Nickp
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| posted on 9/9/12 at 08:44 PM |
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Suspension geometry after polybushing?
I've just attended a 'Tech Forum' on suspension and it got me thinking about something that's been on my mind for a bit.
Manufacturers specify suspension geometry specs for tracking etc, which I think are based on a compromise to allow for movement in a std cars rubber
bushes during acceleration, braking, cornering etc. Once the more performance minded amongst up fit poly bushes, does that same car still require the
same amount or toe in / toe out? Or can it now be set closer to straight ahead with less flex now present? Or am I mising the point? 
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coyoteboy
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| posted on 9/9/12 at 11:28 PM |
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That depends on how good the original bushes are in new form at holding pisitions. Also depends on whether the final suspension layout/settings were
done based on a car with rubber or a car with sphericals, then converted to rubbers in the final design. I'd say most likely you'd need to
tweak settings to work with poly bushes as the play and changes in dynamic settings would be notably different.
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cliftyhanger
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| posted on 10/9/12 at 08:05 AM |
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I would suggest start with the workshop values, and make small adjustments from there. One or two exceptions (maybe camber?)
The manufacturers tend to get things pretty good, and the OE rubber stuff is way better than repro bushes sold now.
And thinking about it, the values they supply are likely to be mid-values.
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MikeRJ
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| posted on 10/9/12 at 10:16 AM |
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quote:
Originally posted by coyoteboy
That depends on how good the original bushes are in new form at holding pisitions. Also depends on whether the final suspension layout/settings were
done based on a car with rubber or a car with sphericals, then converted to rubbers in the final design. I'd say most likely you'd need to
tweak settings to work with poly bushes as the play and changes in dynamic settings would be notably different.
Lost of rubber bushes on new (and not so new) cars are void bushes that are designed to deflect under load. Inevitably the replacement polybush will
lose this function.
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chillis
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| posted on 10/9/12 at 12:05 PM |
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There shouldn't be any appreciable movement in suspension bushes they are to isolate vibration. The some stock bushings are a lot stiffer than
the road springs. Void bushes are designed to move only in certain directions, ie they still control in the direction of load which is why they must
be fitted correctly. They tend to be a cheap alternative to a balljoint these days as well.
Polybushes are less stiff than new rubber bushes but don't degrade as quickly as rubber. There should still not be so much movement that the
suspension moves to the point of significant geometry changes. 1- that would be dangerous 2- that would likely be an MOT fail.
The manufacturers tolerances for geometry are usually quite wide so there is scope for tightening up but unless you have access to a 4 wheel laser
alignment rig and somewhere safe to test out your changes then go with what the book says.
FWIW These settings have always been a good starting point for me in the past when developing geometry.
Rear camber -1.5°, rear toe in 10minutes per side (thats if you can adjust the rear - doesn't apply to live axle/de-dion)
Front castor 5° or max if less, front camber -0.75°, front toe in 5minutes per side.
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Nickp
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| posted on 12/9/12 at 04:54 PM |
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quote:
Originally posted by chillis
Polybushes are less stiff than new rubber bushes
Is that right? I'd always assumed polybushes to be stiffer than even new rubber bushes. Surely poly bushes come in different grades (as rubber
does) so seems a bit of a sweeping statement. Not saying you're wrong, just would like some facts to back it up
[Edited on 12/9/12 by Nickp]
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