Water-cooled Sturmey Archer drum brakes

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StephenW
Posts: 121
Joined: 22 Sep 2010, 11:33am

Re: Water-cooled Sturmey Archer drum brakes

Postby StephenW » 23 Aug 2018, 6:37pm

[XAP]Bob wrote:Water takes 4200J/degree/kg
Air takes 720J/degree/kg

Additionally water is significantly more dense, so you get more water around something, and has better thermal conductivity...


Don't forget the latent heat of vapourisation!

If the water boils, it absorbs 2260000J/kg!

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[XAP]Bob
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Joined: 26 Sep 2008, 4:12pm

Re: Water-cooled Sturmey Archer drum brakes

Postby [XAP]Bob » 23 Aug 2018, 6:42pm

StephenW wrote:
[XAP]Bob wrote:Water takes 4200J/degree/kg
Air takes 720J/degree/kg

Additionally water is significantly more dense, so you get more water around something, and has better thermal conductivity...


Don't forget the latent heat of vapourisation!

If the water boils, it absorbs 2260000J/kg!


And that’s for a zero degree rise...
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Brucey
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Re: Water-cooled Sturmey Archer drum brakes

Postby Brucey » 24 Aug 2018, 10:16pm

some comments;

- yes water is vastly better at cooling stuff, but it also imposes violent thermal stresses on parts too. The latter can cause all kinds of problems in the long run. Air cooling is a lot less agressive, so is to be preferred if it can made good enough.

- if your sole difficulty is dust, you would be as well off just blowing compressed air in through the hole that you have drilled, once a week using a garage airline or something. This would spare you from having to disassemble the brakes (not that this is particularly difficult, just one bolt isn't it?) just as well as adding water.

- if you inject water when the brakes are not hot enough, it can saturate the linings and this will cause the brakes to behave oddly

- you need to carry water up hills so that you have it when you are going down them

- you can run out of water... you can't run out of air...

- the worst overheating issues with SA hub brakes are when they are put inside velomobiles. This increases the load on the brakes (the aero drag is greatly reduced so speeds are higher and the brakes have more work to do) and it also completely bolloxes the cooling.

If velomobile riders want to go at speed down alpine passes then the solution is obvious to me; they need to have variable ducting onto the brakes. These ducts would create drag in their own right (less work for the brake to do) and direct more cooling air towards the brakes.

cheers
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Marc
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Re: Water-cooled Sturmey Archer drum brakes

Postby Marc » 14 Sep 2018, 5:22am

The water cooling/water cleaning drum brakes do fine after 1 1/2 month and 1,902km in daily use.
I didn't observe any of the issues raised so far.

Brucey wrote:- if your sole difficulty is dust, you would be as well off just blowing compressed air in through the hole that you have drilled, once a week using a garage airline or something. This would spare you from having to disassemble the brakes (not that this is particularly difficult, just one bolt isn't it?) just as well as adding water.

I wouldn't be comfortable inhaling the fine brake dust without proper mask on a regular basis. But I'm only a mechanic with extensive training and 25 years experience, what do I know... ;)

Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: Water-cooled Sturmey Archer drum brakes

Postby Brucey » 15 Sep 2018, 2:12pm

I don't remember suggesting that you breathe in that dust or indeed any dust..... :roll: :wink:

BTW if the brake is hot enough to boil the water, the dust is likely to emerge as, er, dust, and if you are purging it out as you go, you might (more than normal) be breathing it in as you ride, depending on how the airflow is arranged over your machine.

Every ~18g of water you add turns into ~25litres of gas at STP, and a much larger volume than this at higher temperatures, obviously.

Another thing that may happen is that the various pivots and cams inside the brake may be stripped of lubricant more quickly than normal, if you are regularly 'steam cleaning' the inside of the brake. Keeping these happy, long term, is difficult enough in the normal run of things (the lubricant has not to melt or pick up dust too much, amongst other things) and this may not help at all.

BTW I have experienced saturated brake linings in these brakes and have also seen thermal fatigue/shock damage in similarly treated parts. But what do I know, I'm only a fully qualified materials scientist with several decade's experience.... etc etc etc....

cheers
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