Where to buy: Adult Daycare Director Aka The Bartender Shirt

Simplifying vehicle dynamics to school level Newtonian physics is a fool’s errand.

Adult Daycare Director Aka The Bartender Shirt

[Reply]  @threehats: I carefully read what you wrote. Its wrong on multiple levels.”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.”How could the contact patch possibly enlarge while the tyre sag stays the same? Aren’t these just different measurements of the same parameter (tire compression/conformance)?? Ie more sag = more patch.Secondly, you seem to be saying that one can increase contact patch, keep the same tire pressure the same, and as a result the “ground force” must increase. This is true, but the “ground force” cannot increase unless the equal and opposite “rider force” is increased. A larger tire cannot magically create a larger contact area and therefore a larger “ground force” independently of the rider force. The “rider force” is defined by the mass of the rider/bike and any dynamic/accelerating forces (g-outs, or pumping through trail undulations). You can assume whatever you’d like for that force, but by definition the ground force must match it. And contact patch surface area at the intersection of that ground force and rider force will be determined by those forces and the PSI . . . Period . . . The tire size has nothing to do with it.

I agree its all about the tire sag . . . Sufficient to get great traction, but not so much that bottom-outs and rollovers are likely. But sag in a MTB tire is almost exclusively determined by air pressure, and to a lesser extent, sidewall stiffness. In good faith, I really think your observations can be explained by the different *shapes* of contact patches. All else being equal, wider tires = wider/shorter patch, and larger diameter tires = longer/skinnier patch. Adding to that, its intuitive to me (could be wrong) that wider/shorter patches may aid more in cornering, while longer/skinnier patches may aid more in climbing. So undoubtedly a rider’s preferences could lead them to a particular tire dimension favorite which cannot be duplicated by a different size tire at the same PSI (despite that tire having the same surface area of contact patch).

At age 74 I’m always so appreciative of those who have the time to “count the number of angels on a pinhead” and debate it ad infinitum, or use their quasi-scientific knowledge to explain their wisdom. Especially if they don’t bill by the hour!! (Apologies to those who do).I’d suggest picking some tire pressures you find useful and seeing if they work for you. Then adjust accordingly.Unless you’re racing at the highly competitive level and/or chasing personal bests, the trail conditions that day, your personal mental/physical/emotional state, and risk tolerance are probably more important.Apologies for digressing from the infinitesimal to the big picture; but a long-term 95% solution has always been my credo! :-):-)

[Reply]  @Inertiaman: “ “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.”How could the contact patch possibly enlarge while the tyre sag stays the same? Aren’t these just different measurements of the same parameter (tire compression/conformance)?? Ie more sag = more patch.”

“ Secondly, you seem to be saying that one can increase contact patch, keep the same tire pressure the same, and as a result the “ground force” must increase. This is true, but the “ground force” cannot increase unless the equal and opposite “rider force” is increased.” Adult Daycare Director Aka The Bartender Shirt

Yes, that is what I said. Read it again. I fixed the other variables so the applied load, either static system weight or static weight +/- dynamic loading has to increase to maintain sag with increasing contact patch at fixed psi. Therefore a larger volume tyre needs lower psi to maintain sag due to the increasing contact patch for the ideal sag.

[Reply]  @jddallager: Well said, Can’t help but think that all these over thinkers even know what it’s like to actually ride a bike and actually enjoy it for being exactly that. Ride your bike [Reply]  Contact patch is proportional to pressure and nothing else. The contact patch multiplied by the pressure equals the upwards force that supports the rider.

A 24″ x 1.25″ BMX tyre with 20psi has same contact patch has 29″ x 3.0″ tyre.

All other effects are incorrect. There difference in contact patch shape, but not area. Knobs complicated the issue slightly, but not carcass flexibility, or any other such nonsense.

This is such simple physics, it drives me mad that no-one understands!!!

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