Bump Steer with Coils

[lemons6245]

I have had my bc coils on for 5k plus miles so far. Has anybody else noticed an excessive amount of bump steer when lowering there vehicle. I figured there would be some, but there is a good amount. As a side not I have had a decent amount of problems with the alignment on this vehicle ever since I installed the coils. The steering wheel never seems to be centered and I have had the vehicle aligned by three different shops (when I installed my mountune lower rear bar I had to move the sub-frame towards the rear of the vehicle to get the bar to install properly and have not had it aligned in the 100 miles I've had it installed) and every bump it jerks the wheel one way or another. My FiST is not slammed, I can still fit a finger in between the wheel and the fender all the way around. Just thought I would get some input from others.

 

[AlrightyThen]

I had BC coils on my car for the last 10 or 15k miles, just uninstalled them. Never had any problem with them and looking back I don't remember ever thinking there was a problem with increased bump steer. So yeah, I know that is not much help. Are you running wheels with really low offset or anything?

 

[DvC]

One finger is pretty dumped

 

[jeffreylyon]

Lowering a car won't change the geometry in such a way that bump steer is changed. Make sure that your strut to upright bolts are tight.

 

[eleven]

Wheel offset and width?

 

[NJBooST]

[MENTION=3405]lemons6245[/MENTION] Bring your car to a dealer and have them reset the Yaw Module. Your electric steering is trying to compensate because it no longer knows where "center" is. The electric power-assisted steering (EPAS) features drift compensation and active nibble control. Drift compensation adjust for strong winds and crown in the road. Active nibble control acts as vibration and shimmy cancelation.
One way to know it needs to be reset is when traveling at over 45mph and after taking a long sweeping left or right turn. Does the wheel not return to center? Is there memory steer? It means you need to reset the yaw module.

Here's an explanation for pull-drift compensation:
View attachment 11153

 

[BRGT350]

Lowering a car won't change the geometry in such a way that bump steer is changed. Make sure that your strut to upright bolts are tight.

Huh? Yes, lowering a car will change the angle of the LCA and the tie rod, which will induce bump steer. The LCA pivot and inner tie rod pivot are not at the same location nor are they the same length. When you lower the car, you are changing the angle of both items. Changing the angle means they will be moving in separate arcs. If the outer tie rod arc under bump is moving different than the lower ball joint, the tie rod at the spindle will cause the knuckle to move inwards or outwards. When cornering, the outside wheel is going into bump while the opposite wheel is in jounce. If the bump steer on the compressed side causes the tie rod arc to pull the knuckle inward, the wheel will go into a toe-out condition. This requires additional steering input to overcome the change caused by the arc of the tie rod. If the knuckle moves outward, the wheel goes into toe-in, which makes the car unstable. You need to reduce steering to avoid going into an oversteer condition. When a car is lowered, you are changing the static geometry of the suspension. When a lowered car enters a corner, there is now a dynamic element added to the suspension. The more static alteration, the greater the change in a dynamic situation.

How to correct the issue on a lowered car; Move the inner LCA pivot upwards to flatten the LCA in the vehicle and lower the outer tie rod pivot. Cycle the suspension in bump and jounce to measure the toe change.

Bottom line, if you lower a car, you change the arcs the suspension moves in, which will affect bump steer.

Here is setting bump steer on my Mustang, which sits on adjustable coil-overs;

April Bumpsteer 001 by Bryan Redeker, on Flickr

April Bumpsteer 003 by Bryan Redeker, on Flickr
Shims on the outer tie rod are used to fine adjust the bump steer. The steering rack and LCA pivots have been repositioned in the chassis.

Bumpsteer in April 006 by Bryan Redeker, on Flickr

Note: The position of the steering rack in the front suspension will change the direction of the bump steer. What is written above is for the Fiesta since the rack is behind the wheel centerline. It is the opposite on my Mustang with the rack on the front.

 

[Siestarider]

I just began learning about bump steer from this thread, read some on net last night. My impression is that small (less than 1") lowering on coils on Fist are barely measurable changes in bump. So am I correct in assuming one does not need to worry about bump steer with "small" lowering?

I have never seen bump steer measured when an alignment is done (I learned basic alignment watching Dad do it in garage, toe and camber were all he worried about). So do alignments include it these days?

 

[BRGT350]

I just began learning about bump steer from this thread, read some on net last night. My impression is that small (less than 1") lowering on coils on Fist are barely measurable changes in bump. So am I correct in assuming one does not need to worry about bump steer with "small" lowering?

I have never seen bump steer measured when an alignment is done (I learned basic alignment watching Dad do it in garage, toe and camber were all he worried about). So do alignments include it these days?

Generally, lowering less than 1" isn't enough to make a significant change in the geometry. Within that 1" range is around the maximum change in ride height due to load. Purpose built race cars sit very low, but their suspensions are designed for that height. If you watch video footage of road race suspension in motion, you will see that the control arms and tie rod move together and are parallel to each other and the ground at ride height. That is due to the change in the pivot height locations to allow for the lowered ride height. Street cars with lowering springs do not move the pivot points, only place the tie rod and LCA at an angle in which the suspension was not designed to be at during static condition.

Measuring bump steer is something not done at any alignment shop. I purchased a special fixture to measure bump steer using dial indicators. The springs are removed from the suspension and a jack is used to articulate the suspension. Front and rear indicators measure the amount toe change per 1/4" of suspension travel from 0 to +/-3" of travel. All of the data is entered into a spread sheet and graphed to measure bump steer. Bump steer within +/-1" of ride height is the most critical since that is the range most experienced during operation, unless you have very soft springs. Given that, lowering within that range means you are still operating in the critical range of bump steer that has been designed from the factory. The problem is that the closer you get away from stock, the less range you have left to work with before you are starting to get toe change due to bump articulation. If I recall the graphs correctly, the toe change is linear the further you travel the suspension, it moves exponentially. More bump compression, the more toe change, the more unstable the car becomes. It is called bump steer because the toe change when the suspension is in bump or compression. When cornering, the tire on the bump (compression) side is heavily loaded and doing most of the work to turn and provide grip. Upsetting the balance of traction with increasing steering due to bump steer induced toe change makes the car turn faster than expected, creating an unstable car at a critical point of the turn. This also makes the car darty over bumps in the road that result in compressing the suspension into a bump condition.

Excessive lowering of a production car by using springs can create a terrible handling car.

 

[Siestarider]

Thanks for the comprehensive reply. I use mine as a DD and monthly tracker running stock suspension. But I do want to add adjustable ride height dampers to play with on track.
Will follow your guidance and lower less than 1" max. As Dad used to say, learn something new every day.

 

[BRGT350]

Bump steer can be tough to grasp at first. I learned from reading a number of racing suspension books, white papers, and setting up the suspension on my Mustang. All of this helped when I was the project engineer lead on a new passive and active steer axle suspension. I spent 5 years as a suspension project engineer and 3 years prior as a suspension control system designer.

 

[RAAMaudio]

Superb responses on this thread, very pertinent information indeed

I did a quick read and did not see the negative effect on roll center from overly lowering a car mentioned, go to low and basically all the weigh transfers to the outside tires so you have reduced grip a great deal.

Using the finger to fender gap measurement is totally meaningless unless you are comparing a similarly setup car with known geometry, tire diameter, etc....

The inner pickup point center of rotation for the control arms must be higher than the ball joint center of rotation which is not always easy to get to and measure.

I did a test to verify the effects of going a bit to low years ago where I had an opportunity to make changes quickly back and forth and the lowest point I could go with good geometry versus just 1/4" lower made a 2 second difference on a 45 second course, that is a huge change.

There is a lot of things to consider to properly set a car up for the best it can be which this car is pretty well sorted from the factory, those that want to make it better need to look into all of them instead of just randomly installing parts, many of which are more placebo than real performance enhancers and some simply make the car slower even if it feels faster.

 

[Siestarider]

Bump steer can be tough to grasp at first. I learned from reading a number of racing suspension books, white papers, and setting up the suspension on my Mustang. All of this helped when I was the project engineer lead on a new passive and active steer axle suspension. I spent 5 years as a suspension project engineer and 3 years prior as a suspension control system designer.

Hell of a resume, thanks for posting that. I gravitate toward racing forums when technical issues arise that I do not understand, so fantastic to have your expertise offered here to clean up questions. My only reason to lower is to gain a little improvement in COG, maybe I am better off relocating lightweight battery to tire well and running stock ride height for my DD/track hobby car?

 

[BRGT350]

Hell of a resume, thanks for posting that. I gravitate toward racing forums when technical issues arise that I do not understand, so fantastic to have your expertise offered here to clean up questions. My only reason to lower is to gain a little improvement in COG, maybe I am better off relocating lightweight battery to tire well and running stock ride height for my DD/track hobby car?

what I have found is that unless you are an outstanding driver, one that races at the club level or time attack on a regular basis, extensive modifications to the car don't really make too big of a difference. Yes, technically, moving the battery to the back will shift the cg rearward to balance the car. Moving it low in the chassis also helps. The thought process is 100% correct, but the vast majority of us can't tell the difference or improve lap times based on the change. The best thing to do is get seat time and spend time with an instructor. A good set of tires, brake pads, and brake fluid will go a long way to improving lap times with proper instruction and practice. Of all the modifications i have done over the years, the thing that made the biggest impact in lap times was spending a day at a driver's school. One day at Team O'Neil taught me more than all my years of driving and time spent on track with an instructor. Take the modification money and spend it on instruction and seat time, that will improve performance more than anything else.

I am not sure if it is still around, but the Corner-Carvers forum is outstanding, but you almost need a master's in Mechanical Engineering to understand and post there. I used it for research. Even as an engineer, those guys are on another level! Most work for either automotive companies, racing teams, aeronautical companies, or are professional race car drivers or crew members. Crazy smart guys!

 

[Siestarider]

what I have found is that unless you are an outstanding driver, one that races at the club level or time attack on a regular basis, extensive modifications to the car don't really make too big of a difference. Yes, technically, moving the battery to the back will shift the cg rearward to balance the car. Moving it low in the chassis also helps. The thought process is 100% correct, but the vast majority of us can't tell the difference or improve lap times based on the change. The best thing to do is get seat time and spend time with an instructor. A good set of tires, brake pads, and brake fluid will go a long way to improving lap times with proper instruction and practice. Of all the modifications i have done over the years, the thing that made the biggest impact in lap times was spending a day at a driver's school. One day at Team O'Neil taught me more than all my years of driving and time spent on track with an instructor. Take the modification money and spend it on instruction and seat time, that will improve performance more than anything else.

I am not sure if it is still around, but the Corner-Carvers forum is outstanding, but you almost need a master's in Mechanical Engineering to understand and post there. I used it for research. Even as an engineer, those guys are on another level! Most work for either automotive companies, racing teams, aeronautical companies, or are professional race car drivers or crew members. Crazy smart guys!

I completely agree with the seat time and learning, making the driver more capable is a lot more cost-effective than making the car quicker. The autocrosser who created the Far North Racing site has some great examples in his book. Being really quick on track requires doing the exact same thing (once you learn what that is) over and over as perfectly as possible, which taxes concentration and is maybe a little less entertaining than being just competent and having fun.

But to be honest about it, the car was so much better than I was when I started tracking, it was easy to focus on driving rather than modding. The mod bug is kind of separate anyway, I enjoy doing my own work and this car is relatively easy to work on. But even then, I tend to look for mods that will improve track quickness in theory even if I cannot feel them yet, or maybe never. Its all good. And great to have forum members who share their knowledge. Thanks.

 

[M-Sport fan]

^^^O'Neil now has tarmac/road course (NHIS?) instruction as well as the rally school?

 

[BRGT350]

I just did the 1 day rally course, but the understanding of car control are the same. Weight transfer is weight transfer regardless of the surface. How to attack corners is completely different, but getting your butt to understand how the car is behaving with different inputs is the same.

 

[Siestarider]

Hence why racing seats are molded to fit torso and legs, with butt exposed to maximum chassis feel and minimum comfort. It would be fun to experiment with bottom seat cushion replacements to see just how much butt calibration/sensitivity is available with stock seatbacks for tracking. Maybe later, have to get Cyborg dialed first. Broke my own rules by changing many variables at one time, but when the car is that far apart hard to resist.

 

[BRGT350]

Hence why racing seats are molded to fit torso and legs, with butt exposed to maximum chassis feel and minimum comfort. It would be fun to experiment with bottom seat cushion replacements to see just how much butt calibration/sensitivity is available with stock seatbacks for tracking. Maybe later, have to get Cyborg dialed first. Broke my own rules by changing many variables at one time, but when the car is that far apart hard to resist.

yep, comfort is still very important to deal with driver fatigue. Good seats provide comfort and support while allowing proper feedback. It is a tough balance. Changing too many variable is something I struggled with and still do to some extent. Hard to resist the urge to modify!

 

[Intuit]

................................. Hard to resist the urge to modify!

Not for me!
0) Too much appreciation for professional engineering teams vs "backyard engineer" / tinkerer.
1) I hate creating even minor new problems with modifications. (I generally only modify only to compensate for design flaws.)
2) Way too damned cheap.
3) Don't track it.
4) All cars seem slow compared to the 0-60 in 3 sec stock motorcycle that very easily hits 160mph.