GTD major chassis mods, opinions please.

Simon · 03-11-07, 01:52 PM

As I'm tearing down my car I'm finding more places where I can make improvements. It's my intention to re-jig the rear of the chassis to lower the engine/gearbox as well as reinforce the sills as Brett has done.
When it comes to room (and as anyone who knows me will tell you, I need all the room I can get!) I think the runners removed from the seats will get them as low as possible.
However, In both my previous GTD's I always thought that the chassis section about 6-7 inches up from the floor at the rear of the bulkhead could move slightly. Now I've got the car partially stripped with the seat (sans runners) in position, I can see that the bottom of the seat as well as the top, both have space behind them to move back, but the middle is held forwards by the chassis section (marked X)
As I will be lowering the engine/box, it will create just enough room to enable me to do this

Simon · 03-11-07, 02:01 PM

Now, I think that the chassis section can be cut and rewelded approx 1.5 inches rearward with no detremental effects on the chassis integrity allowing the seat to move rearwards by the same amount.(thats a LOT in a '40 as I'm sure you know) This would obviously involve lots of new strengthening metalwork and gussets as well. I would not attempt this myself but am going to consult a chassis specialist who will carefully assess the integrity of the chassis before any work is done (if he says it's not safe, I won't do it) I think that done properly, this could be a worthwhile mod. The bulkhead would be moved back to the yellow line in the following picture

Simon · 03-11-07, 02:03 PM

The section between the yellow lines in the following picture would need to be removed

Gregg · 03-11-07, 02:05 PM

Simon, don't forget to take into account how your water pump intrudes into the cabin now versus how much intrusion there will be when you move the firewall back 1.5 inches. Comment also applies to engine clearance to firewall as well as ancillaries. You will probably also have to re angle the top of the bulkhead to match up to the spider. Good luck.

Simon · 03-11-07, 02:06 PM

Lastly, I think the chassis section below the bulkhead window needs to be made a bit more substantial as it will take the loading of the shoulder harnesses

Simon · 03-11-07, 02:16 PM

Thanks Gregg,
I think as I'm lowering the engine, the water pump will move down as well away from the bulkhead slightly
Simon

Phil · 03-11-07, 11:57 PM

Simon;
Those mods you are looking at don't seem too difficult, and someone qualified could do them without compromising any strength. A few gussets and the right reinforcements correctly placed would be no problem. IMHO. Just be sure if you are doing it yourself you measure carefully and often as the work progresses so if problems arise you can correct a small error before it becomes a monster. I do this type of work all the time, and measure, measure, measure is my motto...and I always angst about the cuts, sometimes there are small mistakes, but a careful approach usually produces a quality result. I would bet someone here on the forum has made some mods of this type and may have photos, and be able to give some specific advice.
Cheers
Phil

Malcolm · 03-12-07, 05:45 AM

Hi Simon

If you move the rear bulkhead back are you moving the trailing arm pick up points back too? If so you will affect handling and my suspicion is that it will be worse.

To gain more leg room on a standard chassis, and it has been considered by quite a few people now, the considered wisdom was to move the front bulkhead forward by 1.5 inches. This was the standard way GTD extended the footwell to make what is called a long chassis option. It would also involve far less work.

No doubt the rear bulkhead can easily be moved and strongly so at that. But you are in unknown territory and in my view taking a risk in all manner of ways including value of the car. Instead of measure twice, cut once I would also suggest think twice, cut once!

RichardH · 03-12-07, 06:13 AM

For what it is worth, I have had to have the pedal box moved forwards, and through the bulkhead, to give me enough room (just). It does seem an easier option than cutting the chassis.

Simon · 03-12-07, 07:06 AM

Malcom,Richard,

thanks, no not the entire bulkhead, just the chassis member that runs behind the cockpit, the suspension pickup points will remain unchanged and unaffected. As far as I can tell, this should not affect the fundamantal strength of the chassis.
The bulkhead should look identical from the inside apart from the sloping section behind the seat moving back slightly.
The internal seat pan will be slightly longer and the centre section of the firewall (the access panel section) will be vertical in line with the window, for a couple more inches before it kicks out as it does now.
I'm only considering this because I'm stripping the car back to the basic chassis for some radical reworking of the rear.
Simon