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Premium Member
1,487 Posts
Discussion Starter · #1 ·
Making a different chassis? What's wrong with this one?

Well, not much really, I can't call this the Mark 2 because I went through about 3 prototypes before the last one, but it's time for a redesign for a few reasons:

a) Some of my forthcoming bodyshells are too short at the front, there's too little overhang to accommodate the front suspension mechanism I've got.
I've built a few of them now and fixing the M1.6 washer onto the M2 screw is too difficult when I built a few at one time.
c) I could really do with making it cheaper. To resin cast the parts would take more expertise than I've got so the next best thing is to make it smaller. Smaller volume means less time on the SLS machine so cheaper.
d) I've changed jobs, got a different CAD system so can't modify the old design, just print more exactly the same.

So here's a step by step guide for those who fancy getting busy with a mouse instead of sandpaper

1. Start with what you know
Things like the motor, guide, rear axle, any bearings, wheels and tyres. It'll give you practise at modelling things, and you always design around what's fixed

You can see I've modelled in the wires from the guide blade, it's a common thing to forget but "cable management" is a nightmare whether you're designing slot cars or nuclear submarines.

2. Design constraints
Mine are: compact, small overhang, adjustable wheelbase, more trail steer than before (because there's always a chance it'll work and I won't have to link the guide blade to the steering arm), easy suspension adjust, lower than before because the motor pod was really close to fouling the vacform interior. I considered a sidewinder layout but I think a longitudinal motor makes the car easier to slide around corners.

Write these down where you can see them all the time. You might scribble a few out, you might even write a few of them back but it's your checklist.

3. Make a start
I always start drawing on paper, no matter how easy CAD systems are to use, a pencil is much quicker to get different ideas down. Some go straight to the computer, I invite you to find your own way.

Get going with the things close to your "fixed parts", so I started with the motor pod. You do CAD designs by starting off with a block (mostly) and then adding cuts or other blocks to it. Here's where I started from

Chamfer the sides a bit

And round the outside corners

Then shell the whole thing. You can choose the thickness you want and most CAD programs will let you choose different thicknesses for different faces if you like. I'm using 2mm as a general wall thickness for the SLS nylon material

This is about the stage where you create an assembly and put the motor inside to see if the sizes are correct, and to fit with any existing features.

Cylinder on the front holds the motor and is the pivot to make it roll in the chassis. Hole in the top of the motor pod allows me to get the top of the motor sticking out so the overall height is lower. On the previous chassis I had 2mm of material above the motor, plus clearances and I need to get it as low as possible

Clearance for the rear axle gear, placement for the springs

This is now getting so boring that even I'm yawning so as they say in the best TV shows, here's one I prepared earlier before I completely lose the audience

More later!

Premium Member
1,487 Posts
Discussion Starter · #2 ·

Let's get the other thing that we know will be fixed - the bodyshell. Here's the Big Bad Boss's Black Limo (or if you chop the roof off and paint it white... you get the idea)

With all the parts in place I can get the wheelbase right, this is the longest car so far, and also the track (distance across the axle)

Let's turn the shading off and check things in side view

If like me you were born with a clutch pencil in your hand, you'll want to print this out and start sketching other key items. Print it out at 2:1 to get more space for scribbling but also print one at 1:1 so you can keep the real scale in your mind; it's easy to get drawn in when an M2 nut fills the entire screen.

With your pencil you might want to place the springs perhaps, or draw in where the interior will sit so you know how much space you've got the motor pod

Let's put the motor inside the motor pod and check it's held in place properly, and we've got all the features we need. Space for the axle bushes, a way of retaining them, clips for the motor, holes to let the whole thing pivot etc

From the scale drawing, you can measure to see how much height you've got for the rear chassis which holds the motor pod. This is the time consuming part; the internal parts are getting bigger but you're constrained by the outside shape. As well as the Cadillac I've got a van to do which has even less overhang front and back, that's an outline on my drawing too

After squeezing stuff around, it'll hopefully all fit

Worth keeping an eye on the plan view too. Here I've got no problems but if you do a sidewinder configuration things can get snug. It's also a good way to check where the springs can be so you can get to adjust them easily!

Now we've got the size, we can put all the features in that we need

And check the suspension movement. The cars tend to roll by about 8 degrees either way which doesn't sound like much but it looks massive from the outside. I've allowed 12 degrees but whispered to myself in secret that it could be 10 degrees if I'm struggling to fit something in
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