944 Front Springs
06-29-2005, 11:43 AM
#31
Bill,
A note about the springs: I have run from 250# up to 600# in the front, with various configurations of sway bars and rear suspension. Since I am now in ITS as well as PCA I have deleted the rear springs for t-bars. I did some re-reading of an old book on suspension design recently, and here is what I learned:
The springs should be designed to give you a "suspension frequency" of about 120-150 cpm front and rear. The rear should not be the same frequency as the front, and ideally (for rear drive) should be somewhat lower (5-10%). If they exactly match, the car will porpoise in a straight line. The reason for this "frequency" weight transfer. The higher the suspension frequency, the faster the weight will transfer in a corner. Also, the stiffer the spring, the less the amount of weight that will transfer. The trade of is stiffness. As the car gets stiffer, it will skips over the bumps more and after a certain point the amount of grip will be reduced as you lose contact to the track surface. The other consideration though is dive and squat, under braking and acceleration.
The body roll can be controlled fairly well with a combination of 400# (or 350# springs and a good sway bar, but dive cannot. I have found that you have to get up to 500# or more to control dive. It's important, because the 944 front brakes are so effective, and this causes a lot of weight transfer to the front. if the car squats too much it forces more braking power to the front due to the weight transfer, and makes the rear lighter, worsening the situation. Squat is not a problem for our cars, so let;s forget about it.
Doing some calculations based on weight, unsprung weight, and suspension geometry for a stock 944, you end up with a range of about 450 - 600# at the front. the rear is harder to calculate because the sway bar calculations are somewhat suspect. I use a different formula to the one on the paragon website and get slightly different numbers.
In the end, I have settled for 500# front springs and 28mm rear torsion bars. The suspension is unmatched, but the dive is better (less), and I can comensate for handling with a rear sway bar. The torsion bars are scheduled to be changed out as soon as Jason can get me some new 32mm bars (Vaughan is in the same boat here...). I suspect i will increase front spring size up to 550 or 600#. According to my calculations, I should have a suspension frequency in the right range up front, slightly lower in the rear, and much less weight transfer uunder braking than with the 400# springs I ran for so long.
About sway bars: They work, but they are limited. Rememebr that they pick up the inside wheel. Thsi means that if your sway bar is too heavy the inside wheel loses a lot of grip. For my money i will keep them on the car for tuning, but run them light.
the other important factor is shocks. Buy the best you can afford, and have them tuned for your springs. This makes a big difference that is hard to feel unless you are a great driver (I can't tell the difference by the way). They lap charts showed a huge difference for me (over 1.5 sec) at Road Atlanta after changin shocks and doing very little "tuning." Anybody need a set of old Konis?
As for Jim Pasha, he is an excellent resource and a great guy and I have the utmost respect for him. I think, however, that his information is old.
timo
A note about the springs: I have run from 250# up to 600# in the front, with various configurations of sway bars and rear suspension. Since I am now in ITS as well as PCA I have deleted the rear springs for t-bars. I did some re-reading of an old book on suspension design recently, and here is what I learned:
The springs should be designed to give you a "suspension frequency" of about 120-150 cpm front and rear. The rear should not be the same frequency as the front, and ideally (for rear drive) should be somewhat lower (5-10%). If they exactly match, the car will porpoise in a straight line. The reason for this "frequency" weight transfer. The higher the suspension frequency, the faster the weight will transfer in a corner. Also, the stiffer the spring, the less the amount of weight that will transfer. The trade of is stiffness. As the car gets stiffer, it will skips over the bumps more and after a certain point the amount of grip will be reduced as you lose contact to the track surface. The other consideration though is dive and squat, under braking and acceleration.
The body roll can be controlled fairly well with a combination of 400# (or 350# springs and a good sway bar, but dive cannot. I have found that you have to get up to 500# or more to control dive. It's important, because the 944 front brakes are so effective, and this causes a lot of weight transfer to the front. if the car squats too much it forces more braking power to the front due to the weight transfer, and makes the rear lighter, worsening the situation. Squat is not a problem for our cars, so let;s forget about it.
Doing some calculations based on weight, unsprung weight, and suspension geometry for a stock 944, you end up with a range of about 450 - 600# at the front. the rear is harder to calculate because the sway bar calculations are somewhat suspect. I use a different formula to the one on the paragon website and get slightly different numbers.
In the end, I have settled for 500# front springs and 28mm rear torsion bars. The suspension is unmatched, but the dive is better (less), and I can comensate for handling with a rear sway bar. The torsion bars are scheduled to be changed out as soon as Jason can get me some new 32mm bars (Vaughan is in the same boat here...). I suspect i will increase front spring size up to 550 or 600#. According to my calculations, I should have a suspension frequency in the right range up front, slightly lower in the rear, and much less weight transfer uunder braking than with the 400# springs I ran for so long.
About sway bars: They work, but they are limited. Rememebr that they pick up the inside wheel. Thsi means that if your sway bar is too heavy the inside wheel loses a lot of grip. For my money i will keep them on the car for tuning, but run them light.
the other important factor is shocks. Buy the best you can afford, and have them tuned for your springs. This makes a big difference that is hard to feel unless you are a great driver (I can't tell the difference by the way). They lap charts showed a huge difference for me (over 1.5 sec) at Road Atlanta after changin shocks and doing very little "tuning." Anybody need a set of old Konis?
As for Jim Pasha, he is an excellent resource and a great guy and I have the utmost respect for him. I think, however, that his information is old.
timo
06-29-2005, 12:17 PM
#32
Race Director
Originally Posted by timo944
Also, the stiffer the spring, the less the amount of weight that will transfer.
Originally Posted by timo944
The body roll can be controlled fairly well with a combination of 400# (or 350# springs and a good sway bar, but dive cannot. I have found that you have to get up to 500# or more to control dive.
Also, there are other reasons for using heavy springs. With lowered McPherson strut cars, the suspension is already in a unfavorable part of the camber curve and heavy springs can help avoid this unwanted camber change. This is why the WC touring cars are running ungodly spring rates.
Originally Posted by timo944
About sway bars: They work, but they are limited. Rememebr that they pick up the inside wheel. Thsi means that if your sway bar is too heavy the inside wheel loses a lot of grip. For my money i will keep them on the car for tuning, but run them light.
06-29-2005, 12:57 PM
#33
Race Director
Timo,
Are those calcuations for a stock weight car?
I run a 350/30 with Welt sways on a 2450 lbs (2625 as raced with driver) car.
In general the suspesion works quite well. However I was considering you braking statments about dive. My car is very solid, but could be better in long hard braking zones like turn 3 at California speedway (130 mph to 70 mph).
I really like the cornering balance and know that stiffer front springs will give me more understeer which I don't like. Now what I could do however is to go to 400 or 450 lbs front keeping my 30 mm rear bars. (rules limit me to 30mm bars max). The key will be though to go really soft on the front bar. I am thinking near full soft on welt bar. This way the extra spring will reduce dive under braking (allowing me to get weight transfered sooner and apply more brake force sooner). If I reduce the front bar enough then the spring rate under cornering could be same as why I have now and thus keep my balance that I so like. Hmm.... Swapping my 350's for 400 or 450 is not too bad cost wise. Unfortunartly I must still with Koni's per my rules.
Alest they much cheaper than proper dampers. 
I wonder if stiffer front springs will make the car more stable under brakes and allow me to take out some rear toe. ....
Are those calcuations for a stock weight car?
I run a 350/30 with Welt sways on a 2450 lbs (2625 as raced with driver) car.
In general the suspesion works quite well. However I was considering you braking statments about dive. My car is very solid, but could be better in long hard braking zones like turn 3 at California speedway (130 mph to 70 mph).
I really like the cornering balance and know that stiffer front springs will give me more understeer which I don't like. Now what I could do however is to go to 400 or 450 lbs front keeping my 30 mm rear bars. (rules limit me to 30mm bars max). The key will be though to go really soft on the front bar. I am thinking near full soft on welt bar. This way the extra spring will reduce dive under braking (allowing me to get weight transfered sooner and apply more brake force sooner). If I reduce the front bar enough then the spring rate under cornering could be same as why I have now and thus keep my balance that I so like. Hmm.... Swapping my 350's for 400 or 450 is not too bad cost wise. Unfortunartly I must still with Koni's per my rules.
Alest they much cheaper than proper dampers. I wonder if stiffer front springs will make the car more stable under brakes and allow me to take out some rear toe. ....
06-29-2005, 01:49 PM
#34
Addict
Rennlist Member
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Some thoughts;
- I generally agree with a lot of what Timo had to offer here. I have done cars with fairly close spring rates front and rear (300 coil front with 200 helper rear, 287-ish total), and they did porpoise more noticeably than more disparate setups. I like the frequency idea. Makes sense.
Joe - my car is similar to yours, albeit full coil and a little heavier (me too!). 2500lbs-and-change. I started with 400 front, and later went to 500 with no real noticeable difference except less roll, which is what I wanted. Balance was barely effected.
I also had my Welt front too tight. Wheel lift issues. Too much roll stiffness. I now run it full soft. I really feel the 28mm bar is too stiff for an early NA. I was running toe out front and zero rear with good results. I decided to go zero in front to improve inner tire wear. I also added my home-made control arms that utilize late caster blocks. I had dialed in 3.2 caster, and it made the steering noticeably slower and stiffer. I'm going to reduce that to 2.5-ish.
My ride height is 25 front and 25.5 rear at 12 o'clock on the lip
(not overly low). I'd like to level that and then dial in some more rear brake bias to mitigate front inside wheel lock.
- I generally agree with a lot of what Timo had to offer here. I have done cars with fairly close spring rates front and rear (300 coil front with 200 helper rear, 287-ish total), and they did porpoise more noticeably than more disparate setups. I like the frequency idea. Makes sense.
Joe - my car is similar to yours, albeit full coil and a little heavier (me too!). 2500lbs-and-change. I started with 400 front, and later went to 500 with no real noticeable difference except less roll, which is what I wanted. Balance was barely effected.
I also had my Welt front too tight. Wheel lift issues. Too much roll stiffness. I now run it full soft. I really feel the 28mm bar is too stiff for an early NA. I was running toe out front and zero rear with good results. I decided to go zero in front to improve inner tire wear. I also added my home-made control arms that utilize late caster blocks. I had dialed in 3.2 caster, and it made the steering noticeably slower and stiffer. I'm going to reduce that to 2.5-ish.
My ride height is 25 front and 25.5 rear at 12 o'clock on the lip
(not overly low). I'd like to level that and then dial in some more rear brake bias to mitigate front inside wheel lock.
06-29-2005, 02:26 PM
#35
Race Director
Originally Posted by RedlineMan
Some thoughts;
Joe - my car is similar to yours, albeit full coil and a little heavier (me too!). 2500lbs-and-change. I started with 400 front, and later went to 500 with no real noticeable difference except less roll, which is what I wanted. Balance was barely effected.
Joe - my car is similar to yours, albeit full coil and a little heavier (me too!). 2500lbs-and-change. I started with 400 front, and later went to 500 with no real noticeable difference except less roll, which is what I wanted. Balance was barely effected.
In my case I already have all the rear spring my rules allow. I could add in more rear sway bar, but I don't have a limited slip and too much rear bar will cause me traction issues.
My body roll is acceptable as well as things like tires wear.
How did the 500's impact your car under braking?
06-29-2005, 06:17 PM
#36
Addict
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Hey;
I anticipated push, and so I tightened the rear sway a bit. I immediately put it back once I drove it because I did not need it. It actually acted like I had done nothing but the rear bar adjustment in terms of balance. Strange.
Really, it was a very incremental change. I notice less roll and dive, but it is not dramatic. Just right, I'd say.
I anticipated push, and so I tightened the rear sway a bit. I immediately put it back once I drove it because I did not need it. It actually acted like I had done nothing but the rear bar adjustment in terms of balance. Strange.
Really, it was a very incremental change. I notice less roll and dive, but it is not dramatic. Just right, I'd say.
06-30-2005, 09:49 AM
#37
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Hmmm, tempting... I've got a set of 500's sitting on the shelf... maybe I ought to try them out! Currently running 400's with 30mm t-bars.
