Frame Bottom Bracket Width

[Guest agteros]

I have a frame with a Bottom Bracket with a width of 73mm (Cotic Soul)

I'm looking at cranksets with a BB axle width of 118mm (Funn Soljam - BMX cranks or Funn Thrilla's - MTB cranks), or some of the 'Bore

 

Will these be compatible, anything 'special' to make it work?

Can I just buy a different bottom bracket in the correct width to make it work?

[Johan Bornman]

Bottom brackets require several measurements, depending on the type of BB. It sounds to me as if you square taper BB there.

 

These come in 68mm and 73mm sizes. These measurements refer to the actual size of the frame's BB shell.

 

The other measurement, the 109mm, 113 mm, 119mm or 123mm, refers to the length of the BB's axle/spindle. This has bearing on how far the crank sits from the frame. If it is too close, the front derailer cannot swing inward enough to pull the chain onto granny gear. If the crank is too far i.e. the BB too long, it cannot swing out far enough to engage top gear.

 

One way to see if the crank will fit is to compare the length of your existing BB and that fitted to the intended crank.

 

Ultimately, trial and error is the answer.

 

If you want to get really technical, find out what chainline (a measurement in millimeters) your frame was designed for and what chainline the FD can handle and work from there.

[Guest agteros]

The frame will be single speeded, so there is a bit of play (spacers/9 speed hub) at the rear to get the chain line right?

 

The frame (new one, I have no components for it), BB measurements are as follow:

Width (left to right): 73mm

inside diameter ~34/35mm

 

So this means that with a 118m axle there will be 118mm-73mm (22.5mm each side - if equal distribution) of the axle sticking out for the cranks to attached to?

Is the inside diameter standard across frames still in production? JUST the thread direction differs? Sounds too easy/standard for the cycling industry though!

 

Current MTB BB seems to be ~68mm wide. ~35mm sticking out either side on the axle, making it nearly 140mm long?

So they are referring to the actual widht of the BB on the frame, and not the widht(length?) of the axle going through there the BB?

 

I have no clue about the composition of a BB/crank/axle through there. Are these axles sold separately from the crank/BB set?

[Johan Bornman]

The frame will be single speeded, so there is a bit of play (spacers/9 speed hub) at the rear to get the chain line right?

 

The frame (new one, I have no components for it), BB measurements are as follow:

Width (left to right): 73mm

inside diameter ~34/35mm

 

So this means that with a 118m axle there will be 118mm-73mm (22.5mm each side - if equal distribution) of the axle sticking out for the cranks to attached to?

Is the inside diameter standard across frames still in production? JUST the thread direction differs? Sounds too easy/standard for the cycling industry though!

 

Current MTB BB seems to be ~68mm wide. ~35mm sticking out either side on the axle, making it nearly 140mm long?

So they are referring to the actual widht of the BB on the frame, and not the widht(length?) of the axle going through there the BB?

 

I have no clue about the composition of a BB/crank/axle through there. Are these axles sold separately from the crank/BB set?

 

I dont understand the first sentence.

 

The BB dimensions you cite makes for a standard 73mm BB with English thread.

If you intend fitting a square taper, Octalink or ISIS BB ( all cartridge BBs), you'll have to get the BB axle length right. If you are going with a hollowtech crank, the BB shell width is compensated for by spacers that come with the BB.

 

The axle protrusion from both sides of cartridge BBs are not equal. AS the BB gets longer, the right side gets proportionally longer than the left.

 

68+35+35=138.

 

If you install a cartridge BB, the axle is part of the BB. If you install Hollowtech BB adapter cups, the axle is part of the crank - the right crank on Shimano cranks and the left crank on some others....Race Face, I think. Either way, you don't worry about the axle.

 

On a singlespeed you have no problem. If your crank requires a cartridge BB, install a 109mm BB. If your crank requires a BB adapter cup, install it any old way, putting perhaps one space on each side. It doesn't matter. Either way, you'll fiddle with the chainline at the back where you'll play with spacers until you have good chainline.

 

Chainline or chain line is confusing in this context. A straight chainline refers to a straight line that the chain follows from the front to the rear. This is only possible in one gear on a cassette bike.

 

When purchasing FDs and designing frames, chainline is a mm measurement from the centre of the seat tube to the centre of the FD's swing arch.

[Guest agteros]

Thanks Johan, think I'll stick to the computers. They make perfect sense

[Johan Bornman]

Thanks Johan, think I'll stick to the computers. They make perfect sense

 

Don't give up. Maybe I put in too much info:

 

Lets just say - build your singlespreed from whatever components you have and then try and get the chain to follow a straight line from the front to the back, by fiddling with the left and right-side spacers on either side of your single rear sprocket.

[Guest agteros]

Don't give up. Maybe I put in too much info:

 

Lets just say - build your singlespreed from whatever components you have and then try and get the chain to follow a straight line from the front to the back, by fiddling with the left and right-side spacers on either side of your single rear sprocket.

 

I have no spares, so I need to buy a whole bicycle piece by piece.