Custom Electric Vehicle Build

[lagfish]

Hi,
I am looking to build a custom three wheeled electric trike and have been reading electricbike.com and browsing the various manufacturers sites for the past few days. Some of these sites aren't very well organized so I'm a bit lost in all the information. Perhaps some of you guys can recommend an ideal motor for me. The trike will have the two front wheels driven and the rear will be used for steering. As such, I can use any motor (hub or regular attached to the wheel with a timing belt). The concern here is weight because the motors that I have found are all 15 lbs + which is very heavy. So here are the parameters of the bike:
-Desired max speed on level ground: 35mph
-Wheel size: 14"-16"
-Brake type: disc with regen if possible
-Rider weight: as much as 220lbs
-Terrain: some small hills
-Budget: under $1000 per wheel

After searching, it seems like my options are to either use a heavy hub motor which are usually designed for full sized bicycle wheels and just run them at a higher voltage to get the speed I want. Or get a a much lighter Astroflight and gear it down about 12:1.

Are there some lighter hub motors or other mid-drive motors that spin at a lower RPM and don't need such aggressive gearing? Thanks

 

[amberwolf]

The trike will have the two front wheels driven and the rear will be used for steering.

Is this based on an existing design? Or is it your own creation? You should post pics and details, so we can help you pick parts that will actually be able to mount on it.

Most hubmotors are double-ended axles, and will require a full "fork" to support them. Most tadpole trikes I've seen use single-ended front axles.

Also, rear-wheel steering has been tricky to get right on vehicles over the years, if you are making your own design and havent' already worked past that, you might look up previous designs.

The concern here is weight because the motors that I have found are all 15 lbs + which is very heavy.

Depends on the continous power level you need out of them. DD (Direct drive) Hubmotors are commonly heavy because of the cheap materials they are made from and unoptimized designs, plus the lower RPMs they spin at. Geared hubmotors are no better optimized or materials, but they spin at a higher RPM and use built-in reductions (often around 5:1 planetary) so they are lighter and smaller. Probably half the weight of a similar DD.

But geared hubs shed heat less well due to the multiple "layers" inside them, less direct heat path, vs DD, so they're less capable of the same sustained power as a similar DD would be. There are ways around that (lots of threads about cooling motors) to one degree or another, depending on the compromises or expense/time/hassle you're willing to deal with.

-Brake type: disc with regen if possible

Regen is not possible with most geared hubmotors as they have a freewheeling clutch in them. DD hubs can do it. Non-hubs can do it too, as long as there are no freewheels between wheel and motor. But if there's no freewheel, then there's always a bit of extra drag on the motor (and wheel), so non-electric use (pedalling, coasting) is less efficient than if there was a freewheel. If you do lots of downhill braking, or stop/start traffic, the regen tradeoff is worth it. If not, it's probably better to have the freewheel.

Are there some lighter hub motors or other mid-drive motors that spin at a lower RPM and don't need such aggressive gearing?

Bafang BBSHD, normally goes thru the cranks, so a chaindrive at low ratios is easy. Would require a custom mount on your frame around the wheels.



Regarding the power level you'll need, you might check out the http://www.ebikes.ca/tools/trip-simulator.html to try to work that out.

 

[Drunkskunk]

It would be interesting to see how you have overcome the inherent instability of a rear steering tadpole trike, especially at higher speeds.


Direct drive Hub motors are heavy, but give excellent reliability, high efficiency during cruise, and good heat handling. Direct drive can be used for regen braking

Geared drive hubs are lighter weight then DD motors. They are less efficient at cruising, but more efficient on acceleration. they freewheel, so are not capable of regen braking. They have more moving parts so more potential points of failure.

Chain drive is a possibility. You can use a smaller lighter motor, but will need to add the weight of the gearing/pullys/belts/chains/rollers, or what ever your design uses. it's reliability is going to be based on your ability to design and fabricate a system that can withstand all outside factors. Your efficiency will be dependent on your motor choice and design as well.

 

[MadRhino]

Rear steering trike is a lift you know? To unload the pallets off the truck. It does 10 mph and you know why? It is freaking dangerous to go faster with a rear steering vehicle.

Want a hint? One heavy motor is worth 4 light weight motors for both, performance and reliability.

 

[lagfish]

Thanks for the feedback so far.

I don't have a completed design yet - just some concepts at this point. The idea to get around the rear steer wheel's instability is to set the caster length close to zero so lateral forces do not produce a large moment that can destabilize the wheel. Whether this is practical remains to be worked out as the pneumatic trail changes with respect to vertical load and slip angle. The other possibility is to keep caster length negative so that it is inherently stable when cornering and use alignment springs to keep it stable under low speeds.

Why are geared motors more efficient during acceleration but less during high speed cruising?

 

[MadRhino]

Because of the gears, of course.
Gears are making the motor spin at an more efficient speed during acceleration, but the gears are taxing the maximum efficiency of the hub, like any other mean of transmission. Gears are also making it less durable, and reduce its ability to shed the heat.