# Tyre Rotation Speed **URL:** \ **Category:** Factual Questions\ **Created:** [August 2, 2013, 10:57am UTC](https://boards.straightdope.com/t/tyre-rotation-speed/665144 "2013-08-02T10:57:57Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [August 2, 2013, 10:57am UTC](https://boards.straightdope.com/t/tyre-rotation-speed/665144/1 "2013-08-02T10:57:57Z")
I posted this on another forum and encountered some people who think I am talking nonsense. Can any Doper give me a cast iron argument that will convince the doubting Thomases. This is what I said: Any part of the tread on a tyre, on a car doing 70 mph, is accelerating from zero to 140 mph and back to zero between 800 and 900 times a minute. Even more with a small tyre. I used Wikipedeia - [Trochoid - Wikipedia](http://en.wikipedia.org/wiki/Trochoid) - and logic - _But the part in contact with the ground MUST be stationary (more or less). All else follows._ Any other ideas? ---
**Author:** ![Colophon](https://avatars.discourse-cdn.com/v4/letter/c/f05b48/32.png) [@Colophon](https://boards.straightdope.com/u/Colophon)\ **Post date:** [August 2, 2013, 11:35am UTC](https://boards.straightdope.com/t/tyre-rotation-speed/665144/2 "2013-08-02T11:35:22Z")
Well, that is correct if you are measuring relative to the ground. But if you are measuring relative to the car then it is going from 70mph forwards to 70mph backwards. Or you could just say that it has a given angular velocity, and leave it at that. ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [August 2, 2013, 3:33pm UTC](https://boards.straightdope.com/t/tyre-rotation-speed/665144/3 "2013-08-02T15:33:46Z")
> [@Colophon](#): > > Well, that is correct if you are measuring relative to the ground. But if you are measuring relative to the car then it is going from 70mph forwards to 70mph backwards. Or you could just say that it has a given angular velocity, and leave it at that. No argument there - but I really don’t think my doubters could comprehend the idea of a point on a tyre going backwards…🙂 ---
**Author:** ![leahcim](https://avatars.discourse-cdn.com/v4/letter/l/b4bc9f/32.png) [@leahcim](https://boards.straightdope.com/u/leahcim)\ **Post date:** [August 2, 2013, 5:02pm UTC](https://boards.straightdope.com/t/tyre-rotation-speed/665144/4 "2013-08-02T17:02:16Z")
If you have something like a train wheel with a flange to keep it on the rail, the part of that flange that is below the top of the rail is actually moving backwards relative to the ground. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [August 2, 2013, 5:12pm UTC](https://boards.straightdope.com/t/tyre-rotation-speed/665144/5 "2013-08-02T17:12:57Z")
> [@Colophon](#): > > Well, that is correct if you are measuring relative to the ground. Yea, it’s an interesting bit of trivia. If a pebble is stuck in the tread of a car tire, and the car has a speed of 70 MPH, then… - When the pebble is touching the pavement, the pebble has a speed of 0 MPH relative to the ground. - When the pebble is at the “top” (i.e. the highest point during rotation), the pebble has a speed of 140 MPH relative to the ground. ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [August 2, 2013, 5:35pm UTC](https://boards.straightdope.com/t/tyre-rotation-speed/665144/6 "2013-08-02T17:35:51Z")
We are agreed that the proposition is correct. What I need is an irrefutable explanation for people who cannot grasp the concept. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [August 2, 2013, 6:05pm UTC](https://boards.straightdope.com/t/tyre-rotation-speed/665144/7 "2013-08-02T18:05:25Z")
Didn’t you just do this? > **[Tyre (tire) speeds](https://boards.straightdope.com/sdmb/showthread.php?t=693947)** > > The thing that always amazes me when I think about it; the thing that is hard to get my brain around, is that any part of the tread on a tyre, on a car doing 70 mph, is accelerating from zero to 140 mph and back to zero between 800 and 900 times a... ---
**Author:** ![aNewLeaf](https://avatars.discourse-cdn.com/v4/letter/a/ac91a4/32.png) [@aNewLeaf](https://boards.straightdope.com/u/aNewLeaf)\ **Post date:** [August 2, 2013, 6:15pm UTC](https://boards.straightdope.com/t/tyre-rotation-speed/665144/8 "2013-08-02T18:15:52Z")
Why would you measure tire speed relative to earth, rather than the axle on which it spins? ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [August 2, 2013, 6:17pm UTC](https://boards.straightdope.com/t/tyre-rotation-speed/665144/9 "2013-08-02T18:17:51Z")
> [@beowulff](#): > > Didn’t you just do this? > [Tyre (tire) speeds - Factual Questions - Straight Dope Message Board](http://boards.straightdope.com/sdmb/showthread.php?t=693947) Seems that I did - sorry