Gear ratio top speed?

Jlposey

Member
I am running a much larger gear than we usually run. This weekend in rookie red we are running a 16-65. What do you guys think our top speed will be?

dirt track kart
Clone blaze martin engine
 

Jlposey

Member
Circumference is measured from the ground to the center of the wheel Or the over all circumference like 34 1/2 Max tire size?
 

Jlposey

Member
Or role speed. Thanks guys for the response little faster than we have ran before but I think little buddy ready to get up on that wheel.
 

Skidude

Member
Circumference is measured from the ground to the center of the wheel Or the over all circumference like 34 1/2 Max tire size?
Circumference is the roll out dimension. 34 1/2 maybe in your case. But you’ll need to convert that to feet. So, 34.5/12.
5000/4.0625 x 34.5/12 x 60/5280 = 40.2
 

alvin l nunley

Premium User
And some doubt the usefulness of spreadsheets. I'm a junkie! Keeping everything else the same, And incrementing the tire Circumference by 1/16, the speed goes up about 1.1 mph faster from 33.5 - 34.5.

mph ratio_1 8 28 30.jpg
 
Last edited:

paulkish

old fart
And some doubt the usefulness of spreadsheets. I'm a junkie! Keeping everything else the same, And incrementing the tire Circumference by 1/16, the speed goes up about 1.1 mph faster from 33.5 - 34.5.

View attachment 10928
How do you account for stagger, which tire is driving it forward and the combined effective circumference between the two tires because of tire slip?

As usual because the OP stated it's a dirt oval kart your spread sheet is again totally useless.
Even if it was a sprint kart in a turn with the inside rear not fully unloaded wouldn't the partially engaged inside rear slow the kart because it's engaged?
And then if it's slowing the kart any at all doesn't it then mean the outside rear has to be slipping?

On a straight with a sprint kart going exactly straight yes your spread sheet would be valid.
But in every other case including the stated dirt oval situation your spread sheet will give an incorrect answer because of stagger.
You like to ponder on your spread sheets ponder about it being incorrect in all but one case, which is not what is being asked on here.

Al, you really enjoy asking others to do useless stuff for you and report back.
Have you ever in your real years of kart racing taken a radar reading to see if your spread sheets are accurate on a sprint kart going down a straight?

After thunkin and writing the above I have a thought for the OP.
I think radar guns are not that expensive anymore, wouldn't it be fun to do the calculations noting highest rpm on the straight via the tach and compare it to actual speed taken from a radar gun?

Up front i'd bet actual speed is below every time with a dirt oval kart butt I ain't so sure and I think it would be fun to compare on the track.
hummm... out to look for a cheap radar gun.
Now I'm thinking it would be great to know which variables like stagger, tire compound, prep, air pressure with like top speed calculations will 'really' increase top end speed.
With cars it's common place to reduce stagger when the track gets slick, I think for grip reasons mainly in turns.
But how much change does what and I've been told many times by experienced racers(not really believing them if full) that they run 13"s of stagger no matter what all the time.
I wonder now if a stagger reduction because the track slows is really what's needed.
It's probably one of those things you don't need proof to do because it just works or seems to work, with the real question of did I do enough change or was it too much.

Thanks Al, started out poo pooing your spread sheet calculations and it lead to thunkin about the radar comparison thing and other stuff.
... wow, ain't this dirt racin stuff easy? ... :)
 

paulkish

old fart
I am running a much larger gear than we usually run. This weekend in rookie red we are running a 16-65. What do you guys think our top speed will be?

>>>>>dirt track kart<<<<<
Clone blaze martin engine
Above is post #1 which started this thread

... and I just ordered a cheap radar gun ... :)
 

Skidude

Member
Let’s look at this. An engine turning 5000 RPM with a 58:16 gear (3.625:1), 1” stagger (34.5R-33.5L)
5000/3.625 x 33.5/12 x 60/5280 = 43.8 mph
5000/3.625 x 34.5/12 x 60/5280 = 45.1 mph
Averaging the two you get 44.5 mph
Down the straight we know the stagger is holding us back, but what happens in the turns? If a flat turn has a 100’ radius where the RR tracks, then the LR tracks a radius approximately 98.2’, assuming 34” track width.
So, from turn entry to turn exit, the RR travels 628’. The LR travels 616.7’. A difference of 11.3’ So the RR rotates 218.4 time from entrance to exit, the LR needs rotate 221 times. Drop to 1/2” stagger and the LR only needs to rotate 217.7 times. Much closer to the RR and much less drag on the straights and turns.
Another thing to consider to consider is track size. A short track should hold close to the calculations. A large track is a completely different story and the faster the tires spin the larger they get. And LR and RR will most likely expand at different rates due to air pressure and rim width vs tire width.
To Paul’s point, it would be very interesting to make calculations and check it with a radar gun.
 

paulkish

old fart
Because the axle has stagger I think you need to calculate it for both rears and get an it's likely a speed somewhere in between the calculation for each or likely less then the lowest calculation.

After research I canceled my radar gun order until my ship comes in and I can get a good one. ... :)
And after research I want a lidar one not a radar one. ... :)
 
Top