Don’t risk using sliding subframes — have race-style security for your bucket seats with Jon Betts’ guide to making your own seat mounts.
Once you have purchased a set of base-mounted seats, you are fairly limited as to how to mount them. Most manufactures can supply basic subframes but these are normally of the sliding variety and not really suitable for a Fast Road or track car as they could fail under heavy impact/braking. You could bolt then directly through the floor but that would put your head slightly below the dashboard unless you are freakishly tall. The only other alternative is to construct your own mounts, but as they will be seen, you want them to look half decent, so we took a trip over to see Gary at Gary Martin Motorsport who had just knocked up a good set for one of his customers.
The main part of the mounts are made from 1 inch angle iron and then covered in 1.2mm steel, to not only add strength but also to make them a bit more pleasing to the eye. The flared holes just add a nice finishing touch.
Initially the mounts were going to be square, meaning the front and rear of the frame would be the same height allowing us to use a 45-degree angle for the corners, but after some deliberation it was decided to make the front slightly higher. This provides some ‘anti-submarine’ to the seat which helps to stop you sliding forwards under hard braking. This means that the angles at the rear are not quite 45 degrees but, as you will need a gap for the weld to penetrate properly, it made it easier to just open then angle out slightly and fill the resulting gap with weld making an extremely strong join.
As the front of the mount is angled with the bottom stay longer than the top, the angles are a little more tricky so again to make it easier we simply cut the angle at 45 degrees and then made the fill in piece to fit, as you can see in the pictures.
Accuracy is needed when it comes to drilling the holes for the seat fixing; ours are 13 inches apart but measure yours carefully and drill accordingly. You don’t have to stick rigidly to our plan; they can be modified to suit each individual application so have a little fun with it.
You’ll need around 72 inches of 1-inch angle iron; two pieces of 1-1.2 mm steel at 16.5 x 4.5-inch and a 3 mm piece around 5 x 4-inch. Plan what sizes you will need to cut your angle to then mark it out.
Our top rails are 15 inches long and the base rails are 16.5 inches long, all with an internal 45-degree angle. Use a hacksaw to cut them out as well as the small end filets which are 4 inches long again with an internal 45-degree angle.
The top and bottom face in opposite directions so the floor fixings are to hand but the seat fixings are hidden. It is not critical to get the angles to exactly 45 degrees as we will be making adjustments.
We have drilled three 8 mm holes for each floor fixing point, this will allow for some degree of adjustment later on. Each hole is 1 inch from the next, space them equally so each side matches.
The most critical part of the build is getting the four seat mounting holes positioned correctly. There are two on either side and on our particular seats they were 13 inches centre to centre.
We carefully marked these holes on the top rail and than drilled again using the 8 mm drill, centre-punch the holes first so the drill doesn’t move, otherwise the holes aren’t going to line up!
We want the front of the mounts to sit higher, so instead of using more angle, we made a fill-in piece from some 3 mm plate. Make a cardboard template and then transfer to steel and cut out.
Now tack everything together, use clamps to hold the pieces in place and ensure you have everything the right way round. You need your welder set fairly high to ensure adequate penetration.
Once all four corners are tacked in place, fully weld all the joints. Try and leave a small gap between each piece for the weld to enter — opening the gap by making the front higher makes this job easier.
Clean all the welds on the outside of the frames with a sanding disc in your grinder as we will be panelling it out next and you want the steel plate to be able to sit flush with the frame.
We are using 1.2 mm steel to cover the sides of the framework. Measure this out on the steel of use the frame itself as a guide — remember you will need a piece for each side.
If you have a guillotine then use that, if not a decent pair of tin snips will be fine. Follow the mark as closely as possible and then clean the edges up with your sanding disc if necessary.
To add some style to the mounts, drill a series of 1-inch holes in the plate. Mark the holes where you want them and use a centre-punch before drilling or punching them out.
Use some clamps to hold the steel plate to the frame work, tack it in place and then add some 1-inch welds around the perimeter and a couple on the back to add strength to the piece.
Just you make sure everything is OK, we trial fitted the mounts to the seat. If your holes are slightly out, try running a slightly bigger drill through the hole or use a small round file to elongate them.
Here you can see how they will look once in the car, all we need to do now is use our hole flaring tool to finish off the holes and give them a coat of paint. Not bad for a couple of hours work!
Here you can see them all painted up and flared. Some M8 nuts and bolts will be required to bolt them through the floor and use some large washers to spread the load.
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