Wheel circumference - tables vs measurement

[Rip Van]

But then again think about how hard a road bike tyre is when full pumped up to 7.6 bar (110 psi), the contact area is very small and the deflection cause by the rider is probable within the margin of error of the measuring tape...

110psi? 145psi for me & my Rubinos.

 

[Fiona N]

But then again think about how hard a road bike tyre is when full pumped up to 7.6 bar (110 psi), the contact area is very small and the deflection cause by the rider is probable within the margin of error of the measuring tape...

I can put a number on it - 2082mm when loaded, 2094mm measured with same method (roll down) but no rider, both at 100psi (front tyre, 110psi in the back but that wasn't measured). I measured both just out of curiosity and was quite surprised it was that much.

I guess the deflection varies with the eight of the rider so heavier/lighter riders might have more/less of a discrepancy

 

[Gixxerman]

So with a bit of simple maths you can work out the amount of tyre squish (the flatspot).

C = 2 * Pi * R

Rearranging we get:-

R = C / 2 * Pi.

So:-
radius unloaded (Ru) = circumference unloaded (Cu) / 2 * Pi
radius loaded (Rl) = circumference loaded (Cl) / 2 * Pi
Squish = Ru - Rl = (Cu / (2 * Pi)) - (Cl / (2 * Pi)) = (Cu - Cl) / (2 * Pi).

So for your bike the tyre squish = (2094 - 2082) / (2 * Pi) = 1.9mm
For my bike the tyre squish = (2193 - 2155) / (2 * Pi) = 6.04mm

Yours seems quite beleivable, but mine does not. 6mm squish for 700c X 32 tyres at 100psi seems at lot.
But the 2155 was from sheldons tables, so I will find out the true figure tonight by doing a rollout test.
I would expect it to be closer to 4mm which would give a figure of:-
Ru = 2193 / (2 * Pi) = 349.027mm. So Rl = 349.027 - 4 = 345.027.
Cl = 2 * Pi * Rl = 2 * Pi * 345.027 = 2167mm

 

[MJN]

But the 2155 was from sheldons tables

Therein lies your problem, and the nub of the issue behind the original post - tables can only be approximate at best as there is too much variation between tyres (and tyre/wheel combinations) of supposedly the same 'size'.

Mathew

 

[HJ]

It is always best to measure your own tyres rather than rely on the tables, because there is so much variation between different tyres...

 

[jimboalee]

A Roll-out test with you sat on the bike with tyres at correct pressure is as accurate as you need.

When you are out on the road, where you are going to won't get nearer because you have adjusted your milometer.

Someone else's milometer will have a different measurement, and if you weave around on the road, each trip you make to the same place will be different.

2mm in 2m is what? 1/10 percent. That's 176 yds in an imperial century. Who measured the century anyway, and how accurate was their measuring system?