How'd My i3 Know The Tire Was Flat?

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eNate

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I left work last night, unplugged, strapped in, started up and #BONG# Low Tire Pressure indication pops up immediately.

Sure enough, right rear tire was dead flat.

So question #1 is how did the system know this? Typically these systems require the wheel be rotating for them to function. I didn't have any low pressure warnings when I arrived at work. How did it figure this out while the car was parked?

Question #2 isn't a question, it's a gripe: if iDrive suspects my tire is low, why isn't it pushing a notification to my Connected app? BMW is missing the utility of this service. It would have been preferable to deal with this earlier in the afternoon, not 11 pm.

Final question: why does the i3 take such an awful long time to reset the tire pressure system? I aired up, couldn't locate an obvious puncture, so decided to chance the drive home. I reset the tire pressure monitoring, and it took three miles to "recalibrate" and give me a pressure readout.
 
I had a drivers side rear flat the other day. I got in the car and drove about 2 blocks before the car warned me or maybe it just took me 2 blocks to notice it. The tire was very low but not completely flat.

TPMS is done with internal tire pressure sensors so when the car starts it does a quick diagnostic and that would have triggered the warning.

In some areas, TMPS is achieved with tire rotation but I think all the US i3's came with internal sensors (my 2015 was imported from California and has them). In the last few years BMW starting installing them in Canadian models so when i bought my winter wheel set, they came with internal sensors as well.

Our Mini uses tire rotation and it is a 2013 model.

I parked the i3 and drove the truck to work. Then took the wheel to a tire shop for a patch after work.

When reseting the pressures, it took about 10 minutes for driving around to reset everything.

I don't know how often the app pushes alerts to your phone but I can't see it being very informative for such a situation. It might tell you there is a flat if you log into it after the fact.
 
My understanding of TPMS transmitters is they are activated by centrifugal force, so they only switch "on" when the car is in motion. That's why the timing isn't lining up for me.
 
Oh that makes sense. I guess it would need a on/off mechanism to save the battery. I could not find anything concrete on the interweb explaining the start stop process of the sensors. Perhaps the car sends out a wake up call to the sensors and when parked after your trip they just go to sleep after a certain amount of time being inactive.
 
But that also gets into my final question, why the long recalibration period? Maybe, one activated, the sensors only transmit an update every few minutes, and the car waits to receive a series of transmissions before validating / displaying the results?

That's unlike others I've driven that seem to update almost realtime (Chevy Tahoe @ work), or my Ford Flex, which resets instantly, but doesn't display pressures. My former Audi and my wife's Volvo use the indirect TPMS method that monitors wheel revolutions to predict pressure loss, so I get why those took a little bit of driving before they would function correctly. But even they are both quicker to reset than the i3.
 
Some systems have a receiver at each wheel. BMW tends to use one receiver, so it must use logic to decide which result is which at each wheel location. That can take some time. The sensor tends to broadcast at one point in the revolution, but those timings will be random, but change as you go around corners and one wheel or the other takes a longer path. So, it somewhat depends on how straight the road is and how long you drive to get the sensors broadcasting enough and not talking over each other to determine which one is which. You could probably speed it up if you were to drive in a big circle in a parking lot somewhere as each wheel would be rotating at different rates, and getting unambiguous reports would happen sooner. So, it has to sort out the reports, make a decision on where it's located based on the course taken (steering wheel sensor?) and register the 'normal' pressure. After the reset has completed, once it knows where they are, it can just listen until it's heard from each corner, and that can be quite quick.

You only need to reset the TPMS system when you decide you want to use a different pressure, or you move or change one (like rotate the tires or swap out a winter/summer set). Otherwise, all you need to do is to top the thing up. The logic looks for differences from where you do a reset to decide if a tire pressure is low. IT will retain that value for each location until you reset them, so no need to reset when topping up. It's looking for a delta from the remembered value. The TPMS broadcasts an ID and the pressure it sees...the car does the rest. Reset just tells the car to look for and assign each wheel with a particular ID of sensor and what the 'normal' pressure should be.
 
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