Torque wrenches are rare and expensive, and most mechanics don’t know how to use them. Worse, they need to be calibrated, again, never seen anyone calibrating their torque wrench. So why it should be used anyway?
There are two reasons to use a torque wrench, but with carbon bikes or carbon fiber bike parts, it is actually 3!
Metals are elastic, though it may seem absurd. Yes. That’s why some people CAN complain about rigidity (otherwise it would be totally absurd, like
aquaplaning resistance in road bike tires!). When you tighten a bolt too much, it elongates. This can cause two things, provided that screw holes are made from Adamantium, an imaginary metal that has infinite strongness, hardness, durability, you name it. Or Valyrian steel. Same.
When elongated (or stretched) beyond certain limit, a bolt will snap, causing you too much trouble. Or, it will happily stretch, and it becomes thinner. This can cause issues in applications where unthreaded part of the bolt must tightly fit mating surfaces, which can cause unwanted lateral movement.
So, #1: we don’t want to snap our bolt, or elongate it too much.
#2, the most common problem with bicycles is, while screws/bolts are steel or even titanium, screw holes are almost always aluminum, or alloy in US-speak, which is much softer than steel obviously. So, when you overtighten a bolt, it acts as a crankset or bearing puller, pulling the threads out – stripped holes.
Here comes #3: with carbon fiber parts, correct torque is critical because you’re squishing a carbon part. Unlike metals, carbon fiber parts strength is correlated with the weave and orientation of carbon fiber cloth used in production, which makes it super strong in some directions, while quite weak in others.
Discussing composite materials is a very lengthy subject, so I won’t go into it. Bottom line is, get proper carbon fiber paste and torque wrench if you have carbon parts. Without a carbon fiber paste, torquing parts correctly is generally futile, especially on stems. You have to have them both.
What determines the correct torque for screws/bolts?
I think this is pretty self-explanatory if you didn’t jump to this part directly: it’s elongation limit, and materials. If we had steel stems for example, it would be higher than 5nm, which looks like the norm. You want the bolt tight enough, but not too tight or loose.
For stronger metals than aluminium, also bolt standards matter. If you have a look at a hex bolt head, you may see numbers like 8.8, 10.8 etc. This determines, in laymen’s terms, how strong your bolt is – so if a bolt or series of bolts have to be stressed too much, like carrying abnormal loads like in the bridges, they should be stronger than ordinary hex head bolts like 8.8 – most of the time, more bolts doesn’t fix the problem, as bolt holes also reduce strength, or cause more problems than they solve.
Bolts get loose, so I tend to overtighten them
A bolt has to be tight, but over-tightening it won’t solve the problem, if not create more.
If moving parts are bolted together, bolt(s) must be tight enough, so they don’t deform or elongate with the parts’ movement: it’s not bolt tightness (unless its loose) that causes them to get loose. The problem is, they must be tight enough, so they don’t elongate, move or deform more.
Some steels stretch less, some more. So, bolt standards do matter. In fact, they do matter so much in some applications, like in cylinder head bolts, where 2 different metals mate and abnormal pressure, forces acting almost everywhere exist.
With bicycles, where almost everything is aluminium, you don’t have the option of stretching the steel bolt by tightening it more as the aluminium thread wouldn’t permit, so it cannot elongate easily past that point.
So how can I solve the loose bolts issue?
Pretty simple. This problem is not just related to bike parts. I ran a marine engine service, where especially smaller outboard engines have the same problem. You must use a thread lock compound.
I don’t want to repeat myself; I wrote a lengthy post about thread-locking compounds already…
A word of warning about stainless steel bolts
Do not use stainless steel bikes on aluminium, especially on rear derailleur mounts. Stainless steel and aluminium bolt will weld together, this phenomenon is called galling.
Aluminium has a protective oxide coating, think it as rust, forms immediately even if you sand it. But if you bolt a stainless steel bolt in a aluminium hole, air cannot get in to oxidize, and since aluminium being too reactive without it, they stick together.
It’s so bad that you cannot remove the bolt without damaging threads, or in some cases, breaking it. The latter won’t happen on a bike as there is not much metal to stick. But you’ll have damaged holes.
Stainless steel bolts are weaker than proper bolts and will rust eventually, albeit slower. If you need something shiny still, use titanium. Or, use a thread locker very liberally, and use a premium one.
I don’t need a torque wrench
Do you? Well, I bolted many thousands of stainless steel bolts to aluminium, yet made 1-2 mistakes in the last decade. Yes; my hands are quite “calibrated”, too. Would you risk cracking a 300$ carbon fiber drop bar ?
You have no idea how bad is to extract a snapped bolt. First, you have to drill it with a very tiny drill bit at the center. This is easier said then done, because the surface of the snap is not a regular surface. Then, you’ll have to drill with a larger drill, go buy drill extractor if you don’t have one already, then pray all goes well. If the hole is damaged, you have to repair that, too. Do you have Heli-Coil, or similar ? They’ re expensive.
Do you still think it’s worth the hassle, because…you’ re over-confident ?
Torque Wrenches for bicycles
Torque wrenches are not cheap. “Universal” ones are often clunky and not amateur-friendly.
There are also do’s and don’t about torque wrenches, which you have to follow religiously: when used wrong, torque wrenches do harm better than good. For amateurs, I highly advise the primitive-looking beam-type torque wrenches: these need no calibration: if the gauge reads 0, then you’re set to go. If not, it’s broken and don’t use it. Do not buy “preset” torque wrenches: these are set to only one setting, like 5nm. In the end, you may find yourself getting 10 of them, so the price-conscious-looking tool becomes the most expensive option.
Obviously, the real McCoy of torque wrenches is the adjustable one. These come in many forms. Some “click” when you reach the desired setting, some turns freely, and some varieties even come in spanner form. The “industry standard” is the ratchet type, “clicking” with the adjustment gauge at the bottom. I use these. They are also the most problematic ones: you have to treat them carefully and have them calibrated from time to time.
If you decide to buy an adjustable torque wrench, don’t go after the cheapest crap with huge setting range. These are generally off even brand new, if not, will be soon.
What to look for when buying a torque wrench for bicycle maintenance ?
Be reasonable. Torque wrenches are expensive because it is a precision devices. If it’s cheap, it will either break soon or doesn’t work at all. Most probably, it would be the most expensive item in your toolbox.
Some traits make these devices even more fragile and harder to make; like huge torque ranges. Don’t forget; you’ll be looking for something that covers 4nm to 20nm. 2, give or take. Yes; I know cassettes should be torqued at 40nm; but I really don’t care. I don’t even use my torque wrench, I just torque them fairly tight, which is really tight. You know what? I tighten my car’s caliper bolts with a cheater bar! I don’t remember their torque rating now, but it’s somewhere between crazy to mad region, and buying such a torque wrench *that works* probably cost more than my car.
Don’t buy digital stuff. When it comes to torque wrenches, calipers, micrometers, or similar tools, LCD screens are an indicator of inferior stuff: yes I have a pretty crappy caliper with an LCD screen; I know what it is when buying it. It’s accurate enough for me because I’m not machining precision stuff in a lathe. My eyes are not sharp enough to read tiny markers, so I bought it. You won’t need an LCD display on a torque wrench, believe me.
Yes; even Mitutoyo makes calipers with LCD displays, but that happened after the Chinese made 10$ calipers flooded the market. A decent caliper like Mitutoyo with an LCD screen would cost more than my bike inventory.
Go basic. I repeat: get a beam-type torque wrench if you can. If not, choose one within 5-20nm range.
Using torque wrench
I’m ashamed to write about this…but I saw many videos on Youtube, about bike maintenance, where many guys have no idea what they are doing with torque wrenches.
It’s too easy to spot a helpless amateur: he uses the torque wrench as a ratchet. How? First off, do not start to tighten a screw with a torque wrench. Wrench is a misnomer. Torque wrenches are measuring devices more than wrenches.
First, tighten a screw with your bare hands, until you cannot go any further. (This way, you won’t ever have slipped threads on crank arms!) Then, use your ratchet, bits screwdriver, Allen keys, wrench, spanner, or whatever you use. You should do it very subtly.
Next step is to move to your torque wrench: you have to move it very slowly with an uninterrupted motion. Get even slower when the bolt tightens further. Don’t jerk it off like a ratchet. This is where a “clicky” torque wrench, or beam-type works best: feel it. it will click when you reach the set torque. Don’t go any further. Now, very slowly and gently, try again. Once it clicks or reaches the desired point on the gauge on beam types, you’ re done.
DO NOT EVER USE torque wrenches to undo bolts. I have seen an idiot at a garage that used the torque wrench as a cheater bar. This wrecks the torque wrench. As I said, treat it as a measuring device, nothing more – unless your life is at stake, you can use it as a self-defense weapon, but that’s it.