bigboybiker2007

Can anyone offer advice on which inner tubes to use on a hybrid bike (700 x 35/43C) or ( 27 x 1 3.8) All the ones I find say not to go over 85 psi, but when I am on the bike at 85 psi, it is almost flat. When I pump it up to 100-110 psi, it rides great, however the tubes blow out in no time at the valve stem at this pressure. Please help with any advice. Thanks in advance

(51)

You will have to go to a lower size (I think), like 700X30 or 700X28

And..Welcome to the Clyde board. If you feel comfortable, please tell us a little more about you, your bike and what you use it for.

Carusoswi

I ride a road bike, don't know what you ride, but, maximum pressure in a bike tire is more dependent upon the tire/wheel design than any number stamped on either the tube or the tire. I have never checked a tube for its maximum psi specification, and doubt that anyone using a tube matched in size for the tire in which it is used needs to be at all concerned with the tube's psi rating.

Modern era bike wheels are designed to catch the bead on modern era bike tires to lock the tire onto the rim, thus allowing you to pump to a much higher pressure before the ever expanding tube blows the tire off the rim.

As long as your psi doesn't overcome the resistance built into that design, you may pump your tires to your heart's desire without concern for max pressure on either tire or tube.

My tires are stamped for 115 psi max (8-bar, whatever that means). I never ride without inflating them to 140 psi. For me, this makes for a faster feeling ride (have done no tests to confirm this empirically). If I only have 100-115 in my tires, I notice the annoyingly cushy, less crisp ride.

Now, 140 psi may not be for everyone, but, my point is this: if your wheels are built to accept modern tires, they will lock the bead in place so that you are unlikely inflate your tires to the point where psi will contribute to or cause their failure.

If your tubes are failing at the stem, I would be looking for flaws in the wheel at that point. Either the stem hole is too large or it is inadequately insulated so that the metal edges around the hole are cutting into and damaging your tube at higher pressures.

In a properly designed/maintained system, all the tube pressure is relieved/constrained by the tire and the rim. There is really very little burden on the tube at high pressure.

It would be interesting to see how high you could pump a tube/tire on today's wheels before the bead of the tire fails or the tire, itself, disintegrates or fails at some point.

I cannot perform that task, as my pump's clamping mechanism fails at around 150 psi and the hose blows off the valve stem at that point.

I can tell you that I once experienced a road flat due to a puncture - it was from a bit of glass that worked its way through my tire to cause a slow leak in the rear tire. It amazes me that I can ride on my bike a number of miles before I realize that my rear tire has gone flat. The rims are so narrow that, when a tire goes flat, I may ride for a while before realizing that things are just not right - this is especially true if I am riding a long straight stretch. The high profile of the tire in relation to the narrow rim allows the flattened tire to take on a spread profile that keeps rubber between the rim and the road, so, if you have ridden 50+ miles, you may interpret the drag as physical exhaustion rather than a tire failure.

Anyhow, I believe that during one such run-flat situation, I must have damaged the bead of the tire. I replaced the tube, pumped it with my frame pump, road home (on a mushy tire), then, pumped it using a proper pump to my customary 140 psi.

Within 15 minutes, the tire blew off the rim and the tube failed along one of its seams.

Replaced the tube, reinstalled the tire, pumped to 140 psi, and repeated the whole failure process.

Road for a week at 120 psi until I could get to a bike store and purchase a new tire. End of problem.

FWIW, the bead damage seems to have been no more serious than separation of the rubber cladding along one two-inch section of the tire bead - at that point, the bare metal cords are exposed and the rubber cladding is just hanging loose, not attached to and protecting the cords.

I haven't checked with anyone on this, but, surmise that this condition elimintes friction between the tire and the rim at this point, allowing the beat to ride/slide up over the rim under pressure. If my assumption is correct, this would explain why the tire performs adequately at 120 psi, but fails predictably at 140 psi.

Sorry for the long post. Hope you find my explanations/observations helpful.

Caruso

Bill Kapaun

"If your tubes are failing at the stem, I would be looking for flaws in the wheel at that point. Either the stem hole is too large....."
Possibly Presta tubes in a rim designed for Schraeder valves?

" Modern era bike wheels are designed to catch the bead on modern era bike tires to lock the tire onto the rim..."

DieselDan

Tire and rims limit the pressure, not the tubes. You also cannot overcome the laws of chemistry and physics and have tire completely round while you are riding the bike. If the wheel doesn't drag when cornering, then the pressure is high enough.

bigboybiker2007

Thanks to everyone for the tips. The bike is a Trek 7300 and I am 6 foot 3 265 lbs riding usually 20 mile rides on paved roads/paved trails. The rim is indeed made for a presta valve. I cannot find any rough/sharp edges on the rim where the valve stem sticks through.

jakesford

Just as a note, "Bar" is equal to ~14.5psi... So 8bar = ~116psi

Velo Dog

Could you have an inaccurate tire gauge? Reason I ask is that I used to weigh more than you do, and I ran 700x32 or 700x35 tires at 75-80psi for years--they were never "nearly flat" when I was on the bike. If you're using the same gauge all the time (or the one on the pump), see if you can find another gauge to check it against. One of mine, on an expensive Zefal pump, is nearly 30psi off at 100.
Also, I run presta valves in rims drilled for Schrader all the time, and I've never had problems with that, so I doubt that's your problem. Could you be pushing or twisting the valve stem when you inflate?

jaxgtr

You should be able to find Presta value tubes in the 700x35-44 at the LBS. I know the Trek dealers in my area have them. I agree check you pump as well.

StephenH

Have a knowledgable person look at the tires close up while you're on the bike. When you're on the seat, you can see them bulge out a bit, but it's hard to tell how low they actually are.

I weigh about 270, but am riding an old mountain bike, 26x 1.75 or 2 or whatever they sell at Walmart (they show like 6 different sizes on the box). Anyway, I run about 45-50 psi and that's fine. I can see some sideways bulge in the tire, but there's plenty of spring left, too.

StephenH

I think the Target stores have Presta-stemmed Schwinn tubes- kind of surprised me to see them.