Ultra lightweight/efficient ecart

Aphocks

1 mW
Joined
Sep 21, 2015
Messages
18
Hello all, I'm new to this forum and I joined specifically for help on a project i'm building. I am trying to make to most efficient one man electric vehicle possible, similar to the PAC CAR ii but with only two car batteries. I'd like to set this up as kind of a free discussion to see what we can come up with under a best-case design scenario. Thank you for any contributions! :D
 
I have seen this post, and although it is very nice, I am aiming towards something less utilitarian and more efficient. I mean bare-bones, hardcore engineered, minimalist, one man vehicle. Efficiency is my only goal! :p
 
haha that thing is awesome, have you got any ideas for something with three wheels and an aerodynamic fairing?
 
Sounds like a fun project. the PAC CAR ii was designed for a track. It was purpose built to chase a goal of efficiency at 30kph. As such, it would only be built strong enough to sustain those speeds on a track, not fight real world conditions at higher speeds. Once an design is optimized, making it stronger means adding weight, and killing it's efficiency.

So what design paramters do you have for this project? Are you looking to build a track racer, or do you plan to put this thing in traffic, fighting it out with Tahoes, Durangos, and Explorers in rush hour on the Santa Monica Freeway?

And when you say car batteries, do you really mean you plan what could be a multi million dollar project on a couple of lead acid automotive starter batteries? :shock:
 
I am building a track vehicle for a competition. I am shooting for 20-30 mph. And unfortunately the race parameters require lead acid car batteries. I do have a 5k budget though. I intentially excluded most details about the restrictions because I already have a design I'm working on but I wanted to see what people could come up with without any creative restrictions. I find that once you are told one way to do something coining up with an novel alternative method is difficult.
 
Aphocks said:
I have seen this post, and although it is very nice, I am aiming towards something less utilitarian and more efficient. I mean bare-bones, hardcore engineered, minimalist, one man vehicle. Efficiency is my only goal! :p
What you are describing is a BICYCLE . !
Add a small hub motor and you have the most efficient powered human transport. :wink:
 
Any specific cam be arrangements/materials? What do you think would be the lightest pneumatic tire style/diameter?
 
Starting with the body, think outside the norm. The shell will need to be ultra light, and good aerodynamics are far more important than your wheel resistance. You may want to research the Dacron covered Geodesic Airolite Boats. Some of the boats are as light as 8 pounds. Yes. A row boar that weighs as much as your shoes! The principles involved in building them would work well for building an aero shell within your budget.

Motor depends on the course. a hub motor would win on a constant speed track, but if you have to accelerate and break, then a geared system would likely be of some advantage.

Tire choice depends on wheel choice, and that depends much on the other aspects of the design. Counterintuitivly, many manufactures have found and published data showing a wider tire has less rolling resistance. however, they do have more air resistance. There is also a huge amount of variance in rolling resistance based on things like tire pressure, rubber compounds, and TPI.
Higher pressure rolls with less resistance over smooth surfaces, but lower pressure will role better over rough surfaces. Harder compounds can have less rolling resistance, but if you need the vehicle to handle in corners, or stop under racing conditions, a softer compound might be better. Threads per Inch (TPI) counts over 300 are very siuple, and have a lower resistance, but the higher the TPI, the more prone to damage the wheel can be from road debris and rough road conditions.
etc, etc , etc.
Larger diameter wheels roll with less resistance, but cause more wind resistance.
 
Fantastic info, thanks!

I've selected a "Maxon" dc 250 watt motor with a 93:1 reduction gear box. The motor reaches efficiencies of 83% under normal operating conditions at 24v, 10amps.

That dacron covering is something I've scratched my head at, I love the idea of a lightweight fabric body supported by thin fiberglass rods. One thought I had was a heat shrink-wrap type plastic film. Either polymers would have similar molecular mass so I think weight will depend largely on the thickness required for a specific tensile strength.

The road being a smooth level track, I'd say a "slick" no- tread tire,120 psia, high hardness rubber. I'd like the tires to be as small a diameter as possible to lower profile drag, but a ~20" wheel would probably have to be custom made with the above specs.
 
Aphocks said:
I've selected a "Maxon" dc 250 watt motor with a 93:1 reduction gear box. The motor reaches efficiencies of 83% under normal operating conditions at 24v, 10amps.
.
Aphocks said:
....... Efficiency is my only goal! :p

83% would be efficiency of the motor only ?
a 93:1 gearbox will be either planetary or worm reduction and will knock that efficiency way down at the output shaft.
can you not use a motor with a low Kv /rpm and avoid that gearing reduction ?
 
The only reason I didn't look into them much is that I was able to see the competition last year and one team used a hub motor and they didn't place in the top five, while all of the winning teams used some form of chain/gear drive.
 
Aphocks said:
I've selected a "Maxon" dc 250 watt motor with a 93:1 reduction gear box. The motor reaches efficiencies of 83% under normal operating conditions at 24v, 10amps.


That sounds like a good way to end up last. Ebay hub motors have higher efficiency, and any form of gear box will lose power. A 93:1, you would probably be lucky to only lose 15%.

Look at RC outrunners. you should be able to find some in the low 90%s, and with a KV that will allow a lower gearing for better efficiency.
 
I appreciate the input, I have very little experience working with RC motors, would there be one that you would recommend for my use, again I need a very high efficiency/12-24 vdc/250-500 watt total output range (may be supplied from multiple motors)/and i need my wheels to spin at around 75 rpm so whatever motor delivers nearly that or can be efficiently geared to do so. I'm assuming that no single motor will give me the torque i need so I will likely need two or three working together?
 
Would an outrunner RC motor used as a friction drive be a good idea? Very light, no gears or chains needed and might work quite well on a dry flat and clean track.
 
I'm not sure, it would depend on how the losses from a friction drive compare to the losses from a reducing set. I know you can expect about 20% losses from a gear box, and we can estimate the losses from a friction drive:

Rolling resistance between friction wheel and tire is given as:

Ff=(Cl*W)/R
Ff: friction force
Cl: dimensionless coefficient of friction (I will use 0.1)
W: the force that the friction roller applies to the tire, I will assume 22N (about 5 lbs)
R: radius of wheel (.254m for a 20" wheel)

So Ff=(0.1*22)/.254= 8.66N
With a .254m D wheel that's 2.2 Nm of torque opposite to the motor's.


Lets use my current Maxon motor for reference because I have all of the specs.

The non geared motor produces 501 mNm torque at nominal operation.
Assuming a small (2" diameter friction roller)
That's a 1:10 torque ratio with the 20" wheel meaning that we must multiply the motor torque of 501mNm by 10 which makes 5.01 Nm

net efficiency= 2.2 Nm/5.01 Nm = 44% efficiency for friction
En= 80% for gearbox

from this math (and please correct me if it is false because I am no expert!) it appears that a gearbox is more efficient by 36%
 
After looking into it more, it sounds like the losses for a friction drive don't increase with motor power. what that means is that if i simply use a motor with more torque, the efficiency of the system will go up. basically, the friction losses are set and won't change with power, so they become less in comparison with the force of the motor as the motor power increases.
 
There are efficiency losses similar to a friction drive, the calculation I used is actually a rolling resistance equation. I calculated the resistance in newtons caused by friction between the wheel and the drive wheel. Then multiplying that by the radius of the wheel to find the resistance in Nm (torque) I then divided the torque of the motor by the resistance torque in order to find what percent of the motor's torque was lost to friction.
 
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