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Re: Small wheel blowouts - options?

Posted: 17 Oct 2019, 1:06pm
by hamster
Mick F wrote: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.

Not so. The rim touches the road and is going past the brake at exactly the same speed as the bike is travelling. Only because the rim circumference is smaller, the smaller wheel makes more rotations. But the linear speed of the rim is the same.

But back to the OP:
The problem (after technique etc has been covered) is the amount of mass of metal in the rim to soak up that heat. Look for the heaviest front rim you can: maybe a BMX one will be better-suited.

Re: Small wheel blowouts - options?

Posted: 17 Oct 2019, 1:17pm
by Mick F
mattheus wrote:Given that hard late braking is best, I wonder if better brakes *do* help?
This is the thing.
Unless your brakes are GOOD, you cannot do the "hard and late" thing.

That was my issue with 20" wheel Moulton.
One: the original Tektro brakes were bendy and flexible so they wouldn't squeeze hard enough.
Two: the rims were black.

I would expect that if I were to re-fit the black rims (I still have them) and retain the Shimano 105 brakes as they are now, and do the same ride twice measuring the rim temperature silver rim vs black rim ............ the black rims would be warmer.

Not going to do it, but it would be a valid experiment to compare silver CR18 rims and black Moulton MT20 rims.


hamster wrote:
Mick F wrote: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.
Not so. The rim touches the road and is going past the brake at exactly the same speed as the bike is travelling. Only because the rim circumference is smaller, the smaller wheel makes more rotations. But the linear speed of the rim is the same.
I utterly agree, but you're not catching what I'm saying.

At the moment of braking, the wheel is spinning at a higher speed than on a normal bike. The wheel top is twice the road speed of course like any wheel. The axle is moving at road speed, the bottom of the wheel is stationary, but the top of the wheel is therefore twice the road speed ............................. but the rotation rate is very much higher than a normal wheel.

At the moment of braking, the wheel needs to be slowed from a higher rpm.

Re: Small wheel blowouts - options?

Posted: 17 Oct 2019, 1:26pm
by speedsixdave
Good stuff gents, many thanks. As ever much to chew on.

Re descending and braking techniques, on a 'typical' French mountain there's not much that one can do different: the bike accelerates to a maximum velocity of perhaps 40-50mph and then you brake for the corner. That bit at the top of the Tourmalet (descending to la Mongie in this case) is fairly worst-case as it is steep so speed builds quickly, but the hairpins are only a few hundred metres apart so there's very little time for the rims to cool before you're full on the brakes again. It's also a bit twisty between the hairpins so it takes a very brave man not to touch the brakes a bit between pins, which exacerbates. Steep narrow British descents can be bad too as the surface and visibility is rarely good enough to just 'let it go' down a 1-in-5.

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

Alternative facts welcome

A bit harsh, but there's a degree of truth in this. We cyclists like to push the limit, and as soon as one strays from the mechanical norm (e.g. all small-wheel bikes and all tandems) there's a risk that one can push too far. Whether manufacturers should put a warning on some bikes saying 'caution - do not ride down steep hills with touring loads' is another question!

Re: Small wheel blowouts - options?

Posted: 17 Oct 2019, 1:50pm
by Ant...
I once met a guy who had dragged his brakes so much on his decent of Mont Ventoux he had melted the rim tapes, then had punctures.
That was with 700c wheels and no luggage.

Re: Small wheel blowouts - options?

Posted: 17 Oct 2019, 2:18pm
by andrew_s
speedsixdave wrote:Re descending and braking techniques, on a 'typical' French mountain there's not much that one can do different:

That's why I suggested the parachute.
Slightly tongue in cheek, but the blurb did suggest 50 pounds of drag for a runner, which would definitely make it useful if it's not too much of a bike handling hazard.

Otherwise, it's just a matter of trying to deal with the heat as best you can - more mass to soak the heat up, and components that aren't as sensitive to it, of which drum brakes are probably best

Re: Small wheel blowouts - options?

Posted: 17 Oct 2019, 2:19pm
by hamster
Mick F wrote:

hamster wrote:
Mick F wrote: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.
Not so. The rim touches the road and is going past the brake at exactly the same speed as the bike is travelling. Only because the rim circumference is smaller, the smaller wheel makes more rotations. But the linear speed of the rim is the same.
I utterly agree, but you're not catching what I'm saying.

At the moment of braking, the wheel is spinning at a higher speed than on a normal bike. The wheel top is twice the road speed of course like any wheel. The axle is moving at road speed, the bottom of the wheel is stationary, but the top of the wheel is therefore twice the road speed ............................. but the rotation rate is very much higher than a normal wheel.

At the moment of braking, the wheel needs to be slowed from a higher rpm.


None of this matters; the energy stored in the system is the same (same velocity, weight of rider, bike weight etc). It has to be dissipated as heat and that heat largely has to go into the rim of wheel. Now, here's the thing, the smaller wheel has less mass of metal to soak up the heat, therefore gets hotter. THAT's the problem and you can forget all about the rate of rotation etc.

Re: Small wheel blowouts - options?

Posted: 17 Oct 2019, 2:25pm
by mattheus
speedsixdave wrote:Good stuff gents, many thanks. As ever much to chew on.

Re descending and braking techniques, on a 'typical' French mountain there's not much that one can do different: the bike accelerates to a maximum velocity of perhaps 40-50mph and then you brake for the corner. That bit at the top of the Tourmalet (descending to la Mongie in this case) is fairly worst-case as it is steep so speed builds quickly, but the hairpins are only a few hundred metres apart so there's very little time for the rims to cool before you're full on the brakes again.

Yup. (I was riding UP that in the drizzle a week ago! My problem was getting cold on the descent - I stopped half-way down to do star-jumps.)

The point about technique is that a *wrong* way also exists - and that is dragging the brakes all the way down. This is the "intuitively" safest way - if one does not know the full physics of the situation :) (or of course if your wheels/brakes are heat-proof. Or it's just damn cold.)

There are many french descents with longer straights - but on my week in the Pyrenees I found these were more common in Spain/Andorra than on the French side. The Spanish geology is more spectacular, but the tarmac tends to be more interesting on the French side; I much prefer the windy stuff! It's like Wales with the 'z' axis amplified further.

Re: Small wheel blowouts - options?

Posted: 17 Oct 2019, 2:35pm
by Mick F
hamster wrote:None of this matters; the energy stored in the system is the same (same velocity, weight of rider, bike weight etc). It has to be dissipated as heat and that heat largely has to go into the rim of wheel. Now, here's the thing, the smaller wheel has less mass of metal to soak up the heat, therefore gets hotter. THAT's the problem and you can forget all about the rate of rotation etc.
I agree mostly.

The speed of rotation is much higher.
Yes the energy is the same, but the wheel is spinning faster and has to be slowed. It's that instant that makes it very different for different diameters.

What they don't make, is different rubber brake blocks for smaller wheels. I wouldn't mind betting that the faster/smaller wheel rotations need a different compound. Probably there's not much difference in requirements so they don't. Doesn't mean that they need exactly the same friction material though.

Smaller wheels get hotter due to lack of mass to dissipate it, I am in no doubt about.

Re: Small wheel blowouts - options?

Posted: 17 Oct 2019, 2:43pm
by StephenW
An old french cycling guide recommends tying a branch behind the bike to keep the speed down on downhills. If a forester asks where you got it from, you should say that you brought it with you from Paris.

Anyway...

The Sturmey Archer dynamo is a bit inefficient according to lab tests. This will be exacerbated by small wheels. It sounds like a rear drum brake could be a better option.

Re: Small wheel blowouts - options?

Posted: 17 Oct 2019, 2:50pm
by andrew_s
Mick F wrote:The speed of rotation is much higher.

Because the road speed is the same, that makes no difference to the rate at which the brakes put heat in to the rim.

Where the wheel size may make a difference is that any given bit of rim gets heated more frequently, and has less time to cool between times.
If the rim doesn't conduct heat too well, and a section of rim does cool noticeably during one wheel revolution, you may gain with a larger wheel. Otherwise, you're just back to a bigger wheel having more mass.

Re: Small wheel blowouts - options?

Posted: 17 Oct 2019, 3:05pm
by speedsixdave
mattheus wrote:
The point about technique is that a *wrong* way also exists - and that is dragging the brakes all the way down. This is the "intuitively" safest way - if one does not know the full physics of the situation :) (or of course if your wheels/brakes are heat-proof. Or it's just damn cold.)


And if you go all the way the other way, it becomes safe again. If you crawl down at walking pace, you're only really turning potential energy to heat. My schoolboy physics tells me that stopping from 50km/h takes the dissipation of about 9660J of heat, but stopping from 10km/h takes only 386J. Of course my maths could be wrong. And that's in a vacuum.

Re: Small wheel blowouts - options?

Posted: 17 Oct 2019, 3:07pm
by Mick F
andrew_s wrote:
Mick F wrote:The speed of rotation is much higher.

Because the road speed is the same, that makes no difference to the rate at which the brakes put heat in to the rim.
Agree.

I'm saying it's going faster past the blocks. The friction required is different because it's a different speed. Not saying it gets hotter or needs more energy. Just different, and will effect the overall braking experience.

Having two very different bikes shows up how very different they are in many many respects. Braking is just one difference.

Re: Small wheel blowouts - options?

Posted: 17 Oct 2019, 4:22pm
by Brucey
Mick F wrote: The speed of rotation is much higher......


I'm saying it's going faster past the blocks. ….


at the same road speed, the rate at which the rim is passing the brake blocks is the same. Any given part of the rim visits the brake blocks more often though. The big difference is that heat input per unit length or weight of the rim is inversely proportional to wheel size.

cheers

Re: Small wheel blowouts - options?

Posted: 17 Oct 2019, 4:33pm
by Mick F
Does a small wheel rotate more per unit distance travelled than a big wheel?

Yes.

It's rotating at a higher RPM then.

Re: Small wheel blowouts - options?

Posted: 17 Oct 2019, 4:49pm
by Tigerbiten
speedsixdave wrote:
mattheus wrote:
The point about technique is that a *wrong* way also exists - and that is dragging the brakes all the way down. This is the "intuitively" safest way - if one does not know the full physics of the situation :) (or of course if your wheels/brakes are heat-proof. Or it's just damn cold.)


And if you go all the way the other way, it becomes safe again. If you crawl down at walking pace, you're only really turning potential energy to heat. My schoolboy physics tells me that stopping from 50km/h takes the dissipation of about 9660J of heat, but stopping from 10km/h takes only 386J. Of course my maths could be wrong. And that's in a vacuum.

I do that .... :D
I'm either all or nothing when descending.
Energy go up at the square of the speed.
So if it's so silly steep and twisty so I don't dare to let my bent trike go then I try and keep it to around 10 mph.
At that speed by alternating front to back to front I can keep my brakes cool.