Flats and tires

December 27, 2009

First, the four main kinds of flat:

1 – Pinch

Ride over a pot-hole, stone or something with sufficient force to compress the tire all the way to the rim and the tube gets pinched hard between tire and rim. This often leads to two holes in the tube. It’s not uncommon to get a pinch flat in both front and rear if one rides fast over a serious hazard.

Only clinchers with inner tubes are susceptible. Tubulars and tubeless tires don’t get pinch flats.

Pinch flats are the most serious threat when racing on rough roads, such as unpaved roads, cobbles, or bad pavement. You can’t avoid all the hazards when racing and you can’t hop over them all either. Hence on spring classics like Paris-Roubaix, standard equipment is tubulars on traditional strong alloy rims. In 2009, the Zipp 303 was revised to be stronger and much wider making it an aero carbon wheel suitable for these races.

Tough, armored, low-thread count tires (e.g. Specialized Armadillo, Continental Gatorskin or Bontrager Hardcase) are no better against pinch flats than other tires. The only protection against a pinch flat is the air in the tire. A 28 mm tire at 100 psi has the same resistance to pinch flats as a 23 mm tire at 120 psi but the 28 mm tire will be more comfy on those bumpy races.

2 – Cut

When a sharp edged piece of metal, stone or glass makes a cut across the tire it can also cut the tube or it can make a large enough hole in the tire for the tube to protrude and rupture. A cut is not the same as a puncture.

Tires that better resist cuts have low per-inch thread count in the carcass, which allows the threads to be made tougher, and they include armor layers. However, low thread count makes a tire less flexible and thus less resistant to punctures while increasing rolling resistance. These tires are also relatively heavy.

So in relation to high thread-count racing tires, the trade-off is good cut resistance in return for a speed reduction (due to both weight and rolling resistance) and reduced puncture resistance.

When repairing a big cut in the field, a tire boot is a temporary repair that will let you finish your ride. For boots, I carry with my patch kit pieces of old tire cut to about 20 mm square. The boot goes between the tube and the hole in the tire to prevent the tube from protruding when inflated. Folded-up currency works too. If the tire needs a boot then it should be replaced when convenient.

3 – Puncture

A puncture is when a sharp-pointed spike pierces the tire and tube. Culprits can be thorns, sharp-pointed nails or screws, etc. Think of the difference between a cut and a puncture as the difference between slicing a baguette with a knife versus sticking it with a needle.

Flexible, high thread-count tires are more resistant to punctures because they can deform around the object better than a stiff tire. The stiff tire presents a relatively rigid surface to the spike ,which is more likely to push through than it is with the soft tire. That’s why the substantial armor layers (e.g. Kevlar belt) are important in low thread-count tires, although they add to weight and rolling resistance.

4 – Tube failure

This is sadly a very common cause of a flat. The tube simply fails. Professional bike mechanics know how frequently new inner tubes fail. But a tube can fail at any time during its working life and its failure rate increases with age. A flat that seems to have no cause may be for this reason — just a bad tube. Tubular and tubeless riders are free of this problem.

Like any rubber product, tubes decay with time so it’s best to have fresh tubes around rather than old stock. Read Lennard Zinn on aging rubber. I once got two pinch flats on the Tandem and found that two of the three spare tubes I had with me had gone crusty with age.

One way that tubes often fail is where the value stem attaches to the rubber. A smooth rather than threaded valve stem reduces the stress on this joint when using a Silca pump head (or similar).

What kind of tire to use?

Naturally, that depends on the application. Here are my personal opinions. I use clinchers with tubes.

Training: I use a wide, low thread-count, well armored tire because cuts seem to be more common that punctures around where I live in New England. They are slow but that doesn’t matter in training. When training in a group, I’m working harder than those using their racing equipment and get a better workout. Come race day when I use the racing gear, I have a nice boost that they won’t get.

I’ve had good luck with Bontrager “Race Lite Hardcase”. They are neither race nor light tires but they sure have a hard case. Gatorskin and Armadillo are similar. I use these on traditional wheels with plenty of spokes throughout the long New England winter.

Normal Road Racing or group rides when I need competitive speed: I use the softest, most flexible tires. They are fastest and, for a given width, the most comfortable. Vittoria Evo Open Corsa CX and Veloflex Pavé, which both use of cotton in the carcass threads, are my favorites. (Specialized’s “Open Tubular” tires are similar, being made by Vittoria.) Conti, Michelin and Hutchinson tires are stiff in comparison.

Rough and unsurfaced road racing: Using armored, low thread-count tires like the Armadillo or Gatorskin affords no extra protection against pinch flats, which are the biggest threat on these races. So the trade-off between these and flexy high thread-count tires is cut resistance (armored) versus speed and puncture resistance (flexy). I choose the latter.

On rough and bumpy surfaces, tire flexibility is important for speed. Every time a tire bounces off the road surface the bicycle loses forwards momentum when it lands. So a flexible tire will be faster than a stiff tire that’s more inclined to bounce. And a wider tire at lower pressure can be faster than a skinny at higher pressure since the wider tire won’t bounce around so much. 27 mm race tubulars exist for a reason.

I’d like to use Vittoria Pavé Evo CG 27 mm tubulars on Zipp 303 rims but I have neither the money nor the team support car to help me if I flat. So I use instead flexible high thread-count clinchers. It’s unfortunate there aren’t many with substantial width. Vittoria has the Evo Open Corsa CX in 25 mm and the Vittoria Pavé in 24 mm. There’s also the 25 mm Diamante Pro Light which is significantly lighter than the Corsa CX or Pavé though it’s less flexible.

Some people talk about reducing tire pressure on these races. This improves comfort while seriously increasing the pinch-flat risk. I wouldn’t do it unless also switching to a wider tire. I use the same inflation I do on normal roads.