Go Kart Top Speed calculus

[garolittle]

I am getting different numbers when I run the following top speed calculation. Can you help? Assume the following.

Go Kart weight with batteries, motor drive system and driver: 400 pounds total weight.
Rear Wheels: 6 inch diameter (Rear Tires are Maxxis “Pink” with a 12” diameter)

Motor: AC-9 48V
Controller: 48V 600A Curtis 1236SE-5621

Running 600 amps (max) on level payment in 70 degrees Fahrenheit, what do you think my top speed would be with a sprocket ratio of 4-to-1. Thanks.

VenomKarts, LLC
VenomKarts.com
Augusta, GA

 

[Tpdingo]

I did a quick calc and according to the AC-9 data I see it can go to like 7000RPM and that would mean a top speed around 60+ mph.

This doesn't necessarily mean it WILL get there.

 

[garolittle]

I did a quick calc and according to the AC-9 data I see it can go to like 7000RPM and that would mean a top speed around 60+ mph.

This doesn't necessarily mean it WILL get there.

Thanks. This is good information. I will run a few tests and posts the results when the kart is ready. I may lose 5 pounds just to help with the power to weight ratio. Thanks for the response.

VenomKarts, LLC
VenomKarts.com

 

[karte_m0lim]

Had same plan... But decided that downforce is more important.. So gain few kilos :-D

 

[garolittle]

Had same plan... But decided that downforce is more important.. So gain few kilos :-D

Ha! I struggle with the trade off between higher speed and enjoyment of food.

 

[itsid]

Well, you know all the number, heck there's even a hp/tq per rpm chart
(see attached) for that motor with a similar -albeit more powerfull- controller
slightly crap because of hp values
(but if they used valid scientific numbers a horse power is 735 W)

All you need now is "real scientific values" (aka metric conversion)
and then...
do the math yourself
scroll to the end of post #2
to find this:
Pm = m * g * Cr * v + 0.5 * Da * Cw * A * v³

heck if we stick with 0.7 windresistance coefficient and a wind facing area of 0.9m²
as in the example
you only need to change the weight to 181.5 kg

and you can stick with it
you can calculate backwards from power by slightly shifting the terms
(but be aware you will need to take power @ the needed rpm.. hence I did it this way around )
and such you can get a very close approximation of your top speed.

181.5 kg * 9.81 m/s² * 0.02 * [X] m/s + 0.5 * 1.2 kg/m³ * 0.7 * 0.9 m² * ([X] m/s)³
= [Y] Watts of power

put in [X] values (in m/s) scale up and down step by step and see if you hit the motor limit at it's matching rpm

Oh right you likely don't know the conversions for wheel rpm to motor rpm
one turn of your wheel is 95.756 cm ( 0.95756 m) [assuming it's indeed 12" in diameter]
one wheelturn is 4 motor turns (that's easy right...) soo 1 rpm in the motor equals 0.239389 meters on the road
and 1 m/s of speed need 250.6377 rpms of the motor
so with 6500 rpms your [nothing else taken into account] value is 25.934 m/s as top speed
(93.4 km/h or 58 mph)
is it (doubt it) 7000 rpms on the motor it'll be 27.929 m/s (100.55 km/h or 62.5 mph)

So no need to punch in X values above 28

just for the fun of it:
181.5 kg * 9.81 m/s² * 0.02 * 26 m/s + 0.5 * 1.2 kg/m³ * 0.7 * 0.9 m² * (26 m/s)³
= 7569.596 Watts of power

26m/s need 6516.6 motor rpm (not in the torque diagram but we can safely assume it's less than at 6500)
and since it's 10.3 horses needed of max 8 available.. that is a sincere "NOPE!!"
going down a few m/s..

181.5 kg * 9.81 m/s² * 0.02 * 23 m/s + 0.5 * 1.2 kg/m³ * 0.7 * 0.9 m² * (23 m/s)³
= 5418.163 Watts of power

23 m/s means 5764.7 motor rpms and according to the chart the motor has more than 5500 watts at 5800 rpms.. so we're golden

Now, all you need to do now yourself is narrow down between 23 and 26 m/s of speed

concatenated math terms for easier calculator punching:
35.6103 [X] + 0.378 [X]³ = [Y]

'sid

 

[Quinc]

I use this calculator and it has been very accurate.
https://www.bmikarts.com/Go-Kart-Gear-Ratio-Speed-Calculator

 

[voided]

Only thing I found on building the electric karts is.. with lithium I get initial tests claiming more rpm. And once nominal I get less than rated rpm my karts geared for 26mph. I get average of 22 after 15 minutes of hard core driving .

 

[garolittle]

Now, all you need to do now yourself is narrow down between 23 and 26 m/s of speed

concatenated math terms for easier calculator punching:
35.6103 [X] + 0.378 [X]³ = [Y]

'sid

Dude you are like a professor. I wish I had half the knowledge that you have on this stuff. Thanks for the excellent information.

 

[Quinc]

Only thing I found on building the electric karts is.. with lithium I get initial tests claiming more rpm. And once nominal I get less than rated rpm my karts geared for 26mph. I get average of 22 after 15 minutes of hard core driving .

Overvolt and limit it through the controller.

 

[itsid]

Dude you are like a professor. I wish I had half the knowledge that you have on this stuff. Thanks for the excellent information.

but keep in mind that this too is just an assumption!

A much better one than the crappy calculator BMI uses
since that doesn't include ANY important info it just calculates wheel rotation
and assumes your motor or engine can actually overcome the resistances involved.

Anyways, Those numbers do include some assumed values
like
rolling resistance of wheels [assumed value 0.02] means cartyre on road, not mud not sand not grass mind you
wind resistance [0.7 is a guess.. race karts have lower numbers than yard karts of course]
frontal area [0.9 square meters is also just an assumption]

So yeah it too only ends up spitting educated guesses your way, but they're much closer to real world values than that BMI thing for example

'sid

PS overvolting.. *cough* the most smelled brainfart in EV...

 

[voided]

Overvolt and limit it through the controller.

My $25 dollar Chinese speed controller doesn’t have that capability... it does help in the beginning of a race to get a good start and let’s the group spread out alil.

 

[Quinc]

but keep in mind that this too is just an assumption!

A much better one than the crappy calculator BMI uses
since that doesn't include ANY important info it just calculates wheel rotation
and assumes your motor or engine can actually overcome the resistances involved.

Anyways, Those numbers do include some assumed values
like
rolling resistance of wheels [assumed value 0.02] means cartyre on road, not mud not sand not grass mind you
wind resistance [0.7 is a guess.. race karts have lower numbers than yard karts of course]
frontal area [0.9 square meters is also just an assumption]

So yeah it too only ends up spitting educated guesses your way, but they're much closer to real world values than that BMI thing for example

'sid

PS overvolting.. *cough* the most smelled brainfart in EV...

You sure do have all the answers on things you have no actually experience with.

 

[itsid]

You sure do have all the answers on things you have no actually experience with.

I never lit a cigarette in a sawmill,
never put a cat in a microwave
and never stopped a running table saw with my bare hands either
and still know it's a bad idea

Overvolting (by definition applying a voltage the equipment is NOT designed to run at)
is causing more harm than good.

Manufacturers usually know very well what kind of coils they wound for a motor,
what kind of heat and stress that winding can withstand..
What kind of heat they can dissipate from the motor to prevent
coatings, epoxies from degrading
and most of all the magnets from frying.

If you cook the magnetic field off a magnet, you even NEED to overvolt to get a close to original power of the motor..
and that means quicker battery drainage
(more power in to get the same power out is.. well ... you know the word right?)

And I know, there's a fine border between
"still good", "oh, need to keep an eye on the temp",
"shoot, now it suffers from heat damage"
and lastly "oh jeez it's a paperweight finally.. should've taken more care"

You managed to balance that line?
good for you
[frankly I doubt it.. from what you've claimed you're at the heat damaged state by now]

But telling people to overvolt is BS advice!
a brainfart par excellence

'sid

 

[garolittle]

You sure do have all the answers on things you have no actually experience with.

So I really respect Sid because he is incredibly knowledgeable. I actually like his blunt style and his willingness to call out a dumb question (I have asked several). I believe he just wants people to do the research instead of being spoon fed. As much as I like spoon feeding, I often realize that I have not done my homework after reading his response to my question. No worries. I will take Sid’s “blunt” style in exchange for his superior advice any day.

 

[Quinc]

Common practice in EV is to over volt and limit the voltage via the controller to reduce/remove sag.
For instance have 144v going to controller and from the controller have only 120v going to the motor.
As far as sending more voltage to a motor it is all about resistance.
The line you are talking about comes down to volting and gearing. running a 24v motor with a gear ratio of 1.1 is going to create more heat then running it at 32v 4.1


However, back on topic that monster of a motor/controller setup you have is not going to be affected by any rolling resistance and weight until you get above 60+ mph.

12" tires
4000rpm
4.1
= 35mph..once you stop burning the tires and get traction anyway.

 

[garolittle]

Thanks Quinc. Good information. I appreciate the response.

 

[itsid]

running the controller at a higher voltage than the motor is technically not overvolting ..
but at least it starts to make sense

Aanyways, had a lazy hour and was wondering what's left of my Flash skills
(if anyone can still remember the goto tech from 20years ago)
found it still installed and was curious... procrastination at it's best

Where was I.. right..
So here's a nice flash tool that gives you an estimation for your top speed,
based on the torque/power diagram from the pdf above..
you can change the wheelsize, gear ratio and weight below
aaaand then move the mouse and with it the crosshair as close to the blue line as possible...
the red line is the "flat paved road" power demand looking like this:


it only works for that motor and it needs to have a flash player installed of course
(the projector file [windows exe] is too big to attach)

'sid

 

[Quinc]

^^ That is pretty cool. @Garolittle what size sprockets are you running?

 

[garolittle]

^^ That is pretty cool. @Garolittle what size sprockets are you running?

Thanks Sir. So I am starting with a 60-tooth rear axle sprocket and a 15-tooth motor sprocket. I will try several sprocket ratios to gain an understanding of the impact on acceleration vs. top speed. This is the part of the project that I am looking forward to the most. . Follow me on my journey.... https://m.facebook.com/VenomKarts/?_rdr