Mave
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| posted on 18/6/07 at 08:23 AM |
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What's wrong with Sierra upright geometry? And Cortina?
I'm considering building a new front suspension for my Indy. Nice elliptical tubing and some more adjustment possibilities. Not because I think
the Indy is terrible, but just because it seems like a nice project. Starting from scratch also allows me to take a good look at which upright I will
be using. I've done a lot of searching on here, but still can't come to one clear conclusion.
What's wrong with the Sierra upright geometry? I hear people say that its king pin inclination is too large (it's about 11 degrees?). But
what's the negative effect of that? It leads to a small scrub radius (15 mm), which is a good thing, right? How does it compare, performance
wise, to the Cortina upright? The Cortina upright has a very small KPI angle (4.something). , but a HUGE scrub radius (80 mm or something).
Am I correct in assuming that a Cortina-car will require more effort to steer? My Indy (Sierra uprights) is very light in its steering; also at low
speed (despite huge 17 inch 205 tires and heavy engine (Zetec) up front).
My car doesn't really self-centre, eventhough the top wishbones have been modified to give more caster. I've read that people say that
Cortina uprights are much better in this aspect, but on the other hand I read about people having Cortina uprights, that have the same lack of
self-centering! I'm confused! I would like to have more self-centering though.
I'm really interested in the details of this.
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britishtrident
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| posted on 18/6/07 at 10:19 AM |
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You want to look at the geometry of a car that more nearly matches your cars front end weight --- Catherham, Mazda Mx5.
Using parts from either would better but would leave you with a wheel pcd problem.
Caterham used Triumph (more corectly proprietry Alford & Adler) front suspension parts which are still available in a variety of version
including one that takes a spherical bearing in place of the tradditional bronze trunnion. This suspension was used in many single seater racers in
years gone bye..
You might also want to consider that using narrower tyres on the front would improve your steering 205s on 17@ rims is an awful lot of rubber for for
a very light car.
Steering self-centreing theses days of radial tyres depends more on the deflection of the tryres sidewall generating a self-aligning torque than
castor angle.
With tyres and brakes sometimes more is less.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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v8kid
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| posted on 28/6/07 at 03:29 PM |
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As far as I recall the KPI adds on to the camber when turning the steering wheel. i.e. nice negative camber quickly turns into nasty positive
camber.
high KPI's are used on heavy cars to lighten the steering and ensure safe understeer characteristics. On a light sports car we don't worry
about heavy steering (cos we are lighter) and being better drivers can easily cope with less understeer or even mild oversteer.
So less KPI is best.
Of course my memory is not what it was and it could be the other way round!!
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