New BB??

[Cyc_al]

Having just replaced the front derailleur I am now getting a grinding noise when I put any reasonable power through the pedals. I have dropped the chain and spun the pedals on the stand with no real weight on the pedals and this did not create the same noise I hear when I am cycling. I have looked at other possibilities but am coming to the conclusion that the BB is on its way out. Is this a reasonable conclusion or is there something else I need to look at?
If it is the BB the only specification for my 2011 Defy 3 is an RPM cartridge. A quick look at the internet shows this as a general reference. Is there somehwere I can look to get a more exact spec ref or does anyone know what type of BB I need/would recommend; or is it a case of take the old one out and measure it/take it to the LBS?

 

[alicat]

You will be unlucky to have a bottom bracket failing on a 2011 bike unless you have been round the world twice.

Try googling 2011 Defy 3 bottom bracket - it is showing up lots of fixes that just involving removing the bottom bracket and applying grease etc.

If you have just replaced the front derailleur I would look to that as the culprit before checking other options.

 

[accountantpete]

I would check for play in the cranks and that is ok then I'd take the cranks off (leaving the BB in place) and turn the BB by hand. It should turn quite smoothly.

 

[accountantpete]

BTW - those Tempo cranks will have a JIS square taper.

So to get a new one you will need an English Threaded BB with JIS taper and length X (where x is the length of your old one).

 

[PpPete]

Assuming it is the BB that's the problem, and TBH I'm really not sure, (and assuming the 2011 model is square taper - which I suggest you double check) then
http://www.chainreactioncycles.com/shimano-un55-square-taper-bottom-bracket/rp-prod71369 are as good, and as robust as they come.
You do need to figure out the right size though.
Chances are English Thread, 68mm (but no guarantees)
and as stated above you'll have to measure your spindle length off the old one (easier with the cranks off), and easier still when it's out of the frame.

 

[lay]

http://www.hobsoncycles.co.uk/product/343/RPM_Cartridge_Bottom_Bracket_Set

 

[Cycleops]

Spindle length should be engraved on your old one.

 

[andrew_s]

I'd guess that the grinding noise is the chain scraping on the front mech cage. Things move sideways a little when you are pushing, so it won't scrape when you just turn the cranks by hand.
If the BB needs changing, you will have some play in the BB axle so you can move the end of the crank in and out a little. This can be a cause of chain scrape (other than poor gear cable tension adjustment).

The RPM BBs are a clone of the standard Shimano UN54 or UN55 BB. The Shimano BBs will be easier to find locally.
For a road bike that's not Italian (and likely even if it is), you'll want a 68mm English thread (also called BSA or ISO) of the same length as the old BB. You can measure accurately enough across the axle ends to get the right size, as the BBs only come in 3mm increments.
To get the old one out, you'll need a BB tool Shimano, Tacx and a big spanner (12" adjustable, or 32mm) to turn it with. I prefer the Tacx tool as it screws into the end of the BB axle and won't slip out if you have to get brutal* with it. Remove the left side first - this is just a lock ring, The right-hand side is a left hand thread, so on both sides of the bike align the spanner approximately horizontally and pointing towards the front of the bike, and push down on the end of the spanner.

*
I took out a UN72 this morning that had been ridden through too many floods and left too long. I had to go and buy a 32mm ring spanner (approx 14" long), and then stand 14st of me on the end and bounce to get it to move. When it did, the whole BB came out the left side of the bike still attached to the lockring, having pulled the "fixed cup" off the other end.
Lesson: put anti-seize on the inside of the lockring as well as on the threads.