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rubber on the rim. [Reply] @Inertiaman
Adult Daycare Director Aka The Bartender Shirt
I’d say the truth is somewhere in-between —— static contact patch being one thing, but active/dynamic contact patch is effected by tire width, in my opinion & experience——-
2.0 tire at 30 psi deflects (verticals travel) a certain amount on a certain bump/impact. 3.0 tire at 30 psi deflects much less on the same impact. Not 100% sure of theory, but this is borne out in tons of miles on different tires, and general sense of comfort/ traction when switching sizes @Inertiaman: You need less ‘hoiven’ and more ‘broski’ in your approach. This is not lab work dude, not that hard to grasp honestly, lol. You’re not working with mathematically perfect spheres and planes.. As noted by the part where I stated (and you ignored) “real world where surfaces like dirt/mud have give or ground surfaces are uneven.” Stay in the lab though, I’m sure you have some pencils that need sharpening. [Reply] @AckshunW: Absolutely the contact patch surface area will increase when dynamic factors increase the effective force on the wheel/tire. But that *area* increase must, by definition, be identical regardless of tire size.Cornering, bumps, etc will perpetually change the shape of the contact patch.I’m NOT stating that different tire sizes *behave* identically if at the same pressure. There are many variables at play that will influence the ride characteristics (sidewall stiffness, contact patch shape, etc) and no doubt these will manifest themselves in ways that riders will notice.If you accept the fact that the contact patch surface area is the same at given PSI regardless of the tire size — and it is a fact, not a theory — Adult Daycare Director Aka The Bartender Shirt then it may actually HELP you conceptualize what parameters really are driving your real world observations. [Reply] @mikealive: This is hardly lab work. Its basic high school physics.
Real world dirt and mud do not exempt things from the law of equal and opposite forces. This isn’t even remotely a controversial concept, except in MTB forums. You can literally download a spreadsheet that computes tire patch shape based on tire dimensions and pressures . . . You can visually see how the patch changes shape, but not size, as tire dimensions are manipulated while pressure is held constant.Even things like uneven ground surfaces can be modeled, by using an intersection of a torus with shapes other than a plane. It will absolutely influence the shape, but not the the size, of the patchAs I noted to @AckshunW, please consider that I am not challenging your real world perceptions of how different tires behave. I am simply illustrating that these behaviors are not due to tire-size-driven impacts on contact patch surface area. Rather than mock me (incorrectly, fwiw) as a lab nerd, you could perhaps accept these centuries old concepts as the physical facts they are, and assign your real world observations, more accurately, to other variables. [Reply] @lkubica: And anyone else who disagreed with me – read what I actually wrote.Adult Daycare Director Aka The Bartender Shirt I started that as the contact patch enlarges whilst the tyre sag stays the same and the tyre pressure stays the same, the ground force has to increase. This is physics.
What you have all failed to consider that this statement means that the load on the tyre has to increase – ie the mass of the rider+bike. Therefore as this load does not increase in a static situation, either the tyre sag or tyre psi (or some of each) must decrease. Optimum handling comes from having the right amount of tyre sag for the conditions and your riding style – this is what it’s really all about. Air pressure is how we measure it but it’s the sag that really matters, just like with our suspension Inertiaman: Basic high school physics is a massive oversimplification when we’re looking at the behaviour of pneumatic tyres in the real world.
Again, it’s all about tyre sag. There is an optimum amount of carcass deflection for a given situation. You cannot achieve that optimum amount of tyre sag and identical contact patch sizes when using two tyres of very different volume and diameter – the smaller tyre will either have too much sag or a smaller contact patch.