I forgot that the installation ratio of the rear springs is 0.75, not 1.0. This is especially significant because that value is squared when calculating wheel rate, so the current 400 lb/in springs give roughly 200 lb/in at the wheel – same as the front suspension. Due to the rear of the car carrying twice the weight of the front however, when the car hits a dip, the rear suspension will compress twice as much. As the pictures show, the rear tire was moving as much as 4″, which corresponds to 3″ at the shock. While the upper-most marks may have been caused by the off, there were marks there before and nearly as high. Chalk it up to being distracted by the engine nonsense for why I didn’t pay more attention to this.
The question becomes: how much should rear spring rate be increased? I don’t know how much the rear suspension was trying to compress; it stopped only because it hit the bump stop, so how far would it have gone if there was no bump stop? Since the rear of the car has 4″ of ground clearance, that’s the lowest it’ll ever go, but it still doesn’t answer the question for the same reason. I’m going to guess that worst case it might have compressed 5″ (3.75″ at the shock), so that needs to be reduced to no greater than 3″ (when it hits the bump rubber). That works out to changing the existing springs from 400 lbs to roughly 650 lbs.
In a related matter, Midlana only exhibits understeer (push) at very low speed (slower than 30 mph). Because it never exhibited this on-track, it means the front spring rate needs to be increased as well, more than the rear. This gets into the consequences of picking springs that might be best for the track, but result in an intolerable ride on the street. That wouldn’t be any fun, yet neither is spinning off the road due to the rear suspension bottoming out!
I know some people will say just increase shock stiffness. That’s only a solution in transient conditions; the situation in which my off occurred was in a long right-hand turn, after the shocks came to equilibrium, so they don’t play a part in this. Other people may suggest stiffening the anti-roll bars. Well… I don’t have any – for now. I’ll start with increasing the spring rates and go from there. For what it’s worth, I had heavy duty front and rear bars on my Datsun 1200 that was used for both autocross and track day events. For autocross, the rear bar was absolutely necessary in order to get the car to rotate. Conversely, that same rear bar at high-speed track events caused the car to be a real (slow) handful, the rear of the car always wanting to come around. I ended up disconnecting the rear bar for track events, so if a bar ends up necessary for Midlana, it’ll be at the front only.