chainset

[TAV]

Can I change my shimano 105 threaded 53 39 for a shimano threaded compact ive got a 8 speed 12 28 on the back the bike is a old scott usa anybody help cheers as id rather b spinning than grinding up the hills in wales.

 

[fossala]

Can't see why not, you may need a new BB though.

 

[TAV]

How will I know if I need a new bb

 

[accountantpete]

I'm not sure what a threaded chainset is but basically you will need a new compact chainset (you can't just change the chainrings as the existing crank arms are too long) and triple front mech.

You may have a short cage rear mech on that set up - if so it may or may not work. If you have a long cage then no problemo.

EDIT
ignore the triple front mech bit - i was answering another query in my head

 

[Dusty Bin]

I'm not sure what a threaded chainset is but basically you will need a new compact chainset (you can't just change the chainrings as the existing crank arms are too long) and triple front mech.

You may have a short cage rear mech on that set up - if so it may or may not work. If you have a long cage then no problemo.

'Threaded' as opposed to 'press fit' perhaps - ie a normal BSA threaded BB shell.

The chainsets will be a straight swap, assuming they are of the same type, so you should be able to use the same BB. However, if you are running 8-speed, then you may have a UN55 type square taper, or similar - in which case you might struggle to find a new Shimano chainset, but there are plenty of other options out there from Stronglight and others. They might need a different length BB though.

Not sure why a triple front mech will be needed if all he is doing is changing from 53/39 to 50/34 - you can just use the same front mech, although you might need to lower it slightly. Rear mech cage length is relative to the rear cassette ratios, not the chainset.

 

[RecordAceFromNew]

Rear mech cage length is relative to the rear cassette ratios, not the chainset.

This is not quite true, see this regarding wrap capacity implication. Although in the present case with an increase in range of only 2T and general Shimano spec conservatism the OP might well be able to get away without changing rear mech.

Can I change my shimano 105 threaded 53 39 for a shimano threaded compact ive got a 8 speed 12 28 on the back the bike is a old scott usa anybody help cheers as id rather b spinning than grinding up the hills in wales.

Since Shimano has been making 105 chainsets for donkey years spanning at least 3 different bottom bracket types (all for standard threaded bb shells), it is impossible to tell whether you can do a straight chainset swap without knowing which chainset model you have - you should look it up and the marking should be in the inside face of the crank arms, something like FC-XXXX. The model code hopefully would allow the identification of BB spindle type, and if relevant length.

Whether the existing BB is re-useable or not, this FC-2350 square tapered compact chainset (plus if necessary a matching 110mm UN55 BB of whatever bb shell width your bike requires from e.g. here or here) looks like an absolute bargain to me.

 

[User6179]

When I changed cranks on my square tapered bb bike I bought an external BB and upgraded to hollowtech 2 crank , it transformed the bike as the previous crank with square tapered BB had so much flex in it, once I changed it became as stiff as my carbon framed bike with press-fit BB.

 

[RecordAceFromNew]

When I changed cranks on my square tapered bb bike I bought an external BB and upgraded to hollowtech 2 crank , it transformed the bike as the previous crank with square tapered BB had so much flex in it, once I changed it became as stiff as my carbon framed bike with press-fit BB.

Hope you are not buying the stories of oem's pushing new bb standards, that square tapered is more flexy generally. The case in point is that while track riders generally put far more power on their gear than anybody else, most top track riders still swear by something like this.

It is easy to see how the frame deflects way way more at the bb than the bb or crank arms possibly can when you stomp on the pedals, so imho chance is either you had an unusually poor setup, or it was psychological.

 

[Dusty Bin]

As above - square taper is not inherently 'flexy' - no more than any other BB type. As RecordAce says, track sprinters routinely put 2000w through square tapers. Thanks for the clarification on mech lengths btw....

 

[User6179]

Hope you are not buying the stories of oem's pushing new bb standards, that square tapered is more flexy generally. The case in point is that while track riders generally put far more power on their gear than anybody else, most top track riders still swear by something like this.

It is easy to see how the frame deflects way way more at the bb than the bb or crank arms possibly can when you stomp on the pedals, so imho chance is either you had an unusually poor setup, or it was psychological.

I agree maybe rubbish crank to start with but difference was with original crank and bb It felt like I was going to snap the crank off the bike , I assumed at the time the combination of crank and bb.

I had a spare ht2 crank from a previous bike so I bought a new external BB and the new crank now feels as stiff as my carbon bike with press-fit and ultegra ht2 crank so can rule out frame flex.

For the record am a heavy rider whose snapped two crank legs and a bb shell so I maybe feel flex more than most.

 

[User6179]

As above - square taper is not inherently 'flexy' - no more than any other BB type. As RecordAce says, track sprinters routinely put 2000w through square tapers. Thanks for the clarification on mech lengths btw....

Agreed , would say the stiffness will come from the size of the BB shell rather than type of BB .

Wide shell with wide down tube must be stiffer than a narrower one .