Bottom bracket / chain line

[rb58]

The crank set on my fixed says it's suitable for a 103mm bottom bracket, with a JIS taper. This gives a chain line of 42mm. I've read Sheldon, so sort of understand what the chain line is and how to measure it. However, how important is it to get it spot on for a single speed/fixed?

Secondly, if I want to change the crank set for one that says it's suitable for a 107mm bottom bracket (or even a 110mm) bottom bracket, does this automatically mean I have to change the bottom bracket too? Or could I 'get away with it' and use the existing 103mm bottom bracket?

And, if I do need to change the bottom bracket, how can I tell which type I need? (Threads etc.)

Cheers

 

[GrumpyGregry]

putting a chainset designed for a wide BB on a narrower BB can result in your chain ring gouging huge lumps out of your chainstay. Don't ask me how I know this.

Do you need to get it spot on? Not ime no. It needs to be in a range of tolerance esp on fixed so the chain stays on but to the mm? No.

Threads etc? Existing BB should have a label on it that tells you what type it is. I always use that as the basis for buying a replacement but I've never owned a frame with Italian threading, nor some exotic BB shell size and I won't touch press-fit with a barge pole.

 

[Scilly Suffolk]

The straighter the chainline the quieter and more efficient it will be and your drivetrain will wear out more slowly; if both your chainring and sprocket are 3/32" then you could use a 5/6/7/8 speed chain which will have more leeway as it's designed to work at an angle.

I wouldn't like to say how much you can get away with, it's really just a case of "suck it and see".

A note on JIS vs ISO taper cranks: contrary to what Sheldon Brown states, the difference between them is that ISO cranks have 1mm x 45° chamfer on the end which is included in the length, whereas JIS lengths start where the 2° taper starts. That means, for example, an ISO 111mm axle has the same effective length as a 109mm JIS.

Assuming you're using an internal BB, then the Shimano UN-55 is highly recommended and around £20, so there's no reason not to get a good match for your cranks.

To ID your BB, as Greg says, the easiest method is to check what is written on the fitted BB: 1.37["] x 24 [threads per inch] is British/ISO, 36[mm] x 24 [threads per inch] is Italian; there are several other standards, but you're unlikely to come across them.

The other method is to accurately measure across the BB shell: 68mm is British/ISO and 70mm is Italian.

Finally, the fixed (driveside) cup of British/ISO BB has a left-hand thread (ie you turn it anti-clockwise to screw it in); Italian BB have a normal, right-hand thread which can come undone in normal use due to precession: I use Loctite 22 to resolve this.

 

[rb58]

Thanks both. Bottom bracket will be out tonight as a first step to making sure I buy the right crank set for the 'new' bike.

 

[GrumpyGregry]

Thanks both. Bottom bracket will be out tonight as a first step to making sure I buy the right crank set for the 'new' bike.

If any doubt emerges measure everything twice and then speak to the team at Velosolo. Tis my modus operandi.