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I have been experimenting with tire sizes to determine what is the best tire to use for pavement and sidewalk riding while still have as much speed and comfort as possible. I understand that there may be trade offs between tire size, speed, and comfort.

The best combination I seem to have found is a set of 700 by 40c tires that have a max psi of 50. However, those are on a gravel bike that I have. I have a fixie that I have been working on and want to use as more of a commuter bike. I put some old 700 by 35c tires that I had laying around on it.

The issue is that one of the tires seems to always lose air even though there did not seem to be any holes in the tube (that I can tell). I removed the tube and pumped it and it seemed to keep the air in for days, however, if I put the tube in the tire and inflate it, even without riding it loses air within half a day. So I replaced the tube and that worked fine. But then while riding last night, I hit a slightly raised section of pavement and got a flat.

Upon inspecting the tube there is a small slit along the inside area. Is this what a pinch flat is? The tire was inflated to 60 and felt fine even though it has a max psi of 85. I also checked to make sure that it is not an issue with a protruding spoke or anything like that and it does not seem to be. All the spokes seem to be appropriately recessed in their groves. I want to know how I can avoid this, because I will often be riding over this type of terrain. Would having the tire be inflated to the max recommended have avoided this? Do I need different types of tire? Do my gravel bike tires handle this type of terrain better just because they are bigger or because they have a low psi rating?

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  • "bigger" and "lower psi rating" go hand-in-hand. Allowing for some variation, skinnier tires will always have higher psi ratings (with big, high-pressure tires, both the tire and rim would need to be sturdier to resist hoop stress), and will need higher pressure to avoid pinch flats because there's less air volume.
    – Adam Rice
    Oct 27, 2021 at 15:36

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Upon inspecting the tube there is a small slit along the inside area. Is this what a pinch flat is?

Yes. It’s usually a single slit or a pair of holes on opposing sides (that’s why this puncture is also called “snake bite”). Usually they are fairly big (like ≥5mm long) and result in immediate pressure loss compared to normal, tiny punctures you get from small glass shards or metal pieces.

Pinch flat risk mainly depends on: Real tyre width (it’s affected by rim width and manufacturing tolerances), tyre pressure, system weight (rider+bike+luggage), terrain, suspension, and riding style.

There isn’t any real harm or problem with running the maximum pressure, but it will be less comfortable and will have less grip, especially in rough terrain (which is unfortunately also where pinch flats are more likely).

When I want to run low-ish pressures I usually test by pushing the bike (with my whole body weight) against a sharp edge like a curb stone to get a feeling for how soft it is. If I can compress it all the way down to the rim I know I’ll have to increase the pressure or be careful.

Wider tyres (even at lower pressure) help simply because they put a bigger air cushion between rim and ground.

Going tubeless (you’ll need to put sealant into the tyre and regularly replace it) works as well. In cyclocross where narrow tyres and low pressures are prevalent they use tubular tyres which need special rims and glue to mount.

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