I have a couple questions about bump scrub (i.e. track width scrub).

Question

I have a couple questions about bump scrub (i.e. track width scrub).

ksw100 (Automotive)

(OP)

26 May 24 02:26

For a modern road sports car suspension design, what is the acceptble upper limit for bump scrub?
(I know in a perfect world, the goal is zero.)

Bump Scrub is described here: https://youtu.be/IWMzO70zk_A?t=214

Is there car industry accepted terminology for when the track width changes with bump? The above video calls it "Bump Scrub". I have also seen off-road vehicle videos where they call it "Track Width Scrub".

(Note: The above topic is not about Scrub Radius)

GregLocock (Automotive) · Answer 1 · 2024-05-26 04:41:11

Our FAQ calls it out as

2.6 Track
track gain in roll
semitrack gain, ie lateral displacement of contact patch per unit of jounce travel

Targets are a little harder to find. Obviously it is a function of the FVIC location. My gut feel is that given the relatively low wheel vertical velocities compared with the vehicle speed, that the typical numbers seen, say 10% or less, result in small additional slip angles compared with roll steer.

Cheers

Greg Locock

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GregLocock (Automotive) · Answer 2 · 2024-05-26 07:20:14

Thinking about it the main driver for this on a production car is tire wear. I don't have a good tire wear model so I can't check that out, but it isn't entirely unreasonable to think that continually generating small slip forces as the car bounces along in a straight line is not a great idea. It might also affect rough road handling/nervousness.

Cheers

Greg Locock

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BrianPetersen (Mechanical) · Answer 3 · 2024-05-26 11:01:09

It's going to be linked to the roll center height. Swing axles are not a good sports car suspension design. Neither are Twin-I-Beams (probably where the off-road community is concerned with it). But, neither are pure trailing arms, or beam axles.

Track width motions will be exacerbated by soft long-travel setups. Again, this is something the off-roaders have, and normal sports cars don't.

Tires that have reasonable slip angle behavior in cornering aren't going to be troubled by this. Tires consisting of a rubber band stretched around a solid disk, will be.

IMO if you get your roll centres reasonable, and you have reasonable spring and damper calibration, and you pick tires with sensible sidewalls, this is not something to worry about for a road car. Trophy trucks (high speed off-road) might, because of the need for long travel very soft springing.

ksw100 (Automotive) · Answer 4 · 2024-05-26 19:44:39

GregLocock,
Thank you for the FAQ!!
What does FVIC mean? I looked for it in the FAQ and googled it but it gave me Ford Crown Vic results, lol.

GregLocock (Automotive) · Answer 5 · 2024-05-26 22:43:29

Front View Instantaneous Center, very closely related to roll center height.

Cheers

Greg Locock

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I have a couple questions about bump scrub (i.e. track width scrub).