Inner Tube Blow Outs

[foxychick]

I have tried installing a 700 x 35/43mm inner tube into a schwalbe 700 x 50mm tire onto a 19mm wide rim twice and both tubes have fractured, one of them at the valve, any advice would be appreciated.

[pwa]

Smooth off any sharp edge around the valve hole. And get a larger tube.

But where were the ruptures? Did they face outwards towards the tyre tread or inwards towards the wheel rim? If inwards you may have an inadequate rim tape / lining and / or something sharp such as a projecting spoke head.

[peetee]

Most tubes are less inclined to inflate proportionally at the valve. It could be that the slightly undersized tube just won't expand enough at that point and is rupturing.

[Brucey]

I agree with the above; I think it is best to use a tube that is towards the lower end of the claimed range of sizes because the manufacturers are, uh, 'somewhat optimistic' about the range of sizes that are covered. When you inflate a tube outside the tyre you can usually see that the part around the valve sees some horrible strains and usually inflates less easily than the rest of the tube too.

I'd also note that, very often, the valve stem is reluctant to settle in its final position. This is most commonly seen with Schrader valved tubes, which often have rubber half-way up the stem. If a twinwall rim is correctly drilled, often the thicker valve stem won't settle properly or is damaged in the process. Needless to say if the valve stem doesn't settle properly the tube tries to expand into the space under the valve stem and consequently bursts. I have seen this often.

It is always best to check that the valve stem will pull through the rim OK, far enough not to leave a void by the valve stem, which will surely cause the tube to burst.

cheers

[foxychick]

Is it safe to use a 50mm tyre on a 19mm rim?

[Vorpal]

Is it safe to use a 50mm tyre on a 19mm rim?

I wouldn't, but I'm pretty cautious with such things. At best, I'd say it's marginal. I know folks who would try it. I suspect that if you can successfully install it, it's safe to use, at least short term. It may cause premature failure of the rim.

[Brucey]

according to etrto it is OK to use a 50mm tyre on a 17mm rim with a hook bead (crotchet type) rim. However with a non-hook bead rim a 50mm tyre is the largest that will fit a 20mm internal width rim, and 37mm is the widest tyre that should be fitted to an 18mm rim.



cheers

[kylecycler]

Is it safe to use a 50mm tyre on a 19mm rim?

I do - both my Carrera Subways came with that - it's safe enough, especially given that (or maybe even if only because) the tyres are run at ~42 psi front, ~47 psi rear, and that's allowing for the bikes sometimes being laden. They still roll fast at these pressures but with a compliant ride. At higher pressures you might well get issues with stresses on the rim, as Vorpal expressed concern about, but there's no benefit in running higher pressures with tyres that width - you won't go faster and you'll just beat yourself up.

The slightly older Subway now has 50 mm Schwalbe Big Apples on 26-inch wheels, the newer one runs 50 mm Schwalbe Almotions or Supremes on 27.5-inch wheels, all on 19 mm rims. Actual tyre widths on the rims are ~46-47 mm.

It's not ideal, though. I tried 2.15 inch - 55 mm - Big Apples on the 26-inch wheeled Subway and although they were ok at the pressures above, if I tried to run them any lower, to benefit from their width and give a cushier ride (the Subways' frames and forks have no compliance, so it all has to come from the tyres), they got all wobbly when the bike was loaded, and I figured that was because the rim width was just too narrow for that tyre width. You can also get 60 mm Big Apples and IME they wouldn't work properly on a 19 mm rim; they might even 'blow off' the rim, or at the very least the bead might be liable to 'creep up' the rim and not seat properly. Maybe not, but I wouldn't do it.

You'll see from the charts Brucey just posted that 19 mm is within the approved limits for 50 mm tyres (I started this before he posted his comment!) but like I said it's not ideal. 21 mm would be better, and as an example, WTB recommend 24 mm ideal inner rim width for their 47 mm Horizon 650B (aka 27.5") tyre.

I've just checked my spare (Schwalbe) inner tubes - same as what's on the bikes - and they're both marked 40/62 width, which would be the ideal, I think. Your 35/43 tubes would be stretching awful thin when inflated, which might well be part of the problem. Other than that, make sure that after fitting the tyre you always push the valve up into the rim to make sure the tube isn't trapped by the tyre bead around the valve. Also go around the beads and make sure the tube isn't trapped anywhere else. Apologies if you know that already, but it's a common issue - IME the tube then tends to go BANG!!! - a delayed reaction, usually half way through the next night!