Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Scrub radius and wheel width

Scrub radius and wheel width

Scrub radius and wheel width

2 questions.

1. If a wider tyre is placed on the standard factory rim does it effect the scrub radius assuming the diameter is the same?

2. If a factory wheel is say 8 inches wide and has a offset of +50mm and it is replaced with a new wheel 9.5 inches wide with an offest of +50 and a wider tyre is the scrub radius changed (assumming the diameter is unchanged)?

RE: Scrub radius and wheel width

The academic answer is 'No Change', but the correct answer is 'probably will be different'. How much will depend most on the replacement tire. Quite a few factors affect the actual operating scrub radius besides geometric settings. Run-Flat, unidirectional, brand, pressure, rim width, tread depth, raised letters, odd or even number of belts, and radial vs. bias produce consistently measurable differences in tire Mx (Overturning moment) properties. You need access to raw tire data to observe these tendencies because just a stiffness or a fitting model supplied by a vendor can/will obscure the operating functions of the tire at your loads, alignments and wheels (yes, the test wheel can become a player because its usually chosen to minimise wheel effects for tire validation reasons and to eliminate the wheel as a source of lateral stiffness loss).

If you have some air bearings or grease plates, you can sometimes measure a scrub radius CHANGE under lab, non-rolling conditions, especially if its pretty large (25+ mm) already. You need to determine the arc center of the plate movment and compare it to the center of the tire position. In reality, the center of the tire is NOT the centroid of all the forces and moments, but you can get the hang of it. Since the wheel is wider, the tire spring rate is usually different, so you may have lost the 'same diameter' constraint.

You probably won't read any of this in a book. Get someone to invite you into a laboratory tire test session and have them crank it to the load (Fz), slip and camber angles that your car runs and observe the Mx/Fz math channel.

I can probably come up with some sample results to post if I can figure out how to show pictures/graphs AND after I get my barn's burst water line fixed. Stay tuned...

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close