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Old 10-25-06, 08:17 AM   #1
Sawtooth
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Am I crazy?: my version of Rivendells Quickbeam

I want to build a single-speed-ish bike out of my '82 peugeot road bike conversion that will have a single freewheel gear out back but two chainrings that will yeild roughly 50 and 70 gear inches, respectively. This would allow me to move pretty fast on the road but still climb some pretty serious offroad stuff. I would shift simply by moving the chain with my finger when the bike is stopped so I won't have a front der. I would need a pretty serious chain tensioner such as a singleator or a rear dur stopped with a short backward cable. I have the gearing figured out where I will run a 20t freewheel in the rear and a 52 and 38 up front. I am pretty sure this will work if I don't get the chainline out of wack and can keep tension on the chain.
What have I not thought of? Suggestions, comments?
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Old 10-25-06, 08:37 AM   #2
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No problem! Use a rear derailleur as you planned. Chainline shouldn't be an issue if you're using a road double crank. You can even leave the front derailleur and shifter intact to alleviate the need to dismount and do the manual change.
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Old 10-25-06, 09:42 AM   #3
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I think it's a cool idea. I think I would put a front deraileur on, though. Might as well if you are going to have a chain tensioner anyway.

A similar idea I've been mulling over is one of those White Industries double freewheels with two cogs (16t and 18t). I'd use two chainrings with a difference in teeth count of 2. Put the larger chainring on the outside lined up with the 16t rear cog. I'd probably use a QR rear wheel axle. Manually changing gears would involve releasing the rear wheel and moving the chain to the other cog and other chainring. This would give me an easily modifiable single speed with the rear wheel in the same position for both gear options. That way my wheel stays in the optimum position in the fender for both gears. Only reason I haven't done this yet is the high price of those double freewheels.
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Old 10-25-06, 11:12 AM   #4
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Excellent. Two of the guys I figured and hoped would resond as well. I shy away from the idea of the front der just because part of the appeal of the single speed is the clean lines and I don't want to clutter my cockpit with shifters (I only have the stem mount types).

Squeakywheel: what gear combinations are you thinking of using up front and what gear inches are you trying to achieve? Sounds like a good idea.
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Old 10-25-06, 11:25 AM   #5
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Does a chain tensioner effectively act as a rear derailleur? Will it pick up slack? If the front derailleur moves the chain to the small chainring, aren't you going to have an unusable, saggy chain?
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Old 10-25-06, 01:58 PM   #6
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I was thinking about 75 gear inches for the big chainring / small cog combination and 65 gear inches for the small chainring / big cog combination.

I'm not all that serious about this project yet. My vision is to someday do some touring. I'd maybe do it with a credit card and not carry so much. Ride across the country with a minimal set of packs. I'd do it on a single speed with racks and fenders. Ride west to the Nebraska / Colorado border in the higher gear. Loosen the back wheel and move the chain over to the lower gear combination (without screwing up my wheel position in the fender). Ride from there to the Pacific coast in the lower gear.

Just my crazy fantasy. I want to tour with a single speed and still be able to hump my rig over a mountain. I guess I could achieve the same thing with a flip-flop hub, but the cool thing about the double freewheel is that the chainline is straight in both configurations. Also, I wouldn't have to remove the wheel completely to change gears. I can imagine a flip-flop hub and 60 pounds of gear on the rear rack.
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Old 10-25-06, 02:42 PM   #7
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Got it. Run 65 inches up to the Continental Divide, then basically power down to the Pacific Coast on the big ring.

I thought there was a suggestion to keeping the front derailleur so you wouldn't have to readjust the rear wheel to change the chain length. But it does sound interesting...if a little wilfully, you know, retro nuts. Perfectly Rivendell. Although...why not just one chainring and a cassette of five gears, like ... which Bianchi is it? That sounds good to me. But then you've got that waggling rear derailleur....

Good luck. And don't forget to paint it orange. Or green.
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Old 10-25-06, 02:50 PM   #8
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Sawtooth's plan is probably more sensible than mine. With the chain tensioner, you can keep the rear wheel in one place too. The only advantage my idea has is that you don't need a chain tensioner (if you have horizontal dropouts). The big disadvantage of my idea is that you need to loosen the rear wheel to change gears. Just having to move the chain to a different chainwheel might mean you will actally be willing to change gears more often. Heck, you could even use a triple chainring. I wouldn't worry about the chainline. No matter what, your chain will be straighter than a bike with a rear deraileur.

Edit: I remember someone on these forums had a commuting rig with a single gear in the rear and two or three chainrings in front. He used a front deraileur to change gears. Not sure who it was. Maybe DanO220.


Edit again: Here's a link to the thread I was thinking about. DanO mentions the two chainrings and a BMX cog in his last post.

3 speed hub opinions

Wow. I was surprised to see this old thread resurrected. And I've done nearly just as Dcheifransom suggested. However, I am getting by with two rings up front (38/48) paired with the single 16 freewheel hub in the rear. A Melvin chain tensioner by Paul's components takes up the slack. A vintage Suntour derailleur and thumb shifter let me change gears on the fly. 38/16 isn't much easier than 42/16 when the hills come, but the 48/16 spins down the descents and motors along the flats pretty good.

DanO

Last edited by squeakywheel; 10-25-06 at 03:29 PM.
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Old 10-25-06, 03:23 PM   #9
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I saw a bike with a derailer up front and single cog out back on craigslist. I think he had a rear derailer. I can't imagine why (other than money) you wouldn't just run a 3 speed hub. Much cleaner and easier if you ask me.
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Old 10-25-06, 03:35 PM   #10
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I don't think you can have just a front derailleur without the back. The opposite is not true, though.
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Old 10-25-06, 04:03 PM   #11
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Thanks for the link Squeakwheel.
I will have to take a look at that chain tensioner. I would really prefer not to have the rear der. on but at this time finances require that I do this on the cheap and the rear der. is the cheapest way. A Kore reaction might do the job nicely as well but I can't stand that anodized red color.
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Old 10-25-06, 04:05 PM   #12
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Quote:
Originally Posted by Bklyn

I thought there was a suggestion to keeping the front derailleur so you wouldn't have to readjust the rear wheel to change the chain length. But it does sound interesting...if a little wilfully, you know, retro nuts. Perfectly Rivendell. Although...why not just one chainring and a cassette of five gears, like ... which Bianchi is it? That sounds good to me. But then you've got that waggling rear derailleur....

Good luck. And don't forget to paint it orange. Or green.
Thanks. I think it is the castro valley you are thinking of. And you nailed it on the head. I want it as clean as possible and 2 chainrings swithed with my finger seems cleaner than a rear cluster shifted by a der. and shifter. I know, that sounds crazy.
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Old 10-25-06, 04:49 PM   #13
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Quote:
Originally Posted by Bklyn
Does a chain tensioner effectively act as a rear derailleur? Will it pick up slack? If the front derailleur moves the chain to the small chainring, aren't you going to have an unusable, saggy chain?
Most chain tensioners pick up slack, but if you want to handle a 14T difference in your front chainrings [52 --> 38], you can use any mid or long-cage derailleur. You might be able to use a short cage road derailleur - 105 or Ultegra might do it, but I don't think Dura-ace [especially older stuff] will. The main benefits of a rear der. are cheapness and easy setup.
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Old 10-25-06, 11:23 PM   #14
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A White industries DOS 16/19 freewheel and a 42 tooth chainring would get you 69 and 58 gear inches with 700x23 tires (at least according to Sheldon). If you have horizontal dropouts or a standard tensioner and deep pockets that would be an option.
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Old 10-26-06, 08:17 AM   #15
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Quote:
Originally Posted by marqueemoon
A White industries DOS 16/19 freewheel and a 42 tooth chainring would get you 69 and 58 gear inches with 700x23 tires (at least according to Sheldon). If you have horizontal dropouts or a standard tensioner and deep pockets that would be an option.
Thanks Marqueemoon. The deep pockets thing kind of rules me out. I envision this as a fall/winter rig for all purposes. When the road riding gets too cold to pull off before work (already is too cold) we meet a couple of times a week for a hilly mtb ride in the dark and then I pedal to work. So, given that the low gear is really only going to be used for climbing our local mtb trails and 70 gear inches seems to be really nice for most types of road riding, I really want to keep that lower gear down to 50 gear inches or below in order to keep up with my buddies on their mtbs.
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Old 10-26-06, 08:28 AM   #16
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You should be able to shift from the large chainring to the small one without stopping, if you practice just a little. You just push the chain to the left with your right foot while riding.

I used to get my front derailleur frozen in Montreal winters regularly, and I would get it to move left by pushing it with my foot.
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