Mini 4WD Madness! Part 2: AR Chassis Bumperless Conversion

Edit: This post was written some time ago and after some more research I’ve decided to overhaul this post.

The advantages of a bumperless setup are:

  • Possible weight savings and lower centre of gravity
  • Increased setup possibilities such as lower front rollers
  • Reduced distance between front and rear rollers
  • Improved adaptability with bodies that otherwise would get in the way if you wanted to install a sliding damper for instance.

However, it also has the following demerits:

  • Considerable effort required
  • Front rollers will have upper thrust without further work
  • Possible reduction in strength especially if using FRP plates
  • Increased maintenance demands to maintain the thrust angle of the front rollers

You’ll have to cut up the front of your chassis to leave behind just two mounting holes.

2016-04-24 17.19.46

The front base plate will be mounted to the chassis at the two remaining screw holes.

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As the underside is tapered upwards, the base plate will have an upper thrust. Without further modification, there is an increased risk of the machine going off course.

2016-04-24 21.22.25

To fix this, you can either use adjuster plates or cut up some unused polycarbonate from a clear body set to wedge in between the base plate and the roller mounting plate.

2016-04-24 21.46.08


The original post will be left below.

In this post I’ll describe how I converted my AR Chassis to a bumperless setup. Please do not take my word as absolute truth as there are countless methods to achieving this and they have their own advantages. The advantages of doing this are the possible weight savings and increased setup possibilities including and not limited to allowing the use of FRP/carbon fibre plates as underguards to reduce the risk of the machine getting caught on the track walls when landing. The parts I used were listed previously.

2016-03-02 19.18.23

This pic shows the differences between the multi roller setting stay (top) and the X Chassis rear roller stay (middle). Due to perspective the FRP reinforcing plate on the bottom appears longer than it should, but it’s as wide as the X Chassis stay and the multi roller setting stay is wider than both. If you are using rollers smaller than 19mm, mounting them on the straight plate above will result in a machine narrower than the 105mm width limit. A narrow machine would have a lot of space to move sideways in the track, slowing it down as it effectively travels a longer, zig-zag line.

IMG_7107 - Copy

The front bumper of the car was removed to leave six mounting holes on the chassis. As a guide, the bumper was cut at the notches near the roller mounting points. The roller mounting arms on one multi roller stay were cut off, while one hole was removed from the side with the mounting arms that are further apart to keep within size regulations. This piece was then mounted to the underside of the chassis. In the above picture, I already filed the edges to get rid of any sharp edges left behind after cutting.

To keep the underside smooth, a router with a grinder ball attachment was used to recess the mounting holes and flat head screws were used to mount the stay to the underside of the chassis at four points to distribute the load as much as possible. One straight plate was mounted below the stay as a front under guard. The two mounting holes on the reinforcing plate were also recessed to keep the bottom of the under guard smooth. Using a reinforcing plate as an under guard also allows brake sponges to be applied to the front.

Finally, a second straight plate was mounted on the top side for attaching rollers. Longer screws were used for mounting the stay underneath so that the same screws could also be used to fasten the upper stay. Alternatively, you can use a differently-shaped plate on top instead of a reinforcing plate especially if you’re using rollers smaller than 19mm, and secure it to the chassis at the same locations as the stay underneath.

2016-03-07 20.38.47

Here’s how the front underside looks.

2016-03-07 20.38.59

And here’s how it looks from the front. The setting in this picture is outdated and I have removed the front mass damper, among other changes. Also I used screws that were too long and they caused injuries when trying to catch the car.

2016-03-03 21.35.45

For the rear, I removed the stock rear stay leaving behind one pair of mounting holes like older chassis. It’s also possible to only remove the roller attachment points, leaving behind four mounting points just like the front bumper. Using a multi roller stay as a bottom plate like the front, I cut off only the roller attachment arms leaving behind two pairs of holes on both sets of mounting arms and recessed the necessary mounting holes. By doing so, the rear skid bar/brake can be mounted as far back as possible. In the above picture, the uppermost pair was used for mounting the skid bar; the middle pair holds 30mm screws which link to a second upper stay as well as act as the poles for the body mass damper. Finally, the lower pair of holes is for mounting the rear stay to the chassis. Alternatively you can use a differently shaped stay to mount your rollers.

Since only one stay on the bottom was mounted at two points to the chassis, I decided to use a second unmodified multi roller stay to hold the rear rollers and it was mounted to the upper side of the chassis. Spacers were used to keep both upper and lower stays rigid and parallel.

IMG_7108 - Copy

Here’s an example of how the rear can look. Since taking this picture I have changed the settings quite a bit. Flat head screws were used on the underside throughout to maintain as smooth a surface as possible and to prevent any possible damage to the track. The rear brake sponge was covered with tape. Originally the machine was somewhat stable at jumps, but would slow down severely through a rainbow-shaped lane changer. With the rear brake covered, it acts as a cushion for impacts without sacrificing too much speed.

Conversely, brake sponge was applied to the front under guard which provided a surprising effect. The machine is able to maintain most of its speed at the rainbow lane changer, and is even more stable during jumps, allowing me to use an even faster motor.

For both the front and rear rollers, 30mm cap screws were used for their strength over conventional screws. 17mm stabilizer heads were mounted to the top of the front rollers, while the rear rollers are spaced as far apart as possible. The lower rollers are mounted just above the rear skid bar so that the cap screw head will not get caught on the track walls. If you have hex mounts, you can mount the upper rollers at a higher location than is possible using a single cap screw.

2016-03-07 20.39.36

Rear view. I have since replaced the lower rollers with the same rollers as the upper ones, and the lower rollers went to the front. According to some Japanese, the plastic rings on the rollers have lower friction than the aluminium ones. I’m not sure if this is true, but the dish rollers are definitely stronger than the lightweight rollers and therefore are more suited to be in front.

In the next post, I will share how I built the body mass damper for this machine.


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