No margin for error and none required.
Would a staggered setup have broken the neutral?
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  #32  
Old 10-24-2011, 06:02 PM
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Originally Posted by Squidget View Post
Whenever I asked that kind of question, the "real" drivers always said it was the wrong question. If you have to ask, you couldn't use the answer.

In other words, until you can consistently run constant laps, hot or cold tires, over and over, then worrying about tire tuning is rather pointless. Once your skill has exceeded the limit of the setup, then you can make better judgments about race setups.

Which is why no one wants to give you advice on tires. It depends on so many things that depend on so many things. One of those things being driving style. And without being next to you in the seat to see how you drive, we couldn't possibly tell you the right tires. Even professionals never stop playing with setups based on hundreds of factors.

Even if you have hundreds of hours of seat time, it's difficult to really judge your car setup. Hence the extensive use of telemetry to gather and analyze track data. Motec, etc. First time I saw a traction circle plot, I was blown away.

So I think the only answer is "try different setups, and see which one gives you the best time." It's impossible to know ahead of time what will work for you.

Yeah, that's a sucky answer. But it's the only honest one I have.
Damn you Squidget, you're killing all the fun. Come on, tell us why XYZ is really the best setup in the world and everything else is junk!
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  #33  
Old 10-24-2011, 06:20 PM
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Originally Posted by dtc100 View Post
Would a staggered setup have broken the neutral?
No, and I can tell you that from experience. And of course theoretically it shouldn't. If all four (square) tires are neutral, then all four staggered tires should be neutral. Remember, your scenario was a high speed sweeper. All corners of my car are planted and sticking. Now if you change the scenario so I am running at 99% adhesion and I am just starting to crab (stuttering intermittent sideways movement) then all other things being equal the rear will stay planted and the front may move out more if you're not fast to respond.
For me, the advantage of the wider rears is that I could accelerate harder out of the turns without hopping the back end. That said, I went back to non staggered because I just felt more comfortable with the handling. For my car, on that track, the way I drive, with the tires I have, and the risks I am willing to take, I feel more in tune with the handling. (That's probably a Squidget approved answer.)
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  #34  
Old 10-24-2011, 06:32 PM
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Originally Posted by DSXMachina View Post
No, and I can tell you that from experience. And of course theoretically it shouldn't. If all four (square) tires are neutral, then all four staggered tires should be neutral. Remember, your scenario was a high speed sweeper. All corners of my car are planted and sticking. Now if you change the scenario so I am running at 99% adhesion and I am just starting to crab (stuttering intermittent sideways movement) then all other things being equal the rear will stay planted and the front may move out more if you're not fast to respond.
For me, the advantage of the wider rears is that I could accelerate harder out of the turns without hopping the back end. That said, I went back to non staggered because I just felt more comfortable with the handling. For my car, on that track, the way I drive, with the tires I have, and the risks I am willing to take, I feel more in tune with the handling. (That's probably a Squidget approved answer.)
You can see the cars queueing up to take a look at your handling in your photo...

Someone needs to say something about weight distribution and weight transfer here at some point. [I haven't reached that part in "Tune to Win" by Carroll Smith yet.]
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  #35  
Old 10-24-2011, 06:34 PM
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Originally Posted by ///M-ratedE90 View Post
You can see the cars queueing up to take a look at your handling in your photo...

Someone needs to say something about weight distribution and weight transfer here at some point. [I haven't reached that part in "Tune to Win" by Carroll Smith yet.]
That's a little bit farther down the rabbit hole than this thread warrants, isn't it? Though if we were talking about mid-70s Widowmaker Porsches I think it'd be on point.
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  #36  
Old 10-24-2011, 06:37 PM
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Originally Posted by Llando88 View Post
Somebody needs to define under- over- and neutral steering.

I learned neutral is where the front and rear slip angles are the same, under steer is where the front slip angle is larger than the rear, and over steer where the rear slip angle is larger.

Correct?
Understeer is hitting the wall nose first. Oversteer is hitting the wall ass first. Horsepower is how fast you're going when you hit the wall. Torque is how far you push the wall once you hit it.

Sorry, couldn't resist. Some great info in this thread!
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  #37  
Old 10-24-2011, 07:02 PM
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Originally Posted by ///M-ratedE90 View Post
You can see the cars queueing up to take a look at your handling in your photo...

Someone needs to say something about weight distribution and weight transfer here at some point. [I haven't reached that part in "Tune to Win" by Carroll Smith yet.]
Hey I worked hard composing that photo! (I pretended not to see them on the main straight.)
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  #38  
Old 10-24-2011, 07:15 PM
dtc100 dtc100 is offline
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Originally Posted by DSXMachina View Post
...For me, the advantage of the wider rears is that I could accelerate harder out of the turns without hopping the back end. That said, I went back to non staggered because I just felt more comfortable with the handling. For my car, on that track, the way I drive, with the tires I have, and the risks I am willing to take, I feel more in tune with the handling. (That's probably a Squidget approved answer.)
If I may speak theoretically.

Say your square setup now is 245 at all coners, and your staggered was 235 front 265 rear.

In theory, would a 255 square setup (again assume you can do it and weight gain is not a concern) not be the best of both worlds?
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  #39  
Old 10-24-2011, 07:28 PM
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Originally Posted by dtc100 View Post
If I may speak theoretically.

Say your square setup now is 245 at all coners, and your staggered was 235 front 265 rear.

In theory, would a 255 square setup (again assume you can do it and weight gain is not a concern) not be the best of both worlds?
Please refer to Squidget's post, above.
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  #40  
Old 10-24-2011, 07:51 PM
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Originally Posted by CT View Post
Slip angle and whatnot play a major role in whether over- or under-steer are induced, but I'm not sure it can be summarized quite that simply.

Edit for clarity:

Slip angle is not, as many believe, the amount of traction loss a given tire experiences under lateral load. It is, instead, a measure of the lateral deformation of the tire and contact patch. Slip angle, specifically, is the angular measurement between the vertical flat-plane center-line of the tire and the vertical flat-plane center-line of the wheel. It's not sufficient to declare that zero slip angle is good, nor is it correct to indicate that large slip angle is best - a very exacting range of slip angles are required for various scenarios and each car (notably in F1 racing) has its suspension tuned to this specific range of slip angles.
Damned good post.

Another way I'd put it, at the risk of over-simplification: cause and effect. Slip angle is a attribute of a car setup (a cause, one of many). over/under steering is an effect, it's an event that occurs.

That is, slip angle is a factor of the car setup, and is pretty much fixed. over/understeer is what HAPPENS, it's an event that occurs. And multiple factors can cause steering behavior.

That's why you can "induce" steering. Even if a RWD is "biased" to understeer, you can MAKE it oversteer by stomping on the throttle (so the tires spin and break away).

So, you can bias a car one direction or another by adjusting weight, tires, and a bunch of other factors. But a driver can still induce under and over steer by his actions.

Even the course you take changes the dynamics. Depending on suspension, bias could change depending on the speed and tightness of the corner.

Which is why I tried to very precise when asking DSX about balance. The fast sweeper is a relatively suspension-neutral corner (large radius, relatively constant speed), so it's a relatively reasonable benchmark when talking about bias.
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  #41  
Old 10-24-2011, 07:54 PM
dtc100 dtc100 is offline
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Originally Posted by DSXMachina View Post
Please refer to Squidget's post, above.
Well the way I read his post, my skill had already exceeded the limit of my stock 225/45/17 tires during the first several runs of my first autox event

So I am left to wonder what setup should I have for my future autox experience. I already bought a new set of 18x8.5 wheels.

Should I go 225/255, 235 square or 245 square? Since I don't have the budget in the next 6 months to try all three setups, maybe you guys can pick one out for me.

As for my driving style during an autox, my first run will have a few spin-outs, over the next two or three runs I will adjust my speed and style to just able to eliminate them, barely I will then have a few consistant runs and call it a day.
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  #42  
Old 10-24-2011, 08:02 PM
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Originally Posted by dtc100 View Post
If I may speak theoretically.

Say your square setup now is 245 at all coners, and your staggered was 235 front 265 rear.

In theory, would a 255 square setup (again assume you can do it and weight gain is not a concern) not be the best of both worlds?
Well, one downside is that you don't want the fronts to be any bigger than absolutely needed. Weight really is a concern, because it throws off front/rear balance.

Moreover, the bigger the fronts, the worse the turn-in performance. Wider wheels resist turning and want to go completely straight. The wider the contact patch, the more torque it takes to turn that patch. Overly wide tires lead to a "sluggish, plowing" kind of feeling in the turns. Which makes the car feel like a boat, and is not fun.

And, lastly there's the issue of unsprung weight. The more weight under the springs, the worse the car handles bumps. You really don't want a front of the car taking too much air and getting unsettled. That is a very scary feeling.
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  #43  
Old 10-24-2011, 09:04 PM
dtc100 dtc100 is offline
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Originally Posted by Squidget View Post
Well, one downside is that you don't want the fronts to be any bigger than absolutely needed. Weight really is a concern, because it throws off front/rear balance.

Moreover, the bigger the fronts, the worse the turn-in performance. Wider wheels resist turning and want to go completely straight. The wider the contact patch, the more torque it takes to turn that patch. Overly wide tires lead to a "sluggish, plowing" kind of feeling in the turns. Which makes the car feel like a boat, and is not fun.

And, lastly there's the issue of unsprung weight. The more weight under the springs, the worse the car handles bumps. You really don't want a front of the car taking too much air and getting unsettled. That is a very scary feeling.
That makes sense, thanks.

Now if I may ask about tire profile. There is another thread about this issue, but it has to do with pothole explosion.

I am interested in performance. The standard E90 wheel/tire diameter is 25". Both the 225/40/18 and 255/35/18 fit this spec.

But if one wants a different setup, there are a few options, the 235/35/18 is only 24.5", 235/40/18 is 25.4", 245/35/18 is 24.8" and 245/40/18 is 25.7".

The low profiles (35s) all have weight advantage. What the pros and cons of a low profile vs a taller tire? How important is to stick to the 25" diameter?

Again assume we are not concerned about the pothole explosion issue, only performance on track or at autox.
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  #44  
Old 10-24-2011, 09:49 PM
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Originally Posted by dtc100 View Post
The low profiles (35s) all have weight advantage. What the pros and cons of a low profile vs a taller tire? How important is to stick to the 25" diameter?
You have officially exceeded my practical knowledge limit. I have zero experience with oddball tire sizes. So I speculate!

1) I can only theorize that, above all, you do NOT want a smaller contact patch when tracking a car. And that's what a smaller diameter is gonna give you. The weight savings is probably more than offset by the smaller contact patch.

2) Less rubber = more heat, and your tires will burn up much faster. Melt those shoulders!

3) And even on a track with no potholes, you may have bumps, pebbles, and debris. Thicker walls are safer because they absorb more shock.
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  #45  
Old 10-24-2011, 09:53 PM
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Isn't the question about profiles rather than contact? I thought this was covered in the other thread about low pro tires - and the members sick of them thread. Higher profile may have too much flex in the sidewall which can detract from handling... but conversely too little sidewall can also detract from handling where there isn't enough flex. You have to find the happy medium for your application.
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  #46  
Old 10-24-2011, 10:43 PM
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Originally Posted by hyperzulu View Post
Isn't the question about profiles rather than contact? I thought this was covered in the other thread about low pro tires - and the members sick of them thread. Higher profile may have too much flex in the sidewall which can detract from handling... but conversely too little sidewall can also detract from handling where there isn't enough flex. You have to find the happy medium for your application.
Seeing those race cars with tall tires, I can say that his theory of contact patch is NOT in conflict of your theory of tire flexibility Those very low profiles probably don't have enough flex.

But what about the theory of tire pressureability? It can raise hell between contact patch and flex, can't it?

After that we can discuss tire temperature

As difficult as it to pinpoint what tires to choose, I appreciate the feedback, it does give me some good ideas.
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  #47  
Old 10-25-2011, 11:03 AM
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I'm pretty sure the OP has long ago departed this thread with a splitting headache...

If you're still here, Anthony, pick the best-looking (in your opinion) wheels without worrying too much about the performance aspect -- the difference between "best" and "good" performance is only a few percentage points and only makes a difference if you are on the track or an expert driver driving at way-illegal speeds on deserted roads.

There...does that pass muster with the tire tech crowd?
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