DIY Electric Car Forums banner
1 - 9 of 9 Posts

· Registered
78 Posts
Discussion Starter · #1 ·
Turns out that large heavy, slow, fork truck motors have at least 4 major negatives.

It has lots of constant power robbing dead weight.

The gear reducing or increasing parts, need to be larger & heavier, if no current limiting is available.

It requires a much heavier dead weight frame to carry it.

More space is required.

High voltage, low current, high ( K V ) rpms per volt, motors, are lower in losses with a wider band of efficient rpms.

" It is far better, with greater milage, to carry high voltage, than high current. "

I am sure this has all been stated before. Just a recap for new projects.


· Registered
654 Posts
Ok, but if you are at a set voltage (limited by battery fit), say 120v, is it more efficient to run that through 7", 8", 9", 11"?

Assuming all met your steady cruising speed requirements (60mph aero small car).

My analysis was 7" (6.7) would be right at it's limit. risky
8" chosen for high rpm on low voltage, light weight, small size.
9" has too low rpm at given voltage for highway work, but would be good for a heavier vehicle due to the extra torque. hp is same as 8"
11" the hot-rod option, but comes with a weight and size penalty.

So it really was between the 8" and the 11". All other things being the same, which would get lower watts/mile?

· Admin: 'one of many'
4,841 Posts
I think too many factors come into it for an easy answer though someone may come along with one, or an easier way to decide.
I am using an 11" motor in a light weight trike where a 6" one would probably be fine. However I am also having a high ratio transmission, 2.8:1, instead of a low ratio transmission, 6:1 because I can run low RPM and still have torque to get to a high cruising speed, in theory. Also the bags of torque will probably mean the motor won't draw that much current pottering around town, at least that might be the case.

If I were to use one of my smaller motors I could change the ratio and run at a much higher speed. That is like six of one and half dozen of another.

The biggest benefit, as you pointed out, would be weight.
The 11" motor is 104kg about 1/3 of the weight of the trike without batteries.:eek:
A 6" would be about 1/8 the weight in comparison and a 9" would be about 1/6 the weight.
My pack would weigh an addional 170kg.

The main reason for using an 11" though is for the Hotrod theme. I want a big motor and I am willing to sacrifice a little (ok, a lot of) weight and performance to have it.:D

· Registered
654 Posts
I use this calculator to find needed Watts at desired cruising speed.
It is better to make everything a bit worse than reality when using a calc like this than be optimistic and melt your motor in the real world. So I up rolling resistance, weight, and aero drag a bit, just to be sure.

Obviously, you divide by system voltage to get steady amp input. The motor you choose had better be rated for at least an hour at that amp rating.

Conversely, you can take the 1hr rating of your motor and multiply it by your voltage to see the max watts at that voltage. If you come up short, you need to increase voltage if you plan to keep the motor.

I have a vw Bug chassis with a Jaguar body that is light, but not aerodynamic.

The ADC 8" has a continuous rating of 180amps. The system voltage is 96v. That equals ~17KW (23hp). Worst case scenario the calc says I can go 65mph steady. Best guess numbers put it closer to 70mph. The commissioner of the car wanted a cruising speed of 60mph, so I have a bit to spare. A good thing. If he wanted to cruise on the interstate at 80mph, then I would need to up the voltage to 144v. The current build should do 80mph, just not for an hour.

Forced cooling will allow you to get a bit more from the motor, so if you are encountering heat issues, more cooling and higher voltage are the answers.

Also, 80+% of my energy is going to move air. So aero mods can go a LONG way toward increasing top speed, reducing amp flow, and increasing range. Reducing the aero drag from .5 to .3 would suddenly allow steady cruising over 75mph with the same system, and a 33% drop in the energy required to go 65mph. That means 33% MORE RANGE! So my 100 mile range would suddenly increase to 133 miles.

This list shows the best donor vehicles by lowest cdx:
It is an incomplete list and they omit many such as the Eagle Talon which is just after the 2002 Acura NSX at 5.85.
You can see that a Honda CRX is the best low-dollar vehicle to convert. 88-92 years have best aero, but 86/87 is not much worse and a bit lighter. There are only a couple of crx conversions on the evalbum, and only one of those is lifepo4, so it is a really under-used chassis considering it's potential efficiency. has shown how to radically reduce aero drag. I am surprised the EV crowd is a little slow to jump on the band-wagon since there it would increase EV viability greatly.

· Administrator
6,650 Posts
Hi Ruckus

The ADC 8" has a continuous rating of 180amps. The system voltage is 96v. That equals ~17KW (23hp).

Can you increase the voltage? - more volts = more revs and more power - also the cooling fan will move more air

Heating is ~ proportional to current which is proportional to torque

So more revs and volts is good until it explodes!
1 - 9 of 9 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.