Glossary
What is Scrub Radius? Positive vs Negative and Steering Effects
Scrub radius (also called kingpin offset at the ground) is the horizontal distance between where the steering axis meets the road and the centre of the tyre contact patch. Positive scrub radius puts the contact patch outboard of the steering axis; negative puts it inboard. It determines how braking and drive forces feed back through the steering and how the wheel reacts when one side loses grip or locks.
| Scrub radius | Steering feel | Under split-mu braking | Common use |
|---|---|---|---|
| Positive (large) | Heavy, lots of feedback and kickback | Pulls towards the gripping side | Older RWD cars, karts |
| Positive (small) | Good feel, moderate kickback | Mild pull | Most race cars |
| Zero (centre-point) | Light, little feedback | Neutral | Rare; numb steering |
| Negative | Light, slightly artificial | Self-corrects, car stays straight | FWD road cars, diagonal-split brakes |
How wheel offset changes it
The steering axis is fixed by the upright, but the contact patch moves outboard with lower wheel offset or spacers. A 10 mm spacer moves the scrub radius 10 mm more positive. Wider tyres on the same rim barely change it because the centreline stays put. When changing wheels, calculate the new scrub radius before blaming the handling on something else.
Why it matters on track
Positive scrub radius makes the steering wheel report braking forces and kerb strikes, which many drivers like, but it also means a locked front wheel tugs the steering. Combined with caster it affects how much the steering wants to self-centre. Kart stub axles are designed with large positive scrub so the inside front lifts and the kart pivots.
Frequently asked questions
What is kingpin inclination?
The inward tilt of the steering axis viewed from the front. Together with wheel offset it sets scrub radius, and it causes the car to lift as the steering is turned, which adds self-centring.
Will spacers hurt my handling?
They widen the track (good for load transfer) but increase positive scrub radius and bearing loads. Small spacers are usually fine; large ones can make the steering heavy and kick back.
Does scrub radius affect tyre wear?
Only indirectly through the extra toe change it can generate when braking forces deflect the suspension bushes.
Related
Track every one of these settings per session
Setup Genius records camber, toe, springs, dampers, tyres and more for every event, with calculators and an AI assistant built in. Free to start.
Start free