Tern Link D8 into Electric (MXUS 350W motor)

[luciorossi]

Hi guys!

This is my first post, my first build and long time since I was driving a bike actually. I want to describe all the steps I went through and the problems I encountered, my likes and dislikes about this build hoping someone can find here the useful information I was missing when I started, and have the chance of asking the Forum for helping me solve the problems I'll be surely challenged with in the future.
But first things first. What did I buy for this build? Here is a list:

- Tern foldable bike model Link D8 with 20'' wheels (you can check the stats on the Tern website which is very well organized)
- qbbikes.es "Efficienced" kit 350W HT PB SS 36V 11,6Ah rear wheel (check the website)
- bike repair toolset (on Amazon)
- threaded freewheel sprockets (8s 13-32t) close to the original (8s 12-32t) (the kit doesn't come with the freewheel. Remember to put it in the order if you buy from them)
- 2 bottle cage universal brackets (Decathlon)
- spiral cable tidy, cable ties, a small file, sandpaper
- threadlocker fluid

Extra tools/stuff you may need:

- Wrenches, Allen keys, Screwdrivers, whashers, bolts, M-screws of different sizes...

Problems encountered:
Motor Choice: It was hard to decide what kit to buy but eventually I decided for the high torque (HT) one since I live in a hilly city, and for a 417,6 Wh battery for good autonomy. My daytrip is roughly 20 km I climb only at the return so I should have plenty of "juice" (hope so). The motor is a MXUS 350W I didn't want to put more power since the bike is a foldable and also I didn't feel like buying on EM3ev or Greenbikekits or BMSbattery where I had to make too many choices (maybe next build). I chose the MXUS since I wanted a Geared motor on the Rear wheel (check electricbike.com to know why... very good website btw)
Freewheel: Qbbikes suggested me to take a "freewheel" motor model and not the "cassette" type (don't know why... probably they don't trust the mxus cassette). Anyway this forced me to change the sprockets (and unfortunately the Link ones are non-standard). I still don't know if it was a good idea or not, but remember, once you screw the sprockets on the motor it is very difficult to go back, if you try you will probably unscrew the motor lid and not the sprockets. I tried to do this because at first I noticed something strange in the freewheel (I'll discuss it later) and I wanted to dismount it but I didn't have any grip or vise to separate the motor stator from the freewheel, which made me upset!
Mounting the battery as a bottle cage was no easy task because the threaded holes on the Link D8 are way too close to the folding hinge. I had to use 2 universal bottle cage brackets (Decathlon) to adapt the battery bracket to the frame (I'll show the pictures later).
Dropout dimension: The MXUS motor is meant for 135mm rear fork dropouts the tern Link has 130mm dropout, anyway the MXUS comes with 2 dropout spacers which are (to my opinion) too big for what's needed, so I substituted them with a couple of washers. It was easier to mount the motor, with less strain on the rear fork, which (I think) gave me a problem in the freewheel (making a strange sound and "biting")
PAS Sensor: the sensor coming with the kit has a transducer which can easily touch the magnetic wheel. I had to bend it a little, but it works.
Dropout eyelets: As in any build I saw on the internet they have to be properly filed in order to fit the motor axle in the fork. I tried to to this as little as possible

And now the first picture: the sprockets mounted on the motor wheel

 

[luciorossi]

Step 2. Mounting the battery

The M screw number one is screwed on the frame the other two on the Decathlon brackets. I needed two Decathlon brackets because the Link D8 frame is quite big.

The second Decathlon bracket is screwed on the frame only with one M-screw. The other is not used (but you can stretch the rubber bands enough to get everything tight). Notice that I used a bolt as a spacer for the first screw (the one threaded in the frame)

 

[luciorossi]

Step 3. The PAS Sensor

I had to bend the PAS sensor a little because it was touching the magnets wheel. Notice that the PAS transducer is mounted behind a little threaded stop ring which goes around the crankset. You don't need to unscrew the crankset (which I accidentally did), only the ring. To securely screw it back on you can use a flat screwdriver covered with a cloth and a hammer (don't be too hard on it)

This is a picture of the magnets wheel. When I tried to put the pedals back on it was touching the transducer. I tried to mount the transducer in any possible way, but I had to bend it eventually anyway.
IMPORTANT: When you screw back the pedals use a threadlocker fluid to prevent the bolts from unscrewing when the crank is vibrating! Actually whenever you unscrew a bolt and you notice a colored stuff covering the thread, that's a bolt secured with a threadlocker, you should put the threadlocker back on, or you will regret it! (same as I did)... btw bluish threadlocker means medium strenght red is high (check wikipedia for color codes)

 

[luciorossi]

Step 4. The Dropout problem

I had to file the dropout eyelets. I thought getting rid of the paint was enough but it was not. When I was "happy" with filing I used sandpaper to get it smoother

Though I did file the dropouts more that I could stand there is still a gap between the motor axle and the eyelet bottom. Anybody has an advice on this?

A picture of the spacer that comes with the MXUS. It is too big. Notice the cable is bent upward. The motor axle has a notch on the cable side. I'm not sure if it should be mounted up or down... if you mount it up (same as here: http://motorboard.ru/files/MANUAL-MXUS.pdf) wiring is easier while down is safer because you can make a drip loop to protect the motor from water.

Washers used to substitute the spacers. I could save a couple of millimeters of strain on the fork. With the original spacers the freewheel was not working properly, making strange noises.

 

[luciorossi]

Final

Folded. The total weight is 18.8 Kg

 

[luciorossi]

I have some questions though...

Do you think the gap in the dropout could be a problem? shall I use a torque arm?

 

[iovaykind]

You want the cable down so you have a drip loop

 

[luciorossi]

You want the cable down so you have a drip loop

Thanks for the reply iovaykind... I've been following this manual:
http://motorboard.ru/files/MANUAL-MXUS.pdf

It is a front wheel build but the axle notch is facing up... I'm afraid a drip loop would hook up anything on the ground, do you think there is risk of water getting into the motor?

 

[luciorossi]

Anybody has an advice about the dropout problem?
I don't know if the axle should really sit on the eyelet base or if I need a torque arm

 

[comradegerry]

I would definitely make some sort of torque arm, before you use it.

I found out the hard way what happens when the axle of a hub motor decides it wants to turn in an aluminum frame

It looks like there is a flat area by the drop out with some holes to mount something neat and tidy.

I would say minimum of 5mm stainless steel plate on the drive side, a little thicker would give more piece of mind if the stock was available.

Make a card board template, then break out the hack saw and the files!

Neat project, very practical.

 

[luciorossi]

I would say minimum of 5mm stainless steel plate on the drive side, a little thicker would give more piece of mind if the stock was available.

Hi comradegerry! Thanks a lot for the advice, I think it's a good idea to exploit those holes, but why on the drive side?

Happy that you liked the project :wink:

 

[doc007]

not sure if you have seen these yet but if you don't feel like cutting you can get these:
http://endless-sphere.com/forums/viewtopic.php?f=31&t=29129&hilit=doctorbass+torque+arm

 

[luciorossi]

Thanks for the advice doc007 the TA look very big!

 

[neptronix]

Yeah it would be quite unnecessary
A torque arm made out of 1/8 inch steel would suffice. You've got plenty of places to bolt a custom one on to.

Given 20" wheels and a low pole count geared hub motor, you should have pretty good power, but not enough to warrant what is designed for 1-10kW kinda builds

 

[luciorossi]

Given 20" wheels and a low pole count geared hub motor, you should have pretty good power, but not enough to warrant what is designed for 1-10kW kinda builds

Yeah, it was the right choice for me I guess. I'm not interested in super speeds, more on climbing ability. Maybe in the future I'll switch to a 500W (not more I guess. It's still a 20'' foldable), but now I'm using it on a daily basis and it works better than what I was expecting.

My concern now is about the fact that the motor is sensorless so the contoller measures the speed only when the motor is on. Do you know any way to put a Hall sensor on a sensorless motor? Or is there any way to adapt a reed switch to "fool" the controller?