Do you have an 1/8-scale buggy that is no longer a competitive racer? Or maybe a buggy you bought just for play but are ready to modify for a change of pace? Here's my advice: monsterize it! Adding big monster-truck tires and a tough-looking flatbed body will transform your buggy from a 4-wheel spaceship-looking thing into a something from TNN's "Monster Jam." But there's more to the conversion than big tires and a body; using a Kyosho Inferno MP-6 Sports as a starting point, I'll show you everything you need to know to go from buggy to monster truck (you can use my tips with other buggies, too).
TIRES AND WHEELS
Before you choose rims for your new truck, it's important to know the size of your buggy's wheel hexes. The Kyosho MP-6 and earlier Kyosho buggies have a large, 19mm drive hex; all other brands use 17mm hexes. The Kyosho Nitro USA-1 model has the only rim that fits the 19mm drive hexes, so that's the one I used for my project. For 17mm hexes, you can choose designs from OFNA or DuraTrax. Be aware that you may need to enlarge the axle opening of some wheels to fit certain trucks.
If you don't already have your heart set on a certain tire design, compare a few and go with whichever is lightest; they'll accelerate more easily and will put less strain on the engine and drive train. I used Kyosho Nitro USA-1 tires on my truck; they are lightweight and have a very aggressive tread pattern. For more information on which tires fit which rims and which rims fit which vehicles, check out the September 2000 edition of RC Car Action's "4X4."
Larger tires dramatically raise the effective gear ratio of your buggy. Check your car's parts list to see whether the manufacturer offers optional spur gears and clutch bells for it. If it does, install the largest spur gear offered, and use the smallest clutch bell you can (just be sure the chassis' enginemounting slots are long enough for the gears to mesh properly).
Spend the extra cash for a steel spur gear, if it is available for your car. I used an aftermarket Kyosho steel main gear for my MP-6.
All three differentials will also need some attention.
The goal is to slow down the diff movement to protect the cliffs from heat and wear. If you have sealed differentials in your buggy, fill the center diff with the thickest diff fluid you can find; fill the front diff with 7,000WT and the rear with 10,000WT fluid. Mark the differentials just after you fill them so that you remember which fluid belongs where. If your differentials are not sealed, use thick grease in them instead of silicone. The silicone will seep out of a non-sealed unit and create a big mess.
You can also try eliminating the center diff. Having no center differential cuts down on the truck's steering capabilities, but equal power will go to all four wheels at all times. Nuova Faor makes a differential by-pass unit for Mugen, Kyosho and Thunder Tiger cars. This unit is a direct replacement for the center diff and will ensure that the front and rear tires get equal power. I highly recommend it for a monster truck. You can also try a Torsen-style diff from Nuova Faor, Fioroni and OFNA. It works just like a regular differential, but when one end of the diff has pressure on it, the unit locks up. I just happened to have a Torsen diff from OFNA, so I installed it in my truck.
Unless you run your truck on the Bonneville Salt Flats, braking is very important. If you use a standard servo, or something with a torque rating close to that, it's a good idea to upgrade to a good midrange servo with at least 70 oz.-in. of torque. A Hitec 645MG servo pulled the reins on my monster. Another way to improve your truck's braking is to add an extra brake disc-- an easy modification. Add the extra disc to the front end of the center diff. All you need is an extra brake disc and one brake pad.
If your buggy is equipped with dogbones, replace them with universals. Dogbones work fine, but they have a tendency to pop out. Universal drive shafts prevent this from happening. Most companies offer universals for the vehicles they make. MIP and OFNA offer universals for just about any 1/8-scale buggy on the market. I used OFNA rebuildable CVAs on my converted truck.
If you don't already have an engine in your buggy, go for a pull-start unit. A bump-start mill will work, but it's a pain to line up a monster truck on a starter box with four big tires in the way. A pull-start engine is just a lot more convenient. I used an old, non-pull-start Novarossi racing engine that I had retired from racing. It was just lying around, so in it went.
A monster truck tends to have much more body roll than a buggy because the tall tires raise the chassis relative to the tires' contact patch, and this can cause the inside tires to unload during a turn. Swaybars reduce body roll and keep the tires firmly planted during high-speed turns, but they limit the independent suspension travel needed to climb over obstacles-not a problem, as swaybars can be quickly disconnected. Aftermarket swaybars from Kyosho kept my truck in check.
Most diehard 1/8scale racers probably already have a hightorque steering servo for their buggy; good, because they'll need it. If you don't have a high-torque servo, I strongly suggest you invest in one. Monster-truck tires are not easy to turn, especially at high speeds. A servo with 100 oz.-in. of torque will do, but more is always better. To make the most of your servo, it must receive a full 6 volts from a 5-cell Ni-Cd receiver pack or 4 dry-cell alkalines. Never use only 4 Ni-Cds; you'll have only 4.8 volts, and servo performance will suffer. I used a new Hitec HS5645MG Digital servo to turn the front tires on my monster. Its claimed torque output is 106.9 oz.-in. at 4.8 volts. Good enough for me!
Most 1/8-scale buggies are equipped with plastic dirt shields. When you mount the large monster truck tires, they will interfere with the dirt shields. If you don't care about getting dirt inside your chassis, remove the dirt shields. If you prefer the added protection, just trim the ends of the stock shields until the tires don't touch them. I would have had to remove too much material on my MP-6 dirt shields to achieve clearance, so I left them off.
On 1/8-scale buggies, only two body mounts hold the body on the car, and they are very close to the chassis. Monster-truck bodies ride high on buggy chassis to clear the tires and to provide that monster-truck look. You will probably have to make your own body mounts for your newly converted truck. The easiest way to make these is to attach a 1/2X1/2-inch-piece of aluminum angle to the top of the shock towers; or, use the rear wing mounts, as I did. Once that is done, you can drill holes in the aluminum and use aftermarket body mounts such as those offered by Parma.
Before beating on my newly converted monster truck, I drove it around at slow speed on the dirt track, making tight turns to warm up the engine and break in the center diff. Once everything was OK, I mashed the throttle and had some fun. The center diff certainly did its job and prevented the front wheels from spinning when I opened the carb on the big engine. It was exciting to pitch the truck through turns at full power; all four tires were pulling it through, and I was able to get a little of that cool 4-wheel drift action going on. The gearing was a little on the high side, so I will gear it down a little to give it better punch off the line. The Hitec servo had a commanding hold on the front wheels and kept them pointed in the right direction. Braking wasn't as crisp as I had hoped, so I will add another disc to the center cliff to help slow the big truck down.
As you can see, it is not difficult to convert an 1/8-scale buggy into a monster truck; it just takes a little beefing up and tweaking here and there. You will definitely have more fun with your new ride, and it's sure to turn some heads.
Copyright Air Age Publishing Sep 2001
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