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Old 04-15-12, 07:25 AM   #1
Road Fan
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Differential ball size in a BB?

Has anybody successfully used a spindle for a 70 mm BB in a bike with a 68 mm BB?

I want to try this on an Avocet crank installation I'm doing. The idea is to put 11 0.25 balls on the drive side (this is what it was designed for) and put 22 (or whatever fills it correctly) 0.125 balls on the non-drive side. I hope to reverse the effect of the wide shoulder width of the 70mm spindle.

I did a trial assembly with an Avocet 129 mm spindle, and it suits the bike well with a good taper fit spindle to crankarms, good chainring clearance, and good crank/chainstay clearance, with decently low Q. I want to use that spindle, since I can't find an Avocet 4-68, and the Avocet taper design is unique, neither ISO not JIS. Problem is the spindle shoulder width is about 3 mm too wide, and the adjustable cup is sticking out 3 mm past the lockring. If I make the balls smaller the adjustable cup should thread farther into the BB shell.

The cups I have are the original Avocets, and they have a short threaded area compared to a Shimano cup or a Stronglight cup. The ultimate problem I'm worried about is not enough thread engagement of the adjustable cup into the BB shell.
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Old 04-15-12, 07:34 AM   #2
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Interesting approach and i haven't seen Avocet's name in quite a while except I use their O2 saddles on everything I own. AFAIK, Avocet components were relabeled Ofmega stuff and i'm surprised the tapers aren't standard ISO.

Give your bearing idea a try and see if the spacing comes out adequately. The 1/8" bearing ball won't match the cup or spindle race curvature but may find their own bearing circle and work ok. Could you subastitute a Shimano or Sun Tour adjustable cup for the OEM one to get more thread engagement with the proper size bearing balls?
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Old 04-16-12, 05:56 AM   #3
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Bumping! The smaller balls aren't a fruitful approach. I can type more later on.
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Old 04-16-12, 06:37 PM   #4
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Just use 1/4" balls on both sides. The most salient issue you're likely to encounter is that the adjustable cup sticks out 2mm further than it would with the proper 68mm spindle.
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Old 04-16-12, 07:50 PM   #5
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The entire bearing -- cup, cone and balls -- needs to be designed and built as a set.

Changing balls will lead to having the wrong cup/cone curvature, which will likely lead to wear issues.

New BB's are cheap.
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Old 04-16-12, 07:55 PM   #6
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I'm no expert but I thought the ball size wasn't critical. And thinking about it geometrically, why should it be, within limits of course? Like Hillrider says, balls of a slightly different size will ride on a slightly different circle. The radii of the races and cones are, and obviously have to be, significantly larger than the radius of the balls. A ball of any reasonable size will have 1 point in contact with the race, and one with the cone. The balls just need to be all the same size, and reasonably close to the 'design' size.

Like I say, I'm no expert.

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Old 04-16-12, 08:18 PM   #7
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Quote:
Originally Posted by jim hughes View Post
I'm no expert but I thought the ball size wasn't critical. And thinking about it geometrically, why should it be, within limits of course? Like Hillrider says, balls of a slightly different size will ride on a slightly different circle. The radii of the races and cones are, and obviously have to be, significantly larger than the radius of the balls. A ball of any reasonable size will have 1 point in contact with the race, and one with the cone. The balls just need to be all the same size, and reasonably close to the 'design' size.
Just try it and you'll find out why it doesn't work.
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Old 04-18-12, 05:47 PM   #8
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Originally Posted by JohnDThompson View Post
Just try it and you'll find out why it doesn't work.

What I found out is that the balls are not held securely if they're not the right size. Up to 3/16 they are jumbling and trying to squeeze out between the cup and spindle, and not giving that smooth turn. Usually I'll work with subtracting balls to resolve this, but in this case it's not worth it. I don't believe they are fully contacting the ground surfaces on both the cup and race.

John, please give a more substantive explanation of the problems you've seen in trying an installation like this. I presume you have done so, and that there's more you can say.

So John, I have gone back to the original 11 1/4" balls per side. The adjustable cup sticks out 3 mm. By itself this is not a problem, you are correct about that. However in this case (please refer to the last paragraph of my original post) the Avocet cups have a rather short length of thread, and I'm concerned that there might not be enough threads holding it in the BB shell.

I have two more remedies to try. One is to replace the Avocet BB cup with a Shimano, which I think has a longer threaded area. Another is to give up on the Avocet spindle. I've advertised for the 4-68 spindle (I have the 4-70) in several places, with no positives.

But (@Hendo252) the Avocet spindle is uniquely a match for the Avocet crank, so while BBs are cheap, BBs that fit can be a PITA to find. I've test-fit an Avocet arm on a JIS spindle, and while it will torque up, it leaves only 1 mm clearance for the end of the spindle. This is less than the clearance resulting from the Avocet spindle. So if I give up the spindle I give up the crank and (those who read all of my original post) that crank combination fits with the big and widely-set chainstays very very well while giving me Q of 150mm. Pretty good for a triple!
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