I have been working with Performance Shock Inc. to fit my FIA with Penskes. We worked from drawings as much as possible, but were not able to identify a Penske shock that would fit without using remote reservoirs. They arranged to have Penske ship me a shock so we could fit it prior to purchase. One of the issues is length of hose needed. The aluminum cradle has a boxed cavity on either side at the top. We are going to mount the reservoirs horizontally in the cavities. This will assure the entire cradle and suspension can be dropped and installed cleanly into the chassis.
On to springs -
I have been told ERA comp cars have run 550 and as high as 600 lb springs in the back and it was suggested to run 350 in the front.
I keep looking at Allens car concerned with the amount of roll I am seeing. I have been focused on roll bar rates but while reading Smiths stuff last night it occurred to me maybe we don't have enough spring. I found a reference for ride frequency vs spring rate for un-sprung mass.
http://www.optimumg.com/docs/Springs...Tech_Tip_1.pdf
Non down force race cars run a ride frequency as high as 2.0 hz. I am shooting for a 2315lb race weight car. If it were properly balanced and I use a 49/51 split I would have
Front => 567 Total - 100 (un-sprung) = 467 lb sprung
Rear => 579 - 100 = 479 sprung
From my reference a 2 hz car
Sprung Weight => Needed Wheel Rate
500 => 205 Wheel Rate
600 => 246 Wheel Rate
700 => 288 Wheel Rate
I am shooting for a 10% difference between front and rear with a faster rear. I calculated motion ratios for the front and rear of my FIA. My front is optimized some so this won't transfer straight across, but it is of interest none the less.
Cliffs ERA Front - 467 Sprung Mass needs a 205 Wheel Rate
Spring => Wheel Rate
350 => 160
400 => 183
450 => 205 * Notice how much front spring we need to hit the 2.0 hz.
475 => 217
500 => 228
525=> 240
550 => 251
Gen II ERA Rear - 479 Sprung Mass needs a 205 Wheel Rate. Since I want to be 10% faster 205 * 1.1 = 226. Smith likes to go up so
Spring => Wheel Rate
550 = > 222
575 => 232 * Right in line with ERA suggestions -
600 => 242
625 => 252
650 => 262
So a 450 front / 575 rear would satisfy the 2 hz ride frequency and give me the 10% differential. I've never driven anything this stiff and I hope it does not exceed chassis stiffness. (Shelby did and it worked anyway -
!)
That seems like a lot of front spring, but looking at Allens car and knowing the rear roll bar was maxed and he was picking the inside tire makes me wonder if there was enough roll stiffness in the springs. A stock ERA front would require heavier springs because the motion ratio is less than mine.
There are two holes in the shock mounting tabs. The 3/4" outside placement is worth a lot.
450f/575r
Am I missing something on the front?
x-chr