Does my brand new hub motor (Aikema AKM-128CST) have a bent axle? (4K video)

[Comrade]

How's it look on the dropout on the other side of the wheel?

Other side looks much better. There is wobble, but based on the amplitude I think it is just transferred from the other side. That is if the cassette site didn't wobble, it would be running smooth on both sides.

 

[E-HP]

How's it look on the dropout on the other side of the wheel?

Other side looks much better. There is wobble, but based on the amplitude I think it is just transferred from the other side. That is if the cassette site didn't wobble, it would be running smooth on both sides.

Just my opinion, I don't see any more vibration than if you had a slightly out of balance tire mounted on the wheel. In other words, I wouldn't be concerned, but that's just me. Lace it up and start riding. :thumb:
To me, if something fails, it's just another opportunity to upgrade

 

[Comrade]

I don't see any more vibration than if you had a slightly out of balance tire mounted on the wheel.

It's not the vibration I'm concerned about (at this point at least), it's the bending action that the motor would exert on my dropouts.

 

[E-HP]

I don't see any more vibration than if you had a slightly out of balance tire mounted on the wheel.

It's not the vibration I'm concerned about (at this point at least), it's the bending action that the motor would exert on my dropouts.

In that case, if you can't work it out with the seller, how much would you sell the damaged motor for? Shipping plus some nominal amount?

 

[justlooking808]

I don't see any more vibration than if you had a slightly out of balance tire mounted on the wheel.

It's not the vibration I'm concerned about (at this point at least), it's the bending action that the motor would exert on my dropouts.

In that case, if you can't work it out with the seller, how much would you sell the damaged motor for? Shipping plus some nominal amount?

to me that looks fine, if HP doesn't take the motor, pm me and maybe we can work something out?

 

[Oli.Hall]

Hi @Comrade,

You haven't mentioned yet which model of motor you have, but it looks like it may be an Aikema AKM-128CST, or an AKM-100CST. Is it one of those models please?

I've got lots of experience with those motors and the problem is not in the axle, or the bearings, the problem exists in poor fitting of the freehub carrier into the side cover.

The CST series motors have a central part of the freehub, around which the freehub body rotates, which is press-fitted, or in some motors it is screwed into the side cover. There was probably a small piece of metal swarf in the hole when they pressed it into the side cover causing your part to be fitted at a slight angle. This part holds not one, but two bearings in the drive side, if this part is misaligned then the bearings exert a strong cyclical bending force on the axle, the direction of which rotates with the motor, causing the wobble that you see with each rotation.

It is very important to fix this because the axle in these motors is not just one long piece of steel threaded rod, it is actually split into two short stub-axles which are each bolted to opposite sides of the aluminium motor core just inside each side cover. Any flex that you are seeing at the drop-out is also happening to the motor core and the axle is only held by three machine screws at that end!

TL;DR, you need a new side cover which comes with a new freehub assembly.

Luckily they are really cheap at around $15. Buy one, or ask Topbikekit to send you one free of charge. They are very easy to replace it yourself and if the motor is brand new they probably won't require any special tools. Before you email TBK about it, first make sure you can unscrew the side cover youself. Remove the locknuts and try and unscrew the whole side cover anti-clockwise.

Side covers:
AKM-128CST Side Cover http://www.topbikekit.com/akm128cst...with-cassette-base-for-replacement-p-858.html
AKM-100CST Side Cover http://www.topbikekit.com/akm100cst...with-cassette-base-for-replacement-p-859.html

I have one of these motors apart on my desk at the moment and I am happy to explain more if you want to PM me.

Thanks,
Oli.

 

[Comrade]

They are very easy to replace it yourself and if the motor is brand new they probably won't require any special tools. Before you email TBK about it, first make sure you can unscrew the side cover youself. Remove the locknuts and try and unscrew the whole side cover anti-clockwise.

Yes, it is a AKM-128CST.

Thanks for the detailed explanation. All of it makes sense.

Strangely Topbikekit went into complete radio silence about this issue. So Paypal is handling it now.

Your picture of the open cover explains very well how to open it also. I assume there is a large rubber o-ring that's between motor body and the cover that's not pictured? And that might be the cause of difficulty when the motor is not new?

I saw there is a specialized cover removal tool for sale, and I wondered how it works, but now that I see how the cover is removed by unscrewing the whole thing, it looks like the tool simply fits into the 3 screw holes and provides leverage.

 

[Oli.Hall]

Once PayPal have sent you your refund then you could always try and obtain a spare side cover and fix the motor you have. It's a shame that TBK didn't just offer to send you a new side cover, but c'est la vie!

Unlike some other makes, the Aikema 128/100 series motors rarely have any type of o-rings meaning they are quite easy to unscrew by hand when they are new - As long as they've never been used under load. As soon as you use the motor for the first time under load, the action of the motor torque, and the pedalling forces cause the side cover to screw on quite tightly. After the motor has been used it is extremely difficult to unscrew by hand and usually requires the tool to unscrew it. You are absolutely correct, the tool simply screws into the clutch retaining bolts and provides extra leverage to unscrew the cover. You could make one yourself but I found it easier to buy one from TBK as they are cheap enough.

In-fact, if you are removing the cover by hand just to have a look inside, then you don't even need to remove the three screws in the cover as these just hold the clutch to the other side of the cover. However, if you do plan to replace the side cover, you will need to transfer the clutch to the new cover. You are better off loosening the clutch bolts before you remove the side cover as sometimes these screws have threadlock and are difficult to remove once the cover is off. Make sure you use a properly fitting Phillips No.2 bit to loosen these as they are usually quite tight and are easy to damage the heads if you use the wrong screwdriver bit. Take a photo of which way up the clutch fits before you remove it!

Thanks,
Oli.

 

[khorse]

If the wheel rim is true it will not be bending the dropouts back and forth. The rim's (and ultimately the tire's) relation to the axle is what determines whether you start forcing the dropouts back and forth - not what happens in between.

Sorry no, you can see in the video it forcing the dropouts with no rim or tire, so your premise is faulty.

What you're saying is correct on a wheel with an OK hub but out of true rim, or badly balanced tyre. But it doesn't mean that there cannot be such a thing as a badly balanced hub. We're looking at one.

 

[Comrade]

I made some measurements in case anyone also wants to make an AKM motor cover removal tool.

 

[Comrade]

I've got lots of experience with those motors

Oli, do you have any blueprints (dimensions) for the AKM-128CST? Something that can be used to calculate needed spoke length.

 

[Oli.Hall]

Hi,

I hope this helps:
PCD of spoke holes (H1) 129mm
Spoke hole diameter 3.0mm
Flange spacing (W4) 44mm
Offset (from centreline) 7.8mm
Centre to left flange 29.8
Centre to right flange 14.2

While the specs say you can use 12G spokes, unless you are planning to carry exceptionally heavy loads, 14G spokes will usually result in more durable wheels for a couple of reasons related to spoke tension.

Apologies in advance to JackFlorey, khorse is correct. While you are correct in saying it is possible to true the wheel so that you no longer see the wobble at the rim, it is not correct to say this will solve the problem. Proper wheel truing will be only be successful at covering up and masking the problem at the rim, but it will not cure the problem at the axle and will not prevent continuous flexing of the drop outs and the internals of the motor.



thanks,
Oli.

 

[docw009]

I've got lots of experience with those motors and the problem is not in the axle, or the bearings, the problem exists in poor fitting of the freehub carrier into the side cover.

The CST series motors have a central part of the freehub, around which the freehub body rotates, which is press-fitted, or in some motors it is screwed into the side cover. There was probably a small piece of metal swarf in the hole when they pressed it into the side cover causing your part to be fitted at a slight angle. This part holds not one, but two bearings in the drive side, if this part is misaligned then the bearings exert a strong cyclical bending force on the axle, the direction of which rotates with the motor, causing the wobble that you see with each rotation.

I've got a freewheel version of the Q128, but I figure they're just different side covers. Just curiour. I blew out the speed sesnor. Put 48V onto it. The Halls work fine. I have never installed the motor. When I do, I planned to use an external sensorm but ....

Do you know if the circuit components are easy to access after taking off the cover.

 

[Comrade]

PCD of spoke holes (H1) 129mm
Spoke hole diameter 3.0mm
Flange spacing (W4) 44mm
Offset (from centreline) 7.8mm
Centre to left flange 29.8
Centre to right flange 14.2

Thanks! That's pretty close to what I measured with some calipers. The only difference is I got H1 = 130mm, and 6.5mm offset. That should change the optimum spoke length by less than 1mm.

I found some specs on the manufacturer's website:
https://www.aikema.com.cn/en/product/product-74-730.html

Is it just me or do they not provide enough measurements to lace a wheel?

 

[Oli.Hall]

Hi,

Those measurements don't affect the spoke length, so as long as you set the dish in the truing stand or with a gauge you shouldn't need them to lace the wheel. In case you want them anyway, they can be derived from the other measurements given:

Left 23.7mm
Right 13.8mm

thanks,
Oli.

EDIT - I had used 135mm as the OLD which underestimated the above by 1mm each. Corrected above now for future readers.

 

[Oli.Hall]

Do you know if the circuit components are easy to access after taking off the cover.

Yes, they are all on the same PCB which is screwed to the outside of the motor core. Note these observations are for the AKM-100 motor which I have to hand. The AKM-128 may use different size components, screw sizes, torques etc. so be aware of this.

When you remove the cover and when you take out the motor core, look very carefully for any brass washers that may be fitted on one or both of the motor axles. These washers often get stuck to the inside surface of the bearings. Make a note of how many are fitted on each side and replace them in the same order when you reassemble.

The PCB is on the other side of the motor, so you have to remove the cover, remove the clutch, then pop the motor core out from the shell. Lightly tap down on the other axle being careful of the cable and the motor core should pop out of the shell.

The speed sensor is on the outside. You may be able to replace it without unscrewing the PCB if you are careful. However be aware that it is an SMT component on the AKM-100H. I can't remember if it is SMT or through hole on the AKM-128. However, complete PCBs are available from TBK for only $6.

If you do decide to remove the PCB, you will also need to remove the stub axle. Use the correct PH.2 bit to remove the retaining screws and be careful not to damage the heads. These ones are threadlocked so when you put them back together I always put another drop of blue loctite on them and I do them up with a torque screwdriver. The AKM-74 and AKM-100 are M4x8 and done up to 2Nm. I am not sure if this is the same on the AKM-128. These screws are quite small threads and they are threaded into aluminium and they are easy to strip if you do them up too tightly.

 

[Comrade]

Turns out they did provide enough dimensions to be able to extrapolate the rest. :lol: W1 was the trick.

 

[Comrade]

Well, I took the cover off, and mounted it to a table using axle bearings. It looks like the cassette carrier is screwed into the aluminum side cover using threads. Not press fit. No way it is cross threaded. The wobble would have been much worse then.

Looks like there is 0.6mm between the high and low points, with cap diameter of roughly 120mm.

[youtube]IFFvdEW_QDc[/youtube]

Think the machine that made the threads wasn't holding one of the pieces straight?

 

[Oli.Hall]

I am glad you have been able to confirm the problem as we suspected is a poorly fitting freehub carrier.

Judging by the dial-test indicator that you have, I suspect that you might possibly have access to some kind of machine-shop. If you have access to a lathe, and you can unscrew the freehub (I was unable to unscrew the one I had, but I didn't try that hard!) you may be able to face the side-cover where the freehub interface is.

If you are able to chuck the side-cover accurately, and face the area where the freehub seats, by removing about 0.2mm (given a thread diameter of around 40mm est.) you might be able to improve the run-out at the edge of the cover.

However this is a bodge, and would invariably induce significant asymmetric loading of the threads and once you had tightened it up due to loading in-use, it would likely deform the threads and I wouldn't ever want to try and remove it again! Lol.

Maybe, if you are able to order one, a replacement side cover and freehub assembly would be the best option.

 

[Comrade]

While I had the motor open, I didn't get the idea to run it in my test stand without the side cover. I think it should run at slow speed without the clutch flying off. That would definitively isolate the wobble to the side cover and not the axle. I should open it up again and try it.

I did try to remove the the freehub. Just to look at the threads. Even made a custom wrench for it out of a mild steel pipe of exact needed diameter and welded to an old socket that would fit my impact gun. It would not budge. No idea why it's on so tight. The thread direction would naturally keep it tight all the time, just like the cover. There was no need for any thread lock. But no movement at all.

 

[Oli.Hall]

Sorry, I realised I made a typo above, I was *not* able to remove my freehub carrier, so I was unable to tell for certain whether it was threaded in, or pressed in. Well done on making the tool try and unscrew it! I agree, there's really no reason it should be so tight. I tried to undo one with a pin wrench but it wouldn't move.

One thought might be that the inner bearing gets pressed in after the carrier is threaded into the side cover, which would expand the thread very slightly and lock it in. You could try to carefully tap or press the bearing out and seeing if that made the thread looser. Additionally, as the side cover is ali and the freehub carrier is steel, so you can try putting the whole lot in the oven at 300F for 20min. The ali should expand faster than the steel, loosening the threads. And 300F is usually enough to loosen any threadlock. That said, unless you want to take apart further for educational purposes, I'd really just consider buying a new side cover at this point! :lol:

Running the motor without the side cover may not tell you much. There is no bearing in the middle of the clutch, just a big hole. The clutch relies on the side cover and freehub bearings to transmit torque to the motor shell, so without the side cover, the clutch will just wobble around supported by the planet gears and the motor shell won't turn. But you're right, the axle should not show any signs of deflection without the side cover on.

You would be better served by trying to remove the outermost bearing from the freehub and running it with the side cover *on*. Removing one of the two freehub bearings will remove the vast majority of the side-loading force and should prevent the freehub from bending the axle. Running with the side cover on, but without the outer bearing, would allow everything to turn as it would normally do. At the same time you could observe and measure the eccentricity of the freehub in relation to the axle where the outer bearing would usually be.

 

[Comrade]

Good idea about removing the outer bearing. I will give it a try. If the axle is not bent I'll just buy a replacement side cover.

Hopefully they have not churned out thousands of defective parts that are on warehouse shelves now.

Top Bike Kit Inc. is adamant that there is no problem. They said they contacted engineers at the manufacturer Aikema (AKM) and they see nothing wrong. Paypal refunded me in full, but Top Bike Kit Inc. at the last second before the refund tried an excuse that Shenzhen is on COVID lockdowns and that's why they could not come to a resolution with me over email. I don't know how much of it all is B.S. - sounds like a lot!

They know Paypal is always on the consumer side, and if a client is telling them they sent a bum motor, their strategy is to deny and not offer anything. Very strange businesses practices.

But if the wobble is gone with further testing, this bum side cover is basically a door stop, and I'm determined to take off the freehub.

I just need a wrench made out of hardened steel. It would look exactly like this, but with specific dimensions.



Any ideas if there is an official name for this type of wrench? Would it be universal, or at least have a common size for freehubs?

EDIT: I think this is it ---> https://www.ebay.com/itm/392859431422 :thumb:

 

[Chalo]

Good idea about removing the outer bearing. I will give it a try. If the axle is not bent I'll just buy a replacement side cover.

A bent axle won't make the cyclical movement you see, because the axle doesn't move. The axle might or might not be bent-- you will have to spin it or roll it to find out-- but it is not related to the problem you've identified.

 

[Comrade]

Here is how it looks with the outer bearing removed. Perfectly normal as TopBikeKit said.

[youtube]1OskakwhKBI[/youtube]

 

[Oli.Hall]

:lol:

Are you going to order another side cover?
I'm sure that will turn it back into a good motor!