Tight is Just as Bad as Sloppy

Even the best kits are manufactured to certain tolerances, so you should never expect that the suspension on a new chassis will just slide together without any massaging.  One side may be a little loose, and one corner just a little tight.  The ideal situation is to have each corner of the suspension drop easily under its own weight when fully assembled but without any shock absorbers connected to the arms (we’ll get to the shocks in a few moments).  Too much play in the suspension will result in your tuning changes getting lost in wiggle and shake.  A suspension that’s too tight will bind and mask the subtle changes required to guide your ride into the perfect setup.

Whether the suspension is loose or tight, here are a couple of tricky little fixes that you can use to get things perfect:

• If the arms or suspension mounts are a little tight on the hinge pins, use a barrel reamer of the same diameter as the pin to make the holes as straight and clean as possible.  These are inexpensive and available from better tool distributors like Midwest America for fewer than ten bucks apiece.  They’re very sharp and only cut on a horizontal axis, which makes them better for the job than a drill bit.  Get one in both 1/8 inch and 3mm, and you’ll be set for just about any kit on the market.  Those of you who are really into tools (you know who you are) should pick up a ratcheting t-handle for the reamer.  These give you a better purchase on the shaft and make it easier to turn the reamer (they also do a great job on thread cutters).  Shade tree mechanics will find that a turnbuckle wrench of the correct size does an acceptable job.

 " A barrel reamer like this will make the hinge pin guides precisely the correct size. Note the T-handle that makes it easier to turn the reamer."

• Arms or suspension mounts with pin guides that are too large and loose present a different problem, but this fix will work whether your arms are new or old.  Start by cleaning out the pin holes with a pipe cleaner and some rubbing alcohol.  It’s important that they are free of any dirt, lubricant, or mold release agents.  Next take a rag or paper towel and rub the hinge pins with some silicone lube or shock fluid; use just enough that they’re coated with a thin film and are a little slippery.  Dribble two or three drops of medium-thickness CA into the pin guide of the arm or mount, and slide the pin carefully into place.  DO NOT wiggle the pin back and forth, just slide it into the hole and lay it down on the bench for an hour until the CA has completely set.  The thin film of silicone on the shaft will keep the glue from adhering to it, and allows easy removal from the arm or mount after the CA has cured.  The pin guide will now be precisely the same diameter as the hinge pin, and the slop will be gone.  You may have to repeat this process every couple of weeks, as CA isn’t really designed to take the whacks and pressure of suspension movement, but it’s a simple and easy way to get the pin guides perfect.

• Maybe the pin’s a perfect fit in the arm and mount, but the arm itself slides back and forth on the mount or bulkhead.  This one’s easy to solve: pick up some thin motor armature shims from Hobby Works.  Trinity makes a package in various thicknesses to fit any space, and the shims can be stacked and sorted until they’re just right.  If you find that one combination is just a little too short, but adding one more spacer binds up the suspension movement, make a thinner spacer by placing it under a pencil eraser and rubbing it against some 100-300 grit sandpaper until it’s perfect.  Neat!  It’s important to stack all of the spacers in the same place on each side of the car.  If you put ‘em all on the rear side of the arm on one side, do the same on the other.  That keeps the drive axles in the same plane relative to each other, so that the wheelbase is identical from side to side.

"1/8 inch motor shims in various thicknesses will help you to fit the arm into the mount without any slop."

• Now the arms are juuuuuust right, but the ball ends are either loose or tight.  No problemo.  Loose ends are caused by a nylon ball cup that’s too large for the ball end, so all we have to do is take up that extra space with something that won’t also make the cup bind on the ball.  You can try the same CA trick we did with the suspension arm, but that can be hard to execute (when you snap the cup on the ball, the CA tends to splash out). Some folks use oversize ball ends, and that works if you have the patience to re-size them a little bit at a time with some sandpaper and a Dremel tool.  I prefer a layer or two of thin plastic laid over the ball.  If you just need to take up a teeny bit of slack, plastic food wrap (like Saran or Glad wrap) does a great job.  Sandwich bags are usually a few mils thicker, and freezer bags are thicker still.  You can cut a piece of each several inches on a side, stick it in your toolbox, and have enough to make dozens of ball ends fit just perfectly.  Another alternative is to locate some Robinson Racing adjustable ball cups.  These have a small boss molded into them for a 4-40 setscrew, and a special eight-point cup edge.  When installed, simply tighten the setscrew a little at a time until it takes up the slop.  These work perfectly for installations where the link one works in a single active plane (like camber links), but not so well on multiple active plane installations like steering linkage, where the setscrew drags across the top of the ball every time it moves.

"This Robinson nylon ball cup has a small set screw that makes the fit on the metal ball very precise."

Shock Trauma

Maybe you’re certain that the shocks on the front and back of your new ride have been assembled in pairs exactly the same, but I’ve been around enough to know that that’s seldom the case.  I always put them together as best I can, then go out and pound a few sets of batteries into the car before starting to mess with the shocks again.  That allows the springs and nylon bottom ends to stretch and settle, so they’ll be dimensionally stable and ready for proper setup.   So, having worked with your new car for a couple of practice sessions, it’s time to pull the shocks apart for a rebuild. The seals and guides will most likely be in great shape, so we won’t discuss them here.  If the seals leak, or the guides are sloppy, grab the manual and do a full rebuild so they work properly.  Here’s a checklist to follow:

• Are the pistons exactly the same in both front and both rear shocks?  Many tuners use different pistons in the front and the back, along with different shock fluids.  That’s OK (hey, I do that myself!), but it’s really easy to mix them up, especially if you custom-drill the holes in the pistons with a pin vise.  One easy way to make sure you use the correct piston is to mark them in some way so that you can tell them apart.  On black colored pistons (like the ones that come with Tamiya kits) I use a series of small scratches on the top or bottom.  All two-scratch and three-scratch pistons are the same, and I keep notes in my racing notebook to tell me what the codes mean.  It’s easier with the white or pastel colored pistons used by other manufacturers.  Use a fine point Sharpie to write the diameter of the piston hole (those teeny drill bits that come with Trinity’s Valve-o-Matic work well to measure the diameter) on the top or bottom.  Simple!

"A pin vise with a selection of teeny bits give enables you to make the piston holes exactly as large as you want them."

• Are the spacers you placed under the piston to adjust shock length precisely the same from side to side?  If they’re molded plastic or nylon, probably not.  Use a good set of digital or Vernier calipers to measure spacer thickness, and sand the one that’s slightly thicker until they’re both exactly the same.  Make small adjustments to shock length by placing more of those thin motor shims (like the ones that that you used on the hinge pins) between the piston and the spacer.  Don’t put the between the spacer and the bottom of the shock body, as they can be difficult to find and remove when they’re stuck down there.

• Overall shock length is absolutely critical on the latest generation of sedans, mostly because the chassis are so stiff.  Back in the old days, shock length wasn’t a big deal because the chassis itself flexed both front to back and side to side.  Today’s top o’ the line double-deck chassis are much stiffer, so even a millimeter or two difference in shock length from side to side will evidence itself in odd handling: the car will turn much more easily in one direction than the other, or will slide around instead of tracking when turning.  If you find that your car tends to pull strongly to one side when accelerating hard out of a turn or at the starting line, there’s a good chance that shock length is to blame (or the differentials, but that’s a whole ‘nother article).

• Spring length is equally critical, and I know many racers who purchase several sets of identical springs.  That’s so that they can measure the unladen length of each spring and match them by length in pairs.  If you find yourself having to use a pair of springs with slightly different lengths, simply add the difference in length to the preload on the shock using the shorter spring.  Threaded shock bodies make this easier, but even cheap shocks come with adjustment shims in several thicknesses.  I’ve seen simple spring matching devices that are supposed to measure the relative strength of coil springs, but this seems a little unnecessary to me unless you’re concerned about the quality of your manufacturer’s springs.

• It’s not that difficult to ensure that the finished length of your shocks is identical from side to side.  Trinity makes a nifty Shock Dyno that includes a measuring scale with a pin at either end. You can make your own measuring tool by drilling holes in a popsicle stick that are the same distance apart as the desired shock length.  Stick a 4-40 bolt up through each hole and secure it with a nut.  To check for proper length, simply try to insert the screws into the upper and lower shock mount holes.  Simple! No matter what technique you use, remember that the upper end of the shock body and the bottom of the ball end or nylon cup will vary slightly from part to part.  Measure the shock assembly from mounting hole to mounting hole, rather than end to end.  

• Do you really know what kind of fluid is in those shocks?  Today’s silicone fluids are remarkably stable and available in many viscosities, but it can be hard to keep track of which one you’ve put in the body.  This is where your racing notebook comes in handy, as your notes help you track various combinations of spring, fluid, shock length, and track conditions.  A notebook helps you remember what fluid you put in those shocks the last time, as well as what worked for you the last time you faced a particular combination of temperature and track surface.  If you’re just not the notebook type, use a fine point Sharpie to write the piston/fluid combination on the outside of the shock body.  You can easily remove the markings with motor spray or lighter fluid later on; meanwhile you can see at a glance what you’ve got in your shocks.

Well, that should be enough to occupy you for awhile.  In the next installment, we’ll cover interesting topics like setup boards, droop, rebound, and preload.  Until then, work on squaring up the suspension of your car.  You’ll be glad you did!

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