Measuring tire pressure at the pump

Sunday 1 November 2009

I had an idea the other day for a way to measure a car’s tire pressure that wouldn’t require any action on the part of the driver. Think of it as a remote passive tire gauge. The problem is, I don’t know if it would work.

The idea is to put a plate in the ground at gas pumps. The plate would have sensors that measure two things: the weight of the car, and the area of the tires’ footprint. Dividing the weight by the area gives pounds per square inch, the inflation of the tires. For more useful information, the plate can be divided in four to measure each tire independently.

Will it work? I know the footprint of the tire increases with weight. Imagine putting a large load on top of your car: it will settle lower, flattening more of the tire against the ground, increasing the footprint at the same time that the weight increases because of the load.

But is the relationship direct enough to make accurate measurement possible? One co-worker argued that low-profile tires are much wider than high-profile tires, and so have larger footprints, even with the same inflation pressure. I think it’s possible that the structural support of the tire itself (that is, the rubber) makes the measurement useless. A large portion of the tire’s response to the weight of the car is not related to the air in the tire. Imagine a completely flat tire: it doesn’t have a footprint large enough for the division to come out to 5psi, for example.

So, is this a useful idea? Does it get us any part of the way to alerting drivers that their tires are low? I think it would be useful to tell people that without them having to check the pressure themselves.

PS: if anyone out there builds this thing and becomes fabulously wealthy, I expect a cut!


The DOT already put federal laws in place to require auto makers to add this dashboard light,

And I don't think a passive tire/ground patch sensor + weight could tell you if things were over or under-inflated without knowing the dimensions of the tire (that's the R number on tires like a R15 215/50. I think it might work if you knew the stock tires for a model and could put that into your equation, In that case you would not even need to know the weight of the car, just what the contact patch size should be.

Probably cheaper to just use the already in-place ABS sensors :)
Also, there is differing elasticity to various types of rubber.
I graduated earlier this year. One of the teams doing senior projects for fall 08 built something like this. I don't know all the details, but they used a grid of active piezo sensors. I assume they calculated the area of the footprint by checking how many of the sensors were deflected, and the vehicle weight by the degree of deflection (on average, or something).

They had a video where they drove a car onto the thing and the pressure showed up on an LCD, but they only tried one car in the video so I don't know how well it worked (they might have just fudged it to make it look good for the video).
One problem right off the bat is that tire pressure changes depending on temperature. Higher temperature means higher pressure. In the United States air temperature differences in the cold parts of the country between summer and winter are enough to account for ~5 psi fluctuations in tire pressure.

The pressure is also affected by the warming of the tires as they're driven, the ambient air temperature as the sun heats up during the day, and probably variations in specific rubber materials used. If you park your car in an attached heated garage, for example, and then drive out into a bitter cold New England winter day, you will quickly lose some tire pressure. I have seen the difference estimated to be around 1 psi per 10 degrees fahrenheit.

I think these factors would be very difficult to detect in your system and probably significantly impact its accuracy.
@David: it's good to know that soon (eventually?) cars will all have these warning systems. I didn't find in the link when we could expect all models to have the lights, and I noticed that the system was to tell people their tires were "significantly" under-inflated. If I'm supposed to be at 32psi, will it light up at 30? at 25? at 20? I hope it will be at the higher end.

@Brandon: good to know I am not crazy, and at least some people are pursuing similar ideas.

@Jesse: it's true that temperature affects pressure, but that doesn't affect the feasibility of measuring the pressure, just the significance of the number measured. Any tire pressure system (including the ones that David Pitkin's link say will be present in all cars) would have to deal with the fact that the value they are measuring has natural fluctuations.
Also, the variation in temperature is a problem with a normal tire gauge, too, and probably a big reason why people have chronically under-inflated tires. If you only measure tire pressure at the gas station or in your driveway after driving home from work, you are always measuring them "warm." Most owners manuals list a recommended cold tire pressure. Depending on conditions, these two measurements could vary by 10-20%.
@Ned Oh, I see! The surface area changes of the tire will implicitly account for changes in temperature and thus pressure.
My rx8 has a tire pressure monitoring system. It doesn't warn me when all the tires are low, but evenly inflated. It warns me when the tires have different psi in them. I think under inflated tires is an efficiency issue, but having the tires at various psi is a handling / safety issue.

I've had it come on a few times for me. It is sensitive enough to detect a 1psi difference. It lit up when I had 3 tires at 32 psi and 1 tire at 31 psi.
I think you're forgetting that you can have different types of tires on the car.
When I take mine for example (and I have no clue how you measure tire sizes in the US) I have 195/65 R15 and 205/forgot it.
Well, the 195 are normal tires and the 205 are low-profile one.
So would I be required to specify weight/type of vehicle AND my tire size or would the automaton in question guess from some point?
If I just imagine keeping the database up to date or a UI for people to input their values.. no thanks :P
So what happens if you drive onto mud and that makes your tire look fatter? There are also so many tire models - sometimes I think they make so many so that that they can advertise the low end at $55 but when you actually go to get a tire it turns out your size costs $99.

True story - drove my 3 year old Nissan Altima to TireTown to get the tire changed. Guy says to me I don't have your size I need to order it in. I was stupefied that a company whose sole business is changing tires would not have a Nissan Altima tire in stock. So I guess tires do come in many sizes.
I just want an mpg gauge in my car, then I should be able to tell when my tires are flat implicitly. And if they're not flat, I know to go looking for causes elsewhere...
Here's how you do it: You take continuous measurement. You get the size of the tire when the car comes to a stop. You get the size of the tire when the driver alights from the car. You also get the weight of the driver as they walk to the pump. You compare the shrinkage in tire and the weight of the person and do all sorts of hard math, and figure out the pressure of each tire. Use an electric eye to get the height of the driver, and as an added bonus the customer's receipt will include their weight, height, and BMI!

All bets are off at full-service stations, unfortunately.
People are lazy - simply alerting them that there tires are underinflated isn't enough. After all, they already have an incentive to keep their tires pumped up (better mileage, saves $). I suspect that the real problem is that it's messy and slow to inflate the tires, if you can find a pump at all. Put the same effort required into making this gadget into a device which automatically inflates the tires - and retire a wealthy man.

The tyre maker's recommended pressure is for the air inside the tyre. The differences in characteristics between all the different possible combinations of tyre and vehicle would make this a major project, however, if every vehicle and tyre manufacturer joined in, why not.

OTOH - perhaps you could learn more by measuring the braking characteristics of the vehicle as it comes to a standstill. I suspect that you'd be able to find some metric that is independent of the tyre/vehicle combination. You'd simply be looking for vehicles that were too sloppy, or too taut.
Sylvain Galineau 8:39 PM on 12 Nov 2009
My car's tire gauge is generally pretty good. Apparently, the sensors measure minute difference in rotational speed between the wheels to determine whether any wheel(s) is/are deflating compared to the others and the baseline you initially set up as 'inflated'.

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