Blow out due to Tyre rim miss-match or maximum inflation plu

[Philip Benstead]

Blow out due to Tyre rim miss-match or maximum inflation plus sun

A colleague of mine (yes it was not me) inflated a front tyre 700 x 40c Kenda Cross w/Kshield I assume to it maximum pressure rating.
It was mounted on a Rims Front: Alex R-1000,

The cycle was parked on a balcony with a glass frontage which the sun could shine on.
The following day the inner tube blew with a bang.

I inspected the wheel and found that the tyre was loose on the rim, I could not take it off due security lock, and the owner had lost the key?

I spoke to experience cycle mechanic who said that on budget bikes you should not inflate to the maximum pressure rating because some cheap rim are unable to take the presage. if shone on by the sun it could increase the presser upto 20%.

He said on steel rims do not inflate above 60 pounds psi

I asked where I could find the correct pressure rating for tyre /rim combination, he said it due to experience and that if the tyre it hard to put on the rim then it could take the maximum pressure.


Comments welcomed


http://www.bikepedia.com/quickbike/Bike ... assage+3.5

2007 Raleigh Passage 3.5
Bicycle Type Hybrid
MSRP (new) $285.00
Weight
Sizes 15", 17", 19", 21", 23", women's 16", women's 19"
Colors Silver
Item ID 16315


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Side by side comparison


Frame & Fork
Frame Construction TIG-welded
Frame Tubing Material Atomic 13 Aluminum
Fork Brand & Model Raleigh
Fork Material Steel
Rear Shock Not applicable

Components
Component Group Hybrid Mix
Brakeset Tektro Alloy V-brake brakes, Tektro V-brake levers
Shift Levers Shimano Revo
Front Derailleur Shimano C50
Rear Derailleur Shimano Altus
Crankset SR Suntour XCC208, 28/38/48 teeth
Pedals Comfort Platform
Bottom Bracket Sealed Cartridge
BB Shell Width Unspecified
Rear Cogs 7-speed, 14 - 34 teeth
Chain KMC Z51
Seatpost Alloy Micro adjust Suspension, 27.2mm diameter
Saddle Avenir Deluxe Comfort
Handlebar Steel Riser
Handlebar Extensions Not included
Handlebar Stem Alloy adjustable
Headset 1 1/8" threadless steel

Wheels
Hubs Alloy QR
Rims Front: Alex R-1000, Rear: Alex R1000, 36-hole
Tires 700 x 40c Kenda Cross w/Kshield
Spoke Brand Stainless steel, 14ga. (2.0mm) straight gauge
Spoke Nipples Unspecified
- See more at: http://www.bikepedia.com/quickbike/Bike ... zvYch.dpuf

[andrew_s]

About 20% is right for a wheel left in the sun with no air flow, but it could be higher (23-24%) if it was cold when the tyre was inflated.

As well as a loose-fitting tyre, you can get blow-off through poor fitting. This would typically be where the tube gets pinched between the tyre bead and the rim, but you could also fit the tyre off centre. Normally blow-off would happen relatively soon, but it could delay until something like solar heating sets it off.

Both rims and tyres of the same model can vary in the bead/bead seat diameter, so you can't necessarily say "buy this tyre/rim" or "don't buy that one". This can apply even to quite up-market gear (I've had Conti GP 4-seasons vary between almost impossibly tight and fairly loose, on the same actual rim).

Difficulty fitting can be caused by a rim with a shallow well rather than the relative bead/bead seat measurements, and a shallow well doesn't affect the likelihood of blow-off. A shallow well will make all tyres relatively hard to fit, not just some.

If the rim couldn't take the pressure, it would be visibly damaged - either split or the flange bent outwards half way up the braking surface. It would be unlikely for this to happen even on cheap rims unless the rim was well worn (by the brakes).

[Brucey]

....If the rim couldn't take the pressure, it would be visibly damaged - either split or the flange bent outwards half way up the braking surface....

unless the rim has no hook bead, in which case the tyre can blow off the rim at a pressure lower than the manufacturer's rating. The same thing can happen if the tyre is a loose fit or was fitted badly; in both cases the rim might as well not have a hooked bead at all, 'cause the bead ain't in it....

It would be easy to exceed the pressure rating of these tyres, and the pressure is only going to go up in the sunshine.... Note also that if the rim has no hook bead, the max tyre pressure should be derated; it will usually say this in the weasel words somewhere.

BTW if the tube was pinched badly, this can sit there very happily until one day it suddenly blows out. I don't know if the tube blows out through the gap or if the tyre unseats further before the tube blows out, but something happens anyway, and it can be days after the tyre is pumped up.

cheers

[De Sisti]

I spoke to experience cycle mechanic who said that
.... on steel rims do not inflate above 60 pounds psi

Well that's a mechanic I wouldn't take advice from. Tyre pressures can go to twice the recommended max
on their sidewalls. Tyre manufacturers regularly do that. However, for legal reasons of some sort, they
always recommend a lower max psi.

[531colin]

I spoke to experience cycle mechanic who said that
.... on steel rims do not inflate above 60 pounds psi

Well that's a mechanic I wouldn't take advice from. Tyre pressures can go to twice the recommended max
on their sidewalls. Tyre manufacturers regularly do that. However, for legal reasons of some sort, they
always recommend a lower max psi.

For modern tyres, the max. recommended pressure assumes the rim has a bead hook.
Therefore on ANY rim without a bead hook the tyre can blow off at less than the recommended maximum pressure......I don't remember ever seeing a steel rim with a bead hook.

[andrew_s]

The problem Alex R1000 rim is a normal hooked alloy rim.
steel rims are just a general comment from the bike mechanic. Generally, any tyre with a max pressure over about 60 psi will also say on the side something along the lines of "hooked rims only"

If the tube is pinched between the type bead and the rim, you can get a delay before anything happens because you have to get some air into the pinched off bit of tyre first. What allows air in can be a number of things - cornering loads reducing the pressure of the bead on the rim hook, a couple or more soften/reinflate cycles, extra pressure due to heating, or whatever, but once air gets past the seal that the bead & rim form, it will reduce the sealing pressure and allow extra air to get in more easily. Eventually you get a runaway and the bea is pushed clear of the hook and the tube goes bang.

[Brucey]

probably not relevant in this instance, but very many new tyres have 'mould release compound' on them. This is an effective lubricant; on the plus side it can help a tight tyre seat properly (you may have seen a bike mechanic 'helping' a new tyre to seat using a pair of grips with large curved jaws that grab the tyre; these work best on new tyres, less well on older ones...). On the minus side it can help a tyre that is badly seated and/or on a non-hook bead rim to creep off the rim.

Often the release compound is waxy, which means that when the rim gets hot, the tyre can slide around (or off...) more easily. I have little doubt that many reported 'blowouts on tour' are because of this; nice new tyres, fitted just before a tour, are quite likely to come off the rim the first time they get hot if there is the slightest thing wrong with the fitting and there is release compound present.

Back in the day when I used non-hook bead rims, I knew that if I wanted some tyres to stay put on some rims, I had better use solvent to remove the release compound from a new tyre before fitting it. Until I did this routinely, I had tyres that blew off the rims when new, but then would mysteriously stay put once they were a bit older (and presumably the release compound had been removed through handling).

cheers

[RickH]

Schwalbe don't recommend going above 4 bar (~58psi) with hookless rims in their Tech Info (p 34 of PDF available here). They do note that "According to the ETRTO standard the inflation pressure is limited to five bar" (~72psi) but err on the side of caution.

Rick.