Speedo recalibration

[SeaComms]

Ok, for anyone thats still following, here is the reason for a tyre showing effectively a smaller rolling diameter when on a vehicle. It may bore some of you to tears, but my ever questioning mind got the better of me so had to research it til I was satisfied!! This may not be applicable to drag cars though, as it only effects tyres with tread, and lots of it!

In the really rough drawing below you can see the main outline of the tyre shown as being the surface below the tread blocks, ie, a completely bald tyre. This part of the tyre is fused to the steel belts and will not change in length (or circumference as the case may be) regardless of tyre pressure.

If you look at the tread blocks sticking out from the normal (not on ground) tyre surface, presuming the blocks are perfectly square and they are mounted on the round surface, the outer edge of the blocks will have a larger gap between the blocks than the edge closest to the tyres 'bald' surface.

When you then look at the section of tyre that is flat on the ground under the vehicles weight, these blocks are now sitting on a flat surface of the tyre and are perfectly perpendicular to each other, effectively making the outer edge of the blocks closer together (or the same distance right from outer edge to the bald tyre surface). This has shortened this section of the very outer circumference of the tyre for this flat section. If you let your tyres down, this flat section of tyre gets larger, making more of the blocks closer together and reducing the overall rolling circumference.

If you run your tyres down to almost no tread, this stretching and compressing of the gap between the blocks is no longer relevant and there will be negligible to no difference in rolling diameter or speedo reading at any pressure, as this bald tyre surface is firmly bonded to the steel belts and fixed in length.

There is your lesson in tyre dynamics for today

Anyone still reading at this point deserves a gold star or beverage of choice.

[Banshee]

Bring me my beverage of choice!

With the Aeroforce gauges that I have, it doesn't ask for the diameter, but the rolling circumference in mm. So what I did was make a chalk mark & roll the car one rotation, then measure. Worked out pretty close.

As for speedo accuracy, the specs used to say +/-10% at or above 40kph, but now are a little more complex, from ADR18/03.:

(JK is type M, my bike is type L3)
V1 is the indicated speed
V2 is the actual speed

In the case of vehicles of categories M and N:
0 ≤ (V1 – V2) ≤ 0.1V2 + 6 km/h;

In the case of vehicles of categories L3, L4 and L5:
0 ≤ (V17 – V2) ≤ 0.1 V2 + 8 km/h;

So from this, at 100km/h actual, your speedo can indicate anywhere between 100kph, (must not under read), and 116kph. (100 * 0.1) + 6

My bike speedo can indicate between 100 & 118kph.

So Joe Bloggs can think he is driving down the highhway at the posted 110kph but is only actually doing 94.5kph! And you wonder why so many people seem to be driving so slowly & swearing at the 'speeders' who actually just have accurate speedos?

For the record, at 110 actual, the speedo on my bike reads 120kph. And the odometer is accurate with this setting. The JK, on the gripping hand, has an accurate odometer when the speedo is accurate.


One point, though, Seacomms. The pressure still makes a difference even to a bald tyre's rolling diameter, due to the pressure dependent change in deformation. At high pressure the centre of the tread is pushed out further than the edges but as the tyre pressure drops the centre of the tread begins to bow inwards at the contact patch. (This is why an under inflated tyre wears more on the edges than in the middle, whereas an over inflated one wears in the middle of the tread first.)

[Macca2801]

If your flawed theory of no change to rolling diameter due to tyre pressure changes... Explain how indirect TPMS systems work? You know the ones that work off tyre rotations (wheel speed sensor info) and will alert the driver to a under inflated tyre due to an increase in tyre rotation on high performance vehicle, ie nearly slick steel belted rubber not nobblies!!

Hahahah enjoy!
Matt

Here you go I will help you out, measure the two outside distances of these two tyres.... The deflated one is shorter despite the actual tread lengths being the same..the tread " cuts the corner" so to speak, check the distance A-A and B-B. They have two different rolling diameters due to inflation.


Ps, I can't believe you haven't tested it yet?
Mark the rim point it down perpendicular to the ground, mark the ground and do one full revolution on full pressure. Measure the distance, roll back to first mark and rim mark again perpendicular. Deflate till there is a significant difference in sidewall height and do one full revolution again. post results and shout the beers! Haha
Matt.

[Green Goblin]

Geez guys OUCH my head hurts reading all that

[SeaComms]

LOL. This is fun, and educational too

Firstly no, I haven't tried it myself, but here is a video with what looks to be a tyre with a very small amount of tread on it doing this exact test:



Secondly, I found another web site with an interesting discussion on the same topic:

From http://mgaguru.com/mgtech/tires/ti103.htm

The MGA With An Attitude
MGAguru.com MGAguru.com
ROLLING CIRCUMFERENCE of a Tire - TI-103
for Speedometer/Odometer Calibration

There is occasionally considerable discussion about tire sizes and why the rolling distance of a tire is less than the measured circumference. I believe I have a clearer answer to this "apparent" discrepancy.

At 09:45 AM 6/6/05 +0100, Paul Hunt wrote:
"The biggest discrepancy is that my tyre measured 1940mm circumference whether inflated to 24psi, 10psi, laden or unladen. But it only traveled 1885mm at 24psi and 1876mm at 10psi."

The inner plies of the tire, especially for a belted tire, will not stretch or compress much under any circumstances. This means the steel belt remains the same length regardless of air pressure or deformation of the tread. In the measured example above, the actual measured travel distance divided by PI equates to the diameter of the steel belt, which is at about 3/8 inch less radius from the outer tread. The rolling travel distance is therefore very nearly equal to the circumference of the steel belt.

As the tire tread is deformed to be flat against the ground, the outermost part of the tread is compressed slightly to be shortened to the same length as the steel belt over the length of that flat area. This then accounts for the slightly reduce travel distance relative to the tire circumference, not the length of the flat relative to the initial arc length.

The much smaller variation in rolling distance with the large reduction of air pressure is accounted for by more squirm in the tread as the sidewalls flex considerably. In essence, when you have a gross amount of squirm the tread slips some on the pavement, and the total travel distance is slightly reduced. In the example above the travel difference between 24 PSI and 10 PSI is only 0.5% (representing a lot of squirm on a half flat tire).

"However the rolling radius (which is what is used in speedo calculations) indicated that it should have traveled 1823mm at 24psi and 1615mm at 10psi."

No, that's small point of confusion. The rolling radius noted is likely to be the distance of the wheel center from the pavement, which changes with air pressure but has not much to do with travel distance. If you look at tire stats you will often find a specification for Revs Per Mile, which is the number needed for speedometer calibration. This would correlate closer with the actual measured travel distance in the example above.

Bottom line, yes there is a small difference in the distance travelled on a low pressure tyre compared to a high pressure tyre of the same brand/type/wear etc. It is not due to the diameter of the tyre changing like a balloon does when inflated more or less, it is mostly due to the tread on the tyre distorting as pressure decreases. This is a very small change, and according to the tests done in the ADR speedometer calibration document I linked earlier show this to be as low as 1% from 20 to 40 PSI on the road tyre the test was performed on. This, while very small, would be enough to be picked up as a different speed from one tyre to another for a wheel speed sensor based tyre monitor to detect.

Ok, now I am off for a beer

[wrench]

Macca has hit it on the head. The relevant fact is the difference in C and D, hence my earlier advice to think of it as rolling radius not diameter.
It is the tyre carcass that deforms and causes this. Tread deformation is secondary.

[MightyMouse]

Certainly explains minor differences in calibration... but doesn't explain my experience... there's absolutely no way the tire is deforming by 2.5" at the loaded portion of the tire at normal road pressures.

At very low pressures yes they bag out significantly but not at road pressures.

There's still something strange going on with some variants.