Tire pressure alarms

[Roby]

The local TV news announced that all new cars sold in US after
this summer would include built-in devices to warn about low
tire pressure: more automotive electronics. I found a paper
by NHTSA that describes the two techniques for accomplishing
this. Interesting reading. I decided to either buy a new car
right now, or wait a few years until this stuff is either
improved or eliminated.

Have a look for yourself:
http://www-nrd.nhtsa.dot.gov/vrtc/ca/tpms.htm

and this report linked there:
"Examination of Existing Tire Pressure Monitoring Systems"

 

[Roby]

The government is clearly out of control. The reports points out that
if the tires are not perfectly matched these systems will falsely
trigger based on rotational speed mismatches. The same is true if you
are running chains or just spin the tires a lot on slippery surfaces

That's the solution that compares effective radius of tire pairs by
differencing the wheel pulsetachs (already there for antilock brakes).
It's appealing because there's no added hardware (except display).
But it alarms only in response to wheel pair speed differences: it
would be perfectly happy with all four tire running at 5 psi (or
totally flat)!

I got the impression that the industry was leaning toward the other
scheme: radio telemetry from pressure sensors, either inside each
tire or screwed onto the existing valve stem. Battery powered, with
individual codes for each sensor so that the main computer knows which
tire is low. This also avoids interference from nearby vehicles having
similar blessings. Sensor replacement (e.g., dead battery) requires
a visit to dealer who has the special box that tells the main computer
about the new code. The dealer also has the stuff needed to unmount,
remount and balance the tire if you have the sensor-inside solution.
Bring money.

Oh yes, one reports cautions that these tire pressure monitors are not
intended to warn about blowouts. I'm glad they told us.

 

[Roby]

Sensor replacement shouldn't occur very often - battery life on these
systems approaches 10 years. The bigger issue is when you get your
tires replaced and the mechanic breaks one of your sensors because he
hasn't seen that specific type before. It's a mess, but since the
systems are mandated by law we're stuck. And by they way, it's
illegal to knowingly break of diasble the pressure monitoring system
too...

How would the authorities know? In this state at least, many counties
have no vehicle inspection (it's mandated by EPA air quality). I can't
imagine the system disabling the car until the sensor is fixed, and
without some form of enforcement, a lot of broken systems will remain so.

 

[Paul Hovnanian P.E.]

the lawyers told them to add that to the product announcement (c:

Do you think that'll cover their butts in a civil case? There are
directions in every owners manual that I've ever seen cautioning owners
to check tire pressures or risk hazardous conditions. Nevertheless,
someone gets hurt due to a blowout and there's a lawsuit.

Now, cars are equipped with a 'warning system' that doesn't warn about
all possible hazardous conditions. Owners think they don't need to worry
any longer and forget about checking the tires. Now the auto company
gets sued for lulling divers into a false sense of security.

One condition that worries me is that the wheel speed sensor technology
won't catch a condition involving all tires going flat at the same rate.
Owners STILL have to check pressures manually.