ZERO-DROOP SETUPS

The next question I have is about suspension droop travel limit fwd and aft. I have seen that modern single seaters have no droop at all. Now I am not sure about a historic racecar like my March F2 (year 1971).

The shock stroke (fwd and aft) during hard driving is about 20mm. I know aft shocks should have more travel than fwd shocks but how much?

Corner exit on acceleration the car is pitching fwd up and with no droop can help to get more weight on the car fwd because the unsprung weight will help to hold the nose down. Until now I made the shock adjustment with more rebound but I think that's the wrong way to fix the problem.

The only reason it makes any sense to not let the suspension move freely in droop is to control ground-clearance-sensitive aerodynamic elements. From every other perspective, making the suspension top out prematurely is a bad thing.

For best mechanical grip, we want the suspension to extend freely until the springs reach zero load, and then stop. If the suspension extends further, so that the springs hang loose, that doesn't hurt grip but it can cause the springs to beat up the shocks, or the spring retainers or adjusting collars, or other pieces, if it happens very often.

If the suspension tops out before the springs unload, that abruptly unloads the tires, but it also keeps the ground clearance from growing, at least as long as we don't pull the wheels off the ground. That's bad for mechanical grip, but it's good for aerodynamics if we've got a floor, a valance, a splitter, or a wing that has to be near the ground to work well.

From what I can tell by pictures on-line, the March 712 has a fairly broad chisel-shaped nose, with two small nose wings on the sides of the chisel. The radiator is in the nose, fed by an intake below the leading edge of the chisel, and exhausting through an outlet on the top of the chisel. The little

wings are up fairly high compared to later cars. It doesn't look like the car would be highly sensitive to ground clearance at the front. The car pre-dates tunnels and diffusers; the floor isn't designed to make downforce. It has a rear wing, but this would not be highly sensitive to ride height either. So I don't think the car would realize the same advantages as a more modern car from zero-droop suspension.

There is no hard and fast rule for the relationship between front and rear shock travel, as shown by travel indicators on the shock shafts. In tail-heavy, rear-engined cars, it is common for the front suspension to have a higher natural frequency and smaller static deflection than the rear. That is, the front suspension is stiffer in ride than the rear, relative to its sprung mass. This normally results in more suspension travel at the rear than at the front. Also, it is normal to have more aerodynamic downforce at the rear than at the front. This will result in the shock travel indicators showing more travel at the rear than at the front, if the motion ratios are similar. Looking at photos, it appears that the motion ratios at the front and rear of the March 712 are fairly similar. So if the travels are similar, it may be that the rear springs are a bit stiff relative to the front – but not necessarily.

Keeping the front end from lifting under power will only add a little bit of load to the front tires – and concomitantly reduce rear tire loading a little. The amount of rearward load transfer, for a given forward acceleration, depends entirely on the height of the center of gravity and the wheelbase. How much the nose lifts wouldn't matter at all, except that more nose lift does result in a slightly higher c.g. If the front of the car seems to lift excessively under power, stiffer front springs will reduce that. You may want to combine the stiffer springs with more rear anti-roll bar or less front anti-roll bar.

Some readers may find it surprising that the front end lifts less with stiffer springs, but that is indeed the case. A stiffer spring doesn't mean more upward force. It means less travel for a given load change – hence less extension travel for a given load decrease.

Ordinarily, we don't want to keep load on the front tires under power; we want load to transfer to the rear so we can put power to the ground. This is true unless the car understeers excessively on corner exit. This is not uncommon in rear-engined cars, but a more common complaint in racing generally is that the car spins the wheels and/or oversteers on exit. A car that is too tight (understeers too much) power-on is a problem that racers in many classes would kill to have. If the back tires stick too well, just feed them more power, goes the reasoning.

This doesn't always work, however. There may be no more power available; the driver may have the throttle wide open already. This is often encountered in Formula Fords. Or the understeer may simply increase until power breaks the rear tires loose completely, whereupon the rear end suddenly snaps out.

The March appears to have very short control arms, especially the uppers. That means that either the camber recovery goes away markedly as the suspension extends, or the roll center rises as the


suspension extends, or both. Having that happen at the front would worsen a power push, and stiffer front springs would reduce the lift. Restricting droop travel would keep the nose down too, but the
effect would be abrupt, and the roll resistance would abruptly increase, worsening the push. More spring and less bar keeps the front end down more, without that problem.

how do you think raising the front spring rate would change the squat characteristics?

Let's throw the other extreme, have no spring in the front in place of a solid front suspension, what would happen there?