idl1975
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| posted on 2/1/09 at 09:18 AM |
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Suggestions for geometry and spring rates - Indy (non-R) road car
My Indy came to me from its builder with what looks increasingly like a rather challenging setup for a road car. I'd be interested in hearing
people's educated suggestions for suspension geometry (specifically toe and camber settings) and spring rates for a "three season"
Indy road car that will do a few track days as well.
I haven't been able to find a level (enough) surface at my place to actually measure the geometry accurately, but reckon that at the front it
has at least a degree of front toe out and 3 or 4 degrees of negative camber. The back looks like it's running zero toe and camber.
Tyres are Kumho Ecstas (but going to put some Eagle F1s on it - no, I can't justify super-sticky £400 track day rubber...  ), currently 14 psi
front and 16 rear.
Springs are 350 lb front (!) and 275 rear.
Basically, it's very oversteer-y. The front end responds very well on turn in, with no understeer and a nice neutral feel on a steady throttle,
but when you get on the throttle, it feels like it's balanced on a tenpenny piece on the outside edge of the outside rear tyre. You can power
oversteer it quite controllably if you're prepared for it, get the tyres warm, and conditions are ideal ... but of course conditions
aren't often ideal in the UK! 
Any suggestions for a new starting point on geometry and spring rates?
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Jubal
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| posted on 2/1/09 at 10:05 AM |
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This any use to you? A blade Indy setup sheet. Car was nicely balanced using these settings and defo did not oversteer like yours:
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idl1975
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| posted on 2/1/09 at 10:35 AM |
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Brilliant! No toe out then! 
Any other thoughts anyone? Reasonable spring rates?
quote:
Originally posted by Jubal
This any use to you? A blade Indy setup sheet. Car was nicely balanced using these settings and defo did not oversteer like yours:
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nstrug
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| posted on 2/1/09 at 11:10 AM |
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Ian -
I'm pretty sure those springs are for a CEC, not a BEC. As I mentioned, my Dax has 200 front, 250 back on the DeDion. A car-engined IRS Rush
uses 375 front, 200 back (similar to your setup.)
Really, I think we should be looking at much softer springs - maybe reducing the spring rates by 30-40%.
Nick
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Custardtart
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| posted on 2/1/09 at 12:24 PM |
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Softer rear springs will give more rear end grip and less oversteer.
They're only noodles
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StevieB
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| posted on 2/1/09 at 04:26 PM |
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Comparing springs across different manufacturers is irrelevant because you have to take into account both the leverage of the wishbone and the incline
of the shock.
The rating of spring is only applicable if it's vertcal, and the greater tha angle away from vertical, the further the effective rating is
reduced.
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PAUL FISHER
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| posted on 2/1/09 at 07:26 PM |
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I did quite a bit of experimenting with spring rates with my 1st Indy for the track,I found 175 front,and 275 rear worked best,this reduced the rear
end body roll you see with so many Indys on the track,and help it turn in better eliminating understeer,350 springs for the front of the Indy are too
strong in my opinion for a BEC,the ride on the road with 275's on the rear, is a little firm on the road,but worth the trade off in my opinion
for better track performance.
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nstrug
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| posted on 3/1/09 at 09:02 AM |
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quote:
Originally posted by PAUL FISHER
I did quite a bit of experimenting with spring rates with my 1st Indy for the track,I found 175 front,and 275 rear worked best,this reduced the rear
end body roll you see with so many Indys on the track,and help it turn in better eliminating understeer,350 springs for the front of the Indy are too
strong in my opinion for a BEC,the ride on the road with 275's on the rear, is a little firm on the road,but worth the trade off in my opinion
for better track performance.
That's good to know Paul. When going to a lower spring rate, is it a good idea to also increase the spring length slightly? Otherwise I imagine
there is a possibility of running out of preload adjuster.
Nick
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bigrich
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| posted on 3/1/09 at 11:08 AM |
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I know mines not an Indy but shares similar geometry etc and my springs are the other way round to Pauls at 275 front and 175 rear.
I would say the front of mine is excellent in terms of grip and ride quality and the rear a fraction too hard if out on your own and good 2 up.
I would deffinately reduce the spring weights of your car. the do sound like CEC springs to me
.JPG)
A pint for the gent and a white wine/fruit based drink for the lady. Those are the rules
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mr henderson
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| posted on 3/1/09 at 03:34 PM |
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Worth bearing a couple of things in mind-
The front springs are mounted at an angle, whereas the rears are almost vertical. That means if equal springs were fitted al round, that the
effective spring rate at the front is quite a bit less than at the rear. To get the same stiffness all round, stiffer springs would be needed
at the front.
Softer springs will need a longer unfitted length to provide the same ride height.
Mike Capon has writted a spreadsheet which will help in selecting the correct rate and length of spring
John
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nstrug
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| posted on 4/1/09 at 11:27 AM |
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Right, the effective (static) spring rate needs to multiplied by the cosine of the mounting angle - so the effective rate is 100% when mounted
vertically, and decreases as the angle increases. Of course as the suspension moves, the angle of the damper changes thus changing the effective
spring rate.
Nick
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