Bottom bracket width?

Fignon le Grand said:

So I have a 126mm cyclocross frame that im going to use a 120mm flip flop hub spaced in the axle to fit. I have a campag pista chainset with a standard bottom bracket shell. How do I calculate what bottom bracket width I need, the chainline should be 42mm. Thanks in advance

Here's the belated/tardy answer to your question ...

After you narrow the rear spacing on the frame (I presume it is steel!), you essentially will have the equivalent spacing as on a track frame ...

As you may know, there is some variance in the width/offset of track sprockets and BMX/single-speed freewheels ...

So, the BB spindle length that you want will be whatever the particular chainset/crankset would call for when used on a track frame ... 107mm-to-109mm comes to mind for square taper cranks Campagnolo Pista cranks ... since I recall (without checking) that a ~115mm was the length for a Strada (road) crank.

If you look at your Pista crankset and compare it with a Strada crankset, you will see that the obvious difference is the lack of a "shoulder" on the Pista crankset for the non-existent inner chainring.

If you resort to using a ROAD crankset for whatever reason, you have some leeway in that you can mount the ring on either the inside or outside of the spider on a "regular" ROAD crankset which approximates the chainline of a Pista crankset where the chainring/chainwheel would be on the outside of the spider; so, if you are using an ISIS or Octalink crankset, you just want to get the BB with the shortest spindle available, and simply fudge the fit with mounting the chainring on the inside of the spider.

alfeng said:

Here's the belated/tardy answer to your question ...

After you narrow the rear spacing on the frame (I presume it is steel!), you essentially will have the equivalent spacing as on a track frame ...

As you may know, there is some variance in the width/offset of track sprockets and BMX/single-speed freewheels ...

So, the BB spindle length that you want will be whatever the particular chainset/crankset would call for when used on a track frame ... 107mm-to-109mm comes to mind for square taper cranks Campagnolo Pista cranks ... since I recall (without checking) that a ~115mm was the length for a Strada (road) crank.

If you look at your Pista crankset and compare it with a Strada crankset, you will see that the obvious difference is the lack of a "shoulder" on the Pista crankset for the non-existent inner chainring.

If you resort to using a ROAD crankset for whatever reason, you have some leeway in that you can mount the ring on either the inside or outside of the spider on a "regular" ROAD crankset which approximates the chainline of a Pista crankset where the chainring/chainwheel would be on the outside of the spider; so, if you are using an ISIS or Octalink crankset, you just want to get the BB with the shortest spindle available, and simply fudge the fit with mounting the chainring on the inside of the spider.

Thanks for the advice, Ive actually already done it. Turned out a standard 112mm BB with the chainwheel on the outside of the crank :-) was perfect! Thanks for the reply and all your other mails on my bike,