rieju smx conversion project

[ben--b]

Hi,
I've seen several 50cc conversion topics on ES that seemed very attractive to me . So I decided to give it a try myself.
I bought for a few bucks a 50cc moped with a dead engine and removed all the guts.
Found on ES 2 PEHV battery module to make a 72V 30Ah pack
And purchased a QS138 3kw middrive with votol EM150 which seems to be a popular combo for this kind of conversion.
Here is where I am ATM and I wanted to open a topic to share my thinking about motor mounting.
Everyone fixes the motor to the frame at a location close to the ICE motor it replaces. But I want to try another solution by fixing it to the swingarm , kind of similar to the swingarm QS sells

except I would place the motor in front of the pivot and not behind as QS
I would lasercut 2 steel plates and weld them to the swing arm as on this 3D sketch. I think it can make a stiff attachment.

Here is the layout of the Rieju. I would only have to modify the lower frame by welding some square tubbing to receive the battery

Advantages and drawbacks of this solution, at least as I see it :
advantage :
- easy motor install outside of the bike
- Position of the motor sprocket for a correct chainline is necessarily OK, in line with the wheel axle and swingarm pivo. No need to place the sprocket at the exact same location as original motor
- no need of chain tensioner or lower guide as chain line dosen't change during suspension movement. It would be easy to cover the chain or replace it with a belt
- motor torque and chain tension affects less the suspension behaviour compared to a motor fixed to the frame (in my understanding, maybe I'm wrong ...)

drawbacks :
- it adds unsprung weight but close to the pivot so much less rotating inertia compared to the wheel itself
- it requires clearence inside the frame for the motor movement, so less room for battery
- motor power wires movement may cause their failure on long (or short) term ( but if it's OK on QS swingarm ...)

So what to you guys think of this option ?

 

[SlowCo]

Good idea and worth trying! :thumb:
It seems to counter the unsprung weight of the rear wheel so the shock will "feel" less weight compressing but more in rebound. So you might have to change or revalve and respring the rear. Is the room left in the frame enough to house the battery pack?

 

[ben--b]

For sure the suspension behaviour will be affected but difficult to predict how much and in which way to anticipate the modification on the shock. Field testing will tell ...
I'll have to modifiy the frame to have enough room for my pack, but only the lower small tubbing part of the frame. There should be just enough cleareance also for the front wheel during fork compression.
The battery is quite big and heavy for it's small capacity but beeing a PEHV battery, it can draw a fair amount of amps. And I bought it cheap which helped the choice ...

I would like to anticipate the gearing ratio but I'm not sure how I can trust the figures given by QS. I think the most reliable way would be to compare to what max speed others QS138 users get with a given gearing ratio / wheel size / field weakening.
Thanks in advance for QS138er's who read this post to tell me ...

 

[larsb]

Some data in my rieju thread.
Haven’t had any issues with the belt and the motor attachment - i am not using tensioners or swing slider.

I think you’re solving a problem that might not need solving with the swing mounted motor, my motor position is the same as you’ll get.

Speed is 110 km/h with 22/105 gearing on 17” wheels.
(Field weakening included)

Acceleration is hard with this gearing but not crazy, i’d like more so for the crazy rider a gearing for something like 80km/h would fit better

 

[ben--b]

Thanks Larsb for the info.
I missed your Rieju thread as it was in e-bike section.

I was aiming at 90km/h speed target with FW. I found a huge 70t rear sprocket so could choose a 12 or even 11t front. I'm afraid it will be very loud with such a small sprocket ... umh, maybe I'll directly switch to belt transmission, even if it was not my initial plan. I saw 144 tooth 8m belt pulley at reasonnable price (75€). I'm just not sure of the belt width (25mm, 30mm ?) with the original motor sprocket and can't find the info on QS website. Can anyone confirm ?

I bought the same LG battery as you did, probably from the same guy. I was wondering if it was safe to use it at 150 amps max without the liquid cooling system. Did you notice any heat built up ?

I'll stick to the swingarm mounting mainly because of the ease of implementation. I'm surprised you did'nt face tension problems with the belt but it's fine, I'll switch to the belt option with more confidence 8)

 

[larsb]

I got this belt, it’s 20mm wide
https://www.keilriemenexpress.de/8mgt3-1760-lw-20-mm-powergripr-gt3.html

I don’t have a temperature sensor at the battery but i think 150A will be ok, i’ve used mine to 300A and the voltage sag is not huge.