How do tyre pressure sensors work?

[Old Techo]

Another handy item from Quora.

Q is in English but A is Yankee...


How do car tyre air pressure monitors work?  There appears to be no electronic connection from the wheels yet my car is able to inform me when a tyre loses pressure.

There are one of two ways:

Each tire has a tire pressure sensor mounted inside on the rim. This is usually on the end of the valve stem, but sometimes it's secured to the middle of the rim. It's always inside the tire where you can't see. These little sensors have their own batteries that typically last up to 7 years. Like most batteries though, that's not a guarantee. These sensors are usually activated by the centrifugal force created while you drive. Once activated, the sensor detects the pressure and emits a signal to the vehicle computer. The computer then receives the signal and monitors it. If it gets below a certain threshold, the computer will then turn on the tire light. Some vehicles even display the current tire pressure.

For the second type, there technically IS an electrical connection to the wheel. All vehicles have what's called wheel speed sensors, they do what the name implies, measure wheel speed. There are many designs out there, so I'm going to be basic in describing it. Picture a gear, called a tone ring, this gear is typically located on the end of the axle or in the wheel bearing, regardless of where it is, it is spinning with the wheel.

Now, there is a magnetic sensor that is stationary, mounted right next to the tone ring. As the tone ring spins, the teeth on it pass the sensor. This creates a electronic signal that the computer uses to calculate the vehicle speed. How does this system relate to tire pressure? The wheels on this type of vehicle do not have tire pressure sensors. Instead, it uses the wheel speed to detect if there is a possibility of a flat tire. When there is a tire that is low on air, it creates more rolling resistance. When this happens the affected wheel is spinning a little bit slower than the other wheels.

The computer sees this and then turns on the tire light.

However there is a huge downside to this system. Most people tend to replace tires as needed, or even not at all. When you mismatch tires, in any way, the computer may look at the different signals and say that the tires need air.

Some examples: more than one brand of tire, having tire(s) where the size is different than the other tires, having brand new tires mixed with bald tires (even if the tires are the exact same size and brand), or any other combination of these.

[GerryP]

Mmmm... for the second type mentioned, using wheel speed sensors, I fail to see their logic with "When there is a tire that is low on air, it creates more rolling resistance. When this happens the affected wheel is spinning a little bit slower than the other wheels."
.
I would have thought that a flat or partly deflated tire would have a smaller 'effective' diameter, thus making it spin faster. The difference in speed can still be used to activate an alarm of course, but I'm wondering if they have their reasoning a bit cocked up?

[jr]

I think your right Gerry, a friend of mine has a BMW with the rolling diameter system and it is just measuring the speed difference to decide if the wheel rolling diameter is less.

I have been using the system that has a transmitter screwed outside onto the tyre valve. It is now six years old and works well. The battery usually lasts about three years.


JR

[Old Techo]

Fellas.... I don't agree.

I prefer to refer to the rolling radius as the bottom half is always smaller the the top half.

However neither has that much to do with the circumference, rolling or static.

Your comments could be relevant to the old textile tyres but these days all tyres are steel belted. The circumference is fixed.

Being fixed one rev will travel the same distance whether fully inflated or a bit flat. Sure, the lower radius changes but not the distance around the circumference.

At a stretch of my imagination I can sort of see how a deflated tyre wheel may rotate fractionally slower by tread shuffle.  I mean the bits of rubber between the steel belt and the road.

A bald tyre would not have tread shuffle so I reckon such a deflating tyre would not be detected.

[GerryP]

With a deflated tyre the footprint touching the road becomes longer, so does that mean it travels further? I think not.

I guess there's only one way to know for sure... now there's a challenge OT. 😀

[GerryP]

The circumference is fixed, but not the shape of the tyre or the rolling radius due to tyre deflection.

[jr]

Well don’t agree OT but that is not unusual. I know that the BMW uses the flat tyre more revolutions principle for a alarm of a deflating tyre.

I see it a bit like a bulldozer caterpillar tread very long tread but drive wheel small diameter and needs to go faster.


JR

[Old Techo]

JR,

The track concept is quite different.

The drive wheel (sprocket) walks around the inside of the track and takes quite a few sprocket revs for one track rev.

A wheel does not walk around inside a tyre. One wheel rev is always one tyre rev.

The steel belted tyre is like your track in that it can change shape over undulations (or when flat) but its circumference never changes just like the track never changes length.

[A'van]

In Denmark, when you get new tyres, you got to have them. Maybe that will be like that here as well one day.

I believe that some off the American army Humvee's can also adjust the tyre pressure just by pushing a lever. 

[Jack Driver]

Guys,

Have a look at www.vehicleservicepros.com/service-repair/diagnostics-and-drivability/article/21252922/direct-versus-indirect-tpms-how-they-work-and-how-to-diagnose-them

Cheers,

JD

[jr]

From JD’s article it looks like if pressure drops the tyre circumference will reduce a little, which is enough for the car to see a rotation variation.
With steering angles and tyre frequency and the rest just to complicated.

Having the simple transmitter on the outside is starting to look good as if I rotate my tyres I just move the transmitters no need for any programming.

JR

[Old Techo]

Thanks for that JD

The only way that a circumference change is possible is if the steel belts stretch under inflation pressures and I never considered that likely.

So my theory is wrong and the mystery remains per the original Quora explanation

[GerryP]

The way I see it, it's the distance from the road to the axle. With a flat tyre that distance is smaller so the wheel needs to do more turns exactly the same as having a smaller wheel.

[GerryP]

The more I think about this, the more confused I get. I do get your thinking OT and it kinda makes sense, but not totally convinced.

Can somebody out there go and let a tyre down and compare the distance travelled to a fully inflated one..... Please! 

[Jack Driver]

Another interesting discussion.


physics.stackexchange.com/questions/178235/does-my-too-low-tire-pressure-affect-the-speedometer-of-my-car