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Libra Steering Geometry 
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Joined: Fri Sep 28, 2007 9:50 pm
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Post Libra Steering Geometry
Odd question perhaps.
I'm giving my Libra a long awaited refresh. (My wife has used it for commuting for 4 years.) The only thing about the car I have ever found annoying is the marginal self-centring and low speed tyre scrub on full lock.
I seem to recall there being some dispute from Bryn as to the steering arms and maybe fitting them to the opposite sides of the car to the Build manual.
Any opinions on the low speed front castor behaviour?


Fri Jul 06, 2018 3:18 pm
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Joined: Sun Oct 19, 2014 10:10 pm
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Location: Bristol
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Post Re: Libra Steering Geometry
Can't comment on the Libra but I believe the Spyder is the same in this respect. I know exactly what you mean and mine does that too, but, and make what you will of this, it varies considerably depending on what wheels you use. I have three sets of wheels each with very slightly different offsets and also different tyres fitted so not sure whether its is the wheels or the tyres. I've also changed the tyres on some of the sets of rims and that made a difference too.

My (very unscientific) findings:
Original GTM supplied KN Racing 7.5 x 16 rims with 205/45 Goodyear Eagle F1 - distinct lack of low speed centering/Castor feel and tyre scrub on full lock very evident/noisy
KN Racing rims with 205 45 Yoko AD08R, Similar self centring issue but no tyre scrub noise
MG TF 11 spokes (7x16) with well worn 195/45 Goodyear Eagle F1 - self centring felt much better but tyre scrub felt worse and made a dreadful noise on full lock to the point I had to check if the tyres were rubbing on the arches - they weren't
MG TF 11 spokes with Avon ZZR 195/50 - feel great - nice self centring and no tyre scrub/noise on full lock. Much nicer turn in and steering response generally at all speeds. (note these tyres have a very stiff sidewall and can feel a bit firm and "track focussed though"
MGF 6 square spoke with 215/40 Barum Bavarious (ditch finder specials). I was given a set of four of these wheels for free from an MGF a friend was breaking and there is too much tread on them to throw away so I use these for general road use and as track day wets - to be fair they are actually pretty good. These do have the self centring issue but not as bad as with the KNs

So - rather than changing geometry or steering arms, try some 11 spokes or something with a similar offset to cure the self centring and try different tyres to cure the scrub?

Just my two penneth.


Fri Jul 06, 2018 5:17 pm
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Joined: Sat Oct 22, 2016 12:41 am
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Location: Brisbane Australia
GTM: Spyder
Post Re: Libra Steering Geometry
If you swap steering arms and the toe setting changes, you have also changed the Ackerman.
If you swap steering arms and the tie rod height changes, you have also changed the bump steer curve.

Low speed tyre scrub is almost entirely due to Ackerman not equal to 100%. On a Libra/Spyder with 2311mm WB and 1383mm track, 100% Ackerman will give 16.64 degrees of steer on the outside wheel when the inside wheel is steered 20 degrees. Not saying 100% Ackerman is the best setting for Libra/Spyder but any large variation from 100% will create low speed tyre scrub.

Low speed self centring is a product of caster angle and scrub radius. If either is zero, changing the other has no effect and self centring will be near zero. Increase either to increase self centring. Steering effort will increase. Reduce wheel offset (add spacers) to increase scrub radius.

At higher speeds (when cornering force becomes significant) increased "caster angle" (to increase "trail" or "caster") will have more effect on self centring.


Sat Jul 07, 2018 9:08 am
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Joined: Fri Sep 28, 2007 9:50 pm
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Post Re: Libra Steering Geometry
All interesting stuff.
I'm loathe to swap wheels, I run Compomotive MOs and think they are THE best wheel on a Libra looks wise. I tend to run Toyos on it, but recently changed to some 'unknown' make that was recommended to me and they have certainly improved the scrub. I remember (all them years ago... :-) )self-centering was marginal on SVA.


Mon Jul 09, 2018 7:28 pm
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Joined: Sat Feb 14, 2009 6:06 pm
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Post Re: Libra Steering Geometry
Hi Ferg

My car had really good self centre. I had MGF hairpin wheels 16". Not sure what the offset was. Maybe 35mm?

I have one thought. Are you sure you have your front wishbones the right way round? I remember quite a few cars didn't and they lacked some self centre. One person I know drove like that for many years before Duds noticed the error.


Thu Jul 19, 2018 6:21 pm
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Joined: Sat Oct 22, 2016 12:41 am
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Location: Brisbane Australia
GTM: Spyder
Post Re: Libra Steering Geometry
I checked my Spyder on the weekend and without doing any measurements, the tie rod ends are visibly outboard of a longitudinal plane through the upper and lower ball joints. This indicates there will be at least some toe-out in turns which is what you want.


Tue Aug 21, 2018 6:42 am
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