Dropout Failure Experiments, and a call for Fork Donations

[will_newton]

Justin, I have a Manitou 3 suspension fork with cast aluminum dropouts, if you are interested. It's old and doesn't work anymore and would be a good test of what has been deemed the worst kind of fork to use. Send me a PM with shipment info and I'll send it along at no charge.

 

[ZapPat]

I think it's great you are failure testing all these forks. One question tho, how are you going to account for the effect that regen braking has on the dropouts?

Hey Ben, in this set of experiments we won't be accounting for that. I am very explicitly testing only the failure strength of the dropouts (and torque arms) when stressed to the limit in a single direction. The strength I am sure would be identical in the reverse direction. But the situation of regen in an ebike where you are repeatedly torquing the axle one direction and then the other direction, causing it to wiggle back and forth, loosening the nuts, impacting the torque plate. etc. etc. is a totally different set of tests, and may be subject to another thread here in the future.

I wonder where you found that 10X dropout failure rate when using ebrakes, Ben? This figure seems very high unless you are talking about people using no torque arms at all, and/or not checking their axle nut tightness after installation.

I have been using very intense ebrakes on one bike for a couple months now and have had absolutely no issues with the dropouts, even with the front aluminum alloy suspension fork (but I don't go down 16" steps like dogman!). Mind you I use dual torque arms on both front and back motors - the simple ones come free on 9C hubs now anyways (two per hub), so why would anyone not use them?

One important thing I've found with aluminum dropouts is that there is an initial settling in period after installation, and so it's very important to check the axle nuts during the first few rides. I had to tighten mine up a few times, each time less and less, and now they stay nice and tight even when torque-abused back and forth. Steel dropouts tend to do this a bit too, but much less it seems.

 

[dogman dan]

The settling in I've experienced is the washer deforming a bit. It's inevitable since there is a gap in the bottom of dropout that the washer tries to squeeze into, and it seems to happen on steel too. Sometimes a washer that is slightly big settling into a cup in the dropout can spread the drops some. This can cause the settling to take longer. I hope that isn't what is happening with your aluminum forks, if so, you may have a micro crack in em now. Steel will just bend a bit and keep it's strength better. V shape drops, nuts that keep loosening, and axles that try to spit out as you tighten them are the red flag warnings.

I think the effect of regen is meaninless if the fork, washers and motor fit properly. Tight in one direction is tight in both directions. Able to wiggle is loose, and just as bad with or without regen. No data or experience to back up this statement, just my dog sense. Tourqe arms of course, can save your ass if loose does occur!

 

[Jaybird]

Hi I am totally new to ebikes I have just bought this kit http://shop.crystalyte-europe.com/product.php?productid=16235&cat=249&page=1 I was hoping to install it on my Dahon Jetstream P8 front forks I would be grateful for any advice I was wondering if the torque produced by the motor would be to much for the Jetstream P8 front suspension forks or would I better installing it on a Dahon folder with ordinary forks many thanks for any advice

 

[Affliction]

I wonder where you found that 10X dropout failure rate when using ebrakes, Ben? This figure seems very high unless you are talking about people using no torque arms at all, and/or not checking their axle nut tightness after installation.

I have been using very intense ebrakes on one bike for a couple months now and have had absolutely no issues with the dropouts, even with the front aluminum alloy suspension fork (but I don't go down 16" steps like dogman!). Mind you I use dual torque arms on both front and back motors - the simple ones come free on 9C hubs now anyways (two per hub), so why would anyone not use them?

One important thing I've found with aluminum dropouts is that there is an initial settling in period after installation, and so it's very important to check the axle nuts during the first few rides. I had to tighten mine up a few times, each time less and less, and now they stay nice and tight even when torque-abused back and forth. Steel dropouts tend to do this a bit too, but much less it seems.

I came up with the 10X figure as a guesstimate partly from my own experience with e-brakes and accounting for the fact that people tend to be inherantly lazy. And yes this was for no use of torque arms. Nuts will loosen at even the slightest bit of back and forth movement. e-braking creates the possibility of this movement where driving the axle in only one direction does not. Thus my increased calculation of failure.
Alot of hubmotor kits don't come with torque arms and unless you didn't know any better you wouldn't think of getting any.

 

[dogman dan]

Re, jaybird.
The wattage is pretty low, 250, so it should be fairly safe even if the forks are alloy. But the install needs to be absolutely perfect for it to work. Alloy forks often have a cup on them for retaining quick release hubs that get loose. The washers may need to be modified to fit into the cup, not span over it leaving space underneath. And get a tourqe arm. If you get the tourqe arm from Justin at Ebikes ca you can get a special washer from him for solving the cup problem.

 

[Affliction]

Justin, I had this idea in the past but I never got around to making a stamping die.

The flat surface where the axle rests against the dropout opposite the nut is always smooth on hub motors.
I've frequently seen on regular bike hubs where the mating surface on the inside of the dropout has "teeth" on the hub to bite into the frame for retention. The metal on hub axles is soft enough for easy rethreading; I repair stripped axle shafts all the time for Ridemore.ca by rethreading from 12mm 1.25 to 12mm 1.5 thread pitch. My idea is to make some hardened die's with the negative spline and using a press, imprint teeth onto both sides of the axle where it would meet the frame. Having two die's would make this a 1 step 30 second process you could do to motors before shipping.
Please stamp a few axles this way and test the improvement with properly torqued nuts.

 

[justin_le]

Justin, I had this idea in the past but I never got around to making a stamping die.
The flat surface where the axle rests against the dropout opposite the nut is always smooth on hub motors.
I've frequently seen on regular bike hubs where the mating surface on the inside of the dropout has "teeth" on the hub to bite into the frame for retention. <snip>
Please stamp a few axles this way and test the improvement with properly torqued nuts.

Hey Ben, this is a SUPER idea! I have a feeling that a radially grooved imprint on the ends of the axle could go a really long way in locking the axles against spinout. And unlike most torque arms, this method intrinsically has no slop between forwards and reverse torques, so it could prove especially beneficial with regen systems.

The only slight problem is that a lot of the newer motors (BMC, eZee, Nine Continent) have pretty small diameter 17mm axles. That doesn't leave nearly the same amount of contact surface as the Crystalyte 400 / WE style hubs (20mm) or the 24mm axle diameter on a 5304. See image below for the comparative difference.



I'm doing all these tests with the 17mm eZee axles, where the effects of a knurled end would be the least, but I'll do it all the same. I should be able to score a set of radial grooves in the end of the axle in order to do this experiment, and then we can see if the improvement is worth investing in a die for or not.

Justin

 

[justin_le]

Went to a sports junkies (a used/liquidated sporting goods store) the other day, and got this:



That's seven IDENTICAL GT steel forks. The paint is a little dinged up, but it doesn't look like they have ever been used on a bicycle, as there are no signs of paint wear or chipping at the actual dropouts where the wheel would have been installed.



The shop tech was interested in what I was up to (turns out he has an ebike himself!) and helped me get the entire lot for just $25.

Anyways this means that we can do a set of super controlled experiments quantifying the exact effect that different torque arm designs, axle grooves, nut tightnesses etc. have on the spinout resistance of a motor axle, at least as it applies to steel dropouts. I still don't have any exact duplicate aluminum suspension forks, but probably the lessons that we get in the steel case would apply more or less.

 

[justin_le]

I also finished making the axle clamp and lever arm tonight for performing the actual spinouts. The eZee axle has a keyway in it, (well, two actually one for the stator and another for the freewheel), so I just cut a circular notch in two pieces of 5/8" steel bar stock with a matching groove for a short piece of keystock:



View attachment 1

So everything is in place and I was going to start the tests on all the steel forks this weekend.

But then I realized it would make sense to get more than just a single data point with each test. Previously we looked at the load cell and recorded what seemed to be the maximum value that showed up as the lever arm was pulled all the way down. But when there was a torque plate installed, there wasn't a clearly defined "failure" torque, at some point the dropouts clearly started to give, but it took an ever increasing amount of torque to make the axle actually spin around a full 180 degrees. So it was a little bit subjective.

I'm going to see what we can setup to generate a full spinout profile for each experiment, so we can see the torque on the 'Y' axles plotted against the axle rotation angle. This will be a lot more informative, comparing for instance how steel gives while alloy cracks, and will eliminate some subjectivity as well, but it'll take some time to figure out the best way to set things up.

Justin

 

[Affliction]

Those are awesome forks!

see the steel cup? SAE 5/16" flat washer fits perfectly and can be brazed in to thicken the dropout.
30 seconds of dremmel work finishes the job. I do this to all my forks of similar design.
Kinda a shame you're gonna wreck em all :lol:

 

[Zoot Katz]

Those are awesome forks!
. .
Kinda a shame you're gonna wreck em all :lol:

It's a great score for providing some kind of bench mark.
Being yellow, I was wondering if they weren't all lemons. Possibly recalls sold by an unscrupulous scrap merchant.
There's no tags I can see denoting the quality of the tubing. Is it Hi-Ten or Cro-Mo?
The dropouts don't look like the forged types I'm used to seeing.

Whether any of that minutia was relevant escapes me at the moment.

 

[voicecoils]

Anyways this means that we can do a set of super controlled experiments quantifying the exact effect that different torque arm designs, axle grooves, nut tightnesses etc. have on the spinout resistance of a motor axle, at least as it applies to steel dropouts.

If those cromo GT forks are anything like my PlanetX cromo forks, I'd like to see a test of the dropout strength with no torque arms at all. I suspect you'll find them strong enough to withstand most hubmotors as long as the nuts are tight.

It's the low quality steel or aluminum used in Walmart/KMart bikes that gives me the greatest cause for concern.

GREAT TESTING EFFORTS JUSTIN !

 

[justin_le]

Those are awesome forks!
. .
Kinda a shame you're gonna wreck em all :lol:

It's a great score for providing some kind of bench mark.
Being yellow, I was wondering if they weren't all lemons.

They also had 4 or 5 black ones too, but he wouldn't sell those at a discount. So yeah, the colour in some sense turned them into lemons!

 

[will_newton]

Justin, I sent along the Manitou and also my broken Cannondale Headshock fork. Some folks have asked about using that fork for an e-bike, since the shock is in the headset, but only the steerer is steel, the fork is aluminum.

I don't know if you can test one side at a time, but you could get twice as many tests or could test the same fork with and without a torque arm. I can't remember if you are doing this already...

Have at 'em! I think they should be there by the middle of next week.

Will

 

[Vanquizor]

I would be interested in seeing how much less force it takes to spin an axel the second second time (ie new fork but same axel)...

 

[dequinox]

I'm going to see what we can setup to generate a full spinout profile for each experiment, so we can see the torque on the 'Y' axles plotted against the axle rotation angle. This will be a lot more informative, comparing for instance how steel gives while alloy cracks, and will eliminate some subjectivity as well, but it'll take some time to figure out the best way to set things up.

Justin, quick question:

When you were testing the spin out torque on the dropouts with the torque-arm restraint, which was it that gave way and rounded out the most... the axle or the torque arm?

It seems to me that if the torque arm was the part that "ate it" then some improvements could be made by adding a simple tempering process to the manufacture of the arms. If we could get a manufacturer to laser/water cut out the torque arms from 1080 (or perhaps a T1 or 440c stainless ) steel, and then heat treat them for a Rockwell hardness of about 35-40... it would be much more resistant to "round-out" than your run-of-the-mill 1020 1/8" plate. Hopefully that hardness would also afford some "toughness" so that the torque arms wouldn't just crack out.

 

[Affliction]

I'm going to see what we can setup to generate a full spinout profile for each experiment, so we can see the torque on the 'Y' axles plotted against the axle rotation angle. This will be a lot more informative, comparing for instance how steel gives while alloy cracks, and will eliminate some subjectivity as well, but it'll take some time to figure out the best way to set things up.

Justin, quick question:

When you were testing the spin out torque on the dropouts with the torque-arm restraint, which was it that gave way and rounded out the most... the axle or the torque arm?

It seems to me that if the torque arm was the part that "ate it" then some improvements could be made by adding a simple tempering process to the manufacture of the arms. If we could get a manufacturer to laser/water cut out the torque arms from 1080 (or perhaps a T1 or 440c stainless ) steel, and then heat treat them for a Rockwell hardness of about 35-40... it would be much more resistant to "round-out" than your run-of-the-mill 1020 1/8" plate. Hopefully that hardness would also afford some "toughness" so that the torque arms wouldn't just crack out.

Hardened steel will fracture rather than deform so not really a good idea. The axle's are all soft steel so hardened steel would just destroy them.
Torque arms need to be springy so hardened steel is not the right solution.

 

[justin_le]

Justin, quick question:

When you were testing the spin out torque on the dropouts with the torque-arm restraint, which was it that gave way and rounded out the most... the axle or the torque arm?

It was the torque arms that suffered the most damage for sure. The axles are definitely harder than the stainless.

It seems to me that if the torque arm was the part that "ate it" then some improvements could be made by adding a simple tempering process to the manufacture of the arms. If we could get a manufacturer to laser/water cut out the torque arms from 1080 (or perhaps a T1 or 440c stainless ) steel, and then heat treat them for a Rockwell hardness of about 35-40... it would be much more resistant to "round-out" than your run-of-the-mill 1020 1/8" plate. Hopefully that hardness would also afford some "toughness" so that the torque arms wouldn't just crack out.

For sure, we'll get to know for sure early next week. The local waterjet shop where I've been getting the torque arms made happened to have some 01 tool steel in a 1/8" x 1.25" bar stock. He was planning to make a knife blade from it, but I was like, Nooooo! We need hardened torque plates instead! So I'm expecting those to be delivered this coming monday or tuesday and we'll see just how much of a difference that makes. It might be that the hardened torque plates just cut through the axles at only a little more torque than the axles were cutting through the softer plates, but we'll see.

Justin

 

[Affliction]

It was the torque arms that suffered the most damage for sure. The axles are definitely harder than the stainless.
Justin

The axle's are harder than stainless? I cut threads in these all the time and it's like cutting butter with my tap and die kit.
You can't drill stainless unless you have special drill bits and usually the bit is ruined once you're done.
You sure your torque arms are true stainless steel?

 

[liveforphysics]

It was the torque arms that suffered the most damage for sure. The axles are definitely harder than the stainless.
Justin

The axle's are harder than stainless? I cut threads in these all the time and it's like cutting butter with my tap and die kit.
You can't drill stainless unless you have special drill bits and usually the bit is ruined once you're done.
You sure your torque arms are true stainless steel?

The most commonly used alloy of stainless steel is 304.
It has a tensile strength of of 73,000ksi and a yeild strength of 31,000ksi

Other grades of stainless steel, like alloy 15-5, which is common in aircraft, have a tensile strength of 161,000ksi and yeild strength of 140,000ksi.

All the way up to L-19 stainless, which can exceed 300,000ksi yeild with the proper heat treatment. It's used only in fastener applictions, and it rusts very rapidly dispite being classified as a stainless steel, which technically only means it's >10.5% chromium by mass.



Some alloys of SS are as soft as mild steel and drill very easily. Some alloys are as hard as a tool steel drill, and require carbide tooling (or other means).

 

[GrayKard]

Excellent info so far! I can't wait to see the data from the next set of tests.

What I would want to know is if dual torque arms offer twice the resistance to spinout or do they give more or less protection than that. Not two arms with one reversed for regen but two mounted the same direction.

Gary

 

[dequinox]

The axle's are harder than stainless? I cut threads in these all the time and it's like cutting butter with my tap and die kit.
You can't drill stainless unless you have special drill bits and usually the bit is ruined once you're done.
You sure your torque arms are true stainless steel?

The reason you have a difficult time cutting stainless is its tendency to work-harden. A drill bit trying to push through stainless steel produces a lot of pressure, and like when you bang a welding rod flat with a hammer at room temperature, the surface of the stainless becomes harder and more resistant to cutting. Only stainless has a much much higher tendency to react this way to machining than mild steel or even the higher carbon alloys. You shouldn't be ruining drill bits if you use a proper cutting fluid, feed rate, and speed with a decent drill press. Even with a regular bench top you should be able to get a few holes drilled before the bit goes dull and you have to sharpen it again.

You might try using your tap and die on a SS bolt the same size as the axle sometime...there may (and I say may because I haven't done this myself) be a noticeable difference in the machine-ability. Something like a 1080 steel might be soft if it isn't heat treated...but try machining it after a good temper is put into it. The higher carbon stuff has higher tensile and yield strengths, but that doesn't necessarily vary directly with its machining characteristics.

For sure, we'll get to know for sure early next week. ... It might be that the hardened torque plates just cut through the axles at only a little more torque than the axles were cutting through the softer plates, but we'll see.

Cool, I'm glad that thought already crossed your mind and it sounds like we'll be informed of the characteristics sooner than I thought! You may also want to keep an eye on the profile of the cut edges. A perfect corner (that is, un-rounded) can have a higher tendency to form stress fractures. Slightly rounded corners may help that quite a bit. You probably don't want the torque arms to be too hard...in fact I hypothesize that the R/C hardness should about match that of the axle. Ideally there will be no friction wear on the torque arm/axle at all if the nuts are tight and the axle doesn't move much relative to the dropouts...but sometimes people forget as you've pointed out. :wink:

The most commonly used alloy of stainless steel is 304.
It has a tensile strength of of 73,000ksi and a yeild strength of 31,000ksi

Are your units kg/in^2? I can't remember off the top of my head what ksi is...

What I would want to know is if dual torque arms offer twice the resistance to spinout or do they give more or less protection than that. Not two arms with one reversed for regen but two mounted the same direction.

Two torque arms would double the ability of the torque arms to hold a moment, and probably the total ability to hold a moment by 20-30% (thats a S.W.A.G.), but the real limit relates to the ductility, shear strength, and hardness of the axle and torque arm material. It's a lot like how a crescent wrench doesn't get dented when you strip a nut or bolt head with it...the material of the bolt head didn't have enough hardness so it mushed over instead of tightening further. Does mounting a torque arm backward make a different for regen? I'd be interested to know if it does because I want to play with this on my next build (a few years out of course).

 

[Affliction]

The reason you have a difficult time cutting stainless is its tendency to work-harden. A drill bit trying to push through stainless steel produces a lot of pressure, and like when you bang a welding rod flat with a hammer at room temperature, the surface of the stainless becomes harder and more resistant to cutting. Only stainless has a much much higher tendency to react this way to machining than mild steel or even the higher carbon alloys. You shouldn't be ruining drill bits if you use a proper cutting fluid, feed rate, and speed with a decent drill press. Even with a regular bench top you should be able to get a few holes drilled before the bit goes dull and you have to sharpen it again.

You might try using your tap and die on a SS bolt the same size as the axle sometime...there may (and I say may because I haven't done this myself) be a noticeable difference in the machine-ability. Something like a 1080 steel might be soft if it isn't heat treated...but try machining it after a good temper is put into it. The higher carbon stuff has higher tensile and yield strengths, but that doesn't necessarily vary directly with its machining characteristics.

Ever try and drill through a bearing race? It's near impossible even with Tungsten carbide bits. Yes I know all about cutting oil and how not to ruin a drill bit. On hardened metals I use the old "smoke wrench" to make holes. Then I use a die grinder to make the final shape.
Running a die on hardened steel is a futile effort, you just end up wrecking your tool.
I cut threads all the time and I was just mentioning how soft the axle threads are by the ease of cutting a different pitch for repair.
Hub motor axles appear to be very soft metal.

 

[dequinox]

Ever try and drill through a bearing race? It's near impossible even with Tungsten carbide bits. Yes I know all about cutting oil and how not to ruin a drill bit. On hardened metals I use the old "smoke wrench" to make holes. Then I use a die grinder to make the final shape.
Running a die on hardened steel is a futile effort, you just end up wrecking your tool.
I cut threads all the time and I was just mentioning how soft the axle threads are by the ease of cutting a different pitch for repair.
Hub motor axles appear to be very soft metal.

Perhaps the axles Justin has are of a different alloy than the one's you work with. That might explain the stainless torque arms getting mutilated while the axle was less affected. I wouldn't in a million years try running threads on a piece of metal without first applying a proved annealing process...nor would I ever suggest it. I was hoping the "try machining it after a good temper is put into it" comment would be understood as a bit sarcastic. :|