Hi everyone, I have an RC car I am making as a school project, and need a couple questions answered.

Firstly, do the brand of the transmitter/recievers have to match or will any brand work, i.e Futaba trans + Acoms rec?

Do the crystals have to fit in any specific way, and can you use 29mhz cyrstals in a 27mhz gear?

Also what are the benefits of using 40mhz over 27mhz? Longer range/less interference?

Thanks
Brad

-they do not have to be the same brand, just frequency and crystals have to be the same

-you cannot use different crystals they have to be the same, not sure about which way they sit though

-i dont think thers any advantage there just different channels

if im wrong someone correct me please i dont want to steer this guy in the wrong direction accidentally

alright thanks,
also do you think its worth putting a diff on a this size rc car, its 180mm by 375mm and weighs around 1.3kg, is that heavy too?

On 27MHz AM you can mix and match transmitters and receivers.

40MHz is a frequency used outside of the United States and Canada, the FCC does not allow their use. 27MHz and 75MHz can be either AM or FM. 27MHz can be used for aircraft but 75MHz is surface only (car, boat or robotics). Crystals must match frequency, modulation and channel.

Some radio systems use the positive phase for signals and others use the negative phase. This is in 75MHz and not in 27MHz.

In the USA only 72MHz AM and FM and 2.4GHz are allowable for aircraft.

2.4GHz is allowed for surface and air. (No crystals).

If you find this confusing that's because it is.

its not worth putting a diff on there i dont think unless its going to be doin speed runs down the street or racing u wont really need one. u could always put one on there if you want though its not like it would hurt anything, thats not that heavy iether like i said if not racing or doing speed runs its not goiung to really be important unless its super heavy that were the motor cant even make the thing go

thanks for the advice, and yeah i get it.
Managed to get a 2.4ghz from school so that will do well.
And its not to heavy and runs pretty quick i think atm so i guess theres no point in having a diff.

A diff would improve cornering, add steering and help keep the rear planted in turns.

Interesting car. Don’t recall seeing a twist beam rear suspension on a RC.

Yeah it’s a little heavy, about the right length for a current 1/12 scale, but a touch wide. Current minimum weight for a 1/12 is 730g, but that is with a much lighter 1 cell lipo. I don’t know how close current cars are to the min weight.

yeah i guess the cornering would be better but it turns pretty sharp as it is, not like those cheap knock off rc cars and i think its much more fun with a straight axle.

Regarding the 'twist beam' rear suspension, what is this? Mine works with the two pivoting arms with springs attached at the rear which stretch. Tbh i think it could be better as it is a little stiff but keeps it flat around corners. With more time and better resources i would have made double wishbones all round.

And yeah i made it wider so it handles better and looks cool. It weighs around a kilo without the battery, if i have the time i will remake those arms with ally.

A twist beam is what you appear to have. Not independent suspension. With structure across the axle like the channel across the back, with 2 arms pivoting ahead of the axle on each side. The arms and the structure are all solidly connected, no rubber bushings or joints. (There are rubber bushings for the front pivots on full size cars)

In cornering/side loads, the structure/channel provides roll stiffness, resists body lean, just like you mention. There is some give, as metal is not perfectly rigid. This provides a built in anti-sway bar function.

Probably explains why you apparently have decent turning without a diff. Most similar RC cars have relatively little rear roll stiffness, and without a diff wouldn’t turn too well.

On full scale the structure is tuned for stiffness in the same way an anti-sway bar setup is tuned with different bar diameters. Sometimes an anti-sway bar is added for a performance version for added stiffness/steering, with the base car just getting the twist beam stiffness. Maybe experiment with the rear beam, try different sizes/shapes/thickness to adjust steering.


Always cool to see a scratch built, thanks for sharing!

yeah i guess thats what i have then, seems to be working alright.

Also, I've noticed many motors have 1 or 2 capacitors on them, what do these do for the motor? Mine were hanging off so i just removed them.

I've read they are usually used to reduce interference but with this 2.4ghz setup its fine.
So is it worth putting some on and how should I connect them?

Yes, capacitors were used on brushed motors to reduce radio interference, usually 2 or 3. Probably not needed for your 2.4GHz, if your running around others with crystal radios might still be a good idea.

Does it provide in performance increase? And is there anything i can do to increase the power from this one motor?

I heard it can reduce the noise given off itself, like the humming, is that true?

No performance increase from the caps, don't know about the noise.

Can't see the motor well enough to be sure, but I doubt if big speed increases are practical. Cleaning/replacing the brushes might help if it's got some wear on it.

now have had the mold cut out using the a CNC router, i then smoothed it down a bit and added some smooth curves.
Should be getting a shell vacformed over that sometime this week.
The drawings were drawn so i know what needs smoothing down to get the final shape.
Pics attached....
Also before i say, anyone guess what car I am doing?