Mr Clive
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 posted on 14/7/06 at 10:41 PM |
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Rough Ideal Upright Geometry
Hi
I am currently looking at the design of a single seater road car.
As part of this, i need to start working on the suspension, but I am stuck with the upright geometry.
The final weight (less driver) should be between 250 & 300kg (with luck) It should run on 6.5"x13" wheels with a 215 tread section.
What sort of Scrub radius, KPI and castor angle should I be looking at to give steering thats responsive, but not too heavy and can self centre enough
to pass an SVA
The books I have read explain these factors and their effects but give no quantative ideals
any ideas?
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JoelP
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| posted on 14/7/06 at 11:25 PM |
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scrub radius can be changed via wheel offset i believe, likewise castor angle can be changed if you make an adjustable top wishbone. In fact, kpi
could be adjusted with a clever design of upright, though you would probably have to make it yourself.
However, from hearsay rather than experience, i believe you want around 5-10mm of scrub and 5 to 7 degrees of castor. Im not sure about kpi
unfortunately.
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leto
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| posted on 15/7/06 at 07:11 AM |
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KPI will come from the wishbone geometry that in turn will come from virtual swing arm, camber compensation, track change, roll center (if you care),
available space, scrub radius, and a couple of others that I forgot. A nice little win here to lose there equation with solid headaches usually
included. Best of luck 
“I'm gonna ride around in style
I'm gonna drive everybody wild
'Cause I'll have the only one there is a round”. (J. Cash)
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dozracing
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| posted on 15/7/06 at 08:54 PM |
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There are many ways to figure this out (most will cause you many more headaches than they solve). If you keep within the following limits you
won't be far off, as these are the figures most commonly used on all cars.
Scrub Radius 10-25mm
Castor - 3.5-5 degrees although 7 degress as a maximum seems popular lately.
KPI is usually between 7 degrees and 12 degrees.
If you setup your suspension system to have a flat lower wishbone, and to run around a 1-2" roll centre height above the ground (front) and
higher at the rear. You should now be able to sketch a suspension system that has only a few unknown points. These unknowns usually get solved as a
result of packaging requirements, and or detailed simulation of the geometry with computer programs or string computers.
Essentially if you have a system that is consistent throughout its movement then you are likely to have a system that feels good to drive. You will
find the longer the length of your wishbones the more consistent your geometry will be because the arcs of movement will be far bigger (flatter
movement).
The theory books are great to read, but, no one ever tells you all the real life problems and figures that might get you a good start on a new
design.
Lay something out with these figures first, then pay close attechtion to your wheels and brake package, as these can get messed up in the variables of
the upright geometry.
It used to take me 12 weeks on average to design an F1 upright/wheel/brake assembly package, so take your time to get it right.
Darren
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Mark Allanson
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| posted on 15/7/06 at 09:18 PM |
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Average upright, 8", thats 84 days,thats about 0.1" per day - you could grow them from seed faster!
If you can keep you head, whilst all others around you are losing theirs, you are not fully aware of the situation
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Mr Clive
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| posted on 15/7/06 at 10:42 PM |
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Thanks for that, while I shall try and build in as much adjustability as possible, its helpfull to have figures to use as a rough guideline.
Whats the logic behind running a higher roll centre at the rear?
Is a string computer still a worthwhile method of analysing the geometry, when compared to modern computer programs.
Im not in a hurry, so im not going to rush it.
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JoelP
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| posted on 15/7/06 at 11:22 PM |
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main logic regarding roll centre being higher at the rear, is that most light high performance cars do this, ie elise and caterhams etc. Since very
few of us will ever have the chance to do back to back testing on different setups, it makes sense to copy those who can! 
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Syd Bridge
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| posted on 17/7/06 at 09:04 AM |
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quote:
It used to take me 12 weeks on average to design an F1 upright/wheel/brake assembly package, so take your time to get it right.
Darren
Really???
Takes less than two weeks for a competent man these days. Well, in sports cars anyway. They're not THAT different. Same brake sizes and
hubs, maybe bigger in Sports if anything.
Cheers,
Syd.
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