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      01-21-2014, 10:27 PM   #1
MDORPHN
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1M - Rear toe link

Interesting thread.

http://www.m3post.com/forums/showthread.php?t=768539

Apparently, the stock rear toe link is intended to flex. Consensus in the linked thread is that it is designed to produce dynamic toe-in under hard braking but there are varying opinions about what happens under compression (acceleration). Some suggest it also produces greater toe-in under acceleration, others suggest that is not the case.

Rogue Engineering and Ground Control, among others, make very nice toe links to replace the stock piece. FWIW, I prefer the ones with a pinch-bolt design instead of a jam nut, since it makes precise alignment easier.

They both increase the range of adjustment and, presumably, would also eliminate some of the flex. Uncertain, however, whether this is necessarily desirable.

Thoughts?

Neil
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      01-21-2014, 11:02 PM   #2
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I'll have to look at the angle the trailing arm is positioned at in a neutral position relative to the thrust angle. If it angles down towards the wheel carrier, it is going to create toe-out at the carrier as the wheel starts to load because the link will start to straighten out between the mounting points which will pushed the front carrier mounting point out. On the other hand, more thrust is also going to create toe changes as the rubber mounting bushings flex on the trailing link. I think the net result will be different as you turn in and then start adding power with the stock link setup.
My assumption is based on the realoem pic, I need to look under the car after I get the snow cleared.
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      01-21-2014, 11:59 PM   #3
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Quote:
Originally Posted by MDORPHN View Post
Thoughts?

Neil
I wouldn't trust much of what's being said on that thread, or the manufactures of the toe links, without getting better sources of info. Some background:

BMW uses a five link rear, which is the current sophisticated street-car standard. As such, this means it must have some compliance in the system- if those were all heim joints instead of bushings a 5 link would bind. So there is definitely compliance dialed in, and the vast majority will be in the bushings- if the aluminum flexed significantly it would fatigue out quickly.

To figure out what the compliances are, you ideally want data from a K&C machine, "Kinematics & Compliance". Kinematics means how the suspension moves as it articulates, so ride steer, camber curves, etc. Compliance means how the suspension deflects when it's loaded- does it toe in or out with side loads, etc.

Both of these curves are going to be carefully engineered. Generally the outside rear wheel is kinematically designed to toe in under compression (as it goes up in the wheel well). It is also designed to toe in with some compliance steer under side loads or braking. This gives the car a form of rear wheel steering- when it enters a corner the outside compresses and the kinematics turn the wheel a little, the wheel loads up and the compliance steer turns the wheel a little more... From memory in a Porsche 997, this ends up at around 3/4 of a degree of toe in for the outside wheel at 1G of cornering, while the inside wheel moves the other direction nearly as much. Compare this to the new GT3 and Turbo, which at higher speeds use an actuator to steer the rear wheels in that same direction 1.5 degrees. The front's also moving around in controlled ways, in Porsche's case it's toeing out under side load, which also adds stability.

In both cases the toe-in stabilizes the rear end, basically allowing the rear have an overall lower slip angle and follow the front more faithfully rather than drift tail-out rally car style. I don't know for sure BMW is using similar K&C values as Porsche for similar reasons, but I suspect so. I do know older BMWs would kinematically toe-in under compression.

There are two good reasons why people mess with these curves (and many bad ones). First, on a race car on Hoosiers you can easily be pulling ~1.5 Gs instead of the ~.9 Gs the system was designed for. In this case the car leans over more, the side-loads are higher, both of which cause the wheel to move around much more than you'd like. In this case it can be a good thing to reduce (but not eliminate) these motions with stiffer bushings, etc, usually aiming to cut them roughly in half. The second reason usually is when you've change ride height, spring rate, etc away from the design point, so you need to get some adjust-ability to put things back where they should be.

If we look at the links you're thinking of and where they remove compliance we might be able to roughly estimate what the effect would be. It's a complex system- change any one variable and you're messing with others as well, so camber loss (or gain) in compliance, etc. Real work to figure out, often calling for stiffer bushings, etc.

Your best bet might be to find a top race shop who's campaigned an M3 at a high level, they will probably have had it on a K&C machine and and will have figured out how to reduce the values without upsetting the package. I'd listen to them, where as this stuff will be way over most shops heads. Most shops understand kinematics, and the default answer is to "eliminate bump steer" because it's not hard to do and sounds bad- who wants the rear wheels steering while the car goes over bumps? However once you get your head around the fact that the rear is always steering, bumps or not, and that it's supposed to be, then it becomes clear the correct answer is usually more nuanced.
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      01-22-2014, 01:59 AM   #4
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Quote:
Originally Posted by Pete_vB View Post
Generally the outside rear wheel is kinematically designed to toe in under compression (as it goes up in the wheel well).
According to Malek from MRF it is the other way around in these cars.

Quote:
Originally Posted by Malek@MRF View Post
Read my post. It will clear any confusion.

Car will TOE OUT under squat, car will TOE IN when braking or diving and the rear end goes up.
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      01-22-2014, 02:37 AM   #5
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Quote:
Originally Posted by Nugget View Post
According to Malek from MRF it is the other way around in these cars.
Yes, I read what he said.
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      01-22-2014, 08:48 AM   #6
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Pete -

Really good stuff.

I've reached out to a couple guys I know who are racing E92 M3s in NASA's GTS class, which places no limits on suspension mods.

Heard from one of them and he left the rear toe links alone -- apparently on the advice of some BMW suspension gurus that it was overcomplicated for his needs.

If I learn anything new, I'll add it to this thread.

Neil
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      01-22-2014, 09:48 AM   #7
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Should be pretty easy to know for certain if you have access to an alignment rack. Get the car on the rack and lift and compress the rear end and see where the toe goes.
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      01-22-2014, 09:55 AM   #8
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Quote:
Originally Posted by MightyMouseTech View Post
Should be pretty easy to know for certain if you have access to an alignment rack. Get the car on the rack and lift and compress the rear end and see where the toe goes.
Yes, but far more difficult to simulate lateral loads. And I'm more interested about what happens during high-g cornering than acceleration/braking.

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      01-22-2014, 03:57 PM   #9
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Always happy to talk suspension. I want to say that a trip to a K&C machine costs about 8k for a basic workup. If you get that serious let me know, or I might possibly have another source of info if I call in a favor. Of course having the info and knowing what to do with it are two different things.

When deciding if you want to make a change your best bet is probably to try and listen to feedback from the rear in your favorite bumpy high speed sweeper. That's going to highlight if the combination of your tires slip angles and wheel motions are working together, or if they are causing a problem. Is that why you're looking to possibly make a change in the first place, or what issue are you trying to correct?

As soon as you change tires you also change slip angles, and all of this is engineered with a certain slip angle in mind. It doesn't usually become an issue unless you change something significantly (ie 18s to 19s, street tires to hoosiers, etc). If you take a Porsche 993 around a bumpy sweeper, especially with modern 18" rubber, you can feel the rear end doing its own thing, and it can be disconcerting. Remember that it was designed for 16" rubber with much larger slip angles than what we're used to today. If you look at the progression, as rubber has improved Porsche has taken more and more compliance steer out: the 997 was designed for 18" rubber and the compliance steer was down to ~60% of the 933. The 991 was designed for 19" rubber (along with other changes, obviously) and it's down to ~30%, though ride steer remained relatively constant across all of these cars.

Our 1Ms rear suspension was designed for a) 18" wheels, b) a BMW M3, and c) a street car, not something constantly pulling 1g +, so I'm fairly sure there is room for improvement if one knows what they are doing. The trick is to find someone that does.

BTW, the numbers you are looking for are lateral force compliance steer, usually a single number measured in degrees per KN, and a toe curve vs suspension deflection graph that looks something like this:
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      01-22-2014, 05:06 PM   #10
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Pete -

Blown away by your knowledge and your ability to communicate it to laymen (like me) so clearly.

I'm not really looking to resolve a problem. Instead, I'm simply trying to educate myself while exploring whether there are cost-effective means to improve an already very competent design that would allow me to get my daily driven and almost street-legal car around a track more quickly.

Based on your posts and the research I've done, it looks like it would not be productive to screw around with the rear suspension design.

Best.

Neil
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      01-22-2014, 05:38 PM   #11
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OMG thanks Pete and Neil re relevant and technical, but please speak English in future

So in summary, should we continue to run with slight (static) rear toe in - as the most M3 drivers have done - to allow us to be more aggressive on corner exit?

I am not yet near a skill level where mid-corner speed is important, and besides lateral 1.1g makes me nauseous
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      01-22-2014, 06:10 PM   #12
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If you were to majorly increase the amount of loading the suspension was designed around, it might make sense to replace the subframe rubber bushings with something like Delrin/aluminum to help cut down on the amount of compliance without going into the complications associated with the wheel suspension setup itself; I did this with my 944T when I boosted the power by 50% to help keep the rear stabilized.
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      01-22-2014, 06:29 PM   #13
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Happy you got something out of it, guys.
Quote:
Originally Posted by scotth944 View Post
If you were to majorly increase the amount of loading the suspension was designed around, it might make sense to replace the subframe rubber bushings with something like Delrin/aluminum to help cut down on the amount of compliance without going into the complications associated with the wheel suspension setup itself; I did this with my 944T when I boosted the power by 50% to help keep the rear stabilized.
By subframe you mean the torsion tube mounting on the 944T? Agreed the subframe mounts, assuming everything mounts to one subframe, is probably good and safe if the can be done; I'm traveling and need to look at the subframe to see if that's safe when I get back. The primary difference between the 997S and 997 GT3 is simply that the rubber subframe bushings are replaced with aluminum, it increases NVH but reduces overall play. The 997 RS 4.0 goes on the replace the lower arm bushing with a spherical bearing, cuts camber loss under load, etc, the trick is knowing which bushing to replace and if it's going to do anything ugly.
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      01-22-2014, 06:40 PM   #14
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Quote:
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Happy you got something out of it, guys.

By subframe you mean the torsion tube mounting on the 944T? Agreed the subframe mounts, assuming everything mounts to one subframe, is probably good and safe if the can be done; I'm traveling and need to look at the subframe to see if that's safe when I get back. The primary difference between the 997S and 997 GT3 is simply that the rubber subframe bushings are replaced with aluminum, it increases NVH but reduces overall play. The 997 RS 4.0 goes on the replace the lower arm bushing with a spherical bearing, cuts camber loss under load, etc, the trick is knowing which bushing to replace and if it's going to do anything ugly.
Yes in the case of the 944, it was both the torsion tube to body mounts but I also replaced the trailing arm bushings to tube mounts as well.

Since the 1M uses a subframe to mount the suspension links to which then mount to the body, I would think there might be something to gain besides more road noise for track use, the rest of the setup is going to have some issues doing this given the 5 link geometry setup you had discussed.
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      01-22-2014, 06:49 PM   #15
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Quote:
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Yes in the case of the 944, it was both the torsion tube to body mounts but I also replaced the trailing arm bushings to tube mounts as well.
Yes, the semi-trailing arm in the 944 is a different beast, quite inferior and with toe compliance going the wrong direction and quite a bit of camber compliance. Generally not a bad idea to simply try to limit how much it can move, you might also look at the spring plate to limit camber loss if you have converted to coilover.

Yes, limiting subframe movement on a 5 link is a good idea assuming all 5 links mount to that same subframe.
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      01-22-2014, 06:50 PM   #16
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I remember seeing pics of either Turner's of BW's cars on a K&C machine prior to race season a couple of years ago. Since both of them campaign BMW's in the ST or GT series I would contact one of them for an opinion. James C at BW is wealth of information though he might be hard to get a hold of this weekend

Subframe bushings might be a good place to start removing compliance - there is a thread about this on the M3 forum. Also, BW is marketing some camber arm bearings. Either of these items might be the better place to start reducing compliance than the toe link.
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      01-22-2014, 06:54 PM   #17
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I did find a BMW Motorsport "race rubber" guide rod bushing that was available so at least there is 1 compliance related item in the 1M 5 link setup that could be addressed without a huge cost.
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      01-22-2014, 08:03 PM   #18
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A track buddy who races an E92 M3 in NASA GTS4 consulted James Clay on his set-up. He tells me that the only change in the rear of the car (besides shocks/sway) were what he called "rear spring arm bearings."

Not really sure which bearing this might be. Any guesses?

EDIT: Found 'em! They're also known as rear camber arm bearings. http://store.bimmerworld.com/bimmerw...px?Thread=True


And, yes -- I'd suggest an alignment with slight toe-in in the rear.

Neil

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      01-22-2014, 09:05 PM   #19
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Quote:
Originally Posted by MDORPHN View Post
A track buddy who races an E92 M3 in NASA GTS4 consulted James Clay on his set-up. He tells me that the only change in the rear of the car (besides shocks/sway) were what he called "rear spring arm bearings."

Not really sure which bearing this might be. Any guesses?

And, yes -- I'd suggest an alignment with slight toe-in in the rear.

Neil
I think the "rear spring arm" he referred to is the:
04 Camber link, left 1 33322283885 on real oem,
it does have a sizable rubber bushing where it mounts to the sub frame. Did he say where to find these upgraded bearings for purchase?

I did confirm that the trailing link does angle down towards the carrier from the body so I don't see how it couldn't create more toe out as it swings up under squatting conditions given how it is aligned in the setup.
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      01-22-2014, 09:07 PM   #20
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Quote:
Originally Posted by scotth944 View Post
I think the "rear spring arm" he referred to is the:
04 Camber link, left 1 33322283885 on real oem,
it does have a sizable rubber bushing where it mounts to the sub frame. Did he say where to find these upgraded bearings for purchase?

I did confirm that the trailing link does angle down towards the carrier from the body so I don't see how it couldn't create more toe out as it swings up under squatting conditions given how it is aligned in the setup.
Scott -

You called it right. See my edit above (which I must have added just before you posted).

Neil
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      01-22-2014, 09:26 PM   #21
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Neil, were you planning on changing the sway bar like he had, less body roll will help reduce the degree of geometry changes with the extra loading you have in your setup if the springs don't keep up?
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      01-22-2014, 09:48 PM   #22
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Quote:
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Neil, were you planning on changing the sway bar like he had, less body roll will help reduce the degree of geometry changes with the extra loading you have in your setup if the springs don't keep up?
No, I'm not planning to change sways. As you know, I'm presently running an E93 M3 (vert) front sway with the stock rear.

Neil
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