# tire speed-trick question **URL:** \ **Category:** Factual Questions\ **Created:** [July 21, 2003, 10:18pm UTC](https://boards.straightdope.com/t/tire-speed-trick-question/190022 "2003-07-21T22:18:45Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![Brad\_Amundsen](https://avatars.discourse-cdn.com/v4/letter/b/dbc845/32.png) [@Brad\_Amundsen](https://boards.straightdope.com/u/Brad_Amundsen)\ **Post date:** [July 21, 2003, 10:18pm UTC](https://boards.straightdope.com/t/tire-speed-trick-question/190022/1 "2003-07-21T22:18:45Z")
If a car is travelling at 60 mph down a straight road, what is the speed of the bottom, the top, & the dead center of the tires? ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [July 21, 2003, 10:38pm UTC](https://boards.straightdope.com/t/tire-speed-trick-question/190022/2 "2003-07-21T22:38:33Z")
> [@](#): > > \*Originally posted by Brad Amundsen \* > \*\*If a car is travelling at 60 mph down a straight road, what is the speed of the bottom, the top, & the dead center of the tires? \*\* Speed with respect to what? The only part of this question that makes sense to my 15 watt brain is about the speed of the tire with respect to the pavement. If you are coasting straight ahead, regardless of speed, the tire is going zero miles per hour with respect to the pavement. If the car is under power to maintain 60 MPH there is a small amount of slip between the tire and pavement. So the tire is going very slightly faster say 60.1 MPH. ---
**Author:** ![Brad\_Amundsen](https://avatars.discourse-cdn.com/v4/letter/b/dbc845/32.png) [@Brad\_Amundsen](https://boards.straightdope.com/u/Brad_Amundsen)\ **Post date:** [July 21, 2003, 10:48pm UTC](https://boards.straightdope.com/t/tire-speed-trick-question/190022/3 "2003-07-21T22:48:47Z")
In respect to the road at any given point! ---
**Author:** ![TheLoadedDog](https://avatars.discourse-cdn.com/v4/letter/t/35a633/32.png) [@TheLoadedDog](https://boards.straightdope.com/u/TheLoadedDog)\ **Post date:** [July 21, 2003, 11:06pm UTC](https://boards.straightdope.com/t/tire-speed-trick-question/190022/4 "2003-07-21T23:06:09Z")
I’d guess: Bottom - stationary Rearmost point 60 Top - forwards at 120 Front most point - 60 ---
**Author:** ![TheLoadedDog](https://avatars.discourse-cdn.com/v4/letter/t/35a633/32.png) [@TheLoadedDog](https://boards.straightdope.com/u/TheLoadedDog)\ **Post date:** [July 21, 2003, 11:07pm UTC](https://boards.straightdope.com/t/tire-speed-trick-question/190022/5 "2003-07-21T23:07:23Z")
And which part of a train is always going backwards? ---
**Author:** ![chaoticbear](https://avatars.discourse-cdn.com/v4/letter/c/f07891/32.png) [@chaoticbear](https://boards.straightdope.com/u/chaoticbear)\ **Post date:** [July 22, 2003, 1:51am UTC](https://boards.straightdope.com/t/tire-speed-trick-question/190022/6 "2003-07-22T01:51:44Z")
middle: 60mph top: 60mph+linear velocity of the point at the top bottom: same-extra velocity ---
**Author:** ![Nanoda](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Nanoda](https://boards.straightdope.com/u/Nanoda)\ **Post date:** [July 22, 2003, 5:13am UTC](https://boards.straightdope.com/t/tire-speed-trick-question/190022/7 "2003-07-22T05:13:17Z")
Doesn’t seem _that_ tricky… I’m with **TheLoadedDog** , with the addition that the centre is moving at 60mph 'cause he hasn’t specified. (Don’t know the answer to your riddle though). **chaoticdonkey** would also seem to be technically right, if somewhat vague. 😉 ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [July 22, 2003, 5:31am UTC](https://boards.straightdope.com/t/tire-speed-trick-question/190022/8 "2003-07-22T05:31:09Z")
I’m not so sure about the front most point. Wouldn’t it have a forward velocity component of 60 mph plus a downward velocity of 60 mph, adding up to 84.9 mph? As for the train, the bottom edge of the wheel flange is always moving backwards. ---
**Author:** ![Nanoda](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Nanoda](https://boards.straightdope.com/u/Nanoda)\ **Post date:** [July 22, 2003, 6:41am UTC](https://boards.straightdope.com/t/tire-speed-trick-question/190022/9 "2003-07-22T06:41:45Z")
> [@](#): > > \*Originally posted by scr4 \* > I’m not so sure about the front most point. Wouldn’t it have a forward velocity component of 60 mph plus a downward velocity of 60 mph, adding up to 84.9 mph? The front “point” on the tire isn’t going forwards, just downwards. > [@](#): > > As for the train, the bottom edge of the wheel flange is always moving backwards. Oh yeeeeah! 🙂 ---
**Author:** ![MonkeyMensch](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)\ **Post date:** [July 22, 2003, 7:03am UTC](https://boards.straightdope.com/t/tire-speed-trick-question/190022/10 "2003-07-22T07:03:33Z")
> [@](#): > > \*Originally posted by TheLoadedDog \* > \*\*And which part of a train is always going backwards? \*\* Well that depends on how old the train is. Early trains were built with cylindrical wheels and flanged rails. So in this case, barring the case of the drunk careering backwards from the club car, no part of the train is moving backwards. And who says I didn’t learn anything useful from my last job? ---
**Author:** ![Shade](https://avatars.discourse-cdn.com/v4/letter/s/2bfe46/32.png) [@Shade](https://boards.straightdope.com/u/Shade)\ **Post date:** [July 22, 2003, 1:47pm UTC](https://boards.straightdope.com/t/tire-speed-trick-question/190022/11 "2003-07-22T13:47:39Z")
> [@](#): > > \*Originally posted by TheLoadedDog \* > \*\*And which part of a train is always going backwards? \*\* Doesn’t that depend which way the train’s going? And if it’s a wheeled train or a magnetic train? OK, OK, we all knew what you meant, and someone got the answer… ---
**Author:** ![micco](https://avatars.discourse-cdn.com/v4/letter/m/5f8ce5/32.png) [@micco](https://boards.straightdope.com/u/micco)\ **Post date:** [July 22, 2003, 2:05pm UTC](https://boards.straightdope.com/t/tire-speed-trick-question/190022/12 "2003-07-22T14:05:15Z")
> [@](#): > > \*Originally posted by Nanoda \* > **The front “point” on the tire isn’t going forwards, just downwards.** Huh? Every point on the car has a translational velocity of 60mph. The wheel has an additional velocity due to the rotation around the axle. At any given moment, the front part of the tire has a rotational component that is completely downward, but this has to be added to the 60mph forward velocity that every point on the car has. I think the trick part of the question is that the tire deforms due to the weight of the car, so it’s not a perfect circle. The bottom point has a zero velocity where it contacts the road (if we can assume no slip), meaning the rotational component is exactly 60mph backward to cancel the car’s 60mph forward. However, the radius of the wheel is slightly higher for the top part than the bottom part, so the top part of the wheel has a somewhat higher rotational velocity than the bottom, meaning the total velocity at the top of the wheel is somewhat more than 120mph.