Trial Truck style crawler

[AndyCW]

http://www.brickshelf.com/cgi-bin/gallery.cgi?f=463513

I just posted some pics of my WIP.

I did the axles, suspension, and frame first and am now working on the powerplant/drive line.

I tested it out with two med PF geared down greatly and it would not break any of the axle components. It climbed up a vertical surface and articulated to almost a 90 degree diference between front and rear axles. It doesn't have ackerman or any differentials so it is a little rough on hard surfaces. The two med PF when geared down made the truck too slow so hopefully the two xl will need less gearing and therefore make it fatser.

I am working on a double xl PF with a two speed transmission (shifted by med PF) that also fits inside the box I haxe designed myself into. Hopefully it won't eat gears, axles, or joints when the power is increased.

The body clips onto the frame with a couple of red pins. The battery box is part of the body structure.

It still needs to go on a diet and lose some unneeded complexity.

[DLuders]

This looks good so far! Your Brickshelf photos are huge -- they could "go on a diet" too.

If your design for your 2-speed Power Functions transmission does not work out, you could consider using Sariel's new design for a "2-Speed Heavy-Duty Linear Gearbox" (complete with Building Instructions):

[AndyCW]

Thanks for the gearbox idea.

Are there any concerns with directly connecting the output of two PF motors together or should they be connected with a differential to allow for differences in their speeds?

[T_Tank]

I'm digging your design an I can tell ya sariel has some great info on multi gear transmissions that may give you some ideas how to implement in your build.

[AndyCW]

Thanks.

I just finished putting Sariel's two motor transmission together and need to figure out how to mount it in the chasis. As well as whether or not I like the gear ratios it has and what I should change them to. The axles allready have 8.3:1 in them and are connected to power puller tires.

Is there any advise for specific gear ratios that other people have used.

[DLuders]

I have read (from multiple Trial Truckers) that using Differentials on Trial Trucks is a bad idea.

You may want to read Zblj's 5-part "Trial Truck Tips" on BrickTruckTrial.com .

[AndyCW]

On the first iteration of my truck I had a diferential in the axle, but did not have any hub reduction. It ate a couple of spider gears. I eliminated the differentials and the problem is solved. Watching the trial truck tips is what led me to get rid of the differential.

Will the spider(12t) gears survive if installed after a 3:1 reduction?

I see several designs where people are directly connecting the ouput of multiple pF motors. Sariel even had his double xl gear box like this. I was under the impression that chaining motors together in this manner was not good for them. If one motor is weaker than the other(due to manufacturing tolerances) it will create some load on the stronger motor, but if they are connected utilizing a diferential** this can be worked around.

**which would be as far away from the axles and the majority of the loads

Edited March 1, 2011 by AndyCW

[Sariel]

It's true that there are differences in speed between motors, but they are usually so insignificant, that it hurts less to simply ignore them than to include a differential which is large and much more damage-prone than the knob wheels are. I believe that these differences are proportional to motor's speed, which means that they are smaller for slower motors. I usually run XL motors at 7.2V, so their nominal speed is just around 100 RPM, and I've never run into any problems caused by coupling them without a differential.

[AndyCW]

Thanks for the input. A slight variant of your gearbox fits perfectly between the framerails of the truck.

The penalties from the inclusion of a differential outweigh the penalties of directly coupling the motors together.