Jack Driver
Seasoned veteran
Posts: 298
Tow/Motorhome: Isuzu Dmax Spacecab MY2015
Caravan: Tailgate slide-on camper MY2020
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Post by Jack Driver on Apr 29, 2023 12:29:56 GMT 10
More from another forum. www.exploroz.com/forum/142627/rolling-radius-vs-circumference-of-deflated-tyres-and-tyre-rpm?p=%2fforum%3fpn%3d45I have (copyright) data for the Michelin tyres that I run on the OKA. It quotes (amongst other things) the relationship between Road Contact area (cm2), Laden Radius (mm) and Rolling Circumference (mm) when these tyres are operated at reduced pressures. Road contact:------------- 493,,,,,541,,,,,599,,,,741. Laden Radius:------------- 428,,,,,423,,,,,417,,, 402. Rolling Circumference: 2820, 2810, 2800, 2760. Conclusion? Michelin say the rolling circumference reduces with reduced pressure at the same load. Cheers, Peter OKA196 motorhome ps the extra commas and stuff was an attempt to line the numbers up
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Post by Old Techo on Apr 29, 2023 13:15:01 GMT 10
JD,
Thanks... but there are a few crap theories in that forum
I insist that the distance from the axle centre to the road is not a relevant factor - if the tyre circumference is fixed.
I now except that the steel belts in a tyre can stretch a bit under inflation pressure but if we assume for a moment that they do not change in length. You could think in terms of JR's track except that it is locked to the wheel rather than revolving around a smaller sprocket.
So the first pic is an inflated tyre of distance to ground = R1 and its circumference = C1
That tyre is then deflated such that R2 is much less than R1 but of course C2 is still equal to C1
Thus one wheel rev travels along the road the same distance in both cases as it cannot do anything different. EDIT: You added stuff JD whilst I was typing/posting but it doesn't affect my post.
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Post by jr on Apr 29, 2023 13:29:35 GMT 10
I agree if the circumstance does not change then the revolutions would not change so a steel belted tyre must stretch at full pressure and contract a small amount as the pressure drops. I agree with your circumference theory OT. As the tyre is revolving at high speed it would not take long for the vehicle computer to detect a difference in revolutions.
JR
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Jack Driver
Seasoned veteran
Posts: 298
Tow/Motorhome: Isuzu Dmax Spacecab MY2015
Caravan: Tailgate slide-on camper MY2020
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Post by Jack Driver on Apr 29, 2023 15:47:37 GMT 10
OT & JR, I can follow the argument that two tyres of the same construction and size (including thread depth) will have the same circumference even if one of the tyres is under-inflated (with a larger contact patch) compared to another which is correctly inflated. Thus both tyres will spin at the same rpm when the vehicle is in motion, providing, of course, that there is no slip between the road and either tyre. Then there is the fact that all the information I can find regarding indirect TPMS is that they are based on the principle that under-inflated tyres have a slightly smaller diameter (and hence higher angular velocity) than a correctly inflated one. The obvious inference is then that tyre circumference grows/shrinks as the air pressure that the tyre is subjected to increases/decreases and is not a constant as we would perhaps otherwise assume.Thus an underinflated tyre will in fact spin at higher rpm than a correctly inflated tyre.
If this wasn't the case then indirect TPMS could not work as these are essentially based on differences in wheel speed as measured by ABS wheel speed sensors. Cheers, JD
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Post by Old Techo on Apr 29, 2023 16:14:21 GMT 10
JD,
No argument from me as I conceded a few posts ago that the steel belting must stretch enough under tension (caused by air pressure) to affect the tyre circumference to the extent that is detectable by affecting wheel rpm.
What I was hammering to death was that varying tyre diameter played no part in the process if the length of the steel belting (circumference) remained unchanged.
I also wondered what wheel RPM difference was needed to trigger a tyre pressure alarm.
When we are caravanning and stop somewhere I regularly check tyre temps using my hand. I always notice the tyres along the sunny side are hotter than the shade side. That will affect pressure.
Perhaps the smarts in a tyre monitor system also look at a rate of RPM change.
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Jack Driver
Seasoned veteran
Posts: 298
Tow/Motorhome: Isuzu Dmax Spacecab MY2015
Caravan: Tailgate slide-on camper MY2020
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Post by Jack Driver on Apr 29, 2023 16:44:05 GMT 10
Perhaps the smarts in a tyre monitor system also look at a rate of RPM change.
One thing to remember about indirect TPMS is that they are only concerned with LARGE air pressure differences of around 25%, which is probably a lot more than would be expected from solar heating.
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Post by jr on Apr 30, 2023 9:20:39 GMT 10
I remember seeing different types of tyre wear patterns and some were due to tyre pressure. Over inflated suggested more wear in the centre of the tread and under inflated more wear on the outer edges so this would suggest the circumference length would change. My TPMS transmitters only sends a signal every five minutes for a pressure update reading on the monitor but if there is a large pressure change the transmitter outputs instantly. JR
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Post by nsgnomad on Apr 30, 2023 10:46:02 GMT 10
I am interested in this too. My Peugeot has a TPMS but it the sensors are not the type screwed onto the outside of the valve stem (a la Safety Dave) nor is it the type fitted inside the rim at the end of the valve stem. The valve stems are the normal rubber type. I guess the Pug uses the second type in OT's article. I have no idea how they work but mine has always shown an intermittant error message "tyre pressures not monitored", although once did indicate a low pressure warning correctly for an underinflated tyre.
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Post by Old Techo on Apr 30, 2023 11:06:08 GMT 10
JR,
Decades ago I considered tyre wear on my GTHO speedo calibration.
When I bought the GTHO new in 1969 one of the first things I did was calibrate both the speedo and the tacho. Both were out due to analogue needle imbalance.
One of them had a heavy needle and the other a light needle. I removed both instruments to carefully balance the needle movement such that it held position in all physical orientations. All analogue meter movements have balance weights at the needle shorter hidden end. Said weights may be a few turns of very, very fine wire and/or a blob of glue.
As a Techo often using Test Equipment accuracy and calibration are second-nature. Also at the time I wondered about tyre wear effects.
Back in those 14" rim days with ER70 tyres the tyre radius (wheel centre to road) was 12 inches. Allowing for tread depth wear of 1/4" that was 1:48 or 2%
So from brand new to bald tyre the circumference variation was 2% but of course over the long life of the tyre.
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Post by GerryP on Apr 30, 2023 12:03:50 GMT 10
OK, I shall concede OT that you have convinced me that you are indeed correct in that it is the circumference that matters and not the distance from the axle to the road. That is also borne out by the fact that the speedo reads the same, irrespective of the pressure in the tyre. I might run say 42 on the highway and 22 on the tracks, but the speedo and the GPS always agree. It was an interesting excercise...
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Post by nsgnomad on Apr 30, 2023 13:49:15 GMT 10
Consider a bicycle or wheelbarrow tyre. When there is insufficient pressure in it, the tyre will creep around the rim, evidenced by the valve stem being displaced from the correct position. Admittedly these cases are a tubed tyre and not steel belted so circumference will not be as fixed in the case of a steel belted radial.
The other thing to consider is if it looking for changes in rpm of the wheel, what happens in cornering. The inside wheels are rotating at a slower speed than the outer. Perhaps there is an algorithm that takes normal cornering into consideration, but might a prolonged corner trigger it?? Perhaps as in JR's set, maybe there is an average over a few minutes.
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Post by Old Techo on Apr 30, 2023 14:00:21 GMT 10
Roger,
Early in this thread I had thought about a creeping tyre but quickly dismissed it. Having changed quite a few I know how damned tight they are and hard to move over the rim safety bead.
I had also wondered about cornering... to the extent that maybe sophisticated cars that also monitor steering wheel position would know when cornering.
All of these things are worth thinking about
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Jack Driver
Seasoned veteran
Posts: 298
Tow/Motorhome: Isuzu Dmax Spacecab MY2015
Caravan: Tailgate slide-on camper MY2020
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Post by Jack Driver on Apr 30, 2023 16:06:21 GMT 10
I had also wondered about cornering... to the extent that maybe sophisticated cars that also monitor steering wheel position would know when cornering.
All of these things are worth thinking about
Again, if we're talking about indirect TPMS, they are only interested in LARGE changes. Unless the vehicle is travelling around in circles (like on a race track) the number of revolutions for each wheel will average out over time, i.e. all turns in one direction will be cancelled out by equal and opposite turns in the other direction
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Post by nsgnomad on Apr 30, 2023 22:39:08 GMT 10
In my earlier post, I mentioned that my Peugeot had a low tyre pressure warning. The tyre was not completely flat but considerably low. I can't remember the exact figure but maybe 50 to 70 kpa below the normal 250. Thinking about it, we had been out shopping and had just started driving home. No sign of it when we left home beforehand, and when pumped up no further issue.
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