|Building up bike and torque wrenches||pessot|
Jun 3, 2003 9:03 AM
|I recently destroyed a left crankarm by not tightening the
crank bolt enough. The loose crankarm caused the BB axle
to enlarge the square cutout and the crankarm would no longer
seat correctly. Lesson learned.
I'm now building up a new bike and would like to tighten everything
correctly with the appropriate torque. I have Zinn's
book and the torque specifications.
Never having owned a torque wrench, do I need to buy one for
every size, i.e. 3,4,5,8mm, or do you buy one and use
different sized attachments?
Jun 3, 2003 9:23 AM
|All you need is one torque wrench with appropriate range of inch/pounds or foot/pounds, and adaptors to fit it to your allens and sockets.
But consider this: The cranking action has always encouraged crank bolts to loosen when riding. Like any press fit, the joining should be finally torqued to spec. only after the bike has been ridden once. If you snug down the crankbolts after one ride, they'll probably never come loose.
I have a torque wrench but have always gone by feel with bike components. The alloys are too soft to be over-tightened, as would be likely with a torque wrench.
|Wrong, wrong, wrong...||miposy|
Jun 3, 2003 10:26 AM
|Fredrico says: "I have a torque wrench but have always gone by feel with bike components. The alloys are too soft to be over-tightened, as would be likely with a torque wrench."
Though he is correct that the alloys should not be overtightened, there is no way that he could possibly be more wrong in terms of it being more likely with a torque wrench. Going by feel is just that, going by feel. It is not more accurate than a proprely used and stored torque wrench and is, in fact, much less accurate.
If you buy the correct tool and use it correctly, as with a quality torque wrench, it will torque your crank bolts to the exact proper spec EVERY TIME.
a. tighten the bolts initially with your tq.
b. go on a short ride (say 10 miles or so).
c. re-tighten with your tg.
you will likely not have to think about the cranks again until the next time you remove them.
Regarding your wrench, follow the instuctions in how to use and store it, and it will last you a lifetime unless you buy a cheapie.
|Ok, ok, ok...||Fredrico|
Jun 3, 2003 11:29 AM
|Point about the value of torque wrenches well taken, while not arguing the main point about press-fit crank bolts needing to be re-tightened after a 10 mile ride.
I've known several people who have over-tightened and stripped out alloy bolts--without torque wrenches--and more than one guy who did it with a big, heavy torque wrench designed for automotive use.
Get one if it makes you feel good, but bikes aren't cars. All threaded fittings except cranks and to a lesser degree headsets, pedals, seatpost bolts, and rear derailleur bolts, won't come loose if tightened by feel to a matter of inch pounds. You had a bad experience with the one fitting that always needs to be snugged down after being initially assembled.
|Whats the differance in car/bike tourque!||the bull|
Jun 3, 2003 2:04 PM
|None I can see-they both can be expensive if stripped or broken! All fasteners should be properlly tightened!
I always use my torque wrench on the following.
|Square tapers cranks/spindle suck||Tig|
Jun 3, 2003 10:44 AM
|I still have a set on my old MTB, but will remedy that soon enough. Having spent the last few months as an LBS mechanic, I see destroyed square tapered crank arms (usually the left) each week. Most recreational riders don't know the warning signs and keep riding until the problem is beyond repair. Far too many bikes are built at shops and Walmarts without the mechanic checking the crank bolt torques.
ISIS and Octalink crank arms survive longer when they become loose, and can usually be saved from the trash can.
Jun 3, 2003 12:06 PM
|That's true. Once the flats get rounded ever so slightly, the taper no longer matches the crank arm, the arm loosens, and it will never again stay on. All the more reason to periodically check the crank bolts with a wrench, like when you lube the chain, or clean the bike.
Shimano is so big they can do anything, including coming up with a better crank arm-BB axle joining design, but the four flats have worked very well--providing the bolts are tight. Press fits are tricky. It's hard to feel how tight the bolt should be. Even with a torque wrench, you still have to snug the bolt a second time.
Jun 3, 2003 1:37 PM
|From rec.bicycles.* FAQ
Subject: 8f.11 Installing Cranks
From: Jobst Brandt
> My cranks get loose, quite quickly too; over about 10 miles or so
> from being solid to flopping about in the breeze. Any suggestions?
Your cranks are ruined! Once ridden in the "floppy" mode, the square
taper in the crank can no longer be secured on the spindle. Get some
new cranks and properly tighten them after lubricating the tapers.
Proper tightness can be guaranteed only by torque wrench or a skilled
mechanic. The second of these is less expensive and you might be able
to get a demonstration of what is tight enough.
The admonition to not lubricate the tapers of the crank spindle seems
to find life only on bicycle cranks, of all the machines I have seen.
I have pursued the "dry assembly" instruction by talking to crank
manufacturers and discovered that they apparently had warranty claims
from customers who split their cranks open. It is easy to prove that
cranks cannot split by over-tightening simply by attempting to do so.
It is not possible to split a major brand crank this way, the bolt
will fail first.
Crank failure from "over-tightening" is caused by the re-tightening of
previously properly installed cranks. Once installed, a crank always
squirms on its taper, and because the retaining bolt prevents it from
coming off, it elbows itself away from the bolt and up the taper ever
so slightly. This can be detected by the looseness of the retaining
bolt after the bicycle has been ridden hard.
Grease in this interface does not affect performance, because only the
press fit, not friction, transmits load from crank to spindle. As any
bicycle mechanic can tell you, crank bolts are often appreciably
looser after use, the left one more so than the right. This occurs
because the left crank transmits torque and bending simultaneously
while the right crank transmits these forces one at a time. The right
crank puts no significant torque into the spindle. Either way, the
looseness occurs because loads make the crank squirm on the spindle
and the only direction it can move is up the taper, the retaining bolt
blocking motion in the other direction.
Regardless, whether grease or no grease is used, in use the spindle
and crank will make metal to metal contact and cause fretting
corrosion for all but the lightest riders. The purpose of the
lubricant is to give a predictable press fit for a known torque. If
the spindle is completely dry this cannot be said, and even with
marginal lubrication, some galling may occur on installation.
Lubrication is only used to guarantee a proper press because the
lubricant is displaced from the interface in use. Taper faces of
spindles show erosion and rouge after substantial use, evidence that
the lubricant was displaced.
"Dust caps" aren't just dust caps but retention for loose bolts. It
is not that the bolt unscrews but that the crank moves up the taper.
However, once the screw is unloaded it can subsequently unscrew and
fall out if there is no cap.
Because cranks squirm farther up the taper when stressed highly, the
unwitting mechanic believes the screw got loose, rather than that the
crank got tighter. By pursuing the crank with its every move up the
spindle, ultimately the crank will split. It is this splitting that
has been incorrectly diagnosed as being caused by lubrication. I have
never seen a warning against re-tightening cranks after having been
installed with a proper press fit. It is here where the warning
belongs, not with lubrication.
For the press fit to work properly, the pressure must be great enough
to prevent elastic separation between the crank and spindle under
torque, bending, and shear loads. This means that no gap between
crank and spindle should open when pedaling forcefully. Friction
has no effec
|THAT makes sense! (nm)||Gregory Taylor|
Jun 3, 2003 5:26 PM
|re: Building up bike and torque wrenches||Mazinger|
Jun 4, 2003 6:38 AM
|I've put together a few bikes and the only time I use torque wrenches are for the cranks and BB. Maybe it's habit, but I just can't install those two components without properly torqing it to specs first.
I only time I've overtighten something were bolts (that I broke) for a stem. Everything else I went by feel.