Tyres, tubes and brake clearances

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Tyres, tubes and brake clearances

Postby mountainman531 » 1 Jun 2017, 9:58pm

I've run 700 x 25c tyres for years but I recently decided to try a 700 x 28c Shwalbe Marathon Plus which is also marked 28 x 1.10, 28-622. I used Schwalbe inner tube which is marked as suitable for 700 x 18 to 700 x 28c and the rim is a Mavic Open 622 15C.

I pumped the tyre up hard using a Revolution mini track pump and within a few seconds there was a loud bang and the tube had pushed the tyre off the rim and there was a 3" -4" gash in the tube. My initial thoughts are that the tube is not of a large enough diameter, it looked small inside the tyre. I think it was stretched too much, it's claimed size range seems to be too ambitious. Does anyone have any thoughts on this? I had been using this tube with the 25c tyre with no problem.

Also I'm using Shimano deep drop dual pivot brakes which work well on this rim with 700 x 25c tyres but with the 28 one brake pad only just misses the tyre whilst the other just catches and both are at the bottom of the slot on the caliper. I could be tempted to trim the edges of the pads with a Stanley knife unless there's a good reason not to?

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Re: Tyres, tubes and brake clearances

Postby tim-b » 2 Jun 2017, 5:52am

I'd be surprised if the tyre size has caused those problems, and a couple of things seem likely:
You haven't seated the hub/axle within the front fork properly (it might be that the brake didn't have a deep-enough drop to start with)
And you haven't seated the tyre on the rim properly
Google and Youtube will help you out, or if you prefer the written word the ParkTools website

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Re: Tyres, tubes and brake clearances

Postby Brucey » 2 Jun 2017, 7:06am

Schwalbe marathons can vary a bit in size, and the Mavic rims are usually an easy fit (which you will see when you put the first bead onto the rim; the second is always more difficult because the carcass is stiff). It may be that the tube was (when inflated) trying to push into a narrow gap between the tyre bead and the rim tape. This is usually OK provided the tube isn't too stretched already and it is able to slide around easily. However that size of tube is on the limit and a used tube often won't be very 'slidy' unless it is lubricated with more talc.

If instead the tyre was tight on the rim rather than loose then the tube may have been nipped; checking for nipped tubes is difficult with marathons because they are quite stiff to move around.

Regarding the brake pad clearance, yes if necessary trim the brake blocks (or replace them), but do note that the brake blocks ought not have changed position vs the rim as a result of fitting new tyres.

Also note that the maximum reach of the brake can vary on one side with how the centring screw is set; the RH front and LH rear arms pivot at the centre (so cannot change in max reach) but the LH front and RH rear arms pivot lower down; if you alter the centring screw setting, then manually swivel the whole caliper on the mounting bolt, you can increase or decrease the maximum reach of the arm with the low pivot.

Also note that the brake blocks change in height (reach) as the brake wears; the brake block on the centre pivot arm moves down the braking surface as it wears, but the brake block on the low pivot arm will move upwards as it wears.

BTW a Marathon is a heavy-duty tyre but a (used) Open Pro isn't a heavy-duty rim. Unless you don't use your brakes much, I'd expect the tyre to perhaps outlast the rim, i.e. the rim will wear out on the braking surface. There is no wear indicator on the Open Pro, so you need to keep a careful eye on such rims.


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Re: Tyres, tubes and brake clearances

Postby Bez » 2 Jun 2017, 8:38am

Seems highly likely to me (because precisely this happened to me once and it had precisely the result you describe) that you've accidentally failed to notice that part of the bead had slipped away from the rim hook before pumping, resulting in a few inches of tube pushing out between the rim and the tyre. An undersized tube won't cause the problem you describe.

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Re: Tyres, tubes and brake clearances

Postby mattsccm » 2 Jun 2017, 8:42pm

Frequent hazard with Challenge open tubulars. I now go down a size in inner. Works a treat.