RC suspension tuning for racing is a fine art of balance. We want stiffness for precise handling around turns, yet we want looseness to soak up irregularities in terrain and maintain traction at all four wheels at all times. Now there's a product from RPM that lets us have both of these qualities at the same time.
These little pieces of plastic have a cunning, yet surprisingly simple design. Installing them requires no additional hardware or parts swaps. They are available in two ranges of dampening, and I bought the firmer set. Within each set, there are enough pistons to convert four shocks to each of three color-coded dual-rate setups.
In a nutshell, these little marvels give you more dampening on the compression (up) stroke and less on the extension (down) stroke. So, when you land a jump, your shocks will be firm and soak up the force of impact. However, when you go over a tiny bump in the road, thrusting a wheel up off of the ground for a split second, the shocks on that wheel suddenly loosen up, letting your springs return the wheel back to the ground quickly, thus restoring traction. It's a novel idea that goes a long way towards improved performance.
To date I have only done limited real-world testing on the Valkyrie, but am definitely convinced that these pistons work as advertised. Especially in turns on a rough, gravel surface, I had a newfound sense of control -- the tires' contact patches seemed to be attracted by an invisible vacuum to every single pebble to allow the best use of the truck's all-wheel drive, and the body stayed very level and calm.
I will edit and update this review as the Valkyrie project matures and I get more running time into it.
Specs & Price
Listing at under $7 per set of three piston ratings, with enough at each rating to convert four shocks, this product is a steal. It gives you a direct replacement for stock pistons and requires nothing but simple tools to install.
I feel this is a very worthwhile investment and I will likely try these pistons on all of my off-road vehicles. The theory alone behind dual-rate shock action is very compelling, and from what I've seen so far, the real-world results do justice to the concept. My only complaint is that the packaging says, "fits Losi, Assoc., & Trax. [Traxxas] shocks," yet I was not able to fit the pistons into a brand new set of Traxxas Big-Bore aluminum shocks without modification. Read on to learn how I got around this...
Special Assignment: RPM pistions in Traxxas Big Bore shocks
I tried casually popping these things into a brand new Big-Bore shock and I'm sorry, but they just don't fit. There are a couple of things you need to do:
|Start off by disassembling your shock to pull out the shaft & piston. Remove the piston by taking off the e-clip beneath it and sliding it off.||Do not do this yet! If you were following your instruction sheet, you would now push the two halves of the RPM piston up the shaft and secure them with the lower e-clip.|
|However, at this point you would notice something screwy. The upper e-clip is too large in outer diameter to fit down into the groove that's carved out for it in the upper piston half. This will keep the piston from moving as designed.||So, file that e-clip down to size! Try to keep it round in shape and don't build up a burred edge. It won't take much trimming to get it down to size.|
|This is how the e-clip should fit into the upper piston half. It should be plenty loose at this point -- you don't want it to bind in this position.|
|Now you can go ahead and assemble the two piston halves and secure them with the lower e-clip. When holding the piston up, the two halves will fit together flat and only two sets of holes line up.||Turn it over and you mimic what happens when your suspension is on the down stroke. The halves separate and all four holes are exposed. Beautiful! But it still won't fit! Read on...|
The pistons are still a bit large in diameter to fit into the Big-Bore shock bodies, so at this point you will want to very lightly sand them down around the perimeter with some very fine emery paper (600 or 800 grit). I did this by holding the shock shaft with one hand as close to parallel as possible to a sheet of emery paper laid out on a table and using the index finger of the other hand to roll the piston as I slid it across the surface. Clean it off and test it against your shock body very regularly to be sure that you don't go overboard. It should have a snug fit, but if it binds at all, you need to take off just a tad more material. When you feel you have it just right, wash it out well with your choice of cleaning liquid to get the fine plastic powder out of the holes and from between the piston halves. Let it all air-dry, and you're ready to finish reassembling and installing your new & improved shock. Mount a spring on it and play with it a little, compressing it and letting it go quickly. The dual-rate action is very obvious. Like it? Go run it!