Razor GF Build Initial Questions

[scrotty]

I'm looking to mod a Razor Ground Force for use by my 8 and 11 year old kids.

My reason for modding is two-fold: 1) Increase runtime, 2) Slightly increase performance.

Based on what I've learned from itsid's excellent sticky post, I've made the following observations:

1) The total weight of kart (plus batteries mentioned below) and kid is ~125kg.
2) I'm plenty happy with max operating inclination of < 10%
3) I'd like 1.5 hours of run time per charge
4) The Razor GF has 4.5" wheel diameter
5) I want to limit top speed to 7.5 m/s (~26 km/h or 16 mi/h)

And from this and itsid's formulae my guesstimates:
1) I should be looking for a 24v/500W motor* (the stock model for this model is 24v/250W)
2) I'll need four 12v/18ah batteries in which pairs are to be wired in parallel and those pairs then serially for a 24v/36ah total. (These batteries are ~6 kg each, adding 24 kg to the stock kart minus an estimated 5 kg for the stock batteries which I'll remove.)

My two questions:
1) Does this seem reasonable?
2) How do I cap top speed? Can this be done with a controller? I take it I need some way to limit motor rpms.

TIA!
Sean

* I show how I arrived at this guesstimate a couple posts lower

 

[Poboy kartman]

Welcome to the forum! !!!!

 

[scrotty]

I should show my work...

Per itsid's magical equation for motor power needed is:

[B]Pm = m * g * Cr * v + 0.5 * Da * Cw * A * v³[/B]

[B]Pm[/B] = Mechanical power
 [B]m[/B] = Mass of kart + rider = [B]125 kg[/B]
 [B]g[/B] = Gravitation force = [B]9.81 m/s²[/B]
[B]Cr[/B] = Surface coefficient = [B]0.02[/B] (gravel)
 [B]v[/B] = Max relative velocity = [B]7.5 m/s[/B]
[B]Da[/B] = Air density = [B]1.2 kg/m³[/B]
[B]Cw[/B] = Wind resistance coefficient = [B]0.7[/B]
 [B]A[/B] = Area facing wind = [B]0.6 m²[/B]

[B]Pm = [COLOR=Navy][COLOR=Black](125 kg * 9.81 m/s² * 0.02 * 7.5 m/s) + 
       (0.5 * 1.2 kg/m³ * 0.7 * 0.6 m² * (7.5 m/s)³) 
   = 184 W + 213 W 
[/COLOR][/COLOR][/B][B][COLOR=Navy][SIZE=4][COLOR=Black]
[SIZE=5]∴[COLOR=Navy] Pm ≈[/COLOR][/SIZE][/COLOR][/SIZE][SIZE=5] 400 W[/SIZE][/COLOR][/B]

Since (again per itsid) karting motors almost always list Pm versus Pe, I can just use this 400 W value when looking at motors (versus Pe = 1.25 * Pm → 1.25 * 400 W = 500 W). The nearest common motor power rating that exceeds this value is 500 W (which nicely also matches my calculated Pe making that concern moot).

Keeping to itsid's recommendation to minimize both motor voltage and motor power, a 24v/500W motor seems to best fit the bill for my needs.

But again, how do I keep my kiddos from exceeding ~7.5 m/s?

 

[scrotty]

Hmm, the motor I'm looking at runs at 2500 RPM.

Per this very site's How Fast Is Your Go Kart? calculator, my top speed may be far less than 7.5 m/s.

The Razor Ground Force has an axle sprocket with 32 teeth. Its wheels are 4.5" (11.43 cm) diameter. The motor I'm looking at has an 11 tooth sprocket.

[B]Gear ratio[/B] = axle teeth / clutch teeth = 32/11 = [B]2.91[/B]
[B]Wheel RPMs[/B] = Engine (Motor) RPM / gear ratio = 2500 / 2.91 = [B]859 wRPM[/B]
[B]Wheel circumference[/B] = π * wheel diameter = π * .1143 m = [B].359 m[/B]
[B]Max relative velocity[/B] = wRPM * wheel circ. / 60 = 859 * .359 / 60 = [B]5.1 m/s [/B]

Assuming I'm doing this right, the result of 5.1 m/s is pretty paltry. I can improve this by replacing the motor's 11 tooth sprocket with a 14 tooth socket, thus reducing the gear ratio from 2.91 to 2.28 and thus bringing speed to 6.5 m/s (23.5 km/h or 14.5 mph). This is still a bit disappointing, but I'm really not sure I'm doing all this right anyhow.

 

[Standard issue]

You could put a potentiometer in line with the throttle wires. I know modified power wheels.com has a how-to. I personnaly used a pulse width modulator with a potentiometer to govern speed. Its not variable by throttle input but who.goes any speed but top speed anyway?/you can up the power output as they become better drivers.

 

[itsid]

You can limit the throttle with a potentiometer like Std Issue said,

or just make a special 'key' (think of a car flat fuse but with a resistor inside)

Anyway.. since this motor is too slow for a 11:35 sprocket set...

you should think about finding another rear sprocket with 23 Teeth that should leave you with 16.01 mph. (25.77 km/h or ~7.16 m/s )

or 22 Teeth for 26.93 km/h (16.73 mph or ~7.48 m/s ) depending on what you want

keep in mind that the equation you chose is not taking road inclination into account nor acceleration, it's the top speed on flat ground 'short-version'

When I make a suggestion about what motor to choose I go this way:
Fi = m * g * sin(alpha)
(alpha of 5° which is almost 9% good for most circumstances)

and multiply the result with the wanted top speed to get a Powerout...
125 * 9.81 * sin(5) = ~107 N
107 * 7.5 and we end up with a bit over 800 Watts.

I chosse that path to guess the motor size, because it's just a fraction too much in most cases, so you end up with a slightly nicer acceleration and the ability to climb a slightly steeper 'hill'
which in most cases only costs a shorter range or bigger batterypack which is much less of a shame than a disappointing experience

So, I'm afraid the groundforce has to loose some weight
(has to drop just below 78 kg incl the driver for the same performance with just 500W)

I'm sorry...

The My1020 is rated in POWEROUT already btw, so it's indeed a 500W power to make use of.
it draws a little over 600 Watts IIRC,
and in batteries that means ..

oh god... hang on...

for 1.5hrs of driving time and a hopefully a ~70% powercycle or less.. that's ~650Wh (6fourtysomething)
@ 24V that's 27 Ah of capacity (useable capacity)
high grade batteries should allow for a 90% usage, but let's be safe and say 85% (until the voltage drops below 18V at which the controller will shut down the power to keep the batteries alive and chargeable)
so yeah 32Ah should do. the 36Ah pack is just perfect!
(a minute extra doesn't harm)

the 800W motor though would need 1000Wh (again 9 ninetysevensomething)
that's ~42 Ah so a 50Ah pack is what you'd want to get 90mins of fun out of it, in case you increase the motor power.

I'll keep my fingers crossed that I haven't confused you too much with that (and my motor essence thread.. Oh thank you by the way )

In case you have a question, I'll be happy to answer if I can

'sid

[EDIT]
I was wondering about the "heavy weight" with 125kg for a Razor Ground force..
it's max capacity is 140 lbs and it's own weight is listed as 55lbs
so 195 lbs MAX.. that's 88.5kg!!

and that would mean a 600W motor would be ideal! (well 570W )
So I think you should be good with the 500W motor perhaps.

 

[scrotty]

Thank you so much for the responses! Last month I went ahead and ordered a 36V 500W my1020, three 12V 20Ah batteries, and a YK43B controller. (I also got a 36V 4A 3-stage VeriBest battery charger.)

I haven't put anything on the kart yet. It's all a xmas gift for my two kids and I'm now thinking that the modding work should be something we do as a family - let 'em learn a bit about how a kart works.

I think I'll go with these parts for now and then "test in the field" so to speak to see how the kids' usage might be better served by changes to sprocket/wheel or even motor.

Thank you again for taking the time to help me out! This forum has been immensely useful.

[EDIT]
As far as the weight is concerned, I figured the max capacity of 140lbs would exclude the cart itself (except for the increased weight from new batteries). My older kid is 70 lbs and the batteries and motor add another 50 lbs. So at 120 lbs total I'm hoping I'm okay. No?

 

[wesleyb82]

Don't mean to jack your thread, but wanted to show you our modded drifter with a 1000w motor, check it out:
https://www.youtube.com/watch?v=AOJC02BQ-eI
1000 watt 48 volt Unite electric motor, 4qd Porter 10, 72T/11T, 17mph on paper, 13x5x6 on 6x5.25, 1"x32" axle, 5k thumb throttle, fiberglass seat, 6" led light bar, razor drifter frame.

EDIT: Oh and we ran it was a 450w Currie first and not only did it overheat and almost STB but the kart IMO was underpowered

 

[scrotty]

Thanks, @wesleyb82. I haven't even modded my kart yet and I already feel like I need to upgrade.