Introduction: Actobotics 1/8 Scale Dragster

Actobotics from ServoCity are like a combination of ErectorSet and KNEX (Both of which I grew up on). It lets you build just about whatever your heart desires and budget allows (tip- R&D gets expensive quick!).

The Traxxas 1/8 scale Funny Cars have gotten popular, but it doesn't seem like Traxxas plans to build a 1/8 scale dragster anytime soon, so I decided to build my own. As with anything I build, I had to use the biggest and baddest of everything...which has ended up costing me dearly as I go through the R&D process.

I built this design around a Castle 1515 1Y/2200 motor on 6S, and I'm starting to regret putting a 4 Horsepower motor in an 8.4 pound RTR chassis. I'm not saying that certain parts are just have to factor that some of what they sell simply wasn't designed for the stresses that is about to get thrown at it. Their emphasis is on robots and stuff like that (the name of their parent company is RobotZone) and not stuff that, laws of physics allowing, may hit 300 MPH if I took it to the salt flats...I'll be happy to hit 150 and get it to a stop without destroying itself.

Step 1: Nose Cone Structure

In this section, we'll be building the upper and lower frame rails, as well as installing the front steering spindles.

Something to note throughout this build: I used Stainless Steel Socket Cap Screws from the local hardware store. I found that the major diameter on these is a smidge more than the holes in the plastic brackets that Servo City sells. This actually works out as a good thing.

1 pair 13-hole beam (steering strut)
1 single 21-hole beam (top segment #1)
1 pair 3-hole 45 degree brackets
1 single 7-hole beam (wing strut)
1 pair 21-hole beams (lower segment #1)
1 pair 13-hole beams (top segment #2)
1 pair 13-hople beams (top segment #3)
1 pair 33-hole beam (lower segment #2)
1 pair 13-hole beam (top segment #4)
1 pair 13-hole beam (lower segment #3)
3/4 & 1” hex cap screws

Step 2: Top Frame Rail

1) Lay out the 21-hole beam as a starting point.

2) Lay a pair of 13-hole beams out, one on each side of the 21-hole beam, with 2 holes of length overlapping. Using a 1” long screw and nylock, loosely fasten the 3 beams together.

3) With the 3 beams now laid out, lay out another pair of 13-hole beams outside of the assembly, again with a 2-bolt overlap.

4) Tilt one of the assembled beams up, and insert 2 3/4” hex cap bolts facing outward, place a loose beam over those bolts, and tighten fully with a nylock. Repeat with the other side.

5) Now with both beam assemblies laid back down, tighten the 1” bolt and add another to make the 5-beam assembly become solid. This completes the first 3 segments of the upper frame rail.

6) Repeat the pattern once again with another pair of 13-hole beams and 3/4" bolts. This time, there is sufficient room for the bolts to face inwards. This will complete the 4th segment.

The upper frame rail should now go from the long 1/4” wide beam to 1-3/4” wide at the end of the 4th section, using 1” screws at the front and 3/4” screws elsewhere.

Step 3: Lower Frame Rails

1) Lay out a single 21-hole beam as a starting point

2) Lay out a 33-hole beam next to the 21-hole beam, with a 2-hole length overlap. Fasten them together with a pair of 3/4” hex cap screws and nylocks.

3) Lay out a 13-hole beam at the end of the 33-hole beam, and fasten with another pair of 3/4” bolts.

4) Mirror this assembly with another set of beams.

5) Place a pair of 3/4” bolts in 2 adjacent holes of each 3-hole 45 degree bracket, observing directionality of the photo, then insert to the 9th and 10th holes of the 21-hole beams, and tighten it nylocks.

6) Place 1” bolts through the 4th holes of the 21-hole beam, facing outward. Place 2 washers on each bolt, followed by a tall beam block, 2 more washers, and a nylock.

Step 4: Steering & Assembly of Forward Chassis

1) Place a 1/2” bolt through the center hole of a 9-hole plastic beam. I goofed, so I had to make a substitution. Screw a tall beam block onto this screw.

2) With a 9-hole aluminum beam on one hand, place a 1/2” screw though the 3-rd hole from each end.

3) Screw these 2 screws into the 2 tall beam blocks attached to the lower frame rails. Things may seem flimsy now, but that won’t last long.

4) Place a 1” bolt through the end hole of a 13-hole beam, through the beam block in the center of the plastic beam, through the end of another 13-hole beam, and tighten. This is the upper steering strut assembly.

5) Using a 3/4” bolt, place it through one of the 3-hole 45 degree brackets, the end hole of the upper frame rail, and the other 45-degree bracket. In my case, the SS screw threaded into the bracket, you may need to use locket or hot glue to hold the bolt in place. This creates the front nose cone structure, in lose form.

6) Unify the steering strut assembly to the upper frame rail by placing a 1” bolt through the 10th hole of the upper frame rail. Yes, this will look like a flexible “Z” right now. Or at least, it should…if not, check the photos and see where you messed up!

7) mount the Traxxas Spindles between the 9 hole aluminum beam and the 9-hole plastic beam. Once both spindles are on (leave the screws about 1/4 turn loose), you should have a pretty solid structure with a significant amount of caster. This helps the vehicle track straight and creates a small dead zone in the steering.

8) Place a pair of 1.5” bolts through the 1st and 3rd holes in the lower frame rails. Put a nylock on them, but leave loose. We’ll come back to these later when we do the front wing, they just help hold the assembly together.

Step 5: Cockpit

In this section, we will build the cockpit area. This is the least technical area of the build, but will be the most stressed area of the chassis, as the weight of the batteries in the nose is lifted by thetorque at the back of the vehicle.

On full size cars, it is not uncommon for the cockpit area to bow as much as 4 inches or more upward under the immense forces of physics. Chassis designs have been revisited many times due to chassis failure in this area, but the failures still happen, just less frequently.

Even though this model won't be seeing the same force magnitudes as a full size car, it still has to be designed with similar forces in mind.

2 90 degree channel brackets (seat tray)
1 channel connector bracket
4 channel/beam interface brackets
1 pair 13-hole beams (lower segment 1)
1 pair 13-hole beams (top segment 1)
1 pair 9-hole beams (top segment 2)
1 pair 33-hole beams (lower segment 2)
3/4” bolts

Step 6: Cockpit Assembly

1) Starting with a 90 degree channel bracket, mount a pair of channel/beam interface brackets on like side. You can face them out with nylocks, or face them in with screw plates. I prefer nylocks for something that might be seeing 100+ MPH…just saying.

2) At the open end of the channel brackets, attach 9 hole beams, centered.

3) Place a 3/4” bolt through one end of the 9-hole beams, pointing inward. Attach a 13-hole beam, and nylock. Yes this part should swivel.

Did You Know: On big cars, there are points where the frame is designed to pivot or slide. We call these “Slip Joints”. For this build, because the upper frame rail is attached by a single hinge bolt at the front and a pair of hinge bolts at the cockpit, this allows the frame to flex similarly. This reduces stress on the frame rails themselves, and acts somewhat like a suspension system.

4) Assemble the cockpit’s upper arms to the upper frame rails of the nose cone. It’s not a 100% perfect fit, you’ll be spreading the frame rails about 1/16th or so of an inch- not much.

5) Mount the 13-hole beams to the lower frame rails, repeating the pattern. These beams should stop just short of the cockpit seat.

6) Connect the channel brackets with a channel connector.

7) Attach a 33-hole beam to the bottom of the seat assembly, with a 3-hole overlap to the end of the lower frame rail. I chose to make this connection with all 3 bolts, as this will be a highly stressed area. At this point, the entire front 2/3 of the car should be complete.

Step 7: Math Recess

I would say that this isn't an exact science, but it really is.

3S batteries are 11.1 - 12.6 volts depending on charge
6S batteries are 22.2 - 25.2 volts depending on charge

On a 2200 KV Castle Motor, that comes out to 24,420-27,720 RPM on 3S, and 48,840-55,440 RPM on 6S.

If you think about it, that is ALOT of RPM. Especially when you start laying out tables in TextEdit like I did.

Now deduct about 20% to account for all the various forms of drag.

As far as weight goes, I have a 5.5KG gramscale en route from amazon. Until then, I'm using Tower Hobbies' numbers:

Mamba Monster 2 ESC: 4 ounces
Castle 1515/1Y/2200 Motor: 15 ounces
Venom 5000mah 35C 3S LiPo's: 13.5 ounces each

Assuming the digital bathroom scale is something close to correct, it shows the car as 8.4 pounds RTR. Accounting for the items above, that means the actobotics chassis is about 5.5 pounds.

Step 8: Rear Frame Section

In this section, we will assemble the rear frame sub-section. This section is somewhat ambiguous, as I am still working through the R&D process to find a system that works.

General parts list:

2 pair channel/beam interface brackets
6" 1/4" D shaft
2 pair 3/8" hub spacers
1 pair 1/8" hub spacers
1 pair 1/4x1/2" flanged ball bearings
1 pair dual ball bearing mount
1 pair 21-hole beam
1 pair 7-hole beam
2 pairs 3x3 beam bracket
3/4" and 1" screws

Step 9: Rear Chassis Assembly

1) Attach a 3x3 beam bracket to the 9-hole beam upper rails of the cockpit area using two 3/4" bolts and nylocks, with the bracket on the outside.

2) Insert a 3/4" screw through the center bottom hole of the 3x3 bracket, slide the end hole of a 21-hole beam over the bolt, and nylock.

3) Attach another set of 3x3 beam brackets at the other end of the 21-hole beam, attaching with 2 bolts in the bottom row of holes in the bracket.

This completes the "easy" part of the rear chassis. The rest is semi-ambigious.

In the photos shown here, I have the axle installed using a piece of 1.5" channel, and dual channel brackets as endplates (I didn't have any singles on hand). It took me 3 tries to get to this point and I was still blowing through gears, so I will be doing a 4th revision with a 3.75" piece of channel and single piece endplates. I'll add that in later as a new page with new photos (and leave this here for legacy notes). Functionally not much will change for this step, but it will change when attaching the drive system.

4) Starting with a piece of flat channel bracket, attach channel/beam interface brackets in the same manner as the cockpit area. Face the bolts inward and nylock them. Repeat for the other side. Reference the photos above.

5) Take a piece of channel, and place 2 1.5" bolts through the horizontal pair of .770 hub holes. (Note: Later assembly might actually be easier if you use the vertical pair of holes, for the sake of squeezing the ring gear in place, but you'll need to modify the channel/beam interface brackets to clear.)

6) Place a 1/4x1/2" flange bearing into one of the 3/8 hub spacers, then slide this group bearing-first onto the 2 bolts from step 5. It might take a few tries for the bearing to not fall out. Now slide another flange bearing into the exposed side of the hub spacer, and slide a 1/8" hub spacer over the bearing. You should now have an assembly of 2 flange bearings sandwiched between the channel and the 1/8" hub spacer.

7) Slide the channel bracket with the channel/beam interface adapters over the 2 bolts, to add to the sandwich, with the plastic brackets facing out. The channel bracket should rest flat to the 1/8" spacer.

8) This is where it gets tricky....thread the 2 screws into the threaded holes of another 3/8 spacer. The bearings will probably fall out a couple times, but you'll get it. The spacer won't pull all the way down, it will bottom out on the plastic brackets (see photos).

9) Repeat steps 5 through 8 on the other side of the base channel. This should mostly complete the core structure of the rear axle housing assembly.

10) Screw a dual ball bearing mount into the exposed 3/8 spacer adapter using 3/4" screws. Use the opposite holes from the 1.5" bolts holding the assembly together. Repeat on the other side. This will complete the housing assembly.

11) Install the housing assembly to the end of the 33-hole lower frame rail beam and nylock. This will securely fasten the housing to the rest of the chassis.

12) Mount a 7-hole beam to the top of each channel bracket endplate. Use 1" screws and leave the nylocks loose- we'll be reusing these bolts for the wing struts.

13) The 7 hole beams will line up with the 3x3 beam brackets on the upper frame rail. Connect them with two 3/4" bolts and nylocks. The entire chassis should now be nice and rigid. If not, check the pictures and see if you missed something.

Step 10: Wings

In this section, we'll build the wings. I don't have photos of attaching the coverings, but it was VERY tedious and took me about an hour for the rear wing and 10 minutes for the front wing. The front wing uses 3/4" heatshrink, the rear uses 3/4 and 1/2". The front wing is 2 pieces with a 2" overlap.

Parts List:

1 pair 21-hole beam (rear forward struts)
2 pair 9-hole beam (rear back lower struts)
1 pair 7-hole beam (rear back upper struts)
2 pairs 3-hole 45 degree beam brackets
2 pairs 5-hole 45 degree beam brackets
7 pieces 12" 6-32 threaded rod
1 pair 5x5 beam bracket (rear spill plates)
1 pair 3x3 beam bracket (front spill plates)
2 12" pieces 3/4" heat shrink
1 12" piece 1/2" heat shrink

Step 11: Front Wing

The front wing is the easier of the two, so we'll start with it.

1) Using 2 screws, line up a 3-hole 45 degree beam bracket with the 2 front holes in the middle row of a 3x3 plate bracket. Using a drill press or dremel, drill a hole for the angle bracket through the spill-plate. Do this for the other side as well. This will allow the threaded rod to pass through.

2) Remove the 7-hole beam from the front of the chassis, if present. Otherwise, grab a 7-hole beam. Now stick 2 pieces of 12" threaded rod through the beam and center it roughly. Next, slide a 3-hole 45 degree bracket down the threaded rod, on either side of the beam. These will support the upper element rod.

3) There's 2 ways to do this- the easy way and the hard way. You need to put a nut on each threaded rod next to the plastic brackets. I did it the hard way with nylocks.....I'd suggest using regular 6-32 nuts, double-nutted.

4) Measure the rods out to 7-1/4" centered on the beam, and cut them. Or you can slide them and only cut one end off. Either way. I used a dremel and cutting blade. Just be careful where you send sparks. Servocity loves to send extra kindling in the shipping boxes, and we'd REALLY hate to get sparks near that stuff.....right?

5) Slide a 3-hole 45 degree bracket over each end, followed by the spill-plate that you just drilled out. Add nylocks, and you're done. The nuts in the center will keep the shaft from spinning...or at least, that's their job. I also added backwards nylocks inside the spill-plates to keep them from sliding inward, these really aren't needed if you do the covering.

6) For covering, I used two pieces of 3/4" heatshrink. Cut one piece to cover about 1" past the aluminum beam, slot one side of it to go around said beam (I slotted both sides, that's why mine turned out looking funny), and heatshrink in place. Then scut a piece to cover the other side from spill plate to aluminum beam. Slide it over the first piece and heatshrink, and this should give you a fairly uniform wing covering.

Step 12: Rear Wing

The rear wing is similar to the front wing, but much bigger and more difficult to build. It actually took me 3 tries to get the spacings where I wanted them.

1) Using a 3/4" bolt, attach the end of a 21-hole beam to a middle hole of a flat side of the 5-hole bracket. This is the first attachment point. Repeat for the other side. The head of the screw is on the plastic side, nylock on the beam side, with the bracket pointing up when the mounted side is horizontal-as the shape of the wing. The idea is for the plastic frame supports to be outside of the mounting struts.

2) Place an allthread through the end holes next to the mounting points. Place three 1" nylon spacers outboard of the mounting brackets on each side, followed by a loose nylock- we'll finish this bar in a moment.

3) Start an all-thread through the center hole of one of the 5-hole brackets, going inward. Once it is through the bracket, add a 7-hole beam, two 1" nylon spacers, another 7-hole beam, then the other 5-hole bracket. This will form the dimensions of the inner supports. Thread the rod the rest of the way until it's even with the first rod. These 2 rods together form the lower wing element.

4) Like the front wing, we need to modify the spill-plates for the rear wing to accept the 45-degree hole pattern. Line up the 2 horizontal holes using a pair of screws, then drill the 45 degree holes.

5) Take the nylocks off of the leading rod, slide 2 more allthreads through the upper 2 angled holes (using more 1" nylon spacers on the lower rod), then cap it off with 5-hole 45 degree brackets and the spill plates, finishing with nylocks. Then cut the excess rod off.

6) Covering this wing is very difficult. It requires completely disassembling the wing, notching the heatshrink, and SLOWLY and CAREFULLY feeding the all-thread through the heatshrink one piece at a time. It took me every bit of 45 minutes. It might be worth experimenting as doing it in multiple pieces like the front wing.

Step 13: Attach the Wings

The front wing attaches very simply using the 2 bolts at the front of the chassis.

The rear wing requires adding a pair of 9-hole beams in an overlapping form on each 7-hole strut. The top overlap is 3 holes, the bottom is 2. I added hex spacers to keep everything tied together.

The front strut of the rear wing goes to the back bolt of the 3x3 brackets on the upper frame rails, the rear struts go to the rear-most bolts of the upper frame rails/channel-beam interface brackets. See photos.

(Actually, I changed where the front strut mounts. I started with them 4 holes apart at the fra,e 5 yields a better angle. Some photos may show either way)

Step 14: Install the Rear Axle

To install the drive axle, slide it through one side of the bearings assembly (it will be tight- remember you're going through 4 bearings on each side), I like to slide left to right since that's where the ring gear is. Once the shaft is sticking into the center channel, slide 2 black plastic 1/4" shaft spacers, followed by the ring gear, teeth side in, before sliding the axle through the other side. Again, remember it will be stiff- you've now slid the axle through 8 bearings. Shaft deflection is bad, thus the excessive support.

I picked up some grade-8 1/4" thick washers at the local hardware store, and with one of those and a standard washer on each side, the width was just right to slide on a 1/4" set screw clamp. There will be about 1/16" of free play on either side- this is OK. The end of the shaft should be flush with the set screw clamp.

The traxxas wheels use a 14mm hex, a pattern not offered by Servocity. For R&D, I cut some 17mm hex adapters down on a band saw to about 13.5 or so millimeters. Not perfect, but good enough. Plan is to use the 12mm hex adapters with some 12-14mm adapters being 3D printed. Screw the hex adapter son, and the rear axle is complete. All that is left is the drive system and the auxilary panels and hold-downs seen throughout the photos.