sugino 75's and a 7 speed cassette?
So I have an 80's fuji road bike set up as a fixed gear with sugino 75 cranks. I'm thinking I'd like to put a seven speed casette in the back, but keep one chainring up front and skip the whole front derailer business. sugino 75's have a 144 bcd, but the chainring that comes with it, I believe only takes 1/8" chains. So, does anyone know if this conversion is as simple as buying a front chainring that accepts a 3/32" chain. Is there something I'm missing? I have a 110 bottom bracket. Is that sufficient? Thoughts are always appreciated. Thanks.
me have long head tube
1. single-chainring geared bikes are nice commuters. I've got one myself. Albeit with a much less expensive crank. You could sell your Sugino 75 (people pay lots of money for those things) and get a much cheaper single-chainring crank. Or not.
2. you can get standard-width chainrings for 144 bcd. Likton's has Sugino 42t and 53t rings in this BCD. I assume Harris has this sort of thing as well.
3. you may need to get a new rear wheel if you've currently got a singlespeed rear wheel. Or at least need to redish and respace the rear wheel to take a multi-gear freewheel
4. 7-speed freewheels require a fair bit of exposed axle between the bearings and the dropout, leading to bent axles for many riders, except those with special freewheel hubs (e.g., Phil or American Classic) or Campy hubs. Affects heavier riders mainly. If you're a lightweight, no problem. I weigh 180, and have set up my commuter with a 5-speed freewheel now.
5. Your current setup probably has chainline optimized at 42.5mm if set up for a singlespeed. This will also work about right for a freewheel on a 124mm (or therabouts)-spaced rear wheel.
I won't be selling the 75's. the long term plan is to have a geared and a fixed set up, but for now I have just one frame, so the conversion (or unconversion as the case may be) is more about changing things up for fun, trying to learn some new mechanics, and moving to a new area (raleigh, nc). The one thing in your post I didn't understand was the bit about 7 speeds and bent axles. Shouldn't a brand name hub (specifically the shimano 105) designed to accommodate 7 or more speeds be able to withstand normal riding without bending? i'm about 190.
me have long head tube
I'm sorry, I just re-read your post, and you said 7-speed cassette. I was thinking 7-speed freewheel, probably because I have (until recently) been running a 7-speed freewheel with a single-chainring crank on my commuter bike. Freewheel hubs did commonly suffer from bent axles, especially with bigger riders and especially with 7-speed freewheels. Not so much a problem for 5-speed freewheels, as there's less axle sticking out between bearings and dropout.
If you're going to get a cassette-hubbed rear wheel, you may as well get a standard (modern) one that is compatible with 8/9/10 speed cassettes.
The only reason to go with a 7-speed freehub: if your frame has narrow spacing (124 or 126mm rear dropouts) and you don't want to "spread" it a bit wider to fit the 130mm-spaced modern road freehub in between the dropouts. With steel frames, spreading a frame 2-3mm on each side isn't any problem.
If you change the 1/8" chainring on your crank to a 3/32" width chainring, you can keep the same ring if you go back to a fixed gear set-up. There are plenty of single speed/fixed/track cogs available in 3/32" width.
timcupery pretty much summed things up, but just to chime in...
I did this very thing with an 8-speed cassette several years ago. My road bike had just gone out of commission from a cracked dropout, and there were several races coming up that I wanted to participate in. Using the road crank that was set up on my fixed gear conversion, I spread the 126mm triangle apart to accept my 130mm wheel, hooked up a downtube shift lever and derailleur, and was in business. While I never actually raced on that setup, it was certainly a fun experiment.
Perhaps the only thing that I might recommend would be to take a good look at your chainline once you've got the 7-speed wheel in there. Obviously, since you've only got one chainring, the chain angle is going to be pretty extreme when you ride in the very highest and very lowest gears, thereby increasing your chances for a thrown chain. Assuming that you've got a 42mm chainline with your Sugino 75's, and Shimano's spec for chainline is 43.5mm to the middle of a road crank (41mm chainline inside ring, 46mm chainline outside ring), your chainline will be 1mm off spec in your easiest gear, and 4mm off spec in your hardest gear.
Given this, it might be advisable to invest in a chainkeeper (like the Deda Dog Fang) for a little insurance in your easiest gear. A chainguard would give the same insurance in your hardest gear, but I'm not sure if you can install them on a track crank. If it's not possible, perhaps the best insurance would be to simply avoid all-out sprints in your hardest gear.