Electric assist upgrade / DD hub drag geared hub drag

[wayover13]

Hi. I'm getting ready to upgrade from a direct drive hub motor. The motor has served its purpose of helping me familiarize myself with electric assist, but the drag it experiences when not under power predisposes me against DD hub motors. That, in turn, has to do with the way I ride and use electric assist: I want true assist, not some other source of power to move the bike. In fact, my ideal electric assist would be one that is only engaged when needed, and I need assist only when going up steep grades with a lot of weight on the bike. Outside conditions like that, I want the electric assist to be off and not causing excess drag.

My initial tests with the DD hub motor I got convinced me that I should not use it as I'd like, i.e., only on grades. It was clear to me when I mounted it and tried spinning it by hand that it had far more drag that any but the rustiest/gunkiest of standard, non-electric hubs. Giving the wheel a good hard spin by hand resulted in only about 4 full revolutions of the wheel. An old non-electric hub wheel, on the other hand, gave me about 25 full revolutions.

In any case, I got interested in geared hub motors because they are billed as having no more drag than most conventional non-electric bike hubs. But before actually investing in one and finding out for myself to what extent this is true, I thought I might be able to ask about that here. So, is it true that geared hub motors have far less drag when unpowered than do DD hub motors when unpowered? Furthermore, how do geared hub motors compare in terms of drag to a standard, non-electric bicycle hub? Would my manual spin test on a geared hub motor wheel and a standard non-electric bike wheel give about the same number of revolutions for both?

A sort of a shade-tree way of testing that, which would be a lot more accurate than the manual test, would be as follows. Mount each wheel--a geared hub motor wheel, a DD hub motor wheel, and a non-electric bike wheel--on an upturned bike frame. Have a drill handy with a sanding cylinder tightened into the chuck. Put a piece of reflective tape on a spoke near the rim. Put the sanding cyclinder against the tire on the first wheel and run the drill to maximum rpm's (if a sort of timing light could be rigged up to help count rpm's it would be more accurate). Disengage the drill from the tire and count the number of revolutions the wheel makes before stopping. Repeat the process with each wheel. The results would tell, at least as a rough percentage, which wheel had the most and which had the least drag. Anyone know of any test like this that was ever done?

 

[Rassy]

No need to do the test because the regular bike wheels will always "win". On a geared hub motor there is an internal freewheel that isolates the actual motor from having to turn, but there are gears that still turn. However, with the drag (cogging) from the motor eliminated there is no serious drag from the geared hub motors. Also, the geared hub motors tend to weigh a lot less than DD hub motors which is another big plus for when pedaling without using the assist.

 

[wayover13]

I figured the non-electric hubs would probably win this contest, but it would be interesting to know what would be their edge over the geared hub motors. And I suppose not all geared hub motors are equal in this department; some may have far less drag than others. But my guess would be that geared hub motors would always beat out DD hub motors, correct?

 

[lcyn]

I have both types, geared and DD. I much prefer geared, they are considerably lighter and have no where near the drag of DD, so I can pedal or use the motor, as I wish. I always felt with DD I relied too much on motor, and not enough on myself. I also like that the geared is smaller and less conspicuous, which is impotent to me.

 

[lcyn]

Important. Freudian slip.

 

[lbz5mc12]

I like geared when I want to ride for exercise and I like DD when I don't want to exercise. My geared motor is a front wheel older model Amped Bikes version. I don't have PAS set up with it but when I do use it it's only for getting up hills that I really struggle with. I had it mounted to my GT Aggressor and was running it at 36V. As previously mentioned by others there is little to no drag and since it's smaller it just looks like an over-sized hub. The best way to get exercise with it is to ride as far as you can without using it and when you get winded, pedal with it to give yourself a break every so often, or when you need help up the occasional steep hill.

 

[Samd]

I love quantifiable science experiments like the one you propose. But you'll be wasting valuable time quantifying an infinitesimal amount of friction.
Geared hubs with freewheels have a level of friction you couldn't perceive physically.
Also, the momentum of your body is not included in the experiment you propose.

Bo buy a geared hub and spend the time fitting it instead of measuring an almost immeasurable amount of extra friction versus a bike hub. Seriously, it is tiny.

 

[wesnewell]

You have to learn how to use a DD hub for assist. You always want it engaged when riding. Set cruise control to 2 mph under what you want and then pedal up to that speed. This eliminates the drag and uses almost no electricity, yet kicks the motor in when you encounter uphill grades.

 

[dogman dan]

It's true, a regular wheel will always win the spin test. You will have some tiny bit of drag with a gearmotor, because it just doesn't freewheel as freely as a back wheel on a regular bike.

But the real source of drag you will feel will be the result of the weight of the system. Even the lightest setups often add at least 15 pounds to the bike. Pedaling, you just have to feel that a LOT.

However, as Wes just said, you can set any motor, dd or geared, to assist at a very low wattage. This eliminates both cogging if dd, AND the effort needed to push the extra weight. At 50-75 watts, a decent sized battery will last for hours. To me that is the best of both worlds when you want to ride like the bike is light. By picking your wattage, you can not only ride like you are on a 10 pound bike, but you can ride like you weigh 115 pounds again. 8) I rode like the wind then, but then I graduated, got a job and could afford food. Biking was never the same again.

Do go for the lightweight gearmotor, but also consider giving yourself a light assist all the time when you have headwind or even slight uphill grades. If you don't have cruise control, it takes time to learn to hold a light throttle. A wattmeter really helps with learning to run a 50w level.

 

[wayover13]

I don't really need to learn anything because what wesnewell and dogman are describing is essentially the way I've been using my DD hub motor for the past 2 years. Knowing the added amount of drag it creates when unpowered, I decided I would have it powered almost continuously, the exception to that being on downhill grades. I've used a thumb throttle with it, giving the motor just enough juice to maintain a speed just slightly higher than I would maintain without power. On uphill grades, which is where I need assist most, I would use more juice.

Since this hub motor was built into a 16" wheel though, the place where I needed to use it most was also the area where the wheel was spinning most slowly and thus where the motor was working least efficiently. That's another factor that makes me want to move to a geared motor: since they seem to have about a 5 to 1 gear reduction, they are bound to operate more efficiently under the scenario where I most need assist (uphill grades). Since they introduce far less drag than a DD hub motor, I will also be less inclined to use the motor in scenarios where it's not really needed, such as on flats. That, in turn, should lead to a notable increase in range per charge.

Again, I cannot get on a bike and not pedal. I think only injury or extreme exhaustion could result in me not pedaling on my bike. I love biking in part because I find a hard aerobic workout refreshing, so the hub motor is only there to slightly alleviate the strain for the hardest parts of the ride.

 

[wayover13]

I love quantifiable science experiments like the one you propose. But you'll be wasting valuable time quantifying an infinitesimal amount of friction.
Geared hubs with freewheels have a level of friction you couldn't perceive physically.
Also, the momentum of your body is not included in the experiment you propose.

Yes, real-world conditions could not be fully accounted for in the testing scenario I proposed. But laboratory conditions (actually shade-tree laboratory conditions, given the scenario I described) can still be informative. Given how easy to do this test would be, I'm surprised no one has tried it yet. I for one would be interested to know, as a rough percentage, how the drag on each of these wheels compares. If I ever get around to conducting the experiment myself, I'll post about it somewhwere here.

 

[dogman dan]

Sorry if I came off like you knew nothing. Many have never tried pedaling with 50w assist. I learned how right away, after I found out what 15 miles into a headwind did to my battery. Did a lot of last mile home rides with little left in the battery.

A gearmotor won't make the bike feel light though, nor will it be guaranteed to be more efficient up a hill. Uphill efficiency is had by keeping the motor turning faster. But the gearmotor rotor radius is usually smaller, so you lose some of that 5-1 ratio compared to a much larger rotor dd.

If you want efficiency up hills, you are on the right track with a smaller wheel. Maybe you just need more power to keep it going fast, or at the same power, a slower winding.

I don't say a gearmotor cannot be more efficient up a hill, I just mean to say, "it depends". You can easily stall a gearmotor too, if the wattage won't be enough to go up fast. So get the correct gearmotor for you needs, in this case up a steep hill. Then you can pedal power off to your hearts desire, AND not heat up the motor climbing hills so much.

 

[wayover13]

Thanks for the input, dogman. In terrain we typically confront in my area of the country, we (tandem bicycle) can, on an unpowered bike, drop down to about 5-6 miles per hour on some of the tougher uphills (6%-7% grades). So I'm looking for a motor that's going to operate efficiently in the 5-8 mph range. The upgraded motor will actually be in a 20" as opposed to a 16" wheel, though. I don't care about top end for the motor: I'm resolved that any speed attained above 15 mph on this bike will be either because of my own power (aided by a tailwind, undoubtedly) or because of gravity (steep downhill incline). I do not want a motor to help me go faster: it should mostly just be helping me get up hills.

 

[John in CR]

Do something like a Kepler friction drive, absolute minimum weight, simple system, and when not engaged your bike is 100% natural and free other than the 2-3kg total system weight including battery.

 

[wayover13]

Do something like a Kepler friction drive, absolute minimum weight, simple system, and when not engaged your bike is 100% natural and free other than the 2-3kg total system weight including battery.

Yes, that looks very interesting and very much along the lines of what might be an ideal solution for my needs. But the thread in which it was discussed on this forum is a couple years old. Can these sorts of units really still be bought or custom-ordered? I ride some unusual bikes by the way (recumbent tandems), so something like that would need some customization to work on my rig(s).

 

[veloman]

Where's Neptronix's video showing how well the Mac geared hub spins?

There might be a 2watt difference in drag between a good bike wheel and the Mac hub. You will never notice it. You will notice the additional weight way before any drag of a good geared hub. Some geared hubs like the small q100 or similar have a fair bit of drag, though not quite as much as most DDs.

If you can deal with the 9lb additional weight (over a bike wheel) of a Mac geared hub, that's your best bet. Paul at em3ev.com has them.

 

[wayover13]

Actually, since I'll be replacing a 16" DD hub motor (front) wheel that weighs about 20 lbs. with a non-electric 16" wheel, and replacing a non-electric 20" (rear) wheel with ca. 9lb. geared hub-motor wheel, I'll be seeing a net loss in terms of total wheel weight. That is, if I go with the geared hub-motor solution I'm now considering. I'll see whether I can find the video you referenced--thanks for pointing that out.

 

[Samd]

Yes, real-world conditions could not be fully accounted for in the testing scenario I proposed. But laboratory conditions (actually shade-tree laboratory conditions, given the scenario I described) can still be informative. Given how easy to do this test would be, I'm surprised no one has tried it yet. I for one would be interested to know, as a rough percentage, how the drag on each of these wheels compares. If I ever get around to conducting the experiment myself, I'll post about it somewhwere here.

If I understand what you propose testing it isn't realistic. Turning a bike upside down and testing how long a spinning wheel takes to stop (gear hub or regular) is not the same as putting a 90kg rider onto a bike, getting it up to speed via kinetic energy storage and then seeing how long the 90kg rider takes to come to a stop.

If you did propose testing the latter, I'll wager that you could not measure the difference between a geared q100/BPM/Ezee/Mac and a regular bike axle. Too small a difference.

Whether you took an extra bite of that burrito at lunch would be a larger variable.

Geared hub versus normal bike Axle? The choice of tyres and tyre pressure would be an order of say ten times more important.

 

[veloman]

mxus geared hub, very very similar to mac

http://www.youtube.com/watch?v=JM_rX-toEI4&list=UUZKNOqfkeS1T5skYcBxHBFw&index=35

 

[dogman dan]

To turn it that slow, you will want a slower winding. This is how I have been doing it on my dirt ride.

Since you will be using a 20"wheel, it doesn't have to be an extreme slow winding like I use in 26" wheels. But you do want to choose from the slower range of motors, rather than the typical choice for 20" rims, which tends to be a faster winding aiming for 20 mph or more.

Cellman has a Mac in a very slow 12T winding. 12t might be overkill slow, but maybe not depending on the voltage you run it on. I'd say it might be worth the weight to choose a mac, just to get the right winding choice for your needs.

It's really nice to have a slow motor, allowing crawling up hills without so much motor overheating. Most others just prefer the watt up and blast it approach.

 

[wayover13]

Thanks for the further input, dogman. I've actually been looking at MACs and have been thinking the 12t might be a good choice. I'd sort of been assuming I'd put the hub motor in the rear 20" wheel since I'd not seen any already-built 16" geared hub motor wheels. But now that I think more about it, spokes can be cut to any length, so I should be able to build the sort of hub motor I'd like into a 16" wheel. So maybe I should just go ahead and lace up a new 16" front wheel with a MAC geared-motor hub. That would be an even smaller diameter wheel overall, and would thus allow for higher rpm's up those slow grades. Interesting thought.

 

[dogman dan]

Both would be radial spoked, so not that much advantage to one over the other. I would think though, that 12t Mac in 20" would be plenty slow.

 

[wayover13]

This is something I planned to ask about in another thread, but since you've brought it up here, I might as well ask in this one: why are so many of the drive wheels I'm seeing on this forum radially spoked? My--admittedly deficient--understanding of wheel building seems to indicate that radial spoking would be weaker than cross-spoking. Can you explain?

 

[wesnewell]

Please go to the User Control Panel, select Profile, and then enter your city, state/province, and country into the Location field (country minimum). This will help people help you. Example: Wylie, TX, USA. Without knowing what country you are in it's hard to make any recommendations. Thank you.

 

[slacker]

wesnell,it says wi,usa