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I'm confused about bottom bracket dimensions!

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I'm confused about bottom bracket dimensions!

Old 12-15-14, 09:47 PM
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John Nolan 
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I'm confused about bottom bracket dimensions!

Okay, so I got this Italian frame,(see this thread:https://www.bikeforums.net/classic-vi...me-please.html).

It has a 70mm bottom bracket shell, and I want to get a new bottom bracket for it to fit my Stronglight 99 BIS NM triple (at least I think that's what I have, it looks like that).

Velobase says I need a bb length of "68"..how does that relate to the overall dimension given in something like this Shimano BB with "70x115"?
Shimano Bottom Bracket Square Taper 70x115, Italian

Thanks
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Old 12-15-14, 10:00 PM
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Not sure I'm exactly understanding your question. 68 refers to the width of common bottom bracket shells on the frame of the bike. I understand that a 70 mm width of a bottom bracket shell is for Italian frames.
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Old 12-15-14, 10:02 PM
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You need an Italian-threaded, 70mm bb for starters.

Beyond that, you need to choose the spindle dimensions, likely something like ISO 123mm length with some degree of offset toward the drive side.

A JIS bb could also be used, again it would have to be 70mm Italian, and the thicker JIS-tapered spindle would have to be compensated for by using about a 5mm shorter spindle. Here again, some degree of offset will likely need to be used, possibly by using a fixed cup spacer of perhaps 2-3mm thickness.

At the limit, your chainrings will need to clear the chainstay by a couple of mm. A test-fit using a cartridge-style bottom bracket of known dimensions would get you well on your way toward selecting an optimal bb.
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Old 12-15-14, 10:06 PM
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If you are changing from cottered cranks to cotterless cranks, the new crankset should have a recommendation for bb spindle length.
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Old 12-15-14, 10:25 PM
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Originally Posted by Velocivixen View Post
Not sure I'm exactly understanding your question. 68 refers to the width of common bottom bracket shells on the frame of the bike. I understand that a 70 mm width of a bottom bracket shell is for Italian frames.
Two dimensions and a thread are needed to describe a BB...Velocivixen gets the shell width correct...the next dimension you need is the spindle length...your crankset should come with a recommendation...

finally, you will need to know the threading on your BB shell. Generally, if you have a 70 mm shell, you will need Italian threading and if you have a 68 mm it is generally English threads...
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Old 12-15-14, 11:49 PM
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Originally Posted by daf1009 View Post
Two dimensions and a thread are needed to describe a BB...Velocivixen gets the shell width correct...the next dimension you need is the spindle length...your crankset should come with a recommendation...

finally, you will need to know the threading on your BB shell. Generally, if you have a 70 mm shell, you will need Italian threading and if you have a 68 mm it is generally English threads...
You actually need three. The third is a measure of the offset of the BB - is the drive side spindle longer than the non-drive side, and if so by how much. It can also be important to know if you need ISO taper (such as for Campagnolo and a few others) or JIS taper (by far more widely used, Shimano cartridge BBs are a good choice here). We also assume you do not need Octalink or one of the other odder end systems, because of the likely age of your Stronglight.
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Old 12-16-14, 03:18 AM
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Right: I knew I needed an Italian-threaded, 70mm bb.

The Stronglight crank I have is about 30 years old, and the only info I have about it is what I find on Velobase. I think it is JIS square taper, because in the past I've been able to interchange it with a Shimano crankset, and they had the same fit on the spindle.

I guess what I'm asking is: is there anyway to figure out the correct spindle length and offset for this crank without having manufacturer suppled specs? Trial and error? I don't have anything in my parts box to try with!

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Old 12-16-14, 06:04 AM
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Originally Posted by John Nolan View Post
I guess what I'm asking is: is there anyway to figure out the correct spindle length and offset for this crank without having manufacture suppled specs? Trial and error? I don't have anything in my parts box to try with!
We all wish. There are several tried-and-true methods:

Trial and error, see what works.
Measure the old one.
Borrow a spindle or two.
Ask someone who has the same crank.
Haunt your local bike co-op.
Look up the specs.
Compare specs to other cranks, try to get a sense of whether it is long, short, or average, get a spindle in the ballpark. It will probably be just fine.
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Old 12-16-14, 06:20 AM
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Right: I found the specs for a Shimano crank (the FC-DE30) that spent some time on the same bike as the Stronglight, and it apparently called for a 121.5 spindle. Shimano makes a 122.5 70mm Italian thread bb, so I think I'll go with that.
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Old 12-16-14, 06:50 AM
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Originally Posted by John Nolan View Post
Right: I knew I needed an Italian-threaded, 70mm bb.

The Stronglight crank I have is about 30 years old, and the only info I have about it is what I find on Velobase. I think it is JIS square taper, because in the past I've been able to interchange it with a Shimano crankset, and they had the same fit on the spindle.

I guess what I'm asking is: is there anyway to figure out the correct spindle length and offset for this crank without having manufacturer suppled specs? Trial and error? I don't have anything in my parts box to try with!
For a triple, you'll want a Stronglight 123 or 125 spindle, or a third-party spindle approximating the Stronglight's dimensions:

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Old 12-16-14, 08:22 AM
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Thanks John—most helpful!
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Old 12-16-14, 08:52 AM
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And to go along with JDTs chart, any Stronglight spindle of correct length will work.

Some manufacturers adjust for the English 68mm/ Italian 70mm difference in the spindle itself (in the middle section - 56mm above). Stronglight made the adjustment in the cup thickness. Italian cups are thicker than English/French/Swiss.
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Old 12-17-14, 03:45 AM
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Originally Posted by Ex Pres View Post
And to go along with JDTs chart, any Stronglight spindle of correct length will work.

Some manufacturers adjust for the English 68mm/ Italian 70mm difference in the spindle itself (in the middle section - 56mm above). Stronglight made the adjustment in the cup thickness. Italian cups are thinner than English/French/Swiss.
Seems to me that you perhaps meant that the Italian cups are thicker, to come down to the dimension of the spindle raceway spacing(?).
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Old 12-17-14, 08:39 AM
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Originally Posted by dddd View Post
Seems to me that you perhaps meant that the Italian cups are thicker, to come down to the dimension of the spindle raceway spacing(?).
You're right, adjusted the above.
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