Suspension

Submitted by on February 27, 2018 – 8:32 pmNo Comment

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 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 initially upgrading springs, shocks, or swaybars.

That said, the main goal in improving your Porsche’s handling is actually 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 change the package to one that is specifically tailored to the way you use and drive your car. Some of these components have a negligible effect upon ride quality while others can transform your car into a race car with the attendant degradation in ride quality. We always 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. There are four main suspension components that can be changed for higher performance:

1) Torsion Bars & Springs

2) Dampers (shock absorbers)

3) Anti-Sway Bars

4) Suspension Bushings or Bearings

Other items that are usually considered are Front Shock Tower BracesBump 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 for competition.

Torsion Bars & Springs

These two items perform the same function respectively, for the 911/930 as well as the 964/993 and water-cooled 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 964 and 993-based cars followed suit. All of the water-cooled cars use coil springs.

Torsion Bar sizes can be upgraded to larger bars in a wide range of sizes. The modern 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 so we are quite particular about which ones we use.

Installing larger torsion bars has a small effect on ride quality up to a certain point. 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 with shorter, stiffer versions that reduce lateral weight transfer and lean angles of the 964/993 and water-cooled cars as well as closing the excess gap between the fender and the tire. Porsche Cup suspension system used by the factory for various racing series are too stiff for street use unless you have masochistic tendencies. This stuff is really stiff!!

964, 993 and water-cooled cars gain the greatest handling improvements with the installation of a good aftermarket coil-over system of matched shocks and springs such as Bilstein PSS-10, KW Variant 3’s and Motion Control shocks. This permits ride height adjustments from 0 to-2″, precise corner-weighting, and the fitment of some larger, adjustable swaybars.

Dampers (Shock Absorbers)

Shrader valve on cannister

Shock absorbers (correctly called dampers) perform a multitude of tasks that are critical to optimum handling especially on bumpy streets and race 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 manage the rates of lateral, longitudinal and diagonal weight transfer during cornering and braking. They also dampen spring oscillations and keep the tires in contact with the pavement. 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 (Motion Control, JRZ, KW V3) and some are not (Boge/Monroe). 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 leaps to infinity and the car can spin quite quickly without much warning and this can be quite exciting! Using dampers that are shorter than the stock ones will really help maintain travel when the car is lowered.

There are three ways to set the dampers correctly. First, install double adjustable types, such as Motion Control, JRZ, KW V3, 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.

Shock manufacturers have conveniently painted their products to make identification much easier. Bilsteins are usually green or yellow, Konis are red or yellow, Boges and Monroes are black.

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 some 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, one must use a proper bump steer kit such as ERP or Elephant to allow one to adjust and fine-tune the bump-steer curve for best handling.

Anti-Sway Bars

Anti-roll bars, commonly called “sway bars”, are actually 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 Porsche 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 ‘65 to ‘73 (and ’75-‘76 Turbo), 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 relieving preload which is important during corner-weighting.

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 and water-cooled cars.

Depending upon one’s budget, you can use the swaybars made by Tarett or Smart Racing. These are similar in price, quality, and durability. Like anything else, you get what you pay for.

Adjustable bars 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 20mm to 31mm, 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 and cabriolets 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 water-cooled cabriolets and Targas are far stiffer than their predecessors and do respond very nicely to suspension upgrades.

Suspension Bushings

Porsche installs rubber bushings of varying durometers (hardness) in the suspension pivots and swaybar mounting points and these were an excellent compromise for a car that is used mostly for street purposes.

For maximum performance however, the natural flexibility of the rubber introduces a certain degree of imprecision when driving close to the limit. Replacing the OEM bushings with either harder rubber ones or metallic bushings can make the car handle and turn more crisply, with an increase in noise and harshness. Replacing the rubber swaybar bushings will make the bar function quicker due to less squirm in the bushing. This delays the load into the bar until the bushing has compressed.

Most bushings in use today are made from either rubber or urethane that has been impregnated with a lubricant to help keep it from binding or squeaking., however they still can squeak and make noise unless they are installed correctly. Using the proper greases, zerk fittings with urethane ones, and careful fitting will go a long way toward making these quiet as possible. Using these on the swaybars only, will not affect the ride or noise levels appreciably.

One good choice for dual-purpose cars are the Poly-Bronze ones from Elephant Racing. These are a steel-bronze bushing assembly with a polyurethane outer layer for noise reduction and employ zerk fittings for external lubrication. Suspension friction is far less than rubber or any other plastic bushings for improved ride comfort and control.

Replacing the bushings with spherical bearings, also called monoballs or uniballs, can virtually eliminate all friction and binding that always occurs with rubber or plastic bushings. This is not for a street-driven vehicle, however the handling difference has to be experienced to be believed!

This is not an inexpensive modification, but overall is very worthwhile and cost effective for track-only cars due to the overall improvement in handling and response. Having little or no friction in the suspension also permits very accurate corner-weighting.

Other Suspension Modifications

Three of the most important things, beside what has been discussed above, that make a really good handling 911 are fine tuning the bump steer, corner weight, and alignment.

Tuning the bump steer inherent in every 911 can be as simple as installing a rack spacer kit to installing an adjustable bump steer kit using spherical rod ends, instead of tie rod ends, to precisely position the steering arms. This is only necessary when the car has been lowered from the stock ride height. Rack spacer kits  are comprised of two alloy spacers and longer bolts to raise the rack that levels the tie-rods. The spherical rod-ends and hardware which replace the OEM tie rod ends offer a much greater range of adjustment, albeit at higher cost.

Corner weights must be set precisely so that the car handles the same in left and right hand turns as well as settling correctly over sharp rises that unload the wheels. Accurate corner weights also affect braking since unequal wheels weights create erratic braking characteristics.

Anytime the car is lowered from the stock height, you must place the car on 4 equal scales and adjust the springs or torsion bars so that the diagonal weight distributions are correct. Using computerized racing scales makes this much easier to optimize since the scales’ computer will display the desired wheel percentages and diagonal weights so necessary for a well-behaved 911. The front end of the ’65-’89 cars is easy to adjust and is done with an 11mm socket on the factory screws. However, the rear suspension is quite a challenge since the spring plates must either be re-clocked on torsion bar splines or use adjustable spring plates.

Adjustable spring plates makes this task much easier to dial-in. One can use either the Carrera two-piece plates or the Elephant Racing ones that allow adjustment with a ¼” Allen wrench. 964, 993 and the water-cooled cars require threaded-body struts and shocks to allow changing corner weights.

Alignment settings are critical, as are tire pressures, in the final chassis tuning to optimize your car for the street or each track you drive it on. Tire choices and sizing also determine the optimal alignment due to variations in tire construction techniques.  

Camber, Toe, and Caster are all adjusted for your specific use by a qualified alignment technician. Be absolutely certain that the person doing your alignment knows Porsches and understands the way you intend to use the car. 993’s also have a kinematic toe adjustment that is quite critical for proper handling. Your alignment shop and personnel must be equipped and experienced to align these cars correctly.

Chassis Strut Braces are utilized to tie the front strut towers together to eliminate flex. These bars attach to the top of the upper strut mounting plates and maintain the installed camber setting in a corner when the chassis is under load. These are available in steel, aluminum, and carbon fiber. All Porsches except the 914 and 996 cars truly benefit from the usage of these bars.

Once again, we have just barely touched on these subjects. We would recommend reading,“How to Make your Car Handle” by Fred Puhn and “Prepare to Win” by Carroll Smith as a good beginning to understanding chassis dynamics and how you car behaves. Bruce Anderson’s “Porsche Performance Handbook” also has some good information.

As always, you may direct any further questions to us at Rennsport Systems. We will be happy to help people as time permits.