On the road again
- Is that your new car?
- You like it?!
- Yes, I would like to have a car like that.
- But there is a problem with the tires.
- Do they explode when you sing?
- I have five tires. A front tire will last 29,000 miles while if placed in the rear it will last only 21,000 miles.
- How do you know?
- So the question is, if I rotate the tires as best as I can, how many miles can I drive?
- Where do you plan on going?
Problem source: Dan’s problems.
November 27th, 2009 at 12:24 am
Every mile uses up 1/21000th of the wear of the tyres on your back wheels and 1/29000 of the wear of the tyres on your front wheels, i.e. a fraction (29+29+21+21)/(21*29000) of the wear of 1 tyre. You have 5 tyres, so that gives you enough wear for 5*21*29000/(29+29+21+21) miles = 30450 miles. Whether you can actually get 30450 miles out of them is another matter.
November 27th, 2009 at 12:42 am
If 5 times you drove 30450/5=6090 miles then rotated tyres, then each tyre is used for 2*6090=12180 miles on a front wheel, and 12180 miles as a back wheel. This uses a fraction 12180/21000 + 12180/29000 of its total wear, i.e. all of it, so we have none to spare. One possible schedule:
Call the tyres A, B, C, D, E.
1st 6090-mile stint: A, B on front wheels, C, D on back wheels, E unused
2nd 6090-mile stint: A, E on front wheels, C, B on back wheels, D unused
3rd 6090-mile stint: D, E on front wheels, A, B on back wheels, C unused
4th 6090-mile stint: D, C on front wheels, A, E on back wheels, B unused
5th 6090-mile stint: B, C on front wheels, D, E on back wheels, A unused
(3+3+3+3=12 swaps)
November 27th, 2009 at 4:48 pm
Richard’s rotation should work for any wear pattern. Since all tires spend the same amount of time on each axle, they should each suffer the same amount of wear.
A more standard rotation is Spare -> Right Rear -> Left Front -> Left Rear -> Right Front -> Spare. This would generate even wear, even on a race car, where wear can vary from side to side as well as from front to rear.
And, fwiw, I would suggest rotations every 3045 miles; having tires with uneven wear can be a problem in and of itself, especially between the tire that was spare and the tire at the other end of that axle.