tire air/pressure
 

I've had my masi fixed ltd for about a month now, and I was wondering about tire air. Can I just put some in when it feels low or do I need to measure pressure and stuff?

read ur tires. It will tell u the recommended and max psi.

And u might want to read this.
http://sheldonbrown.com/tires.html

I reccomend getting a decent floor pump with a gauge and topping the tires off with air before every ride. High pressure, low volume tires will lose a considerable amount of air in a short period of time. The last thing you want to do is get a pinch flat because you didn't have enough air in your tire.

When I started commuting rather than riding for leisure I was shocked at how many flats I got right off the bat, not knowing to top the tires. I now fill once a week and it's fine. Filling every ride is better but there's a limit to how much you can eff around every time you get on the bike when the bike is your main transportation.

Good point. If you ride everday then you probably don't need to air up each time. You will learn over time how much air your tires lose per day.

I check the tires every week, they hold air pretty well.

Get a floor pump that has built-in pressure guage, and pump your tires five pounds over pressure every night before you go to sleep.

"Over pressure" doesn't mean over the highest pressure listed on the side of the tire, but, rather, it means over the correct pressure for your body weight.

Let's assume you ride with 700 X 25mm tires.

I specified 25mm instead of 23mm because 25mm most closely corresponds to an inch, and it makes the math easier.

If you weigh 200 pounds, you need a total of 200 pounds of air pressure pushing against the pavement to hold your rims up and away from damaging objects that will pinch flat your tubes and dent your rims.

With a 25mm, or one inch tire, that means each tire should have 100 pounds of pressure pushing against that one inch of contact, so that two tires together would push with 200 pounds of pressure.

If you had a 50mm, or two inch tire, then you would use 50 pounds of pressure, so that two inches times 50 pounds would equal 100 pounds, and two times 100 pounds would equal 200 pounds, your body weight in this example.

Now, to complicate this a little, some of us, maybe most of us, ride with more weight on the rear tire than on the front tire.

For myself, I weigh 225 pounds, and I ride with 125 pounds in the rear and 100 pounds in front.

Every night before I go to sleep, I pump up my rear tire to 130 and my front to 105.

Most people ride with a 23mm tire; close enough to one inch for all the above to apply.

Oh man, you're not kidding.

That really is the last thing I want to do.

Unneeded pinch flats blow. Even when you maintain your pressure they can happen thanks to ****ty roads. Pinch-flatted at 120 psi the other night on the exposed edge of a manhole cover I couldn't dodge thanks to another crappy bus driver.

A decent floor pump with a gauge is worth the investment and then some. Easy to use and fills a very important need exceptionally well.

One of the only benefits of weighing only 170 lbs.

It does depend on your tires. The ones in my bikes lose 5-10 psi over the week which is manageable with one fill. You will also, over time, learn innate flat-avoiding skills, like scanning the road surface ahead of you for danger spots, and unweighting your saddle and pulling up on the bars when you hit bumps.

140 here. Definitely a plus.

What really makes the math easier is the way you did it 100% wrong. If you don't know the difference between an inch and a square inch you probably shouldn't be doing your own pressure calculations. According to Sheldon Brown, you're running your tires without enough pressure:

http://www.sheldonbrown.com/tires.html

To the OP, YES you have to "measure pressure and stuff". When you're dealing with high pressure, low volume tires, you can not go by feel. Just top it up once or twice a week with a good floor pump w/integrated gauge and you'll be good to go.

Thanks for the link to Sheldon.

I've read what Sheldon has to say on the subject before now, and upon revisiting, what I read confirms that I have exactly the right pressure in my tires.

Perhaps Syscrush can help me see how I misunderstood Sheldon, assuming I misunderstood Sheldon.

By the way, what pressure does Syscrush think I should have in my tires?

I weigh 225 pounds and I have a total, front plus rear, of 225lbs of pressure, and I have this distributed over two one inch contact patches.

I think I did it exactly right.

First of all, equating your weight in pounds to tire pressure in psi - and you certainly can't add the pressure in your tires and compare it to your weight.

If your weight is 225 lbs, that's over 100 lbs per wheel, and the link shows that for 100 lbs on a 25mm tire you should be running 110 psi - 120 for a 23mm tire. For ~112 lbs/wheel, your pressure should be more like 135 psi for both wheels if you're running 23's.

If you're going on the assumption that your rear wheel is loaded more heavily (say a 55/45 split), then you'd want something like 120 psi for the front (101 lbs load) and 148 psi (123 lbs load) for the rear. Which would suggest that you may want to step up from the 23's to 25's so you could run 110 front / 136 rear.

I used to only have a hand pump and just pumped up my tires as much as I could. Then I bought a floor pump and found out that I hadn't put even close to enough pressure into my tires.

You should use a pump with a gauge. I keep my 700x23's at 140 psi (Clyde here), and by the time they've lost enough air for me to really feel a difference by squeezing the tire they're usually down to about 80. That's nearly half the air gone!

i remember reading somewhere there's a formula to calculate the best psi base on your weight..anyone remember that?

If ride too soft/slow psi++

If ride too hard/rough psi--

If pinch flats occur often psi++

If random catastrophic tube failures psi--

Pretty easy to figure out, IMO way more accurate than some random internet formula.

That's a good way to get things fine tuned, although the internet formulas will give you a good ballpark figure.

that gives me an idea! I'll write one tonight.
(anyone with windows/a c/c++ compiler can do cross compiles for me?)

Make it an iPhone app!

I don't have an iphone, any willing testers. I'm certainly going to try!
edit: objective cocoa-c? I'll have a look, but I've never used cocoa before.

anyone want to build this for windows. I feel that the numbers that I used gives a pretty decent value for a good pressure.
Its really super simple, just plugging in your values to a formula, but w/e, also, it is only for road tires.


EDIT: added to latest post due to error