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Weighing a Car with Tire Pressures

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Re: Weighing a Car with Tire Pressures

#41

Post by kevin_thibedeau »

mauvehaus wrote: post #34

Relatedly: if you and your bicycle weigh 200 pounds, and your tires are inflated to 90 psi (a very reasonable pressure for a road bike; racers run much skinnier higher pressure tires), your total contact patch is a little over 2 square inches. Think about that the next time you’re watching somebody fly down a descent going 45mph or more. That’s not a lot of rubber keeping you on course. As an added bonus, you can bas…

All the leet roadies are running 25mm tires now. You get jibed for running 23's on an older bike with brakes that don't open enough.

Re: Weighing a Car with Tire Pressures

#42

Post by supernova87a »

kevin_thibedeau wrote: post #41
mauvehaus wrote: post #34

Relatedly: if you and your bicycle weigh 200 pounds, and your tires are inflated to 90 psi (a very reasonable pressure for a road bike; racers run much skinnier higher pressure tires), your total contact patch is a little over 2 square inches. Think about that the next time you’re watching somebody fly down a descent going 45mph or more. That’s not a lot of rubber keeping you on course. As an added bonus, you can bas…

All the leet roadies are running 25mm tires now. You get jibed for running 23's on an older bike with brakes that don't open enough.

Wow, I just road ride for leisure on my own, don't know any racers/university types, and my tires are 5 years old 19mm on an old carbon frame. I'm way out of current trends.

Re: Weighing a Car with Tire Pressures

#43

Post by [deleted] »

JoshuaDavid wrote: post #3

I wonder if you could do even better by putting paint or ink on the outside of the tire, driving over a piece of paper, and seeing what fraction of the paper touched your tire (and thus what fraction of the rectangle boundary actually touches the ground).

[deleted]

Re: Weighing a Car with Tire Pressures

#44

Post by joosters »

Another way to possibly get better accuracy is to pump the tyres to several different pressures, and for each pressure, re-measure the tyre contact surface.

From there, you could then simply average out the results - or better still, the data would help you reason about the pressurearea relationship. Just going by the P=F/A formula, pressure should be inversely proportional to area, but people have pointed out things like the tyre sidewall stiffness will affect this. Taking several pressurearea measurements would help you reason about this - perhaps the sidewall means P = F/A + x, and with multiple datapoints, 'x' can be calculated.

Re: Weighing a Car with Tire Pressures

#45

Post by zten »

kevin_thibedeau wrote: post #41
mauvehaus wrote: post #34

Relatedly: if you and your bicycle weigh 200 pounds, and your tires are inflated to 90 psi (a very reasonable pressure for a road bike; racers run much skinnier higher pressure tires), your total contact patch is a little over 2 square inches. Think about that the next time you’re watching somebody fly down a descent going 45mph or more. That’s not a lot of rubber keeping you on course. As an added bonus, you can bas…

All the leet roadies are running 25mm tires now. You get jibed for running 23's on an older bike with brakes that don't open enough.

25 is already oldschool. Newest aero rims are designed for 30+, running absurdly low pressures with hookless tubeless tires.

Re: Weighing a Car with Tire Pressures

#47

Post by mauvehaus »

supernova87a wrote: post #42
kevin_thibedeau wrote: post #41

Earlier quoted context omitted.

All the leet roadies are running 25mm tires now. You get jibed for running 23's on an older bike with brakes that don't open enough.

Wow, I just road ride for leisure on my own, don't know any racers/university types, and my tires are 5 years old 19mm on an old carbon frame. I'm way out of current trends.

My ass hurts just reading that. I bike toured about 7000 miles on 622x28ish tires, and I can't even imagine riding 19mm tires on anything other than completely immaculate pavement (or maybe a velodrome? What do they run?).

Re: Weighing a Car with Tire Pressures

#48

Post by mauvehaus »

zten wrote: post #45
kevin_thibedeau wrote: post #41

Earlier quoted context omitted.

All the leet roadies are running 25mm tires now. You get jibed for running 23's on an older bike with brakes that don't open enough.

25 is already oldschool. Newest aero rims are designed for 30+, running absurdly low pressures with hookless tubeless tires.

I guess I'm way out of date. I remember hearing that people who ride e.g. PBP run way wider tires than normal road racers because of the road conditions, but I didn't realize that the mainstream road racers were running that wide now.

Also: hookless? Like they got rid of in, like, the '70s or '80s? My beater road bike (that's older than I am) has 630mm hookless rims, and I got a discount because of it.

Are the new ones hell to change tires on? Changing tires on that bike is the most miserable bike maintenance task I've done. And I've done most anything short of a headset or suspension fork rebuild.

Re: Weighing a Car with Tire Pressures

#49

Post by mannykannot »

function_seven wrote: post #9

How much does tire sidewall stiffness contribute to the force holding the car up? I know that can vary a lot. Some cars have run-flats that won't sag when they lose pressure (or won't sag as much, anyway). Low profile tires probably transfer more of the load through the sidewall than doughnut tires. And of course the contribution from the sidewalls (as a percent) will change as tire pressure increases.

When the tire is inflated, the sidewalls (and the tread area) are under tension, right? (They would just fold up under any appreciable compression.) If so, does that mean there's no downward force from the sidewalls? Perhaps we should be asking if there needs to be extra pressure in the tire to keep the sidewalls under tension?

Re: Weighing a Car with Tire Pressures

#50

Post by mauvehaus »

mhh__ wrote: post #31
function_seven wrote: post #9

How much does tire sidewall stiffness contribute to the force holding the car up? I know that can vary a lot. Some cars have run-flats that won't sag when they lose pressure (or won't sag as much, anyway). Low profile tires probably transfer more of the load through the sidewall than doughnut tires. And of course the contribution from the sidewalls (as a percent) will change as tire pressure increases.

Depends a lot on the car and tyre configuration. In F1 cars the suspension is so stiff that the tyre stiffness plays a large role in VD. I have some data on this for a few different tyres but I don't think I can legally quote it here :(

Didn't somebody test a couple years ago with a set of wheels/tires that more closely resembled passenger car sidewall heights? You know, like 80-100mm sidewall heights instead of whatever they run currently? It seems nothing came of that; can you shed any light?
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