Convert hybrid 68/70 Italian bike from 3-piece to cartridge?
 

I've been looking at cartridges to replace my 3-piece bottom bracket, and I'm puzzled by two things:

1. My bike is Italian but with all English threads (a Bianchi Strada). The BB takes cups designed for a 68mm English/ISO system. The fixed cup has left-hand threads, unlike a 70mm Italian BB. But the shell actually measures 70mm. That 2mm difference is no problem with an adjustable BB, but what about a cartridge? I've never seen one in the flesh, so I don't know how the two parts fit together. Will the 70mm width of the shell prevent me from using a 68mm cartridge?

2. With a 3-pc bottom bracket, you pick the overall length of the spindle and also the left/right length. I use a 3N spindle, 32+52+36=120. Going just by overall length, it looks like I would have to take either a 118 or a 122 cartridge. But they don't say how much spindle sticks out on each side. Are they symmetrical? So I should look for a 124mm cartridge? (to get the same chainline as a 36+52+36?)

You won't, or shouldn't have a problem with a typical cartridge BB. The left cup doesn't tighten to the face of the BB, but rather to the end of the cartridge, and so can go sub-flush in the shell without issues. On some, even the right cup is without an outer flange allowing you to float the system to either side for optimum chainline.

Assuming the right cup does have a flange, your wider shell will move it to the right by 1mm, so add that to the BBs published spec. for chainline.

All the less expensive cartridges have a small flange on the left side. On the plastic ones it's very small and could probably be cut off easily, but if it's not intended to be sunk into the shell I have to wonder whether the crank arm would be able to slide on all the way and not contact the shell.

Where do you find that? I think I've looked at all the square-taper JIS cartridges at all the online retailers, and haven't seen a single chainline spec or anything that would allow me to calculate it. They only give the shell width and the overall spindle length. There's never anything about how long the actual cartridge is when it's screwed together, or how much spindle sticks out each side. That would help me not only with the chainline issue but also with that question of getting the crank on all the way if the left cup is sub-flushed.

There's something basic here that I'm missing. With the complete lack of drawings or specs about their geometry, I don't understand how anyone ever knows which cartridge to buy.

There is no chainline spec for BBs per se. The chainline depends on the crank arm offset and the spindle length. Cranks usually spec. a recommended spindle length to generate "correct" chainline.

I'm sorry I confused you. What I should have said is that the chainline will end up 1mm outboard of what's predicted for a particular 68mm BB/crank combination. So if your crank calls for 110 spindle, the displacement to the right will make it as if you used a 111mm . Not a big difference, but could be a consideration if choosing between two close lengths.

I don't have a spec for my crankset. It was made before there were cartridge BBs.

The spindle in the 3-pc system I'm using now is 120mm, but it's asymmetric (32/52/36), and so is the shell (36/34).

The 3-pc system creates a 46mm chainline (the distance from the frame centerline to the centerline between the two chainrings).

The BB shell extends 36mm to the left and 34mm to the right of the frame centerline.

Is there any way to pick a cartridge size based on that information?

http://www.philwood.com/products/bbpages/ssbb.php

Some bottom brackets, like the Phil Wood version linked above, have two 'adjustable' cups so you simply order the correct thread pattern and spindle length and then set the cups however they best give proper alignment and chainline.

Edit: I just read the rest of the details on your current BB - by 36/34 are you referring to the threads per inch on eiher side? If so , I imagine one side is 1.37" and the other is 36mm. If this is the case, there is no stock bottom bracket made to fit. This is very unusual, and even if it was used on a couple of bikes years ago, it is not used on any any more.

The Phil one might still be a good option, but you'll have to specify in the order that you need two different sizes of cups, and probably have to buy two sets.

No, the threads are ISO on both sides. I meant that the BB shell extends 36mm to the left and 34mm to the right of the frame centerline. It's asymmetric only in the sense that it's not centered on the frame.

Oh.

ANother option (although this will not help select a bottom bracket that goves good chainling and pedal-alignment-symmetry) is to remove 2mm of material from the longer side. A frame builder or well equipped shop might be able to help you with this.

It seems to me that overall spindle length can't generate any particular chainline, unless there's some industry-standard rule about how the overall length is divided between left and right.

Chainline is measured from the frame axis. Crank geometry just gives an offset from the end of the spindle to the chain. You need to add that to the right-length (distance from the frame axis to the right end of the spindle) to get the chainline. The right-length is what matters, not the overall length.

But if it's standard practice to specify overall length, then there must be a rule that sets right-length based on overall length. For example:

--If the rule is that all cartridges are symmetric, then a 118mm cartridge has a right-length of 59mm.

--If the rule is that left-length is always 50mm, then a 118mm cartridge has a right-length of 68mm.

--Or there could be a set of rules, for example: all 110-115mm cartridges have a right-length of 60mm, all 115-120mm cartridges have a right-length of 63mm, etc.

But no one says there's a rule. Not even Sheldon Brown.

Just to clarify: I meant that as a question.

i.e. Does anyone know the secret code that translates overall spindle length into information about how much spindle is to the right of the frame centerline?

Perhaps as a diagram, like this one and the table that goes with it, but for cartridges:
http://www.sheldonbrown.com/images/spindle.jpg

There is no secret code or handshake. Chainring position is a function of the right side spindle length and the offset built into the arm, which varies from one to the next. Since you're not changing the crank you have only the spindle to worry about.

I know you want a simple answer, but there is none, except that cartridge BBs measure from center like any other, so a 120mm symmetrical BB will measure 60mm from the center of the BB shell, and a 124 asymmetrical with the right 4mm longer than the left will be 62/58 from the shell's center. looking at the sketch you can divide "B" and add half to A & B respectively since you don't care where the bearings are.

You have an existing spindle, measure it, then purchase a cartridge BB with an equal spindle length, and the same R/L difference, if any. If you can't match it exactly get the nearest one focusing on the right side length. You can also buy yourself fudge room of a few millimeters by using a non flanged bottom bracket like a Phil, which will allow you to float the unit to wither side.

Be aware that even with an, on paper, correct spindle length, there will be small differences in crank position because the distance the crank slides up the taper varies. Allow 1-2mm room for error in chainring-chainstay distance.


BTW- you have another alternative if this is beyond you. Bring your spindle to a bike shop and ask for a comparable cartridge bottom bracket.

But the cartridge suppliers never give any information about R/L difference, or whether it's symmetric or asymmetric. They only give shell width and overall spindle length.

That's why I spoke of a secret code. You agree with me that the R/L difference is something we need to know; but they don't tell us what it is.

If they only give overall length, and don't mention any asymmetry, or difference in right and left side, than assume they're symmetric. Or ask the seller the question about the specific BB you're considering.

Back to Phil Wood , where the ends of the BB shell are don't matter,
because the mounting ring is internal. and in addition the axle is a
tight press fit, without an internal shoulder, so even the offset, if any, is adjustable.

O.k., maybe that's the code I was looking for:
"All cartridges are symmetric unless noted otherwise."

I've never seen one that's noted otherwise, and I think I've looked at all the available JIS cartridges by now. So apparently all JIS cartridges are symmetric, even though the cup-and-cone JIS spindles they replace were usually asymmetric.

Then I can't do what you said about finding a cartridge with the same R/L difference as my spindle, since my spindle is asymmetric. I'll have to buy a symmetric cartridge with the equivalent right-side length.

My spindle is 32+52+36=120. The symmetric equivalent would be 36+52+36=124. So I should choose a cartridge as close to that as possible, which would be about 122.

That should take care of the other issue as well. The extra 2mm on the left should mean that there's no chance of the crank contacting the shell even though the left cup of the 68mm cartridge is counter-sunk into my 70mm shell. I just have to make sure I get one with no left flange, or a plastic left flange that I can trim off.

Thanks very much for your patience and help with this.

Depends on crank design , lower profile designs don't need offset,
as they sort that out when the crank-arms are still on the 'drawing-board',
[or CAD monitor screen, now].

you can buy the symmetric $20 UN 'whatever' for the right side ,
and just live with the left side being outboard a bit, then it will all be adequate..

Thanks, Bob.

Yes, the Shimano UN-54 is more in my price range than the Phil Woods. But I wonder about the plastic left cup, both for the material and for the slight flange or lip, which I might have to trim off.

I was thinking an IRD QB-55 for just $5 more might be a better choice for me. It has a metal left cup, with no flange at all. And it comes in a 68x122, which apparently is the size I need.