Biopace//chain tension...help..
 

For all you biopace riders out there (I know there's a few)...how much slack to you have in the chain at its tightest point in the rotation? I erred on the side of "too-tight" and busted my chain (albeit a patchwork of 3 old chains...), locked up my wheel and almost wound it thru my spokes...:eek: Thankfully I was in the flats. Advice? (besides "put a normal chainring on..")

advice: pull your wheel back so you're snug-ish in the chainring's "tightest" position. the slack you get when it turns is not a problem. people ride with significantly more slack than what it'll produce without ever throwing a chain.
a good rule is, if you can't pull a chain off of a chainring with your hands, you won't throw it while riding.

^^^^ I don't think you read his post

You shouldn't be using biopace sprockets for FG.

^^^^ On the contrary



To the OP, I would say slightly less tension than you used. Between a 1/4 and 1/2 inch bounce in the chain. Also, a pieced-together chain doesn't help...

I'd like to know how you fare so post back...

yes i did. my response addresses it directly.

False. When set up properly (which isn't difficult at all) biopace has no detrimental affects on a fixed gear drivetrain, other than looking unnecesarilly crappy.
superficial asthetics aside, though, it works fine.

In almost 22 years of fg/track riding I have been told by MANY mechanics NEVER to do this.

Why not? An ellipse makes the same overall contact with the chain as it rotates. The only issue here should be the change in the angle between the chain's points of contact with the cog and the chainring as it rotates. That's only a minor variation; maybe an issue compared to super-round track chainrings, but most setups already have a bit of slop.

without asking a single one (of these MANY mechanics) why not?
i can assure you: 1) it works with no issue 2) from experience and 3) can explain why there's no real risk involved.

I respectfully disagree with the late, great Sheldon Brown on this. Somewhere, some months ago, he and I went back and forth on this. He contended that the number of engaged teeth did not vary much, so it did not matter much. I agree, but it does change the amount of chain that is required to stretch from the cog to the ring. Think of a loop of chain as being basically an equilateral triangle: the base of the triangle is the front side of the ring, the two equal legs are the chain running from the ring to the cog on the top and the bottom. The longer the base of the triangle, the farther apart the legs are spread (oh, my!), which requires the legs to be a little longer to reach as far. As an ellipse turns, it will length and shorten the base of the triangle. I know the difference will not be dramatic, but consider that the difference will affect both the top and bottom leg, and it turns out to be more than one might think.

In my limited experience with BioPace on FG, this only becomes a relavent issue with larger rings. I think Sheldon ran smaller rings when he used BP rings, so it was never an issue. With a large ring, it can make for a chain that will unnerve you sometimes. I have never thrown one, but it certainly looks possible.

jim

i ran a 42 tooth bp temporarily, and while there was a slight shift in tension (depending on where you were in your crank rotation), it was never-- including at its loosest-- at risk of being thrown. not even close.
Like i said earlier, if you tighten your chain normally while the chainring is at the position of "highest chain tension", then at its loosest it'll only be slightly slack. It wasn't enough to cause any alarm. When it was slack, the chain was still tighter than the chain of lots of riders who never encounter an issue.
It's really not as easy to throw a chain on a fixed gear as lots of people fear.
Your example is true, but far too exaggerated to be relevant here-- the biopace design is not a triangle, and the difference between the "big" and "small" portions of the gear is NOWHERE nearly as drastic as what would be necessary to run into issues.
Incidentally, resisting, skipping or trackstanding was never an issue either.

that said, i got rid of the chainring because they look like broken ugly garbage.

ps, you have mad fixed gear bikes.

There IS a downside. If stuff on your bike with a round chainring is already very bad in terms of varying chain tension througought the stroke - biopace will make it worse, not better. And it can be bad enough that you WILL risk throwing the chain OR you risk binding in a the majority of the crank travel.

Does that sound reasonable sp00ki? Or is there going to be another thermo-nuclear exchange.

PM Ken Cox. He runs Biopace.

You won't throw the chain if set up correctly, but I hated the feeling of backpedaling a biopace. It was really jerky and had no grace. Pedaling wasn't as bad a feel.

That's a good point. Using a 10% deviation in chainring diameter, I figure the chain path varies close to 5 mm for a 48T chainring and under 2 mm for a 32T ring. Maybe everybody could get along on the whole biopace thing if they just experimented with smaller chainrings.

The issue was the chain line. The mechanic was Chris Williams... of team EDS. I had the privilege of training with them in the summer of 93. What you you doing that summer, sport? ;)

Well, to the OP:
You can run a Biopace ring with no trouble. All you need to do is find the tightest point of the rotation (as you seem to have done :) ), and then set the chain tension to be a little (quarter inch or so) slack at that point. You should be able to run it without any trouble.

I have run a Biopace (46/16 ratio) for almost a year on one of my fixed-gear bikes. It is quite a nice ride. I've never had any trouble, and the chain has stayed on. I sometimes go to quite high cadences (over 120 rpm; sometimes way over, depending on the hill), with no problems. The pedal-stroke is very smooth, though I cannot claim any performance advantage over a round ring. I do like the Biopace better, however.

Trackstands become more problematic, but are still possible. I use brakes, so I don't know about how my setup would react to skidding. On the workstand it looks really strange as the chain angle alternates, but I don't care much.

Aesthetics aside, there is no reason why you couldn't run Biopace on your fixed-gear.

Not listening to rhetoric from uninformed mechanics, that's for sure.
There are many points that can be used validly when questioning the integrity of a biopace drivetrain.
Chainline is in no way shape or form one of them. Show your mechanic this article next time you guys train.

>>>mechanic<<<

>>>uses Biopace on FG<<<<

If set up properly, and by that I mean having a straight chain line and proper tension, there is nothing wrong with running a BP chain ring on a fixed gear.

Setting the chain tension on a BP equipped bike is no harder than setting up the tension on a non BP equipped bike... you always have to find the point where the chain is tightest and then back it off just a wee bit. You do not want to have too much sag on a BP equipped bike as if this happens you could throw the chain.

My Kuwie with a 38 tooth Biopace ring, 16 tooth rear cog, and for good measure, a U brake in the rear (which I have since removed).

I rode the BP ring until it wore out with not a single problem and am now running a 40 tooth Sugino (round) as I wanted a little higher gearing. We're talking about 1000's upon 1000's of km.

http://www.ravingbikefiend.com/bikep...s/kuwiefix.jpg

Sheldon Brown>>>Mechanical Genius>>>Used BP on fixed gear.

Are we done now ?

Oh...I think they look fine and they really mess up the heads of the new mechanics when they see them.

:D

I respect Mr. Brown's opinion on the matter. However, looooong before there was a bikeforum this topic was debated to death in pubs, and coffee shops (you cats are way late on this one). I have heard both sides of the story. I favor the view of a world class track teams top mechanic (among others) over Sheldon's.

I would love to hear the view of a world class track team's top mechanic (among others) because I have also wondered about this issue. However, thus far all we have gotten is your saying that he said the issue was chainline... and that is pretty obviously not an issue. On the other hand, we have Sheldon Brown and several other riders all saying that they have used it themselves with no problems. So... regardless of how long ago the topic died of debate in pubs and coffeehouses, I am going to have to go with experience over you said he said. I'm still open to more info, of course...

Hey man... rock what you want on your bike. I assure you most (not all) old school track cats would drop the same info I gave you.

(edit) One of my favorite velodromes in the country is only a road trip away from you. I'm sure some of the old timers at Alkek could answer your questions much better than me.

So you're still saying the issue is with the chainline? How could that even be minutely affected by a biopace chainring (an more so than a round chainring)?

I take SB for his word on the Biopace.