Fun with TPU
Spring has finally sprung here in Sicily! Rides in short sleeves and shorts for the first time in months. More riding means more chances of flat tires of course and Heather had a good one the other day, whatever she hit dented her bike's rear rim, but luckily the resulting loss of air didn't happen until her bike was hanging in the shop.
So we went out the other day to ride and GGGRRRrrrr!...the rear tire was flat! MAHLE X35 powered e-bikes have one feature Zio Lorenzo really hates - the rear wheel has a solid, threaded axle with flats on it to securely mount into the rear dropouts. It's needed since something has to stop the axle and internal e-guts from turning as the power flows into the part of the wheel that does turn when you turn the assist on.
So removing the rear wheel is best described as a PITA as they say. And of course most flat tires are on the rear, right? The front wheel on this bike has a thru-axle but at least one with a handle on it so no tools are needed. But not the rear! A cut-down 8 mm hex key is in the saddle bag just for this purpose. You also need to disconnect the power cable and make sure you don't lose the nuts while you pull the wheel out, moving the chain out of the way and being careful of the brake disc.
Zio normally hates turning bicycles upside down on the shop floor or roadside but since the rear wheels are heavy AND require precise positioning to drop 'em back in it's far easier getting the wheel in/out without fighting gravity. But at least this flat wasn't on the roadside!
Zio noticed when TPU (thermoplastic polyurethane) inner tubes first came on the market years ago. He'd long ago decided to skip the road tubeless idea, asking what problem did it solve other than making you buy more expensive stuff? Their fanboys claimed you could run lower tire pressure without fear of pinch-flats (which was the case on Heather's flat, complete with dented rim, but with a whack that hard would things have been different? Recent results at Paris-Roubaix would suggest otherwise) along with other supposed benefits like self-sealing small punctures, but Zio remained unconvinced.
When TPU (plastic) inner tubes came around, lots of folks asked why mess with road tubeless when these would replace old-time butyl tubes and be lighter, plus spare ones would take up less space in your saddlebag and be more resistant to puncture? Zio remained unconvinced. Butyl tubes are cheap, easily repaired and don't lose air quickly so you don't need to do more than give a squeeze to your tire before a ride.
After a few years, with more and more seeming to switch to TPU and abandon road tubeless he decided to give 'em another look. The early versions were cheap and seemed to either work great or fail right away. They couldn't be repaired, unlike butyl. They cost more than butyl and it didn't seem they were much more puncture resistant either. But mainstream bike tire companies like Pirelli and Vittoria then got involved, repair kits became available and specific sealant for TPU came onto the market. Zio figured the lighter weight would cancel the added weight of the sealant and improve puncture resistance to better than a butyl tube. The sealant was said to be water based, so before it dried out you could add some water rather than recycle an expensive tube because of a dried up blob of sealant inside.
With those innovations on the mainstream market Zio decided to buy into the concept, ordering some TPU tubes, including a few with the sealant goop already inside. They all sat on the shelf along with the sealant, the injection syringe and the repair kit. Until yesterday. All the butyl tubes on-hand in the size needed had patches on them - Zio usually wants to repair a butyl tube, install it, then air-up the tire and come back in a few hours to make sure the patch job worked. We wanted to ride now, so a TPU tube was installed instead. No sealant, we wanted to ride, not wrench!
Installation was pretty normal, you do need to put some air into the tube to get a bit of shape to install it, but they caution about too much air pressure stretching/deforming the material. Luckily the tires on this bike go on with just the "thumbs of death" rather than needing tire levers, so no issues with possibly pinching the thin material. Tire was aired-up, wheel put back on the bike and off we went for a few hours of cycling in the sun.
This AM Zio decided to finish the job by adding the special sealant. The valve core was removed (Zio's not a fan of the plastic stem, fearing the threads are going to be too easily stripped if the valve core needs to come out again, maybe to add water to the drying-up sealant? Newer versions are available with more durable metal stems) and the sealant injected before pumping the tire back up to normal pressure.
Stay tuned for future developments. Zio doubts these tubes are any more resistant to pinch flats from hitting holes in the road and there's still a standard butyl tube in the saddle bag as spare. If this tube works OK the ones with the goop already inside might go into our gravel bikes the next time we have a rear flat on one of those. We've already tried and rejected conventional butyl tubes that came with SLIME already installed - they blew out way too soon - Zio's guess is due to the cheap tubes more than the sealant not working.
