BB - to torque or not to torque!

4kicks
4kicks Posts: 549
edited February 2010 in Workshop
So Im fitting a new compact crankset to my bike. I wonder if you could tell me if I REALLY need to use a torque wrench or not. Im thinking with aluminium cranks, and given the high torque settings required I just do it "pretty damn tight" but want to know

What % of you with newish carbon bikes use torque wrenches to fit bottom brackets (or do you know if your bikeshops do?
Which is more important to torque exactly right - the bearing cups, or the crank bolts?

I dont want to buy a torque wrench, and can pop down to my LBS and borrow theirs but dont want to do it two or three times, so probably can get them to do the most important one. Yes, I am a bad person as I bought this on EBAY and them am getting them to do a wee bit of work for me on it, but thats a discussion for another day!
Thanks
Fitter....healthier....more productive.....

Comments

  • topdude
    topdude Posts: 1,557
    A torque wrench is not essential at all as long as you have a good feel for doing things up tight enough.
    The BB cups need to be as tight as you can do them with the correct tool by hand.
    The crank bolts need to be tight but not massively over tight, and rechecked frequently.

    I bought a TQ wrench recently and was surprised how much tighter 15Nm was compared to my "by hand guestimate".
    The same with stem bolts at 5Nm which need to be as tight as you can do with an ordinary allen key by hand.
    He is not the messiah, he is a very naughty boy !!
  • a_n_t
    a_n_t Posts: 2,011
    I bought one for fitting my ultratorque cranks but tbh I just do them as tight as! The connecting bolt is meant to be 42nm but it didn't seem that tight to me!
    Manchester wheelers

    PB's
    10m 20:21 2014
    25m 53:18 20:13
    50m 1:57:12 2013
    100m Yeah right.
  • balthazar
    balthazar Posts: 1,565
    One reason for using a torque wrench to tighten old style square taper cranks is that many people naturally undertighten the bolt. This interface transmits a lot of torque, in both directions, and suffers from lash in the system (making it deceptively difficult to design well); without a tight press-fit gaps may open between crank and axle under high load.

    Once you've assembled such cranks to the torque setting a few times (or under the attention of an experienced mechanic), you could use your judgement thereafter. It isn't the specific setting that matters, but the range.
  • 4kicks
    4kicks Posts: 549
    forgot to say its an external bb. So it sounds like do the bearing cups up tighty tighty, then I will get my lbs to torque the crankbolts which MAY be more sensitive to being overtorqued?
    Fitter....healthier....more productive.....
  • maddog 2
    maddog 2 Posts: 8,114
    IMO the cups don't need anywhere near what they recommend, which is something like 45Nm. Just nip them up, they'll be fine.

    If the c/set is Shimano then the two pinch bolts need to be carefully done up though 12-15Nm is what they recommend which isn't a lot, and do them up as the instructions say - one a bit, then the other a bit and repeat.

    If the c/set is SRAM then the end bolt needs the full whack - which is about 45Nm I think

    If the c/set is Campag then I have no idea...
    Facts are meaningless, you can use facts to prove anything that's remotely true! - Homer
  • The first time I put on an external Campag' BB, the outside cups were no problem and, as someone pointed out here they don't appear to be anything much to worry about. The big, fat Allen headed through-bolt is though, and it wasn't long before I was given to ponder by the side of the road why my cranks were loose. A 10mm Hex-key is not something I would normally drop in my repair kit.

    A chum of mine sorted it with his torque wrench. He was really surprised at how tight the recommended numbers were. He was starting to work up a sweat before the device cracked. Phew!

    No prob's since then. :D
    "Lick My Decals Off, Baby"
  • 2alexcoo
    2alexcoo Posts: 251
    I've had six external cranks (Shimano and FSA) over the years on various bikes, and never bothered with a torque wrench.

    If you use a normal sized allen key, like the Park Tools ball ended jobs, and do them up pretty tight but not white knuckle tight you'll be fine. Obviously if you're using a 2' breaker bar it might be harder to judge!
    Alex
  • I brought 2 torque wrenches to build my Planet X:

    BBB for lower torques, £40 (comes with a range of Allen Key attachments).
    Draper for higher torques, £20.
    I also needed a Sealy socket adaptor set (all combinations from 1/4" to 1") to fit the wrenches into various strange shaped bike tools. Think it was £15 and has come in handy for other non-bike jobs.

    A bit of an outlay, but they do cover all bases, and give peace of mind when servicing the 5 household bikes.

    If you are considering buying a wrench, and don't want to spend much, buy one with lower tolerances as these are much harder to judge by hand.

    More advice; if you do, make sure any bike tools you buy can take a socket wrench. Some of the previous Park Tools I brought didn't. Luckily the Lidi bike tool box I brought for £18 doubled up on these and had socket holes on the tools.