Speedmunkey

Honestly, I'd say a good rough estimate would be to figure out the calculated potential top speed, then subtract 15-20% for weight, lack of traction, wind resistance and other environmental variables.. I'll put a few keystrokes towards that tomorrow.

pezz

My Feedback: (8)

So let me make sure that my noob ***** understands this right. I want the tire gearing ratio as close to zero as possible? and slight negative means more torque? and a positive number equals more top end speed? I have a Losi xxx-s running a P2K with a 90T spur, and a 25T Pinion. The internal ratio for Losi is 1.83 I assume that is the transmission ratio in the spread sheet? Using these calculations I come up with a tire ratio of 1.266. Did I do it right? it would seem I need a about a 21 or 22t pinion to get closer to zero.

Thanks for any help.

SkrapIron

That ratio depends on the circumfrence of your tires. If you are using standard 1" touring tires, your tire diameter is 2.1". The circumference of the tire is 2.1xpi(3.14)=6.597 inches.

Optimally, your final drive ratio (spur gear/pinion gear*transmission gear) should equate as close to the circumference of your tire as possible. In other words, your motor should turn 6.597 times for every revolution of your tire. A sum higher than the circumference of the tire will equate to more power, and a lower sum will equate to more speed. I think you may have miscalculated, since my formula shows it is .009.

Just looking at your current setup, and assuming that you are running standard tires, you are perfectly geared. If you want more speed, you could drop your spur to 88 teeth which will give you more top rpm's, but won't hurt your acceleration very much.

Hope this helps!

Speedmunkey

I wonder how well your formula translates for something like a Novak SS5800 that wants to be undergeared? Anyone tried it with a BL motor yet?

The more I think about this formula, the more I like it.

pezz

My Feedback: (8)

I measured the OD of the tire using my calipers, they are 2.50 dia. Is that close enough? or do I need to do it a different way?

SkrapIron

Given your tire diameter, your calculations are right. You have a tire/gear ratio of 1.266. Based on that, you are overgeared. If you drop to an 88 spur 23 pinion, you'll have better acceleration, without sacrificing much top speed.

SkrapIron

Munkey,

I have a Novak 4300 for my T3. The Novak literature suggests a 12.30 final drive ratio for proper gearing that's about an 87 spur, 17 pinion. I am running that setup for backyard bashing. It is a 12.282:1 ratio. I've thought of moving up 2 teeth on the pinion and see what the results are, performance wise, and how it affects the run time. I know that the brushless esc's are not very forgiving when it comes to heat management.... I'm going to stick with the formula for now, and let someone else melt their truck!

YzakB4DDRG

I just finished a mathematics exam....not I gotta go do more math?? Aww man! I though R/C was gonna be fun and rewarding... Well, doing more math is sorta rewarding, it'll help me next year in grade 10...

pezz

My Feedback: (8)

So, I did ok then? lol I am still not sure if I am aiming for a number closer zero, is that what I am after? I could drop the pinion to 22t and that would be real close to zero.

Speedmunkey

zero = dead perfect...

0.001-.999 = more top end, but not overgeared..
-0.001 - -.999 = more torque, but not undergeared.

SkrapIron

Munkey knows the formula! Listen to the Munkey pezz! He is wise! Keep within the ratio, and you'll be assured of a balanced, , predicatble, well handling car what will cut down on your overall maintenance, and keep you running long and happy!

pezz

My Feedback: (8)

Thanks for the help guys, I am new to cars (planes forever) so this will be a great help on my laptop while I am at the track.

Speedmunkey

"on my laptop while I am at the track. "

A man after my own nerdy heart!

sleepy23

My Feedback: (1)

wow, ran the spreadsheet munkey did on the latest speed run i did, i guess i would toast the motor esc real quick since it came out to be 4.668

SkrapIron

Yikes! I'd say that was a bit steeply over geared, Sleepy!

sleepy23

My Feedback: (1)

i got 52mph out of it under no load conditions

jeephead

this should be a sticky

aliens8mycow

Ahh... it's not overgeared until things get hot!

Speedmunkey

jeephead, I agree.

SkrapIron

Sorry for being dumb, what's a sticky?

Speedmunkey

sticky = a post that stays at the top of a forum. It's easily found, regardless of how long ago it was posted. See the thread where Study links to my article on pack building. You should turn this into an article for RCU.

Isaaru

Just 1 little question.
How do I know what my transmission gear ratio is (and what is a transmission gear ratio)?

Speedmunkey

Your manual should tell you that ratio. A Losi XXX-T MFE for example, has a 2.43:1 and an Associated T4 has a 2.60:1..

What it is, is the amount of gear reduction that takes place internally. Using the Losi for example, every 2.43 (I) turns of the spur gear makes the wheels turn 1 complete revolution. So, for every 40,000 (R) revolutions of the motor, if using a 20 tooth pinion and 86 tooth spur (X = 4.35:1), you get 3784.1 tire revolutions...

(R/X)/I = 3784.1

Now, to expand upon this, if you take the circumference of your tires, let's say you have a 3" tire which would have a circumference of 9.424" (C), and multiply that by the resultant from the above formula which we will name (F), you'll find that you travel 35661.4" per minute, theoretically anyway.

C*F = D

Then, you multiply your distance per minute (D) by 60 to give you a distance per hour (H) in inches, divide by 12 to convert inches to feet, and then divide by 5280 (M) to get your theoretical miles per hour.

(((D*H)/12)/5280) = M

So, if you work through all of these simple equations, you'll arrive at the final answer of 33.8mph... I think I might have to make another spreadsheet for this one too. I personally think top speed calculations are pointless, but most people disagree. Feel free to critique my math guys, it's far to early for me to be thinking this clearly, so i'm probably wrong lol

EDIT: Btw, I think I just duplicated Skrap's speed calculation, but with a bit more clarity on the equations used.

C3Racing

My Feedback: (1)

Evader S/T tranny gear ratio ain't in thier manual[:@]

Speedmunkey

Most Duratrax owners can't do basic math anyway.......... LOL J/K! It's the Traxxas guys that can't count....

I looked on their site, and I can't find it either. Anyone know it?