Small wheel blowouts - options?

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speedsixdave
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Small wheel blowouts - options?

Postby speedsixdave » 17 Oct 2019, 11:00am

Hi all,

I've been riding my newish Moulton Jubilee for 18 months or so now and it's a fabulous bike, subject to a few idiosyncrasies that we have covered previously on this forum. There is however one serious issue that troubles me greatly - front wheel overheating blowouts.

I toured in the Pyrenees this summer on the bike with quite a heavy load, crossing a few cols on the way. About 2km over the top of the Tourmalet I had a front wheel blowout. Fortunately I had already lost most speed for a tight hairpin and was able to come to a wobbly but safe halt. The front rim was extremely hot and the tube had a decent-sized burst on its inner side. At this distance I cannot remember the shape of the hole or whether it was on the tube's seam - it was too large to fix happily so I'm afraid I chucked the tube.

The blowout gave me the heebie-jeebies in a big way - I was very happy to get down the Tourmalet safely in the slowest time since Eugene Christophe and I altered my plans out of fear to avoid the Soulour and Aubisque the next day. Over the years I have ridden lots of mountains on lots of bikes and I know how to descend in the approved manner, so I do not think technique is the issue.

I want to ride over more mountains next year and thereafter, and ideally on the Jubilee, so I am looking for practical options to reduce the risk of future front-wheel blowouts. I appreciate one obvious option is 'ride a different bike' but I'm trying to avoid that if possible. My thoughts include:

(1) Changing the brake calipers from the current Tektro 737s (?). MickF claims that this change makes all the difference and I could simply cough up for TRP 957s or the Velo Orange Grand Cru 57mm-drop brakes. Could this feasibly make any difference to the chance of blowouts? My schoolboy physics tells me it's unlikely but that was a long time ago.
(2) Different brake blocks? I was using Fibrax red compound.
(3) Fatter tyres. Should help but I think the current 28mm (Duranos for touring) is about all I'll get under the mudguards.
(4) Adding a front hub brake, which would have to be the Sturmey X-FDD because I like the hub dynamo too much now to go back. I would be tempted to run this from a second lever so I could switch between two front brakes. This seems a good touring solution if a bit heavy, but the geometry of the Moulton leading-link fork makes this more complicated than it might be.
(5) Different rim tape?
(6) Different rims? Currently running the Moulton 406 black rims.

What do we think? Are there other obvious solutions I might have missed?
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pwa
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Re: Small wheel blowouts - options?

Postby pwa » 17 Oct 2019, 11:20am

If it will take a hub brake, that would be my choice. The rims have smaller surface area than 700c but are still going to take the same heat build up, just more concentrated. Heat blow outs are a problem on 700c so they will be even more so on smaller wheels. Get the braking away from the rims and have some peace of mind.

I had heard there was some problem with hub brakes on Moultons, but I'm not sure what.

brynpoeth
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Re: Small wheel blowouts - options?

Postby brynpoeth » 17 Oct 2019, 11:23am

Reduce the tyre pressure before descending?
A colleague mentioned that his car tyre pressures had been uneven, the vehicle hand been standing in the sunshine for a while, the air in the tyres on the sunnyside expanded more :?
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Brucey
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Re: Small wheel blowouts - options?

Postby Brucey » 17 Oct 2019, 11:24am

My guess is that the tube split whilst still fully contained within the cover. You will recall a noise as the tube burst; if this was shotgun-esque then the tyre was possibly coming off the rim and the tube burst into air. If the noise was not so loud, maybe a bit muffled, (a bit like bursting a balloon in a pillowcase perhaps) then the tube most likely split within the cover somehow.

There are various ways this can happen. The tube is always very stretched in places and it is in those places where splits are likely to develop. A lot depends of the fit of the tyre on the rim. If there is a tiny gap between the tyre bead and the shoulder, one possible happenstance is that the tube, when hot, pushes into this gap and can't stretch far enough (rubber is both stretchier and much weaker when hot) and therefore splits.

Even without this, some tubes are very stretched when fitted (imagine how far the 'bottom half' of the tube has to stretch into the corners by the beads and the rim well) and can split anyway. If the tube is lubricated with talc when fitted, it won't be so stretched, so is less likely to split when hot.

If the tube was trapped beneath the tyre bead when it was fitted, then it can swell and burst into the air and make a loud noise even though the tyre isn't 'coming off the rim' per se.

Small wheels with rim brakes are vulnerable to overheating for sure. And bicycle brakes (of any kind) are no heavier/better than they need to be, so repeated braking for hairpins on a steep pass, with small wheels, whilst heavily loaded, is about as hard a test as you can get. Oddly enough the more cautious you are, the greater the risk in some ways; slower speeds require more net braking (there is less aero drag) and worse brake cooling.

Since a rear drum brake is easier to fit and a rear brake makes a good drag brake, it might be OK to fit one of those. The drag brake won't stop you very well but it will reduce the heat load into the rim brakes.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

brynpoeth
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Re: Small wheel blowouts - options?

Postby brynpoeth » 17 Oct 2019, 11:26am

Use a tube of much thicker rubber, are such available?
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pwa
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Re: Small wheel blowouts - options?

Postby pwa » 17 Oct 2019, 11:27am

There is also braking technique, but I expect you know about that. Don't drag the brakes. Let the bike build up speed with no braking, then brake hard and late for the next bend. It heats the rims less than steady braking over a longer time. And just stop every mile or so if you have to. Both those techniques work.

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Mick F
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Re: Small wheel blowouts - options?

Postby Mick F » 17 Oct 2019, 11:27am

speedsixdave wrote:Different rims? Currently running the Moulton 406 black rims.
I was derided on here for suggesting that black rims retain heat more than silver rims.

I still believe it.

I got rid of the bendy Tektro brake callipers and fitted Shimano 105s and at the same time fitted CR18 silver rims.
No overheating in the slightest now.
This could well be the result of having better friction with the combination of good brakes and good rims.

I had a couple of blowouts on the front and the rim was far too hot to touch for a few minutes to even do anything.
Last blowout was going down the back route into Boscastle. Steep 25% hill with not much in the way of good surface or visibility round the corners. I pulsed the brakes as much as I could, but retardation was difficult and got worse and worse. Lucky to actually stop! :shock:
Mick F. Cornwall

brynpoeth
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Re: Small wheel blowouts - options?

Postby brynpoeth » 17 Oct 2019, 11:29am

I submit:
A machine that suffers catastrophic failure as described by Mick F is not Fit For Purpose

Alternative facts welcome
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roubaixtuesday
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Re: Small wheel blowouts - options?

Postby roubaixtuesday » 17 Oct 2019, 11:37am

The hub brake is probably your only reliable option, thereby dumping heat in the hub rather than the rim. We fitted one to our tandem and I wouldn't be without it.

Dumping the heat through air resistance means going very fast and thereby maximising drag and minimising brake usage. Alternatively go very slow thereby maximising cooling through heat transfer relative to heating through braking.

Any theory of braking claiming to reduce heat input to the rim has to explain where the heat goes instead. No alternations to pads or calipers can possibly help if you still descend at the same speed with the new setup.

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Mick F
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Re: Small wheel blowouts - options?

Postby Mick F » 17 Oct 2019, 11:41am

brynpoeth wrote:I submit:
A machine that suffers catastrophic failure as described by Mick F is not Fit For Purpose

Alternative facts welcome
Utterly correct.
It was dangerous with no warnings.

The brakes even now aren't brilliant. They stop well, but not as well as Campag on Mercian.

The issue generally with small wheels, is that they go round faster for any given road speed. Yes, they slow faster too, but at the point of brake application, the rim is going past at a tremendous speed compared to a 700c bike. That speed has to be slowed very quickly.
Feathering the brakes will make them heat up far quicker than the same feathering on 700c.

Heating brakes reduces efficiency. Small wheel calliper brakes aren't efficient.
Hub brakes or disk brakes are the ideal.

I fitted a front roller brake on the Chopper as the chrome wheel brakes were terrible. Front wheel on a Chopper is 12".
The roller brake came off a 27" bike and on that, the roller brake is ok. On the Chopper is was FANTASTIC! I had to be careful using it even though the bike was heavy and for the most part I towed a heavy (un-braked) trailer too. Rarely used the rear brake as it was almost useless.
Mick F. Cornwall

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andrew_s
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Re: Small wheel blowouts - options?

Postby andrew_s » 17 Oct 2019, 11:49am

(Schoolboy physics)
The gravitational potential energy to be dissipated during a descent has 2 places to go - into the surrounding air as wind resistance, or into the brake/wheel interface as heat.
If you descend at the same speed, braking in the same manner, you aren't going to increase the air braking, so the energy going into the brakes is fixed, and if your wheel rim is half the mass it will heat up twice as much.

I assume you know that the maximum air braking is achieved when going fast, so you are best served by sitting in as un-aerodynamic position as possible, freewheeling, and braking hard occasionally (before corners, or as the speed rises too far).

How about attaching a small parachute to your waist?

Other than a hub brake, I'd probably go for a relatively heavy inner tube with no patches on it, and take more photos on the way down.

Note that disc brakes have a fairly low mass, and heat up quite a lot. This can cause problems - eg plastic bits of cable discs melting, or hydraulic fluid boiling if any water has ever got in (if that happens, you've no brake).

brynpoeth
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Re: Small wheel blowouts - options?

Postby brynpoeth » 17 Oct 2019, 12:00pm

Could one unfurl a sail to slow one going down, like a parachute?

I am so scared of descending, I should take my bike back down on the bus :?
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pwa
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Re: Small wheel blowouts - options?

Postby pwa » 17 Oct 2019, 12:14pm

Where the road allows, let go of the brakes, sit up and make yourself as un-aero as you can. If you have a jacket on, unzip it. That means you arrive at the next braking point with less speed and with your rims hopefully cooled.

But I'd still like that hub brake if it is possible.

mattheus
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Re: Small wheel blowouts - options?

Postby mattheus » 17 Oct 2019, 12:21pm

andrew_s wrote:(Schoolboy physics)
The gravitational potential energy to be dissipated during a descent has 2 places to go - into the surrounding air as wind resistance, or into the brake/wheel interface as heat.
If you descend at the same speed, braking in the same manner, you aren't going to increase the air braking, so the energy going into the brakes is fixed, and if your wheel rim is half the mass it will heat up twice as much.

I assume you know that the maximum air braking is achieved when going fast, so you are best served by sitting in as un-aerodynamic position as possible, freewheeling, and braking hard occasionally (before corners, or as the speed rises too far).

I agree with your "schoolboy" physics (I have a degree in the stuff!). It was a Chris Juden article that alerted me to this tactic, so credit to him.

Given that hard late braking is best, I wonder if better brakes *do* help? It's an aspect I hadn't considered before, as my rim brakes have generally not been a safety issue on mountainous descents.

pwa
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Re: Small wheel blowouts - options?

Postby pwa » 17 Oct 2019, 12:27pm

I have blown tyres through heat build up in the Alps, and I have had a hydraulic disc brake fail completely (lever back to the bar with no effect) when the fluid boiled. So heat is definitely an issue in some circumstances. Small wheels with rim brakes don't help.