2WD logic?

[Buk___]

If you put two identical motors on a bike, presumably, you gain in torque; thus climbing and/or load carrying ability, but you are stuck with the same top speed as with a single motor.

What happens if you combine two differently wound motors, say a "201 wind" and a "328 wind"?

Ie. Does the slower motor hold the faster motor back; or does the faster motor carry the slower motor up with it?

Thanks.

 

[d8veh]

I've run with a 201 rpm + 328 rpm Q100s. You get better accelleration and better hill-climbing. The slower motor maxes out at about 18 mph, so you get good power up to that point, then you only have the one motor, but it's right in its efficient zone so does quite well up to about 22 mph, but after that, it depends on conditions.

The main real advantage of two small motors compared with one bigger one is the lower sprung to unsprung mass ratio that you need for suspension to work properly. You slso get much better traction when the surface is slippery, like wet grass, mud and snow. For normal commuting with a hardtail bike, you don't gain anything.

If you want to have a go at it, I found that the best control system was to use the PAS to work only the back motor, and then split the throttle signal wire to both controllers, so that the throttle works both motors. Only use the 5v from one controller to the throttle and join your grounds if using two batteries. This way, you only need one throttle and one PAS to have a nice efficient control system. If you want brake sensors, connect the front one to one controller and the rear to the other to keep it simple.

Obviously, if you use only one battery, it needs to be able to supply double the current, so 30A for two 15A controllers.

 

[teklektik]

If you put two identical motors on a bike, presumably, you gain in torque; thus climbing and/or load carrying ability, but you are stuck with the same top speed as with a single motor.

Assuming identical controllers, wheels, tires, and throttles that's pretty much true. There is a small top speed increase but generally not a lot on the flat. Clearly when the bike is under load (e.g. hill climbing) the terminal speed will be improved because you are not voltage and Kv limited. (Kv reflects the 'wind' or unloaded rpm of the motor)

What happens if you combine two differently wound motors, say a "201 wind" and a "328 wind"?

Ie. Does the slower motor hold the faster motor back; or does the faster motor carry the slower motor up with it?

You will generally compromise performance and efficiency under most if not all conditions. You do not want to mix motors with different Kv in the "everything equal" conditions set out above.

There are two critical factors in play here

• While the motor is controller limited (i.e. under load like accelerating or climbing hills) the Kv or wind of the motor doesn't matter - the 201rpm and 328rpm versions of the same motor will perform virtually identically. (This is always a major point of argument, but it's true. Read this for the why: http://www.ebikes.ca/learn/hub-motors.html)
  
• While the motor is not controller limited, the Kv serves primarily to limit the top speed. Efficiency, etc are not effected.

So the short form is that for hill climbing the two different rpm versions operate the same, but when running on the flat, the lower rpm version will just go slower with no particular advantage. There are some lesser effects from the controller and battery, but from the motor perspective picking the low rpm wind just means you go slower.

So - re-phrasing your question: "What happens if I mix two motors that perform the same climbing hills, but one is a dog on the flat?". The answer: you throw away speed and introduce control problems because of the different performance ranges. Generally a Bad Plan (see battery current remarks below).

I strongly recommend you play with the ebikes.ca simulator which got a recent upgrade to allow it to do 2WD simulations and adjust the Kv to be able to simulate similar motors even if one isn't in the database.

Here's some sample plots to show you what's up. Here I selected the Q100 328r from the database for motor A and adjusted the Kv down by 0.613 for motor B to simulate a 201rpm version. If we look at the 328+201 combination on the flat we get a terminal speed of 27.3mph @ 21.3A (sum the A+B "Battery Amps" values):
(I didn't bother to resize the images - just hold SHIFT when you click so they will open in separate windows for comparison...)

View attachment 5
The same scenario with two 328rpm motors gives a terminal speed of 30.8mph @ 28A


So what about that huge current increase for the dual 328rpm case?
Well - that comes from the higher speed. If we back off on the throttle to 86% for the dual 328rpm case we get the same 27.3mph of the mixed motor build but at about 20A (i.e. same speed, less power)... hmmmm.



So, the whole hill-climbing thing is the next question.
Doing the same runs on an 8% grade we see:

Mixed motor on 8% grade:

Same 328rpm motor on 8% grade:
View attachment 1
And finally same 328rpm motor on 8% grade with throttle adjusted to 81% to match lower speed of mixed motor case:

Again the dual high rpm motor case outperforms the mixed motor case by using less power to achieve the same speed on the hill climb at only 81% throttle - and there's still more speed to be had if you want to open it up.... hmmmm again.

So one big difference here is in the total amps each system will draw at max speed - the matched motor case consistently out-performs the mixed motor case but the particular high rpm wind that was chosen will draw more current to achieve the higher possible speeds. Choosing matched motors with a lower rpm wind will show similar comparative performance but with a reduced top speed and at lower current. The second big difference in motor temperatures. The thermal performance for this particular motor isn't modeled in the simulator, but if you perform similar experiments with something like a 9C 2705 + 2707 you can see the dramatic heat differences that can crop up in the mixed motor cases because of the unequal load sharing. These problems are very much mitigated in the matched-motor case because of equal load sharing under all conditions.

Like a single motor bike, you need to look at the whole system to make a balanced selection of components. Best to start with the battery and work back to the motors that it can support. The simulator will help clear up a lot of questions. For instance, in the example above, if your battery can't support the 45A of the matched-motor 8%grade case, then those motors may not be the best choice if you need to do that more than occasionally. That said, they are not a Bad Choice, since you can crank down the controller current limiting and just force things into an extended current-limited region where that wind will perform the same as any other wind...

There are many schools of thought for controls and controllers...
If you stick with the "everything equal" strategy" (same tires/wheels/controllers) then you can use inexpensive trap or sine controllers and get perfectly matched motor performance in all conditions without any extra electronics or fuss. Unless you are dirt riding or using really powerful motors where the front wheel might spin out, a 50/50 power split works well (I have 14000mi on such a rig - no washouts, no spinouts, no handling issues). There are lots of options, but I prefer a single throttle for easy riding and a CA3 with a single battery and external shunt so I have unified monitoring and can have PAS that runs both motors optimally. There are few circumstances where running a single motor is more efficient than running both even for easy cruising - so there's really no need to go after complicated controls. I set my bike up so I could run FWD/RWD/2WD with various current limits, etc and in the end the simulator was pretty much spot on and now I just ride 2WD, but I have a bunch of pretty controls that seldom get used....

(Thanks go out to Justin for adding the 2WD and Kv features to the simulator - makes this stuff so much easier.... )

 

[Buk___]

I've run with a 201 rpm + 328 rpm Q100s. You get better accelleration and better hill-climbing. The slower motor maxes out at about 18 mph, so you get good power up to that point, then you only have the one motor, but it's right in its efficient zone so does quite well up to about 22 mph, but after that, it depends on conditions.

The main real advantage of two small motors compared with one bigger one is the lower sprung to unsprung mass ratio that you need for suspension to work properly. You slso get much better traction when the surface is slippery, like wet grass, mud and snow. For normal commuting with a hardtail bike, you don't gain anything.

Thanks Dave. That answers my questions as well as several that I would have got around to asking.

If you want to have a go at it, I found that the best control system was to use the PAS to work only the back motor, and then split the throttle signal wire to both controllers, so that the throttle works both motors. Only use the 5v from one controller to the throttle and join your grounds if using two batteries. This way, you only need one throttle and one PAS to have a nice efficient control system. If you want brake sensors, connect the front one to one controller and the rear to the other to keep it simple.

That sounds very sensible and usable.

Obviously, if you use only one battery, it needs to be able to supply double the current, so 30A for two 15A controllers.

Indeed.

My thinking is that I will buy the Q128 rear motor, which run from my existing 36V battery will be more than sufficient for about 90% of my needs -- around town and on cycle paths -- but it isn't really sufficient for getting out to nearby towns and villages in a timely manner.

My thought is to add a friction drive. I think their howl is not nice for mixed use paths and local roads; but would be tolerable for adding some top end on a roads out of town. They can also be very light, and if I use a separate (possibly LiPo) battery pack and their tiny ESC controller, perhaps 2kg or so would be enough to pick up my pace a bit for 10 or 15 miles return.

My thinking is to use two small RC motors with skateboard wheels attached, mounted to the V-brake pivot points on the front forks to drive directly on the rim; and a third brake lever to provide initial contact and spin them up before the power is applied. The idea is that it would only be used once the hub motor is running out of steam, and only out of town. If the battery and ESC are mounted in a small bag on the handlebars, the whole setup should be quickly attached and removed with a couple of screws, some velcro tapes and dis/connecting the extra brake lever, (There's a lot I haven't thought through yet, but there would be no rush.)

Based on what you've said above it should be possible to pick a couple of motors with a kV that are well into their efficient rpm band at the hub's top speed and still have enough headroom to pick the pace up to around 35 or 40kph which I think would be about right.

Anyway, its half a plan. Thanks for the reply.

 

[Alan B]

It is worth emphasizing here that two motors are a huge improvement over one. Doubling the power makes more difference than one would expect. Additionally, torque controllers like the PhaseRunner automatically match the motors (and share the power) by adding torques, whereas the PWM controllers tend to favor the power to one motor or the other because they are matching their effective voltages which doesn't always match the motor speeds.

Two motors, providing that they are both in their useful power range, tend to accelerate and climb better, and to hold their speed much better than one motor (with half the power). As you go up in motor power the front motor tends to have less traction as the rear motor is lifting the front end, so 2WD works best for low to moderate powered motors. When the front wheel is airborne there's not much help from the front motor.

 

[motomech]

My thinking is that I will buy the Q128 rear motor, which run from my existing 36V battery will be more than sufficient for about 90% of my needs -- around town and on cycle paths -- but it isn't really sufficient for getting out to nearby towns and villages in a timely manner.

My thought is to add a friction drive. I think their howl is not nice for mixed use paths and local roads; but would be tolerable for adding some top end on a roads out of town. They can also be very light, and if I use a separate (possibly LiPo) battery pack and their tiny ESC controller, perhaps 2kg or so would be enough to pick up my pace a bit for 10 or 15 miles return.

I would say to you, IMO, the best way to do 2WD is two hub motors. Adding a friction drive to a rear hub motor denies you the traction advantage of true 2WD. And adding a friction drive to the frt. seems overly complicated and remember, any weight off the center-line steering stem axis, will effect the steering response adversely. My experience is, I always use both systems at some point in my ride, if only to get quick acceleration to squirt across a busy street. So I don't see the need to have a second system that is removable and there is very little penality to adding a sm., low-powered Q100 to the frt. If I wanted to reduce the weight, is could simply remove the frt. system battery, which is the bulk of the weight. But, unless one is picking up the bike, overall weight is not much of a factor on an Ebike.
I have using various 2WD systems for 5 years and can now say, for me, the single biggest benefit to using 2WD is to be able to keep the power low to moderate on the main drive wheel, while having the ability to add the frt. system for times a greater overall more powerful bike is desired. This way, I have a bike where the rider's input can be meaningful like an "assist bike", or it can be more like a mini-motorcycle w/ 2WD. Other advantages to a moderate powered rear drive are;
1)The motor reaction to throttle is smooth, no need to "tame" the throttle response.
2)A simple 3-speed PAS can be used in an effective manner. With lower power, there is less tendency for the motor to try and "rush up ' to a higher speed range when the rider changes the limit speed to a higher setting.

I don't disagree w/ any of the above good info, but, if I may, I would add some of my personal observations using 2 Q100's and a Q100H/Ezee V1.

The top speed can be raised in 2WD as the two motors will "help" each other run higher in the rpm zone. How much depends on the motor speeds. "201" and "260" speed motors will see around a 2 mph increase, while two high speed 328's can increase as much 5 to 7 mph. While this seems like a great way to do 2 Q100 systems, much of the down-sides to using a mini-motor in a big wheel remains, like a very lazy mid range and controller over-heating.
This is also true by trying to combine , say a mid-speed mini and a high-speed mini. While the idea of "spreading" the effective over-the-road speed range seems ideal, the bike will be lazy everywhere and without the second motor helping" the high-speed motor, the 2WD top speed may not be any higher than a single 328.
I have found that running 2 mid-speed motors(260's) to be the best overall combo for me. First I ran 2 Q100H's(very good), then tried a Q100H-frt./Q100C-rear. For reasons of the Q100C not fitting well in my bike, I went w/ a larger Ezee on the rear, both 260 motors;



I use two two completely independent systems, 2 packs, 2 controllers and 2 throttles for system redundency.
The 2 throttles are placed like this;





A conventual thumb is placed inside a left-hand, half-twist. Although I usually use the PAS on the rear, they are very intuitive and easy to use.

Overall, this bike has been very reliable and fun to use. Top speed on either motor is around 22 mph and in 2WD, it's around 25 mph.
The rear main pack is 20 Ah's and the frt. is 5 Ah's. This reflects the fact that I only add the frt. motor about about 25% of the time and the two packs almost always discharge at the same rate.

 

[markz]

Looking at the motor sim using the new "add" feature, I am sold on using 2wd direct drive vs mid drive.
Decided to sell the cyclone locally.
For me it was a simple look at 0W human power, 20% incline and 400lbs on a mtb goes from a single motor ~2kph to 2wd setup 18-22kph. Then looking at the column on over heating, what a difference. I mean even using the mxus 3kw with some cheesy wimpy motor in 2wd drastically helps improve everything.