MyRCCar

Maybe people in this group is more in the 3d printing side than in the RC side, anyway i need some help from those of you in the second side...

You know i recently made a gearbox for the Monster/Crawler chassis wich should have solved all my problems in an mega-torque way... As i was having good motor temps in the OBTS chassis config i did not thougth about it initially for the Crawler build. With the sensored brushless motor i was having the low speed i was looking for and quite good torque, but the motor reached 90ºC in about 5min.

Then i made my 1:4 gearbox and of course, the motor is about 30-40ºC after 10min of crawling or more. But here comes the sad part: MY car is sooo heavy and if i make some instant full throttle/full break some of the gear in the gearbox gets "melted".

I have made some research this morning and i found some kind of "magic formula" to calculate a good gearing. It surprises me it doesnt have into consideration the weight of the car, anyway is something like this:

It's related to the circunference of your wheel (in inches, not in mm, explain me why please) and your total gearing ratio. The proportion between this two numbers should be 1:1

2piR is the circunference of the wheel: mine are 2.2 so... wheel circunference = 13.82

My initial config, without gearbox was 44/14 in central diff and 38/13 in FR diffs, so total reduction ratio from motor to wheel was = 9.18

THE FORMULA

wheel circumference - drive ratio = 0 This is the ideal gearing, they say. Anything over 0 is on the speed side, and over 1 will overheat your electronics. Anything under 0, negative numbers, will be o the torque side.

WITHOUT GEARBOX:
Lets see what happens with my original config without gearbox:

13.82-9.18 = 4.63 OMG if more than one heats your electronics... 4.63 could have burned mine in some more minutes, thx i had my termometer with me and i could smell it.

WITH 1:4 GEARBOX:
So before knowing this "magical formula" i decided to make the 1:4 gearbox and applyed it to the car, lets see if we find why it "melts" my plastic gears! we just need to multiply the total reducion ratio by 4. So total drive ratio with gearbox = 36.72

Applying the formula for the gearbox config:

13.82-36.72 = -22.9 OMG Maybe thats why i felt like i could pull from a cow with it!!! If in the example i read -0.72 was quite on the torque side... then -22.9 could be in the "destroy your pinions" side

WITH 1:3 GEARBOX:
If you take a good look the gearbox, there are two 1:2 reductions. I replaced one of them for a Mod 1.5 T11 and T17, so 17/11 = 1.54 The gearbox part reducion ratio would be then 1:3.09 instead of 1:4 Total reducion ratio = 3.09x44/14x38/13 = 28.39 Lets see what says the formula about this:
13.82-28.39 = -14.57 It looks like is also so much on the torque side of the formula.

Other Factors

But now comes other factors into consideration: The Wheight of the car and the kind of motor you choose or have. The actual config of my car is 3.8KG and the motor i would like to use is a 10.5T brushless sensored motor, don't know kV but i think would be about 3600kV and pairs with a 80A ESC. I know this motor is pointing towards revolutions instead of torque, so it is not really appropriated, but i spected to correct it with the gearbox.

I have also read about commercial crawler models or big rc cars to have total reduction ratios of 37 or more, so the formula for those ones should also be a big negative number for any wheel smaller than 2.2" radio.

I have another option, wich would give me more posibilities: building a gearbox with 11T and 22T Mod1 METAL gears, wich would be bought as 1/8 motor pinions. I would use 5x10x4 bearings and 5mm diameter piano wire for gearbox axles. I'm quite sure this would allow me to use 1:4, 1:3, or 1:2 reductions, wich would be always in the torque side of the formula, the motor should not get very hot and i could give full thottle/brake without melting gears, but what could happen then?¿?

I can work also in other direction, wich i "tryed" a little in the 1:3 gearbox: using different pitch or module gears, bigger or smaller teeth. I observed the heaviest cars 1/8 1/10 use Mod1, Mod0,8 or 32p and mostly metal ones. But crawlers seems to work with 48p and tiny gears...

So please, if you can give some advices about this maybe you can save me a lot of working hours (i use to employ brute force instead of other more efficient developement techniques) and you can see a working gearbox for heavy cars soon :)

Now we have 2 diferent chassis with a bunch of configurable options each. Most important one in both cars is the wheelbase.
If you take a deep look to my car makes you will know i used 270mm wheelbase for both of my cars with the OBTS chassis, and i wanted to use 270 in the MTC Chassis but i had to give 10mm extra in the front part to have enough space for my sensored motor.

How would i do it?¿?
As i did with FJ Cruiser, a good start point is a good 3d model, not designed for 3d printing but with as much details as possible. So if you know about a good car model and you can find a good 3d model of it in the internet, we can try from that model to create a 3d printable one.

The goal this time would be to make a enough resistant body to protect electronics and maybe roll without damage, but using as less plastic as posible, you know my FJ Cruiser Body is extra heavy, about 1,2KG.

So, any ideas, any suggestions?¿?

Please somebody, help with this!!!

I have bought another kit to build a new MyRCCar (including Vandal, Trooper center diff., and electronics). As with the already made parts you can build mostly everything from a on-road car to a SCT, buggy or truggy, this time i want to use new parts to build something like a Monster or a Crawler.

I would like them to share the disposition of most elements in a higher chassis made in three main parts, center, front and rear. Changing this front or rear chassis you should be able to build a Monster or a Crawler, or maybe a Hybrid :S !!!

Are you interested in this build?¿? I know there are already 2 or 3 high quality designs shared in thingiverse, some have awesome gearboxes for crawling or variable length transmissions. I think those designs center their attention in being as much printable as possible. That's a very good thing for me, but i always want to be able to get my car nearer a hobby level "thing" than a "toy" thing.

I know many crawling cars are using brushed motors and ESCs and is a very cheap solution. You can buy both for about 10€. But please correct me if i'm wrong, now some low KV brushless sensored motors and ESCs are working good for crawling.

I have experimented moving my Truggy FJ Cruiser at very low speed with a 10.5T sensored brushless, but with the same setup and config it can also run at 50km/h.

So, are you interested?¿? Would you like to take part of the project?

I have already made and tested a new shell... i hope less ugly this time... and also some 5 and 3 mm led lights holders to be mounted in the front and rear diff housings. The cover front side of the led bar can be easily redesigned to fit the number of leds needed and also to modify the aspect of the car.

The new shell can be mounted with the chassis high closed sides and the general height of the car will be a little tall than the previous shell. Now with the truggy wheels the behabour of the car is awesome compared to the same chassis with the FJ cruiser.

I hope you like it... a few days and i will publish it