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Front tire pressure

Old 11-12-10, 08:31 PM
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Front tire pressure

This was mentioned in another thread, but didn't see a number to go by. I have a Trek FX 7.5 (2010) and am looking to increase the riding comfort. I've heard to lower the front pressure, but want to make sure I don't go too low. I do street and bike/multiuse trail riding, and am about 175 pounds. What kind of tire pressure range should I be in, for comfort but still easier going uphill? Max is 110 for my tires.

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Old 11-12-10, 09:18 PM
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If your riding a stock FX 5, those Bontragers should have a minimum amount posted on the side......Just like the Max
PSI, just a starting point for you ..Richard
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Old 11-12-10, 09:26 PM
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Sheldon Brown has great information on tire inflation. Scroll down to the "Pressure Recommendations" section.
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Old 11-12-10, 10:27 PM
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You need enough pressure in the tire to:
  1. ...prevent the tire from rolling off the rim. ("Clincher" tires need a certain minimum pressure to keep the tire's beads seated under the rim's hooks.)
  2. ...prevent pinch flats. (Pinch flats happen when an underinflated tire strikes an object hard enough which causes the tire to deform and "pinch" the tube sharply and thus flatting it.)
  3. ...provide sufficient volume to maintain an effective contact patch during cornering.
The best way to determine the right pressure is to adjust the tire pressure until you find the PSI that works correctly for you. You're after a certain feel. For a comfortable front tire, you don't want so much pressure that the tire tends to bounce and deflect upward when you hit rough sections of pavement; yet you want enough pressure that the tire doesn't feel squishy. A comfortably inflated front tire will deflect very slightly under you weight. When you stand on the pedals and push your weight forcefully downward on the bar, you'll see pronounced deflection of the sidewall.

The max PSI is a guideline. Start with a front/rear PSI combo below this max, and consider that the majority of your weight is supported by the rear tire. To start, with your given weight and the tire you're riding, I'd try 85f/95r and adjust up/down accordingly in small increments (5 psi) until I found the right pressure.

I actually wouldn't be surprised if the pressure that you like is very close to that 85/95. That's about what I ran my 700x28 Bontrager hardcase lites at; while the narrower tire typically requires a bit more pressure, I'm lighter (~160) and don't need as much pressure for a similar ride. My 700x32 tires (other mfrs) have typically ranged from 60/70 to 75/85.
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Old 11-13-10, 10:24 AM
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On my Dahon commuter with 38 mm Marathon Racer tires I run 35 front and 70 rear. On my Fuji hybrid with 32 mm Hutchinson Acrobat tires I run 50 front and 85 rear. Both sets of pressures were set by weighing the load on each wheel as I typically ride the bikes using this article's recommendations. Both bikes feel as efficient at these pressures as at max pressure though max pressure will give a slight efficiency gain in careful testing on smooth surfaces. On rough surfaces a lower pressure is more efficient according to those who have done the measurements. Comfort and handling are both better at lower pressures. I've had no pinch flat problems at these pressures, in fact I have never had a pinch flat in my entire life. Granted that for most of my life I ran my bike tires at the max pressure. The front/rear pressure differential I now use is a lot wider than the conventional wisdom here on BF but it is based on tire engineering criteria and I have seen no issues with it, just a better overall ride.

Ken
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Old 11-13-10, 03:58 PM
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I tried the 85/95 today, I think I like that, or at least very close. Of course the wind picked up and I ended up in an 18mph headwind, but otherwise it felt good.
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