how to work out what gearing to use
 

got my motor mount from here a few weeks ago but i am still awaiting on my tekin RX8 and a 4000kv motor and 1 3S & 4S lipo to test out

but what i amnt sure about is how to workout what size pinion to use

atm it has a 46t spur gear

also dont know the ratio in the front/back diff's but not to sure if i realy need to know that

Wow... What are you running a 4000kV motor in on 4s? Last time I checked that's about 60000 RPM...

Anyways you can use this tool made by BrianG (he is a member here). RC Calculations

Here you go.

http://scriptasylum.com/rc_speed/index.html

That is the link to the calculators and lots of great tools and info. Play around with the caclulators section to get an idea of what it can do for you. THere is also one to solver a parameter. You can put all you info like spur gear, batteries, motor, and how fast you want to go but leave the pinion blank and it will tell you which one you need. The options in the calculators have drop down list that have differential ratios already inputted, just click on yours. You post doesn't say what you are running so that's some info people would need to answer what you diff ratio is.

You do need to know the diff ratios, though if you told us what vehicle you had and any other useful info like tire diameter and intended use (with a motor kv that high and 4s lipo one assumes speed runs?), it would make life easier- if its a common vehicle, all the required info will be there in the calculator, just choose/input the applicable info and it'll spit out the estimated speed with any given gearing- go up or down on the spur and pinion until you get the desired result :smile:

yeah i plan on using the 4S to try and put one of my mates cars to shame the next time there is a 1/4mile drag comp, if i can get this conversion completed in time

dont think he will be to impressed with a RC car beating his 1:1 car

Edit:

car i am converting is a thunder tiger ER-1

tyre dia = 4"

http://www.tiger.com.tw/product/6234-F.html

Assuming you can find enough traction, you'll have no trouble pulling away from a 1:1 with the type of PtW ratio you get with brushless power.

yeah i should do when i had my .28 nitro engine in it with an 18t clutch bell i could take of faster and was quite close but i blew the engine so thought i would try it with electric

If this is your first BL setup you'll probably want to practice driving a bit before you challenge your friend. Driving in a straight line is much harder with this much more torque...