Penulis
| Suspension Tuning |
| Preston Marshall |
| Introduction |
| As with engines, suspensions are a system or combination of parts that are designed to do a specific job under many assumed conditions. The kind of system you choose is greatly dependent on what you can afford, what your goals are and the target environment you anticipate operating the vehicle in. Also, it is pretty well known that a lower cost solution $$-wise will require more effort on the owner's part to be truly effective. This is a tradeoff almost all of us have experienced at one time or another. Common sense, right? Up front, I would like to say that any and all suspension trickery is going to be greatly affected by the tires you are running. As many of you have found in your drag racing experience, tires usually have an enormous effect on suspension performance. Such is also the case for road racing and autocrossing. The science of tire compounding and carcass construction is also highly complex and beyond the scope of this article. Most of the time, your tire choice will be based on application and driving style. With that in mind, my recommendation is simply to try a few different tires and choose the one that fits your driving style the best. I don't think "one size fits all" recommendations are of much value, especially where tires are concerned. However, to give the reader a perspective of the current top performers I will list a few. Current high-performance road race and autocross tires:
First, we need to define the "ideal" case in terms of handling. The ideal car would have the following characteristics:
If you want a more competitive handling car and can spend more money, then you can replace the control arms, suspension and body bushings, springs, anti-roll bars, tires and wheels. Which combination you choose will depend greatly on your goals and what you want to do with your car. Of course, a full tubular space frame with 4130 .080" wall tubing would come in pretty handy as well. |
| Bushings |
| Polyurethane or Graphite Impregnated Polyurethane are great bushing materials choices for compression applications (e.g. your anti-roll bar end-links and frame/body mounts). They are also great for lower priced replacements to stock bushings in control arm applications intended for street/strip use. However, polyurethane bushings in an application that requires rotational deflection (front and rear control arms) and axial deflection (rear control arms) will bind a lot sooner than rubber. Why worry about bind? Well, we want to avoid suspension bind because when it happens the effective suspension spring rate immediately increases to infinity. In essence, the suspension system is no longer compliant with the road surface when bind occurs (definitely NOT a good thing). The GM engineers knew that the stock rear suspension geometry would cause bind at some point in the suspension travel (as all angled arm suspensions do). This is why they chose the specific durometer rubber for the bushings that they did. At the rear you can use either higher durometer rubber bushings from the 1LE Camaro, use polyurethane, or use a Military-Specification spherical bearings. At the front, you can use higher durometer rubber, use polyurethane, or solid bushings (e.g. Herb Adams and Global West). If you want the highest performance, then the spherical bearing at the rear will allow the most consistent geometry and largest amount of suspension travel without bind. If you want the highest performance at the front, then solid bushings (which only allow rotational displacement) are what are required. For autocross, you could easily get away with polyurethane due to the smoothness and flatness of the track surface. In road racing and street circuit racing, solid and spherical bearings provide better performance at higher cost. As for body bushings, we are all very familiar with the General's seemingly haphazard way in which it applied body bushings to the G-body cars. Well, there's nothing particularly wrong with the rubber bushings that GM put in except that one of their main design criteria was noise dampening. For the weekend warrior an upgrade to polyurethane body bushings will help stiffen the platform (remember the direction in which we are asking the bushing to do it's job) and help tie the body to the frame. There are several chassis vendors who sell these for our cars. Last time I checked, Gulstrand and PST had these available. For a much more competition oriented car, one could use the billet aluminum body bushings from Herb Adams. |
| Control Arms |
| Again, it is a price vs. performance vs. application tradeoff. The cheapest, effective solution for all around handling is to box the stock control arms front and rear. We have all heard about how to weld a steel plate to the stock U-channel rear control arms. This is very effective and yields very good results overall. It is less well known that this can also be done to the front lower A arms. It was the common practice of NASCAR and Trans-Am fabricators in the late sixties and early seventies. Generally, a trace is taken of the open section of the A arm and then a sheet of mild steel (1020) is cut to match. Then, the sheet is welded to the open section (the bottom) of the arm with a hole in the middle for the lower shock mount (along with the anti-roll bar and bump-stop mounts). This has the same effect at the front as it does at the rear but requires a little more work too. If you want a slightly more expensive solution (and can accept a little bushing bind and deflection now and then), then you should probably get the Hotchkiss tubular front arms and the boxed rear arms (which have polyurethane bushings). They are stiffer than stock and will provide slightly better camber curve geometry at the front since they are a little shorter than stock. This stuff is fine for street-strip and is okay for autocross applications. If you want a more negative sloping camber curve, a stronger arm and cannot tolerate bushing bind or deflection, then the Global West tubular arms front and rear (with solid bushings) are your appropriate solution. These characteristics are necessary when competing on road racing circuits and street circuits but can also be helpful in autocross as well. |
| Shocks |
| Simply put, you must replace the shocks. If you can afford Bilsteins or Konis, these are the best available. I run on Bilsteins (B36-0949 front and B46-0929 rear) and have always found them to compliment my setup the best. For starters, you should look for a shock with an asymmetric Force vs. Velocity curves between compression and rebound. As a baseline, for road racing you should start out with the dampening forces split 60% rebound and 40% compression. More often than not, you will end up with the ratio of rebound force to compression force hovering around 2:1. Also, a digressive force-velocity (F-V) curve gives better control of the sprung and unsprung masses without being harsh (Konis and Bilsteins have this, KYBs have a linear F-V curve). Basically, a digressive F-V curve gives a "roll-off" of damping force at high shaft speeds. This means you can run more force at low shaft speeds (when you need to control the sprung mass) while trimming off the force at high shaft speed (when you need to control the unsprung mass). Now, the stock replacements of both Konis and Bilsteins for the G-body are not completely digressive. The valving has been updated to provide a slight roll-off of force but neither of them have the new digressive pistons (the new B-body shocks do have the new pistons). For a fee, Bilstein will upgrade your shocks with the new pistons and re-valve them to suit your application. If you absolutely have to have a true racing shock, then you need to call Penske Racing shocks and get the 4-speed (not like a tranny!) adjustable shocks, custom valved for your application. If you are not going to be competing with the car and just want a slightly better replacement shock, then many members have had good luck with Monroe Sensa-Tracs and KYBs etc. |
| Anti-roll bars and Springs |
Again, depending on what you want to do, you need to choose a coil spring rate and anti-roll bar diameter combination to suit your requirements. As Mike Snyder very appropriately said in his autocrossing article, you need to spring the car based on the type of surface and the speed range you will be running. Front spring rates for our cars can vary from 440 lbs./in. (stock) to 720 lbs./in., depending on the application; while rear spring rates vary from 126lbs/in. to 215lbs/in. Most people believe that a coil spring, is a coil spring, is a coil spring. Nothing could be further from the truth and all springs are certainly not created equal. Some things to find out when inquiring about springs are:
As for anti-roll bars, they can be just about any diameter you choose with the stock diameters being the baseline (1 7/16" front and 7/8" rear). I prefer the ADDCO bars that Global West sells since they are good quality and come in a wide variety of sizes to suit many different setups on the G-body. However, Gulstrand, Herb Adams VSE and Hotchkiss all sell anti-roll bars for G-body applications. If you are only going to run autocross, then you want a car that turns in extremely fast and can rotate rapidly (front toe-out also helps this a lot but can make the car feel "twitchy" to the driver). This lends itself to a very stiff spring, larger diameter anti-roll bar and lower ride height. If you also need to drive on the street also, and want to avoid a lot of road harshness, then you should lower the spring rate and increase the bar diameter. Larger bars do increase road harshness, but not as rapidly as springs do. Larger diameter bars also transfer more weight than stiffer springs, which is a BAD thing in road racing and street circuit racing but more acceptable in autocross. Why is weight transfer a bad thing in road racing? Well, vehicle dynamics tells us that the tractive capacity of any pair of tires is greater when both are equally loaded than when they are unequally loaded (i.e. weight/load transfer). This makes a big difference when the corner speeds increase and when the suspension travel increases. In autocross this is not as noticeable since the speeds are generally lower and the courses are flatter. So, in the road racing application, we would only increase the bar diameter a little bit to help balance the front-rear oversteer/understeer characteristics. In this application, we would also want to increase the spring rate moderately to adequately control wheel motion over bumps at higher speeds. Also, in the road racing application the highest quality shocks available are necessary since the wheel motions and suspension travel are increased and must be damped. As long as the car is balanced front to rear (understeer/oversteer), then it is not as important that the springs be very stiff or the bars be very large (the chassis' torsional stiffness and wheel rates are another story altogether). The higher the speed and the longer the suspension travel then the more critical geometry becomes. For all around performance, a balanced car with moderate changes to the springs, bars, shocks and geometry will tend to be more neutral at the handling limits and have more traction. |
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