Driveline Tech
CV Joint Preparation
Measuring And Making CV Axle Shafts
Sprocket Ratio Chart
Successful Clutchless Shifting
Wheel Guide
 

Engine Tech
Engine Buying Guide
Engine Cooling
 

Fab Tech
Bending Tube, Making Tube Benders and Cutting Fishmouths
Wire Mesh Stone Guards
MYO Exhaust System
 

Brake Tech
Buggy Brakes
MYO Brake Bias Valve
The Hydraulic Handbrake
 

Steering and Suspension Tech
Balancing Your Buggy
The Ed McCannick Articles
 

Wiring and Electrics Tech
Buggy Electrics
 

Formulae and General Data
AN & Taper Pipe Thread Sizes
Bolt and Nut Torque Calculator
EngNet Conversion Calculator
Fraction/Decimal/Metric Conversion Chart
Guide to Safety Lockwiring
Hole & Counterbore Sizes
Metals Weight Calculator
Race/Practice Spec Sheet
Rod End Comparison Chart
Scheduled Maintenance
Tube Size vs. Strength
Wire & Sheet Metal Gauge Equivalents
 

Calculators
Length
Online imperial/metric converters
Speed
 

Some basic building tips.
With the plans pinned to a board in the workshop, it's time to prepare a chassis stand, and fabricate the jigs for the A-arms and trailing arms. If you have plenty of space in your shed, the ideal chassis stand has a permanently positioned, perfectly levelled, horizontal steel plate, at least the size of the buggy. Various bits and pieces can be tack welded to the table to support the chassis/suspension as it's being built. These "third hands" and "sky hooks" can be ground off flush again afterwards.

If on the other hand you are restricted for space, a moveable stand with adjustable feet is the answer. I have had one for years, which lives on its side, down the outside of the shed between builds. It's seen everything from buggies to a V8 mid-engined, full tube chassis for a VW Beetle! The top is replaced for each project, and consists of a sheet of 19mm (.75") MDF, on which I mark about five equally spaced lines along its length, and at least five across its width. If this grid is square, then your buggy will be too.

The centre line in the long axis is used to set out the various chassis width measurements, and two of the lines across the width of the table should represent the wheelbase. All the other lines merely act as reference and sighting lines. Every stage of the way, sight through the chassis to these lines. Your eyes will pick up any distortion, or measuring mistakes.

Making jigs for A-arms, trailing arms, or any removable parts is essential, and must be done with the utmost accuracy. Try to design the suspension jigs so they will make arms that can be mirrored from one side of the buggy to the other. Shock mounts etc can be added with bolt on jigs to "hand" the arms. Even make a jig to attach the arm mounting tabs accurately to the chassis. If you ever tear an arm off in a race, or on a rock, simply place a few lengths of tube in the appropriate jig, and weld up a new one that will exactly match the old one, and moreover, fit the chassis perfectly!

Time to start making things. I usually make all the engine mounts, arm mounts, and dozens of other chassis tabs before I touch any round tube. Carefully mark and cut them from RHS tube, to suit any bushes they have to locate. Alternatively, there are a number of specialist companies manufacturing tabs and if all else fails, ask a sheet metal shop to laser-cut the profiles, and fold them into brackets.

The materials commonly used in mini buggy chassis are Ø31.8mm x 2.1mm [1.25" x .083"] CDS/CDW/1020 DOM for the main structure. Elsewhere, 31.8mm x 1.6mm [1.25" x .065"], 25.4mm x 2.1mm [1.0" x .083"] (top A-arms), 25.4mm x 1.6mm [1.0" x .065"], 19.05mm x 1.6mm [.75" x .065"], 16mm x 1.6mm [.625" x .065"], and 12.7mm x 1.6mm [.50" x ,065"]. I use CDW tube for diameters above 19mm. Otherwise I use the cheaper ERW tube (with the weld seam up the inside) for items in either compression or tension.

The CDW tube is commonly 470 MPa, is relatively elastic, and easy to weld with a MIG. I only use 4130 chrome moly tube for the lower A-arms and trailing arms, because of the punishment they take. CroMo is typically around 650 MPa, and should only be TIG welded or brazed, but should then be heat-treated to normalize the metal again.

Chrome moly tube isn't any lighter than mild steel tube, but it does resist deformation far better than mild steel and can therefore be used in thinner sections resulting in lighter overall components.

Don't try and build the entire chassis out of just two tubes. Work out the joints, so the length of a tube, and number of bends in it, are kept to manageable limits.

Draw full size profiles of the tubes on another sheet of MDF, and glue neat wooden blocks at intervals along the outline. Then simply bend the tube, and test fit it to the profile board.

Allow enough material at each end of the tube for cutting the fishmouth. Again, read the centre of the fish mouth off the profile board. Cut the fishmouths with a tube notcher if you have one, or go to your local hardware shop and buy several bi-metal hole saws, matching the diameters to the tubes you are cutting.

Accuracy here is paramount. It's no use cutting a rough end on a tube, which doesn't butt up nicely to the major tube, and telling yourself you'll fill the gap with the welder later on. Unless the fishmouth mates with the major tube all the way around, the shrinkage in a large bead of weld will "pull" a non-mating fishmouth, and distort the chassis. 

I've had tack-welded chassis brought to me for welding with large gaps in some of the joints. Short of re-making half the chassis, I first weld several small "bridges" across the gap, let it cool down completely, and then weld it all around. It's not ideal, but a solution nonetheless.
And you thought this was going to be difficult!

Avoid the temptation to rush the job and finish all the welds. I usually just tack the entire buggy together, and assemble absolutely every last item including throttle cable, wiring loom, seat belts etc. Only then can you be 100% sure that after painting it and assembling it (for the second time), you'll be able to take it straight out and race it without any modifications.

I know of a few beautifully finished chassis that are lying in the scrap pile at the back of some people's sheds, never to see a racetrack simply because they were impatient and hadn't fully planned the build!