Thread together PF bottom brackets: why the recommended torque so high?

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musiclover
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by musiclover

Ninja 25-30 Nm
Wheels MFG 35-50 Nm

Why are the torque recommendations so high? It seems that the 50 Nm of torque may crush the PF frame cup or damage the frame with the tiny lip that sits outside of the frame and where the pressure is concetrated.

Shouldn't torque similar to thru axle numbers (13 Nm) hold it in place?

Press in variants are not pressed in with the same force.
Last edited by musiclover on Wed Dec 01, 2021 8:07 am, edited 1 time in total.

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Karvalo
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by Karvalo

musiclover wrote:
Tue Nov 30, 2021 8:28 am
Shouldn't torque similar to thru axle numbers (13 Nm) hold it in place?
It's not remotely similar. A thru axle is not press fit - how much friction do you think you're overcoming sliding a thru axle in place vs pulling two press fit cups together?

tjvirden
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by tjvirden

Interesting. I've never needed to try of these BBs, but it does seem like an absurdly high torque spec.

I agree that's it's not the same as a thru-axle, but the whole point of a "press fit" is that the part stays in place from friction alone. If the fit is good (the same as a proper press-fit BB) then there's no need for any more torque than will seat the cups. I'm certain that 50Nm will damage some frames.

TwiggyForest
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by TwiggyForest

musiclover wrote:
Tue Nov 30, 2021 8:28 am
Ninja 25-30 Nm
Wheels MFG 35-50 Nm

Why are the torque recommendations so high? It seems that the 50 Nm of torque may crush the PF frame cup or damage the frame with the tiny lip that sits outside of the frame and where the pressure is concetrated.

Shouldn't torque similar to thru axle numbers (13 Nm) hold it in place?

Press in variants are not pressed in with the same force.
No, torque recommendations is at least partially based on thread diameter and pitch. So a bigger diameter thread has more friction due to larger surface area of the threads hence you need a higher torque.

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musiclover
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by musiclover

Standard non threaded BBs do not use this amount of torque, in fact, the two cups are not interconnected with any torque. The BBs bearings cups are fully seated (even before applying 13 Nm of torque) further torque only creates the tension between the cups. I guess, the question is: why does it have to be so high?

RyanH
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by RyanH

When I had a Wheels Mfg thread together BB, I was able to push it in by hand. I thought my frame may be messed up so I measured my shell and the Wheels bb. My shell was actually on the tight end of PF30 spec and the Wheels bb was even smaller. So, my guess is that they need that torque since they're relying on the faces of the shell to keep it in place.

Mine eventually creaked whereas I never had a problem with the delrin/plastic BB's. I'm not a fan of the thread together design and with the amount of play in the threaded together parts, I would guess if your shell faces are not parallel, you'll still end up with bearings that aren't in alignment with each other.
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1llum4
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by 1llum4

The Wheels MFG torque spec is inline with BSA/Italian threaded BB cup torque spec. It is also inline with most cassette lockring torque spec. TwiggyForest explanation is correct.

Nickldn
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by Nickldn

RyanH wrote:
Tue Nov 30, 2021 3:42 pm
When I had a Wheels Mfg thread together BB, I was able to push it in by hand. I thought my frame may be messed up so I measured my shell and the Wheels bb. My shell was actually on the tight end of PF30 spec and the Wheels bb was even smaller. So, my guess is that they need that torque since they're relying on the faces of the shell to keep it in place.

Mine eventually creaked whereas I never had a problem with the delrin/plastic BB's. I'm not a fan of the thread together design and with the amount of play in the threaded together parts, I would guess if your shell faces are not parallel, you'll still end up with bearings that aren't in alignment with each other.
Yikes! I bet most frame designs don't account for that!

Never had problems with PF BBs on several bikes (bb86, bb30) and would steer clear of thread together designs if they relied on the shell faces to keep the BB steady.

My bet would have been that contact with the shell would be increased for thread together BBs, as both sides are 'working together' and as Ryan points out the shell faces are also a adding some positive reinforcement. Cranksets usually go together at about 50nm, so the shell should be able to take that as well.
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tjvirden
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by tjvirden

Nickldn wrote:
Tue Nov 30, 2021 4:13 pm
[snip]
.....Cranksets usually go together at about 50nm, so the shell should be able to take that as well.
I can't work out quite what you mean here - could you expand please?

Maddie
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by Maddie

I think that's a bit of misunderstanding. There are two torque settings: The 50Nm is needed to hold the crank arm to the axle. It has nothing to do with the actual bottom bracket. The second torque setting is that you need to apply some preload in order for the crankset not to move axially. If the frame had to withhold the 50Nm from the cranks, then they wouldn't spin at all.

Cleaner
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by Cleaner

TwiggyForest wrote:
Tue Nov 30, 2021 9:25 am
No, torque recommendations is at least partially based on thread diameter and pitch. So a bigger diameter thread has more friction due to larger surface area of the threads hence you need a higher torque.
[quote/]
Friction force is generally independent of the surface area of the contact area bewtween bodies. Can you elaborate on why for this case the friction force is higher as a reasult of the area of contact?

Ironbarista
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by Ironbarista

That's a pretty big torque range.

Mr.Gib
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by Mr.Gib

Assuming by "thread in" you mean thead together.

I too am concerned about this high torque. That's a lot of crushing force on a bottom bracket shell. However I have never heard of a bottom bracket shell being crushed by a thread together bb though I think it would be possbile.

As for if the torque is necessary, I have a BBright frame that requires a Wheels MFG thread together bb, and unless I torqued it near the upper range, it would make some noise. It is as RyanH says, the design relies on the face of the bb shell to a significant degree for a secure fit.

BTW installing a BBright thread together to high torque requires clever use of presses, drifts, standoffs, etc. The non-drive side has about only 1mm of tool interface - you aint holding a wrench on that at 50nm with your bare hand unassisted.
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TwiggyForest
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by TwiggyForest

Cleaner wrote:
Tue Nov 30, 2021 5:37 pm
TwiggyForest wrote:
Tue Nov 30, 2021 9:25 am
No, torque recommendations is at least partially based on thread diameter and pitch. So a bigger diameter thread has more friction due to larger surface area of the threads hence you need a higher torque.
[quote/]
Friction force is generally independent of the surface area of the contact area bewtween bodies. Can you elaborate on why for this case the friction force is higher as a reasult of the area of contact?
You are correct, it's not due to the contact area, I had the concept right but the mechanism wrong.

Torque required is partially determined by thread pitch and diameter as given by T = KDF where D is bolt diameter, K is torque coefficient (based on thread pitch, angle, diameter, coefficient of frictions) and F is axial force. So to achieve the same axial force with a larger diameter bolt you need a larger torque.

As for the Wheels MFG torque specs, that is a stupidly big range, I would torque it to the lower limit.

jpeterson1012
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by jpeterson1012

I've actually snapped one of those thrad-togethers straight in half by torquing it to the recommended spec.

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