Bump steer.

#1

Has anyone on here measured their bump steer on the their e30? Wasn’t sure if BMW got it right off the bat. No one seems to speak of it so I was just wondering…

#2

No, but if anyone has this sort of data (K&C, etc.) I would be interested so I could build a CarSim model.

#3

Bump steer is change of toe due to compression, right?

I did some experiments in the rear and could not find any change of toe at all with increasing compression. The way I was measuring toe amplified change by a factor of 14 or something pretty big so if there had been toe change, I’d have probably spot it.

I’ve not played with this in front. I figure that since we can’t adjust it, there’s not a lot of use in screwing with it. Besides, trying to measure changes in absolute front toe with changing suspension compression would be pretty darn hard.

#4

I was browsing around today on r3v and came across this thread: http://www.r3vlimited.com/board/showthread.php?t=106577&page=2

In there is a dead link (http://www.e30m3performance.com/tech_articles/susp-tech/rear_curves/index.htm) to what the poster refers to as “the camber curves that Gustave posted on the E30 trailing arm suspension.” I know you asked about bump steer (toe change with compression/jounce), but typically you would find this kind of data (suspension kinematics) lumped together.

Again, I’d be interested if anyone has saved this info from the past. Really, I’d be interested in any sort of vehicle data that will allow me to build a model of the car.

#5

I didn’t save the chart but I did study it a couple years ago when I was trying to understand what the rear wheels did under compression. The camber chart showed a very wide range of semi-trailing arm movement and was therefore really not that useful. Our rear springs bind up after maybe 2" of compression so we don’t have a helova lot of trailing arm movement. Imagine looking at a curve that showed camber change thru 10" of trailing arm movement and trying to glean from it useful info about our 2" of movement.

I don’t recall that the e30M3 site had info re. front camber change. Given the design of McPherson suspension, I figure that if there’s camber change under normal compression conditions, it’s not enough to care about.

#6

[quote=“Ranger” post=71031]Bump steer is change of toe due to compression, right?

I did some experiments in the rear and could not find any change of toe at all with increasing compression. The way I was measuring toe amplified change by a factor of 14 or something pretty big so if there had been toe change, I’d have probably spot it.

I’ve not played with this in front. I figure that since we can’t adjust it, there’s not a lot of use in screwing with it. Besides, trying to measure changes in absolute front toe with changing suspension compression would be pretty darn hard.[/quote]

Front is really for these cars the only one that matters. Dynamically, unless you have large amounts of bump steer in the rain, it won’t make much of a difference (only in feel).

#7

[quote=“swolfe” post=71474]I was browsing around today on r3v and came across this thread: http://www.r3vlimited.com/board/showthread.php?t=106577&page=2

In there is a dead link (http://www.e30m3performance.com/tech_articles/susp-tech/rear_curves/index.htm) to what the poster refers to as “the camber curves that Gustave posted on the E30 trailing arm suspension.” I know you asked about bump steer (toe change with compression/jounce), but typically you would find this kind of data (suspension kinematics) lumped together.

Again, I’d be interested if anyone has saved this info from the past. Really, I’d be interested in any sort of vehicle data that will allow me to build a model of the car.[/quote]
Here you go http://www.e30m3project.com/e30m3performance/tech_articles/susp-tech/rear_curves/index.htm

This is a better article than what I’d seen years ago. It has graphs that do a much better job of showing what’s occurring in our 2 inches (?) of wheel travel. It makes me think about reducing my rear camber.

The toe graph is interesting too. I tried hard to find evidence of toe change with rear wheel position, years ago, but I just couldn’t detect it. I was measuring absolute, not relative toe. Given 3" of vertical wheel movement I should have found, according to the chart, >4/64" of relative toe change…call it 0.8mm absolute toe change. My system of using a laser level to check toe multiplies the toe by 6. So I can easily see 1mm of toe at the wheel because when I’m looking at the laser beam I see 6mm. 0.8mm of toe change would be 4.8mm of movement of my laser beam (because 6x). The laser beam itself is prob 4mm wide but I still should have been able to spot the 4.8mm of movement. Interesting.

#8

Forget about bump steer on the rear…you can’t do anything about it.

On the front, I have found it advantageous to lower the rack about 1/8". Chuck

#9

Is it legal to shim the rack under IT or SE30 rules?

#10

Of course not, Steve. I was repairing wreck damage;)

#11

Toe change during compression is a topic I haven’t seen in detail anywhere but Gustave’s site, unfortunately.

If his toe-curve chart is accurate, then there certainly is something you can do about rear toe steer: raise the ride height via spring pads (good), or raise the subframe (better). Using a correction kit can only adjust static toe, leaving the curve unchanged.

Ranger, it’s weird that you didn’t measure any toe change, but it’s gotta be there; its a semi-trailing arm suspension. You shouldn’t see >4/64" change though:
Gustave measured in inches with a 17" wheel, so the toe will measure a bit smaller with a 15" wheel.
The graph also displays total toe, so you’d halve it for a single wheel.

Does anyone know if the trailing arms are completely horizontal (parallel to the ground) at stock 325i ride height?

#12

Interesting articles. http://www.e30m3project.com/e30m3performance/tech_articles/susp-tech/index.htm

#13

The trailing arms are parallel to the ground at about 6.25" ride height in the rear. From that point upward (compressing shock) the tire toes in.