Why are direct drive hub motors considered best for commuting?

[Hasaf]

In a different thread, there was a bit of a consensus that a direct drive rear hub was best for commuting. However, there was little discussion of why that is so.

I have a mid-drive motor on my DIY commuter, driving a NuVinci N380. I happen to think that IGH transmissions are best for commuting. That nearly precludes a rear direct drive. Yes, I know there is an exception, a direct-drive with a five-speed IGH. I have one here in my office, it was DOA and the support from the Canadian vendor was a form of, "you buy, now go away."

Ignoring that exception, most direct-drive rear motors call for a standard exposed gear arrangement, which I don't consider ideal. In addition, the motor can not use the gearing. My TDSZ2 definitely calls for use of the gearing when starting and riding against strong winds.

In comparison with a geared motor, the direct-drive has more drag when out of use, and is significantly heavier. My first commuter, yes also DIY, used a geared front hub, as I didn't want to remove the IGH and belt drive. It was actually well balanced and hen riding to work in snow and ice (yes, studded tyres) it had a bit of a two-wheel-drive effect that worked well.

So, no IGH, and no running the motor through the gears, and heavier than the geared drive,why is the direct drive considered a better choice, I just don't see it

 

[fooferdoggie]

I think the argument is that they are most efficient at their highest rpms. I don't really know how that relates I do a ton of starting and stopping on my commute. I guess if your commute is flat and not many stops it would be ok. I much rather had a mid drive it lets me spin and keep my heart rate up.

 

[Deleted member 4210]

They aren't best for commuting.

Here is why:
1) Direct drives are heavier than geared drives.
2) Their acceleration is not nearly as strong or responsive as internally geared hubs, since there are no gears internally to leverage the power of the magnetic field generated by the coils.
3) They tend to be more for cargo ebikes, where the magnetic field and no gears are more 'forgiving' under heavy loads. There is really no benefit here for commuters other than POSSIBLY over many years, your maintenance (albeit negligible) could be a little lower.
4) They are typically a lot lower in terms of delivered torque, despite some of their higher watt ratings. Typically around 45 nm for a 750 watt direct drive, versus easily 65 NM for 750 geared drive, and not unusual to see up to 90 nm on geared drives, such as those made by Das Kit. Torque is what gives you the power you feel, and its what you want and need for going up hills.

Direct drive motors are cheaper than internally geared drives, and its why you have seen them many times on the Rad models over the past several years. Though Rad has gone to geared drives on some.

As far as efficiency goes, your mid drive will be superior to both geared and hub drive motors, due to the fact that the motor itself is a) not limited to the rpm of the rear wheel speed, and can be spun at speeds around 4000 rpm, being able to achieve a 'sweet spot' of efficiency combined with high torque - not uncommon to reach 70 to 80 nm of torque depending upon brand b) also leverages your drive train in terms of the gears at your hub, whereas the rear hub drive is going direct to the wheel, and there is no additional benefit from the bikes own gears, other than what is achieved from your own human input. Thus you'll see smaller batteries (36 volts and below 500 watthours) on most mid drives, while being able to achieve much longer range due to the efficiency, versus that of the hub drives. Hub drives can achieve the higher ranges via the flexibility of higher voltages and much greater amphour capacity batteries, and often the OEM's including hub drives on their ebikes, will use non-proprietary (Re-entention is one for example) casings, so you have much greater choice of battery size and availability of sources. The mid drive manufacturers typically use proprietary battery formats, locking you out of everything but what the mid drive manufacturer offers. While they are growing their available sizes as market demand and competition dictates, their proprietary batteries easily fetch more than 70% of the price of a comparably sized non propreitary battery. Also, you can generally choose what battery brand cells, and density you wish from a host of Top Tier Grade A quality battery cell suppliers with the hub drive offerings, including Samsung, Panasonic, LG, and Sanyo.

For commuting you made a good choice.

P.S. Wasn't sure if you were conflating direct drive hub (just the gears) with a direct drive motor at the hub. So I only commented on the motor itself. If you are talking about a direct drive hub, and referencing gears versus bearings (i.e. CVT style such as with your Enviola), those CVT's are actually less efficient than direct drive hubs that have gears. Thats because there is nominal 'slippage' with the internal bearings utilized to provide the seamless ratio changes.

 

[kmccune]

When compared to the better Hub drives, the bottom end standard mid drives are perhaps a bit overrated, however, balance, compactness( on some) and you do not have to juggle the freewheel or cassette and the efficiency, these little motors like to spin, you may be able to get by with a smaller battery to boot.The only disadvantage( besides cost) is you might have to push home if you have some kind of chain or derailleur accident or failure.
One advantage on the DD motor, regen is easily accomplished( saving brakes is the only reason I like regen)"Beware the yeast of the Pharisees" anything that works for you is probably right for you.

 

[harryS]

Radmotors has gone to a geared motor in their big selling City bikes, so I believe the "best for commuting" drumbeat will fade away. Stromer uses DD motor though.

I feel a direct drive motor would take away the fun factor for me. I'm not commuting or trying to get somewhere on time. I want light weight, ability to coast, and could care less about brake pad life.

 

[AHicks]

The redeeming feature of the direct drives is their ability to disapate heat much more readily than the geared or mid drives. This allows them to use more power without issues than the buried motors in the geared hub and mid drives. When talking speeds of 15-20mph plus, the geared and mid drive make great sprinters, but generally can't compete with the direct drives that can run that kind of speed as long as you like - assuming they aren't going to be expected to climb long grades or pull heavy loads....

 

[PedalUma]

I wonder if there is some confusion here? Many internally geared hubs of the type used on non-electric bikes have one gear that is direct drive, not stepped up or down. That gear is the most efficient, because there is almost no transmission loss. Mid-drive electric motors can power these internally geared hubs through the chain.
On the other hand there are electric hub motors. The smaller ones have internal gears. The big ones do not. Neither of these drive the chain, so neither can shift. There may be gears on these bikes but that only effects the rider's pedal power. Not the motor's power.
I almost walked from an opportunity this week because it sounded like the guy wanted a geared hub motor. Nope, he wants a mid-drive motor and for me to install an internally geared shifting hub on a bike that is currently a one speed. He is getting a Nexus 7.

 

[AHicks]

Some confusion could be saved by sticking with commonly used e-bike hardware in our discussions?

Let's not split hairs. There are 2 types of hub drive motors. One uses very few moving parts, with no gears in play. These are generally big (not very stealthy at all) and heavy, and are known as direct drive (DD) motors. The others are geared hubs, with an internal motor similar to a mid drives internal motor. MOST geared hubs offer 5:1 or so internal gearing, and drive the rear wheel directly - no chain/derailleurs in play.

In addition to the 2 types of hub drives, there are mid drives. Sure there are differences between these, but I don't think those differences need to be discussed for the purpose of this conversation. -Al