Folks in the Mid Atlantic states who are fortunate enough to race in the Hobby Works Parking Lot Racing Series and Mid-Atlantic regional TCS (Tamiya Championship Series) Race are blessed with fairly smooth asphalt surfaces. The primo parking lot venues in Laurel and Rockville, Maryland make it a little easier to set up the suspension on a mini or touring car, because ground clearance and suspension travel aren’t much of a concern. But even without pavement heaves, bumpy seams, or dips in the middle of the front straight, a poorly built set of shock absorbers will make it impossible to coax consistent handling from even the most professional pavement weapon.

Since so many of our readers use Tamiya equipment, their shocks are used as examples for this article. However, most of the issues covered here apply equally to any brand or type of oil-filled shock absorber. And while TL01 class participants are excused because their rules mandate stock coil-over shocks that don’t benefit from oil dampening, everyone else should be using oil filled shocks on their ride. Sure, you can get away for a while with using the nail-in-a-rubber-tube spring holders that came with your M03 or FF02, but the sooner you replace them with a set of proper touring car shocks or low-friction aluminum dampers, the better off you’ll be. The non-dampened versions are OK at first, but as soon as you learn how to really drive, you’ll find that the car changes so much from turn to turn that you lose confidence in your ride. And as we all know, that’s a bad thing!

So, How Do These Things Work?

Shock absorbers are really pretty simple devices. They use a coil spring to provide resistance to bumps and surface imperfections; the spring is mounted on a short telescoping tube that’s fastened to the chassis at the shock tower and the suspension arm part way between the lower hinge pins. The location on the suspension arm and the angle of the shock relative to its upper mount determine how much leverage is applied to the assembly when it’s compressed. Because it’s attached with bushings or shoulder screws, the shock assembly can rotate somewhat in a single plane without binding as the suspension rises and falls. Oil dampened shocks take this one step further by slowing the telescoping action, so that the spring is prevented from either compressing too quickly or oscillating uncontrollably on the rebound stroke. This is accomplished by closing off each end of the tube and filling it with a viscous fluid. A small piston fits tightly within the tube and is attached to the shock shaft, which passes through a series of o-rings and spacers that allow it to move freely while keeping the fluid within the tube. 

The resistance of the shock shaft to vertical movement is dictated by the thickness of the fluid within the tube, and the number and size of the holes in the piston. That’s because the piston cycles up and down through the fluid, which has to flow through the holes as the piston passes. Make the fluid thinner or the holes larger, and resistance is reduced. Change the piston to one with smaller or fewer holes, or fill the shock with a thicker fluid, and resistance is increased. Generally speaking, tuners seek to match the resistance of the fluid and the firmness of the shock spring. A firm spring and high resistance are usually chosen when the surface is fairly smooth and has good traction, while a softer spring and less resistance are appropriate for bumpy or low-traction parking lots.

We Don’t Need No Stinkin’ Stiction

The biggest problem encountered with many (including Tamiya) shock absorbers is stiction. It’s caused by a grip on the shock shaft that’s too tight, so that the shaft can’t move up and down freely. This balance between sealing the shock body to contain the fluid-and allowing sufficient freedom of movement for the shock shaft-is difficult to maintain. However, there are a few things you can do to manage the relationship between seal and grip:

· The shock shaft must be as shiny and snag-free as possible where it passes through the o-rings. The best way to accomplish this is to place the shaft in a drill or Dremel tool, turn it at a moderate speed, and polish the shaft with a good quality metal polish like Mother’s or Happich Simichrome and a soft cotton cloth. 

· Reduce pressure on the shaft by making the center spacer smaller. Older style Tamiya shocks used a spacer and two o-rings to seal the shaft passage, and this spacer squeezes the rings tightly against the shaft. Take a millimeter or so off of the spacer, and there will be less pressure on the shaft.

· Newer Tamiya shocks (black, clear, or aluminum ones) use two o-rings with no spacer in a captured compartment at the bottom of the shock. The best way to reduce shaft grip with these shocks is to sand a little bit off of the circumference of the o-rings. We used to simply cut the rubber rings in half cross-wise, but sanding is both easier and safer. Hold a dry o-ring in your fingers, and using a sanding drum mounted in a Dremel tool set at low speed, just barely graze the side of the o-ring a little at a time around its entire outside edge. Less grip is produced when the ring is slightly smaller, and the shaft will slide more easily.

This o ring will grip the shaft too tightly.

Lightly sand the outer edge of the o ring with a Dremel tool.

Now the ring is slightly smaller and will allow the shaft to move freely.

· Pack the o-ring compartment with thick grease like Trinity’s Purple Stuff or Associated Green Slime. This ensures silky movement, yet keeps thin shock fluids from passing the o-ring barrier.

Top O’ the Shock

Now that you’ve got the shaft cycling smoothly, it’s time to pay a little attention to the upper end of the shock assembly. Too often, tuners simply fill the shock with oil, twist on the cap, and ignore a few little steps that keep the dampening consistent:

· Use an upper bladder. These little green doodads seal the oil in the shock body from any air that might enter the shock, and ensure that the oil passing through the piston holes will be free of teeny little air bubbles. 

· Bleed the shock properly. Fill the shock body to the top, move the shaft up and down gently a few times to get any air bubbles out of the area below the piston, and slide the bladder on top of the shock body before you cap it. Yes, you’ll get some shock fluid on your fingers, but that’s OK. Just use a piece of paper towel to clean up the mess.

· Put a small hole in the shock cap. This ensures that the shock will not pressurize when you screw it all together. I use a 1/32 drill bit to accomplish this, but an X-Acto blade tip will do a pretty good job, too. 

Pistons and Spacers

Every Tamiya shock set, regardless of design, comes with a selection of pistons. There are good reasons for that. For one thing, front and rear shocks frequently wind up using different pistons. I often set up my cars with two or three hole pistons in the back for traction, and one or two hole pistons in the front to take out a little bit of steering. Combined with the proper shock fluid (usually thirty weight out back and fifty weight up front) and springs (blue fronts and red or yellow rears), this results in a stable, solid car that goes where you point it without being too darty.

Ride height is usually set by reducing the length of the shock absorbers. Ask ten racers what they use as shock spacers, and you’re likely to get ten different answers! Just about anything that fits on the shaft will work, but I’d suggest that you stay away from o-rings or lengths of fuel tubing (too soft, tend to squish around the piston or other spacers) or pieces of metal tubing cut to size (too narrow, scars the piston, can lodge on the shaft and cause havoc in the o-ring compartment). The best spacers seem to be either the ones Tamiya includes with the shocks as original equipment, or those manufactured by Associated Electrics or RPM Products. No matter what you use for spacers, make sure that you place the same number and size on each of the front or rear shocks, and that you mark them somehow so you know which ones go on which end. 

Tamiya shocks come with a selection of spacers. Small changes in shock length can be made by using metal washers.

You should always measure the finished length of shocks when they’ve been completed. It’s easy to miscount and put an extra small spacer or washer inside one shock. That makes it longer than it’s partner on the opposite side and results in bizarre handling problems that are sometimes difficult to diagnose. Front and rear shocks are frequently different lengths, but your shocks must be the same length from side to side.

Make sure the bottom eyelets are securely fastened to the shock shafts. More than one race has been lost because the shaft pulled completely out of the mounting eyelet! I check these every couple of races, especially if I’ve had an accident or two. There’s nothing you can do to permanently repair an eyelet that’s become elongated or stripped, so when one goes bad just replace it.

If you’re still using the original shoulder screws that came with your kit, consider changing over to bushings or ball links. Tamiya sells an excellent conversion kit for this, and it really tightens up the suspension. Ball links on the bottom end give the shocks just as much freedom of movement, but are much more precise. Yanking the shoulder screws and replacing them with bushings and machine screws will result in a tighter, more precise suspension.

You can do even better by placing a small piece of electrical shrink tubing over the bushing. This further reduces play in the upper shock mount and enables the savvy tuner to make very small, very precise adjustments that can make the difference between just making the A Main and leading the pack home across the finish line.

Preload? What’s Preload?

Preload is frequently misunderstood. It’s very easy to simply slam the shocks on the car and insert a whole bunch of preload spacers between the shock collar and the spring, but it’s not really that easy. To set the shock length properly, add thin spacers until the shock just reaches maximum extension. I do this with the shocks dismounted, just to make sure that I’m not putting too much preload on the car. After mounting the shocks on the chassis, I check to front-to rear ride height and also how much the shocks droop when the chassis is placed on the ground with a battery pack and motor in the car. You want the shocks to compress just a little bit, but no more than a millimeter or two. This ensures that the suspension is not so packed with spring rate that it won’t move when it hits a small bump. The chassis should be level when fully loaded, or slightly higher in the rear than in the front. If the back end is lower than the front end, all sorts of handling weirdness will result.

So, as you can see, it’s not so difficult to do a good job when assembling and adjusting the shocks on your car. It just takes time, patience, and a little bit of thinking in order to get your ride to work the way you want it to. But if this were easy, it wouldn’t be very much fun, would it?