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Looking to delete my torsion bars (I am sick of indexing them ) and go with rear coil overs. Been doing a lot of reading and have some questions.
My setup is:
Koni Sport struts with 275 front springs
26.8mm front sway with KLA added braces
KLA strut brace
Koni Series 30 rear coil overs (need to determine springs)
19 mm adjustable rear sway
Powerflex Black Race Bushings everywhere
Nitto NT-05 Tires 255 front/275 rear
I have seen the effective rate of .42 a lot but Paragon shows an effective rate of .56 for rear coil overs. Which is accurate and why?
What would be a good back to front ratio for a street/DE car? Do I want balanced? I want a slight steady state under-steer I think.
+1 Dimi 944 is right. To do this correctly you need the car corner balanced. It's not just the spring rate you have to think about, the spring height is also important for balance. Once you have the car fitted with some base (about right) springs from your calculations, get the car on some scales. You will be surprised how off the car will be on the scales. Here is an example of a well-balanced car that only a corner balance will reveal.
What is your corner weight?
How much street versus track time do you have with the car?
Do not have corner weights yet. About 50/50 street and track.
Originally Posted by MAGK944
+1 Dimi 944 is right. To do this correctly you need the car corner balanced. It's not just the spring rate you have to think about, the spring height is also important for balance. Once you have the car fitted with some base (about right) springs from your calculations, get the car on some scales. You will be surprised how off the car will be on the scales. Here is an example of a well-balanced car that only a corner balance will reveal.
Corner balance will happen soon, first I need to get my base springs
Shawn, there's that email I have from Karl who goes through the springrate motion ratio and effective rates. I have it at home but I am pretty sure it's around the 42 - 45% at rear.
Wheel rate = motion ratio ^2 * cos(spring angle) ^2
You can determine motion ratio for yourself by disconnecting the sway bars and measuring the gap between tire and fender with car on ground and the compressed shock length. Then repeat with that corner lifted and find the deltas. Many have calculated .9 for the front suspension and .65 for the rear shock/coil so when you square them it's roughly .82 and .4225 but many people seem to leave out the strut and rear coil angles. They're about 15 degrees so you square the cosine of this too - ~.93
There is no standard front to rear spring rate or tire force ratio. There are too many dependencies like the tire/wheel stagger, geometric stiffness and alignment change with wheel travel. The McPherson front seems to benefit more from stiffness by not losing camber compared to the rear so the tendency is for the front to rear ratio to increase proportionally with roll stiffness.
Also, when you lower a 944 the front RC drops more than the rear so you lose geometric stiffness. This is another reason that people go with stiffer front springs on a track car. Your ratio looks like .89 to me and it's a good starting point worth trying but 550 rear will be stiff on the street.
"The info on Paragons site is stuff they got from me a few years ago. In any
case, the 47% is a number that I have calculated By taking measurements. I
am now pulling out one of Porsche's own Motorsport sheets that shows all the
rates of their Turbo and "Cup" cars back when they ran competitively.
Porsche lists the 25.5 mm bar as 31 N/mm which is 177 lbs/in. They also then
give the variable rate coilover helper spring rates at 34-65 N/mm which is
194 lb/in - 371 lb/in. They then give the total Rate at the wheel(T-bar plus
coilover) as 45.4 - 58.5. Back out the rate at the wheel due to torsion bar
which they list as 31 and you have 14.4 - 27.5 at the wheel due to the coil
over. So take your pick, 14.4 / 34 is approx 42% or 27.5/65 is 42%.
Bob is right about them being inboard but his numbers are off. Actually they
are correct I think in that the motion ratio is about 65%. But when
calculating wheel rates from spring rates it is the motion ratio squared
that is uses. So 0.65 ^2 is , guess what,... 42.25% which is the number that
Porsche's own sheet claims as I outlined above. So your torsion bar is 177 at
the wheel, and your helper spring rate is a 285 which is 119.7 lb/in at the
wheel. So working backwards 177 plus 119.7 is 296.7 pounds per inch at the
wheel. Divide this by .42 and that is you equivalent coilover, or 706 lb/in
coilover (initially I had 661 lb/in which is attributable to my measurement
error - I had 47% and Porsche lists it at 42%)."
Wheel rate = motion ratio ^2 * cos(spring angle) ^2
You can determine motion ratio for yourself by disconnecting the sway bars and measuring the gap between tire and fender with car on ground and the compressed shock length. Then repeat with that corner lifted and find the deltas. Many have calculated .9 for the front suspension and .65 for the rear shock/coil so when you square them it's roughly .82 and .4225 but many people seem to leave out the strut and rear coil angles. They're about 15 degrees so you square the cosine of this too - ~.93
There is no standard front to rear spring rate or tire force ratio. There are too many dependencies like the tire/wheel stagger, geometric stiffness and alignment change with wheel travel. The McPherson front seems to benefit more from stiffness by not losing camber compared to the rear so the tendency is for the front to rear ratio to increase proportionally with roll stiffness.
Exactly what I was looking for! Thanks!
Originally Posted by 333pg333
From Karl
"The info on Paragons site is stuff they got from me a few years ago. In any
case, the 47% is a number that I have calculated By taking measurements. I
am now pulling out one of Porsche's own Motorsport sheets that shows all the
rates of their Turbo and "Cup" cars back when they ran competitively.
Porsche lists the 25.5 mm bar as 31 N/mm which is 177 lbs/in. They also then
give the variable rate coilover helper spring rates at 34-65 N/mm which is
194 lb/in - 371 lb/in. They then give the total Rate at the wheel(T-bar plus
coilover) as 45.4 - 58.5. Back out the rate at the wheel due to torsion bar
which they list as 31 and you have 14.4 - 27.5 at the wheel due to the coil
over. So take your pick, 14.4 / 34 is approx 42% or 27.5/65 is 42%.
Bob is right about them being inboard but his numbers are off. Actually they
are correct I think in that the motion ratio is about 65%. But when
calculating wheel rates from spring rates it is the motion ratio squared
that is uses. So 0.65 ^2 is , guess what,... 42.25% which is the number that
Porsche's own sheet claims as I outlined above. So your torsion bar is 177 at
the wheel, and your helper spring rate is a 285 which is 119.7 lb/in at the
wheel. So working backwards 177 plus 119.7 is 296.7 pounds per inch at the
wheel. Divide this by .42 and that is you equivalent coilover, or 706 lb/in
coilover (initially I had 661 lb/in which is attributable to my measurement
error - I had 47% and Porsche lists it at 42%)."
Great info Patrick! Thank you!
Originally Posted by mikey_audiogeek
Agree with all the above. 550lb/in is a good starting point, IMHO.
My setup:
Porsche 944 S2
235/40/18 Bridgestone RE002 front
265/35/18 ditto rear
H&R RSS coilovers all round
M030 bars
Racer's Edge strut mounts
Polybronze rear + Racer's Edge carrier mounts
400lb/in front springs
850lb/in rear springs
-1.8 front camber/-2.2 rear
Still understeers at the limit.
Cheers,
Mike
Thanks Mike! Yes, I will order my base rear springs today.
Many thanks again to the incredible knowledge that can be found on this board!
If you wanted to do something kind of trick at the rear coil, you could emulate what Porsche does with the latest 997.2 RS and 991 GT3 by doing a stacked rear spring setup. This is what I did with my 968 on Moton CS and it takes some of the harshness out of the ride but it's still a little stiff for street use, although that's subjective as others have pointed out.
The nice thing about a helper is that the stacked rate becomes k1*k2/k1+k2, so if you were to add a 250# helper spring in series with a small amount of travel before coil bind, you'd get 250*550/250+550 or a 171# spring rate until the helper binds for a bit of added comfort and rear tire traction. This is about what your stock torsion bar rate is, actually.
In order to make this work, you need a shorter main spring to fit the helper. Both Eibach and Swift have charts of extended length, load, and block length for you to analyze. The load should be at least 2.5x of your unsprung weight at that corner [ie. corner weight minus 110 lbs unsprung or so] to prevent bottoming out the suspension at 1G+ cornering. A 5" 550# main for example with 2" block length would tolerate about 1650# so dividing that by 650 lb corner weight looks like ~2.53x so that would work OK. Depending on your shocks, a 3 or 4" helper could fit with a spring spacer.
My setup is for a car that is driven to the track, but doesn't see much other street driving:
Front:
400in/lbs springs
Racers Edge Camber Plates / High Strung 44 Control Arms with monoball bushings
Ground Control coil over / Koni Sport
30mm front sway bar
Rear:
650in/lbs springs
Elephant Racing Poly Bronze Bearings / Solid torsion carrier mounts and monoball bushings
Ground Control coil over / 30 series Koni
19mm adjustable rear sway bar
Corner balance:
818.....810
782.....747
Cross weight 50.4%
Total weight 3157 including half tank of gas and 215 for driver (2942 without driver).
FWIW, I have almost the exact same set up as chrenan.
I have 440# lbs. springs in front, 600 lbs. in the rear.
It is firm, but not near as harsh as some would expect. We have not yet had it on a track, so I can't comment on tight vs. loose. Drives great on the street. Wish SC had better roads.
corner balance is:
826 816
709 735
150 lb. driver weight
3,086 lbs.
When I picked up the car, was a little surprised car is nose heavy, after hearing much about 50/50 balance on 944's. Maybe turbo's are a little heavier in the front?
07-20-2015, 06:43 PM
Spring Rates
) and go with rear coil overs. Been doing a lot of reading and have some questions.
