Buying a Used Yokomo MR4
Buying a used Yokomo MR4 Electric Rally Car, or any used RC Model, has a number of advantages. It is generally cheaper than new, ready built and may come with a variety of expensive hop-ups already installed. Cheap, pre-loved bargains are always becoming available. However, depending on the age of your purchase, it may need a little tender loving care before you can take it out on the road.
The one thing you will always need is an instruction manual. If not supplied with your purchase they can often be downloaded from the Yokomo website, or purchased separately on eBay. With an instruction manual, any problems with your model Rally Car you may discover can easily be fixed.
When you receive your used Yokomo Rally Car, make a general visual inspection of the chassis, front and rear wishbones, suspension shock towers etc, for anything broken that may need to be replaced. Then, take a screwdriver and box spanner and check each self tapping screw and nut for security, taking care not to over tighten. Next, for those Yokomo models with oil filled shock absorbers, remove them from the chassis and dismantle the coil springs. The damper shafts should push in and pull out with a smooth action. If you feel a jolt as you change direction, this means the oil has leaked out and must be topped up. At the same time, change the O-Ring seals to prevent more leakage. Also check the damper shafts for damage. If they are scratched, change them as soon as possible.
If the body shell of your Yokomo MR4 is broken, ripped or damaged in any way, this can be easily repaired with rubber solution glue. Also, for added protection and if available for your MR4 model, fit an under guard to stop dirt and gravel entering the chassis.
Examine the drive shafts for wear and replaced as required. If possible, change them for titanium. The steel shafts wear and bend too easily.
If you intend to race your MR4 Rally Car model at a competitive level, I would also recommend you obtain and fit titanium pivot shafts, turnbuckles, tie rods and steering rods.
The gearbox of your used Rally Car should be opened up to check for gear wear and lubrication. A thin coat of grease is often used on internal gears and although this is fine for basic running around on the road, if you intend to race your Rally Car at a higher level, this should be removed and replaced with racing oil (ZX1 or Teflon Oil). Of course, this should be reapplied after each race meeting.
Gears are a weakness on all Rally Car RC models. Head on collisions can easily damage the gear teeth on nylon and plastic spur gears. Heavy impacts can also loosen the nuts or self tapping screws that hold the Electric Motor in Position, allowing the pinion gear to pull out of mesh slightly and rip the tops off the teeth on your spur gear. To minimise this possibility, fit bolts with locking nuts to the Electric Motor mount and remember to check them for security after every two or three runs.
Ball joints always cause problems. For top level Electric Rally Car racing, the plastic ball connectors should be checked and if deemed necessary changed after every meeting. A simple thing like a loose fitting connector popping off could easily end your race, so better safe than sorry.
The MR4 steering servo is also prone to damage. In high speed crash situations, the fragile gear teeth of the servo can be broken off, rendering your expensive servo useless, so be sure to obtain a good quality "Servo Saver". Check out my Servo Information article.
If body roll on your Yokomo MR4 is a problem, handling can be improved with the use of stabilizers, anti roll or sway bars, stiffer tuning springs and, or, thicker silicone oil in the dampers.
If your used Yokomo Rally Car comes with plastic and sintered brass ring type bearings, check the shafts that run in them for wear. Dust and grit can get into these bearings and abrade the shafts. Therefore, you should replace them all with shielded ball bearings. If the model has been run with ring type bearings, you may have to change all the axles and driveshafts. For more information, take a look at my article, How to get the best from your Bearings.
Finally, good luck with your MR4 model and good racing.
For More on how to Setup your Rally Car, check out my Hints and Tips page.