Bump Steer - what I never fully appreciated.

Re: Bump Steer - what I never fully appreciated. (TwoDogs)

Mon, 19 Feb 2018 11:30:25 GMT

Found the problem with the steering wheel twitching over bumps and it was my fault. When I installed Roll Centre Adjusters I neglected to install similar spacing to the tie rod ends. This weekend I removed the RCAs and took it for a drive and the twitching over bumps was gone. So I think I have misled people who probably presumed that the tie-rods were also spaced, as only a dope would do the control arms and not the tie-rods.
RCAs are really overkill for a road car, much more relevant for track use. I'm going through the exercise as the car is a project for my son (and I) to learn about cars.

Re: Bump Steer - what I never fully appreciated. (rikkir)

Wed, 14 Feb 2018 18:53:43 GMT

I'm running Eibach Pro Kit springs without any of the issues that you've mentioned. My cars handling is brilliant....

Re: Bump Steer - what I never fully appreciated. (TwoDogs)

Wed, 14 Feb 2018 18:49:25 GMT

Hi Hardtop,
agreed. the only reason my car is down 1" is cause the shockies I got to replace the clapped originals, use Eibach springs which are shorter. If I had decent shockies with original length I wouldn't need to ask people to help me get out of the car when I park.
cheers,
Nigel

Re: Bump Steer - what I never fully appreciated. (92 Hard Top)

Wed, 14 Feb 2018 15:13:40 GMT

After reading all this I'm not lowering my car, as it not a circuit car were I'm trying to get 10th of second faster. I'm also lucky I have a gen 3 setup as it already sit lower from the factory than a Gen2 would. With 17s on 635mm from ground to top of guard wheel arch.

Re: Bump Steer - what I never fully appreciated. (TwoDogs)

Tue, 13 Feb 2018 15:17:22 GMT

Hi Tony,
definitely unique to my car, actually unique is doubtful as many people would have suffered similarly when their geometry was misaligned. Anyway the problem and solution are clear. If you google "bump steer macpherson" you will get several sketches which make it clear, the arcs that the tie rod end and the control arm end "create" when the suspension shortens/lengthens, are no longer parallel. Perhaps lowering the car did it, or my rca on the control arm did it, or both. Either way the solution is to get the arcs parallel again.

Re: Bump Steer - what I never fully appreciated. (Guest)

Tue, 13 Feb 2018 14:28:09 GMT

Have you been in other Mr2 to discover if it’s something unique to your vehicle you need to look into further, like worn bushings, collapses spring, worn shocks, loosened link, steering aspect, worn engine mount, etc...

Had Mr2s with large rims (17-18), lowered by 1inch and more, without rca, highly maintained suspension/vehicle, high speed, and can’t relate to wheel shake/steering flicks that cause an OMG on bumps.

Engineered analysis of the suspension and modifications needs to be matched to the alignment centres settings they degree your suspension at as well as your driving style and other factors. Unfortunately not a simple post and solved IMO.

Re: Bump Steer - what I never fully appreciated. (TwoDogs)

Tue, 13 Feb 2018 13:43:43 GMT

Since I started this thread the car was off the road for a few months being repaired from an accident.
Now that is is back on the road I am noticing again the amount of steering wheel shake on bumps which
led to the initial posting. Now I realise that the source of the problem is a) lowering the car 1" but
b) not lowering the tie-rod junction at the hub. Then possibly made worse by c) the fact that I have
fitted roll centre adjuster to the control arm. So now I understand what is going on, the fix is simple
- lower the tie rod.
The point of my original post was that if you are suffering steering wheel flicks over bumps,
you should fix it, because it can damage steering components (when both wheels hit simultaneously).

Next I move to the rear suspension. I have done nothing to compensate for the 1" lowering yet.....
wondering if it requires tie-rod lowering to reduce toe change..?? with or without RCA ??
I'm getting the RCA anyway to reduce roll and help the SW20s meagre amount of camber.

Re: Bump Steer - what I never fully appreciated. (TwoDogs)

Tue, 30 Jan 2018 18:20:01 GMT

I find the suspension articles over on the Wilhelm site very informative. Especially since several people who have pretty good understanding are collaborating their thoughts, sometimes differing, but usually not.
Many of you experienced track guys will already know the following.
The main thing I get from their findings is that the SW20 camber (lack of) adjustment is not good.
a) The camber changes a lot with body roll,
b) the roll is increased if the car is lowered.
Untreated it means that you have to compromise between straight line grip and wear vs cornering grip.
One contributor points out that the camber change can be reduced moving the top mount of the suspension inwards
with adjustment plates and I have no doubt that is true.
Alex Wilhelm focuses on b), that the extra roll can be removed with his compensating blocks. I have no doubt about that theory, I am not sure about how much extra roll there is, but any, makes the fundamental issue with SW20 camber, worse. I am thinking that perhaps sorting a) and b) first, might make spring/swaybar decisions a bit easier.

Re: Bump Steer - what I never fully appreciated. (cogs)

Sun, 04 Jun 2017 18:16:30 GMT

Falcon
Where yer been Cogs ?

Haha, thanks Falcon! I've always been around but just busy with RealLife™! My '2 still sees daily use but the poor girl is sadly neglected - so many plans, so little time. I'm a little envious of guys like TwoDogs who are just discovering the fun of MR2s, but happy to see they're keeping the scene alive.

Re: Bump Steer - what I never fully appreciated. (TwoDogs)

Fri, 02 Jun 2017 15:14:55 GMT

Thanks Cogs, appreciate the advice. I am indeed going down the path that you and Kameleon have recommended.
My post was really to advise those who may not know and are driving other people's efforts. I picked up a car from a young guy who either had no idea or was dumping all his worn out parts in this car, possibly keeping the good stuff for his main car.
The wheels are wrong, and lacked hub centre rings, the shocks were shot and lowered (cut?) and the geometry therefore needs sorting. So yes the reason for the steering wheel jerking may be not be from the different arcs of tierod & control arm, but my message is really - don't ignore it. If you have major steering wheel action happening on one wheel bump, imagine when both wheels hit some bump, the steering wheel doesn't jerk at all, but the forces didn't cancel, they were absorbed and not by the suspension! Best to reduce those forces with the changes you have listed, which I am doing.

Re: Bump Steer - what I never fully appreciated. (Falcon)

Fri, 02 Jun 2017 07:03:01 GMT

He's Back He's Back !!! Welcome back. Where yer been Cogs ?
I've enjoyed your technical articles in the past and I thank you.
Hope we might see some more good explanations from you into the future.
Cheers.

Re: Bump Steer - what I never fully appreciated. (cogs)

Thu, 01 Jun 2017 21:47:27 GMT

Steering geometry that causes bump steer isn't going to make an appreciable difference to the loads and subsequent wear rates of the steering components. Bump steer is caused by steering angle changes throughout suspension travel, and the steering movement produced is only a small fraction of the suspension travel.

At any time a car is on the ground there are loads on the steering components, adding the dynamics of motion changes these loads in one way or another. The single biggest factor that will increase steering component loads when hitting bumps is the distance between the steering axis at road level and the edge of the tyre, and this is related to the scrub radius which is the distance from the same point to the tyre centre line. This distance is essentially the lever that produces steering forces when a bump is hit. Factors that change it significantly are wider tyres and increased negative wheel offsets. Small changes here can profoundly affect the transmission of bump forces into the steering. Larger diameter tyres, due to steering axis inclination, tend to bring scrub radius back towards better values when offset has been negatively increased. Large scrub radii also cause instability under brakes on uneven surfaces.

Another significant factor that loads steering components is increased caster angles. High caster angles are useful to increase camber change on steering input and maintain flatter tyre contact on cornering, particularly with strut type suspensions where other camber change methods are not available. You can quickly see the effect of caster loads by unbolting the tie rod ends and dropping the car - it immediately goes pigeon toed!

Finally, load on the rack bushes increases when the ride height is not ideal. Ideal means that all four tie rod ends should be in a straight line with the car at rest, ie. tie rods horizontal. When lowered, the outer tie rod ends are higher than the inners causing steering forces to place vertical loads on the rack, and this decreases the bush life.

Take Kameleon's advice and service everything in the car's suspension and steering systems, this is always the best starting point when troubleshooting sloppy handling and dynamics issues. Then pay a bit of attention to your wheel offsets and sizing, keeping in mind they work intrinsically with the suspension geometry in determining the car's behaviour. If you need corrections, make them thoughtfully and try not to just guess. Ultimately if you make the car a fun place to spend time it'll be immensely satisfying knowing that you made it that way. At least that's how it works for me!

Re: Bump Steer - what I never fully appreciated. (TwoDogs)

Thu, 01 Jun 2017 10:09:20 GMT

Thanks @Kameleon

Re: Bump Steer - what I never fully appreciated. (kameleon)

Wed, 31 May 2017 19:23:22 GMT

1-2mm

Re: Bump Steer - what I never fully appreciated. (TwoDogs)

Wed, 31 May 2017 10:38:55 GMT

Thanks Kameleon,

I'm not sure what "slight toe" is ? 1/2 deg ?