Anything wrong w/ this logic? Setting tire pressure using a warm tire with TPMS

I know the conventional wisdom including in the rider manual you are to set tire pressure w/ a gauge on a cold tire. And also I get the sense people trust a $30 digital over TPM's readout but I'm not sure why considering the tire pressure gauge presumably isn't temperature-compensated. You can ride in ambient temps from 40F to 90F and TPM will display the pressure as unchanged because of the temperature-compensation logic that TPM utilizes.

I can't see why you couldn't get accurate pressure adjustment by adding or removing air using the TPM readout on a road-warmed tire. So let's say your road-warmed front tire pressure reads 36psi, and you'd like it to be 38. Simply stop, leave the ignition on and add air until TPM displays 38psi. Why would this not be the most accurate way to set tire pressure? I stopped the bike once and took a little air out once on a warm tire and noticed TPM did drop down in real time w/o the wheels turning as I left the ignition turned on.

The excerpt below from the rider manual seems to be saying they consider TPM to be the benchmark and adjusting pressure using a filling station pressure gauge requires you to take into account the temperature compensation, no?

I have been through this subject over on AdvRider for several months. Too much of the advice from the elves in the Black Forest makes sense to engineers but really isn't practical.
#1 - You can's set your tires cold and have the TPMS reading come out right.
Why? Because you have to ride the bike to 19mph to get the TPMS to display. It really doesn't settle down for maybe 15 minutes too.
What most people really want to know is what the pressure is in the tire. That isn't what you get in a compensated reading.
How do I know? I attached my GS911 and connected my laptop to get the tire temp and pressure from the sensors. It isn't what is on the display because the display compensates.
What I recommend is set your pressure to an amount cold. If it is 68 degrees then 36/42 like the book says. Then go for a ride for maybe half an hour. Depending on ambient air temp take the numbers off the display, write them down or remember them. Then measure the temp at the tire. If you want corrected pressure, add or remove air to the tire using the difference between actual tire pressure and displayed corrected pressure to adjust the actual tire pressue to give 36/42 on the display. OR if you like what the gauge says either use it or correct it to what you want as an actual pressure IN THE TIRE without any corrections to 68 degrees.

IMO, the manual and the engineers are making this subject WAY TOO COMPLICATED. For the last 50 years I've been riding I've used the 10% rule and so have the majority of riders. I find it totally counter intuitive to make the actual pressure in the tire read at 68 degree temp levels whether it is 10 below zero or 120 degrees.

Please don't bring up PV=nRT. That is an IDEAL gas law.

If everything works as presented in the manual then I agree.

A lot of what I have been reading on the system also seems to support your position.

This is how I plan on setting tire pressure going forward.

I find the roller coaster reading of the TPMS a constant annoyance.

If it had been an option on my bike, I'd of opted out of having it.

KISS - just check your tire pressures with the tires cold, set to your preferred psi and go riding.

The idea of using the TPMS to set tire pressure presupposes that they accurately measure the pressure; I do not think they are particularly accurate. Their intended function is to alert a rider to a deflation and from what I hear/read they do that well.

greenwald said:

I find the roller coaster reading of the TPMS a constant annoyance.

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With the 4 BMWs we have owned with TPM the readings do not jump up and down while riding.

I agree with the post that tpms is there for warning of low pressure - and it has saved me.

My pressure will increase 2psi after the tires get warm - its pretty consistent with that. Since I plugged the nail hole I am checking constantly.

I've always used them as a guide and not accurate PSI. My digital hand held gauge is my guide.

greenwald said:

I find the roller coaster reading of the TPMS a constant annoyance.

If it had been an option on my bike, I'd of opted out of having it.

KISS - just check your tire pressures with the tires cold, set to your preferred psi and go riding.

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Neither my '13 F800GT nor '16 RT demonstrates any roller coaster readings whatsoever. There is alway a road-warmed jump of 2psi, then it's rock solid no matter what the ambient temp. The clearest evidence of that in my experience was leaving Bishop CA at 5:30am at 52F and then riding up to St George where the last 3 hours was 113F! Pressure stayed exactly the same--an acid test for temperature compensation. If I looked at actual rear tire pressure it was probably approaching 50psi.

TPM is fabulous--I'd never want a bike w/o it. Saved me on a freeway onramp as I was accelerating to enter traffic when the flashing commenced, I pulled over and plugged the tire. Second instance was after leaving for work so didn't go far from home only to notice a 5 psi drop from normal for me, so turned around and repaired the flat in the garage.

chewbacca said:

What I recommend is set your pressure to an amount cold. If it is 68 degrees then 36/42 like the book says. Then go for a ride for maybe half an hour. Depending on ambient air temp take the numbers off the display, write them down or remember them. Then measure the temp at the tire. If you want corrected pressure, add or remove air to the tire using the difference between actual tire pressure and displayed corrected pressure to adjust the actual tire pressue to give 36/42 on the display. OR if you like what the gauge says either use it or correct it to what you want as an actual pressure IN THE TIRE without any corrections to 68 degrees.

IMO, the manual and the engineers are making this subject WAY TOO COMPLICATED.

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Gee, the manual and engineers made it way too complicated?

I'm afraid nothing said here does anything to debunk the theory I put forth. The very reason you and others and myself follow the 10% rule is because your pressure gauge isn't temperature compensated! Following the 10% rule makes sense when you are checking cold tires to get you in the ballpark, but in the end TPM takes this into account.

This appears to be more of the same ingrained assumption that a $30 gauge and 10% rule somehow trumps TPM--and I have yet to hear any evidence that assumption is at all valid.

I'd instantly be suspicious of anything that says to set the pressure to 36.3 PSI. Not just too low, but nobody's going to make the effort to hit the decimal.

Thanks, but I'll stick with "set it cold and manually".

Lee said:

With the 4 BMWs we have owned with TPM the readings do not jump up and down while riding.

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I agree with the consistent pressure readings on my system. The pressure increases 1-3 psi after the tires are warmed up. That assumes I am using 40/42 cold pressure. Anything lower and the rise from heating is 3-5 psi.


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Complicated?

I see simplicity, not over complication. I set my tires at 38/44 and the TPS says 36/42. OK, now I know. If the TPS is 36/42 I'm good because I'm really at 38/44. Simple.

Pauls1150 said:

I'd instantly be suspicious of anything that says to set the pressure to 36.3 PSI. Not just too low, but nobody's going to make the effort to hit the decimal.

Thanks, but I'll stick with "set it cold and manually".

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I know that's interesting. I think it must have come out of physics/engineering computer modeling which determined the optimal pressure for some such set of criteria, and they published it out to one decimal place.

The set it cold and manually is ultimately in some ways easier to do since you don't need sync it with stopping a warmed bike tire, connect a compressor, and make your adjustment while TPM continues to display and update. But still, needing to do the math to apply the 10% rule is eliminated, and I'll venture a bet quite a few people either don't know about the 10% rule, or don't bother to apply it. You have to do it when you use a manual tire pressure gauge, whereas you don't if you use built-in TPM.

Pauls1150 said:

I'd instantly be suspicious of anything that says to set the pressure to 36.3 PSI. Not just too low, but nobody's going to make the effort to hit the decimal.

Thanks, but I'll stick with "set it cold and manually".

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I think most people would know immediately that 36.3 is simply a conversion from Bar to PSI, and to round it off to 36. And why would you say 36 is too low? That's what BMW recommends for my GSA, and that's what I set it to. Works for me.

As for TPMS, my bike has it, and I think it's very easy to understand, and use.

I'll say this about temperature compensated tpm's. They are good to let me know if there is a drastic change in pressure like when you have a leak. Which is great and I want that information. I even check mine going down the road to reassure myself a "funny feeling" isn't a tire going down.

Here is my beef with them. They don't accurately tell me tire pressure. If I set the pressure at 36 front and 42 rear ( with a Longacre gauge used on race car tires where 1/2 pound makes a difference and the gauge is calibrated yearly ) the tpm readout won't show that amount. It usually shows 2-3 pounds less in warmer temps. So, If I were to "adjust" the pressure in the tire to make the tpm readout more or less correct, I would be over inflating the rear tire. Current Dunlop has "42 psi maximum cold pressure" on the sidewall.

Just give me the tire pressure like a tire gauge would see it. I have plenty of experience with changes in temperature, speed, and load to understand how pressures act.

Realshelby said:

I'll say this about temperature compensated tpm's. They are good to let me know if there is a drastic change in pressure like when you have a leak. Which is great and I want that information. I even check mine going down the road to reassure myself a "funny feeling" isn't a tire going down.

Here is my beef with them. They don't accurately tell me tire pressure. If I set the pressure at 36 front and 42 rear ( with a Longacre gauge used on race car tires where 1/2 pound makes a difference and the gauge is calibrated yearly ) the tpm readout won't show that amount. It usually shows 2-3 pounds less in warmer temps. So, If I were to "adjust" the pressure in the tire to make the tpm readout more or less correct, I would be over inflating the rear tire. Current Dunlop has "42 psi maximum cold pressure" on the sidewall.

Just give me the tire pressure like a tire gauge would see it. I have plenty of experience with changes in temperature, speed, and load to understand how pressures act.

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Exactly.

Here at the race track, TPMS discussions all end the same way - nobody likes them, and none of the racers would trust them.

Realshelby said:

They don't accurately tell me tire pressure..

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Ahh, but they do and better yet, they compensate for temperature, whereas your calibrated manual gauge doesn't. Whether or not TPM accurately detects the base pressure (pre application of compensation logic) is up for debate since I haven't seen published stats on that. Would not surprise me in the slightest if it was calibrated to +/- 1%, no giant hurdle in 2017.

If you're going to go thru the 10% adjustment for your manual gauge have at it. If you're saying 2-3psi lower are you talking about on a road-warmed tire, which I was, or on a cold tire? I always notice TPM reported pressures will be 2 higher after the tire is fully road-warmed over cold. So if I manually set pressure on a cold tire to 36/42, when TPM first displays it will be 34/40, then moves up to 36/42 when the tire is fully road warmed, and stays there regardless of ambient. Here's what I think: people have misled themselves by not applying the 10% rule and end up being perplexed at how their TPM readout doesn't seem to follow any logic!

If you don't have TPMS and you only check tires in the garage before you ride.....Life is good.

Ignorance + Good looks = Happiness

ncpbmw1953 said:

Ahh, but they do and better yet, they compensate for temperature, whereas your calibrated manual gauge doesn't. Whether or not TPM accurately detects the base pressure (pre application of compensation logic) is up for debate since I haven't seen published stats on that.

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Noel, here is where you are over thinking yourself. Yes, the tpm is programmed to compensate for temperature. If I start out in 40 degree weather and ride into 90 degree weather you might say that matters. But it does not. The pressure in the tire is going to vary a certain amount due to the tire heating up in use. Not as much in cold weather as hot weather and vice versa. If I am riding in 60 degrees and below most of the trip, then my "manual gauge setting will be very much in the range I would find acceptable. Same with riding in 60-100 degrees. With compensated tpm you simply have no idea while on a trip EXACTLY what your pressure is. While the tpm system may use the bikes ambient temperature sensor for outside temps, it has zero ability to judge tire temperature. I have checked my tires while on a hard ride and know about what the rise in pressures are and given a certain starting pressure I know if all looks right. All the temperature compensated tpm systems do is let me know the tire might have the right pressure and shows no sign of a leak. You can state all the charts and graphs out there, but the fact is I cannot glance at the tpm display and know what the actual tire pressure is at a given moment. I like the tpm, I would pay for the option again. But simply as a low tire/leak detector.

The biggest thing you are overlooking is accuracy to begin with. Just how accurate is the tpm monitor? If like MOST tire gauges it isn't all that accurate. I'll take my gauges kind of accuracy to base my tires pressures off of over the tpm system. If you want to see how far off your temperature compensated tpm's are, try them against one of these: Longacre temperature compensated gauge

Realshelby said:

Noel, here is where you are over thinking yourself. Yes, the tpm is programmed to compensate for temperature. If I start out in 40 degree weather and ride into 90 degree weather you might say that matters. But it does not. The pressure in the tire is going to vary a certain amount due to the tire heating up in use. Not as much in cold weather as hot weather and vice versa. If I am riding in 60 degrees and below most of the trip, then my "manual gauge setting will be very much in the range I would find acceptable. Same with riding in 60-100 degrees. With compensated tpm you simply have no idea while on a trip EXACTLY what your pressure is. While the tpm system may use the bikes ambient temperature sensor for outside temps, it has zero ability to judge tire temperature. I have checked my tires while on a hard ride and know about what the rise in pressures are and given a certain starting pressure I know if all looks right. All the temperature compensated tpm systems do is let me know the tire might have the right pressure and shows no sign of a leak. You can state all the charts and graphs out there, but the fact is I cannot glance at the tpm display and know what the actual tire pressure is at a given moment. I like the tpm, I would pay for the option again. But simply as a low tire/leak detector.

The biggest thing you are overlooking is accuracy to begin with. Just how accurate is the tpm monitor? If like MOST tire gauges it isn't all that accurate. I'll take my gauges kind of accuracy to base my tires pressures off of over the tpm system. If you want to see how far off your temperature compensated tpm's are, try them against one of these: Longacre temperature compensated gauge

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An excellent explanation of why the TPMS is nothing more than a 'smoke detector' for losing air. Great job!

I hesitated getting too deep into this discussion, due to the usual 'passionate concepts' that float about within this Forum from time to time. But I think your explanation is top shelf, and the OP should still get a pat on the back for jump starting this dialogue.

I've seen many racers disable their TPMS if they had them, but figured this was a 'racing accommodation.' Then I found out they do the same on their street bikes. No faith in them whatsoever.

Even spoke with some HONDA techs at the track once, when I watched them dismantle the TPMS on a personal Gold Wing rim and learned that while the power source is a battery, it is not serviceable and the entire sensor has to be replaced. They confided their skepticism of the system as well.

Warning that you are suffering a constant or catastrophic loss of air? Sure.

Trust it for your tire pressures?