Are there valve caps that show the tire pressure?

Question

I saw these valve caps for cars, set at 2.4 bar:

Is there something similar for bicyles? it would help less careful riders when it's time to pump more air in them.

Answer 1

These types of valve cap gauges aren’t recommended - either for cars or bikes. They work by bypassing the schrader valve in the tire and using a cheap spring-based pressure gauge in the cap.

First, they’re wildly inaccurate even on cars. By the time they show red, you’re riding on your rims.

Second, they’re cheaply made and a significant second point of failure. If the cap is just a bit loose, the o-ring gets just a bit of dirt in it, or the valve cap gets knocked the wrong way, they will start bleeding air. This is most likely why they aren’t sold in the higher pressures that bikes would need.

Finally, as @Argenti noted, many riders adjust their tire pressure depending on the road or trail conditions or their desired degree of ride comfort. Most bike tires can take a wide range of pressures, unlike cars which have a very narrow band.

Conclusions: I’d just get used to squeezing your tires (for the lower pressure of mountain bike and comfort bikes) and/or topping off regularly (especially if you have a road bike).

Answer 2

I think RoboKaren's answer is on point about avoiding those cheap gauges which will probably cause more problems than help. For completeness sake, I point to the fact that the only "bike-specific" tire pressure monitors on the market appear to be high-end wireless digital versions that send data continuous monitoring data to a smartphone.

From the press release:

Quarq, SRAM’s data and digital technology brand, today announced the launch of TyreWiz, the first-of-its-kind tire pressure sensor for road and mountain bike riders. The real-time monitoring device – designed to help riders reduce tire wear, improve compliance, and boost speed – is being debuted at Sea Otter this week and will be available on June 1 exclusively on Quarq.com. The two-sensor package will retail for $199.

It does have a pass/fail indicator lights, with the threshold pressure set by the smartphone app. The application is definitely focused towards "pros" who want to finely tune pressure or monitor it during events (e.g., gravel races).

Once installed, TyreWiz relays tire pressure data to a cycling computer or a smartphone every second. The TyreWiz app provides personalized recommendations and pressure alerts. For the first time, riders will have the ability to use highly accurate real-time information to make decisions that can affect rolling resistance, traction, tire wear, and rider comfort.

The real time continuous data stream suggests they are only intended to be run for events not as an everyday, day in an day out monitoring device. The "long" run times are 300 hours, or 12.5 days of continuous monitoring.

I think the easier option will be to pump your tires up to your desired pressure, give a thumb press to see how hard they resist and occasionally conduct a "squeeze test" as suggested by RoboKaren.

Answer 3

The valve caps in your picture are no good. I appreciate you're asking for "something similar for bikes", but my experience might still be useful as I actually bought ones that look exactly like the ones in your picture and can give you some practical experience.

I was looking for alternatives to the "squeeze tire with finger" method (details in this question) and these valves are very cheap (a few euros) so I just bought them out of curiosity to see if they are useful.

First problem is that most that you find on the internet are for 30psi (around 2.4bar) which is a useful value for cars, but can be quite low for bicycles. It depends of course on your situation, but I keep my Brompton folding bike (for which I wanted these) at 6-7bar (minimum specified on the tire is 4.5); my normal bike is at about 4.5-5.0bar (minimum 3.5). At 2.4bar the bike tires feel already very squishy on both bikes, so I don't need a gauge to notice that they are going flat.

I managed to find ones that are rated at 56psi (ca 3.8bar) online, but it took some searching, and that was the highest value I found. For my folding bike, 3.8bar is sort of ok to be alerted of a leak before it gets too bad.

If somebody produced them for bicycles, they really need a higher pressure rating.

After I bought them, at first I was quite excited; I tested them a couple of times pumping up the tires to various pressures and such, and they seemed to be ok. I found that at about 3.5 bar you start to see the yellow ring, but the red thing only gets obvious below 3bar. So the rating of 56psi/3.8bar doesn't mean you notice the change at 3.8bar.

I think in principle such a system can be quite useful for a quick look if the tire pressure is ok. They are no substitute for checking the pressure properly with a pump and gauge every week or so, but they are good for everyday cycling to see immediately if you have a slow puncture, better than trying to distinguish 6-7 bar (normal pressure) from 3bar (slow puncture) just by squeezing with the finger.

However, on the next day after installing them the front tire was flat. It turned out that one of them started to leak air as a little hole opened in the plastic cap, and I think the reason is that they are not designed for my normal tire pressure but for much lower car tire pressures, so their internal seals didn't cope. Again in principle this could be solved if such gauges were designed specifically for bicycles.

The other problem I found was that screwing them on always lost a fair bit of air before they sealed off. Again this is probably less of an issue in cars where tires have higher volume and lower pressure.

So, to summarise, I think these valve caps for cars are not suitable for bikes due to the higher pressure, but I think that the idea has potential and if similar systems were designed for bicycles, they could be quite useful and popular. They would have to withstand the higher pressure without leaking but at the same time be reasonably cheap. Systems like the Tyrewiz are just too expensive; that might be ok for performance sports, but not for the normal user.

I would also prefer something simple and mechanical like these valve caps, not electronic with smartphone/computer monitoring etc.

If it is actually possible to mass produce something along those lines for the bicycle market that is good enough but also cheap enough, I don't know. Possibly not, otherwise somebody would've done it by now.

Answer 4

Not quite a cap, but it is possible to use a Tyre Pressure Monitoring System (TPMS) from a motorbike. The sender replaces your entire valve stem in each wheel.

Possible gotchas:

1. You'll have to be running tubeless. This won't work with tubes.
2. This may get confused by sealant.
3. I have no idea how you charge the sender units - if they need to be charged at all or somehow generate their own power due to rotation. These appear to have a coin-cell.
4. Unbalancing wheel - that's a dose of mass on one side of your wheel which could give vibrations.
5. The sender may not sit inside your rim's central valley - motorbike rims are wider.
6. Motorbike handlebars are thicker than bicycles, you may need a lot of shims or customise the bracket.
7. Depending on the width of your tyre and pressure, it may be possible to crush/destroy a sender with an unfortunately placed rock impact or landing.
8. This headunit uses USB to charge an internal lithium battery, but motorbikes tend to have 12V power available, so some headunits will expect that. Check the specs.
9. Weight - motorbikes are less restricted on weight than bicycles, so they can waste a few grams here and there. Adding 100~300 grams to a motorbike might not seem like a lot, but on a pushbike it adds up.
10. Survivability. These should be 100% waterproof, but in reality nothing is. Same goes for impact resistance. And they're going to be odd, noticeable, and stealable in a public bike rack. I doubt you could find just a replacement headunit for your senders, or vise versa.

In an ideal world the sensors would use ANT+ or bluetooth and you could report the values straight to a head unit like a garmin etc, but these are all likely to be using a proprietary transmission protocol on a public band like 433 MHz.

Answer 5

These now exist:

https://www.worldwidecyclery.com/products/quarq-tyrewiz-air-pressure-sensor-for-presta-valve-pair (smart pressure sensor)

They talk with your head unit or smartphone.

They have LED lights that tell you too low/high as well.