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Scrub radius

What is scrub radius and is positive or negative better? · Standard part · basis: Design rules and figures from the cited sources; example geometry by Xem Studio

Also called: negative scrub radius, positive scrub radius, wheel offset scrub, steering offset, kingpin offset, scrub radius torque steer, zero scrub radius, scrub radius calculation

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

sizes
4 in table

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.

Scrub radius size chart

parametervalue or rangesource
Older carsabout 100 mm (4 in) positiveWikipedia
MacPherson strutusually negativeWikipedia
SLA double wishboneusually positiveWikipedia
FSAE example27.768 mm front, 56.529 mm rearUniv. of Utah FSAE

Where the files are

Other publishers' files are linked, not hosted. "verify" = check the license at the link before commercial use.

WikipediaScrub radiuswebPublisher's page; dimensions are facts
H&R Springs (Roger Jackman)Scrub radiuswebPublisher's page; dimensions are facts
High Performance AcademyWheel alignment terminology: king pin inclination and scrub radiuswebPublisher's page; dimensions are facts
University of Utah, Mechanical EngineeringFormula SAE Suspension System (senior design)webPublisher's page; dimensions are facts

Free files

★ Free files 1 parts. Xem Studio design: free to use, print and sell things from; not to be republished as files. Xem Studio Design License. index.json

scrub-radius-geometrykpi demo: front axle, track 190 mm, Ø50 tyres, 2.35° (illustrative layout)STEP · STL
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Plain text: sheet.txt · JSON: /api/geometry?object=scrub-radius

Scrub radius, isometric render of a generated part
Render of scrub-radius-geometry from the free STEP; not to scale.

Fits with

Camber angle, isometric render of a generated part
Camber angle4 sizes★ 1 file
Caster angle, isometric render of a generated part
Caster angle5 sizes★ 1 file
Front kick-up (RC), isometric render of a generated part
Front kick-up (RC)4 sizes★ 1 file
Front toe (toe-in / toe-out), isometric render of a generated part
Front toe (toe-in / toe-out)3 sizes★ 1 file
Rear toe-in, isometric render of a generated part
Rear toe-in3 sizes★ 1 file
Ackermann steering geometry, isometric render of a generated part
Ackermann steering geometry4 sizes★ 1 file

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).