Aluminum Fatigue

[mike662]

Can anyone tell me how aluminum fatigue occurs? What I really want to know is will there be signs from the dropouts that fatigue is occurring, or will I just be riding one day and the dropouts will break? Also, is loosening and tightening the nuts that hold the axle in the dropouts on an aluminum frame bad? Will it cause the dropouts to break if I were to keep removing the nuts and then putting them back on constantly? Thanks!

[e-lmer]

The failure is sudden and catastrophic.

If you are lucky it will be before the bike gets going
fast and when it dumps you on your face it will not
including sliding 10 feet on your nose before running
over your broken body.

The best advice I can offer is do not get aluminum failure.

If you are using an aluminum fork use a very carefully fitted
torque arms to take the twist on the dropouts.

[Bob91403]

I agree. Aluminum is very brittle. While steel might start to show some signs, aluminum will fail suddenly, and without warning. Why bother with an aluminum frame or fork? You're adding at least forty pounds to the bike with the motor and battery anyway. You should use a steel frame and go on a diet. You'll look better, and probably save more weight.

[Leslie]

I agree. Aluminum is very brittle. While steel might start to show some signs, aluminum will fail suddenly, and without warning. Why bother with an aluminum frame or fork? You're adding at least forty pounds to the bike with the motor and battery anyway. You should use a steel frame and go on a diet. You'll look better, and probably save more weight.

I hammered an aluminum frame into submission with a rear hub, no torque arm, no rear shock (hard tail), and it still survives.  The only time it failed was with the bike stand bracket with the y bike stand like this one.





And yes it was sudden but not catastrophic and it was torn off at the welds.

With 28kgs of lead on the top bar it put too much stress on the rork (rear fork) reinforcement plate where those stand attach to, it failed when standing the bike up onto the Y stand..  The alloy frame still operated for months with out a hitch, I moved to a steel frame.

I haven't seen a aluminum frame with a rear motor fail yet I could be wrong, but I have seen countless aluminum forks fail.  Usually with no torque arms and or on very powerful motors pushed to the limits.

Most alloy frames and forks are fine with heavy riders going down hill over bumps, its the twisting action the hubs place onto any front fork dropout that amount to the majority of failures.

I have witnessed a 10 km ride with a 36v pack, with a rear motor, with no torque arms, and two loose wheel nuts on my aluminum frame to survive.  I had even a wheel nut pop off due to the thickness of the alloy dropouts and wheel not being centered  making it hard to get enough thread over the axle, still no axle spin out was caused.


I will say my alluminium frame was very thick, the dropouts were heavy plate aluminum.

I honestly get worried even with front steel suspension forks with a rear motor, I look at them moving back and forth when I apply the brakes,  Aluminium maybe even more of a  worry as they don't flex as much. A front hub *gulps*.

I think as long as you have good torque arms and are not speeding at 45kph and bike with rider weighs less than 120kg I think you will be safe..  If your setup feels safe then this is a good sign.  If you feel unsafe then you need to feel comfortable and safe when you ride and this is most important you fix any concerns.

Otherwise

No good changing things if you don't need to.

[Leslie]

Is this your bike here?

Honestly unless you feel like its going to break when you ride or you weigh 100+kgs and like to treat it really harsh, or the back wheel is hit side on by a car,  your frame looks to be as solid as any frame would need to be for general Ebike usage.

My bike aluminium frame was a hard tail too, your rear shock is your best friend here.

Other advice that might be hard to find here now because of the popularity of the MP overcasts the HBS motors in these forums are as follows.





With the HBS motor the wires go into the hub through the axle.

I added a child's stacking cup, that orange plastic thing you see, to protect the wires from getting damaged if the bike fell over.  It had saved me countless times with the failing stand problem I was having with the heavy SLA's.  If something like this saves you even once its worth looking into.

The hose method I thought of worked best.

[Leslie]

Edit!


Moved post to Mod section <<<Click to view

[mike662]

Cool I guess I'll keep things the way they are for now. I don't go past 1/3 throttle anyway but I'll have to add the kickstand to the back some time soon. And yes that picture is of my bike, and it does look solid, but that's why I asked. I keep checking my dropouts and axle and nothing is showing signs of wear at all, so I was worried that there might be more going on than I can see. Thanks for the replies guys!

[Bob91403]

Add a torque arm, and forget about it. It's really just aluminum fork dropouts that are seeing failures.

[Phil]

Some older heaver aluminium bikes are a thicker wall 6061 grade aluminium which will show some crazing lines around stress areas before failure.  6061 aluminium tube will even bend a little before failure. Most modern bikes are made of 7075 grade and have a thinner wall achieving the same (or better) strength than 6061. They do not like to be mistreated however, and any small dent or overload quickly leads to the catastrophic failure mentioned previously. I have seen this tubing (not on a bike) explode in a ragged mess rather than bend. Having said that, if the frame is straight & free from damage it should be fine with torque arms attached. Just don't take drops, dents, or other damages lightly.