Suspention set up computationsto???

[911st]

Our suspention dynamics is a partial result of the combination of weight distribuition, tire size, spring size (torsion bars), shock compression rate, and shock rebound rate.

I have computed below the front to rear distribution of these for stock SC/Carreras/930's and the typical performance modifications that are genneral accepted standards.

I did this to the best of my abilities in the hope of better understanding what we do, to try to understand why given combo's are suggested, and to be able to make a better decission.

It is genneral accepted that our weight distribuition is 40% front and 60% rear.

My calculations are as following:

40/60 Weight dist.

Torsion bars:
47/53 18.8/24.1 stock 78 - 85
44/56 18.8/25 86-89
40/60 Turbos
48/52 21/27's
49/51 22/28's
44/56 21/28's
41/59 21/29's
45/55 22/29's

Shocks:
(stock rates n/a)
34/66 HD's Bilstein's... compression
42/58 Sport"s.............. compression
40/60 HD's................... Rebound
44/55 Sport's............... Rebound

Tires:
49/51 205/225 Apx. tires
47/53 225/245 Apx. tires
45/55 205/245 Apx. tires

60/40 estimated for the sway bars.

Please look at this and make your own conclusions. Some come to me.

With the normal torsion bar recomendations we are stiffing up the front beond what the factory did relitive to the rear.

All Sport shocks have a nice ballance on compression but are stiffer in front on rebound.

All HD shocks are ballanced on rebound but are softer up front on compression.

A ballanced set of shocks might more approach front shocks that are valved as the Sports on compression, and the HD's on rebound. To be used with the out of the box HD or Sport's (My info is they are valved the same which is odd except from a marketing point of view.)

With the normal recomended tuner combinations the front of the car is being stiffened up relitive to the back over factory settings. This may be to ensure the rear maintains it's traction. However, it also reduces the fronts traction. If the front's traction is reduced enough it will brake away closer to the same time the rear dose.

I can understand 21/27 or 22/28 on a non flaired pre SC a bit.

This may leave us with a harsher feeling from the front of the car on the street than is necessary.

Rear tire size is our limiting factor in reaching a ballanced car.

I tire combo of 205/245 would be more ballanced. Adding to the rear tire makes sense. Reducing the front if it is already a 225 would not as you would just loose the extra braking capacity it adds.

If I understand correctly fitting a larger tire to the front is the more chalanging.

A more ballanced set of tires would come to temp more even front to rear and would not require such a stiff front set up.

I am intriged with the following potental set up:

205/245 tires (min 8" rim in rear)
21/29mm torsion bars or possably 21/28's
revalved front shocks with Sport compression but HD rebound
Rebalance brakes. May just have to fit Carrera's to an SC or remove the rear brake presure limmiting valve on the Carrera's.

Then play with sway bars to ballance car. They may be able to be a bit smaller with the larger rear torsion bars. With smaller sways, bumps on one side will not transmit as much to the other side.

Anyone come close to this with there current set up?

Thank you in advance for any help or comment.

[Bill Verburg]

You can calculate stuff to death, bottom line is what works on track.

when improving track performance, starting from stock; the usual goals are
1) less under steer
2) more overall traction(larger friction circle)
3) minimise unfavorable camber & toe change
4) balance in all aspects of performance
5) any change in one area shouldn't adversely affect other areas

The basic plan for a well thought out track oriented 911 is pretty well documented but does depend on the weight and power characteristics of the car.

Assuming a fairly stockish engined SC or Carrera, the recipe is fairly well known
22/28 torsion bars
adjustable sways
8 & 9 wheels there are increasingly good tire choices as you go form 15 > 16 > 17 > 18
225 > 245 front 245 > 275 rear
an effective asymmetric LSD
Bilsteins valved to the t-bars or Fox shocks
930 brakes, Carrera fronts w/ SC rears is a small improvement(but doesn't change bias not that you want to), taking out the p/v on a Carrera is a very bad idea(unless A/X is your thing)

Of course there are more or less aggressive variants at each step.

[911st]

Thank you Bill. I respect and appriciate your knoladge.

Agreed and understood.

With eveloution in things like tire technology, things may evolve. Or if you change somthing in the mix like the front to rear tire ratio, somthing else has to be readjusted.

As you know, sometimes what people belive is just what has been handed down, somtimes it is just created by marketing hype.

I would not advicate the braking change unless the front to rear tire ratio changes. This combined with the stiffer rear may also creat less weight shift and should require some type of balance change. I should have left the braking part out but did not want someone trying this and locking up the front.

22/28's are a well accepted set up for a reasion. Why is this. This set up is heavely biased toward the front with a spring ballance I would more expect to see on a 944.

If this were a coil over race set up we would not put 500/500 springs we would go with somthing like 400/600 which is closer to the actual weight dist. ratio. Then why are we using a combonation with torsion bars that is closer to a 50/50 than a 40/60 set up?

Does this have somthing to do with the design of a torsion bar system that is unmatched front to rear?

If so, this may be a reasion to go the coil overs besides just accessing higher spring rates.

Were these standards developed for different tire charictics. The more flexable nature of the older tire designs would be magnified with our light front. heavy rear. Did we need stiff fronts because the front tires were in effect a long travel soft spring' that had to be accounted for . Now we are running stifer shorter sidewall tires which takes some of the relitive softness out of the front so we migh think of puting some of it back with a softer spring.

I do not know if it is time to adjust the norm.

When we work to reduce understeer are we getting more traction in the back? Or are we reducing the fronts to the point they brake away at the same rate as the rears?

In doing this we are twisting the car to the point we are lifting the front wheel. That has to be effecting the dinamic suspention settings a bunch.

One of the tests used to set up the 911's originally was to put the car in a steady state cornering situition and see if the car brakes away evenly. If the rear brakes first, they readjusted... Is the car then faster? It is more predictable.

Most of these standards came from work done back in the 70's.

Back to the 22/28 standard. I would have to assunm this is derived from the 21/27 narow body, same size wheels, lighter car days.

22/28 is a natural eveolution for the weight increase but it ends up with an even closer to 50/50 distribution than the 21/27's.

Even though we added width to the rear of the newer cars, the fact is the width of the rear was not that much different from the front. We used 205/225 or 225/245's. What may have been more a benifit was the added inch to the rear wheels.

If a change was made to a 205/245 combonation. It seems there should be some changes to set up to compensate. Should it be the shock rates, the sway bar settings, and or the spring set up.

Somthing else I find interesting is that widening the track of a 911 effects the front/rear effective spring rates because of the oriantation of the torsion bars and the levers attached to them.

If the same tires are used and the front it taken to turbo width this would add about 1.5" to the length of the front torsion bar arm lever. It is about 15" long on a normal width car so the added 1.5" makes for about a 10% reduction in effective spring rate in the front. The orintation of the spring lever in the rear is such that adding width dose not effect the efective spring rate.

We do we not change our recomendeds with a wide body car.

A 22mm front bar is rated about 210 lbs. With a wide body it would drop to near 190#. A 23mm bar is 250 on an SC but would drop to around 225 on a wide body. If 210# was the ideal, then which could be do with the wide body 190 or 225?

Sorry, I am having to much fun asking what if's as I try to learn.

[Steve@Rennsport]

Hi,

I'd concur with what Bill offered and add that spring rates (torsion bars, in this case) should be chosen by taking into account several factors.

1) Vehicle weight

2) Wheel/tire package

3) Driver skill, experience, and preference (this is huge)

4) Engine power

5) Usage: street-only, street & track, racing


This is NOT a one-size-fits-all sort of proposition and there are many different solutions tailored for each individual.

[911st]

Thank you much Steve!

Flexibility in design to meet the situation makes sense to me. That is where experience comes in.

I think my questions leads to the consideration that just accepting a "best practices approach" my not always give the best result.

Can you confirm if Bilstein HD and Sports have the same valving in the rear as I was told by Bilstein?

Also, do you have any idea what stock shock rates were?

Knowing a 21mm front torsion bar is 60% stiffer , a 22mm bar is about 90% stiffer, or a 23 is about 125% stiffer helps me make an informed decision about what I my want.

Having this info for shocks would be nice to.

[304065]

You had me at "well accepted setup for a reason."

Truth is, the development decisions that brought us to where we are today were made in the late 1960s. In the early 1970s the factory went to coil-overs because it's impossible to get enough spring rate in the rear end of a 911 even if you went to the largest torsion bar that could fit in the tube (around 33mm).

So the proven combos are not only a function of the ideal calculation, but based on real-world experience in the three decades since the factory left us behind. 22/28 works. So does 23/31 if you are racing. Custom valved Bilstiens to match the spring rate are a must and every shop has their own formula for the ideal relationship between bump and rebound (and they aren't telling, even if you buy the shocks from them-- only Bilstein has all this information and THEY aren't telling.)

Also, trying to figure out the spring rate from the old fourth power rule has a number of limitations-- like the exact modulus of the steel used, the length of the lever arm (the springblade) and the distortions that occur as the suspension goes through its range of motion. Also (thanks Hayden Burwill of WEVO for this nugget) there are actually TWO front torsion bars: the bars themselves, which are fixed at one end, AND the tube portion of the front suspension arm, which acts like a hollow torsion bar fixed at the OTHER end. The result is you always end up with a softer rate in the front than you plan for, another reason for coil-overs.

Anyway, I admire the intellectual curiosity, but there's no substitute for real track experience. For the street I would leave the suspension bone-stock and make sure the bushings are in top condition-- the performance potential of any post-1968 Porsche is well beyond the capacity for safe, legal street use as it is, so you might as well opt for the balance of comfort and handling that the factory determined with thousands of hours of research and testing.

Good luck!

[Steve@Rennsport]

Originally Posted by 911st

Thank you much Steve!

Can you confirm if Bilstein HD and Sports have the same valving in the rear as I was told by Bilstein?

Also, do you have any idea what stock shock rates were?

You are quite welcome.

HD's & Sport's are indeed valved differently. LOL,..They simply need to look at their own charts.

B46-0169 (HD) 194/150
B46-0975 (SP) 311/159

Now,....HD's & Sport's for the late '68 through '72 cars are the same shock; B46-0167 and therefore have the same valving (238/78),...

I've not run OEM shocks on the shock dyno so I have no idea what they are valved at.

[911st]

Thank you, thank you.

[Bill Verburg]

The net handling characteristics of a car depend on the interaction of many components.

You are throwing out a a whole bunch of often conflicting data.

Again, starting from stock.
The basic handling characteristic established by the factory for sale to the general public is high and low speed understeer. As speed builds the understeer decreases but is always there.

Most tracksters want a little less understeer and a bigger friction circle.

things that increase understeer
front
more weight
larger sway bar/more effective sway bar
smaller tires/less pressure
larger sway
more aerolift
less neg camber
narrower track
raise
stiffer springs
more toe out/ bump toe out
narrower tire
bigger tire sidewall ratio

rear
less weight
larger tires/more pressure
smaller sway bar/less effective sway bar
less aerolift
more neg camber
wider track
lower
softer springs
more toe
LSD/more effective LSD
narrower tire
lower tire sidewall ratio

you need to get the basic non adjustable stuff right or at least in the ball park then fine tune w/ the adjustable stuff

Steve is right on the money too, every driver will be be different in preferences and needs.

Most people go through a learning curve, as they use the car they pick up on things that they see as needing improvement or change. It's a lot easier to discuss the effect of a single change than a whole melange of them all at once.

just to pick one thing to start w/. You cannot meaningfully compare spring rates between t-bar and coil over cars. The effective spring rate depends on the geometry of the setup and includes the lengths of the various lever arms and the stiction inherent in the design.

tires have certainly evolved since the 60s, todays street tires are better than 60s race rubber, but the evolution hasn't changed the effect of the tires, only the magnitude of the forces that are now routinely transmitted.