Weight transfer: the master equation

How much load moves to the outside wheels in a corner depends on four things, and springs are not one of them. This single fact kills more suspension misconceptions than anything else in the course.

Module 1 decided where the wheel goes. Module 2 decided how it gets there. Module 3 is about the whole car leaning on its tires, and it opens with the most useful equation in vehicle dynamics — mostly because of what it doesn’t contain.

Turn a corner and load moves from the inside wheels to the outside ones. That’s lateral weight transfer, and the total amount is:

\Delta W = \frac{m \cdot a_y \cdot h_{\text{cg}}}{t}

$m$ is total mass, $a_y$ lateral acceleration, $h_{\text{cg}}$ the height of the center of gravity, $t$ the track width.

Look at what’s in there: mass, grip, CG height, track. Now look at what isn’t.

No spring rate. No anti-roll bar. No damper. No roll center.

What this means, plainly

You cannot reduce weight transfer by changing springs. Not with stiffer ones, not with softer ones, not with a bigger bar. A car with race springs and a car on worn-out originals transfer exactly the same load to the outside tires at the same lateral acceleration.

What springs and bars change is the split — how that fixed total is divided between the front and rear axles. That’s chapter 19, and it’s the whole of balance tuning. But the total is set by four numbers you mostly can’t adjust from the driver’s seat.

This kills a family of common claims in one go. Stiffer springs don’t “keep the car flatter so it grips more” — they reduce roll angle, which is a different thing from load transfer. Anti-roll bars don’t “reduce weight transfer” — they move it between axles. And the car that feels flatter is not necessarily the car with more grip; chapter 20 explains why it’s often the opposite.

Worked example. The layout in the app: 1200 kg, CG at 470 mm, contact-patch track 1.468 m. At 1.0 g:

\Delta W = \frac{1200 \times 9.81 \times 0.470}{1.468} = 3769\ \text{N}

That’s 384 kgf moving across the car — nearly a third of its entire mass, shifted onto two wheels.

The three levers that actually work

Since springs aren’t in the equation, the only ways to reduce transfer are the terms that are:

Lower the CG. Linear, and usually the cheapest real win. Drop the CG of the car above by 100 mm and transfer at 1.0 g falls from 3769 N to 2967 N — a 21% reduction for no change in spring, damper or tire. This is why race cars are low, why engines get dry sumps, and why “seat height” is a genuine performance parameter rather than a comfort one.

Widen the track. Also linear and also effective, which is why race cars are wide. It’s usually limited by regulations or by the width of the road.

Reduce mass. Helps twice over — less transfer and less lateral force needed for the same acceleration.

Notice all three are structural. You decide them when you lay the car out, and they’re expensive to change afterwards. Springs are easy to change and don’t affect this at all, which is exactly why people reach for them and are disappointed.

Why transfer costs you grip at all

If the total load on the axle is unchanged — one wheel gains what the other loses — why does transfer matter?

Because a tire’s grip is not proportional to its load. Double the load on a tire and you get less than double the grip. So taking 384 kgf off one tire and putting it on the other loses more on the unloaded side than it gains on the loaded one. That’s tire load sensitivity, it’s chapter 20, and it’s the reason every one of these chapters ultimately points at it.

Which gives the real chain of reasoning for the whole module:

Cornering requires lateral force, which requires load transfer.
Load transfer costs total grip, because of load sensitivity.
So less transfer means more grip — via CG, track and mass.
And the split of that transfer between axles sets the balance.

Go look at it

Open the strut layout and go to the Handling tab.

Find Load transfer in the results, and the Corner control that sets lateral acceleration. Note the load on each wheel at a given g.

Now the experiment that makes the point. Go to Car Design and raise the CG height from 470 mm to 570 mm. Return to Handling and read the transfer again at the same g. It went up — by about the ratio of the two heights, exactly as the equation says.

Put the CG back and change the spring rates instead — double them. Return to Handling, same lateral acceleration, and read Load transfer again. It hasn’t moved. Body roll has changed dramatically; the load on the tires has not.

That step is the chapter. If it surprises you, do it twice.

Now widen the track by 100 mm and check the transfer once more. Down again, as the equation predicts.

Finally, watch Max grip while you do all of this. Reducing transfer raises it; changing springs alone does not.

Total lateral load transfer depends on mass, lateral acceleration, CG height and track. Springs, bars and dampers appear nowhere in it — they only decide how the total splits between the axles.

Change springs and watch the total not move →

What it costs you

Every one of the three real levers costs something structural.

Lowering the CG usually means lowering the car, which spends suspension travel and — as Module 1 showed at length — drags the roll center down with it, often faster than the CG. Lower is not automatically better, and chapter 24 is about exactly that trap.

Widening the track costs bodywork, regulations, and turning circle, and it changes every geometry number in Module 1 because track is one of the inputs.

Reducing mass costs money, and beyond a point, safety.

Which is the honest reason people tune springs instead: the levers that actually reduce transfer are the ones you can’t easily reach. But knowing they’re the real ones stops you expecting the wrong thing from a spring change.

Rules of thumb

CG height, road car: 500–600 mm. Sports car 450–500. Formula car 250–300. The layout above is 470 mm.
Transfer at 1 g typically runs 25–40% of total mass on a road car. The example above is about 32%.
CG-to-track ratio is the single number that predicts rollover and transfer behavior — under about 0.35 is sensible for a road car.
The cheap win is always CG, because it’s linear and every millimeter counts equally.
Rally-raid and desert: the worst case in this course. Huge ground clearance forces a high CG — often 700–900 mm — and while the track is wide, the ratio is still poor. That’s why these cars roll over so readily, why they carry so much rollcage, and why their drivers treat a slope differently than a circuit driver treats a corner. The equation is unforgiving and there is no suspension setting that escapes it.

Try this

Compute the load transfer for your own car at 0.8 g. Is it more or less than you expected?
How much would you have to lower the CG to get the same transfer reduction as widening the track by 50 mm?
Set the springs as stiff as the app allows, then as soft. Confirm for yourself that the transfer number does not move.

Next: the total is fixed, but the split isn’t — and the split is the single most powerful handling adjustment you have.

Preset values are illustrative starting points, not a measured setup for any particular car.