# Scrub radius

> What is scrub radius and is positive or negative better?

- Category: Car design › Suspension geometry
- Kind: Standard part
- Basis: Design rules and figures from the cited sources; example geometry by Xem Studio
- Page: https://xemstudio.com/objects/scrub-radius/ · JSON: https://xemstudio.com/api/geometry?object=scrub-radius · Plain text: https://xemstudio.com/objects/scrub-radius/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); files Xem Studio Design License (free to use and sell from, not to redistribute as files: https://xemstudio.com/license/)
- Use in every chat: add https://xemstudio.com/mcp as a connector (https://xemstudio.com/use-with-ai/)

**Exact dimensions.** True size, with no printing tolerance added.

## Notes

- Scrub radius is the front-view distance at the ground between the steering axis and the contact-patch centre (Wikipedia). Positive scrub lowers parking effort and adds road feel; negative scrub resists pull under uneven braking or a flat tyre and reduces torque steer.
- Zero scrub causes the tyre to 'squirm', wearing it and reducing stability (Wikipedia; H&R). A small radius improves steering feel; a large one increases torque steer (HPA).
- Wheel offset is the easiest way to change it; more aggressive offset increases scrub radius (HPA; H&R).
- The file is a reference example built by Xem Studio from the numbers above (Reference 1/10 touring car: wheelbase 260 mm (AARCMCC ISTC 250-270), track 190 mm (EFRA width without body), 50 mm = EFRA rim limit.). Sizes the sources do not fix are illustrative; scale the example to your own vehicle.

## Sizes

| parameter | value or range | source |
|---|---|---|
| Older cars | about 100 mm (4 in) positive | Wikipedia |
| MacPherson strut | usually negative | Wikipedia |
| SLA double wishbone | usually positive | Wikipedia |
| FSAE example | 27.768 mm front, 56.529 mm rear | Univ. of Utah FSAE |

## Where the files are

- Wikipedia: [Scrub radius](https://en.wikipedia.org/wiki/Scrub_radius) — web — Publisher's page; dimensions are facts
- H&R Springs (Roger Jackman): [Scrub radius](https://hrsprings.com/technical/scrub-radius) — web — Publisher's page; dimensions are facts
- High Performance Academy: [Wheel alignment terminology: king pin inclination and scrub radius](https://www.hpacademy.com/courses/motorsport-wheel-alignment-fundamentals/wheel-alignment-terminology-king-pin-inclination-and-scrub-radius) — web — Publisher's page; dimensions are facts
- University of Utah, Mechanical Engineering: [Formula SAE Suspension System (senior design)](https://mech.utah.edu/wp-content/uploads/2024/12/5-Formula-SAE-Suspension.pdf) — web — Publisher's page; dimensions are facts

## Free STEP/STL (1 parts, Xem Studio Design License, Xem Studio)

Index: https://xemstudio.com/files/hardware/scrub-radius/index.json · license: https://xemstudio.com/files/hardware/scrub-radius/LICENSE.md

- scrub-radius-geometry: [STEP](https://xemstudio.com/files/hardware/scrub-radius/scrub-radius-geometry.step) · [STL](https://xemstudio.com/files/hardware/scrub-radius/scrub-radius-geometry.stl) — kpi demo: front axle, track 190 mm, Ø50 tyres, 2.35° (illustrative layout)

## Questions

**Why do ABS-era cars use negative scrub?**

It counters uneven braking, keeps steering straight with a flat tyre and reduces torque steer (Wikipedia).

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