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      11-11-2012, 12:21 PM   #1
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Rear suspension design - squat and toe?

I figured I would try here...

Someone was telling me that as the rear end squats, the rear tires actually toe in due to the multilink rear design. So dialing in a touch more rear toe is unncessary.

True?

If it is that is brilliant if you ask me. That is also nice to know for the track.
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      11-11-2012, 05:42 PM   #2
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Sort of. I was told that the converse is true, that when it dives the rear tends to toe out, which is why we toe it in a bit to give it more stability under hard braking.
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      11-11-2012, 06:41 PM   #3
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Think you're right Bill, which is why the car rotates so well. Almost like 4 wheel steering.

Here are some pics of the rear multilinks in action.




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      11-11-2012, 10:07 PM   #4
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Quote:
Originally Posted by Sleeper519 View Post
Sort of. I was told that the converse is true, that when it dives the rear tends to toe out, which is why we toe it in a bit to give it more stability under hard braking.
I know with my E46 that it toes out when the rear squats, assumed the same for the E9X. But I'm being told that the multilink suspension actually toes in.

I haven't looked at it enough but the toe setting on the E46 was in front of the axle. Vrs the E9X where the toe setting is changed on a link that is behind the axle. If so then I would think that it would be the opposite. Its like rear steer/front steer for the front axle. If you lengthen the tie rod for a front steer car, it toes out. Rear steer, it toes in.

I think the rear rotates nicely because of the toe in which keeps the rear end more controllable vs toeing out.

But I would like to see some more info on this.
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      11-12-2012, 08:30 AM   #5
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Usually you would want increased toe out as the rear suspension compresses to help with the rotation of the car.

I have not checked the suspension setup in detail, but from the cars handling behavior, I suspect this is the case withe E9X.
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      11-12-2012, 10:06 AM   #6
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Quote:
Originally Posted by CanAutM3 View Post
Usually you would want increased toe out as the rear suspension compresses to help with the rotation of the car.

I have not checked the suspension setup in detail, but from the cars handling behavior, I suspect this is the case withe E9X.
When I get my E46 back today, I'll snap some pics of the toe arms which is significantly different from the E9X platform.

I always thought toe out in the rear was a no-no becuase it makes the back end unstable and harder to control.
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      11-12-2012, 10:31 AM   #7
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I would think toe in under squat is good and toe out under dive is good for these reasons: would lead to a more stable platform under throttle application and more rotation on turn in for allowing for that power application earlier. Sooner to throttle means more speed on the straight. Get used to the back end of the car moving under braking it is necessary to go fast and frankly it isn't going to spin you it is only going to allow you to point the car more easily. Going fast involves a lot of getting comfortable with uncomfortable situations.
Lift-throttle oversteer is a pretty nasty trait though, just ask every 911 owner before the 996 ... I bet most of them can tell you the part #'s and cost of rear quarterpanels/axles.
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      11-12-2012, 05:54 PM   #8
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Originally Posted by DallasBoosted View Post
Lift-throttle oversteer is a pretty nasty trait though, just ask every 911 owner before the 996 ... I bet most of them can tell you the part #'s and cost of rear quarterpanels/axles.
Actually not.

The best handling cars are neutral with a steady speed. Which means they understeer with more throttle and oversteer with less throttle.

You are probably talking about snap oversteer which leads into a spin, which earlier 911s were inclined to with their high rear weight bias.
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      11-12-2012, 07:24 PM   #9
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Quote:
Originally Posted by bigjae1976 View Post
When I get my E46 back today, I'll snap some pics of the toe arms which is significantly different from the E9X platform.

I always thought toe out in the rear was a no-no becuase it makes the back end unstable and harder to control.
I guess I will retract my earlier statement. After doing some reading, it seems it is better for toe to remain constant troughout the suspension travel. This makes the car more predictable as loads change (front to rear and left to right).
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      11-12-2012, 10:46 PM   #10
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Toe in should increase as rear suspension is compressed. I just can't see it working any other way. Camber should also increase with compression.

That's how the front normally works as well.
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      11-13-2012, 05:54 AM   #11
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Quote:
Originally Posted by CanAutM3 View Post
I guess I will retract my earlier statement. After doing some reading, it seems it is better for toe to remain constant troughout the suspension travel. This makes the car more predictable as loads change (front to rear and left to right).
+1 - main reason why race teams use rigid bushings/monoballs everywhere. This way you only worry about "static" alignment settings, not "dynamic" alignment settings. However, as others have stated, alignment setting change with increased loading, and having these various load-deflection curves are useful.
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      11-13-2012, 11:53 AM   #12
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For the engineering geniuses here...

The toe arm (#1) runs forward and is curved outwards. So if the suspension compresses, camber becomes more positive but I can't explain what happens to cause the rear to start to toe out or in.



Versus the E9X which has the toe arm (#13) at the rear of the hub so use the suspension compresses, that link should elongate and push the back of the out creating toe in?

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      11-13-2012, 01:09 PM   #13
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Quote:
Originally Posted by bigjae1976 View Post
For the engineering geniuses here...

The toe arm (#1) runs forward and is curved outwards. So if the suspension compresses, neg camber increases but I can't explain what happens to cause the rear to start to toe out or in.


Versus the E9X which has the toe arm (#13) at the rear of the hub so use the suspension compresses, that link should elongate and push the back of the out creating toe in?
In my opinion it is not possible to determine anything from those diagrams. The exact location of the attachment points, angles of the arms relative to each other, etc... are all required to make any assessment on toe changes. Even then, it would be extremely complex. A full computer simulation would be the only way to tell besides actual measurements on the car.

I can understand why more camber is required when the suspension compresses, it is to counter the effect of body roll. To keep the tire as perpendicular as possible (or with some negative camber) relative to the road even as the body rolls.

However, from the latest info I have read, toe is better when kept constant throughout the suspension travel. Toe changes can make the car's behaviour less predictable. This is contrary to my original intuition, so will try to dig some more .
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      11-13-2012, 01:19 PM   #14
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Quote:
Originally Posted by CanAutM3 View Post
In my opinion it is not possible to determine anything from those diagrams. The exact location of the attachment points, angles of the arms relative to each other, etc... are all required to make any assessment on toe changes. Even then, it would be extremely complex. A full computer simulation would be the only way to tell besides actual measurements on the car.

I can understand why more camber is required when the suspension compresses, it is to counter the effect of body roll. To keep the tire as perpendicular as possible (or with some negative camber) relative to the road even as the body rolls.

However, from the latest info I have read, toe is better when kept constant throughout the suspension travel. Toe changes can make the car's behaviour less predictable. This is contrary to my original intuition, so will try to dig some more .
My original post was incorrectly worded...fixed.
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      11-13-2012, 01:26 PM   #15
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Quote:
Originally Posted by CanAutM3 View Post
However, from the latest info I have read, toe is better when kept constant throughout the suspension travel. Toe changes can make the car's behaviour less predictable. This is contrary to my original intuition, so will try to dig some more .
Yes, of course less slop in suspension is better but most of us have to compromise. I think BMW engineered a very livable compromise for dual duty cars. You see in other threads, I've wrongly assumed that the rear suspension design of an E9X M3 acts in a similar manner to the E46 (non-M and M) which was pretty much the same as the E36. I know my E46 very well so I can and do maintain input on how the car is setup...just not relying on someone's opinion. I know all of us know the what, what the spec should be. I think all of us would be better served if we knew why the spec is what it is...I surely don't right now.

In the end I know that being intimately familiar with most of the details on my E46 has made me a better driver.
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      11-13-2012, 06:13 PM   #16
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I got this from a trusted source:

Quote:
Yes, it will toe in and out under compression and droop. Next time its on the alignment rack I can show you exactly how much. There is toe change due to bushing flex, geometry and also the rear toe links are thin and designed to bend inward and outward.
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      11-13-2012, 09:16 PM   #17
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My Mcoupe with 1970's suspension did this with a trailing arm rear suspension. This is not new technology.
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      11-13-2012, 09:53 PM   #18
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Quote:
Originally Posted by bigjae1976 View Post
I figured I would try here...

Someone was telling me that as the rear end squats, the rear tires actually toe in due to the multilink rear design. So dialing in a touch more rear toe is unncessary.

True?

If it is that is brilliant if you ask me. That is also nice to know for the track.
So, your original premise is that we tend to dial a little rear toe in when we get (track) alignments, which is true for most folks' setups.

But the reason is NOT for the compression phase (squat) on corner exit under power, but for the droop phase (dive) to give more stability under heavy braking---that is where the rear tires tend to toe out from bushing flex, etc, and get a little squirrelly.

For a purely street setup it's probably unnecessary and zero toe would be just fine.

Just my take on it...
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      11-13-2012, 10:15 PM   #19
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The suspension is designed to reduce toe in during squat and will increase toe in when weight is transferred off the back end.

Meaning if you have 3/32nd inch static toe in, during squat the toe will decrease to lets say arbitrarily 1/32nd-1/16th. It will NOT increase toe while squatting.

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      11-13-2012, 10:19 PM   #20
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Just to throw something more to think about:

Let's assume that the rear suspension yields toe in when compressed and toe out when relaxed, as with BigJae's original assumption.

When under a cornering load the outside tire would toe-in and the inside tire would to-out, this would yield a nice rear steering into the turn. Great for high speed stability but not the best to rotate the car.

Further, when you accelerate, the traction force pulls the rear wheels forward thus increasing toe-in. If the suspension dials in more toe-in as it compresses it would make the situation worse. The same applies to braking where traction pulls the wheels back increasing toe-out. If the suspension increases toe-out as it relaxes, it would also make the situation worse under braking.

This is what leads me to believe that the suspension is tuned the other way around. Just based on intuition though. It is not easy to come to a conclusion here ...

Last edited by CanAutM3; 11-13-2012 at 10:41 PM.
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      11-13-2012, 10:39 PM   #21
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Sheesh...I have no idea which way it goes. I might throw it back on the alignment rack to check it again. My shop is super cool about it...they'd be in!
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      11-13-2012, 10:44 PM   #22
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Quote:
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Sheesh...I have no idea which way it goes. I might throw it back on the alignment rack to check it again. My shop is super cool about it...they'd be in!
Please let us know, you definitely spiked my interest
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