Another tyre question

[graham bowers]

Hi

The question is how much margin is there in recommended max tyre pressures please? Context is:

I recently bought a Dawes Kingpin of 1980 vintage and replaced the tyres and tubes with new ones as a precaution. Both tyres and rims are marked with the French size 500a, which is 440 in ETRTO language.

The new tyres are Shwalbe HS 166 size 37-440, which are within a mm on width as the old Michelins that were on it from, I guess, 1980. The marked max pressure on the new HS166's is 60 psi, so I pumped them up to 60 psi with a trackpump that has a pressure gauge that has been checked (it agrees with the pressure sensors on my wife's new (company) BMW). I've ridden probably 100 miles on the tyres over the last month.

I used the bike to go volunteering last Wed and it was locked up all day within my sight. There was a loud pop around lunchtime and the rear tyre had popped off the rim, bursting the tube. The bead of the tyre is now damaged, see pic. I replaced the tube and the tyre just popped off the rim again but straight away.

I clearly need to replace the tyre but am reluctant to replace it with another HS166, and am also concerned about the front one, but would be OK letting some pressure out as most of the load is on the back one.

Does the panel think I optimistic expecting to be able to pump it up to the max recommended?

Graham

 

[mjr]

Does the panel think I optimistic expecting to be able to pump it up to the max recommended?

No. But on 37mm, I run 45psi front and 65psi rear, so I'm surprised by that marked max pressure. I do lug a lot of stuff around, though

The other things I wonder from the photo are what the rim width was (500A was 32mm width so it could be 15mm rims which wouldn't support a 37mm tyre well), if the rim's steel (it's fun finding brake pads for them!) and if it's true (which is a very quick way to help a tyre pop off...).

 

[Yellow Saddle]

I suspect that the tyre popped off because the bead failed, not the way around you describe. The fact that it comes off immediately with lower pressure confirms to me that the bead is damaged or stretched. However, it is difficult to say from just a photo. I guess that's a kevlar/foldable bead?
Are you sure your rim is hooked and made for a beaded tyre? That looks a lot like a steel rim to me.

My suggestion would be to try with a steel bead tyre, if the rim is indeed intended for a beaded tyre.

To answer your question, a tyre should not come off at the max pressure or even 20% above that.

 

[graham bowers]

Thanks for the replies thus far. At least it seems the failure was not caused solely by the inflation pressure. At the moment I'm thinking it was probably a defective tyre, but detailed answers to questions are below.
mjr
I lug a lot of me around, 15 stone, hence the wish to run at 60 psi as the (my) weight is heavily biased onto the back wheel.
It is a steel rim, 21mm between the inside faces of the flanges. I'll attach a photo of the rim markings.
I had a copy of the ETRTO standards on my company laptop that I now regret not nicking when I retired. Sheldons page (3/4 down) indicates a 21mm rim and 37mm tyre should be OK - but who knows the real truth ?-))

Yellow Saddle
You may well be right in the sequence of the chain of events - the order I described was simply the order of the observations.
The tyre is steel beaded, the exposed bead in the photo is small diameter wire that I imagine is wrapped around a larger hoop. As above, it is a steel rim.
The original Michelin tyres I took off were steel beaded, which I am taking as an indication that the rims are intended for a beaded tyre.
If the rim is not intended for a beaded tyre, what type of tyre would it be intended for please? I'll attach a pic of the rim profile, the flanges gently diverge.

 

[mjr]

Is it true? Watch the gap between brake pads and rim as you spin the wheel, and the relative vertical positions of pads and rim.

 

[raleighnut]

It's a non-hooked rim, you need to run lower pressure unless you change the rim.

 

[graham bowers]

Is it true? Watch the gap between brake pads and rim as you spin the wheel, and the relative vertical positions of pads and rim.

The wheel was duly spun and no "Eureka" moment happened. I'm going to need a bigger clue, I'm afraid.

 

[graham bowers]

It's a non-hooked rim, you need to run lower pressure unless you change the rim.

Is there any specific guidance anywhere on how to determine a suitable pressure value please? I've googled btw and not found anything yet.

 

[mjr]

Is there any specific guidance anywhere on how to determine a suitable pressure value please? I've googled btw and not found anything yet.

http://www.bikequarterly.com/images/BQTireDrop.pdf but I suspect that says something higher than what you were trying. I'm not sure what pressures non-hooked rims would support or how you deal with this problem.

 

[raleighnut]

Is there any specific guidance anywhere on how to determine a suitable pressure value please? I've googled btw and not found anything yet.

I can't find any specific pressure info either. I know that they won't stand high pressures though and will blow off like yours did.

 

[graham bowers]

OK all, thanks for your input. I'll try another tyre and run at 50 psi. I'll clean and degrease the rim and hope for the best, taking great care over the concentricity of the fitting line. If it recurs I'll fit modern hooked alloy rims at 451 size.

 

[Yellow Saddle]

I agree with Raleighnut. That is a non-beaded rim and it will not take kindly to beaded tyres. Unfortunately I lived in an era after steel rims

OK all, thanks for your input. I'll try another tyre and run at 50 psi. I'll clean and degrease the rim and hope for the best, taking great care over the concentricity of the fitting line. If it recurs I'll fit modern hooked alloy rims at 451 size.

I would just fit a modern hooked aluminium rim. You will be amazed at how much better the bike will stop too.

 

[mjr]

And how quickly the

I would just fit a modern hooked aluminium rim. You will be amazed at how much better the bike will stop too.

And how the brakes will wear the rim out!

If you fit a new one, keep the old wheel/rim in case you want to resell the bike to a collector/restorer.