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I ride a cross bike mostly on the road right now and my rear tire is wearing much faster than my front, which is to be expected. I found the following thread which I think is a similar question but not quite the same, Should I change both tires when I need to change one? which says to just buy one tire and replace the front tire with the new tire and move the old front to the rear. As this article says you want a good tire on the front for traction and safety.

My questions are:
If I am riding knobby tires that have been worn, wouldn't that just make them a little more road like and help with the road riding that I am doing?

Why it a bad idea to rotate tires instead of replacing the worn tire? I would like to stress that they are not "bad" just the knobs have been worn smoother or maybe that makes them bad.

Also, I don't use slicks because I do have short portions of several rides that I do which have short stretches of dirt and/or gravel.

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"Don't they rotate when you pedal?" Sorry. Someone had to say it. Now that I've got that out of my system... –  lawndartcatcher Jun 22 '12 at 15:56
    
My tendency (for road tires) is to replace both when the rear is worn out. (But "worn out" to me means close to having the cord showing through.) Usually by that time the front is fairly worn and beginning to show its age. –  Daniel R Hicks Jun 22 '12 at 19:30
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2 Answers

up vote 4 down vote accepted

Rotating tires on a car is usually done to even out the wear (unless you have specific front / back / left / right tires) across the tread - they'll often wear on the outside of the tread, so rotating them presents fresher tread to the gripping surfaces (i.e., they're wearing at the places that make the most contact with the road).

With a bike tire you shouldn't run into this problem (unless you make a lot of left turns, or a lot of right turns). "Rotating" in this case means moving the less-worn one (front tire) to the position where it'll get more wear (to the back) and replacing the more-worn one in the position where it'll get less use (to the front). This means you don't have to buy 2 tires when one gets worn (just one).

A couple of observations. If you play games with your tires (e.g., stickier front tire, deeper groove on the rear, etc.) you should obviously replace the tire with one with similar characteristics, so you won't be able to swap them around.

Typically, one replaces a tire when it either has a huge gash or has worn down to the point where there's a significant decline in the performance (i.e., no tread left or it's about to blow). If you're using knobby tires and you're wearing the nubs down you're correct in that they'll take on the characteristics of road slicks.

It's not necessarily "bad" to rotate them; if you can only afford one tire or the other one is perfectly good it means you can continue to ride without having to miss meals or rent payments. If you're still able to maintain the needed grip throughout your ride (road and gravel) there's no real reason to replace them if they're just a little worn. If, on the other hand, you're starting to have difficulty with traction, control, leaks, etc. it might be time to replace them.

An alternative to "rotating" your tires would be to buy a new set (if you can afford them) and save the less-worn of your two existing ones as a spare just in case you run over a chunk of glass or something.

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I would tend to rotate cross (knobby) tires with less consideration than I would road slicks. Ideally, I wouldn't wait for major wear...just make it a habit of swapping the rubber every month or some such. –  Ken Hiatt Jun 22 '12 at 16:48
    
The traditional reason to rotate them is that a front wheel puncture generally hurts - so you put a new tire on the front and move the older one to the back. Frankly it isn't worth the effort unless you really need to save a couple of $ –  mgb Jun 30 '12 at 16:07
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I dont rotate tires on a bike. I dont think I have heard you should. It will help the tires to wear together but unlike a car, it doesn't cause problems if you don't. I will buy 3-4 rear tires for every front tire. There is also the issue with better performance with some tires on the front vs rear, just not designed to be on both.

That said, I tend to stay away from knobbies, even on my "dirt" bike. The knobs put up quite a bit of wind resistance vs road tires, I found this out bombing a really steep road, I could feel them push the wind. You say there are "short stretches of dirt" in your rides. Even when I raced in dirt I used street tires. Knobbies have better grip on acceleration and in turns but it is not like glue. I would rather have the street tires and know I will slide at 35 degrees than have the knobbies and think I can hit a turn at 36 degrees and slide. Especially when you are only using the knobs on "short stretches of dirt", you might consider street tires to help with better traction and road resistance on the street and better wind resistance all around. And there is a HUGE difference in riding knobs on the street vs road tires, they can cause you to slide out way easier on tight turns.

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One thing I missed: Road tires have a lot more rubber on them then worn down knobbies! Knobbies will wear down super quick on the street because they are meant to "bite", so when they go against asphalt or concrete they wear quick. Once the knobs are gone there is only a thin layer of rubber left. On street tires, the rubber is almost as thick as the knobs but since they rely on the flat rather than the cuts for traction they last WAY longer! –  BillyNair Jun 23 '12 at 7:46
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