Spring rates for Bilstein Cup coilovers with torsion bar delete?
11-02-2012, 09:41 PM
#31
Three Wheelin'
11-02-2012, 09:45 PM
#32
Three Wheelin'
"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 Porsches 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
Porsches 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%).
So it may be a bit stiffer in the rear than I might run but with the ability
to tune sway bars etc... you should be Fine. Your setup is actually much
stiffer in the front than the Porsche cup setup which ran progressive front
springs (200 - 371 lb/in) with the rear setup I described above which is not
that far from what you have( yours is 296 at the wheel and theirs was 259 -
334 lbs/in at the wheel).
Hope this make sense, but believe me, what is above is 100% correct. I can
fax you the Porsche motorsport sheet if you think it will help."
case, the 47% is a number that I have calculated By taking measurements. I
am now pulling out one of Porsches 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
Porsches 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%).
So it may be a bit stiffer in the rear than I might run but with the ability
to tune sway bars etc... you should be Fine. Your setup is actually much
stiffer in the front than the Porsche cup setup which ran progressive front
springs (200 - 371 lb/in) with the rear setup I described above which is not
that far from what you have( yours is 296 at the wheel and theirs was 259 -
334 lbs/in at the wheel).
Hope this make sense, but believe me, what is above is 100% correct. I can
fax you the Porsche motorsport sheet if you think it will help."
Maybe one day... <sigh>
Cheers,
Mike
11-02-2012, 10:41 PM
#33
Addict
Lifetime Rennlist
Member
Lifetime Rennlist
Member
11-02-2012, 11:16 PM
#34
Rennlist Member
Right! Good catch!
I meant it as just the physical movement of shock vs wheel. But for spring rates it is squared.
From Wikipedia:
I meant it as just the physical movement of shock vs wheel. But for spring rates it is squared.
From Wikipedia:
The wheel rate is calculated by taking the square of the ratio times the spring rate. Squaring the ratio is because the ratio has two effects on the wheel rate. The ratio applies to both the force and distance traveled.
11-02-2012, 11:19 PM
#35
Rennlist Member
Thread Starter
I currently run -3 camber up front, + 07 toe, +2.3 castor. Rear is -2.25 camber, +11 toe.
I'm going to start with 450 front, 700 rear springs and have a few other sets of springs that I can swap in as I need to after track testing...
I'm going to start with 450 front, 700 rear springs and have a few other sets of springs that I can swap in as I need to after track testing...
11-03-2012, 12:04 AM
#36
Race Car
Join Date: Sep 2005
Location: TEXAS
Posts: 4,247
Likes: 0
Received 0 Likes
on
0 Posts
So many different ways to say toe. I'm sure even more people are confused now.
I personally run zero toe in front and a very small amount in the back. I run a little toe out in the front for Autocross.
I very much like VAN's specs. Our manual steering needs to be as light as possible, but I would still run a bit more caster and less camber for the longer sweeping corner tracks. - I did add a little anti dive to my front A arms, but only a very small amount with adjustments that can bring me back to stock. - I will probably move to spring rates that will negate the need for any extreme anti dive measures but it doesn't exactly fully work like that. Suspension tuning can be such a daunting topic. I honestly haven't learned enough about this topic just yet and will always search for a new "angle".
I personally run zero toe in front and a very small amount in the back. I run a little toe out in the front for Autocross.
I very much like VAN's specs. Our manual steering needs to be as light as possible, but I would still run a bit more caster and less camber for the longer sweeping corner tracks. - I did add a little anti dive to my front A arms, but only a very small amount with adjustments that can bring me back to stock. - I will probably move to spring rates that will negate the need for any extreme anti dive measures but it doesn't exactly fully work like that. Suspension tuning can be such a daunting topic. I honestly haven't learned enough about this topic just yet and will always search for a new "angle".
Last edited by 95ONE; 11-04-2012 at 01:33 PM.
11-03-2012, 02:15 AM
#37
Rennlist Member
...still waiting to see just what goodies you're coming up with Mike.
11-03-2012, 06:39 AM
#38
Three Wheelin'
Yep, kudos to Van for measuring. Only way to know for sure!
11-03-2012, 11:12 AM
#39
Addict
Rennlist Member
Rennlist Small
Business Sponsor
Rennlist Member
Rennlist Small
Business Sponsor
I posted a little narrative on the other spring rate thread.
In reality ‘real’ racers do not pick springs with a calculator. They go to the track and test. There are a lot of variables that go into spring selection – car weight, shock type and valving, track conditions, driver preference, tire pressure and size, sway bars.
If you start with 450/700 you will be pretty close, it might be a little stiff for street unless you have very nice paving!
BTW – rear toe can be a killer. Toe out on a car with rubber bushings will make the car vary unstable in high speed braking. With the rubber bushing the car will add more toe out under braking.
In reality ‘real’ racers do not pick springs with a calculator. They go to the track and test. There are a lot of variables that go into spring selection – car weight, shock type and valving, track conditions, driver preference, tire pressure and size, sway bars.
If you start with 450/700 you will be pretty close, it might be a little stiff for street unless you have very nice paving!
BTW – rear toe can be a killer. Toe out on a car with rubber bushings will make the car vary unstable in high speed braking. With the rubber bushing the car will add more toe out under braking.
11-03-2012, 06:35 PM
#40
Rennlist Member
The race engineers I personally know would start with calculation to make an initial choice, but then rely on driver feedback, data acquisition and, ultimately, lap times to make a final selection.
11-03-2012, 07:02 PM
#41
Addict
Rennlist Member
Rennlist Small
Business Sponsor
Rennlist Member
Rennlist Small
Business Sponsor
It must be blissful for you to live in a perfect world where you can think that all 944 racers have team engineers to figure things out. I suppose you believe that all racers have the data they need to make such calculations – I am sure every racer out there knows the chassis stiffness data for their car….without that a ‘real’ race engineer will tell you that you are wasting your time calculating anything.
That is why I used the term ‘real’….as in real world. That’s the place where some of us get things done as opposed to just posting about it on the internet…
That is why I used the term ‘real’….as in real world. That’s the place where some of us get things done as opposed to just posting about it on the internet…
11-03-2012, 09:19 PM
#42
11-04-2012, 01:25 AM
#43
Addict
Rennlist Member
Rennlist Small
Business Sponsor
Rennlist Member
Rennlist Small
Business Sponsor
The point is that you can calculate effective spring rates all you want and it does not tell you how it will handle. The "Holy Grail" of equal front and rear spring rates is a worthless. In fact equal effective front and rear rates is a bad track set up.
Nobody I know (and I know a lot of very good 944 chassis guys) have enough technical information to model a useful simulation for a 944 suspension set up. I know of a 944 that was put on a 4 post shaker rig for chassis evaluation and ended up with the effective front rate about twice the rear rate.
So, to answer your last comment - using math to calculate effective spring rates is pretty useless if you don't know how those spring rates work in the 'real' world.
There is enough 'real world' data out there to have a good starting point and then field test and make changes in spring rates to fit the car/track/driver.
BTW - as an engineer that has spent a lot of time 'playing' with particle accelerators I just might have an appreciation for math.
11-04-2012, 07:12 AM
#44
Rennlist Member
Seriously Rob, you just seem to descend upon some threads and just bomb away. You have issues with Tony G, Chris and a few others. I'd seriously hate to be your Cat. All it appears from the outside is that you must have some precedent with which gives cause to these caustic posts. You attack people that have had a fair bit of documented experience in these cars and yet I can't recall seeing much history of your background. Not trying to belittle you and I'm sure you're very experienced but it just doesn't seem so by your outbursts. It makes me think that there must be some backstory but I can never find anything to validate why you lash out like you do. So in the end....I just don't get it????
11-04-2012, 08:30 PM
#45
Three Wheelin'
Last edited by mikey_audiogeek; 12-11-2012 at 05:34 PM.