rear engined suspension tuning (spring rates etc)

[hot66]

I have just been reading an article on suspension tuning focusing on spring rates & anti roll (sway) bars. This article was published in Sept 2001 edition of Cars & Car Conversions magazine (a very long standing & respected UK publication)

Their agument was when cornering in a rear engined car, & asuming front & rear spring rates were the same, more weight would move across to the rear inducing oversteer. The advice in the article for rear engined cars (porsche 911's for example) was to tune the spring rates allowing more weight transfer to the front of the car to offset the problem of oversteer.

It was recomended to run harder springs at the front than at the rear. Anti roll (sway) bars would then be used to fine tune the balance.

Whenever i hear people talking about spring rates on VW's, they seem to always increase the rear spring rates. If the above is correct why fit the larger dia 944 torsion bars?

Does anyone here have real life experience with which spring rates to use at the front & rear?

[Tim]

There are many other factors affecting the handling characteristic of a car. Here is a webpage showing how to tune the chassis to improve handling for a rear engine Porsche. I think it may also apply to an ACVW.

www.rennsportsystems.com/1-e.html

Tim

[kdanie]

That rennsport site is great. You might want to check out Carroll Smith's book "Tune to Win".
ken

[hot66]

really good link

so the article was correct then, soft up the back end to reduce oversteer (in a very general simlified way)

[Supa Ninja]

The reason alot of us stiffen up the rear end is when you have a potent motor and you get on it the rearend will squat, possibly unload the front wheels, and I believe that is not a good thing in the corners. Correct me if I'm wrong.

Nick

[Ron Roberts]

You can stiffen the front as well as the rear. Remember the front end is light. Did the article say the front should be as stiff or stiffer than the rear? or is it in relation to the weight on the springs.

Ron

[hot66]

Originally posted by Ron Roberts
You can stiffen the front as well as the rear. Remember the front end is light. Did the article say the front should be as stiff or stiffer than the rear? or is it in relation to the weight on the springs.

Ron

it indicated the front end should be stiffened, but not to the point where it becomes 'dead' under braking etc..

[Tim]

I am going to build a circuit racing 1303 and am researching what spring rates I should use in the front and rear. I will try to reduce the rear weight by cutting off some metals in the rear, using fibreglass fenders and decklid, replacing the rear and quarter windows with lexans. In the front, I will install fuel cell in the spare wheel area. Battery will be moved to the front as well. I hope to get to less than 800kg. So, what spring rates should I use?
Thanks
Tim

[kdanie]

The prevelent thought on racing suspensions is to use the softest spring rate that prevents bottoming out. I think the 22mm type 1 torsion bars would put me on the bump stops at high speed. I think my 26mm 944 bars may be a little strong but with a lowered rear suspension I doubt it. If you bottom the suspension you will break loose in a corner, very suddenly.
There is a lot to learn about suspension setups, I'm just begining to learn. Another good book is "Chassis Engineering" by Herb Adams. My library keeps growing.
ken

[Tim]

I agree there is a lot to learn about suspension tuning (I am still on the learning/researching stage!). Maybe the solution is to actually do some trial and error with different setups and will finally come to a satisfying one but this will be very costly. Surely different circuits require different setups. Probably drivers have their own preferences as well. I may start trying with 250lbs front and 200lbs rear (coilovers will be used for both front and rear). Would it be too heavy for the front?

Tim

[chigger]

Individual driving style counts alot when you are working with the chassis. Some people like a bit of understeer, some a bit of oversteer, but I prefer neutral steering. Each one of these has its own problems and style of driving to compensate. The whole point of the suspension is to keep the tires in the best position on the ground to get the best traction. How to do that is the complicated part. The tires must comply or follow the contour of the ground. Light wheels and tires and brakes have less inertia or resistance to movement. Light springs and soft shocks will keep the tires on the ground taking into account the weight of the car and the fact you don't want to bottom out the suspension. Also if you change the center of gravity by moving things around and lightening the car, the suspension needs change. If it was me I would get a set of adjustable coilovers with adjustable shocks. Keep a log of what effect different combinations do. The stock car web sites have tons of information on suspension setup and what effects what. Practically none of their products work on a bug, but the information is valid.
One of my pet peeves is seeing a Porsche or bug hot dogging it around a corner and the inside wheel is 6 inches off the ground. Plain old common sense tells me he should have able to go faster around the corner with all four wheels on the ground.
A good test track is two fifty foot circles in a figure 8. 28 mph around the circle is the same as pulling 1G. Crossing one circle to another tests the transistion on the car from one direction of turning to the other. You are not going fast enough to roll the car (hopefully) if the suspension gets weird on you.
I think one stockcar site also tells you how to determine the size and spring rates you need. I hope it helps.

[Richie]

IMHO you have to take a step further back before investing in springs and dampers. At first you'll have to make your car as rigid as possible by welding in a full cage and/or welding all seams that are normally spotwelded.
Then you'll weigh all parts that are responsible for unsprung weight as discs, calipers with pads, hubs, wheels, tyres, uprights, trailing arms, etc. even the wheelnuts. Offcourse you'll have to know the weight of the rest off the car as well.
With that knowledge you can choose the springs and dampers. I prefer stiff dampers, slightly softer springs and rather heavy stabilizers.
Ideally you should have your car on scales and adjust the cars weight evenly over the 4 wheels, not only front/rear but diagonally as well.
Good luck!

Cheers,

Richard

[Tim]

Another good link about suspension tuning, very detailed.

www.se-r.net/car_info/suspension_tuning.html

Tim

[Chris Percival]

Tim, just a side note: Be warned, fibreglass wings may actually be heavier than steel ones. Especially if they are cheap steel ones..

[Tim]

Thanks Chris for your advice. I like those on the J-Sports racer (I would buy if they are selling, including the front spoiler and side skirts). They look light weight and functional. Otherwise, I think those from Kerscher are not bad. I know Remmele makes very good carbon fenders but they are surely expensive. Any other recommendation?

Cheers
Tim