Charging between rotating parts

Hi, I am wondering if anyone can help with ideas on how I could charge between a fixed and rotating part, my initial thought was inductive charge but I amusing this on a racing simulator which are sensitive to EMI issues.

This is my steering wheel…

Currently the buttons and encoders are controlled by a Leo Bodnar USB joystick controller and the screen has a Windows Wireless Monitor adaptor, I have been powering the screen (5" Elecrow RPi HDMI screen) and display adaptor from USB power pack and it works ok but I wanted to have constant charging but avoid having additional USB cables plugged into the steering wheel.

I am also looking at possibly using a Teensey in future version for the USB joystick controller and screen driver and then use just a WiFi/Bluetooth breakout board to have everything completely wireless.

Unfortunately I dont think slipper rings will work because the quick release between the wheel and motor is a solid shaft and needs to be for the strength as the the unit is a 20Nm direct drive system.

I was thinking 2x fixed rings (+ & -) on the motor side and possibly matching spring loaded contact balls that rotate on each ring might work?

This is a photo of the set up, steering wheel - black, QRC - red & motor - silver…

Any ideas or help would be greatly appreciated.
Thanks Justin.


Hi Justin
I think a solid contact will be the only practical solution. Wireless charging might work for a phone where you place the phone in close proximity to the sending coil with a space of 1mm or so. But for any distance as you would need the required transmitting power would I think be highly impractical. Not to mention the legality and potential interference problem.

Another curly cord maybe.
Cheers Bob
Those Leo Bodnar boards are useful aren’t they.


Hi Justin,

Because you probably won’t have more than 360° of rotation (often in motorsport vehicles you’re lucky to have 270°) you can use a fixed connection and a long coiled cable - which is exactly what they do in real motorsport.

If you feel like getting the real deal (at real cost) you could order some steering wheel electrical connectors from an ECU company like MoTec or Haltech, and bundling everything into a single plug.

But otherwise it’s spring fingers and a brass ring. Won’t be nearly as reliable though. Wireless charging is very inefficient, which is why phones using wireless charging have to be so close to the charger (ironically making them less portable while charging than using a cable).


@Oliver33 @Robert93820
Thanks Gents,

This might be a bit clearer with my initial idea, my plan had been to have a fixed transmitter from the force feedback motor side as long as I had a 30mm center hole the shaft of the QRC goes through the center, then what ever receiver is used mounts directly to the steering wheel around the shaft, the air gap between the 2 could be less than 1mm.

Below is the wheel separated from the motor showing the QRC, the QRC is a solid machined aluminum shaft and body and when locked offers no movement what so ever and can be adjusted to avoid movements.

Curly cords are great, but in an accident it is possible for the ffb to rip the wheel out of your hands or oscillate and especially with direct drive units producing 20nm++, the risk of injury is real, therefore an emergency stop is essential to kill the ffb signal. So curly cords can easily get damaged, the same with plugs and sockets etc, Not to mention they can get quite expensive for multiple core cables like HDMI.

Most direct drive systems have 900° rotation lock to lock, (some retail wheels 1080°), this is the same lock as most GT or touring cars, formula/open wheel car generally 640-720°. I will say that you dont often use anywhere near this much lock, maybe only tight hairpins on street courses like Longbeach, but again in an accident the ffb may go beyond normal use.

Interference is my biggest issue to begin with as the ffb motor already produces quite a lot of issues with EMI, thus one of the reasons wanting to go wireless with bluetooth or wifi, the steering wheel controls then have a grounded shield on the rear side facing the motor via the housing for the electrics (atm mine is open as you can see above), this with twisted pairs on wires, grounding of all components and sheilded usb cables.

As I said the usb battery pack idea worked, with a very cheap and old 5000mah I get about 3-4hrs running the screen and display adapter, so increasing the size of the battery pack and having a constant trickle charge to just boost the battery pack while in use and then fully recharge between use, was my aim, so I thought a charger at 1-5 amps might have been sufficient.

@Robert93820, yes Leo Bodnar boards are great but I have to say the Teensey in the Extreme Joystick mode look very promising also.

@Oliver33, I do know of MoTec and Haltech, I was actually using the MoTec telemetry UI to monitor the car on iracing there for a while, it is very good for doing car set-up and monitoring driver inputs during a test session or race. Spring fingers and brass rings was actually what I had pictured in my head as a possible solution now that wireless wasnt looking like a viable option.

And sorry if this all seems very basic to you guys, electronics has never been my strength, I have always been more mechanical, but sim racing has taken me into the electronics side of things and I can see what I want to try and achieve in my ideas, but now that I am getting a bit further into it I dont know what options are commercially available or if any will be a viable solutions.

Thanks again Justin.


Nah I’ve driven and worked on a few different race cars in my time (touring, rally, and formula). 1080° is a road car or quite an old touring car - 1.5 turns left and 1.5 right for 3 turns total - and for this you need a full wheel so you can reposition your hands.

Your style of wheel is designed specifically to never take your hands off while turning which means ~360° max (±180°). If possible, I’d suggest reconfiguring your rotation stops down to this, just for realism (and recalibrating it to make sure you can get to full lock in game).

Re the e-stop - for safety I’d put it on part of the rig that’s hard mounted to move with your seat; maybe even have one on each side. That way you can just release the wheel and hit the stop.

But back to your actual question, fair point about the expense of custom cables. I guess it depends what manufacturing equipment you have and your skill with tools as to what you can create.

Core have a few 5v wireless charging lots I’ve seen. They come.with planar coils but there’s no reason you couldn’t charge them or re-wind them into cylindrical coils and make a coaxial transformer.

Though if everything else is wireless, just one USB cable for power is not too bad and it’ll be much easier. It’s not as neat, but you can always coil your own - you can get flat USB cables, then just wrap them tightly around a broom handle or even a pen.

You could also use one of those magnetic USB cables so that in case of over stretching the cable it just disconnects harmlessly.


Thanks Oliver,

Appreciate what you are saying and I have also been involved in real life motorsport at various levels most of my adult life, the degree of rotation is correct for the application, as I said some retail wheels are capable of 1080° and the higher end equipment is around 900°, mine to be exact calibrates at 881°, iracing then adjusts the rotation available to match the real life car, the steering rotation in iracing is very very close to a 1:1 with real life.

In most case you are correct rotation does not exceed 360° but for some cars like the MX-5 Mazda or circuits with very tight hairpins like Longbeach street circuit you will exceed the 360° in real life and the sim.

The other problem is if the ffb takes off when you hit something then the wheel can and sometimes will rotate to your calibrated rotation setting.

That is why I am looking at what options are available to have the wheel completely cable free and I think your idea for the slip rings and spring fingers is worth exploring.

Thanks again Justin.


Ah yep, fair enough. Let us know how you go with it - I’m keen to see what you come up with. Building a proper sim rig for myself is on my long list of projects I’d like to get around to one day!


Happy to give you some DIY tips.

Also when you do prepared to give up A LOT of free time.

Especially if you want to be half serious about it.