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 :D

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

[juse]

To Tim;
How did you end up with figures like 200/250 lbs on springloads?
I`ve been trying to study these things for a while, and the loadratings seem to variate between 80-150 lbs.
Someone wrote that 80lbs is common with aftermarket struts in U.S., one guy who drives autocross uses 100lbs, one ended on 120lbs and Steve Carter uses 150lbs.
Is it just a matter of opinion or why the scale is so wide?
Yesterday I heard from a Finnish guy who drives rallysprint that he`s not satisfied with the springs he is using now (160lbs), and he tought that he would try 200-220lbs.
Justin

[Tim]

Hi Justin,
Frankly, the figures just come from guesstimation. I assume the stock suspension has 6" travel and 80lb/in spring rate. So 480lb of force will get it bottom out. If I lower the suspension by 4" becoming only 2" travel, at least approx 240lb/in spring will be needed to stop the suspension from bottom out (provided that the vehicle weight is constant). I am not sure whether my assumption is correct or not and that's the reason I post here to ask for opinions.

I would like to obtain a spring rate which I can use as a starting point for my trial and error in the track. FYI, the attached map is the circuit which I will race. Each lap is 4.3km and the longest straight is about 800m. Any inputs for me are appreciated.

Thanks
Tim

[juse]

That sounds like sensible way of figuring out the starting point to springloads, newer thought of it that way myself.
Can`t help you with the track based tuning, since I`ve newer competed in any class myself...yet. I am only planning to try out autoslalom or similar series for roadlegal cars.
I`m very interested of your project and I`m looking forward to learn more from your posts.
Justin

[Tim]

Hello Justin,

I am also a newbie in GL VW and circuit racing although I have owned a standard Beetle for few years and have done some modifications on it. My new project (1303 racer) is still on the sketch. The "donate" car is still sitting in my garage in a kind of "sorry" condition. Currently, I am collecting some used parts, such as Porsche suspension and brake components. I also hope that I can learn from your and others opinions and experiences via this forum. Will keep you posted and will keep asking questions!

Tim

[juse]

Does anyone know the rates of stock rear torsion bars or a formula for calculating the rate?
Allan Staniforth writes in his book "Race and Rally Car Sourcebook: The Guide to Building or Modifying a Competition Car" that the balance between springing frequencies in front and rear is quite important.
So when choosing stiffer springs in front you`ll have to consider the spring rate in rear too.
Justin

[Tim]

Hi Justin,

I've got the book "Baja Bugs and Buggies" which mentioned the torsion-bar spring rate as follows:

For stock torsion bar with 22mm diameter
21-3/4 in bar length - 588in-lb
24-11/16 in bar length - 497in-lb
26-9/16 in bar length - 464in-lb

Please note that this is expressed as torsional force in-lb, not the lb/in as of the spiral spring rate.

Because of leverage, rear suspension stiffness depends on the center-to-center distance from the torsion bar to the axle. In a stock suspension, this distance is about 16-1/2 in. To find the amount of force 1 degree of preload produces in a torsion bar, divide the torsion bar-to-axle distance into the torsion bar spring rate.

e.g. a 21-3/4" stock torsion bar,

588in-lb / 16-1/2in = 35.6lb (change 1 degree)

There are also some torsion bar rate information in the tech article "The perfect suspension" of the GL website.

Hope this helps.

Tim

[juse]

Another comprehensive answer from you Tim, thanks.
But I´m afraid that making those rates comparable to the figures up front, is way too difficult for me. Maths newer was my favourite in school you see...
Justin

[Jeza]

This is something that I've read a bit about following the posts on the forums.

There is one thing that I notice never comes up and that is that Super Beetle front springs are progressive- so the spring rates that Superman posted at Superbeetlesonly.com is for the first inch. The second inch will be stiffer than the first.

This is really important for a car with such a light front end.

I don't know if Steve Carters springs are liner or progressive, but I would imagine that 150 lbs linear springs would be very stiff for a beetle. But then Steve also has a radiator up the front there to.

S1fter has gone for 200 lbs progressive springs, so this is the rating for complete compression, which is about 10 % stiffer than stock. He chose that after testing a stock spring (then carefully filed the info in the bin!!!). But from memory he said that the first inch was about 70lbs, the second was about 150 ish and complete compression was about 180lbs.

Hope this helps - rather than throwing a spanner in the works:)

Cheers
Jeremy

[Tim]

I think progressive springs are good for street use while the linear springs are more suitable for racing.

Tim

[juse]

As I mentioned before; I own a couple of great books of by Allan Staniforth who writes that when choosing the springrate, the most important thing is wheel frequency.
The formula he gives for this uses sprung weight and CPM(= a factor of wheel frequency).
I toyd with these numbers and got springrates for front from 80lbs to 120lbs for serious street vechiles. I believe this is for linear springs only.
Justin

[Tim]

Hi Justin,

Is it possible to calculate the spring rates for racing? I plan to have my car down to about 800 to 850kg and to have the front and rear weight ratio around 45:55 to 40:60. I have no idea about the wheel frequency for racing though.

Thanks
Tim

[juse]

Tim;
I dont remember the formula, although it isnt any complicated, but I`ll look it up for you as soon as I can.
Justin

[juse]

Here it is Tim, the formula for springrates:
Frequency of suspension in cycles per minute(CPM) for racing cars without wings or ground effect is around 100-125. Figures like 160-200CPM are nearer current trends when moving towards the ground effect area.
A formula for McPherson-setup is without suspension leverage, as there is none of it.

CR = ((WF/187,8) x (WF/187,8)) x SW

CR=coil rate; lbs/inch
WF=wheel frequency; CPM
SW=sprung weight; lbs

Its very hard to write a formula like this...hope you get something out of it.
I also found a site that covers the same matter, but much more complicated way:
http://www.theracesite.com/index.cfm?pagetype=2&form_article=1233

Justin