Lotus Elan

Steering geometry - can someone put my mind at rest........

PostPost by: Orsom Weels » Sun Sep 02, 2012 11:32 am

Hello George
I have a friends S3 rolling chassis in my workshop and have recently been looking at this very issue. With springs removed, lower wishbones set roughly parallel with the front cross member & the original steering rack shimmed to set the tie rods roughly parallel with the wishbones/cross member, with original track rod ends, we could achieve plenty of toe in, at least 15mm at the disc, difficult to measure, but with the wheels on (with no tyres) we could measure up to an inch of toe in at the rim. However, with new track rod ends, which measured around a ? inch longer than the originals, we could only just achieve toe in, about 1/16th of an inch. As has been mentioned, the threads on the tie rods are rolled & so slightly larger diameter than the rod so the track rod end could screw over the length of thread until the tie rod bottomed in the track rod, however, this left virtually no thread for the lock nut which was left sort of dangling on the tie rod. Not good. A different set of track rod ends which were much closer to the originals dimensions were sourced & all is now well. One other observation, as the suspension is allowed to drop the toe in increases very slightly, so if you can?t get toe in at full suspension drop, you?ll never achieve it at normal ride height. Much easier to check everything without the body on
The two sets of track rod ends both carried the same part numbers & were from the same manufacturer, or at least were in the same boxes, & were both from the same well known Lotus specialist. Just shows you need to double check every thing.

Regards, Tim
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PostPost by: Elanman99 » Sun Sep 02, 2012 11:52 am

Couple of things occur to me.

The rack (bar) needs to be in the middle of its travel and the tie rods (effective length) need to be equal. Since one of yours is a replacement maybe its not what it should be.

Is the car a LH drive? not that it should make any difference to this problem.

Ian
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PostPost by: George4th » Sun Sep 02, 2012 12:40 pm

Ian
It is a LHD , the replacement tie rod is the same length as the old bent one! albeit slightly different looking with more thread up the rod
There is only one position to bolt the rack so its basically in that position?

If there is a problem and I'm not sure that there is, it can only be the track rod ends as Tim suggested, although I have tried the old ones on and there is no difference. (thats not to say the car didn't have a problem originally)
The thing is the only adjustment you have is to pack the rack or reduce the length of the tie rods or change the track rods and all this can be done when the body is on and everything is at the correct height.

One thing using a bit of geometry ..... the bottom wishbone is about 60mm out of horizontal. We know that from the center of each bolt its 228.6mm (9 inches in the manual)
Using Pythagoras the gain in the width of the track would be about 8mm per side = 16mm overall if the lower wishbone was level. The rack would not alter length as the tie rod would be slightly bending upwards instead of downwards. A bit Heath Robinson but possibly near enough at this stage.

My money is on the track rod ends

Thanks

George 4th
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PostPost by: rgh0 » Tue Sep 04, 2012 12:34 pm

The ball pivot for the tie rods at the end of the rack should lie approximately on the line joining the upper and lower inner pivots of the upper and lower wishbones when in the staight ahead position. It looks like your rack may be to long??

cheers
Rohan
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PostPost by: George4th » Tue Sep 04, 2012 3:42 pm

Rohan
It measures 39.25inches end of tie rod to end of tie rod
Gary (Pissant) gave me a measurement of between 39 and 39.25inches

Do you think it may be my track rod ends not having sufficient thread inside?

Thanks

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PostPost by: khamai » Tue Sep 04, 2012 4:01 pm

Like others have suggested measure the bumpsteer.

Detact the sway bar, undo the lower damper mount and loosen the a-arms at the chassis to allow the suspension to move through its full motion, from full bump to full droop. Position both sides so the lower A-arms are roughly horizontal and measure the toe at each wheel. Now raise the chassis in 1/2" increments and measure the toe at each increment (this droop). Repeat except lower the chassis (bump). Now you've got the bumpsteer curve. It will not have zero change, but generally you'd like to have the same change side to side and more toe going into the bump. Toe-out at bump is not a happy thing. Under brakes the nose dips, both sides go into bump and you don't want toe-out under brakes (bad, or at least it would be "interesting"). Some Toe out in droop is desired. The inside wheel in a corner is in droop so having the inside wheel pointing more "inward" promotes more slip angle and sharper turn-in.

Cheers,
Kiyoshi
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PostPost by: George4th » Tue Sep 04, 2012 4:22 pm

Kiyoshi
Thanks for the info, how many increments up and down should I do?

Thanks
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PostPost by: khamai » Tue Sep 04, 2012 8:50 pm

George4th wrote:Kiyoshi
Thanks for the info, how many increments up and down should I do?

Thanks
George 4th


What ever number is needed to go from full bump to full droop. I typically will set the uprights on blocks and move the chassis up/down with a jack. Note that the suspension will travel beyond what is allowed by the damper. The best way is to remove the spring and mount the damper (shock for those stateside) sans sway bar. But your springs are mounted so not worth the effort to remove them. Just toss out the last inch or so of both bump and droop.

Typically for cars with the steering rack forward of the spindle, when the car is static the lower A-arms are horizontal and the steering tie-rod is slanted upward (rack to steering arm). This generally gives toe-in on bump and toe-out on droop. Of course the relative lengths of the A-Arms versus the steering rod has an effect on the geometry.

Cheers,
Kiyoshi
Last edited by khamai on Thu Sep 06, 2012 10:19 pm, edited 1 time in total.
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PostPost by: khamai » Wed Sep 05, 2012 4:03 pm

One more...

Check this for "how to measure bumpsteer" - http://gglotus.org/ggtech/bumpstr-how2/how2bumpstr.htm

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PostPost by: nebogipfel » Wed Sep 05, 2012 4:47 pm

elansprint71 wrote:If we had a moderator here, suggestions which are likely to kill somebody would be deleted immediately.


Calm down old chap I don't think anyone is suggesting driving it on the road until it is sorted out.

If moderators deleted all the b*llocks and differing opinions on this forum (safety related and otherwise) there wouldn't be much left.....

Having visited this forum for a long time I get the impression that as with any community there are folk who know nowt, folk who know a little about a lot of things and some who appear to have deeper knowledge in some areas. Most advice is given with the best of intentions but as with all these things should not be taken as chapter and verse on the subject.

I have yet to meet someone who is always right all of the time and have never met anyone qualified to make that judgement of others.

I don't know the correct latin phrase in this situation but Caveat emptor is close enough when reading forum posts.

This, as with all my contributions? to this forum is just an opinion for which I make no claims or charge :wink: :)
John

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