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Old 07-08-03, 02:51 AM   #1
jtown
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Converting old mtn. bike to a roadie.

I have a 92 Rockhopper and I'd like to get some higher gear ratios out of it because I'm doing a lot of road biking with it. It's equipped with Shimano 400LX front and rear ders. and some crappy bio-pace chainrings. The fronts are 48/38/28 and the rear is 13x30. I did some reading and I'm not really quite sure what it means to have rivetted chainrings ... maybe someone can fill me in. They look easy to remove, but I've never tried. My front der. cable runs alongh the down tube and around the bottom bracket.

Is a bio-pace 48 any different than a regular 48 ring? Would my best bet be to replace the cogset to an 11xsomething? Any advice would be great.
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Old 07-08-03, 08:37 AM   #2
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My first inclination is to tell you to scrap the whole thing and get a real road bike. But if that were an option you would have done it already

Biopace. Shimano's answer to a question no one asked If you look carefully you'll notice that the rings are almost egg-shaped. It's supposed to eliminate the "dead spot" in the pedal stroke. Just get a whole new crankset (and BB, while you're at it). If you have a modern rear cassette with a lockring you could swap it out for a 105 cassette, but you'll have to get new front and rear derailleurs and new shifters, too. Seriously, this bike is so obsolete you really would be better off gettting a new bike.
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Old 07-08-03, 09:27 AM   #3
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Old school MTBs of quality are always useful bikes to have. They make much better utility bikes than the current crop of sus MTBs.
If you want to roadify it, slick tyres are the first upgrade.
If you really find yourself spinning out, then gear up, but 28/38/48 is typical of a road touring bike and is a goiod selection. I would suggest simply swapping your Biopace oval chainrings for round ones of appropriate tooth count. High quality rings in any tooth count are made by Specialities-TA, and good-enough no-name rings are widely available. Make sure you get the right Bolt Circle Diamter (probably 110/74mm in your case).
You dont need to replace your derailleur mechanisms to run slightly higher gears, just reset the front mech height.
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Old 07-08-03, 10:11 AM   #4
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One of my most useful bikes was a first generetion mtn bike that I converted into a touring bike with drop bars, bar end shifters, etc. I had the exact same gearing that you mentioned and found that it was more than adequate.

Are you sure you really need bigger gears? 48x13 on 26 inch wheels is a 96 inch gear. At a cadence of 90 rpm that's 25 mph. Do you really cruise faster than that?

Dave
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Old 07-08-03, 10:22 AM   #5
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I have a Trek 850 that has drop bars, hooded brakes, barend shifters, and 1 inch Tom slicks. I ride this bike around town and to work, 14 miles, with a high end of 42-11 and have no problems unless I want to go hell bent for leather down hill.
Put some slicks on that bad boy and ride it!
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Old 07-08-03, 10:31 AM   #6
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You could basically keep your existing drivetrain and turn your MTB into a somewhat of a cyclocross bike with skinny knobbys and dirt-drop bars which almost look like road handlebars. You will need to install STIs...like the old 105's, for instance.........they should be compatible with your 8-speed LX rear. You certainly have adequate gear inches to keep up with the roadies.
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Old 07-08-03, 10:33 AM   #7
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Thanks for all the replies. I've already strapped on some slicks and on flat land I'm usually at 48/13 with my cyclo reading 22-23 mph. On slight downhills I find myself spinning my highest gear which is why I thought about a gearing upgrade. I'd love to just buy a road bike, but I don't really know if I want to spend the high end cash for one yet, plus I like my bike ... it's fun to ride.

Does a 48T bio pace ride different than a 48T regular ring?
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Old 07-08-03, 10:34 AM   #8
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oh and btw it's a 7 speed.
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Old 07-08-03, 10:38 AM   #9
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JTown......if it's a 7-speed, you can go with bar-end index shifters on the dirt-drops. And no, chances are you will not feel the dif between round rings and bio-rings...
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Old 07-08-03, 11:45 AM   #10
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It is easier for experienced riders to spin round rings than Biopace rings. You will be able to maintain a higher cadence, which may help with your spinning out problems. Biopace was designed for newbies who pedal too slowly.
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Old 07-08-03, 04:53 PM   #11
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Sheldon Brown, IIRC, actually thinks Biopace rings are not that bad and he uses them on some of his bikes.

Go with the MTB frame. Of that vintage, it's rugged, and will last almost forever.

Rivetted chainrings means you won't be able to swap out individual rings -- the crank and rings are all rivetted together rather than having bolts that you can undo, and you have to buy the whole lot together. Fortunately, they are lower level Shimano MTB stuff such as Acera, and are cheap.

Be careful about swapping out cranks and BBs. Your BB is probably still cup-and-race with spindle. If you do go with a cartridge, you will need to ensure the chainline isn't radically altered, that the Q-factor is even on both sides (ie, the cranks/pedals are the same distance away from the seat tube, and that the smallest chainring has enough clearance on the chainstay. New cranks and chainrings will add another dimension you may have to take into account.

Your derailleur should be able to handle a moderate change in rings as it's already three-speed.

I suspect that you have a spin-on freewheel 7sp cluster on the rear. You may have to hunt around for one that meets the spec you want. I'm not sure they are available with an 11T cog. If you have a freehub set-up, you may be in a better position.

I'd probably go with a chainring upgrade for now, but with the caveats already mentioned.

R
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