I am working on my first bike project (a fixed gear) and am trying to figure out what size bottom bracket I need. I know it is 68mm in width, but I don't know how long the spindle needs to be.
Do I buy wheels first and then a bottom bracket/crank to match that or vice versa? I obviously need to make sure my chainline is straight.

The frame I purchased is an old Paris Sport, and the front/rear dropout widths are 100/120 mm respectively.

Thanks!

most track hubs set up your chain with a 42mm chainline, then depending on your crank you need a spindle length that brings your chainring out 42mm to have a "perfect chainline"

some BBs have a small amount of adjustment that is helpful like the miche and phil woods

here is a helpful reference
https://www.sheldonbrown.com/chainline.html

42 mm out from the frame portion of the BB?

Keep in mind that Paris Sports date back to the early 70s. I would ask in Classic and Vintage. Those guys are very knowledgeable over there.

120mm is the old standard for 5 speeds which will fit the modern 120mm track spacing new hub use and you should be looking for a 40-42 mm chainline which is the distance from the centre of the down tube to the inside of the chain ring.

This will be affected by your choice of bottom bracket and cranks although a 110 mm bb is usually a pretty safe bet with older road frames and will get you pretty close as the stock bb should be 115mm and you will only be running a single ring and not a double.

This may help...

https://www.sheldonbrown.com/chainline.html

Shimano BB-UN54 in 110mm. If you are really worried about it, use the 107mm one.

If, after all your careful calculations, the chainline on paper is off by a few millimeters, don't worry about it. I've used the above Shimano bb with a Sugino crankset and Formula hub and the chainline is dead quiet.

Ok, I talked to the guy I bought the frame from and he said it used a British 1.370 x 24 tpi bottom bracket. So I know what exact size to get, but does that determine the length?
@Carleton: I'll take your advice and ask over in the Classic/Vintage section(s).

Here's the correct response - https://www.bikeforums.net/showthread...for-fixed-gear

Thread over.

Oh and Paris Sports can be sorta hot. Here's my teammate's bike. It's from the early 70s. Just got a fresh chrome job.

- Early 70s Paris Sport made by the famous Francesco Cuevas FREEHAND.
- Sprint geometry.
- Campy cranks, BB, track ends, fork dropouts
- Nitto B125 bars (yes a quill stem would look better, but the bars would be too low, below the tire line which is illegal for racing)
- This bike will be raced, not a street bike.

Dear lord

Now that I realize what you're saying, it sounds so simple!

I also have a paris sport track bike. A friend bought it on ebay about 10 years ago with an almost full Suntour Superbe grouppo (Sugino Mighty Comp 151bcd cranks), Reynolds 531, and I believe Cuevas also built it. I'd love to see more close ups of your friend's frame and like to know if it is a 26.6 seatpost. I'm currently replacing all the parts on it with Suntour Suprebe Pro (waiting for the weather to improve before switching the wheelset). I've pegged my frame as early to mid 70s due to the long wheelbase. Despite several accidents, it still rides great. I had a way to long phil bb in my frame for a while on Sugino Mighty Competition and Sugino Aero Mighty cranks and the chainline was fine. It rides the same but looks nicer on the matching Suntour Suprebe Pro cranks and bb.

OP: is your frame a track frame or a 5 speed? Have you selected a crank yet?