Suspension and aero effects query

[jesfirth]

I put this query on the suspension and brakes forum but had no responses so I thought I would try here. 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 if I hit the brakes at over 100mph. It didn't do this without the front splitter. 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. Btw I have no budget for professional assistance etc. TIA. Jes

[spitfire4v8]

To support your level of downforce at your present ride height you need stiffer springing. How stiff no-one can tell you, you need to experiment.
Home build flat floors are useless at club level, they add weight for no gain and the rake and distance to track are critical. They also compromise gearbox and diff and exhaust cooling.
Rear gurnies and lip spoilers seem to work well, there's a great shot of andy race's griff side on where all the dust from the track is being picked up by the rear boot spoiler. Sounds like you have more front downforce than rear though, I'd work on upper body aero, forget the flat floor .. and experiment with stiffer springs.
Changing to double adjust dampers will also mean you don't have to keep sending them back for revalving every time you make a significant change on springs also ..
You're blazing a lone trail with your spec of griffith though .. no-one will be able to give you definitive answers.
good luck.

[jesfirth]

Jules. Thanks. My flat floor is only partial by in filling gaps and voids wishbones etc all using 0.9mm aluminium sheet. I avoided enclosing the centre to leave the engine, gearbox, diff exhaust to stay cool. It does seem to work well. The rear splitter and boot spoiler are definitely working. Without aero is was getting max 1.1 lateral G. With it all on it increased to up to 1.3 lateral G which is good but as you say front rear balance is the challenge. If I were to go for 2 way shocks which manufacturer is likely to be suitable. Matt has ohlins on his sprint griff and they seem to work well.

[itsallyellow]

Hi Jes,

We went through a very similar thing as we developed the Cerbera. Rear downforce is easy to achieve. We find balancing the from to the rear the real difficulty.

I guess it's even more difficult for you though as I believe you also use the car on the road?

I love the ohlins so these are what I would recommend. Worth the extra money!

To balance front and rear grip we actually run the same spring rates front and back. Were at 950 all round now and will probably go stiffer still. We started at 500 front 400 back and it was no where near enough. As joules has said I think experimenting is the key. Springs don't take long to change so book an open pit lane day at your favourite track and try a few combos.

What I have learnt is that it's sometimes best to book a day at a track and pay for a suspension specialist to be there and help. We did most of it ourselves in the early days and what they would see the car doing over a couple of laps would take us a whole days running to sort out.

Cheers
Mike

[Jetblackonetenth]

I agree with Mike, rear down force is easier to get than front, however do not discount a full flat floor as combined with a good rear diffuser will give front down force.

and by the way, ohlins any day, we have some very old ones which are excellent but the new ones we have on Steve Glynns car are really nice.

We have increased spring rates to allow for the extra effective weight you get from the down force but as with everything you can go too far and make the car worse

[jesfirth]

Thanks for the input it's much appreciated. Mike - yes it will still drive it to the events so it does need to work on the road but not for regular use.

We can't have rear wings so that limits the rear downforce so all it has is a small boot spoiler and a large diffuser. If anything I have too much front downforce but I can easily reduce that by narrowing the front.

I was going to try 600/500 but with your suggestions and having had a chat with Matt I think I will try 1000/700 and see if that is a good place to start. Ohlins sounds good. I assume a sensible choice on a budget would be 2 way with remote?

[itsallyellow]

1000/700 sounds crazy! With 800's I couldn't make an r888 grip at all! I really wouldn't go that high!

Just to put it in perspective we ran with 750's on the from with the splitter at 45mm....

Mike

[wadsapple]

Hi Mike long time no bump into, hope you are well ?

Jez the pogo effect will be the front diving down, the front splitter grounding out, therefore cutting off any downward forces and making the front spring back up then gaining an effect and then breaking again and so on.
I use a flat floor that may well be useless but it does add a smoother air flow under car, possibly making it more stable. also don't underestimate the confidence inducing effect you get with adding stuff to your car. carry on in my opinion

[andylaurence]

A flat floor is certainly worth it. Do it properly though - holes in it will drastically reduce its effectiveness. You wouldn't use a sieve or a colander as a bucket...

It certainly sounds like you need stiffer springs - the pogo effect has already been described. An alternative might be a third spring, if you can find a way of implementing it. It's effectively the opposite of an ARB - it has stiffness when both wheels compress, but not in roll.