I’ll post the details tonight. Sorry, I ran out of time over the weekend.
Here�s how�.
The overall aim is to enhance the performance of the standard ProBax seat by introducing a lumbar support that is both robust and fully integrated into the structure of the seat. I am not advocating external hacks or add-ons.
Note the following advice is supplied without warranties of any kind. If you follow this advice you should end up with a very satisfactory solution that is tailored made to your specific requirements as a driver. However, if you foul up, you�re on your own!
Step 1. Acquire materials. You need two densities of foam; the first is high density to provide the structure for the lumbar support. I found a bargain source at Halfords in the form of a ground sheet that retails for around �6 for a great big (rolled up) sheet. The sheet is about 8mm thick and can be easily cut with ordinary scissors. The second is low density about 20 to 30mm thick. I used some old packing foam from a tool box (it works though). Buy some self adhesive Velcro strip to secure the finished product (sounds crude but its very, very effective).
Step 2. Remove the driver�s seat. Undo the seat-belt anchor and the four Allen bolts at the ends of the runners. Make sure you either remember the positions of the bolts in the floor pan (there are adjacent pairs of tapped holes, and if you use the wrong holes the seat runner will NOT lock properly once you re-install the seat). There are some spacers that you�ll need to take care of.
Step 3. VERY slowly ease the seat backrest cushion away from the seat shell. Use your fingers and slowly work you way in under the foam.
Take GREAT care not to rip the foam or leave chunks of it stuck to the seat shell. The aim is to keep ALL of the original ProBax foam attached to the leather or alcantara facing material. The very top of the cushion is held to the shell by two push-fit trim plugs. Ease these out very gently.
Step 4. Make up a pattern (using paper or card) of the area previously occupied by the backrest cushion. You�ll notice that the very top part of the backrest is filled with a board to support the push-fit fasteners. Exclude this area from your pattern.
Step 5. Now for the fun bit! Cut your first piece of high-density foam to the shape of the pattern. Cut another piece about two thirds of the pattern height. Cut a third piece about one third of the pattern height. Temporarily stick these pieces together (with PVC tape or similar) to form a very rough wedge shape. Remember that the natural shape of a human spine is a very gentle S shape, and the lumbar support helps keep the lower vertebrae in proper alignment.
Place the foam pieces on the seat shell (with the smaller pieces facing the shell to minimise the contour step changes) and fold the backrest back into position over them. The clever bit is that we�re still utilising the original ProBax foam that�s attached to the leather (or alcantara) facing material.
Now try it out. If it doesn�t feel right immediately you now have complete freedom to make as many adjustments as you like. Add layers, remove layers, or change their shape � its up to you.
Use the low-density foam sheet to give a further level of isolation from the step changes in the high-density foam.
Step 6. Once you�re happy with the seat out of the car, get the thing back in the car and give it a thorough road test. You�ll probably need to make further adjustments after that. It took me about 4 weeks of trial and error changes before I achieved perfection (specifically for me, that is). It�s amazing how different the seat feels in, and out, of the car, and when driving.
Step 7. When you�ve arrived at the optimum shape, disassemble it all and trim down the width of the layers so that they can�t be seen at the (now raised) edges of the backrest cushion. Attach strips of Velco to the layers and the seat shell and fasten them all down.
Finally, pop the backrest cushion into place and give the push-fit fasteners at the top a really good push home.
That�s it � enjoy.
I sincerely hope that this provides at least some of you a better Lotus experience.