Why the spring rates don't correspond to weight distributions?
I was looking at the RACE_S setup for the FZ5, which has some 60:40 weight distribution, but the spring rates are like 125:105! If you "drop" the car it is quite obvious that the nose bounces up way quicker than the tail.
So what is the reason for this very strong front springs compared to the weight it bears? If it's to reduce some power oversteer, then why not just use the anti-roll bar?
Because anti-roll only affects the car balance during the corner, whereas a stiff front suspension will also have some influence on corner entry (car doesn't dive as much) and a little on corner exit.
It's partly because it's wrong (the Race_S setups aren't very well researched, and aren't meant to represent a real setup necessarily), and partly because weight distribution isn't thw whole story - wheel frequency, unsprung masses, the wheel rate (and rising rate if applicable) etc.
Probably I asked this because the RACE_S setup is too strange. Still, is spring ratio = weight distribution a good starting point?
AndroidXP:
I think the amount of "dive", or pitch change, is just related to the overall spring stiffness instead of front vs rear ratio. For example, if you take X N/m of spring off the front and increase the rear by X N/m, the pitch change will be exactly the same in straight-line braking. The only difference is that the car (CoG) will sink a bit due to the front compressing more and the rear extending less.
Not really, no. You can do the maths on spring rates quite easily (as a starting point), starting with either wheel rate (N/m or lb/in etc) or wheel frequency (Hz or CPM), but you still need to know what you want in those cases.
Neutral has potentially the best handling but can be tough to drive as it requires smooth inputs. Oy course it's roll stiffness that is important rather than frequencies for handling, but since track widths usually vary only a little (between front and rear), comparing frequencies will give you the same effect. Adding understeer by stiffening the front springs is usual for powerful RWD race cars, as doing it just a little makes the car more stable and steerable with the throttle. Having rear frequencies slightly higher (and now I do mean frequencies rather than roll stiffness) helps keep the car flat over bumps as the rear can catch up with the front, given that it is hitting the bump slightly later. I think this is often why rear track widths tend to be a bit narrower (or at least one reason), as you can use slightly higher rear frequencies without getting oversteer, then adjust roll and balance with the ARBs. I haven't checked, but higher rear frequencies might also make it easier to obtain damper settings that give a good balance between transients and bumps.
In LFS, the "stiffness" for springs and the damper values, I think, are converted to equivalent wheel rates. Because in the FZ5, the front spring is close to perpendicular, but the rear spring is close to 45 degrees, BUT I've set all the stiffness and damping rates' ratios equal to the weight distribution and the car obviously has the same frequency front vs rear. If the "stiffness" meant the spring rate, but not the wheel rate, the rear will be under-springed/over-damped compared to the front due to the large inclination (?), but it's not the case.
So yes, I should have said "stiffness" instead of spring rate, because in LFS we seem to get the equivalent wheel rate directly...
To Bob:
It seems that the front has to hit bumps first could mean that softer front is better. So, did your "neutral" mean that "wheel" rates' ratio equals weight distribution? (Which should mean same frequency for both ends.)
As for adding understeer by stiffening the front... I don't think it's the best method, or can achieve what the ARB can't, because:
a) In a straight line, braking with the same effort, the steady state load increase in the front is only affected by the car's pitch change (on top of the so-called "weight transfer"). In this case, stiffening both front and rear have the same effect of reducing pitch change, which means to add understeer through springs, you should stiffen both ends, not only front.
b) When cornering, stiffer front springs is exactly the same as stiffer front ARB.
Yes, pitching (be it squat or dive) is affected by both the front and rear springs. But to get understeer you have to be turning, not braking, by definition. I'm unsure how this would affect trail braking.
Just found the way to calculate how much psing rate you should use. Let's say you are comfortable with a spring rate of 85 at the back of the XRT. We know that XRT has mass repartition of 52/48 (F/R).
First of all, find how much of the wait is on the rear:
48% * 1223 (Weight of XRT) = 587 kg @ Rear
52% * 1223 = 636 kg @ front
All you have to do is 85 x 636 / 587 = 93 (The spring rate you shoud use to front)
It works for me, as I was using 95 and felt it was confy. Maybe someone can develop on a more efficient method?
Ediit: I made a little diagram, same way you do to convert a result of an exam...
Yes Bob I thank you for giving us the tools, so that we can feel like we are LFS race engineers but one thing you can't give us is the wisdom to use those tools
meh, that's why suspension tunning is often seen as a black art. Which doesn't mean it is hard, it's just that there is a hell of a lot of information out there to assimilate. And often a little information is just as dangerous as no information but hey it's fun, like a game within a game
For me, I find two things helped. First is an understanding of the spring-mass-damper system, so at least things like frequencies and damping constants mean something. The other is some idea in linear algebra about choosing a basis to get decoupled variables. For example, ARB's really shouldn't be viewed in the variables (front, rear) for the stiffness, it should be viewed in something like (front+rear, front-rear), where the first "variable" tells you how much overall body roll you get, and the second tells you about over/understeer balancing.
personally, i started LFS using no tools and guessing at suspension setups.
i got pretty good, but every once in a while i would hit a part of the track and the car would go crazy.
it happened most in the MRT and RAC and i hated driving them.
then i was introduced to bobs thing... project 3 or whatever...
with some common sense and time, the MRT and RAC have become my favorite cars, simply by tuning the dampers.
i really dont understand most of what i am doing here, but a general rule i have established is to tune the front to about 70% of max stiffness, and to tune the rear's FREQUENCY to be slightly higher than the front.
then i make the Front ARB something like 70-90% of spring stiffness
the rears, i make low enough that i have comfortable understeer. this is usually done on KY NAT because it has a good mix of turns.
for tuning the dampers, i initially set the bump damping to be between 80 and 87 percent of critical, and tune the rebound to get the car stable. usually, this means that all the bars for everything are barely visible and green...
as for tires, well, i start off at the stock pressure and usually lower both front and rear pressure until i feel they are heating up too fast.
In "tricky" RWD cars the rear springs are made longer and softer (retaining ride height) simply to make the car less sensitive to road bumps.
Over a bump cars with shorter springs tend to loose grip momentarily (they lift off). This is tolerable at the front but sets you spinning if it happens at the rear axle at maximum lateral acceleration.
but one thing you can't give us is the wisdom to use those tools 
but hey it's fun, like a game within a game 