2-speed Automatic Gearbox
My first automatic gearbox, based on a design by Misha van Beek, but improved and simplified.
I always liked the idea of incorporating automatic gearboxes into large models, unfortunaltely such gearbox designs are unique and I only know two by Misha van Beek. While both are advanced and show great knowledge of the author, they are also large, complex and none changes gears smoothly. I thought that the new, small differential may give me a good opportunity to create a more useable design. I had three simple goals: to make a gearbox that is small, simple and above all capable of changing gears without jamming. The two first were easy, the third one took some time to work out, but eventually I got exactly what I wanted.
The basic idea of Misha’s design is to use a differential to transmit the drive in such a way, that if the resistance of the output grows, it will at certain point trigger the gearbox to switch to lower gear. Thus the motor will have a better chance to overcome the resistance, and once it’s done the gearbox will switch back to the higher gear. Whereas Misha has taken this idea further, and created gearboxes with more than just 2 speeds or with no fixed gear ratio at all, I considered two smoothly operating speeds a better option than three or more jamming ones. The lower gear has 2.77:1 gear ratio, and the higher one has 1:1 gear ratio. I have been experimenting with more different ratios, but as the difference increased so did the tendency to jam while changing gears. The 2.77:1 ratio difference should be sufficient in most cases, and it offers a reasonably smooth functioning of the gear change mechanism.
The construction of the gearbox is really simple. It fits into a 8×12 studs rectangle (plus the middle shaft protrudes out by 1 stud on higher gear), and axles of any length can be put into input and output without taking the gearbox apart. Moreover, the horizontal liftarm which servers as the gear change lever, provides a lot of space to adjust the tension of the rubber band accordingly to the weight the gearbox is supposed to handle (namely, the towballs it sits between can be put closer or further from each other).
You can see the smooth functioning of the gearbox in the video. I have also provided a parts list (just 45), an LDR model of the gearbox at lower gear and higher gear, plus an exact instruction which can be found below.
Photos:
Video:
Media reference:
blog.kockak.hu, Bricks ‘n’ Gears

If you adapt it to, it will.
will this work with the old differential
Of course not. Why would I put a differential in there if it wasn’t needed?
could you make it without the differential ?
I want to use my PF car for smooth surface racing not just for speed trials…the gearing is quite high which makes it a bit sluggish in lower rpm:s.
How do you make an automatic 2-speed gearbox that changes into high gear when the motor reaches a certain rpm and you want the car to keep accelerating?
i made this gearbox but it doesn’t work, can someone please help me?
sorry, i mean the 2nd one.
i really don’t know which one is newest.
@Pontoos
Perhaps you should look at it, because it has the new differential right in it.
will it works with new differential?
@Alex
Firstly, I don’t think this gearbox was ever suitable for a Truck Trial vehicle. It’s simply too complex and fragile. Secondly, it’s not possible to use driving rings. Driving rings have a neutral position, and whenever a gear is shifted, they need to go through it. The problem is, in the neutral position the motor is disengaged from the output, which stops an automated gearbox from working. I actually tried this solution, and the only result was a driving ring jumping from high gear to neutral position and then back to high gear. It never shifted to low gear, because the neutral position stopped it from doing so.
I have just tried your gearbox on a TT. The problem is that I use a PF XL motor which turns at 146 RPM – less than the 180 RPM you advise. Therefore gear change is mo more perfectly smooth.
Do you think you could develope a 2 speed automatic gearbox using the Driving Ring (www.bricklink.com/catalogItem.asp?P=6539) and the 16 Tooth Gear with Clutch (www.bricklink.com/catalogItem.asp?P=6542)?
@Joe
No. Two motors are more powerful and just as fast.
but two moters are fast and just as powerfull
@Alex
In that case you’re right, I’m sorry.
@Sariel
I think you misundertood me. I meant the the gearbox works only if the vehicle is moving forward and it doesn’t work if the vehicle moves backward. This is how it looks like from the video. And it makes sense because I don’t see how the differential could trigger the lower gear moving in the other direction.
@Alex
No, this is completely wrong. I suggest you watch the video.
This gearbox can change to the lower gear only if the motor does not change direction of rotation. Otherwise it will stick to the higher gear. Is this correct? Thanks
@Joe
But you can switch to lower gear and get extra torque.
extra power ? for when you go up a hill
@Joe
I think you could, but what for?
like that could you make it switch on another moter instead of changing gears?
@Jeff Cheng
Try this one: http://www.brickshelf.com/cgi-bin/gallery.cgi?f=104297
could you possibly create a 2 speed auto with the slightly older differential (without the angled teeth), as this gearbox will only work with the newer version. thanks
@Sariel
i will, if ill get that 8297 suspension =)
atleast it would trigger at downhills and when speed would be high enough..