Avon Tiger Understeer Suspension Advice Needed

[kyepan 291]

Hi there,

 

Did a sprint day with my mate russ, who has a lovely avon tiger with a 2.0L zetec R engine on bodies! tis very nice.

But he's struggling with tragic understeer, mid corner, and exit. The initial turn in is good, then as soon as you try and drive through the corner, it ploughs on.

 

Now it's got Gaz coilovers and we think 300lbs springs at the front, we think perhaps a bit less at the rear.

This spring rate sounds way to high for a 500kg car with a reasonably light car.

We wound the dampers right off, and it made no difference.

Considering the gaz kit for the 205 has 250lbs springs, which i consider way too stiff, from my understeer experiences!!!

 

Any thoughts from the suspension experts, i'm thinking he needs more like 150lbs at the front...does anyone own a kit car that they autotest or sprint? or anyone else shed any light.

 

Cheers

J

[Tom Fenton 1,526]

Its no good comparing spring rates directly, what you need to consider is the rate at the wheel. On a 205 the spring is near enough vertical so your wheel rate will be very close to your spring rate.

On most 7 type efforts the spring and damper is inclined at nearly 45 deg, therefore a bit of maths and all of a sudden your wheel rate is probably something more like 50% of your spring rate, so your 300lb spring all of a sudden is more like 150lb at the wheel. Obviously this is very simplified and you'd need to measure the actual car, but you get the idea.

Best thing to do is to get on a 7 forum and see what folk are using. Don't forget that geometry could also be playing a big part.

[kyepan 291]

good point! spring ratio.

 

Thanks tom.

[j_turnell 134 3 Cars]

I've set up a fair few if these, not the best suspension design tbh, bit agricultural! Spring rates don't sound to bad If I'm honest.

 

What's the ride height like? You got a positive rake as this has a big effect on high speed cornering? Should be between 10-15mm

 

Check the front wishbone bushes as I've replaced plenty of these, seem to use cheap rubbish, replaced no end!

 

What tyres are you using and what pressures?

 

Has it got roll bars atall?

 

Open diff or LSD?

 

I'd go over the whole car and check for play make sure the taper front bearings are set correctly.

 

Has it had a geo setup recently? Sounds like it would also benefit from corner weight an alignment check.

[j_turnell 134 3 Cars]

Going back to Tom's point on spring rates as he rightly mentioned the dampers are heavily inclined. Saying that the spring rates do seem to vary a reasonable amount between the various 7 style cars out there. Caterham run a much softer and compliant setup generally, whereas Tiger's seem to be the other end of the scale. Do you know what the rear springs are? If everything else checks out fine, then i'd reduce the fronts slightly to maybe 275-250lb and run around 200lb rears. Also worth checking you have a good amount of droop and compression, a common problem on these. Around an inch of travel on compresion plus the bump stop is fine.