Bomb proof my axle

[shenzhen_ex]

I broke my rear motor axle on my first ever E-Bike ride .

I never want to break another axle. My bike is a DH, I don't want to have to baby it over every bump.

I'm thinking of adding adapters to the axle to strengthen it. This will also repair my broken axle.

I already have an axle adapter on the left rear side that I used to to make my motor fit my bike. That side of the axle did not brake. (Rear adapter thread)

When I get this done I will have adapters on all 4 axle ends.(Front adapter thread)

I have a Bafang CST, it has a splined cassette attachment (image below). That CST is going to complicate the adapter design.

How can I incorporate a Torque Arm?

How can I improve the design?
Do you think this will work?
Any threads that cover adding adapters on the CST side?

 

[bowlofsalad]

Broken axles is something I've been thinking a lot about. A question I ask myself is what the axles are made out of. To summarize some of my thoughts, http://www.newton.dep.anl.gov/askasci/mats05/mats05189.htm this article might help yield some perspectives. I'll copy and paste it to be sure it sticks around here.

Question: What is the difference between hardness and toughness?

Replies: Mohamed,

In technical terms, hardness and toughness are used as qualitative descriptors for how much energy has to be put into a material in order to change its shape. For example, if we were to pull on a bit of plastic along a line (just in one dimension), and the plastic stretches for a bit before breaking, we want to be able to describe in words wether it took a lot of energy, wether the plastic stretched a lot, how much energy was required to break the plastic, etc.

Hardness describes how much energy it takes to deform (stretch, compress, bend, etc.) a material. If the material takes a lot of energy to change only a little, it is said to be hard. Conversely, if only a little amount of energy is needed to make a lot of shape change, then the material is soft. Metals would be considered hard, chewing gum would be soft.

Toughness describes how much total energy has to be used before a material breaks. If the material takes a lot of energy (it may change shape) before breaking, then it is a tough material. If only a little energy is needed to break the material it is weak or brittle (depending on wether the material yielded or not).

There are many words like these in common usage. Mostly it is a way for a scientist to communicate an idea to someone for whom numbers would not mean so much. These words might be used in combination also. Glass might be considered hard and brittle (takes a lot of energy to make small changes in shape, and breaks easily with just a little energy). Rubber might be considered soft, strong, and tough. Soft because it changes shape with a little energy, strong, because it does not yield (get to a point where it suddenly stretches) easily, and tough because it takes a lot of energy to break.

Remember though that these are just qualitative words used to make conversation easier and without using actual experimental numbers.

Greg (Roberto Gregorius)
Canisius College

Hi Mohamed,

While "hardness" is easy to define, "toughness" is a little more difficult. Perhaps the best way to explain this is by examples. Think of a steel hammer, and a diamond. The diamond is obviously very hard. However it is not tough. The steel hammer, on the other hand, is not particularly hard (you can easily scratch it with many hard substances, including the diamond). However the steel hammer is certainly tough. You can hammer things with great force, and the hammer is never damaged. In fact, if you hit a diamond with the hammer, using only very moderate force, the hammer will survive without damage, but the diamond (even though it is very hard) will be smashed into many pieces.

Hardened steel chisels, for example, can easily be heat treated to a very high degree of hardness. Just heat the chisel to a red heat, then plunge it into a water bath. The result will be a chisel that is so hard, it will even scratch glass. Unfortunately, however, that chisel will also be essentially useless. When hit with a hammer, it will tend to shatter. To add toughness (at the expense of slightly reduced hardness), the chisel is "tempered" by heating it to several hundred degrees (not to red heat) and allowing it to cool. The result will be a tool that is still hard enough to be useful (but not so extremely hard as before), but which is now tough enough that whacking it with a hammer will not damage it at all.

Hardness is a property that is necessary for cutting tools. They must be harder than the material they are cutting. Toughness, on the other hand, is the property that allows an object to withstand impacts without damage. You might think of "tough" and "brittle" as direct opposites, just as "hard" and "soft" are opposites.

Regards,
Bob Wilson

Simply put, hardness is the resistance to penetration the surface of a material has. If you were to subject two materials to the same force applied by the same shape, such as a small ball bearing, the one that the ball bearing pressed into the least is the harder of the two (hardness testers do exactly this, using different forces and indentor shapes and sizes for the different hardness scales).

Toughness is the total amount of energy a material can absorb before fracture. So something very resilient, like rubber as an extreme example, would be a very "tough" material, even though it might not be very "strong".

David Brandt

Hi Mohamed,

Toughness is the energy absorption needed to permanently deform an object. In mathematical terms it is the area under the stress vs strain curve to failure. For example, rubber is very tough (you can put a lot of energy into and it still returns to its original shape).

Hardness is resistance to plastic (irreversible) deformation. Elastic deformation is where an object bends, but returns to its original state -- plastic deformation is where it does not return to its original shape. In practice, people use the term hardness for a lot of different things. The classical definition is "what can it scratch without being scratched itself", but that does not give insight into the meaning. In specific fields, people talk about hardness in terms of specific types of plastic deformation -- such as resistance to being penetrated.

I would like to credit Dr. Michael Rauscher, an expert in materials science, for the content of this response.

Hope this helps,
Burr Zimmerman

This is an important detail for understanding materials choice for what you are trying to do. I think a very important matter to consider is the size of your axle. 12mm axle? I would ditch that motor if I could and get a 14mm axle. I don't know how you have made or plan to make some of your parts, but the truness of the mating surfaces are very important, if things approach each other at differing angles, you'll end up with a lot of force focused on a much smaller area than originally intended, this will very likely end up in breaking something and may have been the cause of a lot of broken axles. This is why I don't think clamping dropouts or torque arms are a good idea, it is difficult to engineer that sort of situation so the surfaces mate flush with one another.

Torque arms are a very important matter for ebikes, converting a bicycle into a ebike adds a lot of stress and fatigue to the frame as most bicycles were designed with much less weight and speed in mind. A lot of people use stainless steel closed mouth torque arms. Those are great I am sure, I use them myself, but honestly I don't love closed mouth torque arms. Installing and uninstalling things is a pretty difficult task. Fixing a broken spoke a flat on the side of the road (some people think you can just fix the flat with the inner tube on the bike, but you have to find the cause of the flat and remove it before even considering patching an inner tube or swapping it out) with a closed mouth torque arm is difficult to impossible (without the right tools). A user on this forum doctorbass http://endless-sphere.com/forums/viewtopic.php?f=31&t=29129 makes what sounds and looks like a pretty great torque arm. If your bike frame is made out of steel, welding the torque arm or having it welded to your frame sounds like an extremely ideal situation. He recommends glue for those who cannot weld and such, but I'd really rather not use glue.

For all we know, because you only placed this adapter on one side, this itself caused the break due to the uneven forces or something like that. Who knows. Maybe you just had a poorly made axle and there wasn't much you could have done.

TL;DR I'd look for a full suspension steel bike frame and a new wheel that uses a 14mm axle, and use those torque arms I mentioned from doctorbass and weld them to the frame. If you try to build a house on sand, it's probably going to sink without immense amounts of excessive effort. Start all things with a good foundation and things will be better and easier for you.

 

[Kingfish]

Shen - I would never have cut the axles. But what's done is done.

Though FWIW you're not alone with axle snafus: Once upon a time in 2010 - I increased hole through the axle to accommodate 10-AWG phase wire, but then I was afraid to mount the motor for fear it would break at the wrong moment. The next time I used 12-AWG and after a long struggle it came out fine.

I would try to think how you could solve the problem without resorting to cutting. If you have access to a machine shop - you might try replacing the axle or upgrading the diameter.

~KF

 

[shenzhen_ex]

Shen - I would never have cut the axles. But what's done is done.

KF... Cutting the axles was not a goal, just necessary to integrate the parts I ordered before i knew better.

Ironically, i cut and put adapters on 3 out of the 4 axle ends. The end that broke as the one without the adapter

I would try to think how you could solve the problem without resorting to cutting. If you have access to a machine shop - you might try replacing the axle or upgrading the diameter.

KF, I do have access to a machine shop but I am worried about machining the "spline" features (see image below: credit spinningmagnets). I think I would need an indexing head and special cutter.

 

[shenzhen_ex]

Broken axles is something I've been thinking a lot about. A question I ask myself is what the axles are made out of.

bowlofsalad. Interesting! so what is generally accepted as an ideal hardness for an axle?

Torque arms are a very important matter for ebikes

What do you think about TA's that just prevent the axle nut from spinning vs. the axle?

 

[bowlofsalad]

Broken axles is something I've been thinking a lot about. A question I ask myself is what the axles are made out of.

bowlofsalad. Interesting! so what is generally accepted as an ideal hardness for an axle?

Torque arms are a very important matter for ebikes

What do you think about TA's that just prevent the axle nut from spinning vs. the axle?

All ideals are relative. That is to say, in this case, if you were using a very small, low power ebike kit, having a 14mm or even 16mm axle with very high hardness and toughness might not be at the top of the chart in things to do. I couldn't tell you exact engineering specifications to aim for, and really, unless you are planning on having an axle made, you probably don't have a lot of choices. When I was talking about hardness, toughness and things like that, I was mostly talking about parts you might have control over like torque arms and those axle reinforcers you seem to want to make.

I am not sure what torque arm prevents an axle nut from spinning. Typically the torque arm and axle nut are two separate things entirely. If you are asking me what I think about things like nordlock washers or ideas similar, I think they are great and you should look into them. Maybe you could rephrase the question so I might understand.

 

[999zip999]

Nordlocks help.

 

[shenzhen_ex]

Nordlocks help.

Agreed...But i'm not sure where to find them in China.

 

[shenzhen_ex]

I am not sure what torque arm prevents an axle nut from spinning.

I mean the kind that are like a wrench on the nut that is secured to the chainstay.

 

[John in CR]

All that beautiful work and just to tease you. My first hubmotor spun the axle in the first 10ft of the first ride snapping the dropouts of the fork right off, so I can empathize with you. Once repaired I'd make sure the other side, which has solid axle, is the one to bear the motor's torque. I can't tell, but does your left side on the front have a torque arm included with the adapter? You don't want the same to happen on the front, and while the adapter is super strong on the right it still eventually transfers the load to the relatively weak hollow side of the axle.

The bike looks friggin awesome BTW, outstanding for a first time effort. My only comment is that 1kwh of A123's is going to feel pretty heavy on the front, especially extending so far forward, but we can get used to the effects all manner of weight placement. eg Remember riding a buddy sitting on your handlebars will you rode your bike around? That was a lot more weight and not rigidly attached, so less predictable.

 

[shenzhen_ex]

....Once repaired I'd make sure the other side, which has solid axle, is the one to bear the motor's torque. I can't tell, but does your left side on the front have a torque arm included with the adapter? You don't want the same to happen on the front, and while the adapter is super strong on the right it still eventually transfers the load to the relatively weak hollow side of the axle......

My left sides don't have torque arms. I cant think of a simple design to add them.
What do you think of set screws on the adapters?:

image credit: botz244

 

[John in CR]

That's what I didn't like about wires out of the right side, since the motor axle wants to unscrew from your adapters. If wires were out of the left, then the solid side of the axle would screw into the the adapter to the limit and lock there. Grub screws may work, but I'd get someone to sign off on it being strong enough, since at that radius it can easily be well over 1000 lbs of force.

How long is the axle in the adapter on the left side of each? What's the wall thickness of the adapters where it's hollow for the axle? That was a quality steel wasn't it? What I'm thinking is a two cutouts in the adapter to match the flats, and then a relatively small torque arm like clamp locks the axle to the adapter. The adapter already has torque arms to the bike right?

Something like this, or depending on the spacing, the cutout could be at the end of the adapter too.

 

[shenzhen_ex]

That's what I didn't like about wires out of the right side, since the motor axle wants to unscrew from your adapters. If wires were out of the left, then the solid side of the axle would screw into the the adapter to the limit and lock there. Grub screws may work, but I'd get someone to sign off on it being strong enough, since at that radius it can easily be well over 1000 lbs of force.

How long is the axle in the adapter on the left side of each? What's the wall thickness of the adapters where it's hollow for the axle? That was a quality steel wasn't it? What I'm thinking is a two cutouts in the adapter to match the flats, and then a relatively small torque arm like clamp locks the axle to the adapter. The adapter already has torque arms to the bike right?......

The axle on the back left is aprox 20mm into the adapter.
The axle on the front left is aprox 10mm into the adapter.
The rear adapter wall thickness is 10mm where it is hollow, the front is 4mm.
All adapter steel is high quality 420 stainless.
Both front and rear left side adapters do NOT have TA's to the bike.

I want to incorporate your design concept on the back axle (see image below).

I think i can bolt that green clamp plate to my rear disk adapter to lock everything up :wink: < i have been waiting for a good time to use that emoticon>..

The Front left has very little room so I think I might need to go with the set screw concept (see image below). This will rely on friction on the 20mm outside diameter adapter to counteract torque since I don't have a TA to the bike on that side. BTW I rode my bike with the Front motor only for about 150km and had no problems. YES that's right, I rode it even with my broken rear axle. Once I removed the TA from the rear I had about 4mm of thread to hold the wheel on .

 

[John in CR]

A torque arm and something to lock the axle inside the rear adapter sounds like a good plan. Yeah, on the front it sounds like set screws are all you can do. I'd put one into each flat. You do need a TA on the front left to make the set screws mean something and have the far stronger side of the axle bear the twisting force. I'd even take the torque arm off of the right of both, since the hollow side has already proven weak. It will save a bit of weight and make sure that the hollow side doesn't accidentally bear the twisting torque. eg The axle might shift slightly in the set screws leaving the left side TA seeing no force while the right side does.

After 1 broken axle, you're a braver man than me to go ride that much with it like that, though I guess the front right may be stronger since the force is spread differently.

I look forward to seeing and hearing about your 2wd. As always great work.

John

 

[shenzhen_ex]

I ended up going with 2 set screws for the left side TA.
The set-screws are 3/8" bolts, 2 of them sandwiching the flats. One of them is long and hits against the disk caliper adapter.

It is NOT 1 long screw:

 

[shenzhen_ex]

.... does your left side on the front have a torque arm included with the adapter? You don't want the same to happen on the front, and while the adapter is super strong on the right it still eventually transfers the load to the relatively weak hollow side of the axle...

D U D E .... You are not going believe this sh1t. You totally called it. I busted my front axle today at the god forsaken hollow side .

No problem, I will get the motor fixed and go with 2 TA's next time. And next time I am not going to cut the axles, I'm going to cut my fork brace instead

But I can tell you, I was ripping around this town before it broke, I was bunny hopping, power sliding, doing dual burnouts in soft gravel.
I can beat any car off the line as long as they don't know its a race .

2WD 2 Broken Axles. I'm not bummed out. Building it is half the fun.

I am working on an improved design that will be less likely to break.

 

[teklektik]

With all due apologies to LFP - maybe you should rename this Deathbike II (for different reasons)

 

[shenzhen_ex]

With all due apologies to LFP - maybe you should rename this Deathbike II (for different reasons)

Actually I did not crash. My front adapters held the wheel in place.

The problem now is I cannot get the adapter off. The busted axle is sort of caught in a groove in the adapter. I am going to have to hack saw the adapter to get the motor off of the fork.

My motor is stuck into the adapter as shown below.

 

[shenzhen_ex]

I hack sawed my front hub off of the adapter. I discovered that the axle did not break. Rather the threads stripped. I was using the treads to transmit the torque to the TA's (green item).
This is my improved design. I added set screws to transmit the torque to the TA's. And this time I won't cut the axle, Instead I am going to cut the fork brace.

The white collar is only on the NDS because there is more room on that side.

 

[shenzhen_ex]

My girlfriend sent my front hub motor back to the on-line shop that I bought it from. This is the same shop that replaced for my rear hub motor due to broken axle less than 2 weeks ago. They want 9$ USD to replace the front with a new hub. Maybe its because I hack sawed off the axle on wire side completely and sawed all but 12mm off the other side.

This girlfriend is awesome.

 

[shenzhen_ex]

Got my front motor back from the seller. They did a free replacement of the stripped axle but said "if i hack saw the axles again, they wont fix them under warranty". That is fair.

Anyways, my new axle is 30mm longer than the original, and much stonger. I don't know if it is from a rear motor. It will work better. I will be able to jump this 2WD DH before long.

 

[Chalo]

You know what one of the benefits of re-engineering things in unintended ways is? You get to figure out where you went wrong, and maybe even why it isn't usually done the way you did it.

I gotta observe, combining silly-wide rear frame spacing and a cassette hub motor is a crapshoot at best, and Chinese manufacturers don't deal in "best" things. Hub motors and cassette splines are a very uneasy marriage by themselves.

Here's the deal: When you get away from all the superficial stuff like axle material, thread depth, etc., the thing that breaks axles is leverage. You have chosen a setup that gives you longer than normal levers to break a skinnier than normal hub motor axle with. The axle is skinnier than normal because it must pass through a cassette spline that was originally tightly packaged with a round 10mm axle. But then you stuck it in a frame that was designed for a hub with a much wider body, so the axle overhang on yours is much longer than the already hokey design of your hubmotor was taking into account.

Chinese hubmotors generally seem to be engineered by rough guesses and some trial and error, with a strong bias towards doing things the cheapest possible way that will barely get the job done. And that can be okay, unless you go about making its job harder. My guess is that it may prove impossible or at least impractical to reconcile the compromises of a cassette hub motor with the compromises of your frame, and still do the kind of riding you're talking about.

If my previous experience is any guide, you are also likely to hork up your cassette body, which is a common problem among cassette hubs that have larger than 10mm axles. That might not be an issue if you don't really pedal, though.

You say your bike is a DH bike and talk about jumping it and such, but here's the fact that matters to you now: Your bike was a downhill bike-- which is to say a bike for a relatively new application, with some immature and proprietary engineering involved. But then you changed some things and probably made it unsuitable for its intended purpose.

If your goal was to have a thrashable 2WD e-bike, you'd have been immensely better off to start with a regular rigid steel frame and fork with normal axle slots and conventional spacing. For "regular", "normal", and "conventional" you may substitute "reliable", "time-tested", and "proven" if you like. But like I and so many others here have done at one time or another, you decided to go with what you thought was cool instead of what was known to work. And now, like us, you get to explore the flip side of new and cool. My advice is to carry it out all the way and learn all the lessons you can. Because that's the part of your bike that is the most valuable, and the part you'll get to keep.

I'll leave you with a couple of pictures of one of my own experimental bikes I call the X-Plane. It was a preliminary attempt to develop the cargo carrying ability of a normal bike frame, but I named it because it "x-planed" to me why there are no cargo bikes made that way. (Or are there?)

 

[shenzhen_ex]

And now, like us, you get to explore the flip side of new and cool......

WOW...That is an incredibly accurate summary statement of my eBike build, and in some ways, my life. How do you come up with shIt like that?

My advice is to carry it out all the way and learn all the lessons you can. Because that's the part of your bike that is the most valuable, and the part you'll get to keep.

I will be using that line often except I will substitute the word "Bike" with all sorts of other messed up stuff.

I gotta observe, combining silly-wide rear frame spacing and a cassette hub motor is a crapshoot at best.....

The mismatch between the axle length and dropout width was a noob mistake. I went with the cassette rear hub because I thought it would be stronger and more versatile. Hey, what can I say? I didn't even know ES existed when I purchased the frame and hub motors.

.....Here's the deal: When you get away from all the superficial stuff like axle material, thread depth, etc., the thing that breaks axles is leverage. You have chosen a setup that gives you longer than normal levers to break a skinnier than normal hub motor axle with. The axle is skinnier than normal because it must pass through a cassette spline that was originally tightly packaged with a round 10mm axle. But then you stuck it in a frame that was designed for a hub with a much wider body, so the axle overhang on yours is much longer than the already hokey design of your hubmotor was taking into account......

I certainly made a much longer simply-supported beam out of my rear axle. My disk side adapter (see below) increases the cross section of the non-cassette side, and that side has a solid axle. I doubt the axle will brake there. But as you pointed out, I have a long skinny unsupported axle overhang on the cassette side. That is where it will break if it breaks at all.

You say your bike is a DH bike and talk about jumping it and such, but here's the fact that matters to you now: Your bike was a downhill bike-- which is to say a bike for a relatively new application, with some immature and proprietary engineering involved. But then you changed some things and probably made it unsuitable for its intended purpose.

I agree that this bike can no longer race down rugged mountains and jump off cliffs, but I will be jumping this bike. When I get this running I am going to take a few stair jumps. If the axle brakes I will need a motor upgrade, and maybe a band-aid .

....Chinese manufacturers don't deal in "best" things....

Well, I am guessing that you noticed that I am located in China. You may even know that Shenzhen is the "work-shop" of the world. Perhaps you even suspect that I am working at a Chinese manufacturer. It may be a stretch, but was that a well planned, strategic personal jab ,or just a common comment.

 

[Chalo]

....Chinese manufacturers don't deal in "best" things....

Well, I am guessing that you noticed that I am located in China. You may even know that Shenzhen is the "work-shop" of the world. Perhaps you even suspect that I am working at a Chinese manufacturer. It may be a stretch, but was that a well planned, strategic personal jab ,or just a common comment.

It has no bearing on you, of course.

And to be fair, it's too general a statement to be true. China may be the best in the world at making suspension bridges or something. But Chinese export products are remarkable for their quantity and low cost, not their thorough design or meticulous details. That's truer of bike hub motors than it is of practically anything else, since they have high materials costs and they serve a very price-sensitive market.

 

[shenzhen_ex]

done!