All Wheel Drive AWD - different wheel sizing

[Bikengineer]

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.

 

[turbo1889]

I currently have an AWD tadpole hinge steer trike that has two 20" wheels in front and a 26" wheel in the rear. I just selected a different number of sprocket teeth combination for the front wheels motors then the rear wheel motors. I didn't use hub motors but rather cyclone gear head motors with single speed chain attaching them to the wheels. All it took was just selecting different sprocket sizes to compensate for the different wheel sizes.

If you use hub motors you could do the same thing by using hub motors with a different winding counts in the front and rear to compensate for the wheel size difference.

Also, for proper riding "feel" it is often best to set-up such that the rear motor is just barely slightly more loaded then the front. At least that is my experience so that the front only normally grabs hard and does the majority of the pulling if the rear breaks traction or when pulling into a turn. That is how high performance AWD internal cumbustion sports cars are set-up (think European style high end performance) with the rear gearing being just slightly steeper then the front with a limited slip hydro-clutch on the front so that when going straight the rear loads down the motor and produces most of the torque but if it slips out then the slightly lower geared front grabs and pulls or when entering a turn and the travel distance on the front wheels is reduced due to corner curvature the front again grabs and pulls but otherwise is generally fairly unloaded with minimal torque up front on the straight with the hydro-clutch allowing the front gearing to overrun the motor slightly.

With two (or more) electric motors front and rear it can be done even more easily. Just basically set it up so that if you put the bike up on a stand with both front and rear wheels off the ground and spinning free and separate speedometers front and rear each calibrated to the individual wheel sizes the front speedometer always reads a little slower then the rear at any throttle setting. That will produce the same kind of feel where most of the torque will be in the rear until it slips out or you start to turn at which point the front wheel will then become dominant or at least equal. Makes for a good feel to the ride.

Or in other words you don't have to perfectly match the motors to each other, rather ideally for best ride "feel" the rear motor will be geared slightly higher and will thus be loaded down more and produce the majority of the torque unless it spins out or you start to turn and then the front wheel becomes dominant and pulls instead of pushes. Really can make cornering performance awesome if you set up the two motors to each other just right so that when you start to turn the torque balance shifts from rear dominant to front dominant and the front motor power pulls you into the turn and then when you come out of the turn it swaps back the other way punching you out of the turn into the straight.

 

[turbo1889]

Oh, yah, I had no problem making one throttle work for all three of my motors just used a couple throttle plug ends for the other two controllers and spliced them into the wires.

 

[Bikengineer]

Cheers turbo1889

 

[motomech]

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.

 

[d8veh]

It doesn't matter what wheel-size or motor speed you use. Obviously, if they're different speeds, one will max out before the other, so above that speed, you only have one motor doing the work. Every configuration that I've tried has worked. I've tried two high-speed motors, two medium-speed , two low-speed and one high-speed with a low-speed. The bikes behaved exactly like you would expect. I prefer to use a single high-discharge rate battery, but 2 batteries is OK if you have space to put them. If you want a single throttle to two controllers, a cycle analyst is the easiest way because it buffers them. More complicated is to make a throttle with two hall sensors. It's possible that you might be able to work two modern controllers from a single throttle. Has anybody tried it recently?

I think you should go with your idea, and let us know how you get on.

 

[motomech]

It doesn't matter what wheel-size or motor speed you use. Obviously, if they're different speeds, one will max out before the other, so above that speed, you only have one motor doing the work. Every configuration that I've tried has worked. I've tried two high-speed motors, two medium-speed , two low-speed and one high-speed with a low-speed. The bikes behaved exactly like you would expect. I prefer to use a single high-discharge rate battery, but 2 batteries is OK if you have space to put them. If you want a single throttle to two controllers, a cycle analyst is the easiest way because it buffers them. More complicated is to make a throttle with two hall sensors. It's possible that you might be able to work two modern controllers from a single throttle. Has anybody tried it recently?

I think you should go with your idea, and let us know how you get on.

<<< It's possible that you might be able to work two modern controllers from a single throttle. Has anybody tried it recently?>>>>>>

Whoever supplies BMS Battery has, you have seen the "Dual-Driving Controllers"?
Somewhat hot for this build though.
I have found that two throttles can work well when a single motor is low-power to bike weight.
I "drive" the rear motor more or less with the cruise control, just leaving it on and adjusting road speed with the frt. wheel motor.

 

[Bikengineer]

Thanks again for the well thought out replies. Having used the internet since '94 I'm still amazed and delighted that folks take the time to share and answer the Q's of strangers in other countries. I'm going to get the 24" rear MXUS and start experimenting. Fortunately I have some time before my cargo (6week old twins) are big enough to fit in Yepp seats.

The tele Mundo build is a good target.

 

[dogman dan]

I've often thought about a dual motor but never done it.

My simple dog thoughts have been that a slow wind gearmotor on the front, then a faster wind, gear or dd motor in back might work well.

The idea is, both motors pull at low rpm, and at least one is pulling more efficient. But then at fast cruise, the single fast motor does it all, and the slow motor just freewheels.

That slow motor will still spin under wot, but at no load when the wheel overspeeds the slow motors no load speed.

The reverse, the gearmotor faster than a dd, would make the dd run faster than no load, which could make some drag? It would be fine I'm sure though, if the motors or wheel size was not so far apart.

I was thinking like, fast motor paired with an extremely slow motor, like a Mac 12t.

 

[turbo1889]

Well, the purpose of my AWD cargo trike was for winter traction on snow/ice/slush/etc . . . running studded snow tires on all three wheels and when loaded with cargo up front all three wheels having about the same amount of weight on them.

For that I did find that there was a difference in how the bike "felt" and how controllable it was as far as the options of "Faster motor in back, slower motor in front", "Slower motor in back, faster motor in front", or "Both front and back the same or darn close speed". I did find that having the rear motor slightly but not a whole lot faster so that the front motors would be further up their curve with less torque or at top speed even just mainly freewheeling on the straight but would catch and pull when the rear slipped out or when entering a turn due to the curvature gave me the best handling "feel" when navigating slick winter road conditions using the AWD for that purpose. Granted it is a tadpole trike with hinge steer which is different then a normal two wheel bike so the same may not hold true for a normal two wheel bike.

Obviously, since I didn't use hub motors but rather three chain drives off the cyclone gear head motors I could experiment more easily with different set-ups just by swapping around sprocket sizes. With hub motors not so easy to adjust the speed variances between the front and rear motors and experiment with multiple set-ups.

As to the throttle issue, I just spliced together wires to all three controllers (Kelly) off of one throttle. It works, maybe it ain't supposed too and that ain't the correct way to do it but it worked for me, but maybe I just got lucky.

 

[dogman dan]

That jibes with what I was told when I was considering dual motors for the racetrack. I was told make the rear slightly more powerful than the front.

Mostly so it would not start the power sliding first, so the rear would break into a skid before the front would.

 

[d8veh]

If you have 250w or other low-torque motors, it makes little difference what motors or where you put them, but when you use higher torque, like a 500w geared motor, you have to be more careful. The torque from the back motor lifts the weight off the front so that the front spins and skips. You need to find a way of reducing the torque of the back one on take-off.