I had my bike in the shop for maintenance and the tech looked at my rear tire and said, “You’ve been riding this on a trainer?” I have a trainer that uses fluid resistance. A bracket holds the axle and a spring holds the roller against the tire. I have used it for maybe 4-5 hours, all this year. The tire is probably three years old. I never noticed but the shape of the rear tire is a little flattened compared to the new front tire, which has the same tube and tire.
I also read in a review for trainers that this can happen.
So what is going on? When riding on the road it seems like there would be more pressure on the contact patch than there would be on a trainer. On the trainer the only load on the tire is from the spring on the trainer. On the road there is maybe 95 pounds on each contact patch.
I believe it’s due to contact friction & heat. On the road, you’re always rolling over new ground and you’re rocking so the tire contact patch varies considerably. On a trainer the roller is your only contact patch and heat builds up and doesn’t dissipate like it would on the road.
As noted in the OP, on the road the force is about 95 pounds on each tire. On the trainer there is no way that spring is exerting 95 pounds. I would guess closer to 25, 30 tops.
Checking tire temp after a ride would confirm/refute this. I suspect that on the trainer, the tire cools just fine due to spending most of its time whizzing through the air. OP can check this the next time he/she uses it.
As for the wear…I’d guess three factors:
If the machine’s roller is textured, this may be causing accelerated wear. Different road surfaces have a similar range of effects on motorcycle tires. (If the roller isn’t textured, then this is almost certainly not a factor.)
On a flat(-ish) road surface, the tire’s contact patch is a long, narrow oval. On the trainer, the small diameter of the roller results in a contact patch with a much different shape. The average tread force per unit area must ultimately equal the tire pressure, but it may be that the roller results in a more uneven distribution of load across that contact patch; greater distortion of the tread material on each pass over the roller may indeed result in more heating, or at least more fatiguing of the tread material, than one sees when riding on ordinary roads. If the tire runs hotter when on the trainer, it may be because of this, or because of poorer cooling (in which case my initial suspicion above is wrong), or a combination of the two.
The roller is held in a fixed position on the tread 100% of the time, so ALL of the wear is concentrated there. Contrast this with riding on the road, where you wobble a bit here and there, and turn, and so on. Instead of the contact angle being an absolutely constant 90 degrees from the wheel axle, now you’ve got a histogram with a distribution of contact angles, feathering the edge of the wear band a bit, making the transition from worn tread to unworn tread not nearly so sharp.
Tires with less tread on them are more vulnerable to puncture when riding on roads, but it shouldn’t be an issue for use with the trainer. You’ll want to replace when the cords are showing, lest you risk a sudden blowout - it’ll just scare you on the trainer, but could take you down on the road.
My thinking was more that the roller doesn’t dissipate heat the way the road would. But yeah, comparing the tire temp to a similar ride on the road might be a good way to confirm/refute the hypothesis.
Because a trainer wears a flat spot in the tire it can look more worn down than a tire with more even wear from the road. That said, general consensus does seem to be that they still wear out faster on the trainer. That small contact patch is the most likely culprit. While the weight of the bike and the rider isn’t on the tire, the trainer needs to be engaged with enough force to prevent slipping, and since all the forces of pedaling and the resistance from the trainer go through that very narrow contact patch, that’s probably what’s causing the excessive wear. There’s more flexing of the tire due to the smaller contact patch.
Another thing to consider is that a bike on a trainer is less likely to have the tire pressure topped off before each use than on the road. Lower pressure leads to more deflection and more wear. Most people use their old road tires for the trainer until they completely fall apart. Another option is harder trainer-specific tires that would be too harsh to ride on and too slippery for any sort of cornering. Regardless, don’t ride the trainer on a white carpet if you don’t want to see the black skid marks of shedding flecks of rubber.
Here’s somebody’s (intuitively reasonable) take on it:
My first thought was … the part of the trainer that contacts your rear tire is generally smooth. This requires you to put a whole lot more pressure against the tire than you would if there were some texture on the trainer’s little wheel. I also think the contact patch with the little wheel is likely smaller than with the road.
But I hadn’t thought of the pressure element, largely because I was careful about PSI on my tires when I used my trainer.
ETA: I got lazy. I didn’t read all the replies. I probably would have just seconded what @jjakucyk said
The wheel on the trainer is about 3-4" in diameter and has small grooves around its girth, but no longitudinal texture. And so yes, the contact patch would be much smaller than on the road, and a small almost rectangular shape across the width of the tire rather than an oval.
This happened when I set up my rollers inside a few years back. I was able to clean it up but I put it on plywood after that.
Could it also be that when the bike is mounted to the trainer, and not being used that often, it gets a flat spot from the pressure of the training ‘drum’ on the wheel?
If you don’t switch from indoors to outdoors very often then you can get a trainer specific rear tyre that (I assume) has a harder compound and won’t wear out as fast.
