Tubes HEY! You need air in that tire!

[loquin]

A wider tire CAN safely run at lower pressures than a narrower tire, because the area of the tire in contact with the ground is greater than with a narrow tire at the same deflection. Since the pressure is in pounds per square inch (in the US,) IF you increase the area (in square inches,) you can proportionally decrease the pressure to support the same weight, or increase the weight supported at the same pressure.

The sketch below is a diagram of this effect. A downwards force F deflects a tire where it contacts the ground. The contact area for a tire, and a tire which is twice as wide, are both shown.

The contact area of the tire is equal to the downwards force, divided by the tire pressure. If a tire supports 100 pounds, and the tire pressure is 50 PSI, the contact area would equal 100 / 50, or 2 square inches. A 2 inch wide tire would have 1 inch of it's circumference (average) in contact with the ground. A 4 inch wide tire at the same pressure would only have a half inch of the circumference in contact. For the same deflection of the sidewalls, you could cut the tire pressure in half, to 25 PSI. You would then have 4 square inches of the tire in contact, but, since the tire is 4 inches wide, you would have 1 inch of the circumference in contact...

This is also why narrow tires HAVE to run at high pressures, in order to support the rider's weight.

However, this does not justify underinflating a tire, as the extra flex in the sidewalls can cause a LOT of stress in the sidewall near the rim, especially with today's narrower rims.

 

[loquin]

...
Firstly i agree that a wider rim with a wide tyre is optimal but i think your missing the point regular bicycle tyres now days are made with the understanding that the rims are thinner than years ago.
Manufactures didnt just decide 1 day to make the rims thinner.
i remember my old bmx bike had fat rims, i also remember my tyre wasn't round! more a streched halvemoon.
My mb has 1 inch rims wraped with 2inch Serfas Drifters.

Manufacturers make rims as narrow as possible to save metal costs, and to shave weight (to appease the spandex shorts bunch.) They have gone through the calculations, and have determined that under 'normal' conditions, they can get away with a wheel that's X millimeters wide, and not have too many tire failures that can be attributed to narrow wheels.

Adding weight to the bike (heavier riders... engines & mounts...etc) or running the bike at higher average speeds and/or for longer average distances over their life (as with a motorized bike) may press against, or exceed the assumptions that the manufacturers have used in their calculations.

We should try to get rims that are as wide as possible, knowing that our motors may cause our bikes to go over the limits that were used as assumptions by bike and bike tire manufacturers.

And, the thicker rubber and additional belts at the tire face will tend to distort the shape of the tire

 

[DougC]

Here is one reference I have ran across--

http://thevintagent.blogspot.com/2009/01/beaded-edge-tires-and-safety.html

About halfway down the page is a chart that gives tire inflation pressures and their resultant load capacities. We're concerned about the bottom part of the chart listing "beaded-edge" tires; the "wired edge" is something else.

This chart is probably a bit on the high side really (for our purposes) because it is intended for motorcycles that are regularly run at higher speeds than what motorized bicycles usually are.

---------

I said I ran 22/40 PSI front/rear, I weigh ~275 and the bike weighs ~110 lbs.

The capacity listed for a 2-1/4" tire at 24 PSI is 110 lbs. My front tire might be a tad low at 22PSI.

For the same width tire the chart stops at 36 PSI and 240 lbs, but we can project that inflating my rear tire to around 40 PSI should be hitting around 275 lbs capacity (-the Kendas I use also have a max sidewall pressure of 40 PSI).

110 lbs + 275 lbs = what my bike & me weigh.
~

 

[loquin]

This chart is probably a bit on the high side really (for our purposes) because it is intended for motorcycles that are regularly run at higher speeds than what motorized bicycles usually are.

I would say ... not necessarily, as motorcycles tires are also much heavier, with thicker sidewalls than are bike tires...

 

[DougC]

I would say ... not necessarily, as motorcycles tires are also much heavier, with thicker sidewalls than are bike tires...

Yes, but the thickness requires higher pressures.
Thicker car & motorcycle tires have to be kept inflated higher, because the thicker rubber will heat up internally if it flexes too much.

In non-motorized use, bicycle tires don't ever overheat (chunk) from rolling, so there's not really any danger of tire overheating from running them with the pressure fairly low.

....In motorized-bicycle use, I don't know.... It could happen I guess, but I don't think I've heard of it. Has anyone ever seen heat damage from a motor-bike tire? In car tires for example, if you keep driving on a flat tire it will quickly cause the sidewalls to shred apart, from internal heating.
~

 

[WhizBangAndy]

Dont feel bad Im 330 lbs and I run the cheapie 26x2.5 whitewall balloon tires @ 35 psi. No pinch flats but i dont go far either due to NJ screwed up way they look at these

 

[Pirate88179]

Andy:

You live in Jersey? I was born and raised there but moved to Kentucky about 20 years ago. They use a lot more common sense down here on most things...guns, MB's, taxes, etc. It was a nice place to grow up but, I am glad I am not there now. Ride safe.

Bill

 

[WhizBangAndy]

Yup Yup Still stuck in Jersey..Hopefully I can escape before I lose the rest of my sanity. With all the bond issues hitting now the tax rate is going out of sight for housing here so my sights are down your way as soon as I tie up some loose ends. If I get the callback Im going to be spending the next year in Afghanistan as a contractor so that should solve my problems no matter the outcome (rich or dead..lol) Hopefully then I can pull up stakes before this debt riddled state collapses in on itself..lol