TLit

I blew another innertube on my Motobecane 12 speed tonight. This is now 4 times with the same brand, on the first I used a better "Blackburne" innertube but may have put it in wrong.

Is it me or the innertubes? This time I lined the wheel inner space with electrical tape and checked again for anything that may be a problem. It lasted 1/2 hour after inflating to 80 psi. I'll check around for a better innertube tomorrow.

Trueblood

My guess is that you have hookless rims. I have to be careful with a few of my bikes. I won't inflate above 70 or 75 psi on the older wheels with hookless rims. Had a few blow off the rim while the bike sat in the apartment. Or, if the sidewalls are shot on the tires they can blow out. Any sign of dry rot on the the tires?

TLit

I replaced the tire with a tough unit.

Trueblood

hmm, ok, have a look at this thread - https://www.bikeforums.net/classic-v...questions.html . Does the tire have a bead? Can you determine if the rims are hookless?

davester

If the tire is blowing off the rim it is almost certainly because as mentioned above you don't have a hooked rim and you're pumping the tire up too high for a hookless rim.

Electrical tape instead of a rim strip? I've never heard of that. I use Velox rim tape. It seems to me that electrical tape wouldn't be stiff enough and might allow the spoke heads to poke through. Also, I'm guessing that the glue from electrical tape would probably make a big mess, especially after the rim gets hot from hard braking.

squirtdad

walmart bike shop is part of the issue

but need details on tire, rim tape, wheel size, tube size etc, how hard to install (did yoy need tools), did you check for holes, were they all at the same spot

I personally am a fan of velox tape and continental tubes, but will try some schwalbe tubes soon

madpogue 

Clarify "blew". Did the tire / tube blow off the rim? Did the tube simply pop but with the tire still in place? If it's the latter, did you examine where on the tube it blew? Did it happen in the same relative place on all the tubes?

Trakhak

With straight-side rims such as were stock on many '70s Motobecanes, ignore the maximum pressure number specified on the sidewalls of your tires. Unless specified otherwise, that number pertains to hook bead rims only. As others in this thread have said, keep the tire pressure under 75 psi.

Also, check that the tires are seated properly as you inflate them. I always inflate to about 20 psi and then check to make sure that the tire's witness line is visible and at the same height around both sides of the tire. I then inflate to 40 psi and check again, and finally, inflate to full pressure. I never go above 65 psi for 27" x 1 1/4" tires on such rims.

Interesting that people with 27" x 1 1/4" tires usually seem to try to run their tires at the highest pressure possible whereas people with 700c x 32 mm (essentially the same tires) tend to be more interested in how low a pressure they can use.

TLit

When the innertubes blow the tires are blown off the rim. I don't know the difference between varying rims, the inner part of the wheel is clean. Thanks for the advice on the electrical tape being a problem. So is there a scientific way to know proper pressure? Were all the tires in 1975 or so only getting 65 psi?

I only used my hands to install this time.

stevel610 

Take a cotton ball and run it around the inside of the rim to see if there are any sharp barbs not visible to the eye.

TLit

I don't think it is an internal puncture, as others said these hookless rims are the problem with overinflation.

I spoke to the local shop that said they never heard of an innertube blowing due to overinflation.

Also they get $37 to put an innertube in that they get $17 for. I don't know how they can pay their expenses around here. But I can't afford to spend too much after spending $35 on a wheel truing.

52telecaster

The Walmart tubes are fine in my experience. Also the inflation of tires for touring and commuting was indeed 60-75psi back in the day. It's a good place to be really.

TLit

The innertube has been blowing out on different places. First time this happened it was a small hole. The last ones have been flappy blowouts, like it exploded. Not sure what the mechanism is to cause these to blow.

daverup 

The fact that they said this seems to indicate a lack of experience with hookless rims. They absolutely will blow off if overinflated. A folding bead tire will usually not work, you need wire bead tires, and a lower inflation pressure.

Mr. 66

Let's see a picture of the rim, and tire. I would be a little surprised if the rims are hookless.

TLit

base2 

Psi is Pounds per Square Inch
70 Psi on one square inch is 70 pounds of force
70 Psi on two square inches is 140 pounds of force.
70 Psi on three square inches is 210 pounds of force.
70 Psi on four square inches is 280 pounds of force.
70 Psi on five square inches is 350 pounds of force.
etc, etc...
How many square inches is the inner surface area of the air cavity you are filling?

Your stamped steel rim is holding many thousands of pounds of force. Probably around 6000 pounds or so trying to spread the bead seat apart. Just for normal operation.

Then add to that the absence of a bead retaining mechanism...Of course it's going to blow apart if you ask it to hold 9000 pounds. (100 psi) I don't know why anyone would expect it to hold together.

52telecaster

Definitely want to stay 70 or under on those. That wasn't the tubes fault.

smd4

As someone with a bit of experience with pressure vessels, something about this doesn't seem correct--or at least not the correct way of thinking about pressure. I fire a small boiler that, calculated, has a surface area of about 49,000 square inches. We run at a pressure of 190 psi. According to the reasoning above, that means our little boiler would be encapsulating about 9.3 MILLION pounds of force. That number--even if true, and as large as it is--doesn't concern me. But the 190 pounds per square inch does.

Am I missing something? Maybe some engineer can clarify.

base2 

I was attempting to get the OP to visualize the way the number on the gage multiplies to become the total load being contained by the system. So that he can see for himself how a tiny imperfection, defect, or load concentration can provide an escape route for a whole lot of force.

I've tossed more than a few wheels in the bin from people who jack their 2.125 tires to max pressure & can't figure out why the brake track parted. Or their vertical sided steel rims are now flared & won't hold a tire. Force times area.

I've had Chinesium carbon rims split right at the valley all the way round on first inflation. Force times area.

My Stan's rims have a maximum tire pressure for each tire size approved for use. Force times area.

In this case, the OP's absence of bead seat has probably saved his wheel from catastrophic failure a half-dozen times over. 100psi (9000 pounds) is 50% over what I would guess was determined to be the safe working load of 70psi (6000 pounds)

And you are right. The number on the gage is the number to be concerned about. I haven't run a waste heat boiler or distillation plant in a while, but there is a reason regular, hourly inspections & constant monitoring were the order of the day & failure of any component tended to kill people.
They used the wrong strength of bolt on a single fitting.

cyccommute 

This issue isn’t because a HellMart bike has hookless rims. Hookless are a recent fad and while HellMart follows trends, they are usually far behind the curve. I’ve seen this often on smaller HellMart bikes and it’s due to the rims being poorly made. Their dimensions are off so that the tire doesn’t seat properly. With a compressor, I find that I have to fill with 10 to 15 psi, check to see if the bead is seated, then inflate to 25 to 30 psi, check again, and, finally, inflate to 40 to 45 psi (no more!). Even then, I check it several times to ensure that the tire is going to stay on. It often doesn’t. It can take several tries to get the tire to seat and is often just easier to get another tire.

79pmooney

Most has been said already. My two thoughts are: electrical tape is not going to work. It creeps so eventually it wil slide and you will punture on the rim side of the tube. '

And for air pressure - you can buy a pump with gauge or simply buy a gauge or go to the bike shop, ask them to fill with 65 psi, then squeeze the tire with thumb and finger to see what 65 psi feels like. (A gas station air fill will also work. But be careful, they can overfill your tire very fast!) After you fill with that low pressure, ride and notice what it feels like. If you don't like it, have your eyes open for better wheels that fit your bike. (Rim diameter, hub OLD, freewheel/cassette compatibility.) Aluminum rims will give you radically better stopping power, especially in the wet.

smd4

Ah...you're one of those people with the calibrated fingers! Nice!

Trueblood

I didn't realize that hookless was a recent fad. A couple of my bikes from the late 70's/early 80's, with both steel and alloy wheels, have straight walls on the inside of the rim. I think of that as being hookless. Some of my newer 27 inch wheels, and by newer I mean 10 years old or so, have a lip on the inside of the rim to catch the bead. I thought that meant hooked.

I would definitely recommend a floor pump with a gauge to the OP.

smd4

They may be a recent "fad," but I don't think the design is recent.