The OP of this thread, Simon, kindly agreed to send me a couple of sections from his discarded Tannus tyre. I wanted to see for myself what the hype was all about and I quite frankly, found it intriguing that he hated these expensive solid tyres enough to cut them off after just a few rides. I received the sections, the original instruction brochure and some pins by post last week. Thanks Simon.
My initial thoughts for the request was to devise some sort of comparative rolling resistance test. At the time of my request I had to real idea of how I would do it so I opened a beer and gave it some thought, whilst mindlessly bouncing a
superball against the wall. (That's how I'd like to phrase it for my post-Nobel prize interview). Then inspiration struck and I thought of the days when I played games in my workshop. Whenever I fitted a tyre I would finish off the exercise by throwing it away from me, onto the workshop floor and make it bounce back at me. It would magically bounce back because I put a hefty backspin onto it. It was my private little party trick. The trick revealed how much energy the wheel returned because the bounce is dramatically different dependent on the wheel configuration - inflation pressure, knobblies, slicks, sealant etc etc.
My inspiration was that I would bounce the Tannus and compare it to my own 25mm Conti at 60PSI. My inspiration had a flaw in it in that I only had a section of Tannus, now a whole wheel. Genious prevailed and I came up with the definitive ball-peen hammer trick. Instead of bouncing the wheel, I would bounce a hammer onto the tyre and see how much energy it returns to the hammer. Off came my front wheel and I started hammering it. I laid the Tannus flat onto a hard surface (granite kitchen top) and bounced the hammer on that. Same difference, give or take a large experimental error and poor eyeball technique. (This was after two beers).
Another inspiring moment hit upon me and I disovered that the Continental-shod wheel is actually losing more energy because I'm testing the entire wheel across its entire radius, through a set of spokes and into another complete tyre section. In other words, I should not bounce the hammer on top of the tyre when the wheel is standing on itself. I have to isolate a small section so that I only test one layer of tyre, not two and a set of springy spokes. I how positioned the wheel so that it hung off the corner of a workbench with the corner wedged between the spokes. Suddenly the hammer bounced really well. When compared to the Tannus I could just about convince myself that there was a dramatic difference and that the Tannus definitely had a higher RR than a pneumatic tyre and that I've proved it. I could just "feel" that the hammer bounces 1/10th of a mm higher from the pneumatic tyre than from the tyre. I've honed this "feel" after years of subjective compliance-testing of frames and wheels where I could "feel" that steel frames have about 0.3% more give than carbon frames and that 50gram lighter wheels gave me an edge of 0.5% on hills than heavier wheels.
Before publishing my findings in Scientific Bicycle Today, I procrastinated a bit and mulled over the results.
Another inspiration hit me between the eyes. Of course! Hysteresis (energy losses when rubber returns to a position before deformation), is not just caused by the deforming rubber against the road but also by other squirming, notably with tubular tyres, squirming against the rim itself. In other words, energy is lost by the compression inside the rubber and lost by the rubber (tyre) rubbing against the rim as your ride. This only happens with tyres which are either tubular (as in tubbies) or solid. Clincher tyres don't have these losses because they have to bottom to scrape against the rim in the first place.
Now I'm onto something, I said to myself. I then investigated the interface (fancy word for parts that rub) between Tannus and rim. It is hugely imperfect. The rim's shape only superficially matches the tyre's shape and to make things worse, the pins that hold the Tannus in place are spaced 50mm apart, leaving a large section not securely attached between the two pins. Here the tyre is a bit looser than directly under the pins, causing even more movement. If the rim doesn't have a flat spoke bed (none of them do), then there's a void between the Tannus and the rim itself, into which the rubber can morph in and out as you ride. More energy losses.
The third point of loss (if you've lost me so far, the other two were losses inside the rubber itself which is due to friction as the long curly chains or rubber polymer stretch and rub against other long curly chains of polymer and, squirming against the rim) is thanks to the generous (read stupid) tread on the tyre. It is really deep and causes large losses at the tyre's surface as the rubber squirms in and out of the voids in the tread. Had the tyre been designed smooth, it would have been better. However, then it would not have appealed to a naive consumer who believes that tyres must have tread.
So there you have it. These tyres have high rolling resistance because:
1) A rubber polymer always has higher hysteresis than a compressed gas.
2) The tyre does not fit perfectly into the rim and isn't attached with hard glue that prevents squirm.
3) There is thread squirm.
I have no idea how to test this, but a particular concern would be performance in the wet. Tyre rubber has been improved and improved for 100 years and still no better wet traction compound than carbon black has been found. Unfortunately, carbon black makes the tyre...err...black. Tannus are available in lots of colours, all chosen from the rainbow, which tells me there is zero carbon black in there. (How scientific is that deduction?)
So, (I've noticed on the radio that all scientists start answers with "so", so I'll do it as well), here are some pictures.
View attachment 369445
A
scan from the information brochure showing you how to measure your rim width and choose the correct tyre. This means that fit is an approximation, unlike on a pneumatic tyre that automatically adjusts its fit. Room for improvement could include tyres more precisely matched to rims, better retention devices and perhaps even factory-fitting with hard glue.
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A section of pins. These match the rim's internal width and apparently the tyres are shipped with a selection.
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This is the pin fitted to the tyre. Round end facing downwards and pressed down on to click and hook behind the bead hook in the rim. Spaced 50mm apart.
View attachment 369444
Cross section of very expensive rubber. Note the tiny bubbles. To make the tyres sound like better value for money, these bubbles are called "nano voids". Note how the section intended to fit inside the rim will only approximate a rim's real shape and leave plenty of scope for squirming around, losing energy in the process.
View attachment 369446
A stupid quote hinting that the tyres contain some magic engine that kicks in at speed and somehow managed to reduce RR once in motion. Pure marketing genius. Ignobel Prize on its way.
View attachment 369448
Gratuitous tyre tread that serves just one purpose - to increase RR. Actually, make that two, it also suckers people.
And now I'd like to thank my producers, my dog, my co-author Et Al and of course Simon. Without them I would not be able to waste so much time.
