Let’s talk about torque, not the kind you generate when you pedal your bike, but the kind you use to hold your bike together. In Zen and the Art of Motorcycle Maintenance there is a scene involving a young “mechanic” stripping bolts on the protagonist’s motorcycle because he lacks “mechanic’s feel”. OK, I can relate to that, and mechanic’s feel is great. I like to think I have it. And I use it for lots of bolts and nuts on a bicycle.
But for some connectors, notably those clamping on carbon fiber, I’ll whip out a torque wrench.
I am not going to write a why and how manual on torque; that wouldn’t be esoteric, would it? Instead I’ll poke around the “nuts and bolts” of bicycle assembly.
Why do certain connectors require certain torques? One answer is so they won’t come unscrewed during use. But the more general answer is to generate tension in the connector, thereby applying a compressive load between items to be joined. If there were a convenient way to measure connector tension directly, that’s what we’d all do. But there isn’t, so we use torque as a proxy.
Actually that’s not entirely accurate. In industrial applications it’s being done by several manufacturers, with Direct Tension Indicating Bolts.
The problem with torque as a proxy for tension is that the condition of the threads affects the relationship between torque and tension. Clean vs dirty, lubricated vs dry, old vs new. Generally, torque values assume clean threads in good condition with some level of lubrication, but that still leaves a wide margin of error. That bolts aren’t snapping off or coming loose every day speaks I think to the large window of allowable torque on these things.
By the way, I generally lubricate threads before makeup… generally. I consider the use of grease or anti-seize compound as much a corrosion inhibition measure as a lubrication measure.
Component manufacturers usually include torque recommendations in their assembly instructions. Shimano especially does a great job with their online TechDocs Library.
Now, in esoteric fashion, let’s talk about torque.
I always use a torque wrench for:
- Shimano’s 5mm crankarm pinchbolts. This is a heavily loaded interface, and the consequence of insufficient torque is severe. 12-14 NM.
- Stem/handlebar and stem/steerer bolts, those 4mm bolts that must be tightened to 5 NM. Interestingly the stem/handlebar interface must be tight enough to never slip, while the stem/steerer interface is better off slipping in a crash. I use a preset “torque key”. Mine is made by Ritchey. There are several brands. Park Tools makes one. You do know not to put carbon assembly paste on your steerer tube to stem interface, right?
- Seatpost clamp bolt on a carbon fiber seatpost. 5 NM. Over-tighten this one and you can deform/damage/break your seatpost. Not tight enough and your seat will slide down while you ride. This is what carbon assembly paste is for.
- Sometimes I use a torque wrench for threaded bottom brackets and crank bolts of the 8mm and 10mm variety. I have found that “good and tight” with a full-size socket wrench or a long-handled Park shop hex key is about right for the 25 NM crank threads. I use a 15” breaker bar for 30-40 NM bottom bracket threads.
Things that irk me:
- I hate aluminum chainring bolts. They break. The female half is very thin-walled and loaded in tension (hoop-stress). Check with a magnet to see if yours are steel. I’ve quieted more than a few noisy bikes by replacing a broken chainring bolt.
- 2mm hex bolts like on Zipp wheels and most brake pad holders. OK, the bolts themselves don’t bother me, it’s the people who overtighten and round them out, or force me to round them out trying to remove them. The loads on these interfaces are very small. Be gentle. Do not overtighten. Use your fingertips, not your hands. Shop tip – jam an appropriately sized Torx head into a rounded out 2mm hex to get the bolt out, then get a new bolt.
Above: Itsy bitsy 2mm bolt on the bearing adjustment collar of a Zipp hub.
- Ball-end (Bondhus) hex wrenches. I once had the task of extracting a seatpost bolt containing a broken 4mm ball-end from a Bondhus wrench. Shimano used to warn not to use a ball-end wrench to install their dear derailleurs, but I can’t seem to find the reference now. (See Bad Torque Recommendations below.)
Things I’d like to see:
More Torx bolts. I would love it if Shimano would spec Torx bolts for their Crankarm pinchbolts. Kudos to Campy who have used Torx bolts in lots of places for many years.
A good idea:
If you are riding a thru-axle bike, tighten the thru-axle bolt with the wrench you plan to carry on the road or trail. Some of the torque specs are like 10 NM and higher. You may not be able to loosen a fully torqued thru-axle with that cute little multi-tool. You can apply this same idea to the lug nuts on your car. When you get home from the tire shop, check to see if you can budge each lug nut (one at a time, please) with the lug wrench in your trunk. Better to learn this at home than out on the road.
Bad torque recommendation IMO:
Shimano calls for 8-10 NM on their rear derailleur mounting bolt. That’s just way more than necessary. There’s just not that much load on this interface. I use about 5 NM. I once had the pleasure of working on a damaged Dura-Ace titanium derailleur mounting bolt. It was expanded in six places, corresponding to the six contact points of the hex wrench. The bolt was literally jammed inside the housing and would not pivot. I didn’t see it being assembled, so I don’t know all the details, but the deformation clearly occurred during installation. Maybe the builder used a Bondhus wrench. Like any good garage mechanic I honed down the high spots a few thousandths of an inch until the OD was in tolerance and re-assembled the derailleur.
Above: Dura-Ace titanium rear derailleur post deformed by hex wrench during installation.
Torque wrench calibration:
If you have at least two torque wrenches, use them against each other to calibrate them. Of course they might both be wrong, but at least they’ll be consistent. If I have a torsion bar torque wrench and a “click” type, I’ll trust the torsion bar wrench.
Above: Testing two torque wrenches against each other.
Or you could perform a dead-weight test.
Above: 20 lb load at 6” = 120 inch lbs or about 14 NM (I clamped a 10mm hex wrench in my vise and installed a 10mm socket on my torque wrench.)
Pedal threads are self-tightening due to something called precession. I grease pedal threads for corrosion prevention and I torque them pretty lightly (10-15 NM?). I think what’s important is removing and re-greasing the threads every so often, at least once a year. I have gotten pedals out of cranks for people when no one else could, (remember the mnenomic “Back Off”) but I have also thrown away cranks because I could not get the pedals out.
Brake and derailleur cable pinch bolts:
These are scary. I always feel like I’m gonna twist off the actuation arm or something. What is really important is to route the wire under the pinchbolt properly. There is always a groove. And the cable is meant to lay in that groove, so it is not simply gripped between two flat plates. Take off the bolt and inspect the mounting surface to see where the groove is and route the cable accordingly. Shimano calls for 6-8 NM and that feels really high! I don’t use a torque wrench. I screw ‘em down pretty tight and give the cable a good hard tug.
This is a toughie. To tighten cassette lock-rings I put my cassette tool face up in a vise and rotate the wheel by hand. The freehub body is very thin in the area of the threads, and like a chain-ring bolt, it’s under hoop stress, so I’m very gentle. I have seen a steel freehub split from hoop stress. Many freehubs are aluminum, dimensionally identical to their steel counterparts, so not nearly as strong.
Above: Tightening a cassette lock-ring by feel.
Shimano’s recommended torque is 30-50 NM! I had never measured the torque I apply, so I took a break from writing and went to the garage for a while. I tightened a lock-ring by hand with the tool in the vise as shown above, then I put the torque wrench to it. It took about 25 ftlbs (34 NM) to tighten it further, so I’m in range.
Above: Applying 25 ftlbs torque to a cassette lockring
Now that I’ve got you worried about all the nuts and bolts on your bike I’ll just sign off without further comment.