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Tire pressure idea I came up with
Hi there,

I'm a n00b to this forum, but I've been biking for a good bit of my life. I'm actually coming back off a fairly long layoff right now, and trying to learn a lot of things. I had an idea occur to me about tire pressures, and I figured I would share it.

Essentially, it takes the whole concept of "pounds per square inch" quite literally. You'll see what I mean.

As for the "pounds" part, that's pretty simple. I figure on adding 40 lbs (to cover the bike itself and light to moderate gear) to one's own body weight, at a minimum, and more if you do heavily-laden touring. Those who use metric and know their body weight in kilograms should multiply that by 2.2 to obtain pounds. If in doubt, or if it makes the math easier for you to do, round up.

The "square inches" part has a bit more complex math. I thought about it and came up with half of the tire width (0.5 x width) and one-tenth of the tire diameter (0.1 x diameter) as a reasonable approximation of the size of a tire contact patch. Once you have those two figures, simply multiply them, then multiply again by 2 (because you have two tires).

If your tire size is inch-based, e.g. 26 x 1.75, you're good to go. If your tire size is millimeter-based, e.g. 700 x 45, divide what you got in the last step by 650.

Then, just divide the pounds figure by the square inches figure, and that will give you a good ballpark minimum tire pressure. As a safety margin, I would run a little bit higher than that in both tires, and higher still in the rear than the front.

Let me run through this for my own bike as an example. I'm a bigger guy, and I come up with 280 when I add 40 lbs to my own weight. Rounding that up to 300 makes the math a lot easier.

I have 700 x 45 tires, so the tire part works like this:

[(700 x 0.1) * (45 x 0.5) * 2] / 650 = 4.846

So you're talking 300 pounds (remember, I rounded up) divided by 4.846 square inches of tire contact, or 61.906 psi. Obviously, tire gauges don't read to the thousandths of psi, so we'll round up to 62 psi. Going with my "safety margin" guideline, we'll call it 65 psi front and 70 psi rear.

That's right about in the middle of my tire manufacturer's recommended range of 50-85 psi, and I think my formula does a decent job of taking into account my extra poundage. ;-)

I also kinda like the fact that this formula uses measurements you can read right off your bathroom scale and tire sidewalls, rather than requiring arcane calculations like figuring circumference, and uses familiar constants like one-tenth and one-half instead of a bunch of crazy decimals. Metric measurements complicate things a bit, but there's not much I can do there.

Let me know how this formula works for your body weight and tire size, and what you think of it in general.
I was just out riding when I had one of those "duh!" moments about my formula.

I don't know why this didn't occur to me last night when I posted it, but I realized that the one-half of tire width factor and the multiplication by 2, to account for both tires, cancel each other out. So that simplifies it even more.

The square inches part of the equation now looks like this ...

(tire width) * (tire diameter / 10) if your tire size is inch-based

[(tire width) * (tire diameter / 10)] / 650 if your tire size is millimeter-based
Hi Larry;

The only flaw that I can spot, is that typically, something on the order of 75% of the load is on the rear tire.
(09-29-2010, 03:10 AM)nfmisso Wrote:  Hi Larry;

The only flaw that I can spot, is that typically, something on the order of 75% of the load is on the rear tire.

When your braking won't the load transfer to the front tyre ?? Smile
[font=Trebuchet MS]Ride hard or ride home alone![/font]
"Dammit, Jim. I'm a Bicycle Mechanic, not a Doctor!" (Star Trek) Bones McCoy.
Wheelies don't pop themselves. (from a QBP fortune cookie)
(09-29-2010, 06:39 PM)cyclerUK Wrote:  When your braking won't the load transfer to the front tyre ?? Smile

yes, you are correct; and it could be as much as 100% of the load in a panic braking situation.
What you forgot to allow is a Dynamic Amplification Factor. This is the difference between the load under static conditions and the load under moving conditions (including adverse scenarios).

So you better allow a DAF of around 2 as a minimum. It will be much higher if you hit a pothole but 2 will probably cover normal riding.

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