FC- front/rear spring rate ratio
 

I've seen a number of F/R spring rates posted for FCs, and it looks like the rears are generally 66% of the front. Sound right?

Thanks.

No.
They are 1:1, although the fronts are progressively wound.
An old Trust / GReddy listing in an old Option mag listed them as 2.0 kg-mm front and 2.0 kg-mm rear.


-Ted

My fault- I meant in racing apps with coil over setups.

Thanks
Ben

If it were only that simple... That ratio might work out but it would be pure luck. The thing to remember is a certain set of springs may be matched with a set of sway bars that "they" are trying to sell you too. But they could be all matched up for use on a street car,(full interior, big girlfriend etc.) And won't be the hot set-up on a race car. Race car might be lighter (hopefully) and have the corner weights that are 100's of pounds different than a street car. Race cars end up built differently too so unless you know the car your getting rates from is REAL close to your car you won't be getting YOUR best set-up info. Corner weights, movement ratios (wheel to spring), unsprung weight (suspension, wheels etc), what ride rate you want(what frequency the car will bounce at without shocks), and what speed you will spend most of your time at are all taken into account. Also the smoothness of the track. Not as hard as it sounds and the math is pretty simple. Then you start thinking about anti-roll bars. It's all in one of those "... to Win " books, I forget which one.

When you have all the info on your car, plug in the different rates that you find advertised or offered. They will be probably be all over the place on ride rates.

That's in the range, but a softer rear would be closer. I'd say around 61-62% is closer. Too many variables to list, but unless you are making BIG power(camber gain on squat), around 61% Imho. Alot depends on the quality of the damper too. You can go to higher rates, and bigger variances as you move up the damper "food chain". 450/275 is popular, 500/350, 650/450, 700/500 are rates that have worked for me, depending on about twenty different variables. Carl

I realize that there is a whole lot more to it than what I'm asking, but I was just looking for some starting rates.

The car should have around 425+hp, so I'm guessing it will be a squatter.
I'm going to start on the softer end of things (450/275) and work my way up as I get more comfortable with the car.

I am running the same spring rate front and rear, 400#. it is more shock,tire, and driver dependent. there is no perfect setup. it all depends on how YOU want your car to handle

If I recall correctly, the K2RD setup I have (uses KYB AGX adjustables) is 350F/225R and it is pretty stiff. Unfortunately K2RD seems to be gone, so I have no way to find out for sure short of unmounting the springs and stacking weights on them.

Are the front and rear motion ratios the same? I think they're pretty close.

They felt stiff because The KYBs are WEAK>>>

I've run twice those rates with advanced designs, and they are smooth... Moton, and JRZ are even better I've been told. The difference in dampers is so great, you can tell in the first 100 feet the difference. Carl

yah its 350/225, the # is marked on one end of the spring if you're still curious

what kinda stiffness do you guys recommend for a car setup for drift?

would the difference in "gap" of rates between front and rear be greater to keep the rear end "looser" ?

Actually an engineer will disagree with you. But once you throw in the fact our cars are still driven by humans and not computers you suddenly have to keep the driver's butt happy. Common theory in all the books I've read is that spring rates only need to be stiff enough to keep the chassis from bottoming. Any stiffer and you're not generating as much overall grip. Sure there's a certaining tuning aspect that can be controlled with spring rates but going stiff just for the sake of it doesn't neccesarily create speed.

The split on my ITS car falls into the 69% range. At 400/275 it's free on most tracks but I'm wanting to experiment with going stiffer all around while maintaining the same balance. We've managed to lower it enough where the rear toe links bottom to the tub under hard compression...which creates general havoc.

I am not a supreme expert, but have been drifting FC's/FD for a year and a half now, and from what I've gathered, something along 10KG/8KG F/R is a good starting point. If you are more skilled 12KG/12KG is more slide happy. Make sure you get a really good 2-way Lsd (Kaaz is my personal favorite) and Both FC, and FD need alot of angle up on the steering rack, which are threaded on spacers, goes between your inner tie rod and rack.

Don't forget though, too soft a spring means more body roll and your camber goes all to hell. This can be limited with antiroll bars of course, but they cause extra weight transfer to the outside wheels. It all comes down to testing and compromise.

Yeah, soft is good for ultimate grip as long as you can keep the camber change in check and the chassis off the ground. Increased compliance in the suspension will always offer more grip but the tradeoff is more suspension movement and thus less camber control. No hard and fast rules apply.

The point made of the human driver being involved is not lost, however. If the setup scares the driver it doesn't matter how fast it "should" be. In the end it's the opinion of the driver that matters because he'll go faster with what makes him feel confident.

On a side note that was an interesting point made in Wright's book " Ferrari Formula 1: Under the Skin of the Championship-Winning F1-2000" (awesome book!). We already know that making fighter airplanes inherently unstable makes them more manueverable but they must be flown by a computer since the human pilot cannot cope with the inherent instability. Cars don't have that allowance and as such Ferrari makes no attempt to put the "theoretically perfect" setup on any car as it would be nearly undrivable, even if you do have Michael Schumacher. The performance of the chassis and tires have much more potential in them but a human is incapable of extracting it because his reflexes and awareness are not up to par with a computer.

On these lines.

If you make the suspension too soft the car will be too slow to respond to inputs. As you shift the weight to the outside in a turn there is a certain time element. If it takes too long to take a set you can easily get ahead of the car and get to a point where the car is oscillating at the natural frequency of the suspension. This is not only impossible to drive with it is also somewhat dangerous. Of course shocks come into play in these dynamic situations. However, a shock will not change the natural frequency of the suspension it will only mitigate the effects if you happen to find the natural frequency.

Essentially, a decent suspension will have a baseline at a natural frequency that is higher/faster than a human can give the car inputs....around 2.0 Hz or so is a good starting point.

Interesting stuff about Ferrari F1. How can we be sure that Schuey's not some sort of robot? They could program to lose sometimes.

Carroll Smith speaks alot about setting up cars for specific drivers, so as not to overwhelm a less skilled driver.Carl

<nitpick>Sorry to nitpick, but that's not true. Damping does indeed affect the natural frequency of a system. </nitpick>

Trying to do all the chassis control with damping is impractical, of course, and your error does not really affect the validity of your main point about the system's time constant and drivability.

Well, I suppose I should have been more specific.

The undamped natural frequency of the system is not affected by the dampers. (obviously)

In this application the only reasonable "easy" way to measure/calculate natural frequency is to neglect the affect that the dampers have on the system.

Nitpicks are welcome.