Scratch Build an RC Car With CAD and Rapid Prototyping

Introduction

For a long time I have wanted to build an RC car completely from scratch, but never quite had access to all of the tools required to do it by hand or been able to justify the cost of using rapid prototyping methods. After taking part in the Intel IOT Invitational and the Mediatek Linkit One challenge I found myself with a whole bunch of Amazon and Shapeways vouchers, so I decided it was time to make the dream a reality.

The goal of this instructable is twofold, for anyone who wants to replicate my project exactly, I will provide all of the files (including editable models) and instructions to do it, but more interestingly, I will try and explain my choices and processes so that anyone can take what I have learned and use it to build their very own one-of-a-kind RC car, based on mine, or straight out of their own imagination.

Design Choices

The first step when starting a project like this is to determine your goals and your limitations, these were mine, yours may be different.

Design Goals

• Build as much as possible myself
• Design something unique
• Use lots of metal in the design (I like how realistically heavy RC's drive)
• Solid rear axle hot-rod
• Resiliant enough to actually drive (not a shelf queen!)

Design Limitations

Cost: I wanted to spend as little as possible (some further notes on that below)

Time: I wanted to get it done in 80 days for the 3D printing competition, this was a great motivation to keep working hard on it. The 3D modelling was by far the most time consuming.

Available Tools: No metal working facilities, hence why I chose laser cutting and carefully designed around having to do anything other than drilling and tapping afterwards.

Available Software: I am familiar with Solid Works, but now that Autodesk Fusion 360 is available free to hobbiests I used this as a project to learn on. I can't recommend it enough!

Potential Cost Reductions

I was fortunate enough to be able to fund almost the whole project with vouchers I won from various Instructables competitions, but this means that my choices would not universally be the cheapest options, here are some ideas if cash is tight.

• Design around a readily available body
• Build the body by hand out of Styrene or Wood
• Print the body in pieces on a hobby 3D printer
• Use cheap motor/esc/servos/wheels from an old RC car
• Print the profiles on paper and cut Aluminium parts by hand on a bandsaw/scrollsaw

Step 1: Parts List

These are the parts that I used. Almost all of them were bought from Asiatees, with a handful from Aliexpress, Banggood, and my local hobby shop.

The vast majority of the parts can be swapped out with alternatives that you probably already have if you are into RC.

Part	Link to Supplier	Supplier	Cost (USD)
Front Axle	Aluminum 1:14 Tractor Axle	Asiatees	21.5
Rear Axle	Boom Racing SCX10 Axle	Asiatees	98
Driveshaft	Hardened Steel Adjustable Shaft	Asiatees	20.9
Gearbox	Alloy SCX10 Case	Asiatees	21
Gears	Boom Racing Heavy Duty Gears	Asiatees	24
Wheels (Front)	75mm Wheel and Tyre	Aliexpress
Wheels (Rear)	KRT 108MM Wheel and Tyre	Asiatees	13
Body		Shapeways	130
Motor	7.5T Brushless Sensored Motor	Asiatees	57
ESC	XERUN-120A-SD V2.1	Asiatees	83
Shocks	65mm Aluminium Shocks	Banggood	16
Servo	1:10th Scale Servo	Hobby King	15
Link Rods
Ball Ends

Step 2: CAD Work (Design in Autodesk Fusion 360)

Select a CAD tool

3D CAD tools are generally terribly expensive or terribly terrible but, to my absolute delight, Autodesk Fusion 360 is completely free to hobbiests and students.

I used this project as an opportunity to learn how to use Fusion 360 (I am familiar with Solid Works, which I also am very fond of, but could never afford it for personal projects)

Learn CAD

Teaching you how to use a CAD tool is beyond the scope of this Instructable, but I strongly recommend just diving in and trying it out.

In my opinion there is no better way than to learn than by getting neck deep in a project you enjoy.

You can also download my files (next step) which I have attached to this Instructable and step through the timeline right from the beginning, to see how I created each element.

My Approach to the Overall Design

1. Import a front and side view and scale to 1:10th
2. Make a very rough model of the body shell
3. Model all of the bought parts (axle, motor etc)
4. Design and model the frame and suspension
5. Decide how the body will mount to the frame
6. Model the body in detail, to fit around the frame

My Design Goals

• Body should be removable easily but not come off during a crash
• Motor pinion and spur gears to be hidden/protected
• Motor in the front (most RCs dont obey very realistic motor layouts)
• As few parts as possible (for cost)
• As realistic as possible without overly affecting cost/perfomance
• Sturdy

Modelling the Body

Inspiration and DImensions

First, find some inspiration pictures to work off, they don't have to be much like the finished model, but it is tremendously helpful to have some baseline shapes and dimensions to work from.

I did a lot of searching for photos of '32 fords, which were the general shape I was after. They are not mine to post photos here, but I recommend you look at this article, the profile shots are a perfect example.

I also searched hotrodding forums for dimensions of the wheelbase and length of the frame rails so that I could import the image and scale it to exactly 1:10th scale

Loft the body

The next step is to start drawing profiles which we will "loft" into a solid shape. Lofting is a 3D modelling technique where we draw two or more 2D shapes (closed profiles) and tell the program to create a solid body the flows smoothly from the one shape to the other (for more control, we can use guide rails, which force the shape to adhere to those lines).

Hollow the body

Once the solid shape of the body has been defined, I used a "shell" operation to hollow it out and remove the floor (with a wall thickness of 6mm).

I then sliced out the windows and doors (via extrusions and body-splitting tools, defined by sketches) as well use splitting the body in a few more places where I wanted support braces.

I then performed a second shell operation on the 6mm shell, with a wall thickness of 2mm, after which I recombined all of the parts and used a boolean operation to remove the inner 2mm walls, leaving only the outer 2mm wall and "braces" wherever there had been splits in the model before. The animated GIF attached to this step should make it clear.

3D Printing Considerations (Shapeways "White Strong and Flexible" Nylon)

Design for Cost

When designing something to be 3D printed at shapeways via SLS, one needs to keep in mind the 3 pillars of their pricing scheme:

Material Usage

How much nylon is actually used up. If I had made it all with 6mm walls it would have been

Volume

In other words, how much space the part takes up in the printer. If I had designed this model with the floor and firwall included it would have been incredibly expensive.

Labour

Effectively, how many separate pieces there are to pack (and polish, if you choose that)

I uploaded about 20 different versions along with way, with resulting prices ranging from 100 to 240 USD, all based on slight variations of wall thickness, sizing, etc.

Design for Strength

In the end my model was 2mm thick in most places with additional 2mm ribs around the doors, windows and selected areas across the roof. I am very happy with the strength versus weight so far. It is wise to read the material guidelines very carefully.

I followed the guidelines to the letter and was happy with the result (although slightly dissappointed by the loss of detail around the doors from polishing, but that can be easily corrected).

Speaking of the doors, you could easily make them open, but I felt that it would be too much of a strength sacrifice for a gimmick.

Modelling the Metalwork

I decided right at the beginning that I would make the chassis out of laser-cut Aluminium. In order to keep costs down I designed around only two thicknesses as well as minimising the number of bent parts (bending requires labour and hence is more expensive). In addition to the expense, sheet metal is not currently supported by Fusion 360 so any bends have to be "unfolded" manually for laser cutting.

Suspension Geometry

I learned a lot about vehicle suspension during this project, funnily enough most of the information was from 1:1 car sites and not from RC sites. Here are some of the most interesting links that I read regarding the "triangulated 4-link" suspension geometry that I decided to use.

Triangulated 4-Link

The Dirt Forum: 4-Link

Suspension 101

The Science of Straight Axles

Magnetic Mounts

I decided to mount the motor directly to the firewall, which meant that the firwall must be rigidly attached to the chassis while still providing support to the body.

I could have used screws, but they are a nuisance to remove in the field. I could have used standard RC bodyposts and clips, but they look horrible.

In the end I decided to use a technique that I developed in a recent boat project and have been extremely happy with. I use steel pins that are held in place by magnets. The various mounting holes are placed such that the body will be locked in position when all the pins are inserted. The magnets don't provide any of the holding force, they just stop the pins sliding out.

Step 3: View, Download, Modify and Improve My Design!

I put this in it's own step because i want you to see it! This project represents countless hours (seriously, I'd rather not count them) of my time, so I could either keep it to myself and let it rot on my harddrive, or give it away to people who might just learn something from it.

The following link gives access my entire project, which means you can open it, make changes, see the good choices I made and learn from the bad ones.

If you do make anything with it all I ask is that you let me know about it!

Fusion 360 Project Link: Ossum Hotrod!

Step 4: Manufacture the Parts

Order from Shapeways

In order to order from shapeways you need to export your model as an STL file. Assuming you have designed it in Fusion 360, right-click on the particular body you want to print and select "save as STL", making sure to select the "high refinement" option for maximum detail.

Shapeway's website will guide you through the process and let you know if you have designed anything unprintable.

My last caution is to make sure that you know what the import duties are going to be if you are ordering internationally, mine came up to about 65USD (or 50% of the order value), due to my country's ludicrous rates aimed at stopping the import of chinese goods or something similar.

The STL I ordered is attached to this step.

Order from Lasercutters

Please Note: If you decide to use my drawings, there is about 2mm of error in the transmission mounting holes, so it would be worth measuring yours first and modifying the design. Or just fix it with a file like I did.

As of yet there are no good online laser cutting services that handle metals (as far as I know), so you will have to find somone local, which shouldn't be too hard (I used Vulcan Steel in Cape Town).

The lasercutting folk will want your profiles in a DXF format (since they are just 2D shapes), and if you have designed any bends they will want the bend lines on the drawing too. In addition, it would be wise to supply them with a 3D/Isometric drawing of the bent parts, so that there is no room for confusion. You can look at the drawings I submitted as an example (attached to this step).

Order from 3DHubs

You could order everything from Shapeways, but it is expensive, and some of the other parts don't need such fancy materials and processes. 3DHubs is a website that lets people with 3Dprinters (from hobby level to commercial grade) offer their services. My initial plan was to print the radiator cover and the transmission tunnel cheaply from ABS with a 50USD voucher I had, but so far there have been issues with using the voucher, so I can't recommend them one way or the other.

I have attached the files though. If I end up winning a printer you can rest assured that these will be the first parts I print!

Step 5: Paint the 3D Print

Paint Choice

There is a mountain of information on painting Shapways prints (actually, far too much, and not all of it good), so in the end I decided to go for a Tamiya TS spray paint as the base, since I had read some positive reviews. If I had access to an airbrush I would have chosen an acrylic, simply because I like working with water-soluble paints.

Since the material is porous it actually takes paint very well, and the Nylon is generally fairly non-reactive. I would do some reading of my own if I were you though, since I was paranoid enough myself about destroying my expensive print with the wrong paint.

Technique

I decided that I wanted a lightly rusted ratty look, partly because I like a vehicle that looks like it has a story and partly because I want to be able to run it hard without worrying about scratches and dings.

Base Coat

I started with a base coat of "Tamiya TS1 Red Brown" (Code 85001), with the expectation that it would give a nice rusty look which, since it soaks right into the porous material, would be exposed wherever the body gets scratched up.

Rust Texture

The next step was to sponge on some lighter rust colours to give a little texture. I used Tamiya acrylics here, since they come in tiny pots that are nice and cheap. "Burnt Sienna" is a good colour for this.

Faded Paint

The next step is to lay on the "factory" paint, which will mostly get removed, since it has "rusted off". The trick here is to put something on the rust base coat the prevents the colour coat from adhering properly, most people use salt, adhered to the model with a little water. After the paint has dried it is scrubbed with a hard brush, and will chip off wherever salt was applied.

The technique is explained well here, by someone far more experienced than I.

Detailing

You may want to add some further details at this point, perhaps some hotrod flames (unless you wanted them to be rusted off, in which case you would have done them before the salt chipping), maybe some light washes of rust streaks that have made their way down the paint. I chose to just do a little tribute to the "Elvis" car from Fury Road, which was one of my big inspirations for this build.

Step 6: Modify the Front Axle

The front axle that I chose was actually designed as a hop-up for a 1/14 scale Tamiya tractor, the standard mounting points would not work for my design, so I had to make a few modifications.

Shock Mounting Point

The axle was designed to have shocks mounted to the tapped holes in the blocks spaced about 74mm apart, but of course these would foul my frame rails, so I drilled two new holes further out (see annotated pictures)

Suspension Link Mounts

I used a 5mm drill (see pictures) to mark the exact center of the recess in the top of the axle, since it fit perfectly. I then drilled the hole with a 2.6mm bit so that I could tap it with an M3 thread.

Step 7: Assemble the Chassis

Drill and Tap

My laser cutters could not do holes where the diameter of the hole is less than the thickness of the material, which is probably common to all laser cutters, this means that we need to drill the holes in the frame rails afterwards.

My first step was to print out drawings of the various parts in 1:1 scale, so that I could position the holes.

Use a sharp punch to mark the centers and then carefully drill, taking note of the hole sizes.

• 4.5mm for the mounting pins
• 3mm for untapped holes
• 2.6mm for M3 tapped holes

Doing both frame rails at the same time makes sure that all the holes line up, just make sure you don't mess up the first two, after that you can screw them together and handle them as one piece.

I did the 3mm holes first so that I could then screw the two profiles together.

Mount Frame Rails

Start by attaching the two frame rails to the floor pan, making 100% sure that they are parallel to each other and perpendicular to the floor pan. Make sure to file any burrs off the holes that you drilled, so that the plate cant sit flush on the rails.

Radiator Support

You may find that the bending of the radiator support is not perfect, which means that the holes won't line up without a bit of tweaking. I worked around this by using very long screws (M3x20) to mount it first - just put the tip of each screw through both parts, then slowly tighten them up to bend it into shape. Once the support is mounted neatly to the firewall you can replace each long screw with a short one (M3x6) , one at a time.

Front Suspension Mounts Points

The small plates that make up the front suspension shock mounts and link mounts should be bolted to the frame.

You can see exactly how I did mine from the photos.

Thread Lock

I added this step last because you should really do a test assembly before you start using Loctite, I guarantee you will end up disassembling it at least once.

After you are sure that everything fits, reinstall the various nuts and bolts using a threadlocker, like Loctite, metal on metal parts will vibrate loose if you don't.

Step 8: Assemble Suspension

Assemble Suspension Links

I used the metal ball ends along with some M4 threaded rod, sleeved in a bit of plastic tube from the gardening section, to assemble my longer links.

For the short links I used M3 tapped ball ends, screwed directly into each other (with a short piece of M3 rod) or into M3 hex standoffs.

• Rear Upper: 52mm
• Rear Lower: 40mm
• Front Upper: 105mm
• Front Lower: 84mm

Rear Suspension

Front Suspension

Step 9: Install Steering Servo

Servo Mount Blocks

I made two small aluminium blocks to mount the servo to, simply drilling and tapping holes that match those on the chassis rails and perpendicularly to those that match the servo's mount holes. Take a look at the pictures to see.

The blocks could be laser cut along with everything else, but I didn't think of that in time.

Steering Linkage

The steering linkage is just another pair of ball-ends on a threaded rod, you can see in the photos how I attached it. I used the one that came with my rear axle (since it is a convertible front/rear axle - see parts list).

Make sure to power up the receiver and plug in the servo (or use a servo tester) so that you can centre the servo before fastening down the horn.

NB: It is also important to use your radio's settings to limit the servo travel. The axle has less travel than most servos, which means they will be straining at their limits and either damage themselves or the steering knuckles.

Step 10: Install Driveshaft

This step is going to be highly dependent on the particular drive shaft that you order.

Due to stock issues the shaft I ordered was actually intended for an Axial Wraith and as such the one end was too wide/long to slide over the SCX10 transmission's output all the way. I solved this problem by enlarging the hole in the transmission casing.

I used M2.5x16 nuts and bolts, along with some threadlocker, to fasten the driveshafts to the outdrives.

Step 11: Electronics

The electronics are a completely standard brushless RC setup, so if you are unsure I would recommend reading some of the great information that fills the various RC forums and webpages, also, read the manual that comes with your electronics.

Power

I soldered an XT60 connector to the ESC power input as this is my favored connector, so all my batteries use it. The battery itself is a 2800mAh 2S LiPo.

Connect the Receiver

Channel 1: Steering Servo

Channel 2: ESC throttle

The receiver gets power via the 3 wire cable from the ESC (assuming that you chose one with a built-in BEC)

The receiver passes power and control data to the servo via it's 3-wire connector.

Motor Wiring

The motor wiring depends on what you have chosen

1. Brushed Motor and ESC: Connect the two wires from the ESC to the motor, in any polarity, if the motor turns the wrong way, swap them.
2. Sensorless Brushless Motor and ESC: Connect the three wires from the ESC to the motor, in any polarity, if the motor turns the wrong way, swap two of them.
3. Sensored Brushless Motor and ESC: Connect the three wires from the ESC to the motor, taking care to connect A to A, B to B and C to C. Connect the motor sensor to the ESC sensor input with the supplied cable.

Step 12: Temporary Handmade Radiator Cowl

As I mentioned in the printing step, I had some troubles getting my 3DHubs voucher working, so in the meantime I whipped up a radiator cowling by hand, using some scraps of bit of balsa wood.

It is not terribly elegant, but it does the job for now.

The joy of working with balsa is that it is easy to cut with a sharp blade and sand into shape.

The downside is that it is very soft, so it will dent easily.

I painted it first with multiple coats of acrylic to seal the pores, sanding lightly between each coat, then I sprayed it with the same colour grey as the body.

I also used a scrap of aluminium sheet (about 1mm thick) to make a sort of salt-racer style grille. The edges are hidden by the cowling, so I used the highly precise cutting technique of fastening it in the vise and bending it back and forth until it snapped.

The pictures should explain exactly how I made it, but if you want to see a in-depth project with the same techniques, check out my RC airboat.

Step 13: Room for Improvement

Mistakes I made, don't be like me!

Clearance for nuts and bolts

Model the nuts and bolts. This one is a giant pain, and hence I didn't do it. I have made the mistake before of not leaving enough space for the screwdriver, so I didn't make that one this time. I ended up without enough clearance for the front uppper suspension arms, since the nuts for the lower suspension brackets got in the way. I got around it by countersinking the screws and inserting them from the other side, but still, dumb mistake.

Cheapo Chinese Wheels

I'm all for cheap Chinese stuff, it lets me play with a hobby that would normally be out of my budget, but sometimes you win and sometimes you lose. The wheel and tyre combos I bought came with tyres I really liked but the wheels were total garbage, I broke 3 within the first battery, with no serious crashes.

Design Improvements

There are oodles of possibilities for design improvements, which I will leave up to you. Here are some choices I would make differently.

• More angle between the front suspension arms (there is too much lateral wobble)
• Use black polised nylon (if it is available from shapeways by the time you print, it wasn't for me). White shows scratches too easily.
• More support for the firewall, to ensure it is perpendicular to floor and mates well with the front of the body.

Tuning Improvements

Shock Springs

This is just a tuning exercise, but the springs that came with my shocks are WAY too hard, so the ride is incredibly bouncy (as you can see in the first video), even with the weight of the car. I will be installing new springs soon to try out.

Update 1: I replaced all the springs with the softest Tamiya road springs I could get, it is much better now!

Update 2: I raised the shock mounts on the front and back by 10mm (thereby lowering the suspension) this has also reduced traction roll a lot.

Open Rear Differential

The rear differential is in fact not a differential at all (since it is designed for crawlers), it is permanently locked, this results in huge understeer. If possible I would like to get an open differential, but I haven't found one that fits in the SCX10 axle yet.

Gearing.

The gearing was a bit too low with the stock gears (although, going much faster was uncontrollable until the suspension had been lowered and the steering tightened up - see "panhard bar").

Update 1: I replaced the stock pinion (which had already stripped out due to an imbalance in the spur gear) with a 33 tooth pinion. Much much better top speed and the motor handles it without getting hot.

Panhard Bar

There was too much lateral play in the front axle, mostly due to the slop in the metal ball ends as well as the links being too parallel. I installed a panhard bar made from a bit of brass tube (see attached picture), this was far and away the biggest improvement to handling and therefore enjoyment, it is frustrating to crash because the car is weaving around of its own accord.