Biopace possibilities!
 

I have a 1988 GT Outpost All Terra mtb. I am converting it for use as a hybrid bike for groomed trails and lite road use. Have switched out original tires for Michelin Country Rock 16 x 1.75 tires. Love em! Component set is Exage Country all around. Bike is all original and in pristine condition. My plan is to add trekking bars and use the entry level Exage components until they need to be replaced. They work so well at this point that they may never have to be changed. The triple crank is Biopace (48-38-28) Shimano shifters are indexed. Mynfrench road bike has a double crank (52-42) I believe. I avg 13 mph on the majority of my rides. I've lost a little avg speed when riding the GT. I don't know why other than possibly tire change, french bike is 27" GT is 26".......I'm trying to decide whether to change the Bio pace triple to a Biopace 52-42 double. There are things I don't know here: 1. Can I switch from a triple to a double with no adverse issues? OR 2. Can I just switch the 48 38 chainrings out and add the 52 42 rings with no adversity? OR 3. Are there issues regarding the chain I must be aware of? I really like the Biopace and want to keep it in some iteration. I've even thought of changing to a better quality Shimano Exage double crank 52/42, but don't know the ramifications of that change with respect to font/rear derailleurs. At this point changes aren't that expensive because I'm dealing with an entry level bike to begin with, so, I'd like to make pe manner changes now while it is still cost effective.

Any and all suggestions/help are greatly appreciated!

Thanks folks!

I have Biopace 52/42 chain rings on one road bike. Got 'em because they were cheap on eBay and in good condition. I can't really feel it as much as I remembered in the '80s. The feel is much more pronounced on the smaller rings of a MTB.

Awhile back I had an old MTB set up to get close to road gearing. It had a crank with 110mm bolt pattern (like yours) so I bought 38T and 50T rings from Vuelta USA for it. I also put a 12-23 road cassette on the bike.

A MTB has a wider bottom bracket than a road bike and more dramatic chainstay angle; putting a road crank on is unlikely to work. If anything, I'd guess a compact 50/34 MIGHT work.

Leave it alone!
 

Thanks [MENTION=41945]EricS[/MENTION]. The drivetrain works beautifully. I am just a casual rider who enjoys working on old bikes. I need to have enough sense to leave well-enough alone. Bought the bike for $50 and have only added a new saddle, tires, and a seatpost. It rides and shifts smoothly and crisply. I just need to change to trekking bars, Ergon1 bar grips, and move over the Exage Country shifters and brakes. It rides really well now and these additions will put it over the top I feel. The bike was in wonderful shape for an '88. I just need to finish these simple things and then ride the bejeezus out of it. I seem to want to fix something that ain't broke! :lol:

I've got a 70's Motobecane Grand Touring I'm putting together that I put dura-ace biopace crankset on. Looks nice, but haven't gotten to riding it yet. I'm curious though. I'll have time to complete build next week, looking forward to it.

I'm convinced it helps make the rides less demanding on my surgically repaired knees and shoulders" fwiw.....

Your Biopace double 42/52 is likely on a 130 BCD crank and the rings from your MTB are probably on a 110/74 BCD crank. Not interchangeable from one crank to the other. You might be able to find some smaller 130 rings for the road bike, and almost certainly find larger 110 rings for the MTB, but your choices will be limited. I think I still have larger 44/50 rings from a 110 BCD triple touring crankset.

Biopace in 130 BCD was 52/53 and 40/42. Biggest ring in 110 was 50t.

Top

The Michelin Country Rocks are going to be slightly slower than a 27" road tyre due to their extra width and lower maximum pressure. They're also heavier, which will cause a speed loss going up hills and lower your average speed.

Another alternative to switching chain rings/cranksets is to change the gear ratio on the cassette/freewheel. A 12T small cog with your 48T chainring will give the same ratio as your current 13T with a 52T chain ring.

I think the issues you'll be dealing with are something like this:

1. BCD- that's the Bolt Circle Diameter- basically, the size and pattern of how the chainrings connect to the crank. Most ATB/MTBs have a 110 for the outer ring and 74 for the inner ring. A lot of "road" cranks are 130. Older ones are 144, there are some other gooney ones as well.

http://www.thetallcyclist.com/wp-con...icycle-BCD.jpg


2. Bottom Bracket Spindle length. Generally, the spindle for a triple crankset will be longer than one for a double. Makes sense, right? Three rings take up more real estate than 2. If you switch from a double to a triple or vice versa, you'll generally need to replace the spindle or BB unit.

3. Chain length/Derailleurs. Your triple setup is going to have a chain that's long enough for your current chainrings. If you get bigger chainrings- you'll probably need a longer chain. If you get bigger chain rings, you may need to move your front derailleur to clear the chainrings. You might want a shorter cage rear derailleur because you won't need that extra cage length. Or you can keep it because it looks cool.

Your secret plan about the trekking bars is a good one. What makes it even better is that your components will fit on that bar... I mean the bar diameter is the same from a flat bar to a trekking bar- but road drop bars are different. I'm still working on getting my trekking bar set up to my satisfaction. I may actually start looking for a different style of trekking bar- and there's a huge range of them, different bends, different sizes and stuff... And I really want to clean up the cable routing...

http://i1294.photobucket.com/albums/...ps4v4hu9rr.jpg

http://i1294.photobucket.com/albums/...pswy01wf6u.jpg

The difference is likely to be in the tires and the weight of the bike. Heavier bike, higher rolling resistance in the tires, you will lose some speed overall. The tires can make a real difference.