School me on BB's
 

Hello everyone,

I sure could use some help wrapping my mind around how to upgrade my bb and crankset.

I ride a Mercier Kilo tt. According to Bikes Direct, the bb is 68 x 108 mm. The chain line is 43.5mm.

I understand that the 43.5mm chain line is measured from the center of the bb to the center of the chainring. According to Sheldon Brown that's not a standard chain line for a ss/fg.

It's easy enough to find a 68 x 108mm bb for an upgrade, but the spindle length is only half the equation since the crank spider will place the chainring in maybe the wrong position for an even chain line.

Is there a solution for shopping for the right crankset/bb to ensure I'll have picked the proper spindle length for the chosen crankset or is it a matter of putting spacers in to compensate for buying a setup that throws the chain line off?

Any help would be greatly appreciated. :thumb:

A crankset and BB need to be a matched set. The crankset should specify the required BB type (ISO or JIS) and spindle length. If you pair the crankset to the BB correctly, you won't need spacers or have a chainline problem.

So If I decided to buy the Omniums that come with the bb and install, my chain line will be perfect?... And if I buy the Sugino Messenger crankset, if I buy the Messenger bb I'll have the proper chain line? If that's true then I just need to match the bb dimensions correct? So look for a 68 x 108mm bb and matching crankset and all will be fine? Thanks for taking the time to help!

Doh...

I think I figured it out on my second read through of your post...

You're saying that, for instance, the Sugino Messenger has a recommended spindle length of 103mm (I think)... So buying a bb that's 68 x 103mm will make a matching set as long as the crank connection type is matching as well (JIS, Powerspline, etc.)?

Exactly. The Sugino Messenger is JIS square taper btw. All BBs are going to be 68mm wide and English/ISO threaded unless specified otherwise.

OK thanks a lot it's all starting to make sense now. Last question..

If a crankset says 45mm chain line with 103mm spindle and I have a bike with a 43.5mm chain line, how much of a difference is that 1.5mm going to make? I want my drive train as quiet as possible and I'm noticing that there aren't any 68 x 101.5mm bottom bracket out there for sale which would theoretically make for a perfect 43.5mm chain line.

But you need to make sure the taper matches (ISO vs. JIS) for that to be accurate. Assuming that's correct, a 1mm or even 1.5mm fudge factor is tolerable either way anyway, as long as it doesn't place your chainring right up against your stay.

Look at 1.5mm on a metric ruler. Is it worth losing sleep over?

I've seen conversions with chainlines off as much as 10mm that were dead silent. On the other side of the coin, I've seen proper track setups that were dead straight and noisy as all hell. Components play more of a role in having a silent drivetrain than the "perfect chainline".

Also, cogs vary in their flange width, which also affects chainline. Even hubs can vary in the distance from the cog thread shoulder to the lock nut. Finally, over time as you remove and re-install a square taper crankarm, it will gradually creep inward, altering the chainline.

Oh, just one more thing. You can take specs from bikesdirect with a grain of salt.

Also factor in trueness of cog/chainrings.


That and some combos are just loud.

word, that sugino zen sounds business...but aint silent

Mine sure is.

ahhh put the zen on my BK vs the track pro and yes...silent. something about the tubing on the fuji resonates sound. weird

It's actually not that weird at all.

Science, *****es!

Thanks everyone for the help. The concept has been demystified. Apparently, it wasn't nearly as difficult as I'd led myself to believe. At least I can make an informed decision now when buying drive train parts.

Bugga. Don't know what went wrong but we'll have to try again :innocent: