int19

Hello all,

I'm building up a new crankset, and I have a set of Ritchey Logic Sugino cranks. I have some biopace chainrings which I'd like to put on them. Here's the problem. The crankset that I removed the rings from did not have an integrated 5th arm spider. My Ritchey's do, so the pin on the chain ring which sticks out is no longer behind the crank arm, I can only get it above or below. Does this matter? I assume that it does...

TIA,
Richard

urbanknight

To the best of my knowledge, that only matters in that it keeps the chain from getting stuch between the crank and the bottom bracket should it come off the chainring. So as long as your derailleurs are adjusted properly, you should be fine.

HillRider

Besides preventing the chain from wedging between the chainring and the crank arm, the pin also indicates the correct "phase" of the Biopace rings. So if you can't install it in the preferred orientation, you will compromise the enormous benefit Biopace was supposed to have.

simplify

^^^right, although there are many people who turn them deliberately to get a different effect.

The other solution is to pull out that pin from the chainring. I did this when I used my Biopace on a crankarm that had an integral pin of its own. The pin on the chainring comes right out with a pair of pliers and little effort--wobble it back and forth, and be sure the chainring is kept flat so you don't accidentally bend the chainring. Then you can install the chainring in the intended orientation, instead of having to turn it.

HillRider

If I interpret the problem, the pin isn't an obstruction. The problem is the new crank has the crank arm as the "5th-arm" of the spider like Campy does with some of their cranks. One chainring retaining bolt threads into the crank arm.

The old crank had all 5 arms oriented around the crank arm so no chainring retaining bolt actually went into the crank arm.

Sheldon Brown

Normally, the pin and the index marks on Biopace chainrings are supposed to go behind the arm. For cranks like yours, however, you can't do that, so the rings should be installed 180 degrees from the normal orientation, so that the index marks are _opposite_ the right arm.

Sheldon "The Last Biopace Fan" Brown

TallRider

Yup, just what I was thinking. Biopace rings were designed to be in specific ovalized orientation with respect to the crankarms, and if you installed it with the pin just to the left or right of the crankarm, that orientation would be 36 degrees off what it's supposed to be. However, by flipping the ring around (so the pin is exactly opposite the crankarm) the ovalization is lined up as it's supposed to be because Biopace rings are symmetrical.

I would recommend pulling the pin out. I've had this issue with using Campy Record chainrings (designed for Record crank with integrated 5th spider-arm) on non-Record cranksets. However, since these cranks aren't ovalized, it doesn't matter so much what orientation. But it still does matter since ramps and pins in the chainrings are designed to aid shifting at specific points along the the pedaling stroke, so even in that case I've mounted the chainrings with the pin opposite the crankarm.

dbg

Sounds like a good idea... So if I may hijack the thread breifly... I'm considering trying out biopace on a recumbent and it seems like the relationship of the leg power output and the chainring are about 90 degrees away from that of an upright bike. Do you think it makes sense to shift the biopace oval back by 90 degrees?

TallRider

No, because your legs will be pushing from a 90-degree diff angle than on a regular bike, but at the point when your legs are pushing the most, the cranks are also rotated 90 degrees from what they are on a regular bike. So it cancels out.
If you rotated them 90 degrees (actually 72 degrees, presuming a 5-arm spider which is the only sort of crank for which biopace rings were ever manufactured) you'd be doing the opposite of what Biopace was intended for, and you'd be maximizing pressure on your knees during the power phase of the pedal stroke.

dbg

So I'm thinking I actually want to shift them forward by 90 degrees so that the interaction with the oval on the chain is in the same place during my power stroke as it would be on an upright.

TallRider

Well, first you can't shift them 90 degrees - you could only do that if you had a 4-bolt spider on your crank and chainrings. With a 5-bolt spider, you have to work with 72 degree increments.

Secondly, the point of biopace is not to have the largest chainring diameter at the power phase of the stroke - it's the opposite. (There are ovalized chainrings, including those used by Bobby Julich in time trialing, that are ovalized with the wider section in line with the power phase. But these have historically been hard on people's knees. See Sheldon's article on Biopace for more info here.)

Sheldon Brown

No, it isn't as much as 90 degrees. For recumbent use, you want to move the chainring forward by one bolt, i.e. 72 degrees.

Sheldon "That's How It Is On My Greenspeed" Brown

int19

Hi Sheldon,

Thanks a bunch, I was thinking the 180 degrees; same thing late last night and will mount them that way when I get home. I can't wait to try them out and see what they are like!

Richard