If you’re into motorsport racing you may have heard about “weight transfer”, but never quite understood what it is.
It’s an important concept to understand for both racing and your own street car. The principles of setting up a street car are actually the same as that of a purpose built race car, and done right will reward you immensely.
Let’s take a dive into the concept of weight transfer…
The basics of weight transfer
When your car is stationary the weight will be distributed fairly evenly across all four wheels. You can argue the toss, but for the purpose of a simple explanation just take this as fact.
As you accelerate, the weight transfer moves to the rear of the vehicle. Imagine a hot rod at the quarter mile, the weight being transferred to the rear is the reason the front of the car can lift.
Now, when you brake, the opposite happens. The weight shifts to the front two wheels.
To round off this rather simple intro into the physics of weight transfer, when you corner the weight is transferred to the outer wheels. For example, corner left, and the weight will shift to your right wheels, and vice versa.
Before you phone up the McLaren Formula 1 team and profess to be an expert, I’ll tell you now the concepts are simple but the reality is much more complex. However, understanding weight transfer will definitely allow you to make your car far more capable than it currently is.
What can you do to your car to affect weight transfer?
The three easiest ways to affect weight transfer and make your car handle better are:
- Suspension
- Anti-roll bars
- Roll centre height
Let’s consider these a little more.
When you change your suspension, such as aftermarket springs or adjustable coilovers, you change three things – (1) ride stiffness, (2) roll stiffness, and (3) roll centre height. Anti-roll bars, strut braces, and cages affect roll stiffness, and there are numerous ways to reduce roll centre height. In your road car, laying the rear seats flat and emptying the glove box will lower roll centre height, but we can do better than that!
It’s worth noting ride and roll stiffness can greatly affect how a car understeers or oversteers. The balance affects how your car handles, and your preference will depend on application, but the average road car is setup to marginally oversteer.
Ride & roll damping (shock absorbers)
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
Back in the 1950s and 60s ride and roll stiffness were largely overlooked considerations for car designers. Due to inadequacies of tyres, racing cars had similar ride and roll stiffness to the the road production sports cars of the day. The thinking was 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.
Three important components of weight transfer
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.
Weight transfer considerations
Here are weight transfer considerations. They’re derived somewhat from the theory of Mark Ortiz and Claude Rouelle:
- 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).
A brief history of weight transfer considerations in racing
How tyre development changed weight transfer considerations (starting in the 70s)
Tyre development changed everything, but it took time.
As an Australian, a good example I can use is Formula Ford in Australia.
In the early 70s the tyres used were Goodyear RR12, a treaded race tyre. The most popular Australian cars built were Bowin or Elfin, and these were pretty much designed around these tyres, with an emphasis on maintaining momentum and corner speed.
No car enthusiast of the day considered changing the springs. It was never considered, and all ran factory springs.
The springs on the Bowin were quite soft, especially at the rear. On the Elfin they weren’t much firmer.
Slick tyres introduced (1974)
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 a Formula Ford Avon tyre, and 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 which emphasised 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 became clear the tyres were generating more grip and the car response was much improved with a greater ride stiffness.
So, in the 1960’s, even though weight transfer calculations and the “roll couple” were well known, but 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.
Driving faster in the 1980s meant trying harder
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 70s and 80s, 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 80s 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.
Developments in understanding, mid 90s
Our first weight transfer equations were as used by David Gould in the mid 90s. 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 and theory for suspension set up.
Now anyone can look at the set up of a race or road car and have some thoughts about what they might do to improve the car. You can consider set ups for different cars – i.e. live rear axle vs IRS, strut suspension vs double A arm, and so forth.
Why aren’t weight transfer calculations used more often in motorsport and 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, and as far as I can tell, Chevy R&D didn’t either.
In the UK and 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 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 weight transfer worksheet to do calculations.
