The ultimate torque arm (For <2kW and standard axles)?

Made from 10 or 12 layers of laser cut annealled spring steel and tempered to ~50HRC, it requires drilling & tapping a M4 hole 30mm away from the axle center. (The dimensions are flexible; by adding a series of 4mm clearance holes, an appropriate position can be chosen (and any excess cut off, though that risks people making the arm too short.)



Two (brass?) rivets serve only to hold the layers together for convenince, and play no structural role. The SS shouldered, m4 screw acts as location, pivot and torque sink.

Once initially fitted to the bike, no tools beyond the wrench for the axle nuts is required for use.


The 2mm/30° sweep on the blades gives 0.6mm x2 clearance around the axle flats, allowing the device to be swiveled out of the way for wheel removal; but when the axle nuts are done up, they compress onto the sides of the axle flats and present a resilent, solid 5/6mm face to the flats to absorb/transfer the torque.


They pack reasonably well into rectangular areas for cutting:

The jaw sweep can be applied with a fly-press and the heat treatment (260°C and water or oil quench) should be within the reach of most shops.

The layered approach allows the width to be tailored to the motor requirements and by using 1mm brass rod for the rivets, that can be done by the user to suit their requirements with just a hammer.

Thoughts?

Looks good. Give it a shot...

spinningmagnets said:

Looks good. Give it a shot...

Click to expand...

No need to act like a cunt. I think you'd get a bigger response for making/testing/doing something. There are plenty of people out there with ideas. The interesting part is turning them into a reality.

Ok, then I'll give you feedback.

You don't want to use anything harder than the axle for torque retention. If something gets smushed in the contest between torque arm and axle, let it be the relatively cheap and disposable torque arm.

An open claw is so much less secure than a closed eye torque arm that it hardly seems worth using.

Those relief notches to allow the bent "wings" are huge stress concentrators.

You're obviously thinking on this problem, but not obviously thinking it through.

Agreed with the previous. I should add, that torque plates are much better than a torque arm. Best are through axle or pinch type, if you plan to feed serious power some day.

I apologize for the lack of detail in my response. You've done a very good job on this design, which you clearly have put some time into. Your design incorporates some elements that are not common and they are intriguing enough that I would very much like to see it proceed to the testing phase of development.

As much as I enjoy fiddling with mechanical things, I have never engaged in any professional engineering training. This design has matured to the point where any further debate exceeds my capabilities, and I'm afraid I must confess that I am unable to be of any further assistance, other than to hopefully encourage you to continue on, in its development.

Best of luck. I hope the testing meets your design goals.

spinningmagnets said:

I apologize for the lack of detail in my response. You've done a very good job on this design, which you clearly have put some time into. Your design incorporates some elements that are not common and they are intriguing enough that I would very much like to see it proceed to the testing phase of development.

As much as I enjoy fiddling with mechanical things, I have never engaged in any professional engineering training. This design has matured to the point where any further debate exceeds my capabilities, and I'm afraid I must confess that I am unable to be of any further assistance, other than to hopefully encourage you to continue on, in its development.

Best of luck. I hope the testing meets your design goals.

Click to expand...

I'm the only one who should apologise SM. I should not write drunk! (It's not a common habit with me, it was my birthday yesterday.)

I vented my frustration at the lack of feedback, and you -- as my only responder -- were not really the target, but simply the only convenient hook.

So, please accept my profound apologies for making you the target of my frustrations.

No apologies necessary, I am quite familiar with "drunk posting" (and my best friend, the "edit" feature).

For the casual readers, please be aware that the ES membership are not a monolithic bloc with a single hive-mind. Sometimes...very valuable engineering information is revealed when two posters strongly disagree with each other and "discuss" their difference of opinion in these forums. There is a subtle balance we strive for, to avoid flame wars, but...still allow a vigorous discussion with as much "freedom of speech" as we can get away with.

If nobody is complaining, then...we allow quite a bit to slide by, that might otherwise be moderated. I have been a member of forums in the past that were heavily moderated, and as a result...everyone was very polite. Unfortunately, the consequence was a consistent lack of innovation and fruitful discourse. They became boring and useless.

If anyone has a strong opinion about anything, please post it.

Thanks Chalo. This is the kind of detailed response I was hoping for.

A blow-by-blow response (should you be interested.)

The rocking back and forth of acceleration and regen seems to defeat star (and most other locking) washers and requires wedglock type washer (such as NordLock) to prevent loosening. These require the surfaces they engage (nut and torque arm in this case) to be softer than they are to work properly.

When the nut loosens the forces on the torque arm dramatically increase. There is no guarantee the rider will notice. If it loosens the rocking will increase significantly and perhaps accelerate the damage to the now loose springs, as well it will damage the dropouts which may cause frame failure.

This design does not address the design goals of having a round axle that is the proper size for the dropouts, so it still causes the axle to be off-center and the corners of the dropout to not fit the curvature of the axle properly. This requires filing and still mis-aligns the rim or disc brakes.

Hubmotor axles seem to be made from a variety of steels, some very soft.

The analysis looks interesting, some actual tests would be nice, as Justin is doing on his designs.

Alan B said:

The rocking back and forth of acceleration and regen seems to defeat star (and most other locking) washers and requires wedglock type washer (such as NordLock) to prevent loosening. These require the surfaces they engage (nut and torque arm in this case) to be softer than they are to work properly.

Click to expand...

There is nothing to stop you using a NordLock (or any other type of ) lock nut with this design. It's primary purpose isn't to prevent the nut undoing; that it will act to resist it is a bonus.

Alan B said:

When the nut loosens the forces on the torque arm dramatically increase. There is no guarantee the rider will notice.

Click to expand...

The spring nature of this design means that when the nut loosens, the wings will no longer present an entirely tight-fitting and completely 'solid' face to to the axle flats. The results should be a sensation and noise that would certainly grab my attention.

This should start to happen when the nut is around 1/2 a turn loose, and well before the 2.x turns it requires for the nut to become free of the tension and grip provided by the spring wings.

Even beyond two turns, if fitted correctly, it is unlikely the axle would be free to 'fall out' unless the bike was jumped, and under those circumstances, I would fit a second screw to prevent it; but that is not the scenario I'm envisaging for myself.

More secure than an ordinary nut alone.

Alan B said:

This design does not address the design goals of having a round axle that is the proper size for the dropouts, so it still causes the axle to be off-center and the corners of the dropout to not fit the curvature of the axle properly. This requires filing and still mis-aligns the rim or disc brakes.

Click to expand...

I'm not trying to address that problem.

The motor I have, and AFAIK, any motor I might want to buy -- for now at least -- has an over-sized axle with flats; and that is the target for this design.

In the event that suppliers start shipping motors with 10mm round axles, they will have also have had to come up with some alternative to axle flats for torque control.

Presumably that would be something along the lines of [strike]Jason[/strike] Justin's inside-the-dropout-plate mechanism, and they will either provide is as a part of the order -- as with tabbed washers -- or at least make it available for order at the same time at reasonable cost.

Most motor axles can readily be changed. Thus designing a system of axle and torque arm is not that unreasonable, if one wants to do the job right.

The point made was missed. The spring steel may be too hard for a proper wedglock washer to work.

There seems some requirement to precisely hit a hardness. Is this practical at the cost point of a torque arm?

The precision cutting of the layers makes these expensive to manufacture in small quantity. The heat treatment is also not easy or inexpensive.

The design is interesting but raises many questions that cannot easily be predicted. Testing is indicated.

You might want to get his name right. He deserves more respect.

Buk___ said:

Chalo said:

Those relief notches to allow the bent "wings" are huge stress concentrators.

Click to expand...

Actually not. They are specifically designed to be 'stress distributors'.

Click to expand...

Pay more attention to the force path that remains after those cuts are made. You have a longer lever working on a narrower section of material, with a more abrupt transition from wide/stiff on either side to narrow/compliant at the points of highest stress. It's not a good recipe for endurance. Either the wings will bend away from the arm and out of parallel, or they'll crack and snap off.

I understand the problems that are created by having to pass plugs through a torque arm. But that's a reason to make a pivoted or interlocking nutcracker-style torque arm rather than an open sided one. You don't have to spend much time with tools in your hand to know that a relatively light section on the box end of a wrench is much less likely to spread and round off your fastener than the heavier and chunkier open end. And those things haven't been undermined by slits that radically reduce their sections.

Alan B said:

Most motor axles can readily be changed. Thus designing a system of axle and torque arm is not that unreasonable, if one wants to do the job right.

Click to expand...

Not unreasonable at all. Just not what I set out to do; nor something I have any interest to do. I'll leave that to those with the commercial backing and industry position to make it viable.

Alan B said:

The point made was missed. The spring steel may be too hard for a proper wedglock washer to work.

Click to expand...

Not missed, but rather dismissed.

You see, before posting my response to you, I went to the Nordlock website, downloaded and read most every document that looked liekly to contain the relevant hardness information. Sadly, but typically, they do not make that information available, and I suspect that my email enquiry asking them for that information will either get no response, or a "Nord-lock is an ISO-4001/9000 certified company, and warranties it's products" response, despite that their warranty only covers the first installation (first torquing) of their product:

2. The warranty expires when the bolted connection where the product is initially installed is disassembled.

Click to expand...

Hm! Impressed so far?

I also noted two other things about their products:

1. Whilst they state: "Nord-lock products can be used with steel or aluminium", they go on to say: "if you intend to use them on aluminium, please contact us for further advice".
   
   Now, I am little more than guessing here, but I doubt that that 'contact us' requirement is because Al might be too hard.
   
   Which leaves the alternative, that it might be too soft for the serrated back-side of the ramped washer -- that needs to reliably grip the non-rotating surface it is in contact with -- to provide that reliable purchase, and so deny the user the ability to apply the "600-650 Nm" of torque required to make the product work.
   .
   
2. Nowhere (that I found, and I looked pretty hard, but could have missed something), do they offer a product compatible with any of: M10/M12/M14 threads (regardless of fine/medium/course considerations).
   
   They -- on their official website -- offer two sizes: M22 & 7/8". Which to my knowledge doesn't fit any bicycle/e-bike axle!?

In your last post you mentioned that the hardness of the materials that the Nord-Lock was in contact with [paraphrase] was critical, and went on to list those as "the torque arms and axle".

Now, quite why you included the axle which has to sustain the thread forces of the specified "600-650 Nm" or required torque I'm not sure, but safe to say, if my specification of 50HRC is softer than the 'typical hub motor axle' as I believe -- but identified earlier that I have yet to find documented substantiation of -- then it will (also) be 'soft enough'.

If however, your statement about the softness of the axle was a misnomer, then it all comes down to the required hardness of the contact surface the serrated back face of the Nord-Lock ramp washer interfaces to; and -- as they do not specify that -- I can only suggest that 50HRC should be soft enough for their product to obtain the purchase it needs.

Beyond that -- admittedly somewhat insubstantial statement -- I reserve judgement until (and unless) they respond to me with some actual, relevant information on the subject.

Alan B said:

There seems some requirement to precisely hit a hardness. Is this practical at the cost point of a torque arm?

Click to expand...

The precision required is no more stringent than that of any screwdriver, spanner, or other tool, where too high a hardness means they would shatter, and too low means they would bend. The same sort of products that even non-Chinese manufactures manage to make a profit, whilst producing reliable products.

If you have tempered your own steel -- I did 45 years ago at school -- then you'll know that it is neither a precise science, nor hypercritical. It takes a practiced eye and experience. Neither commodity is hard to acquire.

Alan B said:

The precision cutting of the layers makes these expensive to manufacture in small quantity. The heat treatment is also not easy or inexpensive.

Click to expand...

Agreed. To a point.

The primary costs in laser cutting are:

• Setup time:
  The time it takes to program the cutter and install the material.
  
  A fixed cost that is amortised over the number of components produce per run.
  .
• The time taken, per millimetre, to cut. At 0.5 mm thickness, these can run the cutter at its highest rate; and at <300mm cutting length per item, the cost is minimal.
  
  The heat treatment is simple enough that it can be done with a common-or-garden propane blow-torch -- the hardening phase -- and a (slightly above average) domestic oven capable of 260°C.
  
  The final cost is all about quantity. With 20+ takers, I could break even. With 30, I might even make a small profit.
  
  If I do this as a one-off for my own use only, it will cost me ~£30 -- if I collect the materials and transport them myself. (I haven't counted the cost of propane gas, domestic gas, petrol, or my time.)
  
  Commercially, I'd estimate a unit cost (12x0.5mm) at ~£1.29 if produced in batches of 1000. Isuspect that a 500% markup would sell lke 'hot cakes', if the efficiacy of the design were proven. (Wild arsed guess, with a little experience thrown in for good measure. )
  

Alan B said:

The design is interesting but raises many questions that cannot easily be predicted. Testing is indicated.

Click to expand...

Of course. Testing is a requirement before anything is set in stone.

But better to test a design that has been subjected to adversarial scrutiny, than waste time, money and resources on prototyping and testing a design that is entirely bound up within the mind of the conceiver.

Regardless how erudite the proposer may be, this place has -- amongst its vocalists and fakirs -- an inordinately high proportion of genuine -- identifiable, if not verifiable -- experts in their fields.

No one who has been watching here for more than a few months -- or has researched back more than a few years -- would challenge the expertise of, say, -- so many, yet so few, to choose from -- lewbowski (I apologise in advance if I spelt that wrong, but I don't want to destroy the thread of my thought process by looking it up).

Alan B said:

You might want to get his name right. He deserves more respect.

Click to expand...

This confused me for the longest time. It took reading and re-reading your post a dozen times before the penny clicked and I re-read my post to which you responded.

Sorry Justin, if you were offended by my referring to you by the name -- which happens to be the name of one of my Brother-in-laws -- Jason. No offense was intended.

When I wrote "Jason", it was habit; when I re-read (re-red) it, it was familiar; it did not stand out as obviously wrong. No slight was intended.

I seriously doubt that someone like you would be offended by such a trivial slight -- even were it intended -- you have better things to do with you time and energies; but none the less, having had my error pointed out -- no matter how obliquely -- I apologise (again) for any offense I might have caused you.

(For those, habitually or professionally, offended on your behalf -- not so much.)

[strike]Placeholder. I'll respond to this tomorrow.; too tired tonight (this morning) to make any sense.[/strike]

Chalo said:

Buk___ said:

Chalo said:

Those relief notches to allow the bent "wings" are huge stress concentrators.

Click to expand...

Actually not. They are specifically designed to be 'stress distributors'.

Click to expand...

Pay more attention to the force path that remains after those cuts are made. You have a longer lever working on a narrower section of material, with a more abrupt transition from wide/stiff on either side to narrow/compliant at the points of highest stress. It's not a good recipe for endurance. Either the wings will bend away from the arm and out of parallel, or they'll crack and snap off.

Click to expand...

It's all about the difference between toughness, hardness and strength, which many people, including many engineers, do not really appreciate.

Have you ever wondered how springs like these:

as used in garden secateurs, and those in the pliers and scissors in swiss army knives and the like:


Or those you find in the center of tape measures:
,
and the much larger and stronger versions used in industrial counter-balances:,
survive the constant flexing despite being so thin?

Toughness and resilience over hardness.

Now take that thin but tough material, layer it, and sandwich it in a tight section and use the edges of that section as your energy absorber...

Chalo said:

I understand the problems that are created by having to pass plugs through a torque arm. But that's a reason to make a pivoted or interlocking nutcracker-style torque arm rather than an open sided one. You don't have to spend much time with tools in your hand to know that a relatively light section on the box end of a wrench is much less likely to spread and round off your fastener than the heavier and chunkier open end. And those things haven't been undermined by slits that radically reduce their sections.

Click to expand...

You are describing a different mode of operation that requires different design criteria.

Just as the car industry moved away from building car bodies and chassis like tanks, in favour of deliberately deformable structures to absorb energy rather than confront it head on; that's the idea behind this design.

It isn't the constant torque that damages dropouts, but rather the shock-loading of the initial take-up, and the constant transitioning from positive to negative and back that fatigues the material.

Rather than using brute strength to resist the constant impacting, the design applies basic material science to absorb them with steel tailored to be resilient.

spinningmagnets said:

Looks good. Give it a shot...

Click to expand...

I agree,.... and I mean that nicely. I does looks good. So give it a go, and see how it does.

IMO,... they would be quite reliable for use in under 1000W applications. Being a past die maker, these could be easily stamped and treated with acceptable tolerance for that sub-1000W use. And they MAY be very economical and flexible (using varied layers) for "reasonable" 1000W-2000W use, but others like those previously described would certainly be better suggested.

I'm bravely running 1500W+ on heavy bare 5/16" ALUMINUM plate open-end dropouts!!! Frequently inspected for 2+yrs now,.... still not confident, but absolutely no issues. The heavy plates leave no exposed threads beyond the axle nuts with minimal required washers. And something like this MAY provide me an option to consider.

DRMousseau said:

spinningmagnets said:

Looks good. Give it a shot...

Click to expand...

Being a past die maker, these could be easily stamped and treated with acceptable tolerance for that sub-1000W use. And they MAY be very economical and flexible (using varied layers) for "reasonable" 1000W-2000W use, but others like those previously described would certainly be better suggested.

Click to expand...

Indeed. If these were to be manufactured in bulk, it is easy to see that they could be stamped out from rolls of annealed spring steel stock:

using a small, automated brake press, which could form the wings in the same, single operation and then they could be heat-treated in batches to produce the final components very economically.

The greatest cost would be the production of the stamping die which would probably be several thousand £/$?

For small scale, the advent of laser cutting means it might be viable to produce batches of a dozen or two at a time, if you knew there was a market for them.

As a one-off for my own use, I'd have to resort to laboriously hand fettling them from plate stock -- which wouldn't be accurate or economical -- and an off-the-shelf existing product is more likely what I'll end up using

Regarding the NordLock washers, I'm surprised that (with years of mechanical experience) you have no experience with these. Have you never used or encountered the product?

I found sizes that have worked with 14mm and 16mm ebike axles. They may have been inch marked, but they have been in service here for up to 8 years without issue. After all these are washers, precise fit is not much of an issue. If you were concerned a bushing could be used to insure centering, but I have not found a small amount of clearance to interfere with their locking or unlocking. They need to be retorqued a few times after isntallation, the same as any trailer lug nut. They don't move much after a couple of times. Months later when they are removed you can feel the "tightening" as they are turned in the release direction, then they "pop" and the tension drops off. That's been my experience with the half dozen sets I've used. You can easily test them yourself.

I do recall finding hardness estimates for the washers. In the most stressed case here, they have gone through many tire changes and have performed as designed every time. There is one ES member who was selling them for years, perhaps he has more data.

We have seen cases here on ES where a hubmotor was performing fine, and then the nuts came loose, the axle spun, sheared off the wires, spread and/or cracked the dropouts, shorted the controller, blew the FETs, and in front fork usages, came out of the dropouts resulting in a head forward over the handlebars crash and in some cases fairly serious injuries. Things were fine until the nuts came loose. Keeping the nuts from coming loose is important. Standard locking washers don't achieve this for some reason. Even without regen. Torque arms with holes tend to capture the axle and reduce the problem, but a "C" shaped unit will release the axle immediately.

Many dropouts have lawyer lips. This may need to be compensated for in the torque arm design. Merely clamping against a flat surface can cause the dropouts to crack and fail.

The relationship between the dropout and the torque arm mounting hole is extremely variable on ebikes. This makes a standardized part not really practical. Measuring the relationship between an available hole and the dropout slots precisely enough for manufacture difficult. A group buy would require doing this many times over, and many different cutting patterns. Generally torque arms use struts to adjust their angle, and torque plates are drilled while fitting to the bike frame to adjust for these issues. So they can be mass produced and still fit a range of bike frames. This design seems to miss these details.

I have some concerns about the force multiplying effect of the slots. We've seen beefy chrome moly steel dropouts fail when the axle was not fully seated into the dropout, it doesn't take much increase in leverage to defeat the steel, especially with 12mm axles. 12mm axles do fit the dropouts better since the axle is not as far displaced from center, but as we know the forces are a lot greater due to the reduced flats. Is this steel that much stronger?

I think it is a bit premature to claim this (or any) new design as "ultimate". That kind of claim might apply after actual performance demonstration, but to claim it of an unproven design? There is often an inverse correlation between the magnitude of claims and the actual performance. I would expect an ultimate design to use QR levers and not require a lug wrench that weighs more than an entire road bike tool kit.

This design could make changing tires easier, though taking one screw out (for conventional designs) is not all that big a deal. The torque arm stays with the motor. None of my motors have Higo connectors on the axle wires so the pins can be ejected and threaded through the torque arm hole the one time when the arm is added to the axle. So this is mostly solving a non problem. I like Higo connectors elsewhere, but they are a bit small for the motor wires which may see 3x the battery current or more.

Looking forward to test results.

Alan B said:

I think it is a bit premature to claim this (or any) new design as "ultimate". That kind of claim might apply after actual performance demonstration, but to claim it of an unproven design? There is often an inverse correlation between the magnitude of claims and the actual performance. I would expect an ultimate design to use QR levers and not require a lug wrench that weighs more than an entire road bike tool kit.

Click to expand...

I get back to you on the rest, but I wanted to answer this. The question mark at the end of the title makes it a question not a claim.

It was tongue in cheek and slightly provocative to encourage some feedback; not a claim. Never a claim.

Addy said:

No need to act like a cunt.

Click to expand...

so the c-werd (no, not chalo) iz no longer anymore on the banned word liszt?
good to know.
but the i guess you would have2 delete the entire 80sum page thread about kunt engineering.
george sheds a tear of happiness.

drunk or no, you nailed it the furst time.

Buk___ said:

Have you ever wondered how springs like these:

as used in garden secateurs, and those in the pliers and scissors in swiss army knives and the like:

Or those you find in the center of tape measures:

,
and the much larger and stronger versions used in industrial counter-balances:

,
survive the constant flexing despite being so thin?

Toughness and resilience over hardness.

Click to expand...

Mostly it's about those things flexing across their thin axis, thus greatly attenuating stress levels for a given deflection. The opposite of what you're asking of your spring stock.

Making a torque arm that intentionally flexes open is even dumber than making one that breaks because it's pre-cracked. Allowing the axle to twist back and forth even a little bit will back off the axle nuts and precipitate a larger failure.

Alan B said:

Regarding the NordLock washers, I'm surprised that (with years of mechanical experience) you have no experience with these. Have you never used or encountered the product?
...
I do recall finding hardness estimates for the washers.

Click to expand...

I've never used them, though I seem to recall that something similar was mooted for use on the military version of the Bedford TM truck; but (as I recall) they were rejected (by the UK Army) in favour of bog standard castellated nuts and split pins.

I found some hardness information on them and it would probably require the 254 SMO variant to be used.

Alan B said:

We have seen cases here on ES where a hubmotor was performing fine, and then the nuts came loose, the axle spun, ...

Click to expand...

Doesn't that mean the dropouts had to spread or be chewed away almost instantaneously?

Alan B said:

Things were fine until the nuts came loose. Keeping the nuts from coming loose is important. Standard locking washers don't achieve this for some reason. Even without regen. Torque arms with holes tend to capture the axle and reduce the problem

Click to expand...

I don't see many signs of people -- with legal or relatively low-power (ie. <2k) hub motors -- using much by way of extra precautions beyond correctly torquing the axle nuts. None of the commercially available e-bikes even seem to bother with anything more than tabbed washers.

Beyond a couple of instances of Al dropouts letting go, I haven't seen much in the way of horror stories here either; but I haven't been around here as long as you.

Alan B said:

but a "C" shaped unit will release the axle immediately.

Click to expand...

I'd like to challenge that that is necessarily so, but I don't (yet) have any evidence to offer.

Alan B said:

Many dropouts have lawyer lips. This may need to be compensated for in the torque arm design. Merely clamping against a flat surface can cause the dropouts to crack and fail.

Click to expand...

The design I posted is such that the jaws are the same diameter as the axle nut flange -- based on those supplied with my motor -- so if the flanged nut fits inside any lawyer lips (my bike doesn't have them), so would the jaws. The arm obviously wouldn't as show in my drawings, but it would be possible to move it so that it came out the bottom via the dropout opening. It would mean a circuitous route back to the dropout plate to site the retaining screw but would be doable.

Though for my own bike I'd simple file the lawyer lips away; just as I deepened the dropout. The design was, and is, mostly targeted at my requirements.

Alan B said:

The relationship between the dropout and the torque arm mounting hole is extremely variable on ebikes. This makes a standardized part not really practical. Measuring the relationship between an available hole and the dropout slots precisely enough for manufacture difficult. A group buy would require doing this many times over, and many different cutting patterns. Generally torque arms use struts to adjust their angle, and torque plates are drilled while fitting to the bike frame to adjust for these issues. So they can be mass produced and still fit a range of bike frames. This design seems to miss these details.

Click to expand...

Again, the design as posted is targeted very much at my own bike:

And my intention is to drill&tap a new hole specifically for the TA. I looked at trying to re-use the disk brake mounting bolt/hole, but there is no equivalent on the other side -- hole or the material it is drilled in -- so it would mean having different shapes side to side.

As is, there is no actual requirement for that, though is does mean the wings would face inward on one side.

Alan B said:

I have some concerns about the force multiplying effect of the slots. We've seen beefy chrome moly steel dropouts fail when the axle was not fully seated into the dropout, it doesn't take much increase in leverage to defeat the steel, especially with 12mm axles. 12mm axles do fit the dropouts better since the axle is not as far displaced from center, but as we know the forces are a lot greater due to the reduced flats. Is this steel that much stronger?

Click to expand...

I don't know how to make this clearer -- it isn't about strength -- but I'll have one last try.

The TAs are a supplement to, not a replacement of, the existing dropouts. Their purpose -- as I envisage it -- is to prevent the corners of the axle flats from exerting impactful shear loading on the dropouts.

Given that frame and axle are produced in isolation of each other, there will inevitably be a loose fit between them, with the dropout tending to be oversized and the axle flats undersized, just so that each will fit in the event that the other is "dead-on balls accurate" 10mm.

That 'slop', which might vary from say, 0.1mm to 0.5mm or more, so that the transitions of torque from positive to negative and back again, means that the corners of the axle flats have a (small) distance through which to accelerate before being arrested by the dropout. And it is that acceleration that is the nemesis of dropouts.

Try a little experiment: Take a steel rule (or similar piece of metal), place the edge on a piece of softwood and press it. See how big an indent you can leave. Then lift it off a short way -- a couple of millimeters -- and stab the corner of the rule into the wood. Now imagine performing the same experiment using a torque motor to supply the force.

If there is a 0.5mm overall gap -- that's just -0.005" on each side of the axle and +0.005" on the dropouts -- and a 12mm axle will have 9.204° of rotation in which to accelerate when the torque transitions from negative to positive, or vice versa.

Now the angular acceleration can be related to the torque that drives it, and given the rotating mass of the motor, its torque and the radius of the flat edge, it would be possible to calculate the effective force that would result; but I hope it is sufficient to say that it will be several times higher than if the same torque was applied with the faces of the dropouts tightly abutted to the flats of the axle.

The idea behind the spring-jaws is that they remove that slop, and so remove the space in which the axle can accelerate before contact.

The idea behind the open C-shape and the slots, is that they provide flex -- they are designed to allow the jaws to expand outwards -- slightly, less than a mm -- providing a resilient but flexible buffer to de-accelerate (actually, prevent it from accelerating) the axle, absorbing (some of) the initial torque, before the edges of the axle flats come into contact with the dropouts.

If I've done my homework, they never will make contact with the dropouts; but even if they do, they will have -- through their inherent spring rate, plus inter-layer friction, plus the friction between the front and back layers and the frame and the flange face of the nut -- so reduced the force with which they make contact, that any danger of the dropout failing -- either instantaneously or through fatigue -- is enormously diminished.

By using a material that is designed to flex, and constraining it so that it requires considerable energy to make it do so, it seeks to provide a shock force absorbing buffer, rather than an immovable object. The key is toughness and resilience over strength or hardness.

The ideal way to test the efficacy of the design would be to apply a thin-film piezo-electric impact sensor to the inner faces of the dropouts and measure the forces it experiences with and without the TA in place. Had I the resources, I would do that. I don't.

Alan B said:

This design could make changing tires easier, though taking one screw out (for conventional designs) is not all that big a deal. The torque arm stays with the motor. None of my motors have Higo connectors on the axle wires so the pins can be ejected and threaded through the torque arm hole the one time when the arm is added to the axle. So this is mostly solving a non problem. I like Higo connectors elsewhere, but they are a bit small for the motor wires which may see 3x the battery current or more.

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Your non-problem is my main problem. I designed it to solve my problem; I posted it in case anyone else had the same problem as I.

Alan B said:

Looking forward to test results.

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You may have to wait a while. As with all my non-essential spending, it joins the queue and if it ever makes it to the top, I'll do it. Assuming that we have no repetitions of last year's boiler and car troubles, the weeping leak in the kitchen cupboard proves to be a simple fix that doesn't necessitate ripping the built-in units out or chasing the pipe behind the plasterboard to find its source, I might get to it towards the end of the summer. With luck.

Chalo said:

Mostly it's about those things flexing across their thin axis, thus greatly attenuating stress levels for a given deflection. The opposite of what you're asking of your spring stock.

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Chalo, you tax me

I know you are not a great lover of oval (and other shaped) pressure formed frame elements, but I hope you'll agree that they aren't a complete failure.

If you use an oval top tube, does its greater flexibility in the narrower axis mean that the longer axis is either weak or brittle?

In the thin axis, these springs are capable of being wound tight, and stretched flat and back again many millions of times. Each infinitesimal section of metal can be flexed though 20°. 30°, 90° or 180° without harm.

The biggest arc any part of this design will transition through, is < 4°; maybe 0.5mm/20 thou, before the axle flats will contact, and be arrested by, the existing dropout.

Chalo said:

Making a torque arm that intentionally flexes open is even dumber than making one that breaks because it's pre-cracked. Allowing the axle to twist back and forth even a little bit will back off the axle nuts and precipitate a larger failure.

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The purpose of the TA is to protect the dropout; not supplant it.

And by making the TA completely abut the axle flats, but allowing it to flex, against the internal tension provided by the heat treatment (spring rate) and the friction between the layers provided by the torqued nut that constrains those layers, by the time the axle has twisted enough to contact the dropout faces, the TA has done its job of absorbing the force, preventing the acceleration, and severely reducing -- if not entirely containing -- the forces that would otherwise impart impact shear stresses on those dropout faces.

Like I said above, you have to consider the entire system in-situ.

I'm beginning to realise that what I thought of as a cute and simple design is actually quite sophisticated; and requires rather more than a superficial examination to understand how it operates.

(Update:re the twisting action. Won't that act to loosen one (trivially, <4°) and tighten the other by the same amount? And vice versa when the torque transitions from + to -.

Buk___ said:

In the thin axis, these springs are capable of being wound tight, and stretched flat and back again many millions of times. Each infinitesimal section of metal can be flexed though 20°. 30°, 90° or 180° without harm.

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Flat springs are thin throughout their lengths. Your TA is "thin" (not in absolute terms, but relative to the whole) in a single spot on each side. You'll get material failure there as soon as static friction is overcome and the thing is called upon to do something.