R34 Gtt Rear Geometry (circuit Racing)

[rcs_888]

Hey Guys,

As above I have been building my R34 GTT for the past 12 months i went and did a couple of track days and got hooked so did some suspension upgrades etc then pulled the car and did a full build. Just got out again last weekend and have been battling with rear grip on corner exit.

Currently the car is siting relatively high all round and all the lower arms are sitting nice and level. I am now looking at lowering the car all round but after looking over the rear suspension i am struggling to workout the rear roll centre so hoping someone can help me out. Im not too sure if im overthinking it but as the rear lower arm has squat built in the front pickup is higher then the rear mount so what one do you use as you RC line and then does the traction rod have any effect on the RC too?

Currently im going to be getting some softer springs made up as i have stripped the car so i think that im just not getting enough weight transfer to get it to squat to get the grip but wanting to just drop the height aswell.

Cheers guys

BTW: before someone says use the search I have been trying to search everywhere but noone seems to actually explain the relationship f the arms to the RC and Risking seemed to know his shit but something has happened and he no longer seems to be active and the same with SK so hopefully someone else could help me out please.

[blind_elk]

It will be difficult to show you where you need to go if we don't know where you are starting.

What are the current wheel alignment settings? What springs? What swaybars? What wheels / tyres? What ride heights (centre rim to wheel arch)?

Rear grip is not solely from the rear end. Sometimes you may have too much front grip. It's all about controlling weight transfer.

Frankly, I put RC calcs in the too-hard basket once I saw the complexity of the rear suspension geometry. I take on board the advice of those who have already been there and done that, and trust they have come up with the settings that give the desired outcome in terms of how the car handles. But I suspect that a good book on suspension design will be able to show how to calculate the RC based on the swinging A-arm part of the suspension (the calculation will probably take account of the angular relation of the A-arm pickup points).

[rcs_888]

I know mostly the reason why it is struggling with rear grip and i do believe it to be due to too stiff springs in the rear end. I have taken a heap of weight out the car and still running the same springs so i have softer springs on order so that i can get some more weight transfer.

Unfortunately i struggle putting RC calcs in the bin because they seem too hard.

I know how to do RC calcs on MacPherson Struts, 4 links etc its purely the fact that the lower arm on the R34 incorporating the squat into it means that the lower arm pickup on my car is actually 15mm lower at the rear pickup than the front so do you use the front height or the rear height? The top is simple and you can disregard the Traction arm from your RC calcs as it is there to dial out the bump steer.

At the moment i have lock collars in the bushes and im going to be dropping the cradle soon and machining up some billet mounts to lift the cradle to the body so i can drop the CG and have a smaller Roll Couple so i can have less bar on it

[blind_elk]

From my reading of Puhn, F: How To Make Your Car Handle, determining the RC is not a calculation but rather a construction. In the case of the A-arm:

1. (in plan view) construct a (theoretical) line through the pickup points, and project it to intersect the axle

2. Connect these 2 points to the point where the centre of the opposite tyre touches the ground.

The intersection is the RC. The construction appears to be independent of the relative heights of the pickup points.

a_armRC.pdf

(apologies that it doesn't scan too well)

[Dale FZ1]

It will be difficult to show you where you need to go if we don't know where you are starting.

What are the current wheel alignment settings? What springs? What swaybars? What wheels / tyres? What ride heights (centre rim to wheel arch)?

Rear grip is not solely from the rear end. Sometimes you may have too much front grip. It's all about controlling weight transfer.

^^ Very good common sense, and decent questions to put back at you. Provide more detail so people can get an idea. Do you know what the car now weighs fully fuelled and ready to drive? 2wd R chassis should be somewhere in the 1380-1420kg bracket and about 60:40 weight distribution.

Depending on what detail you can give, there may be some simple setup and experimenting to get it in the ballpark without throwing money and parts at the car. eg you could drop rear ride height 10mm and se what happens. Or adjust the shocks. Or disconnect one of the rear sway bar links. Or adjust tyre pressures.

It's way too easy to over think this stuff. I have not attempted to establish where my roll centres are, and probably don't need to unless I'm out to win a championship (I'm not). But it's proving to be reasonably brisk for a club car, even if it does get some minor oversteer on exit.

Make sure your rubber is the biggest and softest you can fit, and afford. And not old, heat-cycled out garbage.

Make sure your performance as a driver is consistent, and you're using sufficient throttle control if it's got a fair bit of stick.

[Dale FZ1]

^^ The above is working on the assumption you have played with traction arm lengths and established a happy rear end geometry that is not bump steering as you exit corners (ie toeing out under compression through body roll).

[rcs_888]

i dont know what the wheel centre to guard height is but currently the rear lower arm mount is 170mm off the ground and the front lower pickup is 210mmwith the wheel joint at 180mm so thats where my lower arm is at on the rear slightly more important than the wheel center to guard height as alot can vary that like im now lifting the cradle to the body with solid bushes so i will have a lower body height with the same suspension arm angles keeping my RC where it was with slightly lower CG