Suspension Tuning

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Tokico offers several shock options for the RX-7 & RX-8 owner.

Setting up a suspension system requires both objective and subjective inputs, as well as some compromise. You need to decide how you expect your vehicle to perform under varying circumstances and what you’re willing to sacrifice to obtain this performance.

Three broad areas require serious consideration: ride height, handling feel, and cornering power. While all three areas are interrelated, we will address each separately.

Ride Height and Coil Springs

Three common reasons for altering ride height are: tire and/or ground clearance, appearance, and cornering power.

Generally, cornering power increases as ride height is lowered, but lowering a vehicle has a practical limit. Excessive ride height changes can cause the shock rods to strike the "bump rubbers", producing ride, handling and safety problems. To ascertain if this is the case, try the following technique. Attach a bit of clay or putty on top of the shock body and then drive the vehicle vigorously. Stop and inspect the putty to see if it contacted the bump rubber.

While cutting the coil springs is a cheap way of lowering ride height, you may find yourself performing some costly experimentation if the job is not performed correctly.

If you are considering the replacement of your springs, it may be a good time to consider changing the shocks on your vehicle. Since the effort required to change a set of springs usually requires the removal of the shocks, the cost of the installation can be reduced if both jobs are undertaken at the same time. Besides, if you have a high mileage car that is still running on the original shocks, it is probably about time to replace them anyways! Installing a set of new, high performance spring on a set of tired, old shocks can overwork the shock and may produce a "springy", "bouncy" ride.

Tire clearance is not a good reason to raise a vehicle. Do the job correctly and alter the outer or inner fender panel, as necessary. Drag racing may be the one exception where it is possible to improve traction by raising the rear of the vehicle. Exercise caution - you don t want to "top out" the shocks.

Shocks

Handling "feel" enters into an area that can be both ambiguous and subjective. There is a real distinction between handling feel and genuine cornering power. At times they are contradictory, but both are important. In general terms, responsiveness improves with increased stiffness, but only to a point! Ride comfort usually suffers as stiffness increases, and stability can either improve or deteriorate.

Possibly the greatest improvement to handling feel is upgrading the shock absorber. We offer KONI, KYB, and TOKICO shocks and struts. These are high quality, performance-enhancing components that have proven themselves in a variety of street and motorsport activities. If you are looking for a good all-around performance shock at a reasonable price, the Tokico HP Series (non-adjustable) is a good value.

Shock dampening may be non-adjustable or adjustable, depending on the current models available. Adjustable shocks are typically adjusted by a small knob either on the top or side of the shock.

Shocks for the 1979 and earlier models have becoming increasing more difficult to locate as many of these earlier applications are being discontinued as demand decreases.

Sway Bars

Sway bars (roll rate) and coil springs (spring rates) also impact vehicle handling. Again, a modest stiffening in the springs and/or sway bars can improve stability and responsiveness, but ride comfort may be sacrificed depending on the stiffness of the springs and sway bars. If spring rates are excessively increased, cornering power can also suffer.

As a general rule at best, softly sprung vehicles develop greater cornering power than firmly sprung vehicles, up to the point where the chassis "crashes" the ground. One exception to this rule is that as a tire moves up and down, its attitude relative to the chassis and the ground changes dramatically, frequently in undesirable ways. Thus, it is usually desirable to restrict this change, especially in roll.

One way to restrict roll is to change to a higher performance sway bar. However, it is possible to adversely affect the handling of a vehicle by not maintaining the cornering power balance between the front and rear of the vehicle. Assuming the vehicle was initially in balance - and very few vehicles are - a substantial increase in front roll stiffness usually causes understeer (that is, the car goes straight, or "pushes", when you want it to turn). Similarly, a substantial increase in rear roll stiffness usually causes oversteer (that is, the back end starts "coming around" excessively in a corner).

There are few, if any, absolutely true points concerning suspension, especially when subjective concepts such as driver "feel" and "driver "technique" are considered. At times it can become extremely difficult for the driver to determine just what the vehicle is doing at any given time. Vehicles display different characteristics under different situations. Specifically, high speed driving vs. low speed driving; smooth road surface vs. rough road surface; banked turns vs. off camber turns. You will undoubtedly find yourself experimenting until the right compromise is found.

Selecting and Comparing Replacement Sway Bars

We often receive questions from our customers with inquires as how particular Racing Beat Sway bar might compare to another aftermarket bar - or even to another Racing Beat sway bar.

Due to the complexities involving sway bar tubing materials, wall thicknesses, arm lengths, endlink hole positions, etc... it can be extremely difficult for the end user to choose or compare different aftermarket sway bars. For one reason, there is no "standard" test method - no two manufacturers will utilize the exact same test methods to collect and translate their data. For another, it is difficult to convert any numerical rating to an actual in-car effect.

There are many ways of "rating" sway bars. The simplest (and the one Racing Beat uses) is to compare the torsional stiffness of the bar or tube size. (One quick note - all steel bars, whether 1018 mild steel or 4130 aircraft alloy, have the same torsional stiffness - that is, resistance to twisting.)

We do this by measuring the diameter of a solid bar and raising that number to the 4th power - that is, for a .875" OD diameter bar (.875 x .875 x .875 x .875) - which is about .59. This number, the diameter to the 4th power, becomes a "rating" of the bar's stiffness. This is based on the engineering formula for torsional stiffness of a circular bar.

For tubular bars, make the same calculation twice, once for the outside diameter and once for the inside diameter, then subtract the smaller number from the larger to get the bar's rating. For example, compare the previously rated bar (.875" OD diameter) to a tubular bar with a stated 1.125" outside diameter and .75" inside diameter. First calculate the rating for the outside diameter (1.125 x 1.125 x 1.125 x 1.125 = 1.60), next calculate the rating for the inside diameter (75 x .75 x .75 x.75 = 1.60), then subtract the outer rating from the inner rating (1.6 - .32) for a tubular bar rating of 1.28. Comparing this 1.125" OD tubular to the .875" OD solid bar, 1.28 divided by .59 means that the tubular bar is about 2.17 times as stiff as the solid bar.

Now come the cautions: This method does NOT account for the effect of bending in the arms; for differences in shape between various bars; and most important, the differences in bushings and clamps. The good news is that, based on all of our experience, it is still a very good guide. It is especially good when comparing two bars with a similar design and comparable bushings. For example, if you have one set of bars installed, your car is balanced, and you want to change the roll stiffness, just select a new bar for one end of the car, see the change in rating, then find a new bar for the other end of the car that has the approximate same rating change - and the car will retain the same balance it had previously.

Over time, Racing Beat has found that the clamps and bushings that support the bar have a large and important effect on the performance of the bar. Rubber bushings (which Mazda uses) are quiet, durable and require no lubrication. Unfortunately, they also dampen the characteristics of a bar so much that they should not be used in performance applications. When the suspension applies a load to the sway bar, some of the motion is absorbed by the twisting and bending of the bar but some is absorbed by the bushings. The softer the bushings the lower the effective stiffness of the bar. In general, rubber is much softer and more compliant than the urethane bushings used by Racing Beat. Keep these considerations in mind when attempting to compare specifications of various aftermarket sway bars - the specific bushings used may have a significant impact on your bar rating expectations and cannot easily be factored into most comparisons.

We are often asked if there are there standard percentage increases in effective bar stiffness if the endlink mounting position is moved either inward or outward from the recommended center hole position of a sway bar.

The effect of moving the attachment point of a sway bar link on a sway bar depends on the radial distance from the centerline of the bar bushings to the original attachment point and the amount and direction of the movement. Shortening this radial distance makes the bar stronger while lengthening it weakens the bar.

The amount of change is obtained by dividing the original radial distance in inches by the new radial distance in inches. Convert this result to "percent" by multiplying it by 100 (e.g., 8"/7" = 1.14, multiply by 100 = 114 percent). Thus, shortening the arm from 8" to 7" increases the roll stiffness 14% (above 100 percent). This procedure is not exact, but it is a good approximation for the small changes usually available in arm length.

Tires & Wheels

Tire selection is a major factor with regards to handling performance. With the extremely large and ever changing selection of performance tires available we have no specific recommendations. We strongly recommend you speak to other drivers, internet newsgroups, autocross racers, etc... to obtain feedback on currently popular tires and their performance characteristics. As additional reference also speak to performance-oriented tire dealers for guidance.

When selecting a tire/wheel combination, do not tolerate tires that make contact with suspension members or body work.

Although larger wheels may improve the look of your vehicle, beware of increasing the unsprung weight of your vehicle with a set of heavy, but attractive wheels! Lower profile tires can also lead to damaged wheels if you routinely travel over rough roads in your area.

Alignment

After the installation of any suspension component that changes or alters the alignment of the vehicle, we recommend that you have your suspension re-aligned.

It can be common to find that the installation of various aftermarket products (i.e, excessively low coil springs) will not allow the vehicle to be brought back into "factory specifications". For a car that is used primarily for street use, this may result in increased tire wear and a possible change in handling characteristics.

If available in your area, contact an alignment shop that is familiar with performance handling and can assist you with aligning your suspension to meet your specific handling requirements.


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