mikey_audiogeek

You can use those numbers but they're not accurate.

Rear wheel rate is 0.43 * spring rate of spring.
Wheel rate = spring rate * (motion ratio)^2 and the motion ratio is 0.65.

Front wheel rate is about 0.94 * spring rate.

Cheers,
Mike

mikey_audiogeek

Heh heh...
Yep I'm running 400/850 springs which works out to about 365lb/in all round, give or take. Also known as a square setup.

I got the front spring rate/wheel rate number wrong - it's about 0.91.

Cheers,
Mike

mikemyers924s

So you're saying it's best to have a higher spring rate in the back?
I'm definitely in over my head here, but that doesn't make sense to me. Wouldn't that be imbalanced super prone to oversteer? This has been a very useful and informational thread. Looking forward to seeing who else chimes in.

Dawgz83948

If you do the math:

Front = 400*.94= 376
REar = 850*.43= 365

So even though the spring is much higher in the rear, the actual effect on the tire is much lower because of the leverage of the rear swingarm against the spring. Also correct me if I'm wrong, much past 600# in the back requires reinforcement of the swingarm shock mounting hole.

Funn944

I run 250lb front. stock torsion rear lowered all the way down with Koni Sports and Welt Stage II sways front and rear. It's just a daily driver with some track time, and I wouldn't go any stiffer on these SoCal freeways.

xsboost90

if anyone needs a set of 450lb coilovers i have a set 2.5 x 10 .

mikey_audiogeek

You could be right, and it would be prudent to consider it. My setup is the H&R Clubsport RSS kit and I am running the springs as delivered. They have TUV certification for the kit so this would suggest that the shock mounts don't require strengthening.

My car is a daily driver and I have done about 40,000km on this setup. I keep listening for sounds of trouble...

Cheers,
Mike

Oddjob

There really are not many documented cases of rear shock mounting points failing on dedicated track cars (high spring rates), if using correct components, installed properly. Either original mounts and bolts, or using the racers edge lower adapter w/ spherical shock mounts.

For the numbers you are using, has the front motion ratio been squared to = 0.94, or is the motion ratio 0.94, so 0.94^2 = 0.88 effective wheel rate ratio? Ive seen the motion ratio as low as 0.90 front, which would be a wheel rate ratio of 0.81.

The 1:1 front/rear wheel rate balance may not be the best setup for a high speed road course. Most drivers will feel that is too much oversteer in high speed sweeping corners.

Oddjob

Yeah, thats bright - or just use a proper spring rate to begin with.

Dan Shea

I'm not agreeing or disagreeing, but I've heard there are some schools of thought that say swaybars are only there to bandaid the real issue of not having enough spring rate.

Doesn't a sway bar allow you to have a softer ride while cornering flatter?

mikey_audiogeek

Good call. Rear swaybar increases the jacking effect of the high rear roll centre. Standard practice with independent rear suspension where the roll centre is higher than desired is to increase spring rate and reduce roll rate. You see it on VW beetles and formula VW cars - Z-bars instead of sway bars.

Even the new McLaren road car has a rear Z-bar...

Cheers,
Mike

mikey_audiogeek

I was going from memory! Motion ratio of 0.94 seems about right. That would imply zero offset and a 20 degree strut inclination.

The 1:1 ratio is modified by roll centre effects and sway bars. I run much less negative rake than most people which has a notable effect on front/rear relative roll stiffness. I'm also running 235/265 tires.

And of course, on our RHD cars the battery is in the boot!

mikey_audiogeek

Yep +1 on the spring rate mismatch problem. It's usually something that Bilstein owners are much more aware of. Bilstein techs have been saying for years regarding high(er) spring rates and low sway bar rates.

Matches my experience too. Anyone changing from Koni to Bilstein should consider going up in spring rate, down in sway bar rate, and down in tyre pressure.

Cheers,
Mike

coilbox

Just for the record.....I deleted torsion bars and fitted 600lb at the rear.

Nice for trackdays and ok for the street, changing the bump/rebound on the shocks.

Oddjob

First, for a street 944, daily driver type, most of this does not matter.
You can drive around w/ no swaybars. Really doesn't matter what the
spring rates are, doesn't matter what shocks you have, or condition. The
car will still do its job, getting from point A to B.

When setting up a car for competitive high performance driving, whether
for auto-x, or high speed road course DE/Racing, then of course all of
that does matter.

Swaybars are not a band aide, only used to compensate for low spring rates when
trying to improve street ride quality. They increase the roll
stiffness/resistance of the front and rear of the car, which is somewhat
of a different function than springs alone accomplish. Bar size/rates
should be balanced with the overall suspension setup.

Only time I have seen swaybars taken off 944s, are when auto-xing a track
car setup for a road course. Need to get the rear to rotate more in tight
turns, so taking off the front swaybar will help to get the car to turn.
That is a band aide fix, because the spring rates were not balanced/setup
for that type of use.


The use of Z-bars on late 60's VW bugs and Karmann Ghias probably is not
directly applicable to a 944 suspension setup. Nor is the McLaren, as
the design and technology of the suspension differ greatly. Maybe an
interesting theoretical automotive suspension design discussion, but not
immediately applicable to our chassis.



Very seldom will you ever see a 944 pickup an inside tire on a road
course. It will never be a rear, only an inside front. So no reason to
remove the rear bar to avoid this anomaly. And generally will only occur
due to track surface variation at trackout (so may only ever happen at one
corner of one track, and no where else). Older 911s more commonly lift
the front inside, much due to suspension travel limits of very stiff
torsion bars, a huge rear weight bias, and yes stiff front sway bars. The
front wheel drive, front engine cars are the ones chronically prone to
lifting inside rears, especially on auto-x courses.




The Bilstein turbo cup suspension was developed w/ Porsche for the turbo
cup cars. The Cup suspension was designed using 30mm front bars and 21mm
rear bars. No way that the bilstein suspension was intended to be used w/
smaller or no sway bars.

Also, Porsche used koni dbl adjustable 8742 struts and 8242 rear coilovers
on the US Escort/Cups and the Firehawk Clubsports, w/ the same springs and
same swaybars as used on the Bilstein equipped Euro & Canadian Cups. The
konis are actually valved to handle at least equal if not somewhat higher spring rates than the bilsteins.

Tires pressures are not directly used to tune handling. Incorrect tire
pressure will affect handling because it may cause the tire to operate
outside of its designed temperature range. The optimum tire pressures are
based on the brand/model (sidewall construction and rubber compound) of
the tire, not on the brand of shock.




I would recommend that others, before spending money based on any specific
advice from this thread (or any forum thread), look at how 944s have been
setup to auto-x and race by competitive individuals and shops (for
amateur and pro race series) over the years.

The factory Turbo Cup/Escort cars, the IMSA Firehawk S2s, any current club
series where 944s are present in numbers and competitive - PCA Club
racing, NASA, SCCA spec series in North America, or equivalent series
elsewhere. As allowed by rules, you will always find higher spring rates,
higher damping rates, and larger front & rear sway bars.

And look at more modern dedicated factory race cars - 997 Cups and RSRs
use front and rear sway bars, and all P-factory production based race cars
always have. No comfort or band aid compromises on those cars. Even 962s
used sway bars, and it wasn't to compensate for inadequate spring rates.