Sway Bar Stiffness Calculator
Compare two anti-roll bars. Stiffness rises with the fourth power of diameter, so small changes matter far more than they look.
Compare two bars
- Compares bars of the same length and geometry. A different bend or mounting position changes the result.
- Adding bar to one end shifts the balance toward that end. Front bar adds understeer, rear bar adds oversteer.
- A bar too stiff for the surface lifts the inside wheel and costs mechanical grip.
The fourth power rule
An anti-roll bar is a torsion spring, and torsional stiffness rises with the fourth power of diameter. Going from 22mm to 25mm sounds like a 14% change. It is actually a 67% increase in stiffness.
This is why one millimetre matters so much, and why bar sizes are sold in such small increments. It also explains why people who fit a much larger bar often find the car worse rather than better.
Hollow bars are not weak
Nearly all the torsional work happens in the outer material, so removing the centre of a bar costs very little stiffness while saving a lot of weight. A hollow bar of the same outer diameter with a sensible wall can be within 15% of the solid equivalent at around half the mass.
Balance, not grip
Anti-roll bars do not add grip. They move it between the axles. A stiffer front bar transfers more load across the front tires, which makes the car understeer. A stiffer rear bar does the same at the back and adds oversteer.
Because they work by transferring load, past a point they reduce total grip at the end they are on. This is the trap in fitting the biggest bar available.
Frequently asked questions
How much stiffer is a 25mm bar than a 22mm bar?
About 67% stiffer, if both are solid and otherwise identical. Torsional stiffness goes with the fourth power of diameter, so 25 divided by 22, raised to the fourth power, is 1.67.
Should I upgrade the front or rear sway bar?
Depends what the car does wrong. A stiffer rear bar reduces understeer, which is what most front wheel drive cars need. A stiffer front bar reduces oversteer. Changing both together stiffens the car in roll without changing the balance much.
Are hollow sway bars worse than solid ones?
No. Torsional stress concentrates at the outer surface, so a hollow bar with a proper wall thickness gets most of the stiffness of a solid bar at roughly half the weight. Many manufacturers fit hollow bars from the factory for exactly that reason.
Can a sway bar be too stiff?
Yes. Too much roll stiffness at one end starts lifting the inside wheel in corners, and a lifted wheel carries no load and generates no grip. The car gets more responsive on turn-in and then runs out of grip earlier.
Everything on this page is an estimate worked out from the numbers you entered and typical figures for the conditions you picked. Your vehicle, your parts and your conditions will differ. Check anything you intend to act on against the manufacturer's specification, the service manual, or a qualified technician. Verifying the result before relying on it is up to you.