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E34 (1989 - 1995)

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  #1  
Old 11-09-2010, 11:56 AM
robertobaggio20 robertobaggio20 is offline
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Don't get rear differential ratios

Hi everyone,

Kinda embarassed to post this up but i'm tired of cracking my head over this. I don't understand how rear differential ratios work and their effects on a car's performance and fuel economy. I also don't get what's the difference between a limited slip diff and an open diff.
Have been googling this for awhile with no meaningful clarity...there are plenty of explanations as to how a rear differential functions but no useful articles on the effects.

I have figured out that the when you use the word "higher" to describe a differential ratio, you mean that numerically, the ratio is lower. I.e. if the baseline is 4.1, a higher ratio would be 3.64 and a lower ratio would be 4.44.

Furthermore, a differential ratio of 4.1 means the car's long shaft spins 4.1 times (input force) which causes the rear tyres to spin once.

That is the limit of my understanding. Would be most obliged if someone could shed some light on this matter definitively. Thank you.




rgds,
Roberto

Last edited by robertobaggio20; 11-09-2010 at 11:58 AM.
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  #2  
Old 11-09-2010, 03:09 PM
down19992000 down19992000 is offline
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Here is an oldy but a goody.




The drive gear in this example would start out at say 3.42 gear and if you swapped in a 3.92 the drive gear would be bigger taking more revolutions of the motor to turn the wheel over once. Therefore say you normally turned 2800 rpm at 65mph you would now turn say 3300 rpm. With a higher numerical gear you would get better take off out of the hole but lose some topend. I hope that explains it well enough for you.

Last edited by down19992000; 11-09-2010 at 03:22 PM.
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  #3  
Old 11-09-2010, 03:26 PM
down19992000 down19992000 is offline
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Here is a link about a limited slip diff.


http://en.wikipedia.org/wiki/Limited_slip_differential
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  #4  
Old 11-09-2010, 03:30 PM
zaleonardz zaleonardz is offline
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Robert, take a look at

http://www.bmwfanatics.co.za/showthr...highlight=diff
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  #5  
Old 11-09-2010, 03:55 PM
robertobaggio20 robertobaggio20 is offline
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Thanks for the video (saw that yesterday, was a simple and elegant explanation for the thing!) and the thread zaleonardz, unfortunately it doesn't make things clearer which is my point about poor information being out there on google for this subject.

I don't understand this line, from someone who posted in the other forum " Well, if you want a little more acceleration you can go from your 3.64 to 3.91 ratio." How is that possible? If you go to a lower (numerically higher) ratio, isn't your engine going to be turning over more for the same amount of turns on the wheels? Doesn't that mean less power not power? So how can you have better torque? Plus my questions above, restated : "I don't understand how rear differential ratios work and their effects on a car's performance and fuel economy."

I would appreciate if someone actually spelled it out for me here. The links on the web are by and large unclear unfortunately. Thank you.
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  #6  
Old 11-09-2010, 07:37 PM
down19992000 down19992000 is offline
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Like i posted above if you have a factory 3.42 and turn 2800 rpm at 65 when you put a 3.92 gear in it you will turn say 3200 rpm at the same speed resulting in a lower fuel mileage. You will not have anymore torque or horsepower but can accelerate out of the hole faster up to a certain speed but will have a lower top speed. I am not sure how else to explain it to make it simpler. With the 3.92 gear you will shift from 1st to second and second to third (and so on through all your gears) at a lower speed allowing you to get up to speed quicker. You have to balance it out though you could put a 6.13 gear in the rear but might only top out at 85 in overdrive so you can't just shove the biggest gear you can buy in the differential. I mean it makes sense to me maybe someone else can explain it a little better than me??
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  #7  
Old 11-10-2010, 06:35 PM
robertobaggio20 robertobaggio20 is offline
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I guess i'm trying to understand how it works in terms of physics for a lower differential ratio (higher numerical ratio) to produce more torque at the wheels than a higher diff.

It would seem that a higher diff, being able to turn the wheel more times for a given rpm, should be able to outrun a lower diff that will turn the wheel less for the same rpm and would thus travel a shorter distance if two such cars were compared side by side. But, it works the other way in reality. How does that transpire, in terms of the physics of the thing?
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  #8  
Old 10-27-2012, 06:17 AM
robertobaggio20 robertobaggio20 is offline
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And of course, in terms of physics, something that down19992000 is challenged at understanding and thus can't explain, the reason why a higher gear ratio leads to faster accelerations are as follows.

The engine produces x amount of force per rpm. The force eventually ends up at the rear wheels. The more force the wheels get, the faster they can push the car and the greater its acceleration. The higher the gear ratio, the more rpms the engine needs to turn the rear wheels by 1 rotation. The engine does more work and consumes more fuel of course, but then, delivers more force per rotation made by the rear wheels. This greater force is experienced as a higher acceleration than a car with a lower rear differential ratio.
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  #9  
Old 10-27-2012, 06:47 AM
snowsled7 snowsled7 is offline
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So here we are again bobby... Insulting people that try to help you understand

If you have 1000 hp you can pull that 2.73 gear. If you have 150 hp your engine needs the additional leverage of the 4.10 gear ratio. You also ignore that the power peak of the car is not at zero rpm like an electric motor. The torque peak in my car is at 4200 rpm, there just isn't a lot below that point. The lower ratio of 4.10 vs 2.73 allows the engine better leverage over the weight of the heavy car and allows the engine to reach its peak performance rpm more quickly. This is why lower ratios are good for acceleration, more leverage off the line, quicker into the powerband. Low gear ratio's allow semi tractors to move 80,000lbs with 1600 lb/ft of torque.

There is a balance to be found however. You do not want a ratio so low that the tires are easily overpowered. You need a balance of traction, power, and ratio to make it work well. Street cars have ratios designed for all around driving. Often manufacturers offer lower ratios in performance packages. Lots of guys drop their ratio a bit for performance, I intend to. The guys with big power turbo engines can pull taller (numerically lower number) gears. Then the rear gears need to be matched to the transmission gearing. MB used to have top gear be a 1:1 ratio in most of their transmissions. That meant taller ratios in the rear ends. BMW conversly did it like most everybody else and used an OD ratio as top gear, say a .86:1 ratio so rare gearing is lower (bigger number).

It seems like it has been clearly explained to me by patient posters. I never took physics beyond HS, so you will probably either have to accept our laymans explanations, or better yet, go find another forum that has more genuises like yourself. They are sure to understand you better.
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Last edited by snowsled7; 10-27-2012 at 06:57 AM.
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  #10  
Old 10-27-2012, 01:31 PM
robertobaggio20 robertobaggio20 is offline
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Quote:
Originally Posted by snowsled7 View Post
So here we are again bobby... Insulting people that try to help you understand

If you have 1000 hp you can pull that 2.73 gear. If you have 150 hp your engine needs the additional leverage of the 4.10 gear ratio. You also ignore that the power peak of the car is not at zero rpm like an electric motor. The torque peak in my car is at 4200 rpm, there just isn't a lot below that point. The lower ratio of 4.10 vs 2.73 allows the engine better leverage over the weight of the heavy car and allows the engine to reach its peak performance rpm more quickly. This is why lower ratios are good for acceleration, more leverage off the line, quicker into the powerband. Low gear ratio's allow semi tractors to move 80,000lbs with 1600 lb/ft of torque.

There is a balance to be found however. You do not want a ratio so low that the tires are easily overpowered. You need a balance of traction, power, and ratio to make it work well. Street cars have ratios designed for all around driving. Often manufacturers offer lower ratios in performance packages. Lots of guys drop their ratio a bit for performance, I intend to. The guys with big power turbo engines can pull taller (numerically lower number) gears. Then the rear gears need to be matched to the transmission gearing. MB used to have top gear be a 1:1 ratio in most of their transmissions. That meant taller ratios in the rear ends. BMW conversly did it like most everybody else and used an OD ratio as top gear, say a .86:1 ratio so rare gearing is lower (bigger number).

It seems like it has been clearly explained to me by patient posters. I never took physics beyond HS, so you will probably either have to accept our laymans explanations, or better yet, go find another forum that has more genuises like yourself. They are sure to understand you better.

Now snowy darling, where were you when we first had this discussion ? Your contributions are so valuable and have given me (and legions of others) much to consider.

I declare the above post by snowsled7 (tame by his standards) to be the online version of his coming out party.

Snowy ! Welcome to bimmerfest ! I always knew you had it in you.

Last edited by robertobaggio20; 10-27-2012 at 03:04 PM.
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  #11  
Old 10-27-2012, 02:52 PM
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supertech777 supertech777 is offline
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Quote:
Originally Posted by robertobaggio20 View Post
Hi everyone,

Kinda embarassed to post this up but i'm tired of cracking my head over this. I don't understand how rear differential ratios work and their effects on a car's performance and fuel economy. I also don't get what's the difference between a limited slip diff and an open diff.
Have been googling this for awhile with no meaningful clarity...there are plenty of explanations as to how a rear differential functions but no useful articles on the effects.

I have figured out that the when you use the word "higher" to describe a differential ratio, you mean that numerically, the ratio is lower. I.e. if the baseline is 4.1, a higher ratio would be 3.64 and a lower ratio would be 4.44.

Furthermore, a differential ratio of 4.1 means the car's long shaft spins 4.1 times (input force) which causes the rear tyres to spin once.

That is the limit of my understanding. Would be most obliged if someone could shed some light on this matter definitively. Thank you.




rgds,
Roberto
Humility is not for the average minded , my hats off to you Roberto . A fine trait you have there, keeping an open mind so we all remain teachable ..... Right on !

Sent from my Desire HD using Bimmer App
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  #12  
Old 10-27-2012, 03:07 PM
robertobaggio20 robertobaggio20 is offline
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Quote:
Originally Posted by supertech777 View Post
Humility is not for the average minded , my hats off to you Roberto . A fine trait you have there, keeping an open mind so we all remain teachable ..... Right on !

Sent from my Desire HD using Bimmer App
E34s are also not for the average minded.

Thanks mate ! Btw, that post was circa november 2010. So I might not be all that teachable any longer lol.

I recall now. It was Bentley's ghost who finally explained this to me via pm in a way that helped me get it. He used the example of a bicycle and its riding experience, and its different gears and their effects. Thanks sir !
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  #13  
Old 10-27-2012, 04:12 PM
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_Ethrty-Andy_ _Ethrty-Andy_ is offline
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  #14  
Old 10-27-2012, 05:45 PM
robertobaggio20 robertobaggio20 is offline
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Yes Andy yes. Bentley's ghost lives and is a forum member!
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  #15  
Old 10-28-2012, 07:54 AM
snowsled7 snowsled7 is offline
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Quote:
Originally Posted by robertobaggio20 View Post
Now snowy darling, where were you when we first had this discussion ? Your contributions are so valuable and have given me (and legions of others) much to consider.

I declare the above post by snowsled7 (tame by his standards) to be the online version of his coming out party.

Snowy ! Welcome to bimmerfest ! I always knew you had it in you.
Guess I missed the orignal post date, my bad.

The really funny part is that YOU dredged it up to insult another member, and then you ask me where I was?

What kind of game is this bobby?
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  #16  
Old 10-28-2012, 09:58 AM
robertobaggio20 robertobaggio20 is offline
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Quote:
Originally Posted by snowsled7 View Post
Guess I missed the orignal post date, my bad.

The really funny part is that YOU dredged it up to insult another member, and then you ask me where I was?

What kind of game is this bobby?
I was using a heavy dose of irony my friend. Perhaps you should reread my post there. If you don't get it, nevermind. Not important.

And I'm not the one who brought this thread up. My dearest friend down1999200020122022 did in another thread. I didn't know he cared...brought tears to me young inexperienced yet stunningly handsome eyes !


rgds,
Roberto
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  #17  
Old 10-28-2012, 07:15 PM
down19992000 down19992000 is offline
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I had a post typed up but what is the sense in arguing and wasting space.

Last edited by down19992000; 10-28-2012 at 07:17 PM. Reason: Arguing with this guy does no good
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