how much air?

Discussion in 'Bicycle Repair' started by daveet66, Apr 29, 2016.

  1. daveet66

    daveet66 Member

    Can anyone tell how to use a tire gauge and how much air is neededneeded?

  2. FurryOnTheInside

    FurryOnTheInside Well-Known Member

    There are bicycle tyre pressure calculators available on the 'net. You would need to find one that accommodates your tyre size (most calculators are for narrow 700c road racing tyres) and weigh yourself and your bike, and either guess what the weight distribution is or use bathroom scales and something to raise the other tyre to keep the bike level, and sit on the bike to be certain of the weight distribution.

    Easiest way is probably to get someone to measure for you.... Hang the bike in repair stand and measure depth of tyre to rim edge with a tape measure, then put bike on the ground and sit on it (fully dressed) in usual riding position, while your buddy measures the tyre depth. Adjust the pressure by pumping the tyre up. You want the pressure that gives a fifteen percent drop (loaded depth is 85% of the unloaded depth) and then you can make _small_ adjustments from there to suit your requirements if you want to, but harder tyres are unlikely to roll any faster on real road surfaces, and can actually be slower as well as giving a harsh and tiring ride, softer tyres may give better traction in some conditions but may be "squirmy" and affect handling as well as risking pinch flats, and over time, delamination. Pinch flats are particularly a problem with unsprung weight ie cargo, according to my experience with heavily loaded touring bicycles and I would just set up for a 15% drop and leave it at that. You may wish to have _slightly_ less drop in the rear to prevent pinch flats and _slightly_ more drop in front to give better comfort if the roads are rough.
    If the pressure that gives you a 15% drop is lower than the safe minimum stated on the sidewall of the tyre then you need a narrower tyre, if it is higher than the maximum then you need a wider tyre.
    Write the pressure on a sticker and stick it on the rim next to the valve in case you forget.
  3. Steve Best

    Steve Best Well-Known Member

    Measure tire pressure when cold (ambient temperature, not hot after riding). That is the standard.
    There is a MAX PRESSURE on the tire sidewall, never exceed it. Blowouts will result.
    Too little pressure will result in overheating the tire at speed and objects like rocks will pinch the tube against the rim.

    Typical running pressure of most tires is 50% to 100% of the sidewall MAX PRESSURE.
    50% gives smoother ride and more float and traction, but is harder to pedal and wears tires faster, especially at speed.
    100% pressure is essential at maximum loads or speeds, but returns harsh ride, little traction and wears the center out of tires.