- Yokomo MR-4 BX - Global advice from a club racer -

 

Got no idea how many of you have an MR4BX, but over an event recently, some guys were pretty keen to know how I got my BX to handle on a fairly rutty track, red dirt , and why I wasn't continuously rebuilding diffs. I'm not going to say I'm a ledge driver, or my BX was totally dialled, but I do know it was pretty easy to drive, and reliable enough for three runs .

Basically the car was setup so that it went through bumps straight on the straight and without kicking the rear out in the corners.

With a standard setup to start, the following was changed:

  • Yellow yokomo springs front and rear, 25wt front oil, 20 wt rear. Soft rear helped through the bumps etc, softer front actually stopped the front from catching as bad as the stock front when going through corners, which was kicking the back out.

  • Rear shock on inside hole on tower, settles the car in bumpy corners.

  • Removed the shims under front hingepin mounts at the back of the car - helped go through bumps better on power.

  • 2 degree front toe in - helps go straight through bumps

  • 2 degree front and rear camber - more stable through the bumps

  • Curved rear wing trimmed down to the first line - as in highest lip other than full, helps rear traction keeping rear down on straight, and slightly on cornering

  • Front belt tension on the line closest to the centre of the car - on an old belt

  • Rear belt tension on the first step towards the back from the middle of the adjustment, I think it is the 5th notch backwards, with a new rear belt.

  • Slipper reasonably loose, actually quite slow off the line, good enough to get to the corner first, but fine when rolling, helps allow use of loose belt tension and loose diffs.

  • Losi IFMAR Studs silver front, slightly worn Losi Step pins in the rear.
    They were on 5 spoke rims, no idea if that helped (Flex or lack of, no idea)

  • Front diff tighter than rear.

Beware, this setup jumps terribly so may not work if you have triples etc to contend with, all I will say is the 'slap' from landing a 50 cm kicker is pronounced. But it drives well on a damp, bumpy red dirt track. The more you can down side, the better.

And as for reliability, you guys probably do this anyway, but this is what I do/have done.

  • Rebuild the diffs before each day as a precaution, keep the bearings clean too, they sieze pretty easily.

  • Trim the undertray as high as possible, buy some self adhesive window/door foam strip (soft stuff is best) and stick it around the top of the tray above the velcro. Also apply velcro all the way around the undertray.

  • Cut the body as low as possible also.

  • Trim the top lexan cover of the rear diff as long as possible so it sticks into the car under the bodyshell. Stick some of that foam around the belt where it goes through the front of the lexan cover, and on the graphite top plate under the belt. It can be trimmed, or the belt will eventually wear it away so it is nice and snug. Also leave the sides of the lexan top cover as large as possible to fit the top plate and use the four screws to hold it down.

  • Carefully trim a piece of lexan to fit the gap in front of the rear diff between the belts, and as much around it as possible. You can go as close as a couple of mm to the belt as it is pretty close to the pulley and won't move too much up and down.

  • Tape the gaps around the concentric housings for the rear diff under the top plate, kinda like an upside down 'U' so it goes around the diff outdrives and cover the gap across the top of the housing.

  • Use a fine mesh and superglue (careful, it will melt the paint if applied and moved around too much) it to the inside of the bodyshell around the cooling vents for the motor. I used the mesh from a clothes dryer lint collector on a front loading dryer. They usually come in a round disk about 30 cms across, the mesh is a fine as you'll find anywhere, and it will keep the air going through and all but the finest dust out. Any better sources I'm listening.

  • Form a L shaped piece of lexan or use tape to cover up the top gap above the front diff when looking down and the two holes in the front shock tower that are actually under the line of th bodyshell. Dirt will get straight in here.

  • Put some foam across the top of the front shock tower where the top of the lexan diff cover meets it.

  • Put a piece of lexan across the back of the rear diff housings.

  • I also mount the under tray at the back by having an L shaped piece go up and back towards the front, then screw the top of this into the rear hinge pin block holes. I then put some of the foam on the undertray floor all around where the graphite for the rear chassis plate is (avoiding the gap where it might foul with the diff). I also put a strip across on the under tray floor where the under tray goes under the front diff/hinge pin housing and where the lip on the sides starts, this stops dirt getting in when the front end bottoms out.

  • And finally, I slipped a piece of fuel tubing over the aerial and fitted it so it was snug just where the body rests when mounted so it fills the hole in the body.

I wonder if the Tamiya diff covers will fit somehow??

It is pretty involved, but hey, I hate rebuilding diffs at the track, so I did everything I could think of to prevent it.

Sure, it may weigh some extra, but especialy club racing, it makes this car far more manageable.