Link between bottom bracket and rear wheel widths

Question

I am building a bicycle by gathering some components I already have. It is for a basic bike, only used as spare bike.

I have to change the crankset and bottom bracket, as they are scrap.

The crankset I want to put on it comes from a MTB with 135 mm read wheel OLD. The bike I am building is also an MTB, but with 130 mm OLD.

Should I get a shorter BB (by 5 mm I guess) to make it correspond ? Whatever the dimension, I will have to purchase a new BB, because the frame is not BSA, but italian thread.

Answer 1

Yes, if you're going to a 130 rear end, the chainline moves in about 2.5mm so generally you'll want to get a 5mm shorter spindle.

Ideally you would measure the front chainline with the current spindle and do the math from there based on what rear chainline you want and whatever your clearance needs are. In some cases you would find you might want to go more than 5mm.

How much it matters depends a lot on the chainstay length.

You still need to be mindful of clearance with the frame.

Answer 2

Here's a similar topic: Italian bottom brackets and modern cranksets - a compatibility guide?

The documentation for the crankset will indicate what "chainline" it is designed for. The chainline is basically the side-to-side offset distance of the chain from the centerline of the bike/wheels. If you have a triple front crankset, it will probably specify a 50mm chainline. The crankset docs should also indicate what BB specs you need, depending on the frame measurements. If you have a standard English threaded BB shell, it will probably be either 68mm or 73mm wide. If your crankset requires for example a square taper BB with 113mm spindle length, that's what you would get. Square taper BBs normally come with a thin spacer for use on 68mm width frame shells to adjust the positioning to match a 73mm shell w/o spacer, so such BBs are normally sold as 68/73mm BBs. Essentially, spacing for the front and back is handled independently, trying of course to set each to the same chainline value.

Since you have an Italian threaded frame, I would think you would just use an Italian threaded BB with the spindle length indicated by the crankset (looks like 70mm is a common BB shell width for Italian threading, but measure your frame to make sure). More details, such as brand and model numbers of your crankset and frame, would be helpful to provide a better answer.