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Shimano DH2N35 Dynamo DRAG

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Shimano DH2N35 Dynamo DRAG

Old 02-10-23, 04:07 PM
  #1  
rosefarts
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Shimano DH2N35 Dynamo DRAG

Mods, this is an e-bike question as well as an electronic gizmo question. Feel free to move it wherever you want.

Maybe Iím just being picky. Iíve never had a dynamo before and donít really know what to expect.

If I lift the wheel and give it a spin, it goes around a time, maybe 2 and stops. The brakes arenít dragging and neither is the fender. The tires are pretty heavy Schwalbe Big Ben, and really should keep the wheel spinning. I am running a headlight and blinking taillight off it.

With the wheel off, I can turn the axle and itís a very pronounced uhh, indexing. I figure thatís where the magnets are passing and thatís normal for a dynamo.

It sure seems like a lot of resistance though.

Now while riding. This is on a Yuba cargo bike that I recently converted to an ebike. Itís got a 500w rear hub motor and is faster than I ever expected.

When i coast or just turn the motor off, I instantly feel the bike slow down. Not so much that it itíll stop me but enough that I notice it. There is a wattage meter with the lcd display, and from what I see, itís usually around the max watts for this model.

I read an online experiment where the author basically concluded that this hub should slow me down approximately 3 minutes per hour. From what Iíve felt, I believe it would be substantially more than that.

So where does Newton stand on this?

Am I losing more charge and prematurely wearing out the motor to keep these daytime running lights VS the charge lost if I were to run a smoother hub and lights off the ebike battery?

Once warmer weather hits, my wife is going to take this all over. Thatís why I liked the dynamo. Sheís not one to pay attention to whether the lights are on unless itís dark.

But now Iím thinking that if this thing goes dead across town, that after the long tedious pedal back home, sheíd probably never touch it again. If it truly is lowering the range of the bike.

Is the hub out of adjustment?
Should I go with a battery powered lighting system?
Am I losing more power off the battery than regular lights would pull?
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Old 02-10-23, 05:03 PM
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If you're worried about efficiency, it will be more efficient to power your lights direct from the same battery that charges the motor than to translate that electricity to kinetic energy at the rear wheel, then translate it back to electricity at the front wheel, because each step has less than 100% efficiency. There are e-bike specific bike lights.

I've never used that particular hub, but I do have a different brand of dyno hub, and the amount of drag you are seeing does seem unreasonable.
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Old 02-10-23, 05:07 PM
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The sounds about normal for how a hub feels but it doesn't slow you down much in practical terms.

But yeah, it doesn't make a lot of sense to use a dynamo hub on an e-bike. Aside from the added weight...
Your bike battery is powering a motor (losses) that is spinning the bike wheel (losses) that is generating power via a dynamo hub (losses) that is powering lights.
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Old 02-10-23, 05:14 PM
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I am not sure if I have this straight. You have a 500 watt motor to push the bike and you have a dynohub to produce electric power for lights? And your questions are about the dynohub drag?

I assume yes and yes, that is the basis for my answers below.

I have a couple SP dynohubs that I have lots of miles on, so I can't really compare to a Shimano. My dynohubs, if I give the wheel a spin by hand (off the ground) when it is not connected to a light or it is connected to a light that is turned off, the wheel will spin almost as long as a plain wheel with a plain (non-dyno) hub. But with the light turned on, the dynohub works much harder to produce power, the wheel will not spin as long.

This article is about a decade old, it does not list the drag or power for your model hub, but it has a good generic discussion of dynohubs.
https://www.cyclinguk.org/sites/defa...ub-dynamos.pdf

Note that it has a graph with hub drag, dashed lines are measured drag with lights off and solid lines are drag with light on.

When my lights are on or off, I can not tell the difference between the bike drag, so I usually just leave the lights on. But, from that graph, I know that there is extra drag. And from my wheel spin test by hand, I also know that there is extra drag. According to the graphs, my SP PV8 hub at about 25 km/hr would have a drag of a bit over 1 watt with lights off, with lights on the drag is a bit over 6 watts, and it would put out a bit over 3 watts of power to the lights at that speed.

So, if your Shimano has performance that is even close to my hub, the point is we are talking about wattage in the single digits when you have a motor rated for up to 500 watts to drive the bike. I have no clue how many watt hours your battery is rated at, but it probably is rated high enough to push an extra 6 watts of dynohub drag that would go unnoticed.

Maybe we have an electrical engineer here that can provide more specificity?

Regarding the hub not turning easily and feels like what you called indexing, yes, that is the magnets rotating against an armature. That is normal.
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Old 02-10-23, 05:27 PM
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Compared to either of my dyno hubs (Shimano 3N70+/-20 and a Schmidt SON), the notchiness you feel spinning the wheel is normal.

The drag sounds like it's either high or user expectation. The spring after I rode my first hub (the Shimano) through the winter, the notchiness which turned into a perceived terrible buzz around 15-20 mph led me to find out just how bad it was. I decided on a coast down test with the light on and off. After a few rounds of climbing the hill, coasting down with the light on, climbing the same hill, coasting down with the light off, climbing the same hill etc., I concluded the buzz was perceptible but the difference in coasting time between light on and light off was not measurable, much less statistically significant. A plot would have showed the on and off times intermixed. I called off the "test" and had a nice ride the rest of the morning. Since then I rode over the ridge near my house and discovered I'd left the light on 15 miles down the road, turned it off and again, didn't notice a difference in drag.

Maybe I'm just so strong a rider than I don't notice losing 5 W. Or that slow. Or it just doesn't matter. (I prefer the first and last explanations!)
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Old 02-10-23, 10:05 PM
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Thanks for the replies.

The reason I am running a dynamo is because the bike came with it and it didnít interfere with the ebike conversion, so I left it as a zero maintenance way to keep lights on. Removing it would be pretty easy but would require a new front wheel, since at 26Ē, I donít have anything in my stable that would fit.

I do have the light that came with the ebike. It looks like a good one. I would need to attach a rechargeable blinker in the back but thatís easy.

I can coast downhill at a speed of 24-25mph. On the same hills that my road bike would coast at 30 or more.

Thats not apples to apples. Itís a 60lb bike with 60lbs worth of kids on it. 2.15Ē tires vs 38mm, and totally upright. Iíd be crazy to expect it to roll as smoothly.

I am not expecting perfection. I just donít want to seriously shorten my range or the life of a fairly expensive motor.

Do you know if these hubs are basic cup and cone for bearings? Maybe I could make sure itís optimized.
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Old 02-11-23, 03:06 AM
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Some of the Shimano hubs have a cup and cone on one side, BUT before you even think about trying to adjust anything in a dynohub, if you do not know what you are doing, you could break a wire and the dynohub would become useless. It is best to not touch it.

If it was me, I would just leave the dyno powered lights turned on and not think about it. The ebike light might draw more power out of the battery than the extra drag from the dynohub would draw, the reason I say that is I have seen some very bright ebike lights on some of the bikes on bike trails near my home. The motor puts out a lot more power than an average human does, if most of us with dynohubs do not notice the drag from a dynohub, I doubt if your motor or battery will notice the difference either.

You did not say if you have a taillight or just a headlight only. On my bikes with dyno powered taillights, I still use a battery powered flasher, as I usually have the flasher on during daytimes, especially if it is overcast. But the dyno powered taillight is nice for when the flasher batteries ran down and it gets dark outside, it is there and works without batteries.

If you have a dyno powered headlight and no taillight, if you wanted to spend some money I would buy the taillight. Dyno powered taillights should be wired to the headlight because the headlight has overvoltage protection and the taillight does not, the website run by Peter White has a good section on wiring dyno powered lights. But dyno powered taillights are constant on, not a flasher. If you want a flasher, that would be battery powered with its own battery. Most bike shops in USA do not stock dyno powered lights, but many of them will order from Peter White, or Peter White will sell to the public at retail pricing too. Do NOT get an incandescent taillight, only get an LED. A lot of the dyno powered lights are poorly weatherproofed from the bottom, tire spray on a wet day could be a problem on some of them, so if you mount a taillight, it is best to put it where tire spray will not throw a lot of water on it.

If you choose to stop using the dyno powered lights, there is no reason that you could not keep the dynohub. If you unplugged the power connection at the hub, then the drag should be minimized.
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Old 02-11-23, 09:57 AM
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I am a total ebike beginner. In fact, aside from using one as a family minivan, Iím quite opposed to the proliferation of them. Itís totally possible that Iím just being paranoid and the ďwattageĒ meter is usually pegged really high regardless of the front hub.

Here is a before and after photo. It came with the dynamo to both front and rear lights. The ebike kit, should I choose to use it, only has a front.

It sure looks like a factory wiring to the dynamo, and I did see another of the same model with a dynamo too. So itís at least a factory available add on. (The bike is currently discontinued)



Before

After, Iíve since reupholstered the blue pad in matching brown.

Since I decided to use hydraulic rim brakes, the front wheel was not changed.


Iíll double check that the front is LED. It could be anything. Iíve only had this bike a couple weeks, Iíve only really got to know whatís been changed.
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Old 02-11-23, 01:06 PM
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It's a crummy hub, but I don't think the spin test is particularly reliable. Much better hubs also do poorly when you pick the front wheel up off the ground and spin it. Maybe someday they will make lower power dynohubs. I have actually heard from more people that want higher output dynohubs.
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Old 02-11-23, 06:13 PM
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I think it is the electric load on the dynamo.

I discovered an off switch for the light. With it turned off, and the rear still blinking, it spins very well. Not like a regular wheel but close, and considering the application, good enough.

Upon further inspection, the rear blinker is LED but the front is a 6v Halogen. I wouldnít expect such a little bulb to cause so much drag but it seems to.

For the sake of simplicity in keeping everything as it sits, Iím going to try an LED replacement for the front bulb to see if that lowers the level of annoyance for me. Itís also nice to see that the front can be turned off, and since this is a bike for ferrying kids, will be used almost exclusively in the daylight.

If Iím still fussing over it, replacing the front wheel is pretty easy.
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Old 02-12-23, 05:47 AM
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Originally Posted by rosefarts
I think it is the electric load on the dynamo.

I discovered an off switch for the light. With it turned off, and the rear still blinking, it spins very well. Not like a regular wheel but close, and considering the application, good enough.

Upon further inspection, the rear blinker is LED but the front is a 6v Halogen. I wouldnít expect such a little bulb to cause so much drag but it seems to.

For the sake of simplicity in keeping everything as it sits, Iím going to try an LED replacement for the front bulb to see if that lowers the level of annoyance for me. Itís also nice to see that the front can be turned off, and since this is a bike for ferrying kids, will be used almost exclusively in the daylight.

If Iím still fussing over it, replacing the front wheel is pretty easy.
You would probably have better luck with a new LED headlamp than a LED bulb. LED bulbs are usually designed for DC, a dynohub is for AC. The bulbs vary in their ability to handle overvoltage, such as riding down a hill at speed. But the newer LED headlamps were designed with overvoltage protection for that.

If you mostly ride it in daytime, a low budget dyno powered LED headlamp would provide good light for drivers to see you, even though it does not flash.
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Old 02-12-23, 10:21 AM
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There are a number of affordable LED headlamps that would work with that, so I wouldn't bother with a replacement bulb.

I think that's a 2.4 Watt hub, which is only technically suitable for a front headlamp. To power both, you'd want a 3.0 Watt hub. I had a similar (maybe even the same, based on the photos) hub in a cheap wheel that I used on a heavy city bike in Sweden, and I rode that bike way more often than I thought I would, sometimes even for longer distances. I ended up using a rechargeable rear LED lamp and powering an AXA Echo 30 (If I recall correctly...) front LED from the hub without problems. I think it cost around $20, and that was around 7 years ago. The lamp output was at least as good as my nicer looking and older (and more expensive) B&M.
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Old 02-12-23, 11:18 AM
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I think the independent dynamometer testing of Shimano hubs show they pull essentially the same power with the light on or off. I'm not sure what "slow me down 3 minutes per hour" means. I feel like in practice you aren't being slowed down that much at all. There may be other things wrong with your bike, like bearings or brakes not properly adjusted. Unfortunately, shimano dynohubs often have bearing adjustments off, and most bike shops won't touch them.
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Old 02-12-23, 02:48 PM
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So I found an article a few weeks back where they tested a known road, with the same bike, same rider, etc. Some uphill tests and some flat tests. I donít know if they accounted for wind.

This test is where I got the 3 minute number from. For the same effort, the tests on the dynamo were 3 minutes behind the same rider without one, at the one hour mark. Thatís probably .5mph or so.

I actually tried to find it again and link to it, but I couldnít find it.

The more I read about this light system, the more I want to switch away from it.

It is definitely notable how much better itís spinning now, with the light turned off.
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Old 02-16-23, 09:41 AM
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Originally Posted by unterhausen
It's a crummy hub, but I don't think the spin test is particularly reliable. Much better hubs also do poorly when you pick the front wheel up off the ground and spin it. Maybe someday they will make lower power dynohubs. I have actually heard from more people that want higher output dynohubs.
Shimano makes some 1.5w hubs

I'd love higher output
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