Why do you need a Weight Transfer Worksheet? We Discuss The Origins of Weight Transfer Concepts.

 

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What You Will Know if You Do a WTW

10 Biggest Set Up Mistakes

(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).

Ride and roll damping, 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), and also influence the speed of weight transfer in roll and pitch for corner entry and corner exit.  Your shocks should be valved to suit your baseline set up, and we can advise on that.   

Ride and roll stiffness considerations used to be of minor interest to car designers.  In the 50's and 60's, due to the inadequacies of the tyres, racing cars had similar ride and roll stiffness to the 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 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 would take time.  For example, consider Formula Ford in Australia.  In the early 70's the tyres where Goodyear RR12, a treaded race tyre. The most popular cars were Australian built Bowin or Elfin.  These cars were pretty much designed around these tyres.  The emphasis driving wise was on maintaining momentum and corner speed.  No one played with springs.  All ran the factory springs, Bowin softer than Elfin, I guess, but Bowin quite soft, especially in the rear.  In 1974 the tyres changed to a Dunlop slick.  Nobody knew what to do to develop the car for the new tyres, and the cars were only slightly faster.  Then in a move back to the dark ages, they changed again to a road radial tyre.  Suddenly set up didn't matter.  Probably the same old soft springs did service until the end of the decade, when a change was made to an Formula Ford Avon tyre.  Straight away the cars were similar pace to the early seventies.  But then within the second year of the new tyre, some drivers were experimenting with driving style and set ups emphasizing corner entry and corner exit.  Stiffer springs were required and lap records dropped around 1 1/2 seconds per 60 seconds of lap time.  It is clear that the tyres generated more grip and the car response was improved with greater ride stiffness.

So, in the 1960's, 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.  I couldn't get my head round it.  (I was interested in what application there might be in circuit racing - for 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 four part series written by Mark Ortiz, and then around the same time, Claude Rouelle started his race car engineering seminars.  These guys provided the basis for our work in suspension set up.

Now anyone can look at the set up of a race or road car in our WTW and have some thoughts about what they might do to improve the car.  We use it to assist our thinking in determining set ups for different cars eg live rear axle vs IRS, strut suspension vs double A arm etc etc. 

The Weight Transfer Worksheet (WTW) recognizes the importance of ride and roll stiffness calculations in determining a good baseline 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.  We never stop learning.  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).  

Anyone can quickly and easily calculate a balanced set up for their road or race car, by following our methods.  We use a standard Holden Monaro road car as our example, and as expected of late model performance cars - it's balanced.  It's an easy next step to decide what you might do to upgrade the suspension for racing or high performance.

How Does the WTW Work?  

Here is an explanation of the the reasoning behind the WTW.  Mark Ortiz and Claude Rouelle use similar explanations. 

  • The total lateral weight transfer, at a given lateral g in cornering, is a function only of the mass of the vehicle, the C of G and the track width.  In the mid corner, we cannot influence the total weight transfer by any other means eg not influenced by more or less roll. 
     
  • But we can influence front vs rear lateral weight transfer, increase one decrease the other, influence the balance of the car by the following:  Tyre tests show that lateral grip increases with vertical tyre load, but in decreasing increments.  This is referred to as the "load sensitivity of the tyres".  Thus, a pair of tyres more unequally loaded have less grip than two tyres more equally loaded.  It works out that this mechanism gives us an extremely sensitive adjustment for relative grip between the front and rear wheels of a vehicle.
     
  • We now show how the "roll resistance" is used to apportion the weight transfer front vs rear.  Consider the chassis of the car to be a solid object with a compliant suspension at each end.  Mark Oritz's analogy of roll resistance in a race car is as follows:  You are carrying a sailboard along the beach with the sail up, you at one end and your friend at the other.  Say there is a constant force of the wind in the sail, trying to overturn the sailboard.  You and your friend apply counterforce (or resistance), so as to balance the wind force in the sail.  If you decrease your counterforce, your friend must increase his counter force a matching amount and vice versa.  If the force in the sail changes, either one or both of you have to change the counterforce you apply.  See pictures to the right.  We use a plastic ruler to represent the sailboard.  This process is sometimes referred to as the "roll couple".
     
  • The following is now clear:
    The stiffer end in roll (higher roll resistance) will transfer more weight, purely because of the extra twist being applied to the chassis vs the other end.  The other softer end will transfer proportionally less weight.

    We need a stiff chassis to be able to re-distribute tyre load in this way.  But this is only half the story.  We have some weight transfer that goes directly via the suspension links and chassis, not via the springs (see geometric vs elastic weight transfer below).  This still happens on a car with a flexy chassis.  When you fit a strut brace to your car, and get better response, this is in part because you are assisting more positive geometric weight transfer.  

    Through tyre load sensitivity, the stiffer end looses grip and the softer end gains.

    It is the difference in stiffness that counts.  An increase in resistance both ends that keeps the the split the same results only in less roll and no change in the balance of the car.

    It is meaningless to consider what would happen if the front of the car could roll independently of the rear.  The two are inter-dependent.   

    It is the roll stiffness of the "wheel pair" that counts, the combined stiffness of RH and LH springs.  In roll only, there is no affect on the balance of the car with different spring rates R&LH sides, although it does affect balance in pitch and combined roll and pitch (because we are now looking at RH front and rear springs and LH front and rear springs as the wheel pairs of interest).
     

Total Weight Transfer is the sum of three very important components that we can calculate easily in the WTW :

Non Suspended Weight Transfer - due to the component of lateral force applied by the weight of the wheels, uprights, brakes etc.  For live axle, includes total axle assembly weight.  We take the axle height as a close approximation to the centre of gravity, (CG), for the non suspended mass.  

And two components of Suspended Weight Transfer:
Geometric Weight Transfer - due to the component of lateral force, applied directly at the Roll Centre (RC).  Geometric WT is reacted directly through the suspension linkages, and does not induce body roll.
Elastic Weight Transfer - due to the component of lateral force, applied at the Suspended Mass CG, and does induce body roll.  This force is reacted in the springs, anti-roll bars and shocks, and is the only one of the three components of total weight transfer that does induce body roll. 

It is clear that low roll centres give little geometric wt and most of the weight transfer goes through the springs (elastic wt), and is therefore delayed by the time it takes for the vehicle to take a set.  Conversely, with high roll centre most of the weight transfer precedes the body roll, leaving a smaller amount of weight transfer to go through the springs.

The location of roll centre heights and the affect on geometric weight transfer vs elastic weight transfer is of importance in the set up of the car.  Geometric weight transfer is a major influencer for cars of high front weight percentage and/or for FWD.  Also for RWD with live rear axle.  Also for current open wheelers with high downforce and little suspension movement.

Why Arn't Weight Transfer Calculations More Generally Used in Motorsport and the Aftermarket?

I think the main reason historically is that the important geometric weight transfer was not considered in the same light as it is today.  Costin and Phipps didn't calculate it.  As far as I can tell, Chevy R&D  didn't either.  In the UK and the US, the main concern was jacking forces, and these were calculated.  The mantra was "low roll centre height".  The push was on for independent suspensions, front and rear.  Researchers felt live rear axle would no longer be used in performance cars.  Most car manufacturers had a rear engine small or mid size car in the range, (changing to front wheel drive later for better packaging).  But as live rear axle continued in production cars for cost reasons, and were therefore required in racing for many touring and sports cars, interest in weight transfer for higher roll centre height was of renewed interest.

In "Tune to Win" 1978, on page 36, Carroll Smith does his calculations for weight transfer without reference to geometric weight transfer.  Then on page 38, he gives a good description of geometric w.t., but discounts it because of the jacking affect.

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.  In professional racing, the race engineers will use simulation software and live data analysis, coupled with 7 post rig tests (if available).  Yet they might still use a steady state spread sheet like the WTW to do calculations.

 


In the 50's the Triumph factory race cars ran standard springs. Ride and roll stiffness of the road cars was considered adequate for racing.  In this photo, one of today's TR's has uprated suspension , which can be selected easily using the Weight Transfer Worksheet 

 

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 and Tod Selby still make sway bars for our racing business to-day.

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 racing Torana gets from uprated spring stiffness today.

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".  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.

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. 

 

 

 

 

 

 

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Nowadays, these 1950's TR's corner consistently at 1.2G - more than F1 cars of the period.  This level of grip is possible because of  the latest semi-slick racing tyres.  Both cars are exhibiting huge rear tyre distortion as they accelerate off the corner.


Here's a 50's Cooper Climax engined AGP car.  In modern form, it corners around .9G on historic Dunlop racing tyres. 

Below we show how roll resistance works in distributing total roll force between front and rear of the car.
The ruler represents our car chassis (a bit bendy as it happens, but does show the roll resistance happening.)


The hacksaw blade, at our centre of gravity, is applying a force trying to rotate the plastic ruler.   We are applying a counter force at each end of the ruler, representing the front and rear of the car, to exactly balance the rotating force on the hacksaw blade.  Each hand is splitting the resistance required  50-50.

 


If we decrease the counterforce at the front (RH in the photo),  we must increase the rear, so as to continue to balance the rotating force on the hacksaw blade

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Here we have decreased the counterforce at the front of the ruler to zero, and we have increased the rear counterforce to balance the whole of the roll moment applied