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Tire inflation questions

Old 10-04-11, 09:28 AM
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Tire inflation questions

I have two bikes, my Madone with 650 x 23 tires and my Quick with 26 x 1.7 (not the standard Quick tires). The Madone inflation numbers are 90 to 125 psi. The Quick numbers are 40 to 65 psi. I weigh 115 (yes, I was an Athena).

First, the Quick. I went for a 16 mile ride yesterday that was primarily crushed gravel MUP, with rolling hills, a few washouts and a fair amount of loose gravel. It also included about a mile of terrible dirt road because the gravel was so large and loose. The ride went well and was pretty comfortable. My tire pressures when I checked this morning were 37 pounds, slightly less than the minimum. I pumped it up to the 40 pound minimum. Does that sound about right? It seems like more pressure would bring my bike back to its usual harsh ride and lower pressure is much more comfortable.

Next, the Madone. This bike is pretty new for me. I find it comfortable in taking the normal flaws of the road. I am keeping the tire pressure at the minimum of 90. Should I try any higher? Or does this sound about right?

My only loosely related question is about the Quick. My LBS put larger tires on it to make it suitable for the ratty dirt roads I ride on in the summer. The stock tires are 26 x 1.3. The new ones are 1.7. I have a spare tube that has no markings on it, the bike shop threw it in for free when I originally bought the bike. Do you think it will work with the larger tire? I didn't think to ask if they used the same tubes when they changed the tires out.
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Old 10-04-11, 09:33 AM
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The tube will be ok.

Less PSI means a softer ride.
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Old 10-04-11, 09:40 AM
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Yes, at that weight you should be fine with the minimum listed pressure for both bikes. That will give a more comfortable ride and only an insignificant increase in rolling resistance. At higher weights there would be a need to increase the tire pressure to avoid pinch flats when hitting potholes, rocks, or other obstacles.

Tubes expand readily to fill the available space inside the tire so they can be used with a wide range of tire sizes. They frequently have a range of widths listed on the tube, but you can safely use them even well outside the listed range.
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Old 10-04-11, 11:56 AM
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Thanks!
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Old 10-04-11, 02:41 PM
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Keep in mind that your tires will only be at the minimum tire pressure for a short period, maybe even less than a day.

They (tubes) all lose air pressure over time, some worse than others.

If you are in the habit of topping up before each ride, then it obviously doesn't matter.
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Old 10-04-11, 02:50 PM
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Right. Supposedly the rule of thumb is that when you sit on the bike, the tires squish down by 15% in the middle of the contact patch, and there's ways to calculate that out. Those people actually advise a lower pressure in the front tire than the rear, for that reason. Tracking everything on charts and math formulas seems a bit obsessive to me Maybe it'd be worth it to run the numbers once. But yes, at 115 pounds, you will have a better experience at the lower end of the air pressure. I, being a big ol' Clyde, keep my tires on the high end of the pressure scale.
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Old 10-04-11, 03:50 PM
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Given Seve's comment that tires lose pressure, I might bring the Madone up to 100 pounds. I don't want to be checking every day. I think I'll leave the Quick at the absolute minimum.
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Old 10-05-11, 08:02 PM
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I would encourage you to go up to 115 on the Madone. You won't notice any difference in your ride, but will have better protection from pinch flats, etc.

On the Quick, if you are riding on pavement, I would inflate it to near the maximum, but towards the minimum for unpaved trails.

I weigh about 215, and inflate my Madone about 10 lbs. above the "maximum". My cychocross bike I inflate relative to the terrain I will be riding. In my 35+ years of serious cycling, by inflating my tires before every ride, and expecting my tires before every ride, I have had relatively few flats compared to my riding buddies.

More problems come from under inflating than over inflating.
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Old 10-05-11, 08:18 PM
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Originally Posted by goldfinch
Given Seve's comment that tires lose pressure, I might bring the Madone up to 100 pounds. I don't want to be checking every day. I think I'll leave the Quick at the absolute minimum.
I weigh 212

I inflate my 700x25's to 120 psi and check about every other day, sometimes every 3rd day. Of course, I always check to see that I haven't had a garage flat The result is that the tire is always in the 110-120 psi range

On the mtn bike, I inflate to 80 psi, and allow a range of about 60-80 psi. The mtn bike does not lose pressure so rapidly, and I check it about weekly.

In 13 years, I have never had a pinch flat.
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Old 10-06-11, 07:22 AM
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Okay, my plan is the leave the Quick at the bare minimum. The comfort difference is substantial and the risks of a flat with the beefier tires does not seem high. They also do not seem to squish out when I am on the bike.

The Madone I will try upping the pressure to 110. I am going for a fairly long ride on it tomorrow and I'll see how it feels.

EDIT: now I looked at the calculator for tire pressure based on tire size and a person's weight. The suggested number is 91 pounds. So maybe I will use the 100 pounds that I was going to use.

Last edited by goldfinch; 10-06-11 at 10:49 AM.
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