4

Will running a tire at the minimum recommended pressure increase the safe, minimum tire clearance? If I switch to the tire width I have in mind, I will have slightly more than 3mm of clearance. This is the minimum safe clearance according to renehersecycles.com. However, I plan on running this wider tire at the minimum tire pressure, and I worry that the additional "squishing" due to low pressure will mean that 3mm is not enough. Thanks in advance!

update: By clearance, I mean the minimum distance from the tire to the frame.

5
  • ... slightly more than 3mm of clearance. This is the minimum safe clearance according to renehersecycles.com. 3mm of clearance all around? What rectal database did that number come out of? A 3mm change in runout (distance from the hub) is pretty large - there aren't many ways for a wheel to be damaged such that its rim and tire to wind up 3mm or more farther from the hub at any spot and still remain rideable. Conversely, 3mm side-to-side is nothing - pop one spoke on a 20-spoke wheel and it'll likely move 4-5 mm out-of-true. Sep 15, 2023 at 23:38
  • (cont) That 3mm value is probably just a number that can be put on an internet page to get bicyclists to not use tires that are too large. It doesn't seem realisticly based on preventing tire rub - it's too generic and it ignores what happens when wheels do fail - they almost always go out-of-true side-to-side, and the rim can't move too far in or out without making the wheel unrideable. If you're worried about avoiding tire rub, you need to add a lot more details - type of bike, type of riding, wheel details like spoke count, tire details such as size, tread/knobby, etc. Sep 15, 2023 at 23:45
  • (cont) For example, if you're riding an old steel MTB and continually bouncing it off trees and rocks, you're probably not too worried about potential tire rub should you twang a spoke or two. On the other hand, if you're riding an expensive carbon road racer that already has little tire clearance, you would be rightfully concerned about tire rub because if unnoticed it could ruin your frame rapidly. Sep 15, 2023 at 23:48
  • What kind of frame and construction are we looking at? I had a road bike which had a lot of flex and ~3mm of clearance at the chainstay turned to negative clearance when pushing hard because the frame had "compliance"
    – Criggie
    Sep 16, 2023 at 21:59
  • It's an aluminum frame, which according to Cannondale, is of the "SmartForm C3 Alloy" variety. This is the frame for the Cannondale Adventure 1 btw. I brought my bike to my local bike shop, and they said that having such a small clearance would be an issue if one of the wheel spokes break.
    – Peachy
    Sep 17, 2023 at 17:34

2 Answers 2

8

By "clearance" I assume you mean distance between the tire and frame members.

If you run a tire at low pressure, that only affects the tire's shape where it contacts the ground. It won't change its shape where there are parts it could interfere with. I think your tire would need to be at atmospheric pressure in order for its all-round shape to change.

3
  • Yes, I did mean the minimum distance between the frame and tire, should have clarified that!
    – Peachy
    Sep 16, 2023 at 0:49
  • Even atmospheric pressure would not make a difference. Tires simply do not stretch significantly due to tire pressure. If they do, a loud BANG is imminent... Sep 16, 2023 at 9:58
  • 2
    @cmaster-reinstatemonica They may not stretch, but the pressure will change the shape. An uninflated tyre will generally relax to be more spread out. Of course, if your tyre is not inflated, you'll have other problems when trying to ride the bike :-) Sep 16, 2023 at 15:58
5

For the most part it doesn't matter because at the spots where the tire is passing close to the frame/fork, it's unloaded by anything but air pressure and will be dimensionally very close regardless of pressure.

Lower pressure can slightly reduce the effective width and diameter of the tire. This will be by a small amount if any, <1mm, and on most tires it's negligible, but with Herse it could make a difference since they're known to "grow" more than most. Every factor matters when you're playing so near the limit.

Lowering the pressure will probably make the whole tire flex over locally at the contact patch a greater amount under turns and impact, at least if you go low enough. This could have some impact on reducing how much the whole wheel flexes in response to loads, which in turn could reduce the clearance needs up near the frame.

Ultimately when you're pushing tire/frame clearance to the limit, small differences start mattering more than they usually do, and it all becomes an experiment. The 3mm number is reasonable in a certain light, but it can be very badly misapplied. If it's a carbon bike being used with knobbies in messy conditions, for example, it's not necessarily enough to prevent wear to the paint and eventually the frame. Personally I think the 3mm number really only applies to fairweather conditions and riders that are willing to accept whatever consequences might come.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.