Bumpy off road riding is near the top of the list of applications where you'll benefit from less unsprung weight. Here's a page put together by an "expert" who does motocross suspension: http://www.mb1suspension.com/#!unsprung-weight/c15fs In particular:Punx0r wrote:Saying "less unsprung mass is better, therefore mid-drive is better" is a gross over-simplification. You can't judge the overall suitability of a system based on one axiom.
On technical subjects some people know what they're talking about and some people think they know what they're talking about. The latter tend to draw sweeping conclusions based on a single factor, while the former have the knowledge and experience to arrive at the best compromise that works in a real-world application. This is usually non-ideal in most or all factors. That's what you pay an expert for: google will tell any idiot in 10 seconds that "unsprung mass = bad for handling", the expert can tell you whether or not this is relevant in a given application.
Anyway, you don't need to be an expert to understand that, just have some riding experience and spend 20 seconds thinking about a wheel interfacing the dirt. Since the wheel and attached mass are forced to contour directly over the ground they'll be able to do a smoother and more consistent job with less mass. Anyway, even on relatively smooth tarmac tracks race teams have been putting huge effort into making things like carbon wheels and aluminum sprockets.MX site wrote: Factory teams spend BIG BUCKS on lessening unsprung weight. Most works bikes have magnesium hubs, titanium linkage bolts, Ti axles, aluminum spoke nipples, tapered and butted spokes, titanium shock springs, carbon fiber chain guides and magnesium (or beryllium) brake calipers.
No. The benefits of superlight wheels are grossly overstated. Make them strong and reliable first, lightweight and/or aerodynamically efficient second.Punx0r wrote:Should I buy some fragile, expensive carbon wheels and the lightest, skinniest tyres I can find because I will automatically feel the benefit because the unsprung mass will be less?
Yup, look at the mess you made!I have started this discussion, and look how it developed
It's true the speeds would be slow, and I know DD motors are not efficient in low speeds. But just like you can use a DD while going on flat at 15Km/h or so, and it can be efficient at those low speeds because of the low power needed - if there is a second motor that is helping - it wouldn't be enough to allow slow climb together? Each will carry half the load.Alan B wrote:Power is not the correct way to evaluate DD hubs for tough off-road terrain. Power comes into play when the back EMF rises at higher speeds, but at these low speeds, you cannot put that power into torque efficiently, and with the terrain these speeds are not transient, they are nearly continuous.
Instead we need to look at torque and heat production at low speed, and the motor current at the first knee in the saturation curve that determines how much motor current we can use efficiently.
For example, a 9C 10 turn hits the first knee in the Kt graph at about 48 amps, (so 480 amp-turns for other windings of these motors). Above that level of current the Kt (torque produced per amp) drops quite a bit. So you don't want to operate above that current level for very long, it is wasteful which translates to lots of extra heat.
With a 200 milliohm resistance and 48 amps we have i squared r heating of 461 watts, and the torque produced is about 90 Newton meters. So if you need more than 90 N-m you need a larger motor (or a second motor), or you need the gearing to make 90 N-m be adequate (smaller wheels, for example). And you need to dissipate almost 500 watts all the time you are producing this torque. That's a fair amount of heat.
Oh, you were talking about the motor constant. I thought it was something else, because Justin told me once that on E-bike motors, in order to reach magnetic saturation, you would need to force so much current that the I2R losses would burn your motor in seconds.Alan B wrote:Look for the thread by justin_le on motors where he makes those measurements and plots. (Any thread by Justin is worth reading). Kt is torque per amp of motor current, as magnetic saturation begins the torque per amp changes slope. Operating above that kills efficiency. The heat produced by the motor is a function of the torque, from i squared r. Low torque, low heat. If you know the dissipation capacity of the motor you can calculate the continuous torque that it can produce, and the limit of motor current that you can use. Here is the thread:
https://endless-sphere.com/forums/viewt ... =2&t=14494 Justin's thread on motor torque vs current, actual measurements
It is not the speed that causes the inefficiency, it is the motor torque required. You can toodle along all day at walking speed if the torque needed is low. But as the gradient increases the load the heat generation rises with the square of the torque required.
For a given torque, two motors doubles the heat dissipation capacity, and if each motor delivers half the torque the heat in each motor is 1/4 as much. It makes a particularly huge difference if this torque is near the stall torque of one motor. I did pretty much this exact experiment recently with the Bonanza2WD, and the heat went from nearly melting a single 9C motor (it was very hot for hours) to barely warming a pair (one was a BMC gearmotor, so not precisely the same motor, but the result is the equivalent).
A rule of thumb for DC motors is peak efficiency is at about 10% of motor stall torque (not speed). So you want a lot of torque capacity, but only to use a little of it. It makes for great launches, too.
I see.Alan B wrote:Justin clearly shows the saturation reducing the incremental torque production graphs from his measurements. No speed was involved, and he compensated for temperature effects.
It is true that this current is high and leads to lots of heat, so you cannot operate there for long periods of time. People have put 10kw into these hubs and done it, but they heat up rather quickly if done for very long.
It is also the case that on motors up to at least the 9C size that folks operate in this current regime quite often, for short periods of time. Like on every aggressive launch. For example, 480 amp turns is about 48 amps of phase current on a 10 turn (10T) motor, and this definitely is easy to reach during initial launch. On a 6 turn (6T) motor this is 80 amps, and when the motor is starting out there is no back EMF and it only takes a little battery current to generate 80 amps of phase current.
For example, if you have set your controller for 40 amps of battery and the standard 2.5x phase current multiplication factor then the phase current would hit 100 amps on each aggressive start, long before the battery current hits 40 amps. These are common settings. They generate extra motor heat long before the bike's speed is high enough to have losses from wind. These are merely facts that one might want to consider when setting the maximum phase current value.
In my case the PhaseRunner can easily reach magnetic saturation with the 10 turn motor, and probably with the V4TT BMC as well. So I may not want to allow the max phase current. It depends how much you care about efficiency during launch vs torque, and heat in the motor.
Here is one example of magnetic saturation that he measured: