Noonievut

I have an aluminum stem that lists torque setting of 8-8.6NM on the two bolts that go over the steerer tube. It’s a carbon steerer tube by the way. All the other stems I have list 5 or 6. Is the torque setting specific to the stem and can they vary that much? Why would one be that much higher, what makes it different?

PeteHski

What size bolts are they? Torque settings are related to bolt diameter and material e.g M4 is usually around 5 Nm, while M5 would be around 9 Nm. But it varies with material specs too.
But in short I would think they are just larger diameter bolts.

HillRider

The torque you use is dependent on the steerer material and stem material not the bolt size since the real consideration is how tight it has to be not to slip and no more. My Easton aluminum stem specifies 5Nm but doesn't say what steerer material. I find 4Nm on a carbon steerer is plenty to keep the stem firmly in place and higher would accomplish nothing.

Noonievut

Thanks. They’re M4 but look and feel solid. The stem is pretty hefty. I went with 7NM to play it safe.

Noonievut

Just read this after my reply. I’ll dial it down

UniChris

That's kind of... half right, and half wrong.

The clamping force depends on material, but the torque required to produce a given clamping force very much depends on both the diameter and thread pitch of the screw.

But it also depends on things like if the threads have been lubricated with a grease or anti-seize...

veganbikes

Tighten it till it breaks and back off 1/2 a turn, no?

HillRider

Correct but.....given the near universal use of M5x0.8mm clamping bolts makes the recommended torque a useful value. Of course lubrication does matter but I hope most mechanics know to grease these bolts to avoid corrosion if for no other reason.

sdmc530

Maybe a mute question but are you using an expansion plug in your steerer tube? If you are I wouldn't get so worried about 5-8 nm, its a pretty small difference and with a plug its less of an issue. Not saying specs don't matter, I use a torque on everything per rating but think about how many people don't use one and tight is tight enough when the think so. Heck my own shop doesn't even use one. Not saying its a dangerous practice but most don't use and a good majority I bet are over tight.

shelbyfv

I'd be inclined to trust the specs printed on the stem if it's from a legit mfg. If it's some Chinese Ali purchase, I'd go lighter and keep a watchful eye until a proper stem arrived.

HillRider

I look at the manufacturer's torque spec as a "not to exceed" value, particularly for a carbon steerer. If a lower torque holds the stem firmly in place that's enough.

dsbrantjr

I torque steerers and other controls "race tight" so that they cannot be moved with normal forces but can move under firm pressure, so that they will be less likely to be broken or to impale the rider during a crash or spill.

WizardOfBoz

So I was thinking about this, initially disagreeing, but then... Normally you tighten a bolt down against a hard surface, and the purpose of the torque reading is to stretch the bolt to an optimal degree. So when you tighten a connecting rod to a crankshaft, the clamp seats on the connecting rod and torquing stretches it. Because there is variable friction and people use different preps (like locktite, anti-seize, grease) torquing is not the most reliable way to get a precise stretch. Using the connecting rod as an example again, really precise work is often done by measuring the untightened bolt length with a micrometer, and then making sure that the bolt is stretched to a precise, optimal value. A bolt that is less tight actually provides a weaker joint than a bolt that is slightly over-torqued (see Carrol Smith's book on Fasteners). But for tightening two parts together tightly, the torque value is determined by the bolt material, thread pitch, and what has prep stuff been applied.

This all changes if the bolts tighten the clamp onto the bar and the two halves of the clamp don't touch. Then the whole point of the torquing exercise is to get a precise clamping force on the bar. An ideal design would use adjust the bolt size and number so that the torque values were identical - the bolt joint will be stronger with the ideal amount of stretch.

Going back to the OPs question, though, I would use the torque values on the part. They are probably set up for optimal strength. If I had replaced (for example) steel bolts with Ti, I'd check to ensure that the torque values were within the acceptable range of the Ti bolts.

I like Calvin Jone's (Park Tool) videos. Here's one you may like, too:

easyupbug 

Exactly. Lubrication has a huge effect on reducing the torque to achieve desired bolt tension. If you don't grease/oil/threadlocker those bolts you can be 1/3 or more under tensioned as typical manufacturing convention is torque values given are for lubed bolts.

PeteHski

I have to say 8-8.6 Nm is way too high for an M4 fastener. Recommended tightening torque is usually around 4.5-5 Nm for an M4 steel caphead. Most stems with M4 fasteners state a maximum limit of around 5.5 Nm.