Is the "Volt-Up, Gear-Down" Theory Justified???

[999zip999]

I run 55v (61.3v) I would like to bump it up 72v (82v) on a bmc 600hs and 30amp. crsyt. Will be turn down to 20amps. Tomorrow.

 

[The Mighty Volt]

Dogman,

No wonder you continue to melt hubmotors. I've never melted one... not even darkened the color of the varnish on the windings, and 95% of the time I ride a bike with a sealed hubmotor.

After the last debate on this topic, I tried partial throttle on one of my regular climbs. Luckily I stopped to check the motor after just a few hundred yards, because it was much hotter than it gets at WOT on the same climb, and I hadn't even gotten to the steepest segment yet.

Speed is the target not some power level. I could melt my motor at 1000W if I was so inclined, however, if I can maintain 30mph on my primary bike, the motor can handle multi-kilowatts as long as the battery will last regardless of the grade or how much extra weight the bike is carrying. The minimum safe speed at WOT factors in everything but the ambient temperature.

For those playing with the simulator, Justin is still working it, and since it doesn't factor in the fact that cooling is better at higher rpms and higher speed, those overheat times can't be relied upon to make valid comparisons.

This is what I am talking about!!!

Here is how I look at it.......go for a jog. Notice how you break into a particular stride.

Now try to artificially limit that stride, taking shorter "steps" as you move along.

Notice how tiring and uncomfortable it feels??

Your body is designed to jog at a certain rate, a certain length of stride.

Step it up into a sprint, and you burn out. Reduce it to an exaggerated fast walk.......you cramp up.

That's what I was saying about the speed with the X5303. There is clearly an argument which can be made that letting that sucker rip into a certain speed bracket makes more sense than inching along.

I understand the sim is very "2D" as it were, it can't accurately represent everything in a "real-world" sense. But what it does seem to be saying to me is that holding the bike at 15Mph is potentially far less efficient and far more likely to result in overheating than bringing her up to 28Mph.

What struck me then as odd was that 28Mph seemed to be the optimal speed for a 60v system but that 35mph was the optimal speed for a 72v system.....which caused me to query the logic of "Volt Up-Gear Down". LFP then pointed out, accurately, that the motor is in fact ignorant of the system and the voltage it is being fed.

Its obviously a question of not taking the sim too seriously, it needs to be combined with real-world data, which if course is what it was intended for- namely a guide.

 

[miniebiker]

I guess this is the answer.

http://www.youtube.com/watch?v=EZ2_sXwdaiw

Variable gearing is definitely better for hills, but only at moderate and lower power can "through the gears" using bicycle components be reasonably reliable.

What we really need is a proper multi-speed geared hubmotor with proper metal gears and cooling system for the smaller motor enclosed. The reasons we don't have them already are:
1. I'm not a mechanical engineer
2. Ebike design is dominated by pedalists
3. Motorcycle companies either don't understand how fun 5-10kw can be with a lightweight silent bike, or they're stuck in the box with a motor driving a chain because that's what they know. At least we have a member inside one plugging for what's really needed and being ignored.

I have no doubt that a 5kw 2 speed actively cooled hub motor is possible in the 5-8kg range, and if I could, I'd build one to prove it.

Hey, think outside the box and design one that runs cool and does not waste power making extra heat

That is a for sure sign of a well designed system !

And I hear you about bicycle components. Those cost money.

A good chain can cost as much as a Walmart bike.

I think I can design a bracket to hold the geared motor on the bottom bracket.

 

[sn0wchyld]

That is correct. The higher the voltage the higher the speed you need to maintain at that throttle position to avoid overheating. If you're looking at sims, then you want cruising to be at or above the point of peak power to be in the band of better efficiency for the motor, and out of currently limiting to make life easier for the controller. Especially with the higher Kv scooter hubbies, you want to avoid partial throttle under heavy loads like big hills, because it's a quick death for controllers unless you can get above the safe point of speed. If you don't maintain that safe speed, partial throttle just chops of the phase pulses even more, which is harder on the controller.

ive been hunting for a clear answer on this for a while now... do you mean that so long as you get to around 70-80% of no load speed for a particular throttle position then the efficiency will be quite high? or, to put it another way, the efficiency curve (and peak) of a given motor moves according to the position of the throttle, not according to pack voltage?

 

[dogman dan]

I think you just stated what I have been poorly trying to say. John is totally correct that motor speed has to be fast enough or it will make heat. That's why you should pedal your guts out to keep most stock kit motors above 15 mph when climbing a hill ( when used at 36v). Below that speed, they will get hot. In the 5303 example, 15 mph would not be fast enough I think.

But I think you just said the crucial thing. 80% of no load speed , at a given throttle setting. Take a typical 1000w ebike. Ride it half throttle on flat ground and it will go a certain speed, and likely won't overheat. Then go climb a 10% grade hill, ride it half throttle, and pedal enough to go the same speed you went on flat ground. I bet the motor does not overheat. As you pedal the motor fast enough, the load on it drops just the same as it drops as you reach cruise speed on flat ground. The catch 22 is can you add enough watts to get the motor up to speed on that grade. At some point, you can't. But you might be able to lower the throttle some more, and then be able to reach the now slower minimum speed by pedaling. That is, provided the hill is not too steep. For most typical dd motor kits, that critical grade is around 8%

The cruicial thing is not lugging it. This won't work if your half throttle watts is 4 times what the motor was designed for. Nobody can pedal hard enough in that case. Nor will it work if the speed you need to reach is beyond what you can reach by pedaling.

As for pack voltage, changing it will change the curves for the motor. So will changing the throttle position, but in a different way. You can see this if you imput 50% throttle into the sim. In either case, the "sweet spot" will be at different speeds.