The [unnamed] Mini-T Project

4/30/07: Tonight I broke down and started working on the final battery hold-down. Starting from the same block of delrin I've used on the Ultimate Rustler, Daemonsbane, Mini-Giant: Evolution, and others, I first traced the balsa proto's outline on, then got to work with the Dremel and a 1/4" cutter, basically until I couldn't feel my hands anymore. I'd say I got about 2/3rds of the way done. Hopefully in my next stint I'll finish it.

4/15/07: Progress once again. Here's the main chassis mostly assembled, with battery accoutremonts being prepared. The way the batteries hook together is very interesting. Because there is an odd number of cells on either side, rotating just one pack reverses the polarities of the ends along either length of that pack. With my right side pack (the top one in the first photo) fitted with shaved-down female bullet plugs at opposite corners and the left pack fitted with a male plug at the inside rear cell end, I can create either a +/+ or +/- connection at that end by just separating the packs and rotating one. This leaves either two + terminals or one + and one - terminal at the opposite end. The end result is that by just having two wire adapters, I will be able to go from 6V 2800mAh for tame off-road driving at up to ~30mph, to 12V 1400mAh for on-road speed runs in about a minute, without removing anything but the body.

The third photo shows a balsa mockup of what the battery hold-down may end up looking like. I'll drill a hole at the front end and have the whole thing work like the strap on an RC10T3 or XXX-T, hooking at the rear and receiving a body clip at the front to lock it in place.

8/11/06: And here it is, the CF version, with countersunk holes on the bottom and countersunk battery slots on top. Fits perfectly; now I'm waiting for my new controller to come in to figure out wire routing and final placement of electronics.

5/18/06: Making progress! This evening's work product is an acrylic final mockup of the main chassis plate. Fits exactly as planned!

Why acrylic? It was handy and it's clear, which made marking holes easier. Why not lexan? Like I said, it's just a mockup. Now it's time to do the final out of 2mm CF. I'm thinking about redoing the motor plate, too.

4/7/06: What last time was a balsa mockup is now all carbon fiber & Delrin!

It came out nicely, adding exactly the 28mm of additional length I needed. It's now extended just a bit less than 2 1/2" compared to the stock chassis. The new IB1400's from United RC are all soldered up and ready for connectors, to boot. They'll be hooked up in parallel for off-road running, but in series for some speed runs just to see what she'll do.

Next up is the new main chassis piece, which should be a piece of cake (how hard can a flat plate be?), after which the last difficult part, the battery hold-down system, can be tackled.

3/22/06: Well how about that. Here I am more than two years after starting the project, and it still isn't done. I'll spare you all the excuses, but suffice to say I'm not sorry for them, but am nonetheless relieved and excited to be back on track. Here's a picture from the workshop tonight (click to enlarge it):

What I'm mocking up to the left of center is a conversion of the chassis to a hybrid channel/"TVP" (twin vertical plate) design that further extends the wheelbase, but allows me to mount all 10 battery cells flat in two ranks. The speed controller will attach to the battery hold-down strap, leaving its center of gravity right about where it was before. The BEC & receiver will mount on the same plane as the batteries on a "wing" of sorts off to one side. This new design will give less wheelies and make the vehicle more stable in turns.

Now, one of the things that has freed me to move forward with this project is that I finally found a body and wheel/tire combination that works. The amount of searching & experimenting I've done and the obscure bodies I've gotten my hands on that I never before knew existed still leaves me in a daze when I think about it. The fact is, though, my very early decision to use such a powerful motor forced me to use oversized tires, which in turn called for a different scale of body. Hopefully some day I'll find good uses for the rejected items I'm left with.

I won't reveal the body until it's mounted, but the wheels & tires are fair game. At the rear, I'm going to stick to the HPI Super Nitro size, but step down to Super Nitro tires, significantly smaller than the buggy rears I was using before. Up front I'll run 26mm tires on Traxxas Street Sport sedan front wheels (a perfect fit, interestingly). I'm putting together two sets, one with slicks for testing and speed runs, and one with HPI's rally treads to hit the dirt.

The next thing I'm reconsidering is the name of the project. I think if I had gone with a Mamba 8000kV system or equivalent, everything would have gone down a straighter path and I could have ended up with a truck that lived up to the "Ultimate" title. By insisting on bolting in the Lehner 5300 as the founding premise of the project, however, I think I inadvertantly locked myself into an unwinnable battle against problems of traction and scale. I'm extremely pleased with the way the original chassis turned out, but in the end there was no way to do the finishing touches and end up with an "Ultimate Mini-T." At any rate, I ramble. I'll figure something out!

10/11/04: The first two times I've tried actually running the truck, it self-destructed immediately. Even before I had charged the battery at all, just off the residual juice that they had left over from United, and after sitting around for about a month, I completely shredded the stock diff when I blipped the throttle in my kitchen. An MIP ball diff was ordered and installed to fix that. Then, while doing the speed & runtime tests for my United R/C battery reviews, I tried again, this time with a charged pack. This time, one of the axles sheared in half from the torque. Crap.

Today, I finally got the gearing high enough (28/66 with 2.2" tires!!) and the slipper loose enough to actually survive. The batteries are still far from being broken in, but in speed runs, it hit 31 mph. This, again, is on 6V (effectively 5 cells). Yet, it still had enough torque to immediately flip over on its back from a standing start, against the weight of the 10 cells, aluminum front suspension pieces, and oversized wheels & tires on a 1" extended chassis. Of course I will have to eventually try it on the full 12V the motor is rated for, just for the heck of it.

8/29/04: Alright, power, baby, power! What I've created here looks like a 10-cell pack, but it's hardwired as two 5-cell packs in parallel. With the hot United R/C cells I've used, I should be able to sustain over 40 amps of current draw, though I'm limited to around 6V. With the that Lehner Basic 5300 motor, though, the most powerful motor ever installed in a Mini-T, I'm confident that I can gear this baby to the moon (even with the oversized wheels) to get some good speed out of it.

7/11/04: Today, at last, the rear suspension got the same beefing-up treatment as the front did in the last update, with natural finish aluminum arms & towers from GPM. The needed shock mounting holes were drilled & tapped to accept 3mm hardware, and the lower shock eyelets were also drilled out as necessary (they're designed to barely accommodate 4-40 screws). More importantly, though, in what is more the overcoming of a psychological barrier than anything, I got all of the electronics installed.

All three components (brushless controller, BEC and receiver) are simply attached to the sides of the chassis channel with my favorite brand of double-sided tape. This is the most compact way of mounting such a bulky array of electronics and on a more sane setup would easily accommodate a full-sized brushed motor ESC and large RTR-style receiver.

All that remains now is to build my bizarre battery pack and affix some body mounts and the truck will be ready to run under its own power in its full glory!

4/18/04: The truck is really coming together and the end is well within sight. The latest updates to the chassis involve the addition of a handful of GPM aluminum parts for the front (shock tower, arms & bumper) and a rear shock swap to a set of Associated TC3 threaded-body dampers with extra-long shock shafts from a Traxxas vehicle. The Powerline shocks I had before were just fine, but the Associated ones match the color of the VCS Micro fronts for a much cleaner, more professional look. I've also installed thin translucent blue plastic strips on either side of the chassis ladder. These strips, affixed with Shoe Goo, will allow the vertical mounting of my complement of unusually large electronics (including full-sized non-BEC brushless controller and an external BEC unit).

Oh yeah, that's right, the truck has a body at last! It's not yet mounted, but the paint & trimming is done and it's looking pretty good to me. The paint scheme was all done with rattle cans and includes custom window shapes with outlines & tint, Indy Silver fading back into True Blue Pearl with Metallic Black stripes and mottling and a faux carbon fiber bed overlay.

In the next update I plan to install matching GPM aluminum parts on the rear and hook up the electronics in their final positions. One more update after that should see the truck come to life in final form!

2/16/04: This is the big one; it's all downhill from here. I think the only way I could have gone further would have been to start from a solid block of aluminum and carve the whole new chassis out as one piece! What was a bit rough in this stage of the project was that as of yesterday morning, all I had done was draw out the new lower chassis shape on the CF sheet. Not a single cut had been made, yet that evening was the deadline for the ending of this stage in the giveaway contest. Worse still, I was only able to put in a few hours of work due to circumstances beyond my control. Somehow I got through it today, though, ending up with this 5-piece interlocking carbon fiber & Delrin creation:

You can see a few more pics in the Ultimate Mini-T Gallery, where you can also click the medium-sized images to view much larger versions. Here are shortcuts to some detail shots:

This 1.2" extended chassis gives me a flush underside and uses all stock screw holes at the front and rear with the aid of a locking "key" at the upper rear, resulting in a very rigid structure. You may ask why I mounted the servo up so high, when the stock setup gave the lowest center of gravity. Well, simply put, moving 1/3rd of an ounce to a high place was a small price to pay to allow the centerline mounting of six big KAN 1050mah cells in a 6x1 side-by-side configuration. Small electronics could be mounted on the "wings" of the main chassis deck, but I'm using a normal-sized receiver and bulky 18-cell capable brushless controller with external BEC unit, and I have something clever and devious planned for mounting these up in a way that keeps the CG down. I'll also do a proper battery hold-down.

Note to anyone copying this design: I've put this chassis design up here publically, with detailed photos, for folks to not only enjoy viewing, but learn from and perhaps even duplicate in their own projects. If you want to take this design and sell it, however, you MUST approach me for permission FIRST. Nothing gets under my skin more than other people stealing my ideas without giving due credit EXCEPT when they steal my ideas without giving due credit and then make money off of them, to boot. In the past I've let such things slide, but I now have the resources to pursue full legal recourse should the need arise. I'd rather the need not arise. Thank you for your understanding!

2/6/04: Alright, I'm going to be brutally honest here. I've seen folks try using every size of wheel from HPI/Tamiya "Minis" to 2.2" offroad truck dubs. Unfortunately, though, I haven't seen any that really fit well, front and rear, other than stock replacements. With a 540-class brushless motor powering my truck, I can't afford to run no bearings, or just one, or have a wheel off-center because the hole doesn't fit around the axle. A rear axle pin "sorta" fitting isn't going to cut it, either. At the same time, I couldn't use anything stock size as they're just too small for all of the power they need to put down, and Trinity's "MonsTer" tires are barely larger than stock, and don't come in good tread designs, either. I needed to go big, really big, and still have a good fit.

HPI came to the rescue with their Super Nitro 2.2" touring car wheels for the rear, 2.2" buggy universal fronts, and two sets of wheel adapters. On the front, I was able to use the adapter set that came with the wheels (also available separately here). They allowed me to pop in the stock-size 4x6x3mm bearings. This didn't leave me a lot of room on the axle stubs and I had to mount the locknuts backwards, but I'll later go back & fix that by Dremelling down the adapters a bit. At the rear, one of the items in the Super Star rear wheel adapter set almost gave me what I needed, adapting 4mm axles to hex hub wheels. I just had to Dremel slits for the axle pins, and the rear wheels went on great. (Yes, I did have a little slip on the adapter in the picture, doh.)

I first tried on some Pro-Line Holeshot buggy rears and Blade buggy fronts, but found a matching set of "Smash" rear and 4WD fronts on the discontinued parts wall at a local hobby shop and happily switched to them for a nicer effect.

1/15/04: Today I reveal some modifications that were made many weeks ago. I've been holding out on everyone to give folks a chance to participate in the MiniOffroaders.com Mini-T Giveaway! You have entered, haven't you? At any rate, first up is my servo mod, completed & tested in late November '03. I'm borrowing an Airtronics 94091 Super Micro servo that I had purchased to use as an accessory accutator on another project. The 94091 has a whopping 18 oz/in of torque with a 0.12sec 60-degree transit time. This might not seem powerful against 1/10th scale servo standards, but realize that this little bugger weighs only 9 grams (less than 1/3 oz)!

Alright, so there are several issues involved with getting this thing to work. First off, the connector. The stock servo uses a 4-wire setup. This Airtronics uses a standard Z connector. This is actually a good thing, as I plan to use standard electronics. Next on deck is the spline. We all know that the stock Mini-T uses a most unusual and incompatible hex spline and matching servo saver input hub. No problem. That's what Dremels & X-Acto knives are for. Observe:

The screw hole is filled with epoxy (later redrilled to accept the stock 2mm screw) and the spline is carefully shaved into a hex that fits the servo saver just right. This gives 100% compatibility. All that's left is the issue of mounting the thing, since the 94091 has one mounting eyelet per side to the stock unit's two. I explored the possibility of fashioning up some custom mounts from either Delrin or solid aluminum, but when doing a test-fit with the stock servo cover, I stumbled upon a stupifying "Oh" moment. One strip of thin servo tape (ever wonder why they call it "servo" tape?) on each flat face of the servo case provides more than enough stiction to hold the unit in place when clamped down by the servo cover/brace. The cover squishes into the upper piece, molding it a bit. I only tried this setup for one run with stock electronics, though, as I have other plans for a later stage of the project.

Next up, it's time to finish the replacement of the stock spring holders ("shocks"). I decided to go with an Associated VCS Micro shock set, built with Factory Team blue bodies & retainers. Unfortunately when built to stock specs, these shocks are just too long, forcing the ride height up too far and not giving a proper range of suspension motion. I took care of this by cutting down the length of the ball end adapters and also trimming off nearly half of the length of the threaded end on each shock body (one ball end threads directly onto a threaded nipple molded into the shock). The upper mount then worked perfectly by replacing the stock metric brass ball studs with Associated 4-40 studs (in matching blue-anodized aluminum). The bottoms were a bit more of a challenge. I've seen people trim down the lower ball end to fit into the slot in the arm, and then drill a small hole through it to accept the stock screw. I didn't find this to be an acceptable solution, though, as the thin screw would eat into the pliable ball end and make it all go loose in no time, just from regular use. I wanted a proper, fitted joint. I accomplished this by, for each shock, drilling a hole through an aluminum ball stud (to accept the stock lower mounting screw) and then cutting out a flat, roughly 3/32" thick slice of the stud to pop into the remainder of the ball end (after trimming the ball end from both sides). Somewhat confusing to explain in words, easy to show in pictures:

So there you have it, a perfectly tailored set of front shocks. I used 30wt oil and black springs. This is just a little thick on the oil (I would recommend 20wt for most people), but I need to account for the increased weight of the finished Ultimate Mini-T I have in mind.

12/24/03: A picture is worth a thousand words, right?

Yes, that is the same roughly 540-sized Lehner Basic 5300 brushless motor used on my brief XXX-Tyranus project. Overkill? Well what did you expect, a rubber band? Will a 540 BL be too much to handle? Impossible.

The motor mount is a custom piece cut from 2mm carbon fiber with a blue aluminized cloth outer weave for the sake of bling (sheets available from Art's Hobby). The rear is extended more than necessary to allow the mounting of some sort of wheelie-control device at some point. I knew the stock gearing arrangement would not suit my needs, but fortunately many gears made for Associated vehicles work with just a slight trim of the inner diameter. The spur shown above is a 66-tooth Robinson piece (in contrast to the 60-tooth stocker), and I also have 68- and 69-tooth gears lined up & ready to go if the need arises. The slipper clutch assembly did not have to be modified at all. What's going to be difficult, of course, will be figuring out how to protect & conceal the bulky new gears, as the stock gear cover is far too small.

I've done a test-fit of the full-sized Schulze 18.61 controller and it looks like the motor wires will route back quite neatly.

11/28/03: After quickly tearing out the stock electronics and beginning work on a new motor mount, I went off on a tangent for an hour or so to install some actual rear shocks. The stock units hardly deserve to be called shocks at all -- they offer no damping action whatsoever. All they do is hold on the springs. This is what I came up with:

I had some Powerline threaded touring car shocks left over from my 4-TEC Rally Conversion (which has since become a new vehicle) and decided to put them to use. To mount them, I fashioned some custom lower mounts from scraps of 2mm carbon fiber. I mounted these brackets in the stock locations, but drilled 3mm holes to mount the lower shock eyelets (versus the stock 2mm setup). This was a fairly straightforward affair, with the exception of having to grind down down the backing nuts to fit in the very limited available space.

I'm using cut-down Losi off-road springs in the lightest rate they make, 1.6lb/in. With these shocks with their stock 2-hole pistons, I've started off with 35wt silicone oil. This feels just a tad stiff on the damping for now, but I expect it to balance out in the end, as I expect the final truck to weigh a good deal more than stock. I'm just glad to have real shocks at last!

11/25/03: Finally got the little bugger today. Plopped some Duracell AA's in it and drove it for about 10 minutes. Fun. Now it's time for the mods.

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