Bicycle Mechanics - Torque wrench help

Hey guys,

Sorry for such a stupid, basic question, but I'd appreciate a bit of help.

Please tell me exactly what parts I would need to use a torque wrench on my bike. I have a torque wrench with 3/8" drive (not that I have a clue what that means) and I have a hex wrench set. What exactly else do I need to use this and how do I do it?

Thanks.

Welllll, you need hex wrenches that are connected to 3/8" female socket fitting. These look like regular sockets, but have a short hex wrench coming out of them.

I went out and got a basic torque wrench from Sears (the one with the scale).

I then went to tighten my seatpost (which has a specification from the Trek manual of 9.6-14.1 Nm.

The thing snapped and the screw came flying. Half the screw is still in there. Easy / cheap to fix?

I think I finally figured out how to use the damn thing, though. But, I'm scared to do it. I was going to tighten the seat fixing bolt (specifications from manual: 6mm Allen wrench, 17-28.3 Nm), but I don't want to break it. First of all, the 6mm bit doesn't even fit, only the 5 mm one does. Second, I'm scared to go beyond 10 Nm, cause I think I will snap it again.

Are the specs given in the manual absolutely correct? In other words, should they be followed to the numbers? Should I bring the torque to 17 Nm and all will be okay?

Thanks!

You're doing something wrong. Are you sure you are setting the torque wrench to Newton-Meters? 9 Newton-Meters is only about 10 ft-lbs.

My torque wrench is calibrated in ft-lbs and Kilogram-Meters. There are 10 Newton-meters in 1 kilogram-meter. If you mistakenly torqued your bolt to 9 kilogram-meters, that's equivalent to 65 ft-lbs! More than enough to shear off a seat bolt!

My seat binder bolt is supposed to be torqued between 85-125 IN-lbs. Which I believe corresponds to a maximum of about 10 ft-lbs.

You can try using a bolt extractor like they sell at Sears (personally, I think they're useless) or you can drill it out, toss the broken bits and start over.

Although, it sounds as though you might want to get some help with this one.

I'd also return that wrench (I think you have the bar style) and get the better one for about $60. The better one allows you to dial in the exact torque you need.

Good luck and remember: Tight is tight, too tight is broken!
PJ

Bear in mind that the typical torque wrench you find at a hardware store is designed primarily for automotive use. Auto bolts are typically torqued between 35 and 75 ft-lbs (though I've had to torque a nut as high at 250 ft-lbs!) using bolts that are between 3/8 and 5/8 diameter.

As pjbaz indicated, most bike fasteners have torque specs in inch-lbs. One inch-lb is 1/12 of a ft-lb. In metric, torque can be expressed in either kilogram-meters (KGM) or Newton-meters (NM). One Newton-meter is 1/10 of one kilogram-meter.

You can buy torque wrenches calibrated in in-lbs or NM insead of ft-lbs. These wrenches have shorter handles and are designed for lighter torque than the common automotive torque wrench. Sears has two of these listed in their on-line catalog. One is calibrated in NM, the other in in-lbs. There is also a less expensive unit with dual english/metric scales.