qwerkus said:
Title sums it up: what's the most powerfull hub motor under 4KG currently available - APART FROM THE XIONGDA 2 SPEED ? That one does not fit into my aluminium (disc only) frame.
I'm surprised no one mentioned the Aikema 128 in this thread.
- 2.8kg Two basic versions, 500W and 800W "H"
- Sold by BMS battery as Q128 and Q128H
- double planetary reduction, my 128H is 10.66:1 (same as Bafang G310)
I found motor data at http://cyclurba.fr/forum/308685/q128h-courbes-puissance.html?discussionID=14329 I put the data into Excel and charted it:
View attachment Q128H Report.pdf
It took me a while to understand these charts, but once you do they are amazing. To do the test they apply full power to a motor and steadily apply an increasing torque load over a couple of minutes, you can follow the test moving from left to right. At the far left is no-load rpm, the rpm where the voltage applied is balanced by the reverse emf of the motor (think of this as the motor acting as a generator, this is a basic aspect of these motors (Kv) and has a lot of consequences in use). Most motor reports only show the 'linear range' before the motor is loaded so hard that efficiency drops, here is a typical example
View attachment Q100H_Curve.pdf
Linear zone charts don't show what happens when you are climbing a hill and the motor bogs down, but that part is where you burn out your motor, that is the part you need to see so you choose the right motor speed. The Q128H report is unusual, it has it all, increasing load until the motor stops. It really helps me understand how small hub motors are toasted, as they bog down brushless motors keep generating more and more torque. So they are helping you climb a hill, all seems good, but too many watts are going into heating the motor and if you load it this way too long you permanently damage the motor. Often the magnets magnets go first, they partially de-magnetize when they get too hot, now you have a weaker motor that you will overload, they get even hotter, etc.
Motor overheating is the bugaboo of light hub motors, to visualize that I plotted the heat produced by the motor (electrical power into motor minus shaft power out). The difference between these two is heat that has to be shed. Again, no good data on this, as best I can tell a hub like this can continuously shed about 140W, so if you were climbing a mountain pass for a full hour you wouldn't want to use more than 24N-m or so (500W shaft output). On the other hand you have 50N-m on hand to cruise over a nasty steep hill at a good clip (but I wouldn't to push it that hard more than 5 minutes or so).
The closest competitor seems to be the Bafang G310, I can't find a G310 motor report so I can't directly compare them, their internal constructions is very similar, so are their weight and size, so their ability to shed heat should be similar. The main difference is that the Bafang G310 is only available high rpm ("standard wind" is 400rpm at 48V). It is good for speed, but if you slow down to climb a hill the motor rpm will drop to a level where the motor efficiency is low and the motor can overheat quickly. For hills you need low gearing so the motor spins at an efficient rpm when you climb a hill.
The Aikema is the opposite, you can get fast and slow versions of the 500W 128, the 128H is low rpm only. That means it is a torquey climber but assist will fall away as you hit 17-18mph on 700C wheels. The 800W Q128H is freewheel only, the 500W Q128C is freehub. The 500W Q128 seems to be more efficient in the linear zone but efficiency drops faster as it bogs down.
As far as I can tell if you want the most powerful hub motor under 4kg the Bafang G310 and the Aikema Q128 are the two players, and if you are climbing hills and need a lower geared option to avoid toasting the motor Aikema is it.
https://endless-sphere.com/forums/viewtopic.php?f=3&t=97415 has more info on the Q128H
