Suspension Settings - Dialing Out Bump Steer
Discussion
356Speedster said:
Rscocca said:
There is another option to moving the rack up or down.
In the past I have used a bump steer kit. It effectively moves the outer tie-rod up or down to correct the angle between the lower control arm and the angle of the tie-rod.
You need to drill out the hole in the steering arm to accommodate a 5/8 inch wide bolt. This bolt will go thru a rose joint that is screwed onto inner tie-rod.
If you add shims under the rose joint it will change toe out or toe in.
Do you have a link to such a kit? Having adjustment on the tie rod end would be beneficical.In the past I have used a bump steer kit. It effectively moves the outer tie-rod up or down to correct the angle between the lower control arm and the angle of the tie-rod.
You need to drill out the hole in the steering arm to accommodate a 5/8 inch wide bolt. This bolt will go thru a rose joint that is screwed onto inner tie-rod.
If you add shims under the rose joint it will change toe out or toe in.
Admittedly that photo shows a lot of shims, In general its about 1/8-1/4 inch (Sorry I don't speak metric, but I should learn
), You'll need to know thread of outer tie-rod to get correct kit.I think it is about time I put a bit of effort into understanding how to set steering to avoid bump steer.
I am having my suspension laser aligned next week but I suspect it will not cure my bump steer completely.
What is the black art of avoiding bump steer when designing a chassis. My TVR Cerbera suffers the same problem but none of my other cars do. Our SL500 has wider tyres than the Cerbera and different sizes on the front and rear axles but no issues.
I know the height of the Ultima rack is a major factor and I have spent a good bit of time trying to adjust mine and I will be checking it again this weekend before my laser appointment.
Does anybody know of an idiots guide to steering set-up showing how to design front suspension and steering to avoid this issue?
Paul
BTW. I have found that if I 'wiggle' the steering wheel a couple of degrees either side of dead ahead (per second) as I drive down a (bumpy) road I can eliminate the bump steer. And before you ask there is no play in my steering/suspension.
Thoughts?
I am having my suspension laser aligned next week but I suspect it will not cure my bump steer completely.
What is the black art of avoiding bump steer when designing a chassis. My TVR Cerbera suffers the same problem but none of my other cars do. Our SL500 has wider tyres than the Cerbera and different sizes on the front and rear axles but no issues.
I know the height of the Ultima rack is a major factor and I have spent a good bit of time trying to adjust mine and I will be checking it again this weekend before my laser appointment.
Does anybody know of an idiots guide to steering set-up showing how to design front suspension and steering to avoid this issue?
Paul
BTW. I have found that if I 'wiggle' the steering wheel a couple of degrees either side of dead ahead (per second) as I drive down a (bumpy) road I can eliminate the bump steer. And before you ask there is no play in my steering/suspension.
Thoughts?
Edited by Storer on Thursday 21st August 00:08
I had a go sorting this last year. First I had the geo sorted by Mat Jackson of Brit Touring Car fame, at his garage in Arden on his test rig. Bump-steer was not part of the deal though... So I had a few heart-stopping moments when hitting a particularly large dip in the road on the way back, and it was not nice...
After reading up about it, I used the old laser-and-mirror trick (as per you-tube)with a made-up rig, and slotted the steering rack mounts. After about an hr of going back and forth between sides making sure that adjustment at each end did not mess up the other end... I got it very close to perfick!
I took the car back to the duel carriageway that had the humongous sudden dip (A46 south-bound) that had scared me before, and went-for-it. Must admit I did tense up as I got closer to the danger-area, but I need not have worried. Even at xxx leptons, the wheel hardly moved...so sorted!
After reading up about it, I used the old laser-and-mirror trick (as per you-tube)with a made-up rig, and slotted the steering rack mounts. After about an hr of going back and forth between sides making sure that adjustment at each end did not mess up the other end... I got it very close to perfick!
I took the car back to the duel carriageway that had the humongous sudden dip (A46 south-bound) that had scared me before, and went-for-it. Must admit I did tense up as I got closer to the danger-area, but I need not have worried. Even at xxx leptons, the wheel hardly moved...so sorted!
Storer said:
Does anybody know of an idiots guide to steering set-up showing how to design front suspension and steering to avoid this issue?
Without modelling the whole lot up in something like MathCAD or Matlab and analysing the curves, your only real option is to simulate the whole lot with a string plot made to scale. Not overly complicated but relatively time consuming - one of the biggest challenges is accurately measuring everything in the first lace as a mm or two here and there makes a significant difference. For example, a slight difference in the machining of the insert in the upright for the ball joint taper can move your joint a fair amount, changing the results dramatically. Effective wishbone length also makes a difference (for those with rose jointed suspension).Edited by Storer on Thursday 21st August 00:08
You have to consider that the geometry is a complicated set of 3D vectors, all changing dynamically with steering and suspension movement (not only bump steer but also camber changes too with suspension movement) so really everything in suspension design is a compromise - made worse by having to package it in limited space to fit round items such as roll bars, radiators, hoses etc.
Longer wishbones helps significantly as a given amount bump/droop gives a much smaller track change/the angle of the wishbone movement is far less), so your 'curve' becomes far more of a straight line - reducing bump steer. Potentially not so good for camber recovery though.
Storer said:
......What is the black art of avoiding bump steer when designing a chassis.
In very simplistic terms with the car sitting at the required ride height the steering track rod should be a horizontal from TRE to rack.What is happening is as the suspension compresses the upright moves up but the rack position is fixed so the steering turns inwards. If you start back down again the steering straightens as the track rod passes through the horizontal. As you pass below the horizontal the steering again turns in.
The bump steer adjustment is moving the rack up or down to find the horizontal point.
The spacers shown in the pick are probably because the suspension has been excessively lowered on a production car and there is no scope for moving the rack down by the required amount so you have to move the other end down.
Steve
ebeck said:
356Speedster, what ever happened here? Did you get your bump steer resolved? What was the issue?
I think Mark's still dialing... 
I was going to start working on that same issue but I've postponed getting down to it as I'm getting things ready for my future move. Bump stear has now passed onto a back burner for the moment.
Ok I see, so I gather this is typical almost endemic then?
I am just trying to understand this, so bear with me. Seems adding "Bump steer kit" yes ok, so you can shim the tie rod end easier than adjusting the rack up and down or adding threaded height adjusters (like for motorcycle rear wheel) under rack mount so you can make small adjustments with a bolt head and it also prevents movement but that does not resolve it, just makes adjusting easier but seems like the tie rods are just too short no matter how much adjusting goes on?
Is this really just an issue of very small incremental adjustments that have to be dead nuts perfect then there is no BS issue or is it a geometry issue and you have to live with some level of it especially at faster paces?
Ultima offers rose jointed suspension as an option, not clear hos that addresses it either as I read. I see MacG racing offers longer wishbones front and rear though, and double adjustable coil overs does either of those help with the Bump steer. but Issue of mechanical rose joints sans sift mounts, that aside does those longer resolve it?
If still large movement causes toe, could one stiffen the ride, perhaps compression settings on the coil overs and trade ride comfort for BS stability? Seems double adjustable coilovers could then help with tuning?
Just trying to understand.
Thanks
I am just trying to understand this, so bear with me. Seems adding "Bump steer kit" yes ok, so you can shim the tie rod end easier than adjusting the rack up and down or adding threaded height adjusters (like for motorcycle rear wheel) under rack mount so you can make small adjustments with a bolt head and it also prevents movement but that does not resolve it, just makes adjusting easier but seems like the tie rods are just too short no matter how much adjusting goes on?
Is this really just an issue of very small incremental adjustments that have to be dead nuts perfect then there is no BS issue or is it a geometry issue and you have to live with some level of it especially at faster paces?
Ultima offers rose jointed suspension as an option, not clear hos that addresses it either as I read. I see MacG racing offers longer wishbones front and rear though, and double adjustable coil overs does either of those help with the Bump steer. but Issue of mechanical rose joints sans sift mounts, that aside does those longer resolve it?
If still large movement causes toe, could one stiffen the ride, perhaps compression settings on the coil overs and trade ride comfort for BS stability? Seems double adjustable coilovers could then help with tuning?
Just trying to understand.
Thanks
Edited by ebeck on Wednesday 10th December 18:23
Hullo! Sorry, I've not been paying attention to the forum lately... in terms of an update, I've been able to massively reduce the Bump Steer in 2 big chunks. What's left now, I'm hoping will be dailed out with a professional geo setup.
The first big improvement was made by ditching the Kumhos. These are awful on the road (although they were fine on track) and caused horrid tramling, which exaggerated the BS issue, from a feel at the wheel perspective. The tyres themselves clearly don't cause BS, but they were heavily contributing to the wider handling problem.
The second big improvement came from what I know will be a controversial mod, hence why I hadn't posted it immediately. After discussions with some other owners who had done this, an expert on steering geo and some general chassis gurus, I decided to modify the steering rack itself. I nearly had a whole new rack designed, but in the end, I chose to mod my std rack. Given the final cost of the rebuild and machining time, I should have just bought a new custom rack, LOL!
The rack itself was shortened by 50mm (25mm per side) and the track rods were increased by the same amount. The inner track rod pivot point was some way outside the straight line created by drawing a line between top & bottom wishbone mounts at the chassis. This is widely regarded as a key part of good suspension geo. Shortening the rack meant all 3 pivot points are now in line.
I had already invested in a Longacre Bump Steer gauge and spent a lot of time with it before the mod, attempting to scientifically address the issue. No matter what the position of the rack in the chassis, I couldn't get a satisfactory reading.... after the rack mod, one side is almost zero BS and the other requires some final fettling, but it's 90% there.
There are other geo settings / changes I'd like to make, to get more adjustment into the front end and as such, I wish I'd taken the rose jointed suspension, just to give me more fine tuning options. It's a lot of money to change now, so I'm going to have the car properly setup and see if that puts me at a point where I'm happy with the handling. Fingers crossed!
Perhaps I'm more sensitive to this than others, but modding the rack and changing the tyres has dramatically changed the car for the better. I hope this helps!
The first big improvement was made by ditching the Kumhos. These are awful on the road (although they were fine on track) and caused horrid tramling, which exaggerated the BS issue, from a feel at the wheel perspective. The tyres themselves clearly don't cause BS, but they were heavily contributing to the wider handling problem.
The second big improvement came from what I know will be a controversial mod, hence why I hadn't posted it immediately. After discussions with some other owners who had done this, an expert on steering geo and some general chassis gurus, I decided to modify the steering rack itself. I nearly had a whole new rack designed, but in the end, I chose to mod my std rack. Given the final cost of the rebuild and machining time, I should have just bought a new custom rack, LOL!
The rack itself was shortened by 50mm (25mm per side) and the track rods were increased by the same amount. The inner track rod pivot point was some way outside the straight line created by drawing a line between top & bottom wishbone mounts at the chassis. This is widely regarded as a key part of good suspension geo. Shortening the rack meant all 3 pivot points are now in line.
I had already invested in a Longacre Bump Steer gauge and spent a lot of time with it before the mod, attempting to scientifically address the issue. No matter what the position of the rack in the chassis, I couldn't get a satisfactory reading.... after the rack mod, one side is almost zero BS and the other requires some final fettling, but it's 90% there.
There are other geo settings / changes I'd like to make, to get more adjustment into the front end and as such, I wish I'd taken the rose jointed suspension, just to give me more fine tuning options. It's a lot of money to change now, so I'm going to have the car properly setup and see if that puts me at a point where I'm happy with the handling. Fingers crossed!
Perhaps I'm more sensitive to this than others, but modding the rack and changing the tyres has dramatically changed the car for the better. I hope this helps!
I understand shorting the steering rack and having longer track arms, the angle would not be as sharp during bump affecting toe. Similar to longer control arms and longer track rods without changing the factory geometry though. Probably better for regular folks like myself to manage. But unclear how that affects pivot points that are now in line. I am sure it does, as UltimaCH mentions a picture is worth a thousand words though. Thanks!
Morning all. In terms of Bump Steer, this is what I was looking to achieve in the "inner mount plane"....
It's a very hard part of the car to photograph, but here's a pic showing where the inner tie rod knuckle was sitting in comparison to the wishbones mounts....
And here's the "after" shot, showing the new alignment...
I hope this helps clarify the changes I made. I know there are others out there taking alternative approaches to their front suspension geo, so this is just my solution and not the only one!
It's a very hard part of the car to photograph, but here's a pic showing where the inner tie rod knuckle was sitting in comparison to the wishbones mounts....
And here's the "after" shot, showing the new alignment...
I hope this helps clarify the changes I made. I know there are others out there taking alternative approaches to their front suspension geo, so this is just my solution and not the only one!
ebeck said:
I see MacG racing offers longer wishbones front and rear though, and double adjustable coil overs does either of those help with the Bump steer.
In short - sort of! Someone previously posted about the cause of bump steer, and if you think about the arcs formed by the ends of the wishbones through their movement, the longer the wishbones the less sensitive the car will be to bump steer. So - the longer wishbones do help (very) slightly, but are not why we designed them.These were designed to push the upright further 'in' to the wheel, to reduce scrub radius which reduces track change when steering, ultimately increasing grip. It also reduces steering effort which allows us to run higher castor angles without making the steering excessively heavy, which improves dynamic camber.
Having analysed the suspension in depth the rack is fine. Our kit includes extensions for the rack tie rods so that they still 'match' the wishbones. The theory pointed out above is correct for finding the ideal rack height.
To be clear, the dampers make no difference at all.
Again, the short wishbones are the main reason as to why the car is so sensitive to bump steer. There is also an optimum ride height to run the car at too - as soon as you are out of that the curves don't match as well and you get more BS.
Thanks for chiming in Jonny, it's always good to hear your track proven feedback!
You're one of a few people I've spoken with over the last year who have covered wishbone modifications & benefits. It's something I've considered for the future too, if I feel the need to perfect / refine the handling further. How far extended are the wishbones and how far do the wheel offsets have to be changed in response?
You're one of a few people I've spoken with over the last year who have covered wishbone modifications & benefits. It's something I've considered for the future too, if I feel the need to perfect / refine the handling further. How far extended are the wishbones and how far do the wheel offsets have to be changed in response?
Wow.... Thanks for the contribution to the Ultima knowledge base, thats great. However I was secretly hoping for a guide to help us all. However it seems the best advice is tyre choice and.... it sound like get a new rack!!! I wonder if there is a trade off for your mods? IE did the factory decide to design the rack the way it is for a good reason?
after spending alot of time playing with the steering/susspension geo i am still not happy with the set up and from what in can assume from all the info on the subject is there is an inherant problem with using standard susspension arms as opposed to the longer/rose joint type,and standard offset wheels,with the titan rack,i.e the longer arms combined with different offset[see factory demo cars]and a few others on here,don't seem to have the same issue,or it is less severe then the standard set up.so my assumption leads me to think the titan rack is a compromise for normal/standard susspension,but works well for longer arms with different offset wheels,this leaves us with a decision spend lots on changing to longer arms/different offset wheels[poss wider including new tires,]or living with very skittish hi speed control.so in hindsight go for rose joint susspension plus different offset wheels,poss wider and receive better feel/control and alot less of a bump steer issue. just my thoughts.[its about time this issue was cured for all options]-jmwram
356Speedster said:
Thanks for chiming in Jonny, it's always good to hear your track proven feedback!
You're one of a few people I've spoken with over the last year who have covered wishbone modifications & benefits. It's something I've considered for the future too, if I feel the need to perfect / refine the handling further. How far extended are the wishbones and how far do the wheel offsets have to be changed in response?
Even if you go just an inch it is a fairly dramatic percentage increase. It also dramatically reduces steering weight, so to compensate we add some further caster in which is also beneficial for camber recovery when turning.You're one of a few people I've spoken with over the last year who have covered wishbone modifications & benefits. It's something I've considered for the future too, if I feel the need to perfect / refine the handling further. How far extended are the wishbones and how far do the wheel offsets have to be changed in response?
This should not be rocket science. Most car manufacturers manage to solve the issue as do most racing teams.
My car is not as bad as it was after a complete suspension set-up using laser alignment. But any ridges in the road tend to cause the car to 'wander' a bit.
I have to say that our Mercedes SL500 wanders a bit on roads with ridges but not to the same degree.
I have longer wishbones and wheels with a greater offset, so they are not the total answer.
If you are having issues then get the suspension settings checked out (maybe by the Factory if you are in the UK) before spending money on different wishbones/wheels.
Paul
My car is not as bad as it was after a complete suspension set-up using laser alignment. But any ridges in the road tend to cause the car to 'wander' a bit.
I have to say that our Mercedes SL500 wanders a bit on roads with ridges but not to the same degree.
I have longer wishbones and wheels with a greater offset, so they are not the total answer.
If you are having issues then get the suspension settings checked out (maybe by the Factory if you are in the UK) before spending money on different wishbones/wheels.
Paul
Storer said:
This should not be rocket science. Most car manufacturers manage to solve the issue as do most racing teams.
Paul
Don't under estimate the task of optimising the wheel path and suspension kinematics for a modern car! A HUGE amount of work, both theoretical and practical (Test & Development) occur to get modern cars to be so linear across a massive range of conditions.Paul
Edited by anonymous-user on Sunday 8th March 19:28
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