I spent a whole bunch of time getting a pair of new direct-drive chassis built for the IHSR race in Rockville Maryland today. But with the weather so iffy -- a good chance of significant snow later today, I decided to stay home. Rockville is a about a two-hour drive from here, and I didn't want to risk a long drive home in dangerous conditions. There is another race next Saturday and, weather permitting, I'll debut both of these cars there.
I decided to build all-new chassis for my two direct-drive cars. I had too many improvements (I hope they are improvements!) to make to try and salvage my Mark 1 chassis.
For the pre-1962 vintage IHSR class I picked on a J&R Products 'Jack Rabbit' motor -- which is recommended for the class -- and solid brass bar stock 1/16" thick by 1" wide as the basis for the chassis. I also decided to create independent front wheels. I chose to bolt the whole chassis together, as much as possible, rather than solder everything. I wanted to be able to shim the heights of the front axle and guide. I used a whole lot of UNC #4-40 flathead screws, with matching machine nuts, all in 18-8 stainless steel.
One nice thing about using 18-8 stainless is it is non-magnetic. Makes things easier to assemble in the presence of motor magnets.
At some point in the future I plan to trim the length of all those screws. For now, I am leaving them long so I can adjust the shimming.
First, the pre-1962 vintage car -- a Cooper with a single direct-drive motor:
The motor mounting system -- common to both cars. has a flat brass plate glued to the side of the motor, first with superglue, then with JB Weld epoxy. That plate is fastened to an L-shaped bracket with Bo-Link servo tape. The servo tape is a foam tape, rather thin, extremely strong, and gives a bit of flex and shock absorption to the chassis. The body mount is a thin brass plate, loosely mounted to the chassis, so the body has a little bit of freedom to move.
The front axle assembly is bolted on so its height can be shimmed. The wheels rotate independently, held in place by tiny brass washers soldered onto the ends of the axle. The axle sits right above the guide post, which created a clearance problem. Said problem was solved by grinding a bit of the axle away directly over the guide post, and also by using a tiny screw and washer to hold the guide on, rather than the normal machine nut.
For the unlimited class I have built a dual-motor direct-drive chassis, covered by a Mazda sports car body. Both motors are JR Products "M20 Hawk DD" -- a motor used in their own direct drive chassis. I measured their armatures as 4.3 ohms. Assuming perfect, zero-ohm contact and circuitry, that makes the car's start-up current draw 5.2 amps. In the real world it will be a scootch less, hopefully less than 5 amps.
The construction of the chassis is similar to the other car, with a few major differences. For one, the front motor is mounted so it can rotate a bit in the 'roll' direction. (Don't understand roll? See below.) And, inspired by space constraints, the guide is mounted on a drop-arm. The guide is held on by a conventional machine nut.
The pivoting hardware for the front motor is shown here. The thick brass plate has a flat-head screw mounted in a tapered through-hole and soldered in place. A pair of short brass tube sections form a bushing assembly, and everything is held together with a pair of machine nuts. This setup allows the clearance inside the assembly to be adjusted for free movement with minimal clearance.
The drop arm is a soldered-up assembly of 1/4" x 1/16" brass bar and a short length of brass tubing. This all pivots about a length of UNC 2-56 threaded rod, towards the rear of the chassis, secured with machine nuts, which rotates in a close-fitting length of brass tubing soldered to a brass plate. That brass plate is bolted to the chassis.
The above photo shows how the front motor pivots (That's roll, see?). The point of designing in that pivot is to keep all four tires in contact with the road while negotiating banked sections of track, which are quite common in the routed tracks we race on. The ability of the front axle to pivot has proven to be a significant advantage in the custom HO chassis raced in the HOCOC unlimited class. Early testing seems to indicate that this chassis does indeed corner well on banked curves.
That same pivoting action has a second, potential advantage, that I am not going to disclose here. At the moment I have not integrated that feature into this car, and I may never do that. Right now I don't think it is needed.
Both cars have been tested on my 4 x 12 foot high-banked oval HO track. The slots on that track are not deep enough for the guides -- the bottom of the guides drag on the bottom of the slots. Also, the braids on that track are only 1/8" wide, which makes power pickup touchy.
Excellent electrical contact appears to be essential to these cars performance. If the braids are not adjusted just right there is a notable drop-off in acceleration and speed. I am hoping when these cars are run on a 1/32nd scale routed track the wider track braids will obviate this problem.
Regardless, at the moment, from what testing I have done, I feel both of these cars have potential. We'll see how they do in a race, hopefully next weekend.
Ed Bianchi
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