Why do you need a Weight Transfer Worksheet? We Discuss The Origins of Weight Transfer Concepts.
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(There is an explanation "How Does the WTW work?" about half way down the page)
Setting up a purpose built race car, or developing a racing set up for a street car - the principles are the same.
Key set up changes you can make on the race car are to change springs, anti-roll bars or roll centre height. You are adjusting ride stiffness (a spring change), and/or roll stiffness (springs, anti-roll bars and roll centre height all contribute). Ride and roll stiffness are key inputs in determining the understeer/oversteer balance of the car in the Weight Transfer Worksheet (WTW). In the Technical Pages, we look at ride and roll stiffness over a range of vehicles, drawing on our experience, and of others, over a number of years using the WTW.
Ride and roll damping, your shock absorbers, are considered after your ride and roll stiffness have been selected. Your shock absorbers control the tyre contact with the road on bumpy surfaces (high shock shaft speed), but ineffective as a tuning tool, because the slow shock shaft speed forces are too weak to have any effect. The tuning effect we are after is to be able to influence the timing of the weight transfer.
Ride and roll stiffness considerations used to be the exclusive preserve of the car designer. In the 50's and 60's, due to the inadequacies of the tyres, racing cars had similar, or sometimes less ride and roll stiffness than the road production sports cars of the day. The thinking was that you could design suspension geometry to keep the wheel upright in roll, thus maintaining maximum tyre grip in cornering (to the detriment of other criteria, as it turned out).
About the only set up tweaks were anti-roll bar adjustment (most common change), and tyre pressure. It could have been argued that you don't need a comprehensive model to adjust those. Tyre development changed everything. But it took time.
So even though weight transfer calculations and the "roll couple" were well known (see side bar eg Costin and Phipps, 1965), it wasn't that clear what the tyres wanted. I remember looking at some speedway set information about weight jacking in 1983, without being able to get my head round it, for use in circuit racing (Formula Ford). Right through to the 80's, most teams running purpose built race cars, would run the set up very close to what was supplied by the factory. Jim Richards (Australian touring car great) said, in the 70's and 80's, he drove the cars as set up for him by the team. If more speed was required the driver would try harder. A top line CART driver retiring in the early 80's said, "We didn't do much with the shocks". Thus it was not obvious to racing people what set up changes we should seek to understand, and how we might make a systematic approach to the process.
Our first WTW equations were as used by David Gould in the mid 90's. In 1999 there was a landmark 4 part series written by Mark Oritz, and then around the same time, Claude Rouelle started his race car engineering seminars.
Now anyone can look at the set up of a race or road car in the WTW and have an opinion. Everyone in racing, or with an interest in performance cars, should have a look at the WTW numbers for their car.
The Weight Transfer Worksheet (WTW) recognizes the importance of ride and roll stiffness in determining a good balanced set up for the car. It applies for all cars, especially racing, sports, touring, historic cars and late model performance road cars.
Since 1999, we have done many workshop set ups using the WTW, and track testing with our customers. In affect, it is low cost R&D, where our customers also benefit by getting fast race cars. We have no budget to perform this work. Yet we have made progress in developing suspension technology. We have established the direct linkage between the set up of purpose built race cars and performance road cars. This is what Chevrolet R&D and their consultants did in the 60's (*see side bar).
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How Does the WTW Work?
Here is an explanation of the the reasoning behind the WTW. Mark Oritz and Claude Rouelle use similar explanations.
In current open wheeler racing, geometric w.t. can be used because of the reduction in jacking affect: small suspension travel, wide track, long suspension arms to stop the RC height moving around so much relative to the chassis ie you don't get "progressive" jacking as the car rolls more. In fact, you need the geometric w.t. to help reduce the roll angle and suspension travel, while using less rear anti-roll bar, sometimes none at all.
So if you are going to modify the setup of of any vehicle, racing or road, it is clear you need to consider weight transfer numbers. The WTW is of great value to individuals, racing teams, suspension workshops and suppliers of suspension products in the aftermarket.
Spring and ARB changes also influence pitch and combined roll and pitch.
I worked at the British Motor Corporation, Zetland Australia in the mid 60's. The factory supported race cars ran standard springs, with only the addition of the very effective Selby sway bar. Don Selby still makes sway bars for our racing business to-day.
I read the other day that the Bob Jane Torana Sports Sedan (Repco V8 4.4l) started racing on standard springs. This is amazing when you consider the performance advantage a Torana gets from uprated spring stiffness.
*The mechanics of weight transfer was fully documented by Maurice Olley at GM Research and Chevrolet R&D by the early 1950's. Then followed one of the most productive periods in vehicle dynamics history. GM consultants, CAL (Milliken, Whitcomb and Segal) developed a mathematical model of the motions of the car the "equations of motion". This was summarized in the "single track" model, as used in our presentation "How Does the Driver Control the Car?" Through testing on the skid pad and the race circuit, they validated just about everything we know to-day. Before these guys there was no useful tyre data. They started an avalanche in tyre development.
At the same time in England, Mike Costin and David Phipps wrote the bible on racing cars, "Racing and Sports Car Chassis Design". In it, Keith Duckworth's calculations for weight transfer are presented in complete detail. Costin and Phipps is said to be the only racing car book on F1 designer Gordon Murray's bookshelf, in the 1970's.
(I read that when Keith Duckworth died, at his funeral there were F1 cars outside on the street, and a DFV F1 engine in the dooway going into the church.)
Weight transfer calcs are not used in software calculations for vehicle Stability Control Systems. Instead the parameters measured are those in the "linear single track" model, in a very effective closed loop control system. Stability Control is arguably the single most important primary safety development in automobile history, resulting in the saving of 1,000's of lives.