Hub drive torque limitations. Steep hills while towing a child?

[danielrlee]

*sigh*

I'm not interested in getting into an argument over this subject as it's already been discussed in depth on this forum. The following threads make good reading if anyone is interested:

Science, Physics, Math, & Myth
Try and look past the lengthy and unnecessary first post as the thread was started as a continuation of an argument from another thread. The easier-to-digest info starts at post #3
https://endless-sphere.com/forums/viewtopic.php?t=64907

higher or lower kv
https://endless-sphere.com/forums/viewtopic.php?t=83838

 

[ScooterMan101]

For me several reasons,

The main one is that I started out 5 years ago with a high speed low torque hub motor ( Mac 6T ) and live in a very hilly area , with a couple mountains near by.
and
I wanted to try out different voltage battery packs because sometimes I want to go fast ,
sometimes go up a hill slower .
I did go up a long and steep hill with the Mac 6t , at different throttle stettings , on a 48 volt ( 13s pack ) and it melted solder in the motor and I had to get the motor fixed .
Then
My budget only allows a couple of hundred dollars at a time for batteries , so around 4 years ago Hobby King would have sales on Lipo battery packs. I would buy 6s , for a 12s pack and 7s for a 14s pack and 2 8s packs for 16s , each at different times over the course of 3.5-4 years .
In the last year I bought 4 5s packs to experiment with , so I can do 10s or 20s
( I tried 20s on my mid drive/30 amp controller up a long steep mountain and it did not work well , even using lower gearing . )

With 4 different size packs , I want to get use out all of them.

Note : I have been watching Hobby King regularly and for the last nearly year there have been No , sales on packs that are 5000mAh or larger . And Prices have gone up on other items there , so perhaps Cheeper Lipo packs are more rare these days .

For me going up hills on lower throttle settings just does not work, the motor bogs down much faster that way.

Perhaps it would work like you say if I were to buy controllers that go over 30 amps ? even the 40 amp controller that I had with the Mac 6t , although to save the motor I had the Cycle Analyst set to 35 amps max.
Now that that is sold and I am using a DD rear , and Mid Drive I want to modify the 30 amp controllers to get more amps out of them.

? @scooterman101

why not just use the higher voltage battery and lower throttle to get the same motor operation you would get from a lower voltage battery?

i can't understand what you would get by actually swapping batteries simply to get a higher or lower voltage to get a higher or lower motor speed.

the controller converts the one to the other based on your throttle usage and whatever it's programming is so it is completely unnecessary.

 

[dogman dan]

Mac 6t a spectacularly poor choice for climbing steep hills, unless the rider is very very light. I pound the crap out of a similar motor in 10t wind on fairly steep singletrack, and have never melted one, till I put 400 pounds on it. Riding trail, I weigh out at about 260 pounds, and have found it impossible to melt this wind. Typical grade on these trails is 8%, but in places 16%. Of course, I do pedal hard on the 16% bits.

RE overloading, its any combination of grade or weight that slows the motor to an inefficient rpm. So with the fast wind, it can slow to an inefficient rpm that is still pretty fast riding. While the 10t can go slower under heavy load without being quite so much making heat instead of motion. I may not fully understand windings, but I have taken many motors out, and deliberately overloaded them to the failure point, so that E bikekit could set limits on the warranties. The basic results of this testing, was proving that on the same type motor, given 1000w, lower speed winds could stand more load than faster winds. Basically, both would slow to about 10 mph with a 300 pound load up the 3 mile long, 6-8% grade test hill I used. But the slow motor made less heat by the top than the faster wind.

But I never convinced Jason to stop calling the low rpm motor a torque model. It just feels torquey, because it gets to speed quicker than a fast wind at 1000w. Once overloaded, it does produce more torque at a slow rpm, because it makes less of the 1000w its getting fed into heat. But again,,, and again, and again,, max torque of both motors is at 0 rpm, and both are same at that time.

I never did melt test a wind as fast as a mac 6t though. Rode an 8t once, and it felt sluggish to me at take off, but it was fast once going. Fast enough, it won a trophy in a race against a big field of gas bikes. No hill on that race track, so a 6t would have done even better.

I may have said enough to totally confuse, but lets finish with this. That 6t mac is not a bad choice for hills, just for very very steep hills. On a lesser grade, like 5%, it will run up it fine. Its a matter of grade, weight, and the power its given. So limited to 1000w, and assuming at least 250 pounds total weight, its going to struggle on 8%, but not at 5%. At 5% it will climb that hill faster than a 10t.

Back to the OP,,, he wants to ride a motorcycle alone, and still tow his kid up a steep hill.. A motorcycle / moped power level, large motor will do this easy as pie. Again, I recommend a 48-72v controller, so he can play with voltages. And a nice fat powerful motor, of any wind. NO NEED for him to chose a slow wind, if he choses big power. He'll simply never be overloaded.

 

[st35326]

And a nice fat powerful motor, of any wind. NO NEED for him to chose a slow wind, if he choses big power. He'll simply never be overloaded.

This is the best answer. For pulling 4-500lbs up a grade and going 45 mph simply design in a bigger hub motor something like MXUS, Crystalyte 5304 or similar. Not many controller/battery combos or hills have the ability make those motors sweat. I run two of these motors on two different bikes at 5.5kw and 7.5kw with HV packs and flux weakening. Both bikes easily beat my Sur-Ron up every hill weve raced up.
As much as I hate to say it though, for what you want to do and after factoring in the cost of a big hub, controller and battery you may be better off just getting a Sur-Ron. Eco mode for towing your kid while sport mode should get you to about 45 mph (GPS not the lie-o-meter).

 

[john61ct]

Aside from the extra weight,

is installing bigger / more powerful drivetrain to handle **infrequent** but very heavy loading

going to impose much of an energy inefficiency hit when only running at say 20-30% of peak current?

Not talking high speeds in any case, just variability in weight carried and/or grade of slopes.

 

[donn]

I would also point out that there are two speed hub motors that give you a low and a high gear.

Thanks for pointing that out! Though evidently not new at all, it's new to me. From a casual web search I get the impression that Xiongda owns this category, and populates it with relatively low power hubs, which makes sense.

 

[amberwolf]

yes, though there are threads about retro-direct types of drive that include ponderings by varoius people, dunno if anythign was built.

 

[dogman dan]

Other than the effect of just the weight, running a big fast direct drive motor at low speed is quite efficient. So any extra load is pretty minimal, an extra 10 pounds of motor in a 250-300 pound total weight. A bigger motor can mean an extra 15 pounds of battery weight, but still not a very big percentage of the total. And again, the big motor is NOT overloaded.

There can be some aero penalty as well, if the bike itself is set up for big cargo, or the battery alone requires panniers. but that has nothing to do with the motor size, but with bike size. I personally found that at low speeds, a very large pannier cost me about 100w at 15 mph. Still able to cruise 15 mph on about 300w. 20 mph took about 800w.

But a big motor does not run inefficient when the speed is low, and load is less, at all. Its not like a V8 vs small engine car. In fact, in stop and go riding, the big motor gets to efficient rpm so fast, its actually quite efficient compared to a small, and high rpm winding DD.

Even at very very low rpm, the motor goes inefficient only when the load is heavy relative to its magnet size and copper power. It pulls more so easy, it does it efficiently even at lower rpms, because it is NOT overloaded.

 

[pullin-gs]

5% is not that steep.
You could also build a smaller wheel (I use a 20" wheel) if you want to reduce load on your motor/controller/battery combo at expense of top speed.
Run some simulations on ebikes.ca and see what kind of performance and motor load you see for various combinations.
You can adjust weight, grade, motor, battery, wheel, and controller information and view results (power-load, speed, etc.).

 

[donn]

Other than the effect of just the weight, running a big fast direct drive motor at low speed is quite efficient. So any extra load is pretty minimal, an extra 10 pounds of motor in a 250-300 pound total weight. A bigger motor can mean an extra 15 pounds of battery weight, but still not a very big percentage of the total.

It's probably too early in the morning for me to expect to understand anything, but now I'm wondering, for a given level of performance and a more efficient solution, why the extra 15 pounds of battery? If I'm required to throw 2000W at that big motor to get it going, then I'm not sure that's what "efficient" means to me.

 

[dogman dan]

Just assuming you need a big battery, if you run 2000w continuous, or it doesn't make it to the top of a rocky mountain pass 10 miles long. Or across a desert 60 miles long. I tended to pack 2000 wh of battery on my high powered, high weight, high drag cargo bike. This means about 25 to 30 pounds of battery alone. My lighter bikes with no trailer or cargo boxes, carry more like 8-10 pounds battery. So yeah, that rig did mean more battery weight, about 15 pounds more.


So to charge, you'd connect one of the 8s packs bigger wires to an RC charger, and if balance charging, connect the balance plug. Charge one half, then swap to the other half and charge.

Use the balance plugs, or the original bms plug to adjust individual cell groups occasionally, if you just bulk charge the 8 s sections.


I do tend to think as if everybody had a 10 mile long 8% grade mountain in their backyard, because I do.

Re the 5% comment above, no 5% is not that steep. But using a geared motor and 400 pound load, I have fried a motor very quickly. 26" wheel. A similar rated DD motor easily makes it past the 5% part of my test mountain, but its very hot and at risk of melting at the top, where its 8%.

So what I'm saying, is if the hill is 5%, then a typical 1000w kit dd in 26" wheel can handle it at 400 pounds, where I put the load limit at 300 pounds for 8%. Its really just a matter of whether the combination of weight and grade and wind will slow the motor to less than 12-13 mph in 26" wheels. Once you lug it to 8 mph, and ride for 20 min, the motor is about to melt. So you do get away with it on short hills, for 10 min.

20" wheel helps a lot, you can add about 100 pounds to the load limit of these 500w rated motors if you run a 20" wheel. Same lugging at 400 pounds, but the rpm is higher because of the smaller wheel. This makes a 10 mph ascent of a mountain possible, where 15 mph is the goal for 26" wheels.

 

[e-beach]

......cabike.com ......

cabike.com?

 

[donn]

cabike.com?

I bet that's a scramble gram for ebikes.ca.

 

[qwerkus]

For what it's worth: I hauled 2 kids through a hilly city for 2 years with the tiny YTW06 laced into a 28". Wasn't fast and the climbs would last max 2min, but it worked. Thankfully I learned a lot since. Now we live at the foot of real mountains and it's definitely too much for that motor. Yet that experience with the small xiongda (which I would NOT recommend) made me considering hubs again. I'm now convinced that there is a hub motor config for every need

As others said it already, if you want to climb hills with hub motors, the trick is reducing wheel size together with increasing motor mass. First one increases torque, second one decreases overheating.