torque spec and cotter pin holes

[bwringer]

This thread is clear evidence that winter is too damn long around here...


But it is still an excellent idea to give some thought to your threads and fasteners before you just grab a wrench and give it one grunt for little bolts, two grunts for big 'uns.

Over on the V-Strom side, we've found that the official published torque specs are far too high for the lightweight hollow aluminum axles used on the V-Strom. By thoroughly re-thinking the engineering and physics involved, we've arrived at revised torque specs and procedures that are still perfectly safe as far as keeping the wheels attached, yet will help ensure that the aluminum threads don't gall.

What's interesting to me is that there are still lots of V-Strom people who can't bring themselves to distrust the ridiculous number Suzuki published in a book. The mere fact that it's been published is magical evidence that it has to be correct. So they overtorque, and at the next tire change bemoan the cost of replacement axles, or post a frantic plea for help with an axle nut that won't come off.

This is magical, mindless thinking. It's far better to be a mindful mechanic -- develop an understanding of the materials and physical principles you're working with, so that you understand when things make sense and when they don't. Motorcycles are pure physics in action, and every interaction -- even as humble as tightening a nut -- has something to teach us.



As far as the much tougher steel axles used on a GS, the torque values under discussion above aren't even close to the physical limits of the material. So there's a lot of room for errors that won't cause any harm. If the threads are still in decent shape and the axles are straight, don't worry about what some PO did -- the threads would fail long before the axle could deform permanently.

[Dogma]

Aw, now you got me thinking...

A threaded fastener connection only needs to be tight enough to make sure the fastener is never stretched to the point that the clamped parts don't lose enough compressive pre-load to allow slipping. On a front axle, there isn't anywhere to slip to, so you just need to make sure there is enough tension in the axle to hold all the parts snug against each other.

Overtorquing, the bearing spacer will probably buckle before the axle breaks. But adding too much tension could conceivably reduce the axle's margin in carrying the weight of the bike. Tension and bending stresses add up in a way analogous to tire friction under braking and cornering. But I doubt this is a real danger for GS axles. According to my old Holo-Krome slide calculator, an M12 tightens to yield somewhere around 170 ft*lb. And then the axle is larger than the threaded portion on the end. This being a safety critical part, there is probably a huge safety factor in the locading calculations. It wouldn't surprise me to learn that the axle is 8x stronger than the worst-case calculations called for.

Undertorquing? What needs to be under tension? The bearing inner races need to be clamped by the axle and various spacers so they don't spin on the axle due to the drag of lubricants and some rolling friction. You can easily turn a healthy bearing with your finger, so this probably isn't significant either. Probably lubricant drag increases with rpm somewhat, maybe mitigated by warming. I'd be very surprised if this turned out to be significant.

The only thing that could cause the components squeezed by the axle to separate is some kind of load that puts the axle in tension. In the case of a front wheel, that'd be the kind of flexing a fork brace addresses causing the various parts to lever against each other. But that same loading case also forces the individual components against each other. So there really isn't anything that can cause an axle with some tension on it to allow parts to slip.

So, the front axle needs to have a some tension, with a lot of margin on the high side. If you're worried about it, I'd say torque to minimum spec, then advance to the next hole. There's probably no problem with backing off from max spec, or going to the nearest hole either, as long as the axle has some tension on it when you're done.