OK, so with all due deference to those who know better, and based on what I remember from my time at Girling UK over twenty years ago, here is my contribution to the ride height/bump *** debate.
Forget ride height for now, it's a red herring.
These are the basics: Suspension travel is fixed by the length of the hydraulic damper. Imagine a car with no springs. At the extreme limits of travel, the wheel stops moving when the damper hits either abutment. Because bump forces (generated by the the wheel deflecting over the road surface and the inertia of the unsprung mass of the car) are much greater than rebound forces, a bump *** is intoduced to cushion the blow. Actual wheel travel, measured at the wheel, will depend on the geometry of the susension arms/wishbones and the mounted position of the damper.
Now we add the spring, usually a steel coil spring with a linear rate, ie one that deflects by a given length for a given force. In GPL, 90 lbs per inch means that the spring compresses another inch every time you apply another 90 lbs force to it.
This is the graph of spring/damper deflection against force.
Deflection (inches)
__|______________________________________
| * Bump | *
***_|____________________ *
Preload |
Notice the effect of the bump *** is to augment the steel spring
and effectively provide a rising rate near the limit of travel. The
bump *** does reduce the available damper travel but not by much.
Any length bump *** will be flattened to a near pancake at full
bump. Longer ones will be softer because they deflect more and also
affect the spring rate earlier in it's travel, giving a less sudden
rising rate.
Preload exists when the spring is compressed before it is fitted to
the damper. Even off the car, the damper is forced against it's
(rebound) abutment by the spring. This means that a certain force
has to be applied to the spring/damper unit before it begins to
move. Without preload, too much useful suspension travel can be
used up by the weight of the car at rest.
We're still not talking about ride height though!
Let's imagine we have selected a damper length, a spring of a given
rate, decided on our preload and fitted a bump *** of suitable
length. We fit it to the car in a geometrical position that gives a
certain wheel travel. Now we park the car on it's wheels fuel her
up and put the driver in. We look underneath and, oh dear, the
weight of the car has overcome the preload and compressed the
springs to the point where there is only an inch of clearance under
the belly pan! First time we hit a bump, the whole thing will
scrape on the ground.
Insufficient ride height!
So what do we do? Well we're happy with the damper travel, spring
and bump *** effects so we just want to raise the car up. If we
had time we could reposition the mounting points of the suspension
arms lower down on the chassis but that would be too complicated.
So we move the top mounting point of the spring/damper unit instead.
The spring doesn't deflect any more than it did and the damper
doesn't budge but the suspension arms will be at a different angle
and the chassis is further off the ground. Now when we hit that
bump, the spring/damper unit can move through it's travel without
the car hitting the deck and, maybe, the bump *** will come into
play. Of course there are several other reasons why we want the car
as low as possible so it all becomes a matter of compromise as are
all engineering matters.
Now, I have no idea which elements of all this the boffins at
Papyrus have managed to model, but in order to make sense of the
simulation I think we need to know. Papyrus? Are you listening?
Until we know, we can't make sensible suspension adjustments on the
car setup screens. For my part, I am assuming for now that the
model has a fixed length travel, the bump *** length just affects
the spring rate curve and ride height is as described above. By
this reasoning, a ride height of 1 inch and a bump *** length of
2 inches makes sense, although the bump ***s will never come into
play.
Sorry this has turned out so long, but let me wind up with a couple
of very relevant questions for the developers:
1. Are long bump ***s just *** or are they a combination of
packing and ***, ie do they reduce travel?
2. Do you define ride height as the distance between a reference
point on the chassis and the ground?
