Here is the car as I received it. It came with an AM radio that was apparently modified to get power from an external battery pack (see wires).
Yeah, I know. The wing decal is on backwards. I didn't build it!
Damage was pretty standard issue as Madcaps go: Rear chassis breakage/splitting around the gearbox mount (can't see it in the photos), a broken C3 rear gearbox mounting post (first time I'd seen that), but surprisingly no broken front chassis posts. The motor was a stock Mabuchi RS540 with a broken wire tab that couldn't be repaired and it stunk like a piece of machinery that had been extremely hot at some point with that "hot oil" smell. No visible damage, just smell.
I decided to install the optional metal motor plate in order to reduce flex caused by a (hopefully) more powerful motor and perhaps absorb a little bit of heat as well. Ball bearings were used throughout, but I kept the stock gear set with a Robinson Racing 23T 0.6mod pinion and the standard 70T spur. I figured I'd test whatever motor I chose with the standard ratio first and go from there.
I didn't take any pictures of the build, but it wasn't very eventful anyway since it was basically a standard stock build following the manual. I used black CVA shocks again (50519 and 50520) like Madcap #2 since the performance is improved so much versus the stock friction shocks.
The only interesting part of the build was the use of Avante aluminum posts as a drop-in replacement for the broken C3 gearbox mounting post. I'd heard they could be used on the forums, so I bought a bag of parts from eBay to test the theory. Each bag comes with three posts and a bunch of other junk, so two bags will refit three cars. It turns out that the parts are exactly the same dimensions as C3. Just the standard self tapping screws have to be swapped out for 3mm machine threaded ones and that's it.
Now for the motor choice. I wanted something with a bit more power and speed. I have a small selection of motors that I purchased from others over the years, almost all well used. Many of these motors are standard 27T stock variants which means the performance is fairly known and predictable within a certain range, which is to say rather boring. But a few of the motors have an unknown number of turns on the armature, so it is fun to try them out to see what they are capable of. The Demon Power motor that I used in Madcap #2 was one of the "unknown winds" that unfortunately turned out to be rather disappointing once I ran it, so this time I was determined to find a motor that had more power. After looking at the various choices in my collection, I decided to use a Fantom motor with seemingly no other identifiable markings. I did not even power it up to test before installation to see how fast it spun. I mean, all of the used motors in my collection have been disassembled (if it is possible), cleaned and oiled before being put away, so I knew the Fantom was in good working condition, but I did not remember how fast it was when powered by the 4 AA pack that I use for motor testing. Basically I was blind to what the motor was capable of. It could be another slowpoke like the Demon Power or it could be a real powerhouse. I figured it would be fun to find out and compare. I forgot to take a picture of the motor by itself, so here it is fully installed with the label partially hidden by a heatsink.
Here are some more pictures of the finished chassis and original body. The body actually cleaned up a little and got a bit whiter which was nice. There is paint missing from a bunch of spots, but it is still usable. The wheels and tires are from Carson Motor Sport in the UK. The chrome rims are bright, but I think they go with the body color well enough.
I used a Futaba S28 servo for steering along with a G to J conversion cable to connect it to a Futaba R113ip receiver. The speed control is a Futaba MC330CR.
Performance
With the stock 23T / 70T gearing and the Fantom motor, this car was a rocket ship! Massive power and incredible speed. In fact, it was almost too much speed. It was hard to hit full throttle without running out of pavement. Not only that, but just letting the car coast felt like the brakes were on because the motor magnets were so powerful. After a few minutes of driving, I touched my finger to the motor can to feel how hot it was and I could tell that I needed to gear it down or I'd damage the motor. I swapped the 70T spur gear for the 77T and also switched out the 23T pinion for a 21T pinion (the largest that could fit given the motor mounting screw slot size on the metal motor mounting plate). This changed the pinion/spur ratio from 1:3.04 to 1:3.67. The new ratio definitely reduced the heat output of the motor, but I felt that it needed one less tooth on the pinion, so I replaced the 21T with a 20T. This changed the gear ratio to 1:3.85 and resulted in great motor temperatures while still providing incredible speed and amazing power. I added a black vertical finned heatsink to help dissipate heat and I believe it does help a little. Getting the heat to transfer through the motor label via a clip-on heatsink is not ideal, but I think it's worth it.
Launching the car from a standstill or a slow forward roll results in the back end sinking down to help get more traction thanks to the black oil filled CVA shocks and their smooth movement. Conversely, hitting the brakes actually lifts the rear end and drops the front down a bit. The power provided by the Fantom motor is quite amazing considering it is a well-used example that might be over 15 years old and it's being powered by NiCd batteries that are probably close to 10 years old. If I had to guess, I'd say the Fantom is a 9T or something simply because it is so fast and powerful.
Given all of that power, I know what you're thinking. The poor Madcap ball diff cannot possibly stand up to such a powerful motor without disintegrating. Maybe this is true, maybe it isn't. All I can say is that the diff has not even given me a whimper through over 8 battery packs of running. No squealing, no loss of power, just great power delivery and the awesome whine of the Madcap gearbox. I also purposely do not thrash on any of my cars and try to deliver smooth power commands. Also, since I'm not really doing much off-roading or jumps, drivetrain shock is minimal. I did use the optional shims when building the diff because they certainly do help. I know because when I first built the diff without the shims and tested the car, it squealed a bit under high acceleration. I added the shims and it's been great ever since. I should mention that I used Tamiya ball diff grease, not just any old grease when rebuilding the diff. Ball diff grease is specially designed to not let the balls slip whereas other greases would allow slippage. This is a key thing to remember. Also, I made sure all diff parts were thoroughly clean before rebuilding it. All parts went into a motor cleaner bath until everything was clean and then the rebuild was started. All it takes is a single grain of grit or sand and the diff can be ruined.
Of the three Madcaps I've rebuilt so far, this is my favorite one to run. But I still have one more Madcap rebuild left, so stay tuned!
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