Tpms how it works and its quirks...

[spiderman302]

I am using an external sensor that screws on to the end of a METAL valve stem. (SPY tpms)

On my daily commute my front tire increases by 3 psi and my rear tire increases by 4 psi. The only reason the tire pressure rises is because of internal tire heat. Using 1 psi change = 10 F change. Then 4 psi increase would equate to a temperature rise of +40 F (+22 C) and 3 psi would be a temperature rise of + 30 F (+17 C).

Now the front sensor temperature only reads an increase of +2 C and the rear sensor reads an increase of +12 C which is does not correlate to the pressure changes. The front sensor is out in open fast moving air so it is quickly cooled. The rear sensor is in "blocked" air flow by the saddle bags and is getting some engine heat as well as both side catalytic heat and muffler heat. There is still enough air flow that it still reads 10 C below what it should. So the temperature reading is NOT very useful with external sensors. Do NOT set limits based on temperature. I have no data on the internal type sensor.

The pressure increase is only what you should watch. Make a mental note of your starting pressures then see how high it gets near the end of your trip or where you are going fast at the hottest time of the day. This will give you a working range based on your set up. There should be less change on cold days and more change on very hot days.

Now my set up is different than yours since I am running double dark. I have set my cold rear CT to 26 psi and my front to 35 psi in order to get a smoother ride. This is a worse case set up since lower pressure will cause more stress in the tires but I am only seeing about 4 psi increase in the normal outside temperatures of 20 C to 30 C. I just missed getting data when it was 48 C last month and it will be a few months before I will get data at 0 C... I am guessing that at 0 C there would be a 2-3 psi increase and at 48 C there would be a 6-9 psi increase.

So if you get a TPMS Just keep track of your pressure operating range.

 

[cdnemsguy]

Very good write up! I have to get out to my hangar to do a small experiment with my tires and TPMS. I am going to remove the air and run nitrogen in the tires. I will fill them, purge them and fill again with N2, as N2 should not be effected by the tire temp and therefore not change the PSI. This is one reason we use it in some aircraft tires, and some cagers as well. I don't see why it wouldn't do the same in the bike tires, but as for the sensors, I am not sure where the temp readings come from. Mine are not accurate I believe, although I do trust the PSI reading with air in the tires. N2 to follow.

 

[spiderman302]

Oh. good point. The hidden reason why N2 is better than "air" is that there is no water vapor. Humid compressed air will "expand" more than dry air as it gets heated. So that may be the real reason why there is a wider range in pressure variation....

 

[mlheck]

And this is one of the things I love about the internal TPMS on my K1600. It also reads the temp and corrects the pressure reading to compensate for it.

I ran the Zumo TPMS on my 1300 and the problem was that if the tire pressure grew by 4psi because of heat then my cold alarm set point was now much lower when hot.

And has a tire loses air it starts to run even hotter, thus giving you an even more erroneous cold PSI reading.

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[T_C]

One of the reasons I like the Orange TPMS over my Fobo units. You get truer temperature readings. It's surprising how fast the temperature will drop a few degrees once you get off the high speed runs. Rain, or just wet roads, really keep the tire cool. But summer or winter, 40° over ambient for my GYTT rear and 30°for my BT-45 front.

I recently heard some other person talking about nitrogen and it's benefits. They thought that the pressure also would not increase with temperature. Umm.... sorry, there is a basic law of physics here.

 

[wjbertrand]

Dry air and dry nitrogen will expand with heat increase following the exact same curve (air is ~79% nitrogen after all). The ideal gas law applies and the chemical composition of the gas phase has no bearing. As pointed out above however, water which can change phase from liquid to gas, can change the quantity of gas (as vapor) and cause measurable differences.

The process of separating/purifying nitrogen from the atmosphere has the effect of making it very dry, herein is where the advantage (though tiny) lies.


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[Mellow]

I don't really care that much about temperature w/tpms.. yeah, it's interesting but if you look at it too much you start worrying about it when it's really a moot point and has probably always been a high temp in relation to ambient temps and the friction being applied.

I use a tmps for 2 reasons and that's it:
1 - let me know how much air I have 'cold' before I ride
2 - alert me if one of my tires drops below a certain level quickly.

Nitroge? well, that's just crazy talk..

 

[cdnemsguy]

Helium...now we are talking. That would make the bike lighter, actually counteracting gravity as it is lighter than the air in the tires, and therefore better mileage as less surface area and friction on the ground. Might be some issue with traction and cornering tho? Also if we get bored around the campfire, we can take a hoot off the valve stem and be funny for a few seconds like Donald Duck.

 

[Mellow]

Helium...now we are talking. That would make the bike lighter, actually counteracting gravity as it is lighter than the air in the tires, and therefore better mileage as less surface area and friction on the ground. Might be some issue with traction and cornering tho? Also if we get bored around the campfire, we can take a hoot off the valve stem and be funny for a few seconds like Donald Duck.

Yeah but... would the engine sound like weeeeeeee, weeeeeee

 

[dwalby]

helium molecules are very tiny, and leak out easily, so you'd be topping off your tires every day to keep the pressure set where you want it.

 

[wjbertrand]

helium molecules are very tiny, and leak out easily, so you'd be topping off your tires every day to keep the pressure set where you want it.

Maybe. Permeability has two factors though. First is simple diffusion as you suggest, the size of the molecule vs. the size of the pores in the gas barrier material. The other factor is solubility of the gas into the barrier material. For example CO2 is a larger molecule than either N2 or O2, yet it permeates rubber much faster than either of the two smaller molecules. Besides diffusion, the CO2 has the ability to more heavily saturate the material, thus moving through and escaping via an amplified, additional mechanism than just simple diffusion. Without researching the solubility in rubber and comparing molecular, or in the case of He, atomic radii to the other gasses one can't be sure. A cool experiment would be to fill rubber balloons with different gasses and see which one deflates the fastest.

 

[STGuy]

I believe there is plenty of air around the rear tire no matter what. The rear tire is your drive tire and does more work therefore more friction when going in a straight line. The front wheel just rolls along and is not "pushing" against the road to make you go down the road. Reason why you might change the rear out twice as much...?

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[dwalby]

A cool experiment would be to fill rubber balloons with different gasses and see which one deflates the fastest.

did your parents never buy you helium filled rubber balloons for your birthday Jeff, they went flat overnight. That's why they use the mylar balloons now, they're more helium-tight than the old rubber ones.

 

[adamk]

I have an Orange TPMS and had a low battery warning on one of my sensors on my last trip. It happened as I was riding over Snoqualmie Pass in cold weather. The display turned red and it was flashing voltages of something like 2.4 volts if I remember right. I didn't even know the system would warn of low sensor battery levels?? I installed the system years ago so it doesn't surprise me that the batteries are starting to wear out. Anyone know of a good source for replacement sensors?

I was reading a few old threads on this site and there was discussion that Orange was going under and also warnings that the Orange systems were flaky. I've had no issues until now.

 

[Mellow]

I have an Orange TPMS and had a low battery warning on one of my sensors on my last trip. It happened as I was riding over Snoqualmie Pass in cold weather. The display turned red and it was flashing voltages of something like 2.4 volts if I remember right. I didn't even know the system would warn of low sensor battery levels?? I installed the system years ago so it doesn't surprise me that the batteries are starting to wear out. Anyone know of a good source for replacement sensors?

I was reading a few old threads on this site and there was discussion that Orange was going under and also warnings that the Orange systems were flaky. I've had no issues until now.

I had issues with the earlier versions but then Gordon started supplying them and it was a newer version, I bit and mine have been good for a couple years so far. I think you have to buy a new sensor and re-pair it.

https://www.st-owners.com/forums/threads/motorcycle-tpms-tire-pressure-and-temperature-monitor-system-rrr-tools.150463/

 

[Mellow]

Hmmm.. looks like he doesn't sell them anymore...

 

[T_C]

And the new sensors (lower price then then old ones) will not pair with the old head units. (or so he says). You can't buy the old senders... so maybe take a dremel and see if you can release the 3032 from it's epoxy sealed housing?

 

[Mellow]

And the new sensors (lower price then then old ones) will not pair with the old head units. (or so he says). You can't buy the old senders... so maybe take a dremel and see if you can release the 3032 from it's epoxy sealed housing?

Yup, maybe be a simple as that.. I didn't know what batteries they used but might as well given they are throw-away otherwise.

 

[wjbertrand]

Seems like they are still available. I got a couple new sensors from him earlier this year:

http://www.rrrtoolsolutions.com/articles/motorcycle-tpms-tire-pressure-monitor-system/

I couldn't figure out how to order directly from the website so I emailed him. I bought my system from him originally as well and he sent me two new sensors for free!

 

[Mellow]

Seems like they are still available. I got a couple new sensors from him earlier this year:

http://www.rrrtoolsolutions.com/articles/motorcycle-tpms-tire-pressure-monitor-system/

I couldn't figure out how to order directly from the website so I emailed him. I bought my system from him originally as well and he sent me two new sensors for free!

How long ago? I did the add to cart and it said no longer available.