I think the main problem with hub motors is the unsprung weight. That
is, the motor is adding weight to the wheel and that weight has to go up
and down with every bump in the road. If it doesn't, the wheel will
bounce off the roadway which means less traction and control.
In this case, the motors may be light enough and the speeds slow enough
that the unsprung weight will not be an issue.
The other potential problem is *not* having a differential. With a
mechanical differential, one wheel is allowed to turn faster than the
other when going around corners. With hub motors, that process will
have to be done electronically.
Peri
------ Original Message ------
From: "fred via EV" <[email protected]>
To: "[email protected]" <[email protected]>
Sent: 27-Sep-14 8:39:29 AM
Subject: Re: [EVDL] Building a true road going solar EV.
I'm not qualified to comment on the power requirements of a particular
system, nor the power sources previously discussed, but I would like to
make a suggestion to the builder of this vehicle.
Research in-wheel motors (hub motors) before you commit to that design.
Look specifically for references to handling and braking. You'll find
key words such as un-suspended weight frequently used in this region of
expertise.
Also consider that a typical hub motor of the kind used for bicycles
has a very narrow range of efficiency and a similarly narrow range for
power. Range of rpm is the relevant point here. Bicycle hub motors
throw away energy as heat when traveling slower than optimum and can
get away with it, in part, because the human bean is providing
compensatory energy via the pedals.
I suggest also to research the Stokemonkey e-assist power system or one
of its clones. The Stokemonkey is a slightly modified hub motor which
drives the left side of a crankset, allowing the cyclist to select the
appropriate gear in the normal bicycle power train, matching the
cyclist's cadence preference to conditions. If the system is correctly
installed, the cyclist's cadence is closely matched to the motor's
narrow range.
If efficiency is your goal and it appears that is the case, you may
benefit from using a similar design in your EV, including some form of
transmission. My first velomobile was equipped with a Stokemonkey. With
pedal assistance, I was able to enjoy two watt-hour per mile
consumption. Flipped around, that's 500 miles per kwh. Without my pedal
assistance the energy use skyrocketed to nearly ten watt-hours, or only
100 miles per kwh. I suspect there are few EVs on the road, other than
the hyper-efficient solar models that can get close to those figures.
There is a similar design to the Stokemonkey, a one-off for the
personal use of the builder that weighs under five pounds with
batteries, with a 100 watt motor. He built it solely for the purpose of
assisting hill climbs. At that weight and that level of power, it's
perfect for his requirements.
Your vehicle could do well enough, I believe with a pair of 1 kw hub
motors driving a common transmission with suitable gearing. My current
velomobile has a Rohloff 14-speed hub transmission, but it is not
covered under warranty for electric assist. I doubt that it would
tolerate the forces of two 1 kw motors. The big advantage of the
Rohloff is the linear progression of the gearing, a 426% range with 12%
steps for each click of the shifter.
You would not have the space constraints in your EV that exist in a
bicycle wheel and might find a more effective transmission design, but
I doubt that hub motors are going to be a suitably efficient answer.
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