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Slightly awkward question... A pressure gauge is good and useful, especially having a pump with a gauge, but some air escapes at each measurement, so I wonder if there are other methods that give reasonably good estimate.

The reason for asking is that one of my bikes is a Brompton, the small tires contain very little air and also the valve is a bit hard to get at because the spokes are so close. So when I measure the pressure with a gauge, I lose some air just from the measurement, at best perhaps a around 0.3 bar, but if I don't quite manage to push the gauge in properly, it can easily be 1-2 bar (it only takes a second for most of the air to escape). Also pumping it up is hard (especially when travelling without a big track pump), so I want to avoid losing air.

Normally I just use the "can I squeeze it with my finger" method and the "look at how much the tire bulges when I sit on the bike" method (and then a pump with a gauge occasionally when it feels too soft), but these are not very precise.

Basically these are hardness tests for the tire, so I wonder if there are any (light, cheap) instruments that can measure the hardness of the tire in a noninvasive way.

Obviously, relating the force to the pressure depends on the tire material, so it wouldn't be calibrated to pressure, but I'd be happy with a result like "the tire is softer than a certain threshold for my particular tire that I defined earlier".

My tires have a pressure range of 4.5-7.5 bar, and I usually pump them up to 7 bar. So I'm not too worried if it's 7 or 6 or even 5.5 bar, but it shouldn't really be below 5 bar.

Update: I also experimented with a cheap valve cap pressure monitor and wrote about my experience in the answer to a different question.

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  • Well, in theory you could weigh the tyre without air, pump it up and re-weigh it; that will give you the mass of air in the tyre, and then you can compute the pressure using the gas law PV = nRT. (You'll need to know the volume of air in your tyre, but you can use that by taking the difference in displacement between a mounted, inflated tyre and the dismounted tyre and wheel. You'll also need to know the local density of air, which will depend on temperature and humidity.) :D
    – DavidW
    Feb 11 at 14:51
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    DavidW: do you live in a vacuum chamber? Feb 11 at 14:59
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    There are valves with built-in pressure gauges. You'd need to have a presta valve with a removable core, or a tubeless tire.
    – Adam Rice
    Feb 11 at 15:20
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    There is quite a number of pump chucks that are more or less easier or harder to put in place. With a Brompton I've had the least trouble with a screw-on chuck. Whereas with the standard press-on and use lever type it's quite messy, given the restricted 'operating theatre'.
    – Carel
    Feb 11 at 16:40
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    @Michael For a pump, yes, but the pressure gauge just opens the valve to let some air out. When I do repated measurements, each one is a 0.2 or 0.3 bar lower than the previous, or even more. The problem is that the volume inside the tire is really small. Feb 11 at 18:26

3 Answers 3

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Squeezing with your fingers is very inaccurate.

I’ve found that pressing the whole bike against a sharp corner gives a much better indication of low tyre pressure. Put the front wheel against a sharp-ish corner (e.g. stair, curb stone), engage the front brake, and press down on the handlebar with your whole body weight. If you can compress the tyre all the way down to the rim without too much trouble it’s probably too little pressure. With the rear wheel it’s a bit trickier, you have to engage the rear brake and press down on the saddle at the same time.

The nice thing about this method is that it takes tyre width, rider weight and luggage weight into account.

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  • Thanks, that's a good suggestion, I hadn't thought of using a curb but you're right this makes it easier to see. Feb 11 at 18:35
  • I suspect this is a better suggestion for off-road bikes than for road-only bikes, but upvoted anyway.
    – Weiwen Ng
    Feb 17 at 15:52
  • @WeiwenNg: I’ve found that it works surprisingly well for road bikes as well. At least it gives you a warning if your pressure is too low and you are in danger of pinch flats. It probably works worst for heavy, puncture resistant city tyres which one wants to run close to maximum pressure (to avoid excessive rolling resistance and cracks in the sidewalls).
    – Michael
    Feb 17 at 16:31
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  1. From first principles: If you have, for example, a tire inflated to 70 psi, and you put a 70-lb load on that wheel, by definition the contact patch will be 1 square inch. So in principle you could ink a patch of the tire, put the inked patch on a piece of paper, and put a known weight on the wheel, and measure the area of the contact patch. You could then work backwards to the pressure that would produce a contact patch of that size. This seems like a lot of work, and there would probably be some error induced by tread and casing thickness, but it is possible.
  2. By throwing money at the problem: There are valves with built-in pressure gauges. You'd need to have a presta valve with a removable core, or a tubeless tire.
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  • Most Bromptons use tubes with Schrader.
    – Carel
    Feb 11 at 16:42
  • That may be, although I have used 349-mm tubes with presta valves.
    – Adam Rice
    Feb 11 at 17:51
  • Thanks. Solution 2 is a good idea, but at the moment a bit over the top for my needs. Solution 1 could be something to experiment with, perhaps I can use my body weight (roughly constant) and mark the size of the expected contact area on the rim. Have to try that... Feb 11 at 18:33
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While reading the answers a "serious joke" came to my mind, so bear in mind I haven't tested this.

Squeezing with fingers can be good for a quick check before a commute, but also, not trying to determine a pressure value, only to test if "safe to ride" at the moment.

I think the problem with squeezing is that: a) You may press harder or weaker than you think and b) You may misjudge the push back, both because tiredness or fatigue.

So, eliminating those factors, I think a "Spring clamp" can be converted to gauge, by placing a protractor-like dial centered in the pivot, fixed to one of the levers and placing an indicator on the other, then clamping the tire and judging the angle that the clamp achieves. The first time you can calibrate with inflating the tire with a known good gauge. You may make many markings or just the minimum and maximum that you want for a particular set of tires.

The cons: this would serve only for that particular set of tires, a spring clamp is bulkier and heavier than a pressure gauge and you would need to place the clamp on the tire exactly at the same distance from rim. (some line or labeling on the tire may serve as a guide)

A spring clamp should have more than enough force for the task and it should press with the same force every time and should be cheap enough and accessible on a hardware store.

A regular sized clamp may be good for MTB tires, but I think the smaller ones are enough for a road tire. I would clamp the tire only, side to side, leaving the rim alone or clamp the tire against the rim only if the rim is metallic. I've never used carbon wheels so I don't know if such clamp would damage them.

But returning to seriousness, a ready made solution may be a pump that has a gauge and an extension hose. These hoses have a valve chuck on one side and a valve-sized threaded fitting on the other. (some have a check valve but other are a plain hose). In order to measure you would try to push air in and see the pressure in the gauge so it should eliminate or minimize the air escaping problem. The hose also helps with the difficulty of using the pump in the tight space that small wheels have.

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