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Bump steer yet again.

PostPosted: Tue Jul 08, 2014 8:30 pm
by G4ILN
I've been setting up the bump steer on my Plus Two while the body is off the chassis.

I'm using a laser beam aligned with the stub axle centre. The beam points forwards (ie perpendicular to the stub axle axis) onto a target about six feet away. The chassis height is fixed and the springs and shock absorbers have been removed. I'm moving the suspension arms through their full range of movement.

Ideally the beam should produce a straight vertical line on the target indicating no bump steer as the suspension moves. The best I can achieve is a curve, a tangent to which is vertical. This curve is symmetrical about a horizontal line drawn through the ride height position. ie toe in increases by equal amounts on both compression and droop.

Should I be able to do better than this?

I realise that my measurement method is flawed because the track changes as the suspension moves up and down. Tomorrow I plan to reflect the laser beam off a mirror onto a target mounted below the laser beam. As the beam and target will move together, this will cancel out the change in track. However this will produce a horizontal line on the target rather than a curve. I'll have to measure deflection at various points and plot a curve from that.

I'm having to use a lot more shims than Lotus ever did, but reading through previous posts this seems to be quite common.

Cheers, Graham.

Re: Bump steer yet again.

PostPosted: Tue Jul 08, 2014 10:14 pm
by ericbushby
I have recently done mine.
I fitted a laser pointer to the hub pointing sideways to the garage wall. Cardboard with a vertical line on it was stood against the wall. I have the advantage tha the wall is 5 feet away. The car was raised up with the rear wheels on blocks and the chassis level. Front wheels removed. No front springs or shocks as you say. Adding shims I could adjust it until the laser followed the line on the board instead of running out as the suspension was raised.
Being knock off wheels the laser pen was just held in the hub recess with a piece of foam (pan scrubber type)
The chassis was marked 0.040" and 0.050". I finished up with 0.130" and 0.180". Of course, as this is the first time I have done this, I do not know if this is correct but it is a lot better to drive than it was.
I hope this helps,
Please let us know how you get on, we need to learn all the time.
Eric in Burnley

Re: Bump steer yet again.

PostPosted: Tue Jul 08, 2014 10:45 pm
by G4ILN

I actually started using a method the same as yours. I turned up a hardwood plug to fit the hub centre and bored it to take a keyring laser. When using this method it is obviously vital that the chassis is level fore and aft. I found that the laser traced out a straight line on the target and by adjusting the shimming I could get this line to be vertical.

The method seemed fool proof to me and I couldn't understand why people were using mirrors etc. Perhaps king pin inclination negates the results? I did have a couple of problems though. Although the bore in my plug was concentric with its outside, the laser beam came out at an angle to the stub axle. This could be demonstrated by rotating the hub and watching the beam trace out a circle. This shouldn't have mattered so long as the hub was locked in position though. The second problem was that the laser batteries only lasted a few minutes. Therefore I changed to using a laser spirit level pointing fore and aft which resulted in the laser tracing a curve.

I had placed a spirit level across the rear lower inner suspension mounting pins (nuts removed) and adjusted the chassis with wedges to get it level from side to side. I then placed 1/4" thick plates where the rack shims would be and put the spirit level across these. The plates are necessary to prevent the spirit level grounding on the press outs under the rack. This showed that the shim mounting pads weren't level with each other. By adding shims to the low side I was able to determine roughly what the difference in shim pack size would be between the two sides.

I actually needed a 435 thou shim pack on one side and rather less on the other to achieve the best results.

Re: Bump steer yet again.

PostPosted: Wed Jul 09, 2014 5:43 am
by john.p.clegg
Your first post sounds good to me apart from one point,the ride height,have you obtained this with the body off or with the body on and typical body weight added where necessary as the ride height alters due to weight/springs /time etc from the book figures...

John :wink:

Re: Bump steer yet again.

PostPosted: Wed Jul 09, 2014 7:45 am
by robertverhey
I've a quick question, as I'll be tackling this in due course. Is it feasible / worthwhile to do this with the shock absorber /spring unit removed and a suitable tube used as a spacer in its place, to keep the lower wishbones apart? Seems to me that would make it very easy to move the suspension up and down through its full arc......

Re: Bump steer yet again.

PostPosted: Wed Jul 09, 2014 7:57 am
by john.p.clegg
That's just the way to do it..( or just remove the spring)

John :wink:

Re: Bump steer yet again.

PostPosted: Wed Jul 09, 2014 10:09 am
by andyelan
Hi Graham

I?ve done quite a bit of investigation into bump steer on my Plus 2S130/5 using a commercial bump steer gauge and I?d say what you finding is quite normal. I reckon the minimum toe change that?s achievable is about 0.020? as the suspension moves from nominal position to +/- 2?

The key point here however is that the toe change must be symmetrical as the suspension moves from nominal to full compression and nominal to full droop as this affects ?roll steer?.

If the rack height is significantly higher or lower than it should be, then the total amount of bump steer for each wheel will be greater than it needs to be, however, more significantly, the toe change will always be in the same sense, i.e. as the suspension moves from full droop to full compression the wheel will continuously either toe-in or toe-out. While this is not too much of a problem when the car squats or dives under acceleration or breaking, it is very significant when the car rolls during cornering. What happens in this case is that as the car rolls the suspension on one side compresses while the suspension on the other droops. This then results in one wheel toeing- in while the other toes-out, or in other words, both wheels steer in the same direction. This roll steer will either add or subtract to the drivers steering input and can be very disconcerting, if not downright scary, if the car is suddenly swerved left then right as first the toe change oppose the steering then will add to it. It can be noticed if the car is tipped by either a strong side wind or a large truck going passed and gives a feeling of general instability. When the rack set to the correct height, then the wheels will toe-in irrespective of whether the suspension droops of compresses. Now when the car rolls the toe change of each wheel will be opposite the other hence any overall effect on steering will be cancelled out.

On a more practical note.

As also noticed by Eric, I too found the marking on the rack mount significantly different to the shimming required. I ended up using 0.188 RHS and 0.217 LHS whereas the chassis was marked 080 on the RHS. (I wasn?t able to read the marks on the LHS). I can?t explain why there should be this discrepancy but it seems common.

And on a final point. It is of course not the height of the rack at the mounting that?s important but the height at the inner ball joins at the rack ends. This makes setting the rack height a little tricky and time consuming as each time a shim is changed at one side, it affects height at the other side. One thing I?d recommend is the solid aluminium rack mounts. I have fitted and have noticed absolutely no detrimental effects whatsoever while driving (to be honest I didn?t notice any improvements either) however their big advantage is, as far as I?m concerned, that they make fitting and changing shims very much easier and also give far more consistent and repeatable bump steer readings than when trying to use the original type rubber mounts.

Hope this info is of help


Re: Bump steer yet again.

PostPosted: Wed Jul 09, 2014 5:51 pm
by ericbushby
With the spring/shock absorber assembly removed, I bolted the wishbones up again.
A 2ft length of wood clamped to the bottom wishbone made a convenient handle to lift the suspension full travel easily. With the laser pen loosely attached to the hub it was obvious that the line on the target was leaning forward at the top with no shims fitted. This showed that the wheel was turning inwards under suspension compression. Adding shims showed a clear improvement. Then it was a case of refining the process carefully.
It may have helped that I have the solid aluminium clamps so a light G clamp could be used to hold the rack between tests.
I cannot see the need for a tube spacer as the wishbones seem to be correctly positioned when bolted back up to the trunnions but if in doubt it can only be better to fit one.
Eric in Burnley
1967 S3SE Type 45

Re: Bump steer yet again.

PostPosted: Wed Jul 09, 2014 8:55 pm
by G4ILN
Thanks for all your replies everyone.

I've spent a great deal of time on this today. As I'm getting old this is likely to be the last chance to do the job "body off", so I might as well get it right.

Yesterday I was using a forward pointing laser at stub axle height, with the beam horizontal, pointing at a target about six feet away. This enabled me to adjust the shimming to achieve two nice symmetrical curves, one from each wheel. There was equal toe in on droop and bump.

Today I pointed the laser at a mirror six feet away and adjusted the angle of the mirror to reflect the beam back onto a target fixed just below the laser beam. This nulls out the effect of the track changing as the suspension moves. The method is shown in this Youtube video:-

The results showed slight differences between the amount of toe in at ride height plus and minus 3 inches. I chose 3 inches because I had lots of one foot lengths of 3" by 3" fence post which it was easy to slip under the trunions.

However I found that increasing or decreasing the thickness of the shim pack, by 10 thou, only made things worse. Not wanting to make custom shims enabling adjustments of 5 thou or less, I left things as they were.

I conclude that using a forward pointing laser pointing directly at a target is a good method of measuring bump steer and there's no need to use a mirror. Unless of course you want to try 1 thou shim increments! It also gives you nice curves to look at.

Having decided to leave the shimming alone, I made measurements using a laser aligned with the axis of the stub axle and pointing at a target three feet away. This produced a near vertical line. So this is a good method too. It does depend on the chassis being horizontal in the fore and aft direction. If the chassis tilts down at 5 degrees, so will the line.

Everyone seems to find that they need a thicker shim pack than Lotus specified for a particular chassis. So either Lotus got it wrong or they decided that some bump steer was desirable.

It's a long time since I driven the care, but I do remember that steering consisted of stopping the car going where it wanted to go, rather than steering where I wanted to go. Perhaps matters will be improved.


Re: Bump steer yet again.

PostPosted: Wed Jul 09, 2014 10:22 pm
by ericbushby
Thankyou for the feedback.
I am still slightly anxious and waiting for someone more experienced than me to come on here and tell us why it is not so simple.
Mine now drives a lot better than it did, but unless I get a chance to drive another I connot tell if it is good enough.
Eric in Burnley
1967 S3SE DHC (This weather is brilliant. I am using the Lotus for shopping. Any excuse)