Spring rate advice

[jesfirth]

I sprint a griff 500. It has nitrons which are fine for what I do 400 front and 350 rear I think. I have been adding aero, a large front splitter, mostly flat floor, rear small boot spoiler and rear diffuser. At much over 120 leptons it turns into a pogo stick on any bump or unevenness on the track. I am assuming that this is because the springs are being compressed by the additional downforce and therefore the spring rates are too low. I am also getting a lot more dive under high speed braking. On uneven tracks like North Weald the splitter which is set at 130mm form ground level actually hits the track. The car weighs about 1000kg

I have no idea how much downforce is being created. As a rudimentary check if I were to fix a ruler to the wheel arches outside the wheels and drive alongside a friend at differing speeds he can take photos and we can see how much the body drops then in the garage add weights front and rear until the same amount of sag occurs. Will this give me an idea or am I a complete idiot?

If it does work then with the aero downforce do I speak to nitron and get the springs updated and shocks revolved to suit?

Any advice, assistance, rules of thumb etc etc welcomed.

[330p4]

Jes are you hitting the bump stops on compression due to the aero preloading the spring? If not sounds like your rebound damping is not enough also those spring rates are pretty std road rates for the 500 I think certainly my 500 on std shocks admittedly pitches you up in the air on rebound
Ian

[jesfirth]

Hi Ian,

I reposted this on another forum the TVR Dunlop challenge one and have received lots of advice. The springs are far too low for aero - I am going up to 1200 front and 900 rear to solve it. What's actually happening is that at 120 plus the aero is forcing the body down, crushing the springs meaning that are, as you say, in preload, but much worse than that the front spoiler gets very low cuts the air out from under the car, therefore it loses the downforce pops up and the process starts again, I think a couple of times a second,. It's very scary, the car literally jumps around and needs big steering corrections just to keep it in a straight line.

I remember Matt Oakley going through this at goodwood at 155mph on the straight when he first added aero. He came back in after a lap, white as a sheet, said his car was trying to kill him and took all the aero off it for the rest of the day!

[330p4]

Thank god for brown Nomex then Lol. A friend runs an Elise in time attack and they have to use 1000 lbs on the rear because of the wing good luck
Ian

[The Wookie]

Unless your aero is particularly huge and you've strengthened the chassis I'd be careful about going too high on spring rates; I can't imagine a standard TVR is going to be particularly good in terms of torsional rigidity and you might start getting snappy handling from chassis flex instead.

If you were running road spring rates I'd probably look at splitting the difference with what a full on caged race car would run and trying hold the thing up with the dampers by taking a load of bleed out of the compression and increasing rebound to cope with the higher spring rate and bump stops.

That way it'll be supported but will still have enough compliance to cope with bumps and not overload the structure. You might also find that the tyres are worked harder too which could be beneficial for a sprint/hillclimb car.

Another thing to consider would be using longer, more progressive bump stops or packers to close the gap to make the transition into them more progressive.