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I'm trying to DIY a sort of a "smart rack" for a bike that can alarm me of problems like low tire pressure. Problem is that tire pressure is measured via the valve, which isn't a practical solution for this use case. Is there is a way to arrive at a similar pressure (doesn't have to be as accurate, but more accurate than the typical +/-20psi "squeeze test") via some device that works with the tire directly instead of the valve (something that squeezes the tire and or laser/ultrasound)? This is for personal use, so I don't mind if the solution relies on hard-coded tire/bike dimensions.

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    Possibly not what you have in mind, but there is a product that replaces a Presta valve core with a pressure monitoring system that transmits data over Bluetooth.
    – Adam Rice
    Sep 9 '20 at 13:41
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    Cyclists know when their tires are going flat because they can feel it almost immediately. Replacing skills with technology doesn't make people happier or their lives better. Also I bet you could do it by using accelerometers sensitive enough to track vibrations from the road and observing sudden changes in the sharpness of the peaks. Sep 9 '20 at 16:21
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    sram.com/en/quarq/series/tyrewiz
    – Paul H
    Sep 9 '20 at 17:37
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    @PaulH that's correct, I was about to mention Quarq's Tire Wiz. And unfortunately, they are US$200 per pair (I think ... the listings I've seen don't explicitly state this, but every performance oriented cyclist runs two wheels).
    – Weiwen Ng
    Sep 9 '20 at 18:28
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    Assuming you always use the same model tires (to control for sidewall stiffness), I think a system that measures force vs movement could work. Have a little finger push on the sidewall with a known force and see how much deflection you get.
    – MaplePanda
    Sep 9 '20 at 19:00
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If you were prepared to use Schrader valves, you could install TPS valves used in car and truck tires. Size and possibly weight might make this idea impractical, and is most likely to work with MTB tires. I suspect that road tires would be too small. You can get external sensors that screw onto the valve, or you would need to go tubeless to use the (probably better) internal sensors (presuming you can get a sensor that would fit a bicycle rim).

With such a system, you would simply read the display and have a very accurate pressure reading.

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  • +1. I would have suggested exactly the same thing: sensors that screw in like if they where a valve cap. There are some that transmit via bluetooth, so that can be a solution for someone else not building the entire solution themselves. Other sensors transmit on proprietary protocols. Maybe one of those is even more fun for a DIYer.
    – Jahaziel
    Sep 9 '20 at 21:45
  • There are Presta to Schrader convertorrs. They are a brass ring that's screwed onto the Presta valve, and have a Schrader diameter top. The Schrader pump's needle pushes onto the Presta valve, opening it. If you can't find a Presta pressure meter, then you can use these to usea Schreader meter instead.
    – CSM
    Sep 10 '20 at 8:35
  • you could install TPS valves used in car and truck tires - not unless you're prepared to convert the bicycle wheel to tubeless. Car/truck TPMS valves are the whole valve stem; your suggestion to use a TMPS compatible valve cap is better
    – Caius Jard
    Sep 10 '20 at 10:11
  • Can such TPS valves handle bike tire pressures? The typical car has no more than 3bar, while bike tires easily reach 6bars or more. Sep 10 '20 at 13:10
  • I expect many will be able to. Trailer tires often get to 65PSI, Trucks use in excess of 100PSI and its all the same technology. You would need to check the specifications of the particular product you were looking to purchase.
    – mattnz
    Sep 10 '20 at 19:57
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I wonder if an atmospheric pressure sensor would work inside a tire, at least on principle.

At least for science fair projects, I've seen a sealed container with a partial vacuum and one flexible wall or lid. The difference in pressure deflects the lid which is attached to a pivoting needle or similar. The needle rests over a dial and thus allows for a reading.

A more modern application of the same principle maybe using an electronic pressure sensor and a battery operated sender unit that encodes the voltage reading and sends it via radio.

If the device needs a pressure differential, then expose one side to a sealed container and the other to the air inside the tire. (A vacuum inside the smaller container won't be necessary, as the tire pressure is higher than atmosphere's)

If the device is small enough, it can be placed inside a tire but indeed it would need to be a tubeless one.

For a regular tubed tire, here is a similar idea:

enter image description here

A force sensor placed between the rim and the rim tape should be able to detect how hard the tube is pressing against the rim. The signal can be picked, encoded and sent with a battery operated module that may be attached to the spokes. I'm assuming the cables can be run through very small perforations in the rim between the spoke holes. (small enough not to compromise the structural integrity of the rim).

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    Risk of the sensor interfering with the hook/bead/seat of the tyre on the rim. What about putting the sensor under the rim-tape, or between tube and tape ?
    – Criggie
    Sep 9 '20 at 22:57
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    They are not suited for continuous pressure sensing.
    – FarO
    Sep 10 '20 at 7:55
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An option could be to use small strain gauges epoxied to the inner or outer side of the rim, on the plane of rotation (basically, between the spoke nibs).

Since the rim "widens" as result of tire pressure, you can detect that stretch.

Of course, it will be affected also by normal rotation, so it makes sense only with the bike at rest. And you need electronics.

You can also, more easily, prepare some 3D printed "C" gauges: with increased tire pressure the tire widens measurably, so you can check the C gauges in series from the widest and see the first one which does not fit anymore to get an idea of the pressure. Since the sidewalls are not subject to wear, this will work for a long time.

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Technically, the main reason we care about pressure is because it controls tyre deformation.

We don't want pressures so low that an inner tube is prone to pinch flats, or that the tyre squirms when cornering, and we don't want a pressure so high that the tyre transmits vibration too harshly, or fails to conform to the road surface (or that it blows off or breaks the rims).

So, if you figure out what load force & deflection correspond to your preferred pressure, you can simply apply that load force (radially instead of laterally so sidewall behaviour is included) and measure deflection. You might be able to do this with a linear transducer, a servo motor, or just an appropriately selected spring.

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