Tire Pressure Control - K-Bikes.com - Excellence in Motion
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post #1 of 28 (permalink) Old Feb 22nd, 2009, 8:56 am Thread Starter
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Tire Pressure Control

My old GT did not have the TPC feature. The new one does, and when reading about it in the owner's manual, I was surprised to see how accurate this is supposed to be.

It seems the readout is temperature compensated, and if I understand correctly, the implication is that this is accurate enough to use as an adjustment standard. They even give an example showing that if adjusting pressure, you should merely add the difference between what the display shows and what the value should be, disregarding the actual readings on the external pressure guage!?

If this is true, then here is my question. When I rode the bike home from the dealer, in very cold outside temperatures, the display showed 46/48. If this is a "temperature compensated," accurate readout, does that mean I should have pulled over, and bled out enough air (even though the tires were now "hot"), so that the display would show close to 36/42???

Normally, I would not bleed air from a "hot" tire, but if the info in the owner's manual is to be taken literally . . . ?

Don G
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post #2 of 28 (permalink) Old Feb 22nd, 2009, 10:35 am
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Quote:
Originally Posted by dongold
My old GT did not have the TPC feature. The new one does, and when reading about it in the owner's manual, I was surprised to see how accurate this is supposed to be.

It seems the readout is temperature compensated, and if I understand correctly, the implication is that this is accurate enough to use as an adjustment standard. They even give an example showing that if adjusting pressure, you should merely add the difference between what the display shows and what the value should be, disregarding the actual readings on the external pressure guage!?

If this is true, then here is my question. When I rode the bike home from the dealer, in very cold outside temperatures, the display showed 46/48. If this is a "temperature compensated," accurate readout, does that mean I should have pulled over, and bled out enough air (even though the tires were now "hot"), so that the display would show close to 36/42???

Normally, I would not bleed air from a "hot" tire, but if the info in the owner's manual is to be taken literally . . . ?

Don G
Don,

I can't answer your question directly, but here are things that I do know, based on actual experience:

1. Each morning, as my tires warm up, the TPC (TPM) readout increases by 1 (rarely 2) psi in each tire. My guess is that the metal wheels and rims are still cold near the TPM, so the TPM isn't getting an accurate reading of the true air temperature inside the tire.

2. According to the dealer, the TPC readout is 3 psi too low in each tire, compared with their calibrated gauge. Apparently a 10% difference is "within acceptable margins" for the TPC readout. I have seen many posts from others on these forums confirming that assertion.

I note that the TPC matches the several cheap digital gauges that I have. This implies that my cheap gauges are also 3 psi low for GT tire pressures. Nevertheless, the dealer insists that their pressure gauge is right, and they see a lot of GTs that have TPCs that read lower than actual.

If true, that implies that the TPC low pressure warning may appear on a cold morning when the actual tire pressure shouldn't otherwise trigger the warning. Others have reported this issue on the forums as well.

3. The TPC has been VERY consistent for me. That consistency allows me to depend on the TPC to check tire pressures instead of the manual approach, which loses air each time I check. A TPC is a "must have" feature for me, especially because I ride nearly every day.

4. Different riders deal with the TPC accuracy issue in different ways. Some make sure that their TPC reads correctly (36/42) even though they know the actual pressures are a little high. Others learn the TPC readout that yields the correct pressures in the tires.

For me, I still wonder why the OBC can't be tweaked by the dealer to give correct TPC readouts.
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post #3 of 28 (permalink) Old Feb 22nd, 2009, 10:48 am Thread Starter
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Well, I do know this. When riding home yesterday, as I said, the TPC showed high pressure. (48/48).

A little while ago, I went out and measured the cold tire pressure with a relatively expensive, supposedly accurate dial guage. The pressure was indeed too high. I let air out to my usual 38/42.

I'll see what the readout (TPC) says if I can take the bike out tomorrow. It's snowing here right now in NJ.

Don
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post #4 of 28 (permalink) Old Feb 24th, 2009, 5:59 pm
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My experience is identical to XMagnaRider's. TPC is temperature adjusted, and does read about 3psi low. I usually keep the TPC reading around 36/40.

The system does respond rapidly, it's really handy when using a portable compressor to adjust the pressure. The compressor is pretty slow, so I just leave the ignition on and watch the TPC readout, turning off the compressor once it's where I want it.

The only gotcha I've experienced with the TPC is when stopping on a cold day. The air in the tires seems to cool a lot faster than the TPC temperature sensor, and so the pressure reads low for a while until everything adjusts. The same thing might happen as the system heats up, but that doesn't trigger the dreaded yellow triangle, so I don't worry about it as much.
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post #5 of 28 (permalink) Old Feb 26th, 2009, 1:25 pm
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[QUOTE=dongold]My old GT did not have the TPC feature. The new one does, and when reading about it in the owner's manual, I was surprised to see how accurate this is supposed to be./QUOTE]

Other posts suggested that the TPC was not that accurate; it was not to be used to determine your tire pressure before you went for a ride, rather, it was just supposed to be trusted when there was a sudden loss in pressure.

How accurate is it?

I didn't order it originally but still have time to if it can be used instead of daily checking the pressure with a handheld gauge.

122 days and counting
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post #6 of 28 (permalink) Old Feb 26th, 2009, 1:39 pm
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It is not very "accurate" in that it does read about 3 psi low. But it is very "repeatable" in that it consistently reads about 3 psi low.

So it is useful as an instant check of your current tire pressure, as long as you know it always reads a bit low. I don't bother with a hand held gauge any more, as the TPM tells me what my current pressure is with a simple push of a button.

And if I do get a sudden loss in tire pressure, I'll get the warning triangle. That saved me once on a "spirited" ride when just as the bike was beginning to feel a bit squirrelly, the warning flashed and I was able to pull safely to the side of the road, where I found the nail that I had picked up.

Overall, it's a small added expense that I definitely think was worth it.

Ken
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post #7 of 28 (permalink) Old Feb 26th, 2009, 2:02 pm
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Hey Ken, how did it end up with the replacement on your TPM? Were they able to do just the batteries, or was the whole tire unit replaced?

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post #8 of 28 (permalink) Old Feb 26th, 2009, 3:22 pm
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Quote:
Originally Posted by grifscoots
Hey Ken, how did it end up with the replacement on your TPM? Were they able to do just the batteries, or was the whole tire unit replaced?
The batteries are soldered and potted in place so you have to replace the entire sensor, then use the MoDiTech computer to match the new sensors to the bike's computer. Dealer-only job, I'm afraid.

My tire sensors were swapped out at 65K, although they had been getting weak for the last 10K or so miles. At the time they were changed, the front would register a pressure about two-thirds of the time, while the rear would read about half the time. So you shouldn't have to worry about it until around 50K miles, which is probably 4-5 years for most riders.


Ken
Pacific NorthWet
'13 Dark Graphite Metallic K16GTLD, 24K miles and counting...
'09 Magnesium Beige Metallic K13GT, 60K miles miles and counting...
'02 Mauve Metallic K12LTC, 106K miles and sold
BMWLT#145, IBA# 366, MOA# 111996, SCMA# 24032

All lower 48 states plus Alaska on the K13GT in two weeks . . .

Some people see the gas tank as half empty. Some see it as half full. All I care is that I know where the next tankful is coming from...

Last edited by Meese; Feb 26th, 2009 at 3:27 pm.
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post #9 of 28 (permalink) Old Mar 22nd, 2009, 3:25 am
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For the record, the TPM on my 2007 bike was still working fine at 97,000 miles when I traded it. The only issue I had with it was the front tire took about 1 - 2 miles to register a pressure sometimes.

My Problem, I got very used to having the system and now need to add it to my current bike, a 2008 K1200GT again, of course.
Does anyone here know if the factory system can be retrofitted to the bike?
Has anyone tried?

Jim
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2009 K1300 GT Magnesium Beige all options, Amazing bike. 110,732 miles... Oops, crashed.
2008 K1200 GT Dark Graphite Starting over. 26,197 miles when hit from rear & total loss.
2007 K1200 GT Dark Graphite Traded 97,000 miles


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post #10 of 28 (permalink) Old Mar 23rd, 2009, 1:56 am
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Quote:
Originally Posted by QuickSilver
Does anyone here know if the factory system can be retrofitted to the bike?
This Saturday I asked this very question of the dealer. The mechanic said that TPC requires a different computer that most bikes through early 2008 did not come with. Before that, the factory only included that computer for bikes with the TPC option. So, depending on when the bike was made, it is possible. However, he also indicated it would be very expensive, more than the $600 safety package that was a factory option.

Personally, I'd do it for $200 or so, but my bike is a late 07 build so likely can't be done. Anyway, at some point it gets too expensive and you ask yourself why not just check the pressure manually.

Last edited by Action_Mango; Mar 23rd, 2009 at 2:59 am.
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