Columbus Futura fork / 3T Arx Pro stem

[Ben689908]

Got a Columbus Futura fork on my steel bike. Carbon steerer - paperwork says max stem torque should be 5Nm.

Also got a 3T Arx Pro stem, the one with the single Torx bolt. Printed on the stem is Max Torque 8Nm.

Never had any trouble with this setup until I dismantled the front end recently, to travel. Reassembled everything, using 5Nm on that single stem Torx bolt.

The headset keeps losing preload. I know how to preload a stem and what to look for when having problems, such as slipping bung or insufficient steerer/bung clearance to the preload cap. All of that is fine.

I’m asking myself the question now “do I creep closer to that 8Nm?” and “do I actually need to because there’s only one bolt?” although the bolt does influence a larger surface area of the stem so one would think there would be sufficient “grip” at 5Nm.

Cheers!

[BigBoyND]

I would think a single 8Nm bolt clamping the same surface area would create less stress than two at 5Nm. I'd just just torque it to 7Nm (I usually go max minus 1)

[Ben689908]

I would think a single 8Nm bolt at I'm clamping the same surface area would create less stress than two at 5Nm, and just torque it to 7Nm (I usually go max minus 1)

Yeh I think you might have a point. Thanks!

[tjvirden]

Got a Columbus Futura fork on my steel bike. Carbon steerer - paperwork says max stem torque should be 5Nm.

Also got a 3T Arx Pro stem, the one with the single Torx bolt. Printed on the stem is Max Torque 8Nm.

Never had any trouble with this setup until I dismantled the front end recently, to travel. Reassembled everything, using 5Nm on that single stem Torx bolt.

The headset keeps losing preload. I know how to preload a stem and what to look for when having problems, such as slipping bung or insufficient steerer/bung clearance to the preload cap. All of that is fine.

I’m asking myself the question now “do I creep closer to that 8Nm?” and “do I actually need to because there’s only one bolt?” although the bolt does influence a larger surface area of the stem so one would think there would be sufficient “grip” at 5Nm.

Cheers!

Question: so why is it losing preload now, when it was fine before?
Suggestion : A proper (non-abrasive) carbon paste between stem and steerer could make the difference.

As BigBoyND has said; there will be room to go above 5Nm, but if it's not holding at 5, I think you must be sure of why.

[Ben689908]

Got a Columbus Futura fork on my steel bike. Carbon steerer - paperwork says max stem torque should be 5Nm.

Also got a 3T Arx Pro stem, the one with the single Torx bolt. Printed on the stem is Max Torque 8Nm.

Never had any trouble with this setup until I dismantled the front end recently, to travel. Reassembled everything, using 5Nm on that single stem Torx bolt.

The headset keeps losing preload. I know how to preload a stem and what to look for when having problems, such as slipping bung or insufficient steerer/bung clearance to the preload cap. All of that is fine.

I’m asking myself the question now “do I creep closer to that 8Nm?” and “do I actually need to because there’s only one bolt?” although the bolt does influence a larger surface area of the stem so one would think there would be sufficient “grip” at 5Nm.

Cheers!

Question: so why is it losing preload now, when it was fine before?
Suggestion : A proper (non-abrasive) carbon paste between stem and steerer could make the difference.

As BigBoyND has said; there will be room to go above 5Nm, but if it's not holding at 5, I think you must be sure of why.

My suspicion is it never was holding at 5Nm and the shop built it up (yeh the shop built this one for me) using 6, 7 or even 8Nm as per the stem max.

The only thing is that the shop doesn't recall.

I have fibre grip, but am loathe to use it on the steerer in case it causes wear.

[tjvirden]

I suppose if nothing has changed then that must really be the answer, and it wouldn't be so much of a surprise given the stem (I'll get to that further down). Presumably the steerer is in good condition - no obvious wear or any possibility of damage?

As pointed out, the single-bolt stem does put less clamping force onto the steerer; so you can increase the torque on that one bolt and still be within the steerer design spec. The annoying thing is that the 5Nm rating on the steerer doesn't take a single-bolt stem into account. It would be worth seeing if you can get clarification from Columbus on it.

There's another thing: an additional factor that isn't often recognised (in the world of bicycles) is the size of the bolts on the stem (or anywhere else) - in general, the larger the diameter of the thread, the higher the torque needed to induce a particular clamp force along the bolt axis. This is approximately a proportional relationship, so for an M6 threaded bolt to induce the same clamp force as an M5 threaded bolt it must be tightened with about 120% of the torque (e.g. 4.8Nm instead of 4Nm). I think that design of 3T Arx uses an M6 steerer clamp bolt doesn't it? So 5Nm on an M6 bolt is about the same (clamp force) as 4.2Nm on an M5 bolt. That would explain your trouble.

I understand your reluctance to use it, but Fiber Grip should be quite safe; there should be no significant movement between a properly fitting, and tightened, stem and steerer. No movement means no abrasion is possible.

[edited for my forgetting that bolt yield is a cubic function of torque]

[Ben689908]

Thanks for your post, it's really appreciated!

The steerer looks fine - I've had it all apart to clean and regrease bearings etc.

The clamping bolt is actually an M7(!) - T25 Torx - and I must say, using a torque wrench, it feels like you've hardly imparted any force at all by the time it clicks at 5Nm.

I've written to Columbus (and one of their distributors, here in the UK) and I'm still waiting to hear back. I'll have time to meddle at the weekend, so will give 6, 7Nm a go. Failing that... fibre grip or a new stem with the standard 2x bolt arrangement.

[Ben689908]

Columbus have written back and suggested using a different stem with 5Nm torque spec. “For safety”.