Wheel, Fork, Engineering Experts, lend me your ear.

[dlearl476]

Even if you just slept at a Holiday Inn Express last night, I have a question.

In the course of modding my front end to accept a 25mm wheel, I've discovered a few things:

1.  The axle on an older 20mm is torqued into the threaded insert on the left side @ 70Nm.
2.  The axle on a newer 25mm SBK fork has an aluminum nut that is torqued to 15Nm.
3.  Both the above have steps on the left side that locate the left side of the wheel in the fork.
4.  The right side of both types are located by a spacer built into their respective speedo drives.
5.  Both the above are affixed to the fork by 4 pinch bolts (2 per side) torqued to 25 Nm.
6.  The hole on both sides of the older fork is 25mm.  On the right side, the step takes this down to 20mm for the bearings, and speedo drive. On the right side, the threaded spacer goes from 25mm OD to 12mm X 125 threads.

From the above, I use my very limited engineering knowledge (I stayed at a HIE, but it was months ago, but I did work as a machinist for a couple of years in the 70's  ) to deduce that basically the torque is used to put some load on the bearings but basically it's the pinch bolts that HOLD the axle on.  I'm at a loss to explain why the huge difference in torque from one axle to the other, but also understand there is no way you could torque the alloy bolt much past 15Nm.

So here's what I'm proposing:  Having a hollow axle made that is 25mm from one end to the other.  Threaded on the right side to accept an OEM axle nut, machined on the left side with a hex-head and flange (just like the bolt and washer on the other end, only fixed.)  To that I will add a 25m X 30 mm spacer (just like the step on the 25mm axle) to locate the right side, and a spacer on the other end for the speedo drive.  I'll torque this axle nut to 15Nm and tighten the pinch bolts to 25Nm, just like a SBK fork.

Am I missing anything?  Any flaws in the logic?  Is there a reason the 20mm axle is torqued to 70Nm that I'm unaware of?

[Howie]

You are correct in your statement that the pinch bolts are what hold the axle.  The nut locates the axle, and might possibly act as a safety.  I can't speak to the 15 Nm spec for your alloy nut, but if you are not comfortable with that you could use the OEM steel nut with a torque of 63 Nm. 

[Duck-Stew]

Sounds like you've got the logic down and that you're approaching it correctly.  I'm not an engineer, but sounds OK to me.