Foiling the flat (or Preventing the Puncture)

FRANK BERTO, a leading American cycle engineer and technical writer, surveys the latest ‘solutions’ and shares his experiences after thirty years of pedaling.

Getting a flat tyre and fixing it on the road is one of those defining experiences that separate the serious cyclist from the beginner. In any place, at any time, the moving majesty of the bicycle can be brought low by a miserable shard of glass or unyielding thorn. Failing that, an impact puncture can pinch the tube between the rim and rock, causing two ‘snake bites': one on the top and one on the bottom of the pinch. Then there is the depressing list of everything else that can go wrong: the tube can deteriorate and start to leak all by itself. The valve can leak. A patch can come loose. The tyre can wear through. The sidewall can fail. The rim tape can move, allowing the tube to be cut by the spoke heads or the holes in the rim. The tyre can blow off the rim. You can usually look at the hole and figure out what caused it.

Prevention can take a number of forms. Use a wide enough tyre size for your weight and riding style. This is essentially a trade-off. Skinny tyres save weight and they can carry higher inflation pressures. It's the high inflation pressure that lowers the rolling resistance, not the skinny tyre. A wide tyre has slightly lower rolling resistance than a skinny tyre if they're both inflated to the same pressure. Wide tyres usually have thicker tread bodies than skinny tyres. The thicker tread will often reject a thorn or a glass shard before they penetrate through to the tube. You'll get more punctures if you try to get the last few hundred miles out of your tyres. In fact, some riders use this as a wear indicator: replacing a tyre when they get two flats in a row. Finally, wider tyres get fewer snake bites.

The tyre makers hate the fashion of fitting fat tyres onto skinny rims because they stress the sidewalls of their tyres. However, the latest fat tyres have been redesigned to reduce sidewall failures where the tyre takes a sharp bend at the rim flange.

Keep your tyres properly inflated. This, again, is a trade-off. Under-inflated tyres get more snake-bite punctures. Highly-inflated tyres get more punctures. The smaller contact patch on a skinny high-pressure tyre presses harder on the road, so thorns and glass shards are more likely to penetrate.

Use a good rim strip and file off any protruding spoke ends. I've had good luck with plastic bands, cloth tape and with fiberglass strapping tape. Rubber rim strips are less effective. The key is to use the right width. Cover the entire drop center of the rim but don't cover the bead seat.

Butyl inner tubes come in three qualities, based on thickness and weight. ‘Standard' tubes are about 1mm thick. ‘Lightweight' tubes are about 0.8mm thick.  ‘Ultralight' tubes are about 0.6mm thick.

The following remedies don't work very well against punctures by glass or thorns:

Kevlar-belted tyres. These protect only against dull penetrators.  Shards of glass and sharp thorns go right through the ‘armour'

Latex or polyurethane tubes. The thinking here is that the compliant tube will be pushed inward by the point of the penetrator. This may work with dull penetrators but not with the typical glass shard or thorn.

Thorn-proof tubes. These act like a thicker tyre and they help if the thorns aren't too long. You pay a significant penalty in weight and rolling resistance. It makes more sense to buy a wider, thicker conventional tyre.

Polyurethane tyre liners. These help mainly because of their extra thickness. Sharp penetrators still go right through but need to be longer than the norm to do any damage. Tyre liners work better against thorns than against glass, because they break off the point of weak thorns. Tyre liners significantly increase rolling resistance and they can cause flats if they move around or the ends pinch the tube. It makes more sense to buy a wider, thicker tyre.

Fitting wider tyres is, in general, the most effective response to the puncture problem. Their lower pressures and (usually) thicker tread both reduce punctures. They are also often cheaper. Cars don't get many flats because they have thick, fat, low pressure tyres. Mountain bikes rarely get flats on the road. The problem lies in finding the happy medium: for most people an occasional puncture is preferable to a bike feeling like a motorless moped.

What else works? There are puncture-sealing tubes which allow you to use a lighter, higher performance tyre because most of your puncture sealing tube repairs itself. I've been consulting with the company that makes the Airlock puncture sealing tube. I've been testing Airlock tubes for more than a year (about 3000 miles) and I've cut my punctures in half.

Airlock tubes have ‘saved' all of the tack and thorn punctures and about half of the glass punctures. It depends on the size of the glass shard. If it was a small shard and I pried it out when the tyre went soft, the tube repaired the hole. If it was a big piece of glass and the slit in the tube was longer than about 1/16 inch, the tyre went flat in spite of the efforts of the liquid goop to seal the hole. If the cut in the tyre was big enough to require a boot, then I had to patch the tube.

[Ed's note: Many other inner tube sealing products similar to Airlock have come onto the market since this was written, including Slime and Stan's No Tubes. Tyre technology has moved on so much that some tyre manufacturers now have enough confidence in their products to offer a puncture-proof money back-guarantee]