Magnesium suspension forks with hub motors - safety factors?

[neptronix]

Hey folks.. have been thinking about going to dual hub motors lately.

http://www.sram.com/rockshox/products/xc-32-tk

I am noticing that some manufacturers are now selling suspension forks with magnesium lowers, and steel upper tubes.
I know that aluminum has a failure mode that involves shattering and cracking, making running a front hub motor on a aluminum suspension fork pretty damn unsafe, but how does magnesium react to such forces?

Could something like a mag/steel fork be safe enough to depend on?

 

[d8veh]

I've fitted a few motors to Rockshox and Fox forks. Two of them were 36v 500w Bafang BPMs with 30 amps. Both had custom twin torque arms, and I ran them like that for more than a year before changing to other ideas.
Here's one in a set of Rebas:


Heresa Q100 I have in Rebas as well:

 

[neptronix]

Interesting - those rockshox rebas also have magnesium lowers, right?

1000w peak is a good amount of power on one motor. did you notice the fork bending under acceleration or anything?

 

[neptronix]

Oh, i just had an idea, in regards to a 2WD system.
Program one controller to have a ~2:1 phase-battery ratio on the front. Program the other controller to have a 3:1 phase-battery ratio for the rear.
Set the front motor to push 30A batt. peak, and the rear to push 40A batt. peak.

From a stall, this means that the rear motor will do the majority of the pushing, then the power from the 2 motors will eventually equalize at the mid-high RPMs of the loaded speed.

Curious to hear more magnesium suspension fork success/failure stories though first

 

[etard]

Coincidentally, I just watched this video about a magnesium bodied car:

[youtube]zTI5seuGi20[/youtube]

It doesn't look good...

 

[d8veh]

I've never noticed the forks bend. Rockshox ae qite strong.

If you have too much power on the rear, it takes the weight off the front wheel during acceleration, so the front wheel spins and skips off the line. You might want to think about a longer ramp on the power to the front if you can.

 

[chas58]

It is that spinning and skipping that is going to cause the front fork to brake. If you can put smooth power down, great, but if the wheel spins and suddenly catches, that torque will tear out a magnesium dropout pretty easily. It is the sudden jerk that causes the problem.

 

[dogman dan]

I don't think you will find enough difference between mag and alu in the fork to care about. Chances are, most forks of any quality have been some type of magnesium aluminum alloy for some time.

Two torque arms, two c washers, good for 1500w I'd say. DH riding is pretty tough, and lots of force is put on forks by disk brakes. I don't see a problem with any good forks. Cheaper forks will deflect under power, bind up, and turn into a rigid fork till you let off the throttle. But good quality forks are less prone to binding.

I have thousands of miles on shock forks and front hub with 1200w, using the Grin Cycles torque arms.

If you crack your dropouts on medium wattage, it's from lack of c washers, loose or shoddy torque arms, or a nut loosened from regen. Do whatever it takes to use an inner washer if the motor has a thin axle shoulder. I had to file down the shoulders, to install an inside the fork washer on both sides.

 

[speedmd]

Magnesium failure mode you say.

 

[neptronix]

The failure mode of cracking like that ain't pretty. But you've got two examples where cracking would be easy to occur. A hollow suspension component, and something that looks like it has a weak junction to carbon fiber?

Thanks for your input, dogman.

 

[speedmd]

Never mind the 1000 hp it took to twist it apart. :wink: It certainly was not anywhere near strong enough. None of the attachments to the carbon failed however. Never liked those sexy thin spoke wheels.

The suspension carriage looks like it went off roading or had some sort of accident to me. It is widely used and not a problem until you go way too far.

I would not worry too much unless you start seeing some bagging out of the seal areas or dropouts/ torque arm attachments. Cast mag is better than most cast aluminum in failure mode IMO. It has a bunch of advantages that make it perfect for the lower fork section. Making it in one piece is a big one. Forks are over built for the most part so they should hold up for quite a while.

 

[dogman dan]

Exactly, the main reason people crack alloy fork dropouts is because they install washers wrong, don't use a c washer etc. I believe Justin agrees with me on that, it's why he produced the first c washers.

That kind of noob mistake won't happen to you I'm sure.

 

[chas58]

Keep in mind that nothing on the fork drop out was designed for a hub motor. Disc brakes are not part of this equation. A shock load (like a spinning wheel that suddenly gets traction) is plenty of load to crack a non steel fork dropout. If built properly, it can be reinforced to spread the load. Still, its a kluge job, and one needs to be warhy and careful both when building and when riding, and when performing maintenance (check for wear, tear, cracks).