Small wheel blowouts - options?

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drossall
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Re: Small wheel blowouts - options?

Postby drossall » 18 Oct 2019, 8:15am

Same reason as why car-bike/pedestrian survival rates in collisions get dramatically worse with car speeds. The kinetic (moving) energy that you're absorbing/dissipating goes with the square of speed. That's roughly why emergency-braking distances also go up dramatically with speed (although there's a component of those, reaction "time" [distance], that's linear with speed).

Unless RT is going to point out again that I've forgotten my first principles :lol:

roubaixtuesday
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Re: Small wheel blowouts - options?

Postby roubaixtuesday » 18 Oct 2019, 8:54am

pwa wrote: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.


The heat a rim gets too depends on the balance of heat in vs heat out.

The total amount of power is simply the rate of descent - how fast you are gaining energy through gravity.

This heat either goes into heating the rim, or heating the air.

Heating the air can come from air resistance, or heat transfer from the hot rim.

Now, consider a very slow descent.

There is very little heat entering the rim. That heat is transferrred to the air through heat transfer, which easily keeps up without the rim getting very hot.

Next, consider a very fast descent, with no braking at all.

No heat enters the rim. All the heat is dissipated in air resistance.

Finally, consider a medium speed descent. Perhaps half of the energy from gravity enters the rim. Heat transfer can't keep up without the rim gaining a lot of temperature.

So there is some rate of descent for any given hill, cyclist and bike which will produce a maximum temperature in the rim. Any descent either slower or faster will result in cooler rims. At the limit, either a very, very slow descent or a very fast descent without braking, will result in rims at ambient temperature.

pwa
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Re: Small wheel blowouts - options?

Postby pwa » 18 Oct 2019, 9:05am

So if MickF goes out on his Moulton and finds the longest, twistiest steep descent he can, one where letting go of the brakes is not an option, he should be alright if he descends at 3mph? A tedious way to descend, I know, but in extreme circumstances, to avoid a blowout.....

roubaixtuesday
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Re: Small wheel blowouts - options?

Postby roubaixtuesday » 18 Oct 2019, 9:08am

pwa wrote:So if MickF goes out on his Moulton and finds the longest, twistiest steep descent he can, one where letting go of the brakes is not an option, he should be alright if he descends at 3mph? A tedious way to descend, I know, but in extreme circumstances, to avoid a blowout.....


Yes, staying below a critical speed will solve the problem. Although whether that critical speed is 1,3,5 or 10mph I can't say.

The other thing, of course, is that going slowly will make the consequences of any blowout much less severe.

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Mick F
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Re: Small wheel blowouts - options?

Postby Mick F » 18 Oct 2019, 9:11am

drossall wrote: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.
I get all that.
Basic and simple, and even my simple mind can get it.

What about the rpm?
As I said, at 15mph in the real world riding side-by-side, small wheels are rotating at 275rpm and big wheels at 190rpm.
Mick F. Cornwall

reohn2
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Re: Small wheel blowouts - options?

Postby reohn2 » 18 Oct 2019, 9:13am

pwa wrote:So if MickF goes out on his Moulton and finds the longest, twistiest steep descent he can, one where letting go of the brakes is not an option, he should be alright if he descends at 3mph? A tedious way to descend, I know, but in extreme circumstances, to avoid a blowout.....

Needing to be aware of a blowout to such an extent to me indicates a design flaw in the machine.
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Mick F
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Re: Small wheel blowouts - options?

Postby Mick F » 18 Oct 2019, 9:16am

There are many design flaws in a Moulton. :wink:
Sir Alex believed that his small wheels and suspension would conquer the world and all bikes would be like them eventually.

Sadly, he was wrong.

Still fun and interesting though.

Read this, and see if you agree with it all.
http://www.moultonbicycles.co.uk/features.html
I certainly don't! :lol:
Mick F. Cornwall

roubaixtuesday
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Re: Small wheel blowouts - options?

Postby roubaixtuesday » 18 Oct 2019, 9:16am

reohn2 wrote:
pwa wrote:So if MickF goes out on his Moulton and finds the longest, twistiest steep descent he can, one where letting go of the brakes is not an option, he should be alright if he descends at 3mph? A tedious way to descend, I know, but in extreme circumstances, to avoid a blowout.....

Needing to be aware of a blowout to such an extent to me indicates a design flaw in the machine.


I guess blowouts are possible on all bikes. It depends how *much* more likely they are on small wheelers.

I've no idea what the answer to that is. I would guess (but it is only a guess) that a typical Moulton rim would be heavier are therefore less prone to blowout than a carbon fibre racing rim, for instance?

pwa
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Re: Small wheel blowouts - options?

Postby pwa » 18 Oct 2019, 9:26am

reohn2 wrote:
pwa wrote:So if MickF goes out on his Moulton and finds the longest, twistiest steep descent he can, one where letting go of the brakes is not an option, he should be alright if he descends at 3mph? A tedious way to descend, I know, but in extreme circumstances, to avoid a blowout.....

Needing to be aware of a blowout to such an extent to me indicates a design flaw in the machine.

It puts small wheel bikes alongside tandems, which have similar problems with braking heat.

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RickH
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Re: Small wheel blowouts - options?

Postby RickH » 18 Oct 2019, 9:35am

pwa wrote:It puts small wheel bikes alongside tandems, which have similar problems with braking heat.

Of course you can always combine the potential of both problems in one handy package...

sketch-1571387415016.png
Shared recently on Tandem Club Facebook group

:D

reohn2
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Re: Small wheel blowouts - options?

Postby reohn2 » 18 Oct 2019, 9:36am

roubaixtuesday wrote:
reohn2 wrote:
pwa wrote:So if MickF goes out on his Moulton and finds the longest, twistiest steep descent he can, one where letting go of the brakes is not an option, he should be alright if he descends at 3mph? A tedious way to descend, I know, but in extreme circumstances, to avoid a blowout.....

Needing to be aware of a blowout to such an extent to me indicates a design flaw in the machine.


I guess blowouts are possible on all bikes. It depends how *much* more likely they are on small wheelers.

I've no idea what the answer to that is. I would guess (but it is only a guess) that a typical Moulton rim would be heavier are therefore less prone to blowout than a carbon fibre racing rim, for instance?

Or perhaps better with a completely different braking system thought through at design stage,disc or drum brakes would solve a known problem when riding Moultons in big hills.
There's a similar problem with tandems fitted with rim brakes and hydraulic discs,the answer is a third drum brake with a large heat sink(George Longstaff in his infinite wisdom thought it a good idea to machine off the fins to save weight :? )on a friction lever as a drag brake keeping speeds under control and using the two rim brakes to scrub off any further speed but not overheating the rims,a distinct possibility on large wheel tandems due the all up weight.
IMO Moultons have inherent design flaws that are possible to design out but Moulton choose not to,why not is beyond me :? .

On the question of blowouts being possible on all bikes,that's right but the safety margin all things being equal is far,far more likely on small wheelers.
Last edited by reohn2 on 18 Oct 2019, 9:53am, edited 1 time in total.
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pwa
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Re: Small wheel blowouts - options?

Postby pwa » 18 Oct 2019, 9:40am

The reliance of Moultons on rim brakes has always put me off them.

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Mick F
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Re: Small wheel blowouts - options?

Postby Mick F » 18 Oct 2019, 9:56am

reohn2 wrote:IMO Moultons have inherent design flaws that are possible to design out but Moulton choose not to,why not is beyond me :? .
The clues are in the script I linked.
http://www.moultonbicycles.co.uk/features.html

the resulting bicycle is improved in every way - lighter, stiffer, faster

The combination of the lower rolling resistance of high pressure tyres and the lower aerodynamic drag of small wheels, allows the bicycle to go faster with less effort.


Neither of these things are completely true.

They are heavy bikes, even the lighter more expensive ones. Adding drum brakes would make them even heavier!
Aerodynamic drag reduction by small wheels is a moot point, but the frame itself is very draggy indeed so negates anything to do with the wheels.
The suspension slows you down climbing hills, and this is made worse by the heavy design of the bike, and even worser if you had to lug drum brakes up the hill with you!
Discs would be lighter than drums of course, but still heavier than simple calliper brakes.

Modern diamond frame bikes are lighter, stiffer and faster.
Modern diamond frame bikes have high pressure tyres.
Modern diamond frame bikes have more efficient transmissions.

.............. but you don't ride a Moulton to be be fast and efficient ................
Mick F. Cornwall

reohn2
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Re: Small wheel blowouts - options?

Postby reohn2 » 18 Oct 2019, 9:58am

pwa wrote:The reliance of Moultons on rim brakes has always put me off them.

It's the small wheels that put me off,that's mainly because if it takes my fancy I like a bike that can handle a rough track or two.
I think it's fair to say Moultons have limitations that most people find they aren't willing to compromise on,they're niche and have their following,their owners are happy with them and it takes all sorts to make a world.
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reohn2
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Re: Small wheel blowouts - options?

Postby reohn2 » 18 Oct 2019, 10:14am

Mick F wrote:............. but you don't ride a Moulton to be be fast and efficient ................

But the OP's problem is one of safety,loaded touring is one of enjoyment and appreciation of the varied lansdscape and the challenges of riding through it.
When safety is threatened by bad design on an expensive touring machine it takes the enjoyment out of touring.
To be fair many people touring on Moultons may never experience the OP's problems because they may never venture into big hills,but you have experienced blowouts on your Moulton on the short but steep hills of Cornwall and Devon,which IIRC you put down to inefficient brakes and black coloured rims.
If that is the case it's a far more worrying problem for Moulton owners than previously thought and one which isn't being addressed by Moulton themselves.
Last edited by reohn2 on 18 Oct 2019, 10:21am, edited 1 time in total.
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