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-   -   SRAM Drum Brake Hub on 20" Wheels? (http://www.bikeforums.net/folding-bikes/430986-sram-drum-brake-hub-20-wheels.html)

JonathanG 06-18-08 07:10 AM

SRAM Drum Brake Hub on 20" Wheels?
 
I've been thinking of putting a front drum brake on my 20"-wheeled folder and have read good reports about the SRAM front drum-brake hub. However, on the QPB catalog entry it says "should not be used with wheels smaller than 24"." Does anyone know why? Has anyone tried a 20" wheel with a SRAM front drum brake hub?
Any comments and advice will be really appreciated!
Thanks,
Jonathan

rhm 06-18-08 07:26 AM

I put a Sturmey Archer drum brake hub on my Raleigh Twenty, because it was an easy way of getting a reliable brake onto the bike. Caused me no problems; I can't imagine why the SRAM shouldn't work just as well.

Rob_E 06-18-08 07:46 AM

Quote:

Originally Posted by rhm (Post 6900389)
I put a Sturmey Archer drum brake hub on my Raleigh Twenty, because it was an easy way of getting a reliable brake onto the bike. Caused me no problems; I can't imagine why the SRAM shouldn't work just as well.

Cool, I was thinking of doing the same thing with my folders. Actually I was thinking that for just a little more, I could go with the SA drum/dynamo combo hub. I wonder if anyone has tried that? I'm curious about drag. I'm not too worried about it, since these bikes are for casual, vacation, leisure riding moreso than full-time commuting, but I've never used any dynamo hub, so I don't really know what to expect.

I'm curious about the OP's question, too. I thought I had read somewhere that drum brakes didn't always provide the best stopping power on a full-sized bike, but that they were very good on something with smaller tires, which makes SRAM's 24" limit even more curious. Of course I could be remembering wrongly about small wheels and drum brakes being a favourable combination.

rhm 06-18-08 08:08 AM

Quote:

Originally Posted by Rob_E (Post 6900493)
Cool, I was thinking of doing the same thing with my folders. Actually I was thinking that for just a little more, I could go with the SA drum/dynamo combo hub. I wonder if anyone has tried that? I'm curious about drag. ...

Yes, that's exactly what I did. Drag is an issue, but it's so hard to measure, and whether you feel it or not is really a matter of opinion. More of an issue is that most dynamo hubs are designed to give you 6v from 26" or larger wheels; smaller wheels means faster rotation and therefore higher voltage. My small wheels have fried a few lights, not all of them cheap. But my 26" wheels also fried their share of lights, not all of them cheap, so I don't know what's up with that. I have my Downtube Mini set up with a Shimano Nexus dynamo hub, an older one that's not as efficient as the new ones; it powers a 12v 6w halogen headlight and a 12v LED automotive taillight, providing pretty adequate lighting. I used to have a switch on it, but when that broke I just hardwired it all together, adopting the slogan "lights on for safety!" (or laziness). No doubt it slows me down a bit, but I ride too fast anyway.

griftereck 06-18-08 12:40 PM

had Raleigh RSW bikes with either drum or Dynamo hubs.

The have 16" wheels and roll ok.
Even blew a bulb going up hill on one.
But the bulb may have been 30 years old

My Blue Falcon has a Dyno hub with a normal light with no switch.
Same as on a Viking racer I used to have
People called it a Volvo. As lights on all the time

jur 06-18-08 02:42 PM

I wonder if the SRAM drum brake is TOO sharp/strong to be used on small wheels? I can't see any other reason... email them. This is curious.

invisiblehand 06-18-08 02:50 PM

I recall that older Bike Friday tandems had drum brakes. But I would ask SRAM before continuing.

griftereck 06-18-08 11:57 PM

SA made the S3B 3 speed drum brake hub with a tiny brake. So that it would work in small wheels
The hub flange was no bigger than a 3 speed hub
To give the spokes an easier time.

But then Raleigh put Dyno and proper sized drum brake hubs in small wheels.

bicyclridr4life 06-20-08 10:57 PM

Quote:

Originally Posted by JonathanG (Post 6900307)
I've been thinking of putting a front drum brake on my 20"-wheeled folder and have read good reports about the SRAM front drum-brake hub. However, on the QPB catalog entry it says "should not be used with wheels smaller than 24"." Does anyone know why? Has anyone tried a 20" wheel with a SRAM front drum brake hub?
Any comments and advice will be really appreciated!
Thanks,
Jonathan

Should work, look at all the Schwinn Crates with 16 inch fronts with ATOM or SA drum brakes made in the 60's and 70's.

Only reason I can think of is maybe the brake anchor arm is too long for a 20 inch wheel or smaller fork?

Nice to know I am not the only one thinking of putting drum brakes on my folders. I think a nice SA or Sram internal gear rear with drum brake, and a front drum brake would be a great setup.

pm124 06-21-08 10:59 AM

The primary problem is probably that the spokes will break. You need a narrow flange!

JonathanG 06-29-08 04:45 AM

I followed Jur's advice and emailed SRAM, and after some wait, they've gotten back to me. Here's the response in full:

"Indeed, QBP was correct in directing that the 24" wheel is the smallest compatible wheel for the SRAM drum brake hubs.
"The hub was designed around specific torque, heat dissipation, and brake function. 20" wheel is not compatible to this design.
"For further technical requirements for this hub, please see the technical portion of our web site:
http://www.sram.com/_media/pdf/sram/...GHS_MY06_E.pdf"

The website doesn't really add anything.

So, can any of you who are more technically learned than I (all of you?) explain this? Will a 20" wheel not disapate enough heat and cause brake fade? Will, as pm124 suggests, the spokes brake? If so, why? There are no wheel size restrictions on the rear drum brake version of the S7, and Alex Wetmore has put one on the rear of his 16" Tikit with no reported problems.

As always, any help will be appreciated.

Jonathan

bangkok 06-29-08 05:26 AM

Reminded me of this unique kit available in the UK ... a front wheel hub brake kit for Brompton using a SA brake.

http://www.wrhpv.com/small_stuff/hub.html


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