I have a bit of concern about the seat belt postings.
Doing a quick google search I found
Car Crash Example that suggests a 30 mph crash puts about 1.5 tons of stress on the seat belt anchor points. At 60 mph this jumps to 9 tons, and it presumes a stopping distance of 1 foot. If the car hits a concrete wall with 0.01' stopping distance, the calculator jumps to 960 tons. Obviously the crumple effect of a Bristol will mitigate these numbers in real life. But if the belt gives way while the car is coming to rest, the g-forces on the body hitting the windscreen and dashboard will be horrible.
The problem with seatbelts is they only are tested once, and if they fail the results can be life altering. I am not sure if we have statistics on seatbelt-testing crashes in Bristols. I know that the BMW C1 web site had a page for crash reports, which we filed when the chairman of my charitable trust hit a car at 40 kph and walked away uninjured because the C1 has two shoulder belts and a lap belt. I wonder if we should not have a similar web page for Bristols. Kevin?
The problem with photographs of location in the Bristols is they do not tell us the most crucial part, what is holding the belts to the car and how strong is the mounting. This is a problem for both front and rear, as old Bristols lack a B pillar. If I ever get my 411 back on the road, it needs to pass VIN on seatbelt mountings, as approved by an engineer. The challenge is to determine how to do that best.
It would be helpful if Bristol Cars provided a certified instruction set that could be followed and signed off by the New Zealand inspector. Lacking that, it would be helpful if an engineering-oriented Bristol owner could provide an engineered solution that would not carry legal liability, but provide the crucial real-life information.
From my racing days, of course, a simple roll bar does the trick, a heavy piece of bent round steel with four mountings and a total of 16 large bolts braced in the floor. It may be that for the rear of a Bristol a similar U shaped piece of steel, perhaps in box form to lower its profile is way to address the rear.
Incidentally, in my Bristol 409 (under the old NZ rules), the engineer simply charged me $500 to advertise in an official NZ registry that the car had no place to mount shoulder belts in the front. Ironically, the car came with original Bristol aeroplane seat belts (the driver's one had a parachute icon on it). But those belts were deemed to not meet the NZ standard, so rather than allow them to remain and give some protection, the regulation required they be removed, giving no protection. Even more annoying the engineer decided they were surplus, and took them as a souvenir. Recovered one, the other one was "lost".
Claude