Bikengineer said:
My current rig has a MXUS motor upfront on 26" wheel. It also has a xtracycle freeradical sub frame extending the wheelbase at rear making it a cargo bike. As such it's tempting to take heavier loads. I'm considering swapping rear wheel for another MXUS motor and running it all with a single throttle. My goal is not more speed or range but more traction and torque. BUT I'd like a 24" wheel on rear which offers lower cargo (centre of gravity), stronger rim and more torque.
Will it work having different sized wheels??
Am thinking 2 batts, 2 controllers, 2 motors, 1 throttle, 1 PAS, 1 dashboard.
I have some experience with AWD and the MXUS geared, although, not an AWD MXUS system.
I did 2 Cute Q100 328 fast wind motors in 24"wheels and a MXUS frt.(26"wheel)and a 328 Q100 rear.
First, some generalizations:
There is basicly two approaches to AWD:
1)Extending the overall system (both wheels together) useful RPM range, ie; adding a faster wind motor to weight the top end of the scale (speed), or adding a slower wind motor to weight to lower end of the scale (climbing performance). This is done with disimular systems, ie; different size wheels, different motors or winds, different battery Voltages and\or currents(Amps). The more disimular the two systems become, the farther the two useful efficiency curves move away from each other and one wheel system falls greatly in efficiency(although that system is consuming less current, which mitigates a lowering of overall bike efficiency).
2)Or, to increase climbing and acceleration performance by using simular or equal systems. The nice effect of going this route, is the extending of both the bottom and top ends of the overall system performance. The amount has much to do with whether the no-load speed(talking top speed here) is a result of motor speed "hitting"the wall", or just running out of power(current limited). On my two Cute build, the very fast wind motors were very much power limited(when used solo) and when used together, they worked together, letting the 328's "wind-out". I picked-up an amazing 5 mph!
I take it that you are are looking at number 2, rightly so. According to the Ebike simulator, the MXUS is motor speed limited, and two working in conjunction would increase the top speed 2 mph, 18.5 to 20.5 mph @ 36V. Of course, power consumption will increase by about a third! D8veh (who has done a number of AWD builds) felt that two smaller motors operating in their "sweet spot" would be more efficient than a larger motor at the same power levels. I didn't see that, but I wasn't looking that aspect of my build and didn't record power consumption data. And yet, for pure economy, I strongly suspect that a single motor would be the champ, even with the slight drag of the other motor. This speaks to the need to deactivate in some way one motor when the conditions are ideal(more on throttles in a moment).
Of course, what you are really wanting is the ability to deal with cargo(precious) and hills. Again, looking at the sim., you can take your steepest hill, halve the grade and see the dreaded "time to overheat" double! Priceless! Efficiency could increase here as the two motors help each other hang into a more efficient rpm zone, but the real benefit is the piece of mind that you are not going to "cook" your motors.
A note on the sim.: Of course, the MXUS is not on the list of motors, but I have found the the Ezee Orig. is a close match. Their No-load rpm speeds are almost identical, but the Ezee weighs almost a Kilo more. indicating the ability to absorb more heat, and corraspondingly, make more power. But at moderate power levels (stock MXUS controllers), this is not really a concern.
This really points out, that you need not match the motor exactly(same model, etc.). In fact, I am currently working on adding an Ezee geared to my MXUS frt. whl. drive MX bike. Battery capacity being the bug-a-boo. What you do have to pay attention to, is the no-load speed, keep them close to keep the efficiency curves close.
On throttles-many ways to do this. Initionaly, I went with the simplest system possible. I simply divided the throttle signal and sent it to the two controllers. It worked, but almost right away, I noticed a "roughness" in the motors that wasn't there with running a single motor. I think what happened was this: The Cute's kit with a relatively cheap controller, which might have some roughness that didn't show up when run alone. but my two identical systems compounded the effect. My sense at the time, was to change something and I went with two throttles and since the motors were no longer in lock-step, the problem went away. Or, it could have something to do with dividing the 5V signal. Perhaps It could have been, that had I wanted to retain one throttle, I could simply run one signal thru the Cycle Anayst . Not sure if this abnormality was particular to the equipment I was using or not. If you want to get into more advanced throttle control, take a look at Telklektik's gorgous AWD Mundo build.
http://www.endless-sphere.com/forums/viewtopic.php?f=6&t=36959&hilit=mundo
Lot's of great ideas there!
Final note on the MXUS:It is a "larger" mini, 2.3 Kg.s vs. 1.8 Kg.s for the Cute and it has nicely tapered covers that allow clearance for brake caliper and disc. It's easy to mount and has a "middle of the road" wind, all of which made it, I.M.O., the best mini out there. But recently, some other interesting larger mini's are coming on line. There is an auto-switching 2-speed(Mofo?)and Bafang's revamped Sxw?... Both of these require wider than the standard 135 mm drop-outs(or some stretching), so If you have wider chain stays, you might look at them. Then there is the new CST models which allow the use of a real quality cassette, instead of a DNP freewheel. There is a Q100 version, but that is too sm. for your app., but MXUS has two, one a little larger than your current motor. If you like to pelal and would like to run a nice 9 or 10-speed cassette, this could be the one for you. The best way to read-up on these new models is to search D8veh's posts, both here and at Pedalic UK.
Edit: I never answered your original question. Running a 24" rear wheel would be perfectly fine. You will loose about a half mile-an-hour top speed, but pick up a little better climbing performance.