Small wheel blowouts - options?

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andrew_s
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Re: Small wheel blowouts - options?

Postby andrew_s » 17 Oct 2019, 7:16pm

Mick F wrote:
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.

It's not going faster past the blocks.
The rim moves past the brake block at the your riding speed, the tyre aside.

The axle is stationary with respect to the brake blocks. The axle is moving at your riding speed relative to the road. The bottom of the wheel isn't moving relative to the road. Therefore the bottom of the wheel is moving at your riding speed relative to the brake blocks, and, since the wheel remains the same shape, the top of the wheel is also moving at your riding speed relative to the brake blocks.
Wheel size and RPM has no bearing on this at all.

In fact, it's likely that the rim is going marginally slower past the blocks than with a larger wheel, since the tyre takes up proportionally more of the total wheel diameter.

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andrew_s
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Re: Small wheel blowouts - options?

Postby andrew_s » 17 Oct 2019, 7:35pm

Tigerbiten wrote: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.

.5mv^2 has nothing to do with it. You still put more total heat in to the rim & brake when dragging the brake to go slowly.

Going slowly works because you're putting the heat in slower, and giving the rim & brake longer to dissipate the heat into the cooling airflow.

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Mick F
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Re: Small wheel blowouts - options?

Postby Mick F » 17 Oct 2019, 7:38pm

andrew_s wrote:
Mick F wrote:
andrew_s wrote: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.

It's not going faster past the blocks.
The rim moves past the brake block at the your riding speed, the tyre aside.

The axle is stationary with respect to the brake blocks. The axle is moving at your riding speed relative to the road. The bottom of the wheel isn't moving relative to the road. Therefore the bottom of the wheel is moving at your riding speed relative to the brake blocks, and, since the wheel remains the same shape, the top of the wheel is also moving at your riding speed relative to the brake blocks.
Wheel size and RPM has no bearing on this at all.

In fact, it's likely that the rim is going marginally slower past the blocks than with a larger wheel, since the tyre takes up proportionally more of the total wheel diameter.




I repeat.
Mick F wrote:Does a small wheel rotate more per unit distance travelled than a big wheel?
Yes.
It's rotating at a higher RPM then.
Mick F. Cornwall

mattheus
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Re: Small wheel blowouts - options?

Postby mattheus » 17 Oct 2019, 7:52pm

andrew_s wrote:
Tigerbiten wrote: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.

.5mv^2 has nothing to do with it. You still put more total heat in to the rim & brake when dragging the brake to go slowly.

Going slowly works because you're putting the heat in slower, and giving the rim & brake longer to dissipate the heat into the cooling airflow.


A perfect analysis!

(I wonder what the curve looks like of "safety" (i.e. max rim temp maybe?) vs speed - it's obviously a hump of some sort, but I doubt it's a very simple shape. When I'm retired I'll work it out properly ... )

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Mick F
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Re: Small wheel blowouts - options?

Postby Mick F » 17 Oct 2019, 8:02pm

andrew_s wrote:Going slowly works because you're putting the heat in slower, and giving the rim & brake longer to dissipate the heat into the cooling airflow.
Take the argument to extreme.

Go at much less than walking speed down a steep hill.
Keep the brakes on hard.
Wheel rotation is slow.
No heat builds up, or very little heat.

Go very fast.
Brake very hard to reduce speed as soon as possible.
Big heat build-up and maybe too much heat.

Go very fast.
Brake hard and pulse-brake then speed up again.
Big heat build-up and maybe too much heat .......... time and time again.
Mick F. Cornwall

brynpoeth
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Re: Small wheel blowouts - options?

Postby brynpoeth » 17 Oct 2019, 9:34pm

Going slow means braking a lot, I think the rims would get hot
..
Driving tip: use the same gear going up and going down an alp (don't use the brakes much)
Wouldnae work for cycling :?
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drossall
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Re: Small wheel blowouts - options?

Postby drossall » 17 Oct 2019, 10:36pm

Mick F wrote:I was derided on here for suggesting that black rims retain heat more than silver rims.

Black bodies radiate heat faster than white/silver, all other things being equal. They also absorb heat faster, but the heat here is coming from friction, where colour is probably not a significant factor. The heat loss is also by convection to the passing air, but again it's not evident to me that rim colour would have a major effect.

So, I'd expect black rims to cool (somewhat) faster, because of the increased rate of radiation.

If you left your bike out in the sun while you had a coffee at the bottom of the mountain, black rims would indeed absorb heat from the sun faster. But on the descent, that heat source is not a major factor in causing blow-outs.

mig
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Re: Small wheel blowouts - options?

Postby mig » 17 Oct 2019, 11:07pm

riding fixed would provide a solution. a touch of gentle back pressure every now and again would slow the bike easily down the tourmalet :wink:

Brucey
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Re: Small wheel blowouts - options?

Postby Brucey » 17 Oct 2019, 11:11pm

brake of last resort could be a land anchor. Wheel size would be irrelevant.

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andrew_s
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Re: Small wheel blowouts - options?

Postby andrew_s » 18 Oct 2019, 12:40am

mig wrote:riding fixed would provide a solution. a touch of gentle back pressure every now and again would slow the bike easily down the tourmalet :wink:

I tried leg braking only going down a 1 in 6 one time.
It didn't work, and got quite frightening. Fortunately the bottom of the hill arrived before things got completely out of hand.

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andrew_s
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Re: Small wheel blowouts - options?

Postby andrew_s » 18 Oct 2019, 12:51am

Mick F wrote:I repeat.
Mick F wrote:Does a small wheel rotate more per unit distance travelled than a big wheel?
Yes.
It's rotating at a higher RPM then.

Of course, but the higher RPM of a small wheel doesn't affect how fast the rim is going through the brake blocks, which is what you seem to be claiming.

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Mick F
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Re: Small wheel blowouts - options?

Postby Mick F » 18 Oct 2019, 6:43am

Done some arithmetic over an early morning cuppa! :D

Moulton's wheels are 18.3" diameter.
Mercian's are 26.2" diameter.
I know these facts as I've measured them.

Say 15mph ...............

Moulton's wheels are rotating at 275rpm
Mercian's wheels are rotating at 190rpm

I can't my head round the fact that the tops of the wheels are going through the blocks at the same speed if the rotation rate is higher .............

............... although I do know that the bottom of the wheel is stationary on the road, the axle is travelling at bike speed, and the top of the wheel going at twice bike speed irrespective of wheel diameter.

These two facts need another cuppa for me to grasp this! :lol:
Mick F. Cornwall

roubaixtuesday
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Re: Small wheel blowouts - options?

Postby roubaixtuesday » 18 Oct 2019, 7:08am

drossall wrote:
Mick F wrote:I was derided on here for suggesting that black rims retain heat more than silver rims.

Black bodies radiate heat faster than white/silver, all other things being equal.


Interestingly, this is not true, at least not for black/ white.

Emmissivity in the infra red is independent of colour in the visible spectrum.

Polished metals (silver) do generally have lower emmissivity in IR than other materials.

In the visible spectrum, where most energy is received in sunlight, black materials do have much higher absorbtivity, which is why they get hotter.

Absorption and emission are always the same at any given frequency, Kirchoff's law.

Here's a table
Emissivity-table-of-some-common-materials-This-is-not-a-comprehensive-list-and-should.png


For a rapidly descending bicycle, most heat transfer will likely be via convection rather than radiation. This has been explored before here:

viewtopic.php?f=1&t=123534&p=1259520#p1259520
Last edited by roubaixtuesday on 18 Oct 2019, 8:47am, edited 1 time in total.

drossall
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Re: Small wheel blowouts - options?

Postby drossall » 18 Oct 2019, 8:02am

Thanks, I am supposed to know this stuff, but it's a long time since I studied physics :oops: I did realise that convection was more important anyway.
Mick F wrote:I can't my head round the fact that the tops of the wheels are going through the blocks at the same speed if the rotation rate is higher .............

............... although I do know that the bottom of the wheel is stationary on the road, the axle is travelling at bike speed, and the top of the wheel going at twice bike speed irrespective of wheel diameter.

Imagine that your small and large wheels are both rotating, and their respective bikes are travelling at (say) 20mph. The bikes are moving at the same speed of course. Now imagine that you could turn the small-wheeler upside down as a sort of mirror image in a puddle, and put its front wheel in contact with the front wheel of the large-wheeler. You do that at the bottom of each wheel, which you've agreed isn't moving. So, the two wheels are now rotating in contact with each other, and won't change speed because of being in contact.

Now get rid of the rest of the bikes, the ground, and everything else. The two wheels are rotating in contact with each other, and must be travelling at the same speed.
Last edited by drossall on 18 Oct 2019, 8:11am, edited 1 time in total.

pwa
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Re: Small wheel blowouts - options?

Postby pwa » 18 Oct 2019, 8:05am

I remember reading something some years ago about braking tactics on descents to minimise heat in the rims, and the conclusion was that there are basically two things that work.

One, which I find more practical in most circumstances, is the traditional fast descending, letting the brakes go and the speed build up, then braking hard and late as a bend approaches. That leaves the rims cooler than if you have been dragging the brake all the way down.

But there was a second suggested solution, and that was to descend very, very slowly, at 2 or 3mph, which it was claimed did not produce the heat build up that constant braking at 15mph would produce. I can see that being useful on some twisty lanes in Wales. Anyone got any ideas why that might work? Maybe it is to do with the fact the descent will take much longer and the rim will be able to lose heat at the same rate it picks it up.