Bicycle tyre testing notes
Bicycle tyre testing notes
There’s no room for error
Posted 16 August 2012
Testing bicycle tyres is a serious business; it’s serious enough, for example, for every major tyre manufacturer to invest substantial amounts of time, funding and factory floor space in the process of assessing their product.
Tyres in pairs
Besides the obvious performance parameters, manufacturers look into one or two that are less so, such as ease of fitment to rims that are slightly oversize or the pressure needed to blow a tyre off a slightly undersized rim.
The obvious ones? Wear rate, puncture resistance, grip and rolling resistance are all important considerations for the buyer when it comes to making a purchase; if the manufacturer can test them, why not us?
We can, of course, and many cyclists do so by recording, for example, the mileage they get out of a tyre or the incidence of punctures, which tends to increase as the tyre tread wears down. One correspondent tells me, for example, that his last pair of Continental Grand Prix 4000S tyres lasted 2,000 miles before he had his first flat, which preceded two more in the following 100 or so.
Some of this may be put down to pure chance, which is why the testing carried out by a manufacturer, which may involve forcing a sharpened tip against the tyre and measuring the pressure at which it penetrates, is probably a better indicator.
A man from Michelin tests wet grip
Grip? Sure, test away. The trouble is, the limit of grip is what is interesting and, by its very nature, testing grip’s limit has its dangers. Exceed the limit and you may – probably will – fall off. One way around this is to build a bike with outriggers and wear protective clothing – or both – in order to be able to explore with confidence. And that’s before working out a way to measure the limit when you find it. Worth the trouble? You decide.
We, the public, can, however, test rolling resistance fairly easily and with negligible risk unless it is that of falling foul of the irate homeowner outside of whose house you decide to carry out your testing procedure.
This type of testing is one I do a lot, not least because it offers the promise of immediate and worthwhile results. Manufacturers do it by putting a tyre and wheel on a huge spinning drum and, usually, noting how long the apparatus takes to slow from rotation speed A down to speed B.
What it is looking for is, or course, some difference between tyres in terms of how much energy they absorb as the cycle rolls along. Or it might look at tyre inflation pressure; how much is the optimum and, indeed, is there one?
No drum to hand? Then find a gentle slope. There are several variations on the test technique, which essentially involves riding the cycle and allowing it to freewheel or ‘coast’ along at low speed while attempting to find and record measurable differences between tyres or pressures. Recording the time taken to cover a set distance – my personal favourite – requires accurate timekeeping; noting maximum speed achieved on a coast requires an accurate speedometer and rolling to a standstill on an upslope and noting which test subject gets furthest requires a road with a dip.
As with any testing procedure, the key to getting worthwhile results is accuracy, or the elimination of anything that might get in its way. This means changing nothing between tests other than what is being tested and it means repeating each specific test several times in the hope of obtaining the same result each time. Failure to do so means there’s something amiss with the test, not the tyres.
Tyres are tested in pairs, so that any difference between tyre models is doubled, and at the same pressure, so that any differences in carcass and tread deflection that absorb energy show up. Nothing else changes: wheels, clothing, riding position, which pedal is down, around which side of each manhole cover to go… all stay the same.
As does the force of gravity, which provides the motive force. If anything, this aspect of testing is most problematic unless you can find the right hill, which for tyre rolling resistance testing should not really be a hill but a shallow slope. Too steep a slope and ride speed goes up to the point where wind resistance becomes significant, in which case the usually minute differences in tyre rolling resistance we are looking for can become too small a part of the total resistance to measure.
As it happens, I have found the right slope not far from my house. It is about 2percent or 1:50, which is enough for a coasting speed of about 15mph. Littered with useful start and finish points and boasting a newish tarmac surface, it is near-perfect for the task. Or will be when I can get a still day. Last week I thought I had one and managed several test runs before a slight breeze got up. Its effect was to add some four seconds or eight percent to the run duration, which is more than enough to render worthless the earlier results. And then it died down.
Ah, well, that’s the other requirement for successful tyre testing; limitless patience.