Optimal bump steer?

[baileyman]

I'm not sure I should care much about toe-in changes during droop. So I preliminarily set my 26R style frame for the bottom of the nearly-zero range of toe-in change. (It takes a shockingly high stack of shims to do that with bent lower links!) My intention is that bumps will be initially neutral, but inevitably toe in at the upper extreme. Droop will toe in immediately, but as that tire is unweighted, I think maybe it is of less concern. Anyway, all this is static in the garage, so I expect to report back what actually happens when the thing gets on the road!

John

[Elanman99]

So 3 questions come to mind.
What is the bump steer with the factory shims?
What is the bump steer with the 2.190" rack height?
What is the rack height with the bump set to zero?

?

So three points,

Assuming the chassis, wishbones and all relevant components are as they were when the factory set up your car, the bump steer will be too small to be noticeable. I suppose that is the 'optimal' value, as per your subject line.

Rack height is one of the factors that affect bump steer but its not a means of directly setting the amount. Rack length (between rack ball joints) and the height of the track rod end ball joints, amongst many other small details all have an effect on the geometry.

You cannot 'set' bump steer to zero, in the sense that there is no settable adjustment like there is for ignition timing or clutch actuator free play. Remember too that a rack may need different shims at each side, not to make it 'level' with the ground or chassis, but to put the ball joints at each end of the rack in their best (optimum) position to minimise bump steer. Raising one side of the rack also lower the opposite side so fine tuning the front suspension bump steer involves a series tests and adjustments. I suppose by the time someone has done that they are not going to be too bothered about dismantling to measure the shims, just to keep note of details that only apply to that one vehicle.

Ian

[Craven]

Probably another Lotus myth but been around for a while, the cars were set with a small amount of bump under steer, easier to control when hard cornering than an over steering car.

May I suggest you guys take a look at the effects of ?Roll Steer?.

[661]

I made a bit of a mission of it, but I enjoyed the process.
It did take 5 days ( short days - I have a life).
Setting up a chassis with 26R suspension and using this bump gauge:
https://www.vmep-ltd.co.uk/bump-steer-gauges

I could have taken an educated stab at the rough size of the shims required and shortened the process, but decided 'for the journey' to start with none and work up.
Part of the reasoning was to hope that by the time I got to the crucial bits I was competent with the gauges.
My workings are on an excel spreadsheet and so can't be attached here but if anyone is interested I'll forward or will post if someone can convert to a useable format.
The points to take home are, it is possible to get virtually no change in bump steer over the race suspension range.
But in answer to your question, (3) every chassis is different and needs its own appraisal. I understood lotus did this by hanging a beam over the upper spindles and measuring to the rack. This doesn't measure bump steer, merely estimates what the ideal shim size would be if all other variables were as per the expected tolerances.

[nmauduit]

I made a bit of a mission of it, but I enjoyed the process.
...
But in answer to your question, (3) every chassis is different and needs its own appraisal. I understood lotus did this by hanging a beam over the upper spindles and measuring to the rack. This doesn't measure bump steer, merely estimates what the ideal shim size would be if all other variables were as per the expected tolerances.

I share that opinion, and believe that the "quick fit" approach at the factory would be sufficient for a road car on high wall tires not pushed to the limits, but if fitted with stickier and stiffer wall tires on harder suspensions, it is worth being meticulous to push the bump steer all the way to nil or just about, so as to remain confident that at the end of a straight you can slam on the brakes side by side with someone and won't get a scratch on the new paint even if the track is a bit uneven.

[snowyelan]

I have once again not put to text what was in my mind. I should have clarified that for question 3 the height is above the lower inboard wishbone pivots, not the rack mount pads, which will be inconsistent chassis to chassis.

I still would like to see what the results are with a 'as set by the factory' steering rack.

[Andy8421]

I spent some time in the past looking into this, and frankly its all rather complicated.

As pointed out above, much of the adjustment of bump and roll steer is out of our hands as it depends on the fundamental positioning of the suspension pivot points, rack length, and wishbone sizes. I guess we have to take it on trust that Lotus put those in the best place they could.

Having gone round in circles a few times myself, a few observations which may be of help to others.

The positioning of the inner track rod ball joint is a key part of this process - I have often wondered whether all racks are exactly the same length. Equally, the two track rods need to be set to be exactly the same length when the steering toe is adjusted (I never bothered about this until I started looking at bump steer) or the inner track rod ball joints will be in different places for each wheel relative to the wishbone pivots in the straight ahead position.

Bump steer adjustments only apply in the straight ahead position - as soon as the steering is turned the inner track rod ball joint positions move and all bets are off.

One thing that had occurred to me in the past, and with reference to the comment above that zero bump may not be the optimal setting, in a corner when the car rolls, assuming that a steering angle has been applied, the inner track rod ends will be in different places to the straight ahead position. As the suspension on the outer wheel will be in bump and the inner wheel in droop, the effects of bump steer (roll steer at this point) will be different than straight ahead. It may be that there is a compromise setting between zero bump steer at straight ahead and zero roll steer for a given steering angle, that gives better overall handling.

I never reached a conclusion on this and figured out that minimising bump steer in the straight ahead position was about the best I could do.

[prezoom]

Just be grateful that our cars are front steer, rather than rear steer. With front steer, bump or droop result in toe in rather than toe out, like rear steer.

[Quart Meg Miles]

Just be grateful that our cars are front steer, rather than rear steer. With front steer, bump or droop result in tow in rather than tow out, like rear steer.

Explain!

If the steering joint is too far inwards bump and droop will generate toe-out, too far outwards will generate toe-in.

One assumes that the wishbone pivots align with the rack ball joints which is why the +2 turrets are in the same place as the Elan's.

(Edited as I forgot the steering arms are forwards, not trailing, so got my toes the wrong way round.)

[rgh0]

To generate zero bump steer with a wishbone suspension the steering rack inner ball joint pivot point needs to be on the line that joins the upper and lower pivots and the outer tie rod ball joint pivot point needs to be on the line joining the upper and lower wishbone outer pivot points. The Elan and Plus 2 suspension achieves this aim generally with the rack set at the specified height.

However this can only apply in straight ahead driving where the pivot points all align and what happens in real world conditions when the suspension is loaded with braking and cornering forces is more important. How much toe in is set on the front suspension also affects the bump steer geometry as it moves the location of the tie rod outer pivot point versus the suspension out pivots.

Everything is also moving about on rubber bushes and mountings so the actual geometry out on the road at any one time is a long way from being precise. The art of suspension development that Lotus have been so good at for so long is to find the right compromises so that it all actually works in practice. One of the problems we all face is that the suspension settings that Lotus developed originally for our cars no longer really apply unless you are using tyres of the same geometry, flexibility and poor grip that the car was developed for back then. Maybe Lotus or McLaren ( since their boss is a fan of Elans) could work up a modern set of suspension settings to keep their suspension developers and test drivers occupied on their slow days

cheers
Rohan

[patrics]

Hi,
Whether I am right or not, on my race car I set the bump-steer in a very simplistic way and didn?t use any gauges.
First I replaced the track rod ends with spherical joints ? made a mounting piece from the steering arms.
With the car on its wheels ready to go and with my weight in the driver?s seat.
Alignments, heights, weights etc all set.
Measure the angle of both steering rack arms ? probably did this by eye.
Turned up spacers to set the steering rack arms parallel ? obviously as only me in it, the height / angle of the arms is significantly different side to side.
With both arms set to parallel then any up or down movement should only result in toe-in.

To simple?

Cheers
Steve

[prezoom]

With front steer, In bump, as the axle moves upwards from rest, and the link from the steering knuckle to the steering rack rises from parallel with the ground at the outer end, the link, in effect becomes shorter pulling the steering knuckle inwards, causing toe in. Same applies for droop. With rear steer, with the steering knuckle pointing towards the rear, the link will pull the knuckle inwards, resulting in toe out. While toe out can be useful in racing, causing increased turn in, it can also cause the vehicle to pull one way or the other under braking as the nose drops. Toe in makes for a much more stable condition under braking. Then there is ackerman and roll center height, and the chase goes on.........

[JonB]

Just to add some devil's advocacy into the mix.

If you think Lotus went to great efforts to get the suspension and steering geometry absolutely spot on for every car that left the factory, you are most likely mistaken. I doubt Uncle Colin gave a monkey's, as long as there was enough money to fund his F1 aspirations. Why do you think they deleted all the relays from the wiring loom on the S130? Graeme is right, each chassis needs to be set up differently and I think this is because they weren't terribly accurately made in the first place.

However.. my experience, FWIW, is that the car's handling is pretty good as it is, given its age.

[baileyman]

Hi,
Whether I am right or not, on my race car I set the bump-steer in a very simplistic way and didn?t use any gauges.
First I replaced the track rod ends with spherical joints ? made a mounting piece from the steering arms.
With the car on its wheels ready to go and with my weight in the driver?s seat.
Alignments, heights, weights etc all set.
Measure the angle of both steering rack arms ? probably did this by eye.
Turned up spacers to set the steering rack arms parallel ? obviously as only me in it, the height / angle of the arms is significantly different side to side.
With both arms set to parallel then any up or down movement should only result in toe-in.

To simple?

Cheers
Steve

On setting my 26R setup I also had to raise the rack so that the lines imagined by Rohan intersected. Standard elan link lines intersect outboard on a line through the level lower links. 26R link lines intersect lower due to the kinked lower links. Standard rack height projected the rack lines to well below the link 26R line intersections. Standard rack height put the 26R geometry well into bump toe-in whereas raising it brought it toward neutral.

Level rack lines would pass well above either standard or 26R link lines intersections. Could that put the geometry into bump toe-out? I'm not sure my imagination is firm enough to see that. Could it be?

John

[661]

Hi,
Whether I am right or not, on my race car I set the bump-steer in a very simplistic way and didn?t use any gauges.
First I replaced the track rod ends with spherical joints ? made a mounting piece from the steering arms.
With the car on its wheels ready to go and with my weight in the driver?s seat.
Alignments, heights, weights etc all set.
Measure the angle of both steering rack arms ? probably did this by eye.
Turned up spacers to set the steering rack arms parallel ? obviously as only me in it, the height / angle of the arms is significantly different side to side.
With both arms set to parallel then any up or down movement should only result in toe-in.

To simple?

Cheers
Steve

On setting my 26R setup I also had to raise the rack so that the lines imagined by Rohan intersected. Standard elan link lines intersect outboard on a line through the level lower links. 26R link lines intersect lower due to the kinked lower links. Standard rack height projected the rack lines to well below the link 26R line intersections. Standard rack height put the 26R geometry well into bump toe-in whereas raising it brought it toward neutral.

Level rack lines would pass well above either standard or 26R link lines intersections. Could that put the geometry into bump toe-out? I'm not sure my imagination is firm enough to see that. Could it be?

John

An additional consideration is that the steering arm position is slightly moved towards the midline with the TTR set up to prevent the ball joints from fouling the discs.