Sunday, May 22, 2011

Advices to build a competitive RC Car. Part 1: Gear ratio

This is the first in a series of articles in which I shall give advice to build a competitive RC car

In this first I'll show advices for choose the correct gear ratio



I know sometimes choose the correct transmission ratio is not easy, so I will try to make it as easy as possible

For choose it are very important this things:

- Wheel diameter
- Motor type and quantity
- Preformance type: more speed, more acceleration or an average point
- Approximate total weight

For determinate de correct gear ratio you must use this formula:

In kph  ->   Speed = M x R x Wd x 0.0001885

In mph ->   Speed = M x R x Wd x 0.0001177

Note: the numbers are only for change units.

Where:

M = Speed motor under load(rpm), using information of Philo's website
        - RC= 940 (ext.) o 1270 (int.)
        - PF XL=140
        - PF M=280
R = multiplication ratio (1: R) from motor to wheels axle. For exemple if you have 3:1 ratio, divide by 3 --> 1:0.33 --> R = 0.33
Wd = wheel diameter (in mm)

Example 1: This kart http://www.sheepo.es/2011/04/kart-rc.html


- XL motors for traction
- 43.2x22 ZR wheels
- Gear ratio: 24/8 x 24/12 = 6

Speed = 140 x 6 x 43.2 x 0.0001885 = 6.84 kph
Speed = 140 x 6 x 43.2 x 0.0001177 = 4.27 mph


Example 2: This car http://www.brickshelf.com/cgi-bin/gallery.cgi?f=371117


- RC motor in external connector
- 68.8x36 ZR wheels
- Gear ratio 12/20 = 0.6

Speed = 940 x 0.6 x 68.8 x 0.0001885 = 7.31 kph
Speed = 940 x 0.6 x 68.8 x 0.0001177 = 4.57 mph


For this table I have used a standar weight car around 700-800 grams, if your car is heavier speed will be slightly smaller, and  if it's lighter a little higher.

By my own experience the correct final speeds are this:


I hope this wiil be usefull for you!!

And comming soon the next parts

11 comments:

Anonymous said...

you should also do a tutorial on how to build 4x4 off roaders. that would be really cool!!

Ray said...

I think that too. Thx fOr these advices. Help me a lOt!

tuur said...

wich are the 2rc, rc,2xl and 2m motors

Sheepo said...

I built that cars but I didn't take pic :(

Anonymous said...

Where can I buy an rc motor?

Anonymous said...

24/8 x 24/3 = 36 , not 6.

Anonymous said...

how do you connect two XL motors and one M motor to the same battery without the overload prevention system kick in and stop all the motors?

Sheepo said...

If you built the car woth correct gear ratio, low weight and not change the direction of the motors while the car is moving you shouldn't have problems with that...

Anonymous said...

hi sheepo, love your models.i want to ask you one thing. is it posiblle to make an rc car that goes off road ato the same or higer speed of your xr7 using only one XL motor?

Sheepo said...

No, If you want to go as fast as the XR7 (or ever faster) you need more power than only one XL motor, you can try with one RC motor or two L motors, but the result will be similar...

Anonymous said...

ok thanks a lot. So what could I make? I have 1 XL, 1M, 1 IR Reciver, 2 sets of lights and a controller. I know that there are a lot of videos on youtube that show you how to make cars like that, but I don't like them much. If you have anything in mind please tell me. I whould prefer it being an off roader, but if it goes fast on tarmac, no problem :). I have already made your recomended front axle with return-to-center steering.
Thanks again.

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