Several months ago I replaced my double with a triple biopace crank. For these several months I have only ridden in the small and middle chain ring with no problem. Yesterday I went to a place to ride where I could use the big chain ring. It didn't take long for me to find a problem. It seems that when in the big ring and cross chaining only one or two cogs the chain wants to shift back and forth between the middle and large ring. It does this not on every pedal revolution but maybe one out of 10 revolutions. The shift is very quick like 1/2 crank or less.

To minimize this automatic shifting I let the deraileur rub on the chain a little. So it seems to me that the shape of the biopace chain ring works like a great shifting ramp.

Have you guys ever noticed this with a biopace chain ring or am I doing something wrong which is causing this.

I once rebuilt a bike having Biopace rings. I had no such problem as you're having. Thus, I suspect some sort of adjustment or wear issue.

Post your query in the "Bicycle Mechanics" section of the forums. Lots of experienced people hang out there.

I think Jonathan is on the right track here. the biopace are pretty widely used, still, though not in great numbers. A triple has to have the FD set right. I understand that the Sheldon Brown site has a triple FD set up page that works really well for those special FD they take. I am assuming that you went with with a triple specific derail. on the front, also.

Bill

If it's not in time with your revolutions, then it could be a chain issue. Check for a stiff link somewhere. Over in mechanics section someone just wrapped their rear derailleur around their cassette because of that.

As for adjusting the front derailleur. Well, even following the guides, be prepared to be sucked into the VORTEX OF TIMEWASTING DESPAIR!!!! *echoechoecho effect* It took me many hours of gimlet eye observation, rotating that blasted FD till it was just right. Remember to have the crank arm parallel with the seat tube for the penny gap test, since that's where the biopace cog is at it's longest radius. After that, it's just a case of never-ending adjustments, with the middle chainring being the most problematic between shifting and chainrub in highest or lowest RD settings.

Really, if you can afford it, save yourself and have the LBS deal with this headache for you. Though after my FD came back from the LBS insufficiently tightened, I may be starting to get paranoid about checking the general tightness of all bolts and nuts personally...

Oh, and it's nice to know that I'm not the only one who almost never finds a use for that outer chainring. Generally, anymore, if the hill is steep enough to get me fast enough to need the outer ring, I'm probably coasting or braking anyway. Maybe I should file off the teeth, and turn that cog into a bash ring! I'm tired of my pants being chewed up by that ring anyway.

I used Biopace chainrings in triples on two bikes for years without trouble. The only reason I quit using them was that they wore out and replacements weren't available. Is someone making Biopace parts again?

LC,
Others are making oval chain rings, such as Rotor and O'symetric, they are not backwards compatible, but the Shimano isn't making them any longer. I see a lot of them on eBay, both used and N.O.S., not sure if they are triple suitable or not.

Bill

I don't think so. Mine are original to my 1992 hybrid, and are still in excellent shape. (EDIT: No they're not. My middle chainring at least is displaying the signs of being worn, looking just like the Sheldon Brown example. Sigh. Mo money...)

I think the Biopace rings were different from other oval chainring, in that the longest diameter is set to encourage "spinning" rather than giving more power to the power stroke, so it's about 90 degrees out of phase with other oval chainrings.

Hello.

No, no similar problem.

My ol'reliable 1990 steel bike had Biopace Triple as standard. No problem at all in 22 years and a LOT of miles.

The set up on mine was index shifting 7 speed Deore on the rear, friction shifting on the front. Works like a charm.

You mention that the biopace was a recent change. I would suspect that the front changing mechanism that worked well on your previous double chainring has the reach for 2 chainwheels, as you state, but not for the additional one - another 3/4 inch or so of cable reach.

I'm no mechanic, but that's my hypothesis.

I hope you get it to work fine and enjoy it!
(In short, I think its a 'double to triple' issue, not a 'biopace' issue

Thanks for the replies. Since no one has had this problem, I will look further into the problem. I will start by checking to see if the chain ring is running true and is not bent.

Off-topic: Thank you for the info, Bill. I didn't know anyone was making those.

I think you may be on to something here!

Is the FDER at the max range of it's outer travel?
If "centered" properly over the large ring, the chain should hit the inner cage before "falling" to the middle ring.

You are certainly welcome, the O'symetric and Rotor chain rings are more pronounced iin their oval shape than the Biopace rings were. Braddley Wiggins ran an asymmetrical or oval ring on his time trial bike today at the TdF, you could see the oblation of the sides where it "flattened" clearly. Hope you get the shifting problem sorted. I think the Mechanics forum may be the place to ask, as said.

Bill