Seems to me that the pressure monitor on the bike is more of a fancy warning light than a accurate reading of the "tire pressure" as we have all come to understand. I was always taught to check the tire pressure when the tires were "cold" meaning before you began to ride. And the TPMS on the bike gives a warning signal only if it reads a figure plus or minus 5 psi I think.
I don't use the TPMS to determine accuracy of the tire pressure. I use a gauge before I ride.
[QUOTE=105258;809133]I took the bike to the dealer who confirmed the issue. BMW's response was the same as the fuel strips, the guage is not expected to be accurate, but is meant to be a warning for a tire going flat. They never did address the issue of the apparent sudden change in accuracy of the sensor.
For such an expensive bike you would think they would be better than that. Why would they put the readings in the computer and not just use an idiot light.
Many cars do that, but then you know motorcyclists ... .
Yes, the point of the system is to advise you of a significant change from the pressure YOU intitially set (within reason). Rather than testing against a guage, you should test whether the change creates the notification properly.
And, it's not an expensive bike ... it's a German bike.
BMW's assertion that the TPMS readings are not designed to be accurate sounds like typical BMW BS. The US DOT test procedure for TPMS on automobiles requires that the warning light illuminate when any tire pressure is 1 psi below the threshold. The same procedure specifies confirmation of pressures with an accuracy of 0.5 psi. Therefore, the systems need to be fairly accurate.
While the test procedure in not applicable to motorcycle TPMS, the BMW TPMS manufacturer, Schrader, is the largest manufacturer of automobile systems, so I would expect the same accuracy. Also, the tire pressure setting procedures in the BMW Owners Manuals reads like the TPMS display is spot on. If all BMW intends is a warning for low presures, why didn't they simply install an idiot light like most automobile manufacturers do?
Ride with confidence.
Use a pressure gauge.
And invest in a small 12 volt inflator.
It makes adjustments effortless.
Like the fuel strip, don't rely on the pressure indicator for an accurate reading.
Here is my take on TPM: I always rely on a manual gauge. The one I use is the one that will be less troublesome fitting on both the front and the rear wheels of my RT. There aren't many, and it's the pencil kind. I chek my cold tires, inflate if necessary, and ride on. I take note of the pressures indicated by the TPMs on my OBC, ant that will be my reference fron then on, even if they differ from the gauge to start with. If the indicated pressures change by more than a couple of psi down, I will check the pressure again. Other than that, I don't bother, knowing the TPMs will warn me in time. That's their only useful purpose, not to give exact pressures.
I'd generally agree for the stock BMW TPMs - it's basically a warning system. The BMW readout is temperature compensated.. meaning what you read with your pencil gauge and what reads out on the display will only match at 20C or wherever BMW set their base line. On your pencil gauge - if the tire is warmer then 20C you'll see a higher pressure, colder - lower pressure with your pencil gauge.
One thing I've noticed - since getting a good TPMS on my bike (aftermarket that gives me actual pressures and actual tire temperatures) - I never lose any air. The topping off that used to be necessary almost every time I checked tires before is gone. I can only theorize that the loss of air was involved with the manual checking of the pressure. It takes very little loss of air on a bike tire to drop the pressure 1-2 PSI. Plus since the readout isn't temperature compensated, you're always chasing pressures.
I find a TPMS valuable enough that I went to some trouble and expense to add it to my bike. YMMV - and to each his own, but I'd expect to see this mandated by some safety organization (perhaps in Europe first..) before too much time goes by.