Ocelot

Ah, thank you; I thought it was a new part that I had somehow missed!

Hi hellhax! I can't comment on the trucks, but as general advice; remember you can buy most parts individually on Bricklink, so maybe you want to get whatever sets have the best value, then order your wheels and motors separately to supplement that.

@qwest70 No problem, glad to help! Look at the work-in-progress photos on Sariel's site, rather than the videos of finished MOCs; they usually show the insides; for example: Sariel's Tortoise seems similar to what you are doing. Also, this video by Kelkschiz is extremely helpful; it's about suspensions but you can see a few different chassis and wheel designs in there too! There are also plenty of excellent designs by other members of this forum, like this Sherman by Tommy (just as a random example). Ironically, most of those don't connect the motors directly to the wheels. But I stand by that advice if you want performance! @Doug72 I've been searching on bricklink for ages now, and I give up: please tell me what that grey curved gear rack (at the top of your MOC) is?

Hi qwest! I've built a few similar things and have the following suggestions: 1. If you want performance, it is better to forget the gearbox and connect the motors directly to the sprocket wheels (maybe with one gear reduction). If you look at the popular MOCs of this type (eg- Sariel's (?) ), you'll notice they almost all do this. It's because any advantage you get from the different gear ratios (high gear, low gear) is usually far outweighed by the extra friction that all the gears introduce. 2. Consider having the sprocket wheels (that drive the treads) be non-structural, so the weight of your MOC is held up by the rollers at the bottom of the treads. This is so that the axles in your drive train are not also supporting the load of your MOC, and will make the inevitable addition of suspension easier later on. Also leading to: 3. Those little rollers don't look like they are interfacing very nicely with the treads. It might be better to have them on either side of the tread, rather than in the middle (there are various solutions to this, some people use pulleys or even wheel hubs without the tire). 4. The small gears, especially the thin beveled ones, are not recommended for high loads. I'd definitely want to replace the those, at least with the wider beveled versions; and maybe instead with 36 -> 20 tooth gears for reduction; again, to avoid using small gears for high torque. 5. When using the 'tank' steering (left and right treads moving at different speeds), there is a a lot of shear force when turning (sideways force the tries to 'pull' your treads away from the chassis), so try to take that into account when designing the connection between treads and chassis. This may not be a problem for you depending on weights/ terrain. Let me know if there is anything more specific you need, or if the above need clarification.

@skppo and @Gray Gear: Thank you! @aeh5040: Absolutely, the simplest realization of this would be with an actual clutch gear. Unfortunately the white clutch gears (ones I have at least) require a fair bit of torque to slip, so they can't drive the gear rack directly. However, gearing down to the clutch (for eg, with input -> Z8 -> Z24 clutch + Z24 gear rack driver gear) does work; thank you! The one (unexpected) advantage of using a differential is that one can chain these mechanisms together easily for sequential motion with only one input drive.

Thanks for the replies! I'll try C1 and C2 at 9V (it is quite strange to see control signals at the same voltage as the motor supply, but I guess it made sense with the modular PF system).

Hi all, I have been controlling PF motors from a Raspberry Pi with no problems. Recently I acquired a PF servo motor. I know that the servo motor is powered on the 9V and GND rails, and the position commands are PWM via C1 and C2. What I need to know is the voltage of the PWM signals, which is difficult for me as I don't own the PF control blocks. So, if anyone has an oscilloscope and the PF control blocks, I would be very grateful if you could record the PWM frequency, voltage, and duty cycle passed to a servo motor. Apologies if this is already answered but I have tried searching and also read Philo's site; yet nowhere do I find the specs for the C1 and C2 signals.

Update: I've managed to solve it! I'm attaching a picture of my (prototype) solution here in case it helps anyone in the future: as you can see, when the gear rack is pushed to the top, the gear driving it is stopped by red bricks, and the rotation is passed through the differential to the 24 tooth gear instead.Moving the friction pin gear to the fixed frame helped a lot. gear_rack_small by Sal Ahmad, on Flickr

Hi Jim, Thanks for the reply. The video link was starting at 1:50 which was confusing. I've fixed it now. I didn't repost the video since you have that in your reply. Details (my words are going to sound confusing, video clip at 2:50 explains it best): when a gear rack is driven upwards and the end of the gear rack moves past the driver gear, the end of the rack jitters and skips up and down since the driver gear is still turning (+gravity). This jittering motion is very unappealing. In the video, they use a 24 tooth gear connected to a friction pin to 'lock' the rack in the raised position; this works, but doesn't generalize well to other models/ different sizes. What I'm looking for: Idea for a mechanism that raises a gear rack to predefined height, then keeps it fixed at that height while the drive is still connected. When the drive is reversed, the rack needs to retract downwards.

Hi all, Long time lurker, first time poster here. I have reconstructed the mechanism shown here at 2:50 , and it works well; specifically, the way a 'clutch' gear is used to prevent the gear rack skipping once it has been pushed past its driving gear. However, I am having an unusually awful time adapting that mechanism for MOC purposes (it turns out to be quite sensitive to small changes in gear placement or size). Can anyone suggest a similar mechanism for me to try instead? (Or even the name of the mechanism that I'm looking for). PS: Thank you; this forum is very inspiring!