HPI Racing - Micro RS4 Building Tips

Remember, these tips are for racers - they may seem to go too far with modifications, but for some R/C hobbyists, too much is never enough!

Rear Axle Spacing
In order to make sure you do not have too much drag in the rear axle assembly, make sure you have enough space between the rear axle housing and the purple hubs! Use a regular piece of paper as shown below to space the hub away from the axle housing and the bearing. The resulting amount of side to side play is all you need - no more, no less!

Mounting a Transponder
Many future Micro RS4 racers have asked how they should go about mounting a transponder. To respond to this problem, we are suggesting that you mount the transponder either on the upper deck or the rear deck with an old pan car racer's method of using a "box" of polycarbonate plastic. This material can be scrap left over from a 1/10 scale car body or from a sheet of plastic you can get at your local shop, and it's very secure and easy to use.

Cut a piece of polycarbonate about 25mm (1") by 125mm (5"). Bend the plastic as shown, making sure the center of the folded plastic is about 40mm or 1.5" across, so it will fit a standard transponder. The edges are highlighted with a marker so you can see them better. Make the short side about 18mm (3/4") and bend the ends so they overlap each other.

Make a mark about 7-10mm (1/4-3/8") from each end with a marker, in the middle of the plastic strip.

Use a reamer or drill to make a single 10mm (3/8") hole centered on each mark.

Attach the transponder by unfolding the plastic and putting the transponder post through the holes in the plastic. Secure a body clip in place. Attach a square of double-sided servo tape to the bottom of the transponder holder and secure it in one of the three locations shown below. Make sure to attach the transponder holder down the centerline of the chassis for equal left-right handling.

In front of the electronics (more steering)

On the rear deck behind the electronics (more rear traction)

In between the ESC and receiver (balanced weight)

This type of transponder holder is cheap, easy to replace and looks better than drilling a hole through the body. Now get out and race!

Suggested Spare Parts
In all the testing we did on about 10 cars during the month leading to the release of the Micro RS4, only one part was damaged, that was a front plastic universal dogbone. We suggest that any potential Micro racers keep a spare #73404 part tree in their Micro kit box/carry case. This part tree has spare axles, bones, servo arms and a steering arm. During the 2001 National Model and Hobby Show in Chicago, four Micro cars were run almost nonstop for more than 4 days by visitors, suffering much abuse and crashing, and only a steering knuckle was damaged. Steering knuckles are on the #73407 part tree, which also includes the lower suspension arm for the front end.

Hide Your Antenna
If all you do is run your car around your house and you want to have a more "scale" appearance, you can conceal your antenna under the body by wrapping the antenna loosely around the body posts, under the body. This presents a more realistic appearance when you're showing off your Micro!

If you are racing your Micro, you should continue to use the antenna pipe, because of the other radios that will be in use during each race.

Make a Micro Track!
We've had many questions about setting up a race track for Micro cars. We've recently added instructions and pictures showing how you can make a track of your own! Check it out here!

There are many types of barriers you can use to make a track. Listed below are a few suggestions, listed in order of least to most cost.

Masking tape - a single roll of masking tape or colored duct tape can work wonders to make an interesting track layout on a driveway, garage floor or living room floor. Use fountain drink cup tops for corner dots and tape them down. Pros: very cheap, disposable, never gets knocked around or rearranged; (real) cars can drive over it with no damage. Cons: racers can cut the track (you can add any of the methods below), peeling up the tape can get you sticky.

Common garden hose - easy to find, just look in your back yard, garage or ask a neighbor for some old garden hose. Use duct tape or fiber strapping tape to extend pieces, create corners or "T" junctions. Tape the hose in place for a more secure layout. Pros: can't cut the track; cheap, usually free; easy to cut and adjust. Cons: heavy to carry around; can be hard to make long straight sections.

PVC pipe - thin 1'' diameter PVC pipe is very durable, light, and cheap if you buy it in 10 foot lengths. You can tape it together with duct tape or use pipe connectors to make "T" junctions, large radius turns and more. Pros: easy on the cars (PVC pipe is flexible); cheap; light. Cons: cars can move the pipe easily (use bricks to hold in place); white PVC shows tire marks pretty quickly.

1" x 2" wood - wood is a universal building material and is pretty durable for R/C racing purposes. Although it's cheap, it is probably the most expensive alternative of those presented here, and it can be difficult to make rounded turns unless you cut the inside of the turns from thin plywood with a jig saw. Pros: durable; nice looking; easy to paint. Cons: heavy; tough to make some track layouts; turns will be tough on cars.

Track size
A track built specifically for the Micro RS4 should have lanes about 1.5m (5') wide and a straightaway from 20-50 feet long. Racers usually like to go full speed for a top speed run for a few seconds each lap, so adjust the length of your track for stock motors or modified motors, depending on what the racing group decides to run. Try to have a "sweeper" turn (long, wide turn) at the end of the straight so drivers have less wrecks, throw in a chicane for a slow section, and include a hairpin or two. These are the basics of making a track!

Rules
Decide before the racing starts what type of batteries and motors will be allowed. We suggest you start with rechargeable Ni-Mh batteries and stock motors. If racers are encouraged to buy new motors and batteries every time one of them upgrades, the racing will soon be priced out of everyone's reach and enthusiasm will drop off. Make the vote either unanimous or majority rule, and allow racers time to get the motor or batteries they just voted in (i.e., take the vote, then allow a week or two for everyone to get the items).

Changing the Wheelbase on your Micro
When looking for new bodies for your Micro RS4, remember that if you change the wheelbase of your car, you must install a new belt as well. If you don't already have a spare belt to change wheelbases, the part number for the 140mm (short) WB belt is #72314 and for the 150mm WB (long) WB is #72316. The wheelbase for each car is noted on the header card of the package and in the Micro RS4 Bodies page.

EXTRA SPEED: Build a 5-Cell Battery Pack!
We'll show you how to build your own battery packs for maximum performance! Longer runtimes are not the only way you'll benefit - with the extra voltage (1.2v more than a 4-cell pack) your Micro RS4 will also be much faster!

Start with 5 Ni-Cad or Ni-Mh batteries that you can get from your local hobby shop. At most shops these are usually sold "loose"; in other words, in singles by themselves. Ni-Cad batteries are cheaper, but Ni-Mh batteries generally have more runtime (look for the Mah rating) and don't have any special handling needs and are a better long-term purchase.You will also need 14 gauge wire, battery braid or battery bars. In our example we are using 14 gauge wire left over from the ESC.

Use a thick tube-based adhesive like Shoe Goo, silicone or an epoxy/resin mixture to attach the batteries together. This is just like building a side-by-side battery pack for a touring car kit. If you think you'll have trouble building the pack, ask a more experienced friend to help you. Make sure the ends of the batteries are line up properly, as shown to the right.

Let the glue or epoxy dry completely. Read the instructions on the glue to find out the curing time.

Now, "tin" the ends of each battery with fresh solder, using a soldering iron at about 315-370 degrees Celcius (600-700 degrees Fahrenheit). The very hot iron will allow the metal contacts of the batteries to heat very quickly, but be very careful to not to leave the iron touching the battery for more than a few seconds. At these temperatures, the rest of the battery can get heated up very quickly and be damaged by the heat. Clean the tip of the iron on a wet sponge, then apply a small amount of solder. Touch the flat part of the iron to the end of the battery for a few seconds, then touch the solder to the end of the battery. Let a small amount of solder flow onto the battery end, then move on to the next battery. Repeat this for all the batteries. The battery pack should now look like the pack at right.

Cut the wire to fit the batteries exactly - you don't want the wires overlapping to other cells! Strip the insulation about 3mm from each end. Use a rubber band wrapped around the handle of a pair of needlenose pliers to hold the short lengths of wire while you tin the ends of each length of wire. Apply the wires one by one to the battery as shown in the picture to the right, then cut the battery plug wire that came with your Micro RS4 kit to fit the battery pack. For the thin gauge wire used for the battery plug, reduce the temperature of the iron to 260 to 290 degrees Celcius (500 to 550 degrees Fahrenheit). This will prevent the wire insulation from melting but still melts the solder.

Your completed pack will look something like this - be sure to note where the positive and negative wires go! If you attach them backwards you may damage your ESC, and it will not be covered under the warranty! You can shorten or lengthen the battery plug wires to fit your particular car's setup, depending on where the ESC is mounted. Again, before plugging in anything, make sure that your positive and negative wires are soldered to the corresponding ends of the battery pack. This is extremely important.

Finally, for the ultimate in efficiency, use a high-efficiency battery plug such as the one shown in this picture. In the US this plug is sold under the "Dean's" brand name, however it is also available under other brands as well. Other high-efficiency plugs are available, see your local hobby shop for their favorite brands. We don't suggest direct-soldering the battery pack to the ESC because you'll need to be able to charge the pack easily and also replace a dead pack with a fresh one.

Use Shoe Goo, epoxy, double-sided tape, hook-and-loop tape or fiber-reinforced tape to attach your new battery pack to the #73409 optional battery mount. Make sure the pack is centered in the chassis (see picture at right) before permanently glueing it in place. It's easiest to center the battery from the bottom, where your eyes are not distracted by the upper deck and any electronics you have already mounted.

Have Extra Battery Packs Available
Purchase extra #73409 part trees for spare battery packs. With only a $4 retail price you can easily use the optional battery mount parts to keep extra batteries on hand so you can keep racing for hours!

See our "Build a 5-Cell Battery Pack!" instructions, above, or purchase rechargeable Nitro car receiver battery packs from your local hobby shop. In the US, rechargeable battery packs are required to have color-coded shrink wrap: yellow for Ni-Cad, green for Ni-Mh. Ni-Cad batteries are cheaper, but but Ni-Mh batteries generally have more runtime (look for the Mah rating) and don't have any special handling needs and are a better long-term purchase.

Direct-Solder the ESC to the Motor
It is unlikely you will be replacing the motor very often; in this case it is recommended that you directly solder the motor wires to the ESC. This helps to "clean up" the chassis of the car by removing excess wire and also helps you lower the weight of the car. After all, on an off-road truck or touring car the extra wire doesn't make that much difference, but on a car the size of the Micro RS4 it really does help!

For Racing, Use Micro-Size Components
In very small cars, excess weight is a far worse problem than in the larger 1/10 scale touring cars or off-road kits. Because you don't need a super strong servo, we suggest using a "micro" servo, offered by all radio component manufacturers for electric planes and gliders. These typically cost just a little more than "standard" servos and are a little faster and a lot lighter.

Most racers may have a spare small receiver to use, however a micro ESC may be hard to justify in a car like the Micro RS4. All you need to do is compare the weight of a typical ESC with non-replaceable wires to the weight of a racing ESC that you can take those 12 gauge wires off and replace with 24 gauge wires!

Shorten the Wires to Lose Weight
Use a good soldering iron or a friend who has one to help you shorten wires on your car. For the thin gauge wire used for the motor and battery plugs, keep the iron at 260 to 290 degrees Celcius (500 to 550 degrees Fahrenheit). This will prevent the wire insulation from melting but still melts the solder. Use a quality 60/40 solder and a little bit of patience.

Keep the Electronics Centered in the Car
This will help with the handling of the car in racing situations. The picture below shows how the electronics should be placed in the car. The rear deck does not necessarily have to be used for mounting the ESC, however if it is, use the antenna mount and rear shock screw to help center the weight of the ESC.

Mount the ESC Close to the Motor
This will help prevent electronic interference from the power going through the motor wires. Because of the Micro RS4 car's size, it is impossible to completely isolate the antenna from the power wires and ESC (as you would be able to do on a 1/10 scale truck or car), however it's best to minimize the contact as much as possible. In the picture above, note how the receiver antenna goes along the side of the upper deck and around one of the upper deck screws to keep it as far away from the ESC as possible.