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Weighing a Car with Tire Pressures

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Re: Weighing a Car with Tire Pressures

#51

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.

Oops that's a great point. I completely forgot about sidewall stiffness. My messing around with treads was probably just me adjusting things to make myself look good then!

It's useful to consider the extremes. If your car was somehow supported only by party balloons on the rims, then their stiffness would contribute nothing and the calculation would be spot on.

On the other hand, if you had hollow steel "tires", the stiffness would be everything and the contact patch would change only imperceptibly with a change in tire pressure.

Car tires are obviously somewhere in this range.

Re: Weighing a Car with Tire Pressures

#52

Random thought. Could you compare acceleration going uphill, downhill, and on a level street to estimate mass, somehow keeping the RPM the same on the engine?

You would have to know the rolling resistance of the tires, aero load, and the exact amount of horsepower the engine is outputting to the wheels. It is impractical, once you consider engine wear, air density, and probably a few other factors required to get a correct reading.

Re: Weighing a Car with Tire Pressures

#53
Commercial trucks with air suspension do this, there are various "on-board" scales that tap into the air lines and try and give a weight. It's obviously much more of an issue when you have regulations involved in how much you can carry, so weighing is a big part of trucking.

Here are a few:

https://www.air-weigh.com

https://rwls.com

https://www.appweigh.com

There are many.

Re: Weighing a Car with Tire Pressures

#54
post #45

Earlier quoted context omitted.

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I guess I'm way out of date.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 on…

Well, a lot of this is driven by the gravel racing gang... the people partying at gravel events are already happy running 40+mm width, or MTB tires, on very wide rims. But there's also a growing segment of people who have figured out that you can have a bike that's a little bit more comfortable off pavement by going wider, without totally compromising speed on the pavement, and it also makes tubeless less likely to blow up in your face.

Zipp and ENVE seem to be in some kind of internal width arms race with the release of the Zipp 303-S, and the ENVE SES AR series rims. I was exaggerating a little bit :) You'll still find many, many road bikes with disc brakes maxing out with clearance for 32mm width, but that's an improvement over their rim brake cousins. But look around in the growing "all-road" segment and you'll find wider tire clearances on road geometry frames showing up.

I am unsure if this hookless is the same hookless that is from way back in the day. With the way things go in the bike industry (cyclically, no pun intended), I wouldn't be surprised if it were.

Re: Weighing a Car with Tire Pressures

#56

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.

>How much does tire sidewall stiffness contribute to the force holding the car up?

A car? Not much. At any reasonable driving air pressure the air is doing most of the work.

A pickup/van with E or better rated tires? Eh, some but not much in proportion to the weight said tire is expected to carry, especially if the tires are anywhere near the top of their pressure range.

A skid steer or some other off highway machine with "bajillion ply" tires expressly designed to be resilient to sidewall punctures that runs low pressure because all of the suspension is in the tire? Yeah, sidewall stiffness accounts for a lot in that case.

Re: Weighing a Car with Tire Pressures

#57

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).

>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.

Yes you can. It's called "chalking" tires and is commonly used to figure out what pressure achieves optimal contact patch.

Re: Weighing a Car with Tire Pressures

#58
> The first thing I wanted to do with my Saab 96 was to establish a baseline weight. The cheapest scales used for racing are over $700,

You can do this for (almost) free if you go to the local dump. They usually charge you based on weight of what you dump by weighing your car on the way in and on the way out.

Just ask them what the reading is on the way out. If you bring a couple of pounds of garbage, it will cost a few dollars at most.

Re: Weighing a Car with Tire Pressures

#60
I don't often open HN and see a Saab 96 post. I have many Saabs but by far my 96 is my favorite.

A V4, yes V in a car A compact 4 speed transaxle with 4 on the tree

So much fun. Torquey. Enjoyable. Gets good looks and sparks conversation

Easy to work on!

I do love my 900s as my daily drivers but the 96 is a masterpiece

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