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Some more general info: | QUOTE |
How to Make Your Porsche Faster, Part 1:
Suspensions
The best way to make your Porsche faster is to learn to drive it at its fullest potential! As delivered, Porsche automobiles are capable of performing far in excess of most owners’ ability. Investing in Driver Education and formal racing instruction is much more cost effective than any piece of hardware that you can install on your Porsche. Driver Education and track time will make you a better driver than purchasing springs, shocks, or swaybars.
That said, the main goal in improving your Porsche’s handling is making it easier to drive and more controllable when driven near or at the limit of your ability. The factory suspension settings are a compromise between handling and ride quality. Therefore any upgrades will change this set-up to one specifically tailored to the way you use and drive your car. Some of these components have a negligible effect upon ride quality, while others may transform your car into a race car with the attendant degradation in ride quality. We will indicate which items have the smallest impact on maintaining a streetable ride.
911’s, 930’s, 964’s, 993’s, 996's and Boxster’s all have different components and suspension design differences. We will discuss the various parts that may be upgraded with specific recommendations for each model for street and occasional track use. Full racing specifications are available by e-mail for each car and application.
There are four main suspension components that can be changed for higher performance:
1) Springs or Torsion Bars
2) Dampers (shock absorbers)
3) Anti-Sway Bars
4) Suspension Bushings or Bearings
Other items that are usually considered are Front Shock Tower Braces, Bump Steer kits, Spring Plate kits, and complete suspension systems for racers. This last item usually consists of replacing torsion bars with coil-over suspension pieces. There are some specialty parts such as ERP Products that replace the entire front and rear suspension with custom made pieces that are derived from the Porsche 935. These are very high quality components that save weight and make suspension adjustments much easier and more precise.
Torsion Bars & Springs
These two items perform the same function respectively, for the 911/930 as well as the 964/993/996/986-series of Porsche automobiles. Porsche used torsion bars of varying sizes on the 911 from introduction in 1964 to 1989. The 930 also used torsion bars from 1975 to 1989. In 1989 the Carrera 4 (964) was equipped with coil springs and the rest of the Carreras followed suit. All of the 964-993-996 and Boxsters use coil springs.
Torsion Bar sizes can be upgraded to larger bars in both solid bar and hollow versions. The hollow ones help to alleviate the added weight penalty of using larger, stiffer torsion bars. Some of the aftermarket bars have not been of particularly high quality and have exhibited sagging as well as not being the same spring rate as the factory offerings.
Lately, there are some excellent torsion bars available in various sizes that are manufactured here in the USA. Installing larger torsion bars has a small effect on ride quality. You can install bars that are 25% stiffer without a serious degradation, unless you live where the roads are really terrible. Removal and installation require just basic tools.
Coil springs can also be upgraded to shorter, stiffer versions that reduce the lateral weight transfer and sway of the 964-993-996-Boxster series Porsches. They also enhance the appearance by closing the gap between the fender and the tire. The progressive type springs used by Eibach and H & R minimize the effect upon ride quality. The Porsche Cup suspension system used by the factory for European racing series are too stiff for street use unless you have masochistic tendencies. This stuff is really stiff!
993's-996's and Boxsters gain the greatest handling improvements with the installation of aftermarket coil-over struts & shocks. This permits ride height adjustments from 0-2", precise corner-weighting, and the fitment of some larger, adjustable swaybars.
Dampers (Shock Absorbers)
Shock absorbers, correctly called dampers, perform a multitude of tasks that are critical to optimum handling especially on bumpy surfaces and tracks that have many changes in direction.
Dampers MUST be matched to the springs for the suspension to function properly. This is accomplished by selecting components from a manufacturer with a great deal of experience. The manufacturer must offer several choices that are very close to optimal, or adjustable dampers that will allow a range of adjustments. These may not have very wide range of adjustment. You may need to choose the type of damper, specifically for street, sport or racing, to get the range of adjustment that you need.
Dampers that are properly selected and valved, actually dampen spring oscillations and keep the tires in contact with the pavement. They also manage weight transfer during cornering and braking. Dampers control both the amount of weight transfer and the rate at which this happens.
Basically, this means that all weight transfer transients, such as when entering and leaving a corner or changing direction, are controlled by the damper settings. The rate of weight transfer caused by braking and acceleration is also affected by damper valving. Dampers come in different valving combinations depending upon whether they are for street use or track. The damping rates are expressed in two numbers, rebound over compression. These number are expressed in kilograms or pounds at a certain velocity, depending upon the maker. For example, an RSR shock for the 73-89 911 has 180/170; rebound/compression damping rates. The rear Turbo(930) dampers are 136/65 for comparison.
Some dampers are adjustable (Koni-JRZ-Moton) and some are not (Boge). Either way, the object is to have the dampers adjusted to match the street/track surface, spring rates, and sprung/unsprung weight of the car.
A common mistake in setting up a street 911 is to have too much compression damping in the front end. Many people install sport-type dampers on the front needlessly, and suffer the consequences of an insensitive and skittery front end. Another common mistake is to have the ride height too low in the rear. When the rear dampers bottom in a corner due to surface changes or weight transfer, the effective spring rate goes to infinity and the car can spin quite quickly without much warning. This can be quite exciting! Using dampers that are shorter than the stock ones will really help.
There are three ways to set the dampers correctly. First, install double adjustable types, such as some of the Moton or JRZ’s, and experiment with settings until the car feels better and the lap times confirm this. Second, use off-the-shelf components that have been optimized by the manufacturer. Third, use those same parts that have been re-valved with a custom setting optimized for your car by someone with experience and knowledge in this area.
All three are quite acceptable methods and vary only in the time required to set the car up.
The MacPherson struts used on the front suspension present a different set of challenges. Porsche has used Bilstein, Boge, Koni, Fichtel & Sachs, and Woodhead struts on the front of these cars since 1965.
The manufacturers have conveniently painted these components in their proprietary colors to make identification much easier. Bilstein’s are always Green or Yellow, Koni’s are Reddish Orange or Yellow, Boge’s are usually Black, and Woodhead's are usually Blue.
All but the Bilstein struts share the same basic configuration and design. The High-pressure DeCarbon design used by Bilstein allows the damper to be operated in any position. To lower unsprung weight, these attach to the body at the top of the strut so that the unit ‘s heavy end is bolted to the car instead of the suspension arm. Bilsteins basically operate in the "upside down" position, compared to the other brands of dampers and H&R struts employ the same technology.
Bilstein’s design has another advantage for the 911/930 owner, the ability to place the spindle in a more optimum location on the strut tube for better geometry and handling. Since the damper cartridge is "upside down", the tube body is a uniform diameter with no hydraulics behind the support tube. This allows one to relocate the spindle upward, to restore lost suspension travel after lowering the car. This will raise the front roll center back up where it belongs and restore lost suspension travel. The spindle height limit is determined by the choice of wheel diameters. 15" wheels allow no more than 18mm change in spindle height due to ball joint interference with the inside rim. Using larger wheels will allow higher spindle heights.
When the spindle is raised, the steering arms must be reshaped to correct the inherent bump steer. This must be done very carefully in a jig made for this purpose. There are also modifications to the tie-rod ends that allow fine tuning of the bump steer curves.
Anti-Sway Bars
Anti-sway bars, commonly called "sway bars", are transverse torsion bars that attach to each side of the suspension arms. These function by offering varying degrees of resistance to body lean and lateral weight transfer. Changing the position of the attachment point or the length of the moment arm that the torsion bar acts on, allows you make adjustments in the effective stiffness of a given bar diameter.
Some bars offer slider adjusters, spaced mounting holes, or rotating blades to allow you to fine tune the stiffness. There are even some bars that feature cockpit adjustable units that allow you to adjust the chassis balance for fuel load and track conditions. These are for racing only since the adjustment tower is placed where the passenger’s feet are. Installing bars of larger diameter will also increase the lateral stiffness.
Using the swaybars as part of the overall suspension system is necessary for chassis balance. There are several opinions on this, but we feel that the vehicles’ springs should carry the majority of the desired roll stiffness, not the swaybars. These should be used as tuning tools, not primary roll stiffness components.
To that end there are several factors to consider when selecting and using swaybars to make your 911/930/964/993/996/Boxster handle better. One is swaybar mounting methods, the other is chassis stiffness.
Mounting methods vary most with the 911/930 series. The Porsche factory offered two types of front mounts, the through-the-body style used from 1965 to 1973 /Turbo 75-76, and the bottom style, used from 1974 to 1989. The rear bars also varied slightly where the droplink attached to the suspension. The early style is superior, and in fact necessary if you wish to install an adjustable front bar. The Factory bottom-mount bars have no provision for adjustment and have unfavorable geometry for doing so.
The earlier, or through-the-body swaybar must be mounted correctly to prevent damage to the body. Also, the rear swaybar mounts need reinforcement if large sticky tires are utilized, otherwise they are liable to be torn off due to stress.
Several manufacturers provide different swaybars to the Porsche aftermarket, including Porsche Motorsports for the late 964-993-996 based cars.
Depending upon one’s budget, you can use the swaybars made by TRG, Vision Motorsports, and the famous "Charlie Bars" designed by Wrightwood racing. These all vary in price, quality, and durability. Like anything else, you get what you pay for.
Generally speaking, these should be installed in pairs so that you can adjust the roll stiffness and roll couple of the car at each end. These are available in sizes from 19mm to 25mm. Custom sizes are also available at extra cost from the various vendors.
Adjustable droplinks are extremely important for removing swaybar preload. Pre-load is caused by the car not sitting perfectly level on its suspension and pre-stressing the swaybars so that the bar is loaded more on one side of the car. This can be adjusted away by using spherical-bearing adjustable length links between the swaybar end and the suspension part that it attaches to. Adjustable droplinks are available for the front and rear and allow the swaybars to be adjusted to function uniformly in right or left turns. This also removes any cross-weight that has occurred as a result of pre-loading.
The performance of any aftermarket swaybar is dependent upon chassis stiffness. Targas, Cabriolets, and 914s may not respond the same way as a Coupe when using large swaybars, due to chassis flex. These cars will require additional chassis reinforcement to take full advantage of suspension upgrades. The 996 Cabriolet's are far stiffer than their predecessors and do respond very nicely to suspension upgrades. |
Note: This is a 911 article. Relevant to 914´s is information up until the 1989 model
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Suspension swap, Swap 911/930 front suspension to my 914
Feb 9 2003, 02:14 PM
spyder9014 Suspension swap Feb 9 2003, 02:14 PM
