“It’s nice to see Professor Pivot using ‘real world’ testing for tyre comparisons, and some believable results. A couple of observations though – we tend to quote pressures in pounds per square inch (p.s.i.), but only notice the ‘p’ and ignore the ‘s.i’. For the same indicated pressure, a larger tyre will have more pressure inside and give a harder ride. In the same way, the smaller cross-section Stelvio will have a smaller outside diameter.When I tested 700C against 27″ of the same tyre, the difference was clear.
Finally, what’s all this nonsense about ‘faster up hill’? When climbing hills, the power input is as variable as the descending force is consistent – please don’t mix science with witchcraft!”
Mike Burrows
Norwich
Professor Pivot replies: Narrow section tyres do require higher pressure, and the effect can be quite marked. For example, the cross-sectional diameter of the Brompton 37-349mm tyre is 34mm and the Stelvio 28-349mm tyre is 30mm. If my limited mathematics is correct, the larger tyre has a cross-sectional area 28% larger, so where 80psi might be adequate in the large tyre, it seems reasonable to assume that a little over 100psi would be required in the smaller example.Verification of the theory – and an entertaining parlour game, no doubt – can be found by inflating a bicycle inner tube and a heavy goods vehicle inner tube to the same pressure and sitting on them…
But will the narrow tyre at 100psi react in exactly the same way as the wider example at 80psi? I suspect that if the two tyres strike the same bump under the same load, the cross-sectional area will be temporarily reduced by the same percentage in each case – let us say 20%. Once again, I must apologise if my thesis is running ahead of my mathematics, but I believe a reduction of 20% in the cross-sectional area for the big tyre would result in a deflection of 3.48mm, but for the small tyre the deflection will be only 3.08mm? This suggests that the smaller tyre might transmit smaller ‘choppier’ deflections to the bike – a harsher vibration, in other words.This might increase rolling resistance, but would also tend to reduce energy-wasting bounce when pedalling hard.
A good rule of thumb, whatever the bike, rider, road and tyre combination, is to inflate the tyres (individually, to avoid confusion) until the ride becomes harsh, then set the pressure a little below this point.These personal figures should give a reasonable ride/handling/rolling compromise specific to you and your bike.
Mike also makes an excellent point about keeping ‘seat of the pants’ observations well separated from repeatable and reasonably scientific results such as roll-down tests. But observations – however implausible they might seem at the time – do occasionally lead to a new avenue of enquiry, so they’re worth making, with the proviso that they are treated with the necessary caution.
When testing the Stelvio and the Brompton tyre back-to-back on repeated roll-down tests, I obviously rode back up the hill a number of times. On these return runs, the Stelvio tyres certainly seemed to make smoother progress, allowing the rider to hold on to a higher gear. But with no means of measuring the power input from the rider, I fear this casual observation must remain unproven!
