|
|
| Installing Four Chainrings on the Tandem
|
 | Our Tandem |
| Bob Holmberg's Tandem |
Our '96 Burley Duet came stock with 54/44/28 chainrings (110/74 bolt pattern) and an 11-30 8 spd. cassette. Before our last loaded tour, we replaced the 11-30 with a 12-32 and replaced the 28 with a 26. In the middle of our tour, we found out that the 26 wasn't quite low enough, so we replaced it with a 24 (I had wanted to go lower, but we were limited by the 74 mm bolt pattern). After our tour, we changed out the middle ring to a 38 (resulting in 54/38/24) to even out the jumps between the chainrings.
Later we upgraded to 9 spd. and an 11-34 cassette, giving us the 54/11 without having to sacrifice on the low end, although the 32 to a 34 change wasn't too significant. But in order to go lower, I would have to get something smaller than a 24.
I was originally going to replace the 24 with a 22 or a 20 with an Avid Microadapter, but my search for a Microadapter led me to Tony at Chicago Bike in Newport Beach (California). He showed me some of his quad conversions, so I decided to give it a go on the tandem.
We used a Mountain Tamer Quad, which mounts to the crank in place of the chainring spacers. Two splined Suntour freewheel cogs are then used for the granny and the "super granny" gears. Obstacle number one was our Sugino Fuse crank which didn't have chainring spacers, so that meant carefully cutting down the granny chainring bosses so that the granny and super granny gears wouldn't be too far away from the middle and large ring.
Rather than use two freewheel cogs, I mounted a conventional granny chainring on top of the Mountain Tamer, using 2 mm spacers to orient the granny ring far enough away from the middle ring. The advantage of this setup is that I don't have to worry about wearing out any splines on the granny, but that gave me the problem of having to find a Suntour freewheel spacer to take up the space that was meant for the freewheel cog. I couldn't find a 2 mm spacer (the thickness of the cog), so I had to settle for (carefully) cutting a 3.5 mm spacer in half. After that I was able to slide on all of the spacers and the super granny freewheel cog, giving four 54/44/28/19 chainrings.
The old bottom bracket was 117mm, which was more than wide enough for three chainrings, but definitely too narrow for four. We replaced it with a 121mm bottom bracket - not too much more noticeable in terms of Q-factor, but gave enough elbow room for the four rings.
The body of the Mountain Tamer Quad fit nicely over the oversized Burley bottom bracket shell, allowing the super granny to fit above the shell. An Avid Microadapter would not have worked well, since the diameter of the 58 mm bolt pattern was a little to small to fit over the bottom bracket shell - that would have required a much longer bottom bracket to get it to work.
The left barcon shifter wasn't able to move the front derailer (Shimano LX) through the required range across all four chainrings. That problem was solved by mounting the cable to the right of the bolt where it attaches to the front derailer. This moves the cable attachment point closer to the pivot point of the derailer, allowing more derailer travel per amount of cable pulled (thanks for the tip, Alex).
The super granny gear ended up above the bottom bracket shell - far enough inboard that front derailers don't have enough range of motion to make the shift. Rotating the cage a little bit clockwise (as viewed from above) helped. For some reason, the shift from the 44 to 19 is easier than from the 28 to the 19. As a last resort, there's always the pedal backwards while the stoker kicks the chain trick to get it into the super granny. The only problem with rotating the front derailer is that it's a little tricky to adjust it to avoid rubbing in the 54/11 top gear.
With the new fourth chainring, we didn't need the 34 tooth cog anymore, as the 19/30 low gear was plenty low enough for us. Since we were having problems keeping the 9 speed in adjustment, we switched back to 8 speed, with an 11-30 cassette. See the upgrading the tandem to 9 speed page for more info.
We've had 4 chainrings on our Ibis Cousin-It for about 6 years now. 54-42-24-19 The derailler doesn't get us into Super-Granny we either get off or push on the chain while pedalling backwards (on the bottom). Not the best setup but that 19 is nice offroad or towing a trailer. We need a new hub since we combined offroad and towing a trailer, White Rim trail, near Moab, UT.
Here's a description I wrote a while back, let me know what questions you have. I think I did put a longer BB in but I'll have to check the length when I get home.
I've installed a Mountain Tamer Quad on our Ibis Cousin It. The fourth chainring is a 19. This makes offroad climbing really nice. We plan on doing some touring and my knees weren't real happy after the last tour with a 28 tooth inner chainring.
The Quad replaces the spacers behind your inner chainring and accepts two Suntour A cogs with a spacer between them. It's made by Gios Team in Albuquerque. I got their number from their ad in Mountain Bike Action. I first tried a 30t and a 20t, but the 20t rubbed on the frame so I put in the 19t (Yes it's a close fit). You can install a longer spindle (which Gios advertises) but I didn't want to push the pedals farther apart or alter the chainline for the three legitimate chainrings.
I modified our XT front derallieur so that it would make all the shifts and not rub with the chain on any of the four chainrings. This involved a little cold forging (repeated hammer blows) to achieve a more curvy shape. Since the derailleur travel ends (ie. d. is retracted until it is against the downtube) before the shift into the fourth chainring; the bottom part of the cage needed to be bent toward the downtube. In order to eliminate the rubbing on the bottom of the cage I broke the cage and bent the bottom out of the way. This of course eliminates much of the strength of the cage and leaves the front of the cage with a dangling end. The setup worked well for a few rides until that dangling end got caught in the chain which promptly ripped off that face of the cage. My wife had to make the chainring downshifts with her foot for the rest of the ride.
I have a 24t and a 19t on the Quad. I'm trying to see if I can get along with just 3 chainrings by testing out the 24t which I could get to replace the whole experiment. Preliminary rides tell me having that fourth ring for a bail-out is nice. I also went with the 24t because it has 8 splines instead of the 4 the 30t had. I noticed some deformation of the splines on the 30t which I'm a bit concerned about. No such problems with the smaller 19t (4 splines) so far. Gios (the manufacturer) also sells an adapter which uses ONE inner chainring and ONE Suntour A cog (instead of TWO A cogs) which I guess would eliminate this worry. This is of course is more expensive but might be worth it for a tandem.
We have a new derailleur which works normally for the top three chainrings. I have carefully aligned it so we can use the fourth chainring with the innermost two cogs (28 & 24) on our freewheel without rubbing. We have to stop and move the chain over by hand but that's not much wasted time if we're going slow enough to need it. I am in the process of designing a cage which will work flawlessly (I hope) with this setup. The cage will be longer and have all the right bumps at the shift points. I will take the rivets out of the broken derailleur (see above), replace the cage, and still have the XT mechanism.
This is all experimental, I'm trying to use my Mechanical Engineering training (I'm a grad. student in ME), but I have no formal training in bicycle component design. Even if it's not the best idea I'm still going to make it work, because HEY -- It's a hobby.
Copyright
|