The Ideal for a moderate-powered ebike

[xyster]

Remove PWM and you virtually eliminate the problem of excess torque.

Remove the weakest link -- the multiple gears, balls or what-have-you, replacing the transmission with a motor that has enough power over the whole powerband in the first place, and you virtually eliminate the problem of excess torque.

 

[safe]

Remove PWM and you virtually eliminate the problem of excess torque.

Remove the weakest link -- the multiple gears, balls or what-have-you, replacing the transmission with a motor that has enough power over the whole powerband in the first place, and you virtually eliminate the problem of excess torque.

But think logically for a second...

Gears can provide any torque you want at any time. (at near "ideal" efficiency)

PWM provides torque in a part of the powerband that is TERRIBLE from an efficiency and heat buildup standpoint.

So if you remove PWM (and Fechter said that some controllers can do this) then you basically eliminate a "Rogue Wave" situation of low rpm's combined with pedal powered peak torque. Factor out the "Rogue Waves" of PWM and you have a hub that can't break.

There are NO benefits at all to PWM for a geared bike... (think and you will "get" what I'm saying because I'm digging through the numbers all the time on the spreadsheets and it really "jumps out" at you)

Take a second look at the chart... the PWM literally TRIPLES the torque in an area that is "useless" for a geared bike because with gears you can upshift or downshift to get things back into the "sweet spot" on the right. You never want to be on the left. Never.

 

[Reid Welch]

(snip)Now as long as you back off the throttle when you shift(snip

No! You did not read the specification, you're missing the basic point:

The Nuvinci DOES NOT REQUIRE BACKING OFF THE POWER FOR SHIFTS
THE whole point of my aim is to have ratio changes, seamlessly, under full power as we may please.



I'm frustrated, I'm sorry.

 

[safe]

You catch us all on a technicality... okay.. it was a cute idea...

 

[safe]

A final note for tonight about the "PWM Issue" as it relates to torque and hub damage:

The "easy" way to relieve torque is just to gear the bike upwards. A taller gear ratio automatically eases tension on the hub and protects it from breaking. But there are times when it would be "nice" to run really low gears and be able to climb a hill that is 15% and still be running at the efficiency peak. In a case like that you could climb all day long and the motor would not heat up much and it will do it without much of an energy drain.

If you are running a motor alone... no pedals... then the torque is very "controllable" because you can simply run the calculation and know what the peak values will be. But when you combine the motor with pedal power you all of a sudden find the "nightmare scenario" of low rpms combined with PWM (which triples torque) and at the same time you have the potential of very big spikes coming through because of the pedal activity. When these "waves" of energy are allowed to align all at once you get enough excess torque to break something.

You can in these rare cases get 4-5 times the torque of the motor running in it's "happy" efficiency zone...

 

[Reid Welch]

You catch us all on a technicality... okay.. it was a cute idea...

Hello again.

Not a "technicality" and not a cute idea,

The matter is that the NuVinci (or other CVT transmission) does not create shocks to the system.

The electric motor armature or rotor has vast instantaneous -force-.
A step-shift, such as by conventional gear hubs, make an instantaneous change in gear ratio, requiring either a spring-buffer (human legs are that), or a slip-clutch of some sort, OR we must back off the motor power, at the least, for the shift.

The NuVinci wouldn't need any babying; needs no extra operation steps,
it would just be velvet and efficient and kindly to all of the parts.

That's the essence here. All other discussions, such a "no need for gearing with PWM power" beggar the point. We were speaking of mechanical torque conversion for ebikes, not electronic.

But yeah, OK, if you guys think the "torque multiplication" of PWM powering is equal to, or better than, mechanical torque conversion,

I am looking and listening, yet unconvinced on that point's superiority
as it might apply to my currie/Unite powered bike.


I do know that my PWM controller with 36V will slog the bike up pretty steep ravines at the local sinkhole park
...and draw 30+ amps in doing-so.
And this is fairly termed lugging a motor at near-stall speeds.
And it has no back emf to speak of at such low rpms. And it sucks current
and heats like mad.

But if I gear down like we would for a -manual bike- or an ICE engine,
then the motor is free to spin and be happy holding back some of that current, while still developing HP -by the usual means-.

My little gearmotor needs to spin to be both a billygoat in the sinkhole park, and a 25mph road warrior;
it simply requires changeable drive ratios for the real world of mechanical drive.
It is all, after all, mechanical and simple. It's not an X5 or other such big motor. It's a buzz motor, whereas the big BL hubmotors are like 16 cylinders in comparison.
PWM torque conversion 'magic' may work better with them, than with this tiny brushed motor. See, I only concern myself for now with what I have (a modest, cheap, Unite motor), and how to best exploit its capability.
The NuVinci drive, for my needs, is clearly the path to explore.

I hope the Fallbrook people reply to my inquiry with good news regarding a tandem-input NuVinci:

"yes it can be done...and we'll do it in time."

I won't hold my breath.

 

[Matt Gruber]

if u r willing to gear it down and go slower
1st try a switch from 36 to 24v

 

[Toorbough ULL-Zeveigh]

I'm tempering any enthusiasm over the Nuvinci until I get some feedback oboot it's durability. This TX strikes me as working on a similar principle to a roller clutch, also known as a one way bearing. Tho instead of cylindrical rollers the Nuvinci is using spheres. Tell me if my take on this is way off base but don't both rely on some sort of friction grip or binding to clutch?

I have a couple of stepless ratcheting screwdrivers that incorporate a one way bearing. Both have stripped & are now useless. In discussions at the PA regarding one way bearings, the Lashout (IIRC) had a problem with them stripping. Chalocolina pointed out that roller clutches are rated for certain torque loads. That it's simply a matter of proper sizing & there is nothing inherently wrong with using them. Perhaps that is why the Nuvinci hub is as big & heavy as it is.

So that is my only caveat & I remain cautiously optimistic. It's a unique piece of kit that I want to experience first-hand to get a feel for how well it operates. I hope it succeeds but even if it falls short I hope it will at least have opened to door for imitators to improve upon & have introduced into the world a new kind of transmission.

Who knows, perhaps the best thing that can happen is that the conventional biking community will snub the Nuvinci because Shimano doesn't make one. While scratching their head over to whom they can sell it to, they might see your letter & tweak it for the ebike market. I hope you manage to get one soon & look forward to hearing your impressions.

 

[safe]

The NuVinci Hub sounds like a great idea. But as far as I know you can't even buy them yet and I shudder to think of the initial startup price. You can buy an 8-speed Sturmer Archer hub for $122 and it weighs only 3 lbs verses the NuVinci 9 lbs.

http://aebike.com/page.cfm?action=details&PageID=30&SKU=HU2206

But getting back to the issue of STRESS.

Here's my observation about torque on the hub... If you could somehow focus the motors usage so that the only areas of the powerband that are used are the "good rpms" (which are the high rpms) then you avoid the spike that exists because of the low rpm PWM effect. As a simple way to "fix" the problem if you could simply "phase out" everything that the motor does below 1/4 of it's maximum rpm speed you would "solve" the problem without a doubt.

Maybe some sort of "low rpm" power inhibitor?

A cutoff valve of sorts?

The core problem is that due to the PWM effect each gear ratio has a WIDE RANGE of torque rather than a more desirable CONSTANT torque. If the torque were constant across the powerband then it's easier to ensure that "torque spikes" don't happen. (it's the "torque spike" that would break the hub, not the "regular" stuff because that's way below the capabilities of the hub)

 

[TylerDurden]

While the NuVinci presents innovative technology, it is interesting to note that the auto industry is still using the centuries-old belt and pully gag, even though other cvt methods exist.

A better belt provides more bang for the buck methinks. But, that doesn't mean the NuVinci don't work... It looks good to me, and it seems the bike is the first area for development by Fallbrook.



Let's flog this deceased equus just a bit more...

If gears are purported to reduce torque at the drive-wheel's hub, where does the force go?

How does the rogue wave (harmonic) arise, given the frequency of PWM compared to the frequency of a rider's pedalling?

How does the rise-time of torque a motor generates compare to the rise-time of torque a rider generates?


Safe?



:?

 

[safe]

If gears are purported to reduce torque at the drive-wheel's hub, where does the force go?

How does the rogue wave (harmonic) arise, given the frequency of PWM compared to the frequency of a rider's pedalling?

How does the rise-time of torque a motor generates compare to the rise-time of torque a rider generates?

There are two things... torque and power. Torque is the "instantaneous" amount of leverage that the rear hub experiences. Power is the "time based average" of torque applied over time. Since time and rpm are related we see that power can increase without having torque increase if we allow the rpms to increase.

Power = Torque times Rpm

As for the "Rogue Wave". I'm not saying that PWM and Pedal torque "peaks" are in any way similiar.. in fact they have nothing in common. PWM simply distorts the torque based on rpm, so it creates a situation of vulnerability when the rpms drop because the torque shoots up really high. If while this is going on (PWM at low rpm) you add on top of this the "peak" of the pedal "spike" then you have two factors that combine together in a lethal way against the health of your hub. If the bike were in a different gear at that moment when the pedal "spike" came through you would have no problem (because gearing changes the baseline) or if the PWM was not there at low rpms you would have no problem either. So this is a "correctable problem" if you simply have good gearing and protect against low rpm PWM.

"Rise-time" is really not a worry. The torque is WORST at bottom. It's the "stall torque" that destroys the hub. Once the rpms rise you get out of the danger zone is a hurry and really can ignore it as a problem. The danger is 100% in the low rpms... nowhere else... at high rpms there is literally "No Load" on the motor or hub anymore...

 

[TylerDurden]

There are two things... torque and power. Torque is the "instantaneous" amount of leverage that the rear hub experiences. Power is the "time based average" of torque applied over time. Since time and rpm are related we see that power can increase without having torque increase if we allow the rpms to increase.

Power = Torque times Rpm

Okay... torque is instantaneous... power is "time based average" of torque applied over time....

So the faster torque is applied to a hub, the more power is applied. Wouldn't that be essentially a shock-load?


So, how do your gears allow rpms to increase, a bigger gear at the hub?

As for the "Rogue Wave". I'm not saying that PWM and Pedal torque "peaks" are in any way similiar.. in fact they have nothing in common. PWM simply distorts the torque based on rpm, so it creates a situation of vulnerability when the rpms drop because the torque shoots up really high. If while this is going on (PWM at low rpm) you add on top of this the "peak" of the pedal "spike" then you have two factors that combine together in a lethal way against the health of your hub. If the bike were in a different gear at that moment when the pedal "spike" came through you would have no problem (because gearing changes the baseline) or if the PWM was not there at low rpms you would have no problem either. So this is a "correctable problem" if you simply have good gearing and protect against low rpm PWM.

Could you show us a graph of that... how the PWM and pedal torque combine to make a spike?

"Rise-time" is really not a worry. The torque is WORST at bottom. It's the "stall torque" that destroys the hub. Once the rpms rise you get out of the danger zone is a hurry and really can ignore it as a problem. The danger is 100% in the low rpms... nowhere else... at high rpms there is literally "No Load" on the motor or hub anymore...

So, low rpm and high torque will break a hub... and if torque is applied over a longer period of time, less power will be applied to the hub at any given point in time. So, what might apply torque more gently: a rider whose torque is greatest only when the pedals are at 90 degrees, or a motor that can apply torque instantly?

 

[fechter]

In my experience, what gives you a bad shock load is when you're under full throttle and your rear wheel momentarily loses traction due to a bump and the wheel has a chance to wind up under no load, then slams when the tire regains traction.

The inertia of the motor armature has built up a substantial amount of energy that gets dumped in a very short time. The amount of torque from this kind of condtion may be many times the continuous torque.
It's like dumping the clutch in a ICE vehicle.

 

[safe]

So, what might apply torque more gently?

You've written many things, but let me just instead try to "present" in a fresh way the problem...

What we have is a problem of PREDICTABILITY.

In a mathematically "perfect world" you would be able to keep the torque applied to a hub within a vary narrow limit. But in the "real world" of electric motors there are these two really harsh deviations from the "ideal" that when compounded together generate torque forces that magnify the stress on the order of 3-5 times above the "desired" levels.

PWM actually INCREASES torque as the rpm's decrease below the power peak. So while power (as the product of torque and rpm) is actually DECLINING (you are slowing down compared to if you were in the "perfect" gear) the torque induced stress is INCREASING. This is a result of PWM and (Fechter please jump in here) this effect could be "masked" so as to not be a problem with the right electrical circuitry.

Pedal powered "peaks" occur as one cycles through the pedal rotation and there are two "spikes" per full revolution, one for each leg as it hits the downward midpoint of the forward part of the rotation.

So let's think numbers here...

Let's say the "peak power" torque is 30 Nm at 2500 rpm based on the loweest gear ratio.

And let's say that the peak pedal "spike" torque is 70 Nm based on the lowest gear ratio.

So in the "perfect world" your overall peak torque would be:

70 Mn + 30 Nm = 100 Nm.

So that would not break anything.

Now let's add PWM to the mix. Instead of your peak motor torque being only 30 Nm at the 2500 rpm, now you have three times that much torque at 90 Nm at about 500 rpm. Now when you combine these torques you get:

70 Nm + 90 Nm = 160 Nm

This very well might break the hub because the limit is supposed to be only 100 Nm. So to compensate you would have to increase your gearing in order to LOWER the overall stress level on the hub. This inturn means the pedaling and the motor have to work harder which eventually makes the machine unable to climb the hill. (you get to a point of "I can't to that anymore")

So to get the FULL PERFORMANCE out of your hub and motor and get the best efficiency and lowest heat creation you need to find a way to "phase out" the PWM effect or eliminate pedaling and just run a motor alone. If you could eliminate pedaling AND eliminate PWM then you could conceivably run 2500 Watts through a typical 8-speed bicycle without any risk of failure.

So this is "it", the absolute limit of power that can ever be run through a standard 8-speed hub is 2500 Watts and never any higher. This is a line that cannot be easily crossed without fear of damage... (well, gearing can bring it back down again, but you get the idea :wink: ) 2500 Watts is "within reason" for a motor if you remove PWM.

 

[TylerDurden]

I'm confused...

If power (watts) is the product of torque and rpm (or "Power is the "time based average" of torque applied over time") , how can there be any maximum of wattage applicable?

Can't the rpms or the torque be applied over a longer duration?

 

[safe]

Can't the rpms or the torque be applied over a longer duration?

Yes, you're right... that's why I threw in that:

"well, gearing can bring it back down again, but you get the idea :wink: "

...comment.

For the bike I had envisioned (using a 5304 Hub Motor) I had set the gear ratio to hit a top speed of about 60 mph and the result at the end of all of it was that the hub would work.

You can always increase the gear ratio and decrease the torque... (but at some point you defeat the purpose of gears if you can't pull them at all)

 

[TylerDurden]

So this is "it", the absolute limit of power that can ever be run through a standard 8-speed hub is 2500 Watts and never any higher.

...so there is no truth to the above statement.


:?

 

[TylerDurden]

You've written many things, but let me just instead try to "present" in a fresh way the problem...

...or in other words, you don't have any good answers for some simple questions.

 

[xyster]

Can't the rpms or the torque be applied over a longer duration?

Yes, but then the power is less, and that's how the upper boundary is set.

Energy = power X time (i.e. kilowatt-hours)

therefore

Power = Energy / time (i.e. kilowatts)

The longer the time, all else the same, the lower the power.

 

[safe]

Useless Gears?

Pictures help a lot...

At 60A you get a power output of 2500 Watts and a top speed of about 67 mph. At 120A you get a power output of 5100 Watts and still a top speed of 67 mph. This is all on a very aerodynamic "lowracer" or "road racer" bike. The motor is actually the PMG132 at 48 Volts by the way and not the 5304 as I thought before. (big is still big)

So here's the scoop... look at the max speed lines and the one that has the "jagged edges" is geared so tall that it's essentially a 4-speed rather than an 8-speed. You really can never use the tall gears when the power is increased so much because the wind resistence does not allow you to get there.

So the "practical limit" for an 8-speed with gearing that "makes sense" is about 2500 Watts. When you get above about 10,000 Watts you are stuck with a single speed (first) and seven gears you can't use.

The hub limit equals the wind resistance... (that's the "answer" ultimately) No matter what motor you use you can't push a hub any harder than the wind resistance pushes back. The gearing simply makes sure that the motor peak never exceeds the hub torque peak.

A better statement might be:

"An 8-speed hub that can handle 100 Nm can push a bike to a top speed of about 67 mph (if the aerodynamics are about as good as a lowracer or road racer) no matter how large the motor that is used assuming that it is above the minimum required to achieve 67 mph."

 

[safe]

Oh Crapola!

I just realized my lookup tables for power end at 2500 Watts. (so that's why things seem to get chopped off there)

I'll have to check this again tomorrow and see what is really possible.

Maybe there is no upper limit?

 

[safe]

Maybe there is no upper limit?

Well, there still is an "upper limit" in practical terms because the ability to use all the gears diminishes as the speed goes up, it's just higher than I previously thought. I've got three charts for the PMG132 at 48 Volts:

60A - 2503 Watts - Top Speed 58.4 - Useable Gears 8
120A - 5114 Watts - Top Speed 82.8 - Useable Gears 6
175A - 7329 Watts - Top Speed 94.6 - Useable Gears 4

So the idea is basically the same, but the speed limits are WAAAAY up there...

Note: you have to "remind yourself" that we are assuming that the PWM effect is somehow "filtered out" otherwise the hub breaks at very low levels because of the tripling of low rpm torque. And if you add pedals it makes things even worse.