High speed gearbox

[F41RF4X]

Hiya was just wondering if anyone could recommend a high top speed gearbox I want to build a buggy but would like 3 or 4 gears also where can you buy the rc buggy motors from any help is much appreciated thanks in advance F41RF4X

[allanp]

When you say high speed gearbox, that really depends on the gearing you have before and after it so there really isn't any such thing as a high speed gearbox. I think what you are really asking for is high performance. In real life this will depend on various factors but in Lego Technic the key thing is efficiency. That is, how much of the motors mechanical output power is being "wasted" by driving the gearbox as opposed to just having a single drive axle. In the world of Lego Technic, this loss of power is your greatest enemy when designing a "high speed" gearbox. Because of that, there isn't really any that I could recommend because so much of it's performance depends on how rigid you build the surrounding structure as twisting due to torque can greatly reduce it's efficiency. I can recommend a basic gear setup:

this can work very well but like I said, you have to design your own chassis around it, the more rigid and resilient to twisting the better. this design is also compatible with the new clutch gears and drive rings, you just make it 1M longer. One thing to bear in mind is that when clutch gears and drive rings are under load, they try to push apart like two magnets which causes friction and reduced efficiency. The newer ones appear to be less prone to that problem but it is still there. So another option may be a gearbox that doesn't use those clutch gears/drive rings. They aren't quite as elegant and I've found them to be less reliable, but thy do seem to be less power wasteful.

In this design the red and blue axles rotate where as the green axle rotates and moves back and forth to select between different speeds. The example above is a 3 speed gearbox. This has the potential for higher performance however gears can slip and slide on their axles which for me is where the unreliability comes from. If you are gonna use a buggy motor (available from bricklink but they are pricey) then the gear teeth might get tore up a bit in this design as well.

Edited September 1, 2015 by allanp

[F41RF4X]

Yeah I ment a high performance gearbox I like the idea of the first one just how to make a gear selector does anyone have instructions to make a tried and tested gearbox with a reliable high output doesn't need to be 4x4 but could turn my buggy into a supercar just scratching around for ideas at the moment

[allanp]

Best selector is this one, found in 8880 or on bricklink:

If you want it RC Then you can modify this idea as others have done:

For a 3 speed, connect a servo motor to where the yellow lever is. For a 4 speed use an m-motor and worm gear drive in place of the servo. There are other more complicated and clever ways you can also try, such as this one posted by DurianObried:

Edited September 1, 2015 by allanp

[powerwindows83]

allanp that transmission design doesnt shift properly using a servo due to the allowed dead zone around a target position. depending on how the levers are oriented it gets stuck between gears, ie. it stays in 3rd too long shifting into 2nd thus the whole system locks hard momentarily mid shift. i tried putting a spring "clutch" between the buggy motors and the gearbox to give it a cushion, but it didnt have any effect. honestly i would stick to either two separate 2 speed boxes, or 1 steeply geared 2 speed while throttling the motors. i like to make fast cars (now with a buwizz unit) and i really dislike "sand" and bent gear teeth, so i put a lot of effort into making transmissions take a heck of a beating at high speed :)