RT Metro

I use 27x1 1/4 Pasela TG's on Weinmann rims with Forte cloth tape. I've wasted several tubes on the rear wheel because they would blow out by the valve stem. Sometimes it would be shortly after installation, or sometime after a 5+ mile ride, or overnight. I've checked the rim, and an REI mechanic even added a small strip of cloth tape near the valve hole. The common theme so far is that these puncture happens when I pump them to the max rating of 95 PSI. So far no problems with 85 PSI, or front tires at the max. Any suggestions as to why these happen?

Mauriceloridans

I have noticed something similar. A small (5mm) rip in the seam of the tube where the valve stem reinforcement ends. Shraeder valve. This has happened on several tubes on several wheels. I suspect manufacturing defects.

FBinNY 

The two problems above are different but related, and are unlikely due to defects. A single incident can be bad luck or a defect, but multiple problems call for finding a cause.

1- tire blowing off. First note that modern rims designed for HP tires have a hook edge, which traps the bead, or a molded bulge under it for increased blow-ff resistance. If your 27" rims are straight sided you need to use tires designed for straight side rims. These have heavier bead wires to resist blow-off without the hook edge.

Also given that your problems are always at the valve vs. random, it's likely that your mounting technique is contributing to the problem. The tube is slightly thicker at that valve and it's easy for it to get trapped under the bead keeping the tire from seating properly. After installation push the valve all the way into the tire to push the tube fully into the tire, then pull the valve back into position so the tube is laying on top of the tire. Then inflate to 20-30psi, and spin the wheel around checking for even seating as indicated by the molded line. If necessary lower the pressure massage the tire around until the line is even all the way around. There's little forgiveness of poor seating with straight side rims.

2- tube blowing near the valve. You probably have narrow rims, so it's difficult to pull the valve all the way down past the narrow gap between both sides of the tire. When you inflate the tire the tube fills the space within the tire first, then the section across the bottom expands past the tire down into the rim. That means that the narrow section of tube stretches much more than the rest of the tube.


If the valve isn't pulled down all the way, the gap below it is larger, and the tube overstretches as it blows down into it. The section near the valve is reinforced, so the tearing happens right past it. Make sure the valve is pulled down all the way before inflating, and you shouldn't have problems.

Also many tube makers are now using a modified valve, which has a narrower (or no) base flange in the tube so it's easier to seat the valve with narrow rims. Look for these next time you buy tubes, (you can feel it by pinching the base of the valve) but be aware that there's a trade off. Without a decent base flange, it's easier to break the valve off when wiggling it with a hand pump.

RT Metro

Well both tire and rim are modern hook type. The rims are double walled and narrow. Ill be more mindful of the valve seating. Its a presta valve and from different manufacturers so I doubt its a bad batch of tubes. These kinds of punctures are expensive because the tear is unpatchable. I'd almost rather deal with goatheads. So far my last replacement has no problem other than its a smaller than prescribed tube. Which tube can I share between a 630 x 32 and a 622 x 35 tire?