Where to buy: Adult Daycare Director Aka The Bartender Shirt

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threehats: It's exactly opposite. Bigger contact patch means there is less force per square inch and thus you can get away with lower pressure, which means less counter acting force.Which also means that tire pressure depends on wheel size, since 29er has naturally more contact patch for the same tire width. 

 I disagree. The total tire carcass toroidal surface area (which is what you seem to be referencing) is largely irrelevant. Contact patch surface area, of course, is critical. And for a given bike/rider effective mass, the contact patch surface area is determined *solely* by the PSI, and has nothing to do with carcass size. If the PSI is the same, a lower volume tire and a higher volume tire will have identical contact patch surface area. That is a simple byproduct of equal & opposite forces. The shape of the contact patch at the same PSI may differ somewhat between tire/rim examples (ie, longer/skinner vs shorter/wider), Adult Daycare Director Aka The Bartender Shirt but the actual surface area will be identical.A larger tire doesn't enable lower PSI because it has more total carcass surface. It allows lower pressure, in part, due to the balloon analogy (larger cylindrical cross section in region adjacent to ground = greater distance from trail to rim to absorb cumulative force before bottom out) and, importantly, the wider rims that typically come along with larger tires. Wider rims support a sidewall "geometry" that is less vulnerable to collapse under cornering pressure, so we can run lower pressures before negative consequences creep in.Wide rims, stiffer sidewalls and larger tires geometrically suited to those rims have all combined to make lower pressures viable for riding compared to 10+ years ago. Adult Daycare Director Aka The Bartender Shirt   @threehats: "bigger contact patch from a large tyre" . . .A large tyre does not equate to a bigger contact patch. Only a lower pressure will result in larger contact patch. (assuming the same bike/rider mass). [Reply]  @lkubica: "29er has naturally more contact patch for the same tire width"Not exactly. At same PSI, a 29er may have a different shape contact patch compared to a 26er (probably longer/skinnier if same tire width), but the surface area of the contact patch will be identical. [Reply]  @Barrold: "

I know it's not that simple, with change of pressure with volume when the tyre deforms"Meaningful changes of tire pressure when MTB tyres deform is essentially a myth. The rise in pressure as a tyre conforms, over even a large-ish obstacle, is insignificant.It comes down to PV/T = PV/T. The rise in pressure when the tire is deformed is directly proportional to the reduction in volume caused by the deformation. Even in the case of a bottom-out event, the reduction in internal tire volume is on the order of 1-2% (I won't bother you with the toroidal volume math to reach that; its fairly intuitive if you just imagine the size of the deformation compared to the other 100 inches of circumference). So tire pressure would only rise 1-2% during the deformation event. Insignificant. [Reply][Reply]  @Inertiaman: This is false. All you need to do to test your theory is to take your example to the extremes. We'll use a 26" 2.4 tire, and say like a 200" 2.4 tire--imagine a monster truck sized tire in a 2.4 width. Because of the angle of attack of the larger wheel, *much* more of the tread will be in contact with the ground at a given tire pressure. Especially in the real world where surfaces like dirt/mud have give or ground surfaces are uneven.

Now a person can have the debate that between a 27.5 and a 29" wheel the difference in contact is minuscule and thus negligible, but larger wheels do, in fact, have a larger contact patch.

[Reply]  @mikealive: Sorry, you're wrong, not because I say so, but because Newton says so.Assuming your two examples have the same bike/rider mass, then a very direct application of Newton's Third Law dictates that a 26x2.4 at 30psi and a 200x2.4 at 30 psi will have an identical contact patch surface area. The dramatically different angle of attack in these two examples will absolutely yield a different *shape* of contact patch, but the actual surface area will be identical in both cases.The contact patch surface area is solely determined by the force and the PSI. Say 90 pounds of force (150 lb rider + 30 lb bike equally distributed across two wheels) at 30psi: 90p/30psi = 3 sq inches.As a contrary example to help you imagine this, take your monster truck that weighs 6400 pounds, distributed equally on 4 tires, so 1600 pounds per tire. If those tires are 40 psi, the contact patch must be 40 square inches per tire (40 x 40 = 1600). There is no other possible answer. The tire size is not even part of the equation.This, of course, ignores corner cases which would not actually represent a pneumatic tire system. For example, a 700c x 23 road tire at 5 psi will completely bottom out, at which point its not a pneumatic tire system, but essentially just a strip of

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