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E36 /7 Z3 (1996-2002)
E36/7 Z3 roadster and coupe talk with our gurus here.

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  #26  
Old 05-07-2012, 07:59 AM
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CJ645Bern CJ645Bern is offline
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Quote:
Originally Posted by BeemerMikeTX View Post
I'm curious. What did the dealership tell you, since you have "rear" tires mounted on the front?
well i actually just talked to my seller who i personally know and he said to just leave it like that because "the shop did it and they know what they are doing" still a little sketchy but ill take it
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  #27  
Old 05-07-2012, 08:41 AM
tohbi tohbi is offline
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Originally Posted by Pinecone View Post
What these cars really need is more negative camber. The camber curves are not sufficient with the body roll. That is why virtually every racing BMW picutre you see shows the front wheel off the ground at the apex of the corner.
i've seen these pics and assumed it had to do with the car not having a frame but a unibody causing flex.

my 2.8 has been lowered. not sure what all has been done to it but, when i was considering the 2.8 or a "m" model, i could crawl under the m to inspect the differential but not under the 2.8. presumably, lowering reduces wheel lift?
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  #28  
Old 05-07-2012, 10:51 AM
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CJ645Bern CJ645Bern is offline
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Originally Posted by Pinecone View Post
I differ.

Running mismatched tires is NOT a good idea. It can lead to severe understeer (not that bad) or severe oversteer (VERY bad). And you won't know what it will do until it does it.
not to sound like a noob, but would this be when the steering wheel moves by itself when youre driving normally? Because this happens to me at times

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  #29  
Old 05-07-2012, 05:42 PM
tohbi tohbi is offline
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Originally Posted by CJ645Bern View Post
not to sound like a noob, but would this be when the steering wheel moves by itself when youre driving normally? Because this happens to me at times
maybe your car is haunted.

oversteer is considered dangerous because the rear end swings out, ala porsche and other rear engined cars. some of us like a car that oversteers a little bit.

others prefer understeer, a car that pushes, like a z3. one can neutralize this by driving faster into the turn to swing the rear end out, doing a little "lift" with the steering wheel when entering the turn then sharply turning into it, or finding the perfect tire combination when the front tires grip and the rear breaks away a tiny bit.

on the street you may find the difference when you put a front wheel in the rain gutter on a slow speed turn. the understeer car probably won't clip the curb with the rear wheel while the oversteer car will.

your experiences may vary. most manufacturers look for that perfect 50/50 balance.
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  #30  
Old 05-07-2012, 06:51 PM
comsult comsult is offline
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Originally Posted by CJ645Bern View Post
not to sound like a noob, but would this be when the steering wheel moves by itself when youre driving normally? Because this happens to me at times
This is called Tramlining. When you car follows a groove in the road,
We have lots of heavy logging trucks using our highways up here, and they make depressions in the pavement.
It's a real PITA when driving and your wheelbase is different from the trucks, so you are always fighting the steering wheel, which wants to follow either the right depression or the left depression. Gets real interesting in the fast tight corners.
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  #31  
Old 05-08-2012, 05:10 AM
dougmcintyre dougmcintyre is offline
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Quote:
Originally Posted by tohbi View Post
maybe your car is haunted.

oversteer is considered dangerous because the rear end swings out, ala porsche and other rear engined cars. some of us like a car that oversteers a little bit.

others prefer understeer, a car that pushes, like a z3. one can neutralize this by driving faster into the turn to swing the rear end out, doing a little "lift" with the steering wheel when entering the turn then sharply turning into it, or finding the perfect tire combination when the front tires grip and the rear breaks away a tiny bit.

on the street you may find the difference when you put a front wheel in the rain gutter on a slow speed turn. the understeer car probably won't clip the curb with the rear wheel while the oversteer car will.

your experiences may vary. most manufacturers look for that perfect 50/50 balance.
Here are fuller definitions of understeer and oversteer -
http://www.drivingfast.net/car-control/understeer.htm
http://www.drivingfast.net/car-control/oversteer.htm

And my experiences definitely vary - almost all stock cars are set up to understeer.

That's because it's generally thought that understeer is easier for the average driver to cope with than oversteer. The instinctual reaction for an untrained driver in a sliding car is to lift off the throttle and hit the brakes, transferring weight and increasing traction to the front end, helping an understeering car to recover control. Doing the same thing in a car that's oversteering will usually make the situation worse by unloading the rear tires and further reducing their traction.
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  #32  
Old 05-08-2012, 06:31 AM
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All stock cars (at least fairly recent ones) are setup to understeer as that is safer for most drivers. If a driver enters a corner too fast, the normal response is to lift off the accelerator and possibly brake. In a car that understeers, that increases front grip and the car turns better.

In an oversteering car, that action causes the rear to lose traction and at a minimum step out, and it could cause a spin.

The pessimisitc view of understeer versus oversteer is: Understeer is when you leave the road facing the direction you are traveling, Oversteer is whan you eleave teh road going backwards.
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  #33  
Old 05-08-2012, 06:38 AM
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Originally Posted by tohbi View Post
i've seen these pics and assumed it had to do with the car not having a frame but a unibody causing flex.

my 2.8 has been lowered. not sure what all has been done to it but, when i was considering the 2.8 or a "m" model, i could crawl under the m to inspect the differential but not under the 2.8. presumably, lowering reduces wheel lift?
Nope, it happened before. The strut suspension setup is that as teh car rolls, the outer wheel does not get enough camber gain to offset the body roll, so the outer wheel can go to positive camber in relation to the road, which is NOT good.

This is why, when tuning a BMW, you INCREASE frotn roll stiffness to reduce understeer. Which is backwards of most cars. Many BMW race cars run no rear sway bar.

And watch over lowering a BMW. Aftera bout an inch or so of lowering, the front control arms go past horizontal. meaning that as the suspension compresses, the camber goes more positive. This causes MASSIVE understeer.

The rear frame issues have been seen in 1.9L Z3s. It is NOT engine dependant.
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  #34  
Old 05-08-2012, 08:24 AM
tohbi tohbi is offline
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gonna have to look at some suspension pics to understand all this but i've learned alot here. thanks.
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  #35  
Old 05-09-2012, 02:39 AM
Z3Papa Z3Papa is offline
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I am almost afraid to wonder too deeply into what all can cause front inside wheel lift but would say that it's not limited to negative camber. I get -4 degrees or more of negative camber up front with my Vorshlag camber plates and still can induce inside tire lift if I have too much sway bar, rear spring rate, rear compression or rebound or the corner balance is significantly off. It's all about excess load on the outside front tire. A car's balance when pushed extremely hard is easy to reveal what all needs to be address, whether it is the car or the nut behind the wheel. I have multiple pictures of my car with 3-4" of inside wheel lift on street tires (normally induced more easily on r-comps) and the first thing going through my mind is how to correct it. Some extremely fast DSP/BSP/SM BMW drivers indicate there is no point where reduction in COG will adversely affect suspension geometry and express without reservation that having inside wheel lift is does not adversely affect corner speed especially on exit where you are in rapid unwind of the steering wheel.

This is not the most extreme example nor a very good picture but I am too lazy to upload a better shot --

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Last edited by Z3Papa; 05-09-2012 at 02:44 AM.
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  #36  
Old 05-09-2012, 06:33 AM
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Camber does not cause inside tire lift. Inside tire lift occurs because of very high front roll stiffness. Usually combined with low rear roll stiffness.

The BMW front suspension requires high static front negative camber and high front roll stiffness to avoid the frotn outer tire from going to positive camer in relation to teh road surface.

And that is exactly what you are saying, you have very high static negative camber, and when you run high fron roll stiffness, you get inside tire lift.

But where you err, is deciding that inside front tire lift is necessarily bad. The only way to determine that is by data logging to see what is faster. And considering the number of pro team cars that show inside front tire lift at the apex, it is not always bad.
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  #37  
Old 05-09-2012, 08:03 AM
Z3Papa Z3Papa is offline
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Quote:
Originally Posted by Pinecone View Post
Camber does not cause inside tire lift. Inside tire lift occurs because of very high front roll stiffness. Usually combined with low rear roll stiffness.

The BMW front suspension requires high static front negative camber and high front roll stiffness to avoid the frotn outer tire from going to positive camer in relation to teh road surface.

And that is exactly what you are saying, you have very high static negative camber, and when you run high fron roll stiffness, you get inside tire lift.

But where you err, is deciding that inside front tire lift is necessarily bad. The only way to determine that is by data logging to see what is faster. And considering the number of pro team cars that show inside front tire lift at the apex, it is not always bad.
I guess you didn't read this part of my post --Some extremely fast DSP/BSP/SM BMW drivers indicate there is no point where reduction in COG will adversely affect suspension geometry and express without reservation that having inside wheel lift is does not adversely affect corner speed especially on exit where you are in rapid unwind of the steering wheel. I honestly think having four wheels gripping around a very fast corner will produce fastest results but have also run data acquisition on a number of setups which have led me to accept some lift as inevitable.
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  #38  
Old 05-10-2012, 04:51 AM
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Interesting.

I know the wheel lift does not reduce cornering, because virtually every rpo setup BMW lifts in the inside front wheel.

But if you look at the suspension geometry, if you go too low, as the suspension compresses, the lower control arm outer end moves INWARDS, causing a positive camber change.

If you can get enough negative camber, it might be OK. But I know TC Kline does not agree. And RRTS used to sell modified lower control arms that raised the outer ball joint allowing lower ride height with proper LCA geometry.
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  #39  
Old 05-10-2012, 05:28 AM
tohbi tohbi is offline
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Originally Posted by Pinecone View Post
And watch over lowering a BMW. Aftera bout an inch or so of lowering, the front control arms go past horizontal. meaning that as the suspension compresses, the camber goes more positive. This causes MASSIVE understeer.
imo, my 2.8 has too much push. of course, i like a little oversteer and almost bought a boxster because of it. but, thinking my car has been lowered, what you write has me thinking the understeer may be related to too much lowering.

i need to find an expert around here who can look the car over and advise.
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  #40  
Old 05-11-2012, 01:07 PM
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Just look under the front of the car and see the angle of the lower control arms. They should go down from the inner to outer end. If they are level or point up, you may have a problem unless you REALLY control body roll and have a lot of static negative camber.

Yes, the cars have a lot of push from the factory. To reduce that you need to add negative camber in the front. 4 degrees was mentioned above. I used to run 3.5 on my E46 M3.

And add a heavier front sway bar.
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