Another BB Question

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XRHYSX

A Big Bad Lorry Driver
After 5 years of abuse its time to replace the BB on my bike,
Its an old style cup type which I would like to change to a sealed unit,
WP_20131007_003.jpg

I'm having trouble on deciding which size I need:scratch:
My bike is nothing special, Its a Freespirit Conquest and has a Shimano tripleset.
The original crank seems to be 120 mm. This is the closest I can find.
Is this what I need/is my tape measure just carp:huh:
The left side of the crank is inline with the tape measure;)
 

RecordAceFromNew

Swinging Member
Location
West London
Those traditional spindles usually have slightly different asymmetry compared to say a Shimano JIS cartridge bb of the same length, so replacing with one of the same spindle length would not necessarily give you the same chainline (distance between centre of frame and middle chainring for a triple), which is what you want to maintain or else your front mech could run out of travel.

In this case since you have a Shimano triple chainset, what is the model code on it (usually stamped FC-xxxx on the inside face of the arms)? That information and the original chainline (which you can either measure or if no longer possible guesstimate based on the type of bike it is) should allow us to work out which cartridge bb you need.

The only other thing you need is to measure the width of the bb shell on the frame - it should be either 68mm or 73mm.

Since the chainset code is a more useful piece of info, this is not really necessary if you would prefer having a cartridge bb, but there should also be a code stamped on the existing spindle, probably something like 3P, 3N or 3S, which would allow you to acquire a replacement spindle if that was the preferred way forward.
 

RecordAceFromNew

Swinging Member
Location
West London
There seems to be some inconsistency somewhere.

A 3A spindle should be 117.5mm in length. It might be good if you could check your spindle with the dimensions here.

On the other hand according to Shimano's tech doc the standard bb for your chainset is 122.5mm (such as this or this) and delivering either 47.5mm or 48.5mm chainline depending on the model variant. Further such bb's are more asymmetric - the drive side is 3.5mm longer than the non-drive side.

47.5mm or 48.5mm chainlines are typical for mtbs and most hybrids, while 45mm is typical for road bikes (and certain hybrids). If your existing setup's chainline is more like 45mm then you should not get those 122.5mm Shimano bb, but 118mm ones (like this or from here).

If I were you I would play safe, put the old one back together, check the chainline to be sure before ordering.
 
OP
OP
X

XRHYSX

A Big Bad Lorry Driver
Right, Erm I may have replaced the chainset awhile ago thinking they were all the same, :blush:

Now come to think about it, I do struggle to get it onto the lowest cog at times

I don't want to put the old crank back on as its pitted worse than a teenagers face :eek:

I'm going to take the chainset and BB shell measurement to the decent LBS and see if they can sort out my mess:whistle:

Thankyou for explaining a few things :thumbsup:
 

RecordAceFromNew

Swinging Member
Location
West London
Right, Erm I may have replaced the chainset awhile ago thinking they were all the same, :blush:

Now come to think about it, I do struggle to get it onto the lowest cog at times

I don't want to put the old crank back on as its pitted worse than a teenagers face :eek:

I'm going to take the chainset and BB shell measurement to the decent LBS and see if they can sort out my mess:whistle:

Thankyou for explaining a few things :thumbsup:

Now it makes sense! Given a) the type of bike (seems to have flat bars, 26" wheels, triple chainset as standard), and b) the difficulty you have in dropping the chain from the middle to smallest ring (the 3A bb spindle is too short for the FC-M171, making the chainset too far inboard for the front mech to operate), unless the front mech is not original and wrong, I am 99% certain that you need a Shimano 122.5mm bb for 68mm shell. Good luck!
 
OP
OP
X

XRHYSX

A Big Bad Lorry Driver
What a difference! Took it to The Bike Warehouse in Faversham and they recommended a 122.5 bb,
I had to Index the front mech but can now access all the gears with ease,
also when in top gear it would jump off the small rear cog, no more it really is a different bike... for the better ^_^
 
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