Speed calculation (check my math)

[Scout]

Just to make sure I am doing this right I'd like someone to check my math.

1 mile = 5280 feet
5280*12=63360 inches

Max engine RPM = 3600
Gear ratio = 6:1
3600/6=600
Wheel rpm = 600

600rpm*60 minutes= 36000 revs per hour

Tire diameter = 13inches
13*pi=40.84

Good so far?

36,000*40.84=1,470,240 inches per hour
(in/hr)/12=122,520 feet per hour
(ft/hr)/5280=23.20 miles per hour

Is all that correct? If so then I can work it backwards and figure what I need for a desired top speed of 40 mph

(ft/hr)= 5280*40
211,200fph*12=2,534,400iph
2,534,400/40.84=62,056rph
62,056/60=1034 tire rpm
1034*6=6204 engine rpm

So if everything is correct, if I build my motor for 6500 rpm, and the motor makes enough power at that speed, I can hit 40 mph.


I'm using a hemi Predator, with this box stock kit http://www.arcracing.com/servlet/the-14/Box-Stock-conversion-parts/Detail and this exhaust http://www.arcracing.com/servlet/the-453/Box-Stock-Project-ARC/Detail. I won't be using the valve springs until I get a flywheel and rod.

 

[Scout]

I also tried it out using gps speed of my current setup with the briggs fun power motor, and the math says it was turning about 2800 rpm at 18 mph. I suppose I'll be looking into some sort of tach for this kart too.

 

[itsid]

you can ease the math a bit:

engine rpm * clutch sprocket / axle sprocket = wheelrpm
pi * wheeldiameter / 12 = circumference
wheelrpm * circumference / 88 = mph

but yours is correct

And backwards that means:

mph * 88 / circumference = wheelrpm
(40 * 88 / 3.403 = 1034.26)

So yes.. if you keep that ratio you'll need the engine to do 6200+ rpm

OR you get yourself a series 30 torque converter and a slightly different ratio
and to reach 40 mph you'll only need a 4500 rpm
with a 1:4.8 ratio

that includes the overdrive of 10% and to get you moving the initial reduction of 2.7
so your gear ratio will vary between 12.96:1 and 4.32:1
(given that the engine has enough power to reach the upper end of course)

since it's a predator..
I guess it's save to assume you can reach 5500 rpm with just governor removal and highflow airfilter and exhaust (and rejetting of course)
And with that you can drop in a 5.5:1 ratio to hit 43mph or keep the 6:1 and hit 39.4 mph
(again with a series 30 TC; 6" driven)

if you want to have a clutch only, a 5.3:1 reduction will get you to 40 mph as well
for a predator with the governor removed.

'sid

 

[Scout]

I am unsure about the TC, seeing as the motor will be built up some and a 30 series won't handle a lot of power. Plus, this kart besides being a yard kart, may also see some dirt road/trail action.


40 mph is a goal I want to hit because I'd like to take it to some well known curvy roads and have at it. (Tail of the Dragon)

 

[itsid]

with just a governor removal and eased breathing, you don't add too much HP.. you'll end up with maybe 10HP max..
something a series 30 can handle;
yes I know it's rated at 8HP.. but just go and ask Tihm how his is doing..
and his GX200 is more likely in the 12-13HP region as it is now

And a TC is not only perfect for going off roads, since it can provide almost three times the torque to climb a hill or get out of a gravel pit;
it's also providing said torque to get you at speed and then shifts down to provide the top speed you're after.

But I agree if you intend to heavily modify a 212 predator, you can add enough power to burn a series 30 belt in an hour or two;
and that's not going to be fun anymore.

But be careful: with a 5.3:1 ratio and just a clutch, you'll need lots of power for the yard part of the ride, plus you'll need a clutch that'll withstand some bad treatment.

Maybe it's easier to get yourself two split sprockets (60 and 53 teeth) plus two matching chains, and change the gear ratio according to your underground

'sid

 

[Scout]

I don't know how much I'll modify it, maybe throw a cam at it, but I don't think I'll do much more, aside from the rod and flywheel, but those are a given.