Need help on precise RPM calculation regarding a car's tire size on a road

[HawaiianPi]

This is a whole lot of hubbub over nothing. The error factor here is not worth the trouble of calculating, not only because it will be relatively small, but also because tires of the same specs from different manufacturers (and sometimes even different models from the same manufacturer) are not all exactly the same. There is some not insignificant variation when comparing tires, so worrying about the small difference temperature makes is, in my opinion, silly.

While tire sizing is not as useless as clothes sizing, one 305/30ZR19 may not be the same as another. Anyone who has spent as many years as I have stuffing big tires under the fenders of little cars will know that (and probably learned it the hard way).

[Imperf3kt]

HawaiianPi, go watch some slow motion videos of what happens to a drag cars tyres when it suddenly begins rotating at 100mph, the tyre lags behind the rim creating a swirl of sorts. The tyre sidewall has no steel belting, otherwise it _would_ tear apart..
At the same time, the tyre diameter expands, and you expect me to believe that this does not follow the laws of physics and increase the circumference?


Lets imagine the difference it makes is 1mm and for arguments sake (because I don't have measurements), the tyre's circumference is 1000mm.
For every rotation at this inflated size of just 0.1% larger, you are travelling 1001mm. So, after 10 rotations, it stands to reason, you have travelled 10010mm.
After 10000 rotations, that will be 10010000mm. An error of 10000mm, aka 1 meter.

Imagine this tyre does 300,000km worth of driving (yeah, its immune to wear, which also shrinks the circumference)
Can you add up how many km out your odometer is?
Might not affect the speedo much, but it'll definitely affect the odometer.

[rpdom]

An error of 10000mm, aka 1 meter.

10000mm is 1000cm is 10m

[HermannSW]

I think a basic problem with this whole thread is the assumption that the wheels always have contact to street. Typically the wheels have some amount of slippage at high speeds, so that the real RPM is higher than that measured by street distance divided by tire circumference.

I assume that you are interested in straight line RPM determination, because with driving curves you will have two different RPMs for both sides. and The car/robot will drift in addition:

slowed down by factor 3.6:


Even if both wheels drive in same direction of circle slippage occurs at high speeds (>50km/h)


You may take a high fps video of the car and measure the (real?) RPM by distance and single frame stepping+counting for time measurement. Pi cameras (v1 and v2) allow for 90fps 640x480 video recording, and PS3 Eye even allows for 187fps 320x240 recording with Pi (Zero).

[HawaiianPi]

HawaiianPi, go watch some slow motion videos of what happens to a drag cars tyres when it suddenly begins rotating at 100mph...

LOL! Drag tires are designed to behave like that. Street tires are not. And you didn't even address my main point. How are you going to compensate for tires that have the same spec's but different dimensions?

[jadatis]

As "pigheaded Dutch Selfdeclared tirepressure-specialist"I google dayly for tire-pressure.
Registered to this forum to give you info.

If the pressure rises , with the same load on a tire , for instance by heating up of inside tire temp, the deflection gets less , and by that the roling-circumference.
I claim to know how this RC can be calculated and made some spreadsheets for it.
What you want is to determine this roling circumference to calculate the RPM for the same speed.
From temp difference in tire you can calc the pressure rising.
From pressure rising you can calculate the new surface on the ground, and for radial tires the length of that surface, because width of surface stays the same.
From new surface length /old surface length you can anproximately say that new deflection/old deflection= ( new surface/old surface( length) ) ^2 ( power 2 so squaire).
Then when you calculate the roling circumference for those deflections , and devide them you can determine the new RPM ( rotations per Minute).
If you mail me at my Hotmail.com adress with same username as here ( like this to prevent spamm), I can sent you the spreadsheets and some explanation.
For you then to make a programm for the raspbery py.