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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Suspension | Brakes | Chassis > Bump Steer on lowered E9X



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      08-20-2018, 03:40 PM   #1
dwashy
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Lightbulb Bump Steer on lowered E9X

So my E90 330i has been lowered for a good while now, and fine tuning and tweaking it to my liking as been "fun". However, I have been ignoring what I'm interpreting as bump steer for quite a while. The car is aligned, has M3 front control arms, Koni Yellows, HR Sports, M3 sway bars, e36 M3 front bump stops, z4 rear bump stops, etc etc blah blah

At high speeds the car becomes very darty on bumps and rough pavement. Car doesn't lose stability or balance, but will want to shoot off pretty hard anywhere over 100 mph. While riding over bad pavement at ~85mph I can release the steering wheel and it will jerk sharply left and right, settling once the rough pavement has passed. Conversely, If I take my hands off on smooth pavement, the car tracks straight and true. Taking my E90 over 115 mph is scarier than riding my buddies R1 to 185 mph which I wouldn't have thought to be the case.

To resolve this issue I ordered this chap here. I haven't been able to get my hands on any suspension geometry books, but it looks like a common issue with lowered cars is putting the tie rods out of alignment with the upper suspension geometry. These parts I've ordered are supposed to push the outside end of the tie rod down, getting closer to the stock geometry (or more importantly, reducing bump steer!)

I had been searching for info on this issue, but never found much. This M3 guy here asked a similar question, but the gurus never got back to him. This post is meant to gain input from anyone who has a better understanding of suspension geometry, and to provide a resource for anyone who had the same question I did. Hopefully this is the "fix", and I can move onto something else on the car
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      08-20-2018, 03:49 PM   #2
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Originally Posted by dwashy View Post
To resolve this issue I ordered this chap here.
Looks like there's actually an updated design which looks better here

Last edited by dwashy; 08-20-2018 at 03:50 PM. Reason: gRaMmEr
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      08-20-2018, 03:55 PM   #3
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SPLParts (whom I've never heard of before) actually has all sorts of cool stuff for E9X chassis
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      08-20-2018, 04:18 PM   #4
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You say your car is "aligned" but to what specs? You'd be surprised how much a ~1/16th of inch in toe, front or rear, makes with high speed stability.
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      08-20-2018, 05:30 PM   #5
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Are your front wheels very low offset and your rear wheels very high offset? Staggered? Square?
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      08-21-2018, 09:32 AM   #6
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I have noticed a little bump steer on my e92 also. Iím only lowered maybe 3/4Ē up front.

Iím wondering how those fk heim joints hold up in a daily driver scenario. Thinking if I used them maybe I would make a boot to cover them out of a bicycle innertube. I do have a FK Teflon heim joint on the trackbar of my Jeep that has lasted 2 years so far uncovered and exposed to sand, mud, and 25k miles of daily driving.
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      08-21-2018, 10:43 AM   #7
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OEM Style 162's but with 235 up front. Stock 255 size out back. Toe out .12* on the front which I'm sure contributed to the feeling. Not sure how the rear is looking off the top of my head.

Last edited by dwashy; 08-21-2018 at 10:18 PM. Reason: correcting numbers after referencing alignment sheet
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      08-21-2018, 05:37 PM   #8
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Toe out? Yeah, that obviously isn't helping.
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      08-21-2018, 10:15 PM   #9
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Quote:
Originally Posted by FCobra94 View Post
Toe out? Yeah, that obviously isn't helping.
It definitely helps turn in response. I do think however I am experiencing bump steer, which is worsened by having some "toe out"
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      08-22-2018, 10:06 AM   #10
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Quote:
Originally Posted by dwashy View Post
It definitely helps turn in response.
Well, obviously, but you can't have one without the other.

In this case it's a moot point though because there is no toe-out to be found anywhere on that alignment sheet.
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      08-22-2018, 01:17 PM   #11
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Quote:
Originally Posted by FCobra94 View Post
Well, obviously, but you can't have one without the other.

In this case it's a moot point though because there is no toe-out to be found anywhere on that alignment sheet.
Ah, so (+) values for toe indicate "toe in" it seems. In that case I have terrible bump steer. Parts should get here Thursday, we'll see if I can get around to installing them that night or Friday. Thanks for pointing out the toe haha
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      08-22-2018, 02:14 PM   #12
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When reading your post the first thing I though was bad alignment. What you posted shows a pretty standard stock alignment setting which shouldn't the reason for what you're experiencing. My first thought is worn suspension bushings that flex too much while under load (high speeds) giving you the strange "bump steer" as the wheel moves out of alignment. How old is your suspension, sounds like you upgraded some components but are you still on the original tension struts and tie-rods? These two items could be worn giving you the strange feeling while performance driving. My 2 cents.
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      08-22-2018, 04:07 PM   #13
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Quote:
Originally Posted by Volasko View Post
When reading your post the first thing I though was bad alignment. What you posted shows a pretty standard stock alignment setting which shouldn't the reason for what you're experiencing. My first thought is worn suspension bushings that flex too much while under load (high speeds) giving you the strange "bump steer" as the wheel moves out of alignment. How old is your suspension, sounds like you upgraded some components but are you still on the original tension struts and tie-rods? These two items could be worn giving you the strange feeling while performance driving. My 2 cents.
Appreciate the input. Everything on the car suspension wise is mostly new. Full TRW M3 arms, new sway bar endlinks, new tie rods, new bump stops, koni yellows maybe 10k ago, new HR Sport Springs, m3 f/r sway bar, rear subframe bushing inserts, new upper/lower shock mounts in rear. Not sure what else is really left
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      08-23-2018, 11:31 AM   #14
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Quote:
Originally Posted by dwashy View Post
Appreciate the input. Everything on the car suspension wise is mostly new. Full TRW M3 arms, new sway bar endlinks, new tie rods, new bump stops, koni yellows maybe 10k ago, new HR Sport Springs, m3 f/r sway bar, rear subframe bushing inserts, new upper/lower shock mounts in rear. Not sure what else is really left
Thats why I mentioned tension strut (BMW also names it radius arms). Its the only thing on the front end that isn't new from what you listed. If it is still original it could be the weak link in your suspension thus the strange handling characteristics. Everything else is tight and upgraded, so it is exposing the flex in the old component. Just a thought.
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      08-23-2018, 11:48 AM   #15
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Quote:
Originally Posted by Volasko View Post
Thats why I mentioned tension strut (BMW also names it radius arms). Its the only thing on the front end that isn't new from what you listed. If it is still original it could be the weak link in your suspension thus the strange handling characteristics. Everything else is tight and upgraded, so it is exposing the flex in the old component. Just a thought.
I could be incorrect, but please check the TRW M3 arms link I had in my previous post. I believe the tension strut is included in that kit. From this guy's DIY here I can confirm those parts have been replaced
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      08-23-2018, 01:17 PM   #16
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I don’t know why everyone is debating whether dwashy has bump steer. It’s a common issue when you raise or lower a vehicle.

Since he has lowered his car it’s a natural assumption he will have bump steer. No one has even asked how much he lowered the car lol. What if he lowered the car 2”... you don’t think he will just simply have bump steer?!?! I’m only lowered like 3/4” and I have minor bump steer. The more you raise or lower the more severe the bump steer.

When the tie rods are no longer resting where the designer placed them at vehicle ride height (parallel to ground) their ends travel much more distance laterally when the suspension travels up/down. This is true for all vehicles, and this is why you see bump steer kits made for our e9x cars.
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      09-24-2018, 11:54 PM   #17
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Hey dwashy did you get the new tie rod ends installed? How does it feel now?
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