rear sprocket nut torque question...

[mattsz]

I'm mounting a new sprocket on the wheel, and I'm curious about the fastener torque listings in the Honda 350F - 400F service manual...

The four "final driven sprocket" nuts are 10mm, 1.25mm thread pitch - as are the "engine hanger bolt" and "rear suspension."  In the 350 section (pg 75), torque values for all three of those are the same, 300-400 kg-cm or 21.7-29 ft-lb.  But in the supplemental 400 section (pg 105), there's a change: the engine hanger bolt and rear suspension values match the 350 values, but for the 400 the same size sprocket nuts show a higher value, 400-500 kg-cm or 29-36.2 ft-lb.

Does anyone know why this was changed?  I thought torque values were based on fastener specs, so matching fasteners would have matching torque values regardless of their usage - within reason, of course!

[TrickyMicky]

Torque values will change depending on the workload of the pieces being held together, the material composition of the relevant pieces, not just the diameter of the bolt. Suspension and engine mounting bolts do not actually move, and the engine bolts are quite often screwed into alloy castings. The sprocket bolts are screwed into steel inserts mounted inside rubber bushes, and the whole lot spin round with the wheel so there are centrifugal forces to contend with. But don't forget that the sprocket is actually secured by a damn great circlip to the hub, on a nice new bike with no corrosion (ha ha), when the circlip is removed, the sprocket lifts of complete with the rubber bushes attached. Centrifugal forces are quite something to contend with, until the onset of modern hi-tec wheel fixings, I have encountered many large commercial wheels where the front left hand wheel nuts had a left hand thread to combat self-loosening.

[Bryanj]

Not just the front one, when comercials had taper seat wheel nuts all lh ones were lh thread, now they have spigot mounted wheels with integtal washer flat nuts and all are normal thread
You want to see the size of the torque wrench for those!!

[mattsz]

Torque values will change depending on the workload of the pieces being held together, the material composition of the relevant pieces, not just the diameter of the bolt. Suspension and engine mounting bolts do not actually move, and the engine bolts are quite often screwed into alloy castings. The sprocket bolts are screwed into steel inserts mounted inside rubber bushes, and the whole lot spin round with the wheel so there are centrifugal forces to contend with. But don't forget that the sprocket is actually secured by a damn great circlip to the hub, on a nice new bike with no corrosion (ha ha), when the circlip is removed, the sprocket lifts of complete with the rubber bushes attached. Centrifugal forces are quite something to contend with, until the onset of modern hi-tec wheel fixings, I have encountered many large commercial wheels where the front left hand wheel nuts had a left hand thread to combat self-loosening.

Sure... but why change the spec between model years if the hardware size, composition of the metals, and application are the same?

[taysidedragon]

Matt, I guess that's what they call model improvement,  learning from past experience, etc.

[TrickyMicky]

Not just the front one, when comercials had taper seat wheel nuts all lh ones were lh thread, now they have spigot mounted wheels with integtal washer flat nuts and all are normal thread
You want to see the size of the torque wrench for those!!

Oh Yeah! 460ft/lbs if I remember correctly, and weren't they a ba---rd to remove after they had been on a while!

[mattsz]

Matt, I guess that's what they call model improvement,  learning from past experience, etc.

Guess so...

Oh Yeah! 460ft/lbs if I remember correctly...

 

[K2-K6]

The difference comes from how the torque is used along with the structural design of the components. Although torque is seen as the constant ( the numbers make it look comparable across diverse fixings) it's application can be completely different.

As an example,  the cylinder barrel/ head stud is set within a very specific torque range to provide an even tensile loading from one end to the other,  and within the material's elasticity.  It's most easily appreciated if you think of it as a very strong bungee strap as often used to secure luggage.

The rear wheel assembly has mostly (spindle,  sprocket fixings etc) that are not pulled into elasticity to anything like the degree given in the previous example.  They effectively have the material in the fixing structure able to be tightened sufficient that it clamps the components it's trying to secure (leaving them in location to work in a shear plane as opposed to tensile) prior to going into stretching.  In this way the components remain way inside their structural elasticity,  but the securing nut ultimately gets compressed during torque loading.  This results in the nut's thread pitch reducing and providing a mismatch to the pitch on the bolt, as this happens the nut gets harder to turn as the differential pitches start to bind against each other.  It's this that you are measuring with the torque wrench (same as first example) but because you're only compressing the nut and not effectively stretching the components you're tightening, then there's effectively spare capacity to revise the torqued level.

The difference to the cylinder stud is that to raise the clamping on that you have to increase the within elastic total capability to revise, whereas on the sprocket one an appreciable gap to ceiling load is present.

As to why they did it,  probably durability trials on the 400 during r+d showed a loosening that they needed to respond to.  Discussion with the design engineers would allow them to evaluate if they could safely up the torque or would have to go for a redesign.  We can see what they agreed on with the spec change.  It does mean that it could also be applied to the 350 installation,  but the text obviously stays the same as already published.