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I've decided to have my local machinist make up one of these axle inserts for an upcoming wheelbuild.
I was wondering:
Do you mind if I borrow the design?
Did you do it in stainless steel or in aluminium alloy (and if the latter, what grade did you opt for)?
Did you find the 119.5mm length allowed for adequate tolerance when tightening the screws? I'm assuming elastic extension of the axle under tension is negligible; I'll do the calc but can't recall a good rule of thumb to relate tightening torque to axial force just now (and haven't chosen my material, see above)
It looks like you've used off the shelf button screws with regular washers - is this the case? I was considering using countersunk machine screws with corresponding countersunk washers, but if -say- you found that serrated washers were needed to avoid slip, I would rethink.
Thanks in advance.
Cupcakes-
Do you mind if I borrow the design?
I'd rather you stole it
Did you do it in stainless steel or in aluminium alloy (and if the latter, what grade did you opt for)?
The tandem one is 316 stainless, the 464 one is 7075T6 aluminium. This design for a solo should be plenty strong enough in cheap 6082T6
Did you find the 119.5mm length allowed for adequate tolerance when tightening the screws?
Yes, as long as your spacers are sized accurately. The hub should be very close to 100mm, if it isn't then send it back.
I'm assuming elastic extension of the axle under tension is negligible; I'll do the calc but can't recall a good rule of thumb to relate tightening torque to axial force just now (and haven't chosen my material, see above)
If your bolts aren't on the point of breaking, you haven't got enough tension in the axle to get anywhere near closing up that 0.5mm gap.
It looks like you've used off the shelf button screws with regular washers - is this the case?
Yes
I was considering using countersunk machine screws with corresponding countersunk washers
If you use M6 screws as I do on solo bikes, you need chain tugs, and then you need to change the axle design to give the tugs something to hang on to. Anything bigger doesn't need the countersunk screw/washer combo to create a large enough bearing surface, the head on an M10 button is already plenty big enough.
On the tandem, all the interfaces are smooth and the M10 bolts hold without any assistance (although there's no harm in using tugs if you have some which fit), and I didn't even gronk on them, they're only done up to 20Nm.
gbj_tester
For something that simple, you can sketch it with a Sharpie® on the back of a fag packet. I think I paid the machinist £40 for the axle + spacers, using my own material, and the bolts were about 50p from the local shop.
You can either leave the spacers loose or spec the ID to give an interference fit on the axle, according to preference. The hub gets squeezed between the spacers when you tighten the bolts, which is why the axle needs to be a smidge under 120mm.