Optimal bump steer?

[snowyelan]

Hi All,
Over the holidays I went through the exercise of setting bump steer to near zero. One thought that crossed my mind was that Lotus had a target for bump steer settings. I am basing this on them supplying shim dimensions for each chassis.
Has anyone measured what this was (toe angle) at full bump and droop when using the specified shims? It would be interesting to try both neutral and Lotus specified settings. Unfortunately there were no shims or numbers with my car when i bought it.

[john.p.clegg]

Scott
IMHO
I don't think figures exist,Lotus will have supplied shims for the steering rack , to put the arc of "bump steer" at 0 degrees at static ride height therefore giving toe-in for either bump or droop .

Altering things like weight , spring strength , ride height, etc will need it all re-calculating/re-setting.

John

[Gray]

Dimensions for rack height as per Lotus design are I'm fairly sure in the Brian Buckland manual.
Having corrected a couple in the past it did make quite a difference to straight line stability.
I attended a talk by John Miles in about 2002, when he started to set up cars for other manufacturers he found zero bump steer didn't work and they were very unstable. Discussing further with him he suggested trying a slight increase in rack height from 0.5mm on my Elan to see if stability was improved. I put some additional shims in but never had time complete the car.

[nmauduit]

I attended a talk by John Miles in about 2002, when he started to set up cars for other manufacturers he found zero bump steer didn't work and they were very unstable.

I respectfully doubt this as stated, there must be a confusion of terms (toe in? caster angle?) ... 0 bump steer means exactly the opposite : no jerk steering action when hitting a bump or a hole in straight line.

[Geoffers71]

Probably more than 30 years ago I attended a local Club Lotus meet where a Lotus factory engineer gave a talk which included a discussion on bump steer. I wrote down what he said on a post-it note and it has been stuck in my workshop manual since that time. The measurement to make ( he said) is:-
Rack height formula
From the centre line between lower wishbone pins to top of rack mounting : 2.19? + or - 0.005?

My S3 SE that I had at the time did suffer from bs, but I can?t remember now whether I used it or not or whether, if I did , it made any difference. Sorry Anyway here it is for what it?s worth.

[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.

[snowyelan]

Hi All,

Sorry, I think I've created some confusion here.

The question I have is what did Lotus ACTUALLY set it to? The only way I can think of to check this is to measure a completely original setup (unlikely now) or use the scribed shim dimensions.

There are several processes for measuring and setting bump steer here. I used the method with a laser pointer tracing suspension movement at the caster angle then comparing it to the line generated with the rack connected. Most of them indicate setting it up for as near to no change as possible or slight toe under bump conditions. Adding shims changes bump toe in to bump out (eventually), which is supposed to decrease stability....

[Gray]

nmauduit - there is no confusion (I am a Chartered Engineer and did work on suspension analysis for a F1 team in their championship wining year - many years ago) - if you have no geometrical bump steer (geometric bump steer being the road wheel turning slightly from straight ahead as the wheel moves up) the vehicle can still be diverted from its trajectory by a bump, introducing bump steer to counteract this can keep the trajectory straighter - go the wrong way and it would increase the problem. John Miles found that some conventional saloon cars he was setting up would be unstable deviating from the straight ahead with zero geometric bump steer, so he introduced some bump steer to assist stability over bumps.

[nmauduit]

nmauduit - there is no confusion (I am a Chartered Engineer and did work on suspension analysis for a F1 team in their championship wining year - many years ago)

good for you. As for my cars, and short of prior knowledge of the occurence and magnitude of the bump or hole, or even side, if it'll happen car loaded or unloaded etc, I can't know in advance which way to anticipate the effect of said unexpected bump or hole, so I will still tune my steering rack to minimize bump steer. This for a given setting of the chassis (body height, toe in, caster etc).

[snowyelan]

I seem to be starting arguments without intending to do so. I'm not challenging whether to use bump steer or not, just trying to determine what Lotus intended the amount of bump toe to be. I struggle with the idea of using 2.19" +/- 0.005"above the lower arm pivots when the rack is mounted by its cast surfaces in bits of rubber.

[Elanman99]

I'm not convinced that bump steer is or was, ever 'used' to change the straight line handing of a car.

The ideal amount of bump steer is zero (if bump steer is defined as unwanted change of wheel direction with suspension travel).

Over the full range of suspension travel I suppose a small amount of BS is probably unavoidable but below a certain value will be undetectable by the driver. A noticeable amount of BS, if it identical on both wheels, may not affect straight line performance as the two sides would cancel each other out, however if the road was wet and grip between rubber and road differed across the width of the road then the car would probably not be or feel stable.

I can envisage deliberate bump steer being used to change cornering characteristics (but it seems a weird concept to me) as when cornering and the outer wheel suspension is compressed the bump steer could be engineered to increase or decrease the amount of lock so giving the effect of under or oversteer.

Rack height and tie rod length affect bump steer as do all wishbone lengths and mounting positions, the whole suspension needs to be geometrically correct (and symmetrical side to side) to eliminate, or at least minimise bump steer. Shim thickness (or rack height really) is set to either what Lotus stamped on the chassis or if that is not known is then best determined by actually trial and error testing (laser pointer method or whatever).

It maybe me but I fail to see how deliberately introducing bump steer (as Gray has just described) can improve stability over bumps!

Ian

[john.p.clegg]

+1 the above...

John

[Elanman99]

I seem to be starting arguments without intending to do so. I'm not challenging whether to use bump steer or not, just trying to determine what Lotus intended the amount of bump toe to be. I struggle with the idea of using 2.19" +/- 0.005"above the lower arm pivots when the rack is mounted by its cast surfaces in bits of rubber.

In my last reply I was a little remiss as I did not actually answer your original question. The straight answer is (IMHO) that Lotus intended bump steer to be zero, nil, none.

The shim thickness is not a measurement of bump steer, its just the value that worked for that chassis, presumably measured with some sort of jig the Lotus factory used.

I agree with you regarding +/-5 thou tolerance in conjunction with cast surfaces, folded and welded sheet metal but I suppose it all averages out in the end so the bump steer become zero, or at least undetectable by the driver

Ian

[Quart Meg Miles]

It maybe me but I fail to see how deliberately introducing bump steer (as Gray has just described) can improve stability over bumps!
Ian

Striking a bump can slow that side of the car thereby changing its direction which might be compensated by a bit of temporary toe-in. Also cars with soft bushing may allow the whole wheel assembly to swing back on its linkages and change the wheel direction too. We are talking fractions of a degree at high speeds.

[snowyelan]

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?

I can answer 2&3 if I bring home my machinists straightedge and guage blocks this weekend. Anyone able to answer 1?

I would like to propose a common method of measurement that seems simple, to me at least.
Vertical board 600mm from the hub face.
Run the suspension thru its travel with a laser pointer attached to the upper wishbone. Trace the line which is along the castor angle.
Repeat with the pointer attached to the hub.
Take a measurement between the lines with the lower arm level, raised 25mm and lowered 25mm.

Does anyone have an original rack setup to test?