<<Cutting the Chassis Apertures>>

The chassis comes with the central steering holes (left) already drilled. You need to cut several more holes yourself, for the chain, trackrods and cables to pass through. The best tools to do this are an electric drill, an electric jigsaw, twist drills and a set of cylindrical hole saws. Ordinary drills and sawblades work well on the glass/polyester composite that the monocoque chassis is made from. One last tool, and possibly the most important: a pen that will make a decent line on the chassis surface.

The chassis is a beautiful moulding, and you might feel reluctant to attack it with a saw. I suggest you draw the apertures clearly, then check back with these notes to see you have got them right, before you start cutting. It is best to start with the trackrod apertures, because these cutouts are big enough for you to practise cutting within the waste material you are removing. A medium-ish saw should work best, not too fine, or the teeth will clog and overheat.

The black line on the picture (right) shows the dimensions of the left-side trackrod cutout. The four corners are best done with a cylindrical saw of about 1-1.5" diameter. The centres of the corner holes obviously need to be offset within the aperture, by a distance from the sides equal to the radius of the holesaw you are using. To make the drill start in the place you have marked, you need to make a little pit, perhaps using a sharp drillbit turned by hand, as in picture 2 below. The holesaw pilot will wander if you don't do this, and the result won't look pretty. Do not try to use a centre punch for this purpose.

Use a slow rotational speed on your holesaw and stop if you smell overheating - you do not want to degrade the composite around the edges of the holes. If you use protective tape, try to get something that does not leave a sticky residue on the surface of the moulding. And keep your fingers out of the chassis while you are using the jigsaw!

The rightside trackrod aperture is basically a repeat of the left. (The GNAT used to have unequal length arms which made the right aperture smaller, but the steering geometry has now been made more conventional. Don't be confused by odd pictures remaining still showing this old layout)

The front aperture, which the chain passes through, is low down on the extreme right of the front face of the chassis, one inch wide and 2.5" high. You should cut two holes with a one-inch-diameter cylindrical saw, one centred 1.5" above the floor, and the other at 3" height, and then use the jigsaw to join the holes along the black lines in the picture (left). Note the second hole in the picture on the right. This is for the cables from the joystick to pass through. It may be best to delay cutting this hole until you have an exact idea of where the cables want to go.

The rear chain cutout is wider than the front, because of the width of the cassette block, and taller, owing to suspension movement. Two holes of about 1.75" diameter need to be cut one above the other, on a centre line 4.0" from the right side of the chassis. One should be centred at about 33mm above the floor, the other at about 76mm (these distances are measured, as in the picture, along the slope of the rear face, not perpendicularly).

The chassis has an internal wall on the righthand side, which requires a hole drilled in it to support the chain pulley. The hole should be about an inch from the front of this wall, and the chain should be within about 2mm of the floor as it passes under the pulley. So you will need the pulley when you mark the position of the hole. A length of chain, an 8mm drill, and a piece of something about 1.5mm thick - the steel strip in the picture (right) - will enable you to use the pulley as a guide to mark the position of the hole in the right place.

The wall is mostly empty space: two layers of composite separated by thin filaments, the same material that the floor is made from. It is not possible to get a conventional drill into position to drill the pulley hole, but neither is it necessary. You can start the hole with a sharp drill of about 1.5mm diameter, turned by means of a drill chuck held in the hand. It won't take too much turning to penetrate the wall. Then you can work out the hole with increasing sizes of drill until it is 8mm in diameter, the diameter of the bolt that supports the pulley.

When you have made the hole, you need to fill the cavity between the walls around the hole with car-body filler, or something similar. A good way is to put tape over the back of the hole, then poke filler into the hole from the other side with something like the blunt end of a pencil. You don't want to fill in the hole you have just made, but you do want to be able to tighten the pulley bolt without squashing the two surfaces of the wall together. When you have filled around the hole, you can further strengthen it by epoxying a composite washer on each side - using two of the blanks you have removed with the hole saw, as in picture 6 above.

If you look carefully at this shot, you will notice that the thickness of the composite is uneven. This is deliberate; the chassis is thicker where the attachments are bolted on, giving it local strength and overall stiffness. You should choose flat washers to reinforce the chain pulley, else find some other way (filing the washer, packing with filler) to give a true surface around the pulley hole.


<<The Angle Brackets BUILDING THE GNAT Chassis Attachments>>