It was apparent since last year's Bo'ness that the tyre revision had helped tremendously. But it also meant that I had begun to rely more on the suspension - that was telling me the front was underdamped and that I was hitting the back bumpstops, inducing breakaway oversteer. So the first thing I needed to do was get the backend sprung.
I painted neat little datum marks on the body at an arbitrary height on all four corners from whihc to make wheel centres measurements. Then pulled the rear coilovers while wielding a simple tape measure. This showed a total travel of about 8cms in the rear, and just 2.5cms to bump. But it also helped me understand how even this was so-quickly consumed.
The progressive rear coils (which had been installed upside down) were standard 12" length 2.25" diameter. But the felt-tip markings indicating the ratings were practically illegible. A progressive spring is just that, it is one rate that translates at some point during compression to another. So this spring never made it past 'soft' before the car moved onto its bumpstops and the rate became (almost) infinite.
New springs were called for. But I had no understanding of the spec of the existing. Hence back to basics on a simple estimation of what might work. The coilovers have fixed spring pans, so the size was apparent, they would have to be the same as before. But I really just had to take a punt on initial spring weights understanding they were likely to be a sighting shot - and that they may not be only set I ordered.
The coilovers sit a quite a steep angle from the vertical of about 30 degrees. A little trigonometry shows that the spring will operate at an effective 87% (Cos 30) of its rating. The Special Tuning MGB spring (which would share roughly the same attachment pount on the axle) is rated at 124 lbs/inch, a standard one at about 100 lbs/inch. The Gilbern is clearly lighter, but I have race-weight springs at the front, and better damping with these AVO telescopics. So I was happy to throw caution to the wind and try a Faulkner spring of 185 lbs/inch although I did not even have any scale weights for the back - so this was real suck it and see.
Anyway, I put it back together again after reducing a new. standard set of MGB bumpstops by about with a hacksaw. And counted myself a lucky guy to find that the ride height seemed visually correct and a fraction of a cm increased. Overall, total rear travel had grown to almost 10cms, and the car exhibited about 4.5cms to bump.