Corner Weights

[Graham Perry]

One for you racers. I am about to embark on setting up my corner weights on my Di-Dion car over Xmas, as I have made so many changes to the car since it was last done professionally I suspect they are miles out.

 

Does anyone have any experience of this ? How close can I expect to be able to get the weights assuming the driver is in the seat (I suspect rear left/right weight distribution to be the problem with a 90 kg driver). Also is it possible to get close to 50:50 Front/Rear weight split without moving things about with the driver in ? I will be using new Digital scales and have plenty of time and patience.

 

Any opinions/advice ?

 

 

[Peter Carmichael]

>>Does anyone have any experience of this ?

Yes.

 

>>How close can I expect to be able to get the weights assuming the driver is in the seat (I suspect rear left/right weight distribution to be the problem with a 90 kg driver).

 

I had a 20 kg difference between total LHS and total RHS figures. Where you put that weight difference, front to back is the whole essence of corner weighting. Read on:

 

You are not moving weight!!!!!!!!![!] (I never use exclamation marks, so I must be making a salient point) You are not moving CofG. Individually, the sum of the LHS, the sum of the RHS, the sum of the front and the sum of the rear will remain the same whatever you do without moving masses around in the car. The sums of the diagonals will change. Corner weighting is like wedging a knife under the restaurant table which is rocking on two of its legs.

 

>>Also is it possible to get close to 50:50 Front/Rear weight split without moving things about with the driver in ?

 

No. You don't want to either. All talk of 50:50 being a perfect balance is marketing bollocks and journo oversimplifications. Journalists are wordsmiths, not engineers. It really is a crap, half-arsed way of considering the problem. Why on earth would 49:51 be bad. The loadings on the system are assymetric front to back. You have steering front wheels and pushing rear wheels. You have a de-dion axle giving you no camber change on suspension movement and you have double wishbones at the front with static camber settings, castor and camber compensation in roll. Are your tyres the same size?

 

A simple working approximation is: Tyre size proportional to weight distribution with a bit extra thrown in at the back to cope with the power. All the weights concentrated at the back, as far as it is practicable.

 

>>I will be using new Digital scales and have plenty of time and patience.

 

Enjoy it. I will try and dig up some of the words I have previously written on the subject. In the meantime, aim for even diagonals, with the car running straight and level.

[Matt Prior]

Peter, you are as always quite right.

 

However, as an HPC-owning Motoring Journalist with a BEng in Automotive Engineering, I just want to say we're not (quite) all thickies tongue.gif who know too much about feelgood interior trim and too little about chassis dynamics.

 

[Slipper man]

Peter. Is this the topic on which you wrote a piece for LF? If not, please do it. It may be quite technical, but if nothing else, the Curborough test proved to me how important correct setup is to the handling of a car.

[Chairman Roger]

Keith

 

I wrote a small article on cornerweights for LF and perhaps this is the one you are referring to ? I am of course flattered that you felt that it was good enough to come from Peters keyboard !

 

 

[StewartG]

Having aquired a reputation for a shall we say, slightly different way of looking at things, here's my thoughts on this subject.

It doesn't matter what four fancy bathroom scales indicate it's how the thing feels on the road that matters. I've read several comments on these pages saying something like "had my corner weights set up by x but it still understeers" what do I do now? What's needed is to tweak the platforms so the thing feels right for you.

Here's what I did in four nutshells.

1) Set the front ride height to what the book says or whatever it takes to get over the speed bumps at the bottom of the road.

2) Set the rear ride height to give the preferred under/oversteer balance.

3) I had standard,rear always locks up first, brakes so I fiddled with the rear platforms until the grip was balanced across the axle. More spring tension on the side that locks up.

4) Next came big brakes for the front so now the front locks before the back so at least you stop in a straight line. Adjust the front platforms to balance the grip between the front wheels. I suppose this may slightly effect the previously set rear balance and the front to back balance but another trip around the loop can check this.

The car now accelerates and stops straight, turns equally well left or right, understeers a little if I go into a corner too fast and oversteers a little if I use too much gas coming out of a corner. If I want more under or oversteer it's available up to a point.

I don't know how many kilos are on each wheel and I don't care it's how those tyres grip that matters.

Yes I know there are probably dozens of other variables I haven't mentioned and they all inter-react but that's another story and there's always more to learn.

Over to you PC.

[Graham Perry]

As ever Peter has come up with an excellant response. What were your corner weights Peter ?

 

I have heard it said that there is more than one way to do this, Peters being one and Stewart G's another. A third theory I have also heard stated is that you should aim to get the fronts to within 5KG of each other and then don't worry too much about the back except to try to get a bit more weight on the right hand rear if you do clockwise tracks and the lefthand rear if you do anti-clockwise tracks. It was a britsh hot-rod champion who told me this. I suspect there is a certain amount of trial and error in all set up.