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Discussion Starter · #1 · (Edited)
I was experimenting with the TPC this morning and had a few observations and questions.
I understand that the readings on the dash have been adjusted to display pressure relative to 68°F. This morning my garage was 40° so, using the estimation of 1 psi per 10° from 68 I adjusted the pressures down 2.8 psi from recommended pressure using an accurate Intercomp oil filled gauge. I then went down my driveway and got up enough speed for the sensors to turn on. They each indicated 2 psi lower than the gauge or about 5%. After driving a few miles each wheel went up about 2 psi as indicated on the dash. Looking at other posts on the subject, this behavior is about normal. It's pretty obvious that these sensors shouldn't be used to accurately monitor pressure as I had hoped. They clearly are intended as a safety device to alert the rider of a loss of pressure.
I'm curious why they only activate when the wheels are turning. Is there some centrifugal force switch or does the control module turn them on when it senses wheels are up to speed? Do these sensors have batteries and they turn off to save power? If battery operated, what is the expected life? Just curious...
 

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I'm curious why they only activate when the wheels are turning. Is there some centrifugal force switch or does the control module turn them on when it senses wheels are up to speed? Do these sensors have batteries and they turn off to save power. If battery operated, what is the expected life? Just curious...
Yes, Yes, and depends on life when assembled and installed and how much time spent actually riding. The batteries deteriorate over time and IMHO tend to take longer to activate and eventually die, leaving a "---" on the dash in the position that failed.

Batteries are generally not replaceable (I tried and failed) but others have had success. They are expensive (~$175 each).

I got maybe 6 years out of a pair covering about 75K miles if I remember correctly.
 

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Your OEM TPMS sensors (part of what is called a 'direct sensing' system) do not transmit data if the tire isn't rolling; this is to extend the service life of the sensor battery, as pressure data really isn't needed while the vehicle is parked...

Typical direct-sensing units have a service life of around 5-10 years, depending on the sensor type, battery type and hours of operation...



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This is my first bike with TPMS and it sure is nice not having to check your tire pressure before each ride (daily sometimes). I would replace them just to save me the hassle especially if I get more than 7+ years on a unit, jmho. Of course, it's a lot easier now with the horizontal valves to check and air tires than before.
 

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Also remember that just because the system is set up to show you pressure at 68 degrees, the proper tire pressure doesn't change and is set at the current ambient temperature (with cold tires). In other words, if you set it properly at 40 degrees, it will appear to be high if corrected to 68 degrees. This is not an issue.
 

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Discussion Starter · #6 ·
So to make sure I did it right, if the recommended pressure is 40 psi, that would be at an ambient temp of 68. Therefore if ambient is 40, the pressure should be lower when checked with an external gauge, right?
 

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So to make sure I did it right, if the recommended pressure is 40 psi, that would be at an ambient temp of 68. Therefore if ambient is 40, the pressure should be lower when checked with an external gauge, right?
No, the recommended pressure is the same at any ambient temperature. You don't adjust what you put in the tire because it's colder or warmer out. That's the whole thing and what makes the adjustment done by the computer ridiculous. It should simply show the current tire pressure and let us deal with the fact that it will read higher when the tire warms up.
 

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Discussion Starter · #9 ·
No, the recommended pressure is the same at any ambient temperature.
With the greatest respect, I don't think that's right. Let's forget about the sensors and just look at the actual pressure inside the tire. If it's 40°F outside and I fill the tire to 40 psi, when the outside temp rises to 68° won't the pressure in the tire increase by 1 psi for every 10° or a total of 2.8 psi? The attached image shows a graph of temp vs. pressure.
 

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No, the recommended pressure is the same at any ambient temperature. You don't adjust what you put in the tire because it's colder or warmer out. That's the whole thing and what makes the adjustment done by the computer ridiculous. It should simply show the current tire pressure and let us deal with the fact that it will read higher when the tire warms up.
Agreed. If I'm riding at 40 degrees, WTF do I care what the tire pressure would be at 68 degrees? The same if I'm riding at 100 degrees.

This "correction" for the display is unnecessarily complicated and confusing.
 
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With the greatest respect, I don't think that's right. Let's forget about the sensors and just look at the actual pressure inside the tire. If it's 40°F outside and I fill the tire to 40 psi, when the outside temp rises to 68° won't the pressure in the tire increase by 1 psi for every 10° or a total of 2.8 psi? The attached image shows a graph of temp vs. pressure.
You are correct but if you're riding in ambient 40 degrees you're riding in ambient 40 degrees and not ambient 68 degrees. That's why you need to adjust your tire pressure during the year to accommodate the change in pressure due to ambient temperature. The fact that the bike shows you pressure adjusted to 68 degrees makes little sense when you're not riding with an ambient temperature of 68 degrees. Perhaps if you live where the ambient temperature doesn't deviate much from 68 degrees it's ideal but many of us live elsewhere and temps can vary a great deal so we must adjust the pressure accordingly and the display on the bike doesn't do much to help with that unless you want to do the math on the offset from the display (which is hardly worth the effort when my handy pressure gauge shows me exactly what I need with no offset required).
 

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Discussion Starter · #12 · (Edited)
You are correct but if you're riding in ambient 40 degrees you're riding in ambient 40 degrees and not ambient 68 degrees.
Oh yeah, now I get it. Thanks for setting me straight. Makes perfect sense now. As should be obvious, I have a tendency to overthink things. Anyway, I also agree that the best thing to do is use a good pressure gauge and only use the TPC as a safety device.
 

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I decided to see the difference in pressures before and after a ride to work. 40 miles to work at mostly highway speeds on a warm day. I started out at 42 psi on the rear tire. Arrived at work and I was up to 46 psi. I don't recall the front tire pressure but I started at 38 psi back then.
My onboard display read 42 psi when I arrived at work. Difference of 4 psi. So does the computer facture in OAT or actual tire temperature when adjusting the display to read 42 psi instead of actual pressure of 46 psi?
 

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I used to follow the algorithm of considering ambient temp to come up w/ the correct 'cold' tire pressure AND then considering what the ambient outside temp will be WHILE I AM RIDING, which of course varies quite a bit not only from tire warming from riding but from sunlight hitting the tire and so forth. But something puzzled me: the TPMS readout was quite far off in doing this method, in fact if for example ambient temp was 48F, and the bulk of my riding that day was say 58F, I would manually set rear pressure w/ my analogue gauge at: 42psi - 2psi offset + 1psi, therefore 41psi. What I would then find reported in TPMS was 41 psi while riding. But then I realized that since BMW's TPMS is already temperature-compensated there is no need to go thru this calculation. Since realizing this I have discovered if I just manually adjust to my preferred pressure regardless of ambient or riding ambient temp I have perfect reporting on the dash: I'm always showing 42/40 after the tires are road-warmed, which is what I set them to, it's always spot on unless big elevation changes impact this, which of course it does.
 

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You don't need to consider ambient temperature at all and that's the point. All you need to do is to fill your tires to the recommended pressure when the tires are cold (or more precisely, at the current ambient temperature). The warming affect on the tires from rolling resistance and the coincident rise in tire pressure are already taken into account by BMW and the tire manufacturer. The on-board computer being set up to display the pressure at 68 degrees when you may be riding with ambient temp's in the 20's or 90's makes it of little value other to let you know when you've got a leak or have picked up a nail/screw/whatever in one of your tires. That's at least something and I'm glad it's there for that (has actually alerted me to such an incident in fact shortly after I got my RT). Just don't use it as your only guide to proper inflation pressure.
 

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Pressure is all relative anyway. Put in 40psi at an altitude of say 10,000 feet. Ride down to the sea and then check your pressure and see the difference. Also it varies hour to hour as the local air pressure goes up and down.
Tire pressures are never going to be precise so close enough is good enough.
 

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I put in the desired tire pressure while tires are cold without regard to ambient temp and ride. The TPS usually shows whatever my gauge said after riding a few miles within 1 lb. I don't get all of the unbelievably complicated stuff being talked about. I then just note if the numbers change drastically, which happened once when I had a leak. They usually stay very consistent. BMW knew what they were doing. Just forget about it.
 

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I don't get all of the unbelievably complicated stuff being talked about.
Not hard to get--the reason it's non sequitur here is that this TPM readout is temperature compensated. This has value because you then have an valid means of differentiating air loss as the source of decreasing TPM reported pressure in the tire versus temperature-induced pressure change. I do like the fact as it turns out TPM readout is always spot on the recommended values which seemingly suggests my tire gauge is calibrated the same as TPM is. Great gauge, Flaig, made in the Fatherland.
 

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Timely thread, I was just going to ask about why the TPS sensors would be reading "----", this is my first bike with these units in them. Thank you for the great explanations.
 

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I put in the desired tire pressure while tires are cold without regard to ambient temp and ride. The TPS usually shows whatever my gauge said after riding a few miles within 1 lb. I don't get all of the unbelievably complicated stuff being talked about. I then just note if the numbers change drastically, which happened once when I had a leak. They usually stay very consistent. BMW knew what they were doing. Just forget about it.
You hit that right on the head.
Funny you mentioned about numbers changing. It happened to me last week on the way home from work.
My rear tire was reading low after a few miles and started to drop slowly. When I realized I was losing air I had time to plan for a safe place to pull off the highway at night. I plugged it up and back on the road in about 10 minutes. The TPMS system gave me a heads up and time to look for a safe place to pull off the road. Priceless option.
 
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