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I would like to tell if my tires are correctly inflated, in this case road tires, but I do not have a pressure gauge.

Is there a handy heuristic for checking the pressure? Could I do it, for example, by trying to squeeze the tire between two fingers and seeing how easily it gives? (I can do that anyway, but it wouldn't give me any idea about the pressure level.)

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I have a pump with a built in gauge, and I check my tire pressure often. I also press from the top of the tire towards the rim and have learned from practice what about 100 psi feels like for my particular wheel and tire. But this is useless if you don't have a gauge to practice with. –  Joe Mar 14 '13 at 6:48
    
I really recently upgraded to puncture resistant tires, and I don't think that I could guess the pressure like I could in my old, thin tires. –  Joe Mar 18 '13 at 6:16
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7 Answers

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Unless you have got correctly inflated tyres then you do not know what you are missing. No rule of thumb can make up for this. Tyre inflation is really important and your best bet is to get a track pump with valve to check your tyres with. Do this every fortnight, leave it to every month if your journey times are not important to you. I cannot stress enough the importance of getting a track pump, however, that is not what you asked.

Several answers here have mentioned that you should take a peek at how much your tyres splay when you are riding the bike. This is definitely a useful check, however you must bear in mind that a correctly inflated tyre is not a solid tyre and it will bulge out sideways anyway. Only when drastically under-inflated will it show an easily discernible extra bulge. But do watch for that, particularly if you go over a bump and the bike 'thuds' more than normal.

There is also the ride-through-the-puddle test. Go through a puddle and onto a dry surface, e.g. a pavement or a hallway. If you look at how much water has been left behind you can get a comparative idea of how much of your tyre is in contact with the road. If you know what you expect and if the trail is thicker than that, e.g. with a lot more than the centre ridge making contact, then you may want to get the track pump out again.

Returning to the weight-on-the-wheel and how much the tyres splay idea, you can also roll your bike slowly over a kerb and see how much it deforms. At speed it will do so more than under the static situation, so try it slowly and there should be no danger to your rims and 'snake-biting' your tyres.

As for squeezing the tyre after inflating it as best as I can with a mini-pump, when I get to the track pump with gauge I am always surprised at how many tens of P.S.I. I was off the mark. Squeezing the tyre is a waste of time.

If you had car-type valves (which you don't) then you could get those valve caps that some car-part shops sell. These go red when you lose 5-10 P.S.I. They are good but not available in Presta to my knowledge.

Personally I find the best gauge to be how the bike feels and how fast it goes. On some parts of my commute I like to go quicker than the 20 m.p.h. speed limit (as cars cannot over take me then), however, there are times when I am not able to hit my expected speeds, as measured on the bike speedometer. With a bit of extra air in the tyres I can usually get back to where I expect to be.

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Haven't heard of valve caps with pressure indicators before, think I'll get some for my car and my bikes. –  Tom77 Aug 12 '11 at 15:45
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When you say "track pump" I think you really mean "floor pump"?? A track pump is just a floor pump with silca head (no locking lever, just a hunk of brass with a gasket inside that is pushed onto valve), silca heads are an "accquired taste"!! –  Angelo Aug 12 '11 at 16:29
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@Angelo: I think that may actually be a regional language difference since I've seen "track pump" used to refer to any kind of floor pump by UK and AU folks before. –  freiheit Aug 12 '11 at 16:51
    
Tire squeeze works good on bmx tires (2.20 at ~90psi). I can easily tell if I'm below 80 psi by squeezing or riding on them. I imagine a thinner tire at higher psi would be a waste of time though. –  dotjoe Oct 26 '12 at 20:18
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Lean most of your weight onto the seat or handlebars (depending on which wheel you're trying to check) and watch how much the tire bulges. Do this on a road-like surface, not dirt or carpet. Generally the tire should bulge out a little next to the contact patch, but not much. Try bouncing your weight, no matter how vigorously you bounce the tire shouldn't get close to bottoming out. There should be at least a little bulging, especially when you bounce hard.

This works better, of course, if you've used a gauge with those tires before and know how much bulging to expect with proper inflation.

It's just too hard to judge with your fingers. Odds are good that a little under inflated and correct inflation both feel almost rock hard.

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+1 This works fine once you know the bulge for the right tire inflation. I use this method while riding, too (I have a MTB background, so I cannot "feel" so well a flat on 700x23 tire with top pressure - its regular feel is too similar to riding on the rim of a dead flat mtb tire). Otherwise, on 50mm plus mtb tires, feeling the pressure with bare hands works better, I think. –  heltonbiker Sep 23 '11 at 13:58
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The best "heuristic" is to observe the rear tire under full weight conditions. It should deform noticeably, but not to an extreme. Deformation should be roughly 10-20% of tire height from ground to rim edge -- less deformation for road conditions, more for off-road.

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The most important thing is to have a pump with a pressure gauge. Just top off the tire pressure on each tire before each ride. You can then squeeze the tire and over time get a feel for what is the right pressure for you.

The "right pressure" depends on your weight and what type of tire you're dealing with, and your preferences. And the only way to determine this is to ride your tires at different pressures and see what feels right. It is somewhat subjective. I DO NOT think that 2-3 psi makes much difference, and I know that some people will vehemently disagree with this.

Generally speaking (to give you a starting point for experimentation): If you're a "clydesdale" (over 200lbs), you'll want to be above the recommended pressure that is printed on the tire. If you're fairly lightweight the recommended pressure is a good starting point.

Another thing to try is to run your front tire at a lower pressure than the rear. This will dampen the road shock to your hands. Again, how much lower is dependent on experimentation.

Some people have more rigid rules and heuristics than me but if I had to pick one set of rules I'd choose these from rivendell, they describe what type of tire and tire pressure is appropriate depending on rider+bike+usage.

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Good point on running lower pressure in front tire. –  heltonbiker Sep 23 '11 at 14:01
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If you use a hand pump and know it well, you can get a rough gauge from how much the resistance increases as you pump. I find that gets me within 10-15 PSI when checking later.

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The general rule on a hand pump is to pump until you're exhausted, then top of the tire at the next gas station. –  Daniel R Hicks Aug 15 '11 at 11:25
    
That would make tire go bang :-) –  Karl Aug 15 '11 at 13:42
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Get a pressure gauge. They're not expensive (I had one of these cheap plasticky ones as a kid. Not the easiest to use without losing air, and no good for racing tyre pressures, but served me fine for many years. These days I use a pump with a built-in gauge.)

Heuristics are unreliable, especially if you're not already used to how your tyres look and feel at the correct pressure. Inflating your tyres sufficiently is key for reducing punctures and decreasing rolling resistance, while overinflating them will give a harsh ride and could eventually be dangerous. Why take the risk?

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Separate gauges are a PITA. You tend to lose 5 pounds of pressure each time you measure, since it's so hard to get a good seal. A floor pump with built in gauge is a far better approach (and, especially for those of us with older eyes, it's best if the gauge is at the top of the pump rather than the bottom). –  Daniel R Hicks Aug 12 '11 at 13:54
    
Agreed, though it's considerably more expensive. And less portable. –  onestop Aug 15 '11 at 7:25
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You can indeed check by hand, however you will only be accurate if you practice it often and cross-check with a tire gauge whenever possible. Realistically you should always use a tire gauge if possible--however knowing how to check by hand is a good backup when changing a flat, or when away from home and in need of a quick check.

As others have said, buy a gauge! A good digital presta/schrader gauge can be bought online for roughly $25 US. Digital gauges are more consistenly accurate, however a less expensive analog gauge is still much better than guessing by hand.

Now, once you have a gauge, use it after every long ride on your road bike--or at least weekly (road tires lose pressure quickly, so daily is better yet). But! Before putting your fancy new gauge on, give your tire a squeeze and push the tip of your thumb into the side of the tire. Make a guess if the tires are "ok", "low" or "really low". Follow up with your gauge, add air if needed, then repeat the squeeze to see how they should feel at pressure.

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But, as I've said, reading bike tire pressure with a stand-alone gauge lets out about 5psi every time you do it (at least on reasonably narrow tires). The way to go is a good floor pump with built-in gauge. And digital isn't all that important -- you only need accuracy within 5-10%. –  Daniel R Hicks Aug 13 '11 at 12:28
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