Club 80-90 Forums

bigbadbob76 wrote:Getting back to true rolling radius vs calculated tyre diameter...
This would give me slightly lower gearing than the maths says.
Which might account for my tacho appearing to read slightly high at 70MPH. (4kRPM vs 3800RPM.)
I'm still not going to worry about it but just thought I'd throw it in there.
Vehicle gauges are not laboratory instruments.

I was wondering about this. I'm not sure that the rolling radius, which is smaller because the vehicle weight compresses the sidewall, actually changes the circumference, i.e. the distance all the way round the tread pattern. The tread pattern would have to compress or stretch for the circumference to change, and I'm not sure it does. Imagine that tread detached completely from the sidewall - its circumference would stay the same regardless of the shape. I could be quite wrong of course.

bigbadbob76 wrote: Vehicle gauges are not laboratory instruments.

That's getting ever more apparent, though I have always found maths on tacho does give a decent indication of road speed.

You made me check one of my vans, the CU was easiest, it's running 195/80 tyres on 14" rims. The maths says 82.57", I measured it at 82 1/2" so no real discrepancy there

CJH wrote: The tread pattern would have to compress or stretch for the circumference to change, and I'm not sure it does.

it's not that simple, the wheel rotates around the contact point as it moves forward...

https://www.tut.fi/ms/muo/vert/11_tyre_ ... radius.htm

Hmm, I like going back to school. I was thinkng along the following lines:

Assume a flat tyre of negligible thickness and infinite strength. The effective rolling radius would be the radius of the rim.

So I was compelled to drop the CU off the jack to take its own weight. The bottom of the tyre deflected by around 10mm. Doing the maths that equates to ~3% of nominal rolling radius when static. As the article says, it will be less at high speeds (maybe like 70 mph, or maybe that's not sufficient )

I just checked the tyre pressure, only 26 psi so 3% is overstating matters somewhat

No time to remeasure at the moment, sorry

Does that make sense

Actually it says it rotates around a point below the surface, not the contact point, implying that the effective radius is greater than the loaded tyre radius. This would make it somewhat like an uncompressed tyre:

It means that the centre of rotation of the wheel usually lies somewhere below the surface.

I got a bit lost with their graph, but they also say this, which backs up what they said above:

The effective rolling radius is not the same as the loaded tyre radius

So my feeling is that it IS that simple - one turn of the wheel causes the vehicle to move by one complete 'length' of the tread - it has to. It doesn't matter where the rotation centre moves to due to tyre walls being different depths above and below the axle. The question I'm not clear on is whether the 'length' of the tread changes due to secondary effects: is it shortened by having the vehicle's weight compressing the sidewalls? I'd guess 'not'. Is it stretched by the wheel rotating at speed? I'd guess 'yes' to that one - think of a drag racing funny car launching off the start line, the rear tyres get noticeably bigger when they spin up, so the tread must get stretched lengthways. Normal car tyres a more rigid though, and the speeds are lower, so I'd guess this effect will be very small.

Now, if you can find me a reference that says otherwise, and that I can understand, well then I'm prepared to change my mind.

937carrera wrote:
Assume a flat tyre of negligible thickness and infinite strength. The effective rolling radius would be the radius of the rim.

It would, but I don't think that's a fair comparison, because the tyre would have to buckle and distort. One turn of the wheel would not cause the vehicle to move by one tread length because of folds in the tyre.

937carrera wrote: No time to remeasure at the moment, sorry

What this discussion needs is definitely more measurements. Let some air out, repeat your rolling test. 10 sample points should do it.

937carrera wrote: You made me check one of my vans, the CU was easiest, it's running 195/80 tyres on 14" rims. The maths says 82.57", I measured it at 82 1/2" so no real discrepancy there

Less than 0.1% discrepancy. 3% would have been a 2.5" difference.

Yes, but that was with the wheel in the air, no compression from the weight of the van. When I dropped the van back on the deck I compared the sidewall height before & after, then did the maths allowing for the 14" rim (7" radius)

Next time I'm with the vans I'll try and remember to pump the tyre up and remeasure

937carrera wrote:Yes, but that was with the wheel in the air,

Ah, I misunderstood - I thought you'd done a rolling test. My drive slopes too much for a rolling test, and my van's going up on ramps tomorrow so I can finally sleeve my flywheel, so it's definitely your job to do the tests.

i'll second that. Haha.
I live at the top of a hill, if it gets away from me and makes it across the A87 without being hit by a truck, it'll land in the sea leaving a flock of flattened tourists in it's wake.


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I got stuck behind a slow moving gin palace tonight, when the coast was clear I dropped down to 3rd gear and pulled out to overtake.
As i drew level with him a strange orange light came on the dash.
"pooh" what's that!!
Then i had a wee chuckle to myself and changed up.


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Serves yourself right, clever ***

Tomorrow I might be pushing T25's around for you

937carrera wrote:Tomorrow I might be pushing T25's around for you

Not on my account, I'm not taking the rap if you pop a disk in your back.

Posting this while I'm in a queue in casualty...

Just kidding, my T25 rolls quite easily. NOte, first measurement was on my CU, this is on the WBX, running 185 tyres.

So, first thing was to measure tyre circumference, 80.6", just as the maths would say. I'm sticking with imperial in line with tyre sizes. Tyre was inflated to 45 psi.



Then marked the tyre and floor and rolled forward 3 complete turns, giving a distance travelled of 232.75", which rounded is close enough to 77.6"

That means that actual road speed is 3.7% slower than pure mechanical gearing would indicate.

So the calculated 3798 rpm for 70 mph with your gearbox Bob would actually need to be 3943 rpm, ignoring any effect from the centrifugal forces acting on the tyre and making it apparently more rigid