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Tracked Cat delta skidsteer

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In my stash of "Really Important Stuff", read that as junk that I haven't thrown away because I MIGHT find a use for it in the future, I found the original 3 volt motors that Tamiya supplied with the gearbox.  Now I just have to fabri-cobble together something that will allow me to solder the motor leads onto the motors.

I have started to fit the cab and driver.  The cab I got to fit around everything without doing too much of a hack job on it.  I only had to remove most of the seat to fit around the 2nd TREX Jr. board and the receiver.  All of the cab floor had to go, plus a couple of mm in order for the cab to fit over the battery.

The driver's not gonna be so lucky.  While it looks like he's not going to have to lose any of his butt to fit over the TREX board and receiver, he's probably going to have to lose both his feet at the ankles to clear the battery.  Not to mention that sitting on those servo connectors is going to be damned uncomfortable.  Probably seem like some Medieval torture device.  He's sitting higher in the cab and farther to the front that he was originally, but it doesn't look out of place - he just looks very tall.

I haven't given him the good-news/bad-news yet, I dunno - waiting for divine inspiration to offer up another solution?  (So far Nada, Nuttin, Zipski.)



It's been almost a year since I worked on this thing last, obviously no pictures since it hasn't changed since last time.  I wasn't happy with the way things were going on this project, I just kept running out of room in the little beasty - there's not much in there to start with.

It's been sitting on the shelf of my computer desk and I've been glaring at it, because I just know it's laughing at me - or maybe that's just the driver laughing?  I'd been trying  to sorta copy the layout that Smallhaul used in his build over at RC Trucks and Construction, but it's just not working out for me, I'm losing too much of the original Bruder for me to be happy.  I think it's time to go back to the 3D model and start rearranging things.

I've also seen the work that RC Modder has done on YouTube and got a quite a few ideas from him.  I especially like his homebrew linear actuators mostly built from 3D printer parts.  I ordered some 5mm and 8mm leadscrews and lead-nuts, so we'll see what I can come up with.  They are literally on a slow boat from China so it'll be a couple of weeks before I see them.  I also like RC Modder's idea of using small individual ESC's and doing the mixing in the  transmitter instead of the specialized TREX mixer/ESC that I had been using.(Be a HELLUVA lot cheaper when I fry one too - don't wanna talk about it.)  I'll have to get some of the small 10A brushed ESC's to see how big they are and if they'll save me any space.  They might work out better than the TREX just because they'll be smaller and easier to tuck away in the nooks and crannies that are available.

I'll let you know how this works out, I promise it WON'T be another year before that happens.


I started looking at rearranging stuff in the skidsteer last night, I'll probably end up moving everything,.  I started with the gearbox.  I knew that the skidsteer was moving too slow and I WAS considering replacing the original 3V motors.  Until I starting looking at the current draws of the 130 series motors, the 6V motor pulls 800mA stalled, while the 3V can pull up to 4A.  I'll keep the 6V motors thank you very much.  I thought that I had the Tamiya 4 speed gearbox in this thing so I figured that I'd just change the gear ratio in the gearbox.  Turns out that I've got the Tamiya Twin gearbox.  That's got two ratio options too slow, 202-ish:1, and WAY too fast, 58-ish:1.  But I DO have the option of two different output shaft locations in the high speed range.

I started looking at the gearbox last night, and I started thinking that I DO have a 3D printer...  I could reuse the gears, print my own gearbox and get my own ratio that's somewhere between 58:1 and 202:1 with the output shaft where I want it.  I'm not sure what the actual gear ratio will be.  I'll probably wind up counting teeth to figure that out as I can't find any info in the instructions regarding gear tooth counts.  I can probably even lose several mm on each side of the gearbox, there's a LOT of air in this one.  I'll have to stretch out the gearbox, but using one of the existing gearbox configurations I found I've got 40+mm of clearance behind the motors.

I also ordered several 10A brushed car/tank ESC's with no brake.  The size those things are I could put 4 of them in the space occupied by 1 TREX board.  I'll clip the hot lead from 3 of the 4 ESC's so that only 1 ESC will act as the BEC for the receiver.  I can stretch out the gearbox and STILL have room for 2-4 ESC's behind/beside the motors.  I'll let the angry pixies in the transmitter do all the required channel mixing, no need for the TREX.

I also ordered a smaller 2S LIPO battery, 2400mAH.  I've currently got a 3500mAH pack that takes up most of the bottom of the skidsteer.  I ordered it because I wasn't sure how long the battery would last, never really thought about to tell the truth.  I've got a Heng Long Walker Bulldog tank.  Cut me some slack, I wanted an RC tank and didn't want to spend a lot of time building it.  I also didn't want to have to sell an arm, leg, kidney, or other appendage to buy one.  It came with a 2200mAH NiCad pack in it.  Couple of weeks ago I took it along with me to one of our group functions, to keep the kids of various ages busy.  That thing was running pretty much constantly for a couple of hours before it got too dark outside and we had to call it quits.  I didn't see any signs that the tank was slowing down, so I think a smaller battery in the skidsteer will be OK.


Pololu lists the gear ratios of my gearbox at 204:1 and 58:1, my numbers don't jive with that.  Counting the teeth on my gearbox, I get ratios of 209:1 and 63:1.  A 6V series 130 motor has an 11,000rpm no load speed.  Using my ratios I get output speeds of either 53rpm or 175rpm.

As a sanity check I decided to work it the other way around and see what the output speed should be for a realistic scale speed.  The speeds I found suggest that a tracked skidsteer will have a top speed of 10-12mph.  [Insert RANT here.]  No I'm not going to convert it to metric for you.  You guys can convert to metric measurements just as easily as I can convert to Imperial measurements. [End RANT.]  This scale 12mph converts to a linear speed of 66 feet/minute. The tracks on my Bruder skidsteer have a 10mm pitch and my drive sprocket has 10 teeth, so this means the tracks will move 100mm per revolution of the sprocket - close enough to 4 inches for me.  Crunching the numbers... drum roll please, we get a total of 198rpm of the drive axle to move at 66 feet/minute.  That's close to my 175 calculated rpm for the 63:1 reduction ratio.  The 175rpm works out to a scale 10.6mph, right about where it should be.  I guess the 63:1 ratio isn't way too fast after all.

Looks like I may have to buy another copy of this gearbox, I've already got 2 of them, one in the model and another in a parts box.  Unfortunately I haven't been able to locate the 12 tooth pinion gears or the additional 3mm shafts that I'll need for the 63:1 ratio output.  The 3mm shaft I'm not worried about, but that 12 tooth pinion needs to be a press fit on the hex output shaft.  I've still got a couple of parts boxes that I need to look in yet, if I can't find them I'll need to get ANOTHER gearbox.  The 2400mAH Lipo pack arrived Monday, the smaller physical size will make finding a new home for it much easier.


I'm going to put this project in indefinite hold, not due to a lack of interest in it on my part, but rather a desire to do the best I can.  I've stockpiled a number of other Bruder models that are a little larger and they should easier to convert.  The stockpile includes a Manitou MRT 2150 telehandler that I picked up used on Ebay for about $10 because it had some broken and/or missing parts.  I'll practice on them and come back to the Cat later, work my way down to it as it were.  I think that I'll start with the Manitou.  That way if it gets irreparably trashed during the conversion I won't be out that much.   I'll start a new thread when I've actually got something to show.



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