Kevlar and pinch flats
 

I have the Hutchinson Scorpion 26x2.0 tires on my bike. These tires are made with a Kevlar beads which makes it what is known as a folding tire. I'm curious as to whether the Kevlar beads decrease the chances of getting pinch flats at low pressures. I haven't gone Mtbing with these tires yet so I'm kind of cautious about what tire pressure I should use. The sidewall says Recommended competion pressure: 29 PSI. Maximum 85 PSI. I haven't gone as low as 29 but I have gone to about 38 and, while looking at the tire while biking around, was scared to death to continue on at such a low PSI. I'm going MTBing this weekend and would like to settle a few things once and for all.

What is the safe zone of PSI for me? I weigh ~140lbs with all my gear on including the bike. Maybe 120lbs without the bike.

Do Kevlar beads increase or decrease the chances of getting pinch flats? What about Kevlar sidewalls.

What exactly causes a pinch flat in the first place?

Thanks in advance,

Josh

The type of bead, steel or Kevlar, has no effect on pinch-flat susceptibility of a tire. Pinch flats occur when you hit an obstruction hard enough to collapse the tire far enough so it hits the edge of the rim. The tube, caught between the tire and the rim, gets punctured in two places right across the rim width from each other (the resulting two closely spaced holes are why they are also called snakebite punctures).

The preventative is to run sufficient pressure so you never collapse the tire completely. That required pressure depends on rider weight, speed, what you hit and tire size and pressure. Larger tires and higher pressure both work to prevent them as does a careful riding style.

Keep in mind, if you are getting pinch flats, how close are you to bending the rim lips?

I'm not getting pinch flats, and I'd like to keep it that way!!! I just don't want to have that fear of getting them while I'm riding on the trails. The trails are hardpack, dirt, some sandy areas, some rocky areas, and a couple drops maybe about 2 feet high.

Josh

+1 the bead is safely tucked away below the rims edge, so it can't affect anything outside of the rim. Pinch flats are caused by a combination of inadequate cross section, and/or pressure for rider weight and road conditions. Most people who gt inch flats get the most improvement by using wider (also taller) tires rather than trying to solve it with more pressure alone.

Where more pressure is impractical, such as mountain biking or rocky terrain, many get the best improvement by going tubeless.

What about adding talc powder inside the tire? I've read that that helps prevent pinch flats but it seems kind of half and half. Some of the people say it's nonsense, some say it's the best thing to prevent pinch flats.

I have looked at getting tubeless but decided not to because I didn't want to spend money for them because I'm saving up for a new bike right now.

Josh

If you have suspension, take the pressure up a bit. You have plenty of room on the upside, with the key issue being "bounce" off rocks and the like.

Otherwise, at your weight, I suspect the issue is how you ride. On rough surfaces you have to get your rear end out of the saddle, use your knees as shock absorbers and relax your arms, so the bike can float over bumps without your body going up and down. Developing this technique slashes the G force of impacts and will do your tires and wheels much more good than trying to dial in pressure.

BTW- be sure to lift your ash clear enough of the saddle so it doesns't come up and goose you in a sensitive place. Practice on level ground with roots or rocks, then work your way to progressively faster descents until you're comfortable with the technique.

Just a suggestion: Use reasonable real-world pressures and stop being scared. You'll never compete on these tires anyway.

[clue: Hutchinson baits suckers with the word “competition”on the side of these tires. Any actual competitor would determine their own best pressure.]

Talc won't do anything to prevent pinch flats since they are due to mechanical damage, not slippage. What talc does is eases initial installation as it makes the tube slip into the tire more freely and make it less likely to get hung up between the tire bead and the rim.

It will also keep the tube from sticking to the inside of the tire after it's been installed for a while. Some tubes, particularly thin, light ones, bond to the tire so badly they tear when you remove them.

You can't sort it once and for all, because tire pressure should be set according to conditions. Pinch flats usually happen at higher speeds and in rockier conditions. Of course you'll want higher pressures then. At 120 lbs bodyweight, maybe 35 to 40 psi. If you ever go on slower, more twisty or wet trails, you may want 20-25 to get any traction at all.
My advice: carry a spare tube, and learn how to change it. Practice a few times if it will settle your nerves. Trailside flat repair should not be a frightening proposition.

I'm going to get some spares today and also a hand pump as well. I already know how to change a flat tire so that isn't a worry to me.

Josh

Good stuff! Now you can play with different pressures to see what suits you on a specific day.
Too much pressure, and you'll be spinning out trying to climb on loose or slippery material. Too little pressure and you snake bite. If you ride enough, you will get a flat, and so will all the people you ride with.
It's part of the sport. :thumb:

Yes!!! Seems so few know...

I tried running low pressures on the front as this calc (and others) suggest. The tires get too smooshy when you stand or otherwise put more weight on the front, the bike doesn't handle as sharply in technical descents, and I got too many pinch flats. I wound up increasing the front pressure incrementally from around 70 psi and settled on 90. Rear is 100. 25mm tires 170 lb.

In my experience, if you hit something hard enough, you can pinch flat even at full rated pressure. One of the hazards of riding at night. I'm far from a heavyweight, too, at 6'2" and 155 lbs. This was on my winter bike with heavy Weinmann rims, so the rim survived. I always check my pressure before starting a ride.

I've never ridden tubeless, but I wonder if you could still break the bead seal by hitting something that you don't see?