buildingtwin21

STAGE 1 - STRIPDOWN

The Ho-Bao Hyper-7 buggy comes virtually ready to run, really a bit of a negative point for the RC purist. Personally I would prefer it if they kicked it out for a few quid less and it came in kit form. To make up the custom super truck, I had to strip the ready built car down to a single bare chassis plate. Luckily this only took a few minutes as the car is of modular form - the rear gearbox, front diff and front gearbox sections, once removed, leave you with practically nothing on the chassis.
Then I had to mount up the 2 .21 engines. Running optional large 15T clutch bells to increase the engine spacing, there are still sub-millimetre clearings everywhere! This was pure luck, the engines could have touched each other and I would have had to make loads of major modifications. As it is, I was OK.
The engines have two pairs of mounts each. One engine was fully mounted up in the kit. For the second engine, I had some old Thunder-Tiger engine mounts I could use, and the secondary mounts have to be custom machined. These mounts allow transverse (gear-mesh) adjustability, pretty essential. The major problem here is that the chassis has side kick-up, which happens to coincide exactly with the outer engine mount of the second engine. Some fancy angle machining on both the secondary mount and the chassis itself will be required to solve this... and I am about to do that now.

Engine mounting complete, and the weight of this beast is really coming home. What a lump this thing is. The aluminium chassis is the thickest I have ever seen on an RC car, which is a good thing because there is no longer space for the rear chassis brace - a rear 'top deck' will have to be machined to replace this rod.

Stage 2 - Throttle and brakes

The front brake calipers are now going to be machined. A complex piece, with inside-turned conical opening to fit the kit aluminium hub carrier. Have to get the angle just right or it will rock when the brake is applied.

Change of plan. Since I have to return to uni at the weekend, I will leave the individual wheel brakes till last. Moving on to the throttle, some 1/16" wire was soldered up to form a double-link connecting both carbs so they open exactly the same amount each. I then machined up an aluminium pillar to act as a rocker bracket for the bell-crank. This was a standard servo horn. The throttle servo was located at a position just in front of engine no. 2 and a link sent straight through the rear brake arm and fixed to the bellcrank with a spring. This servo operates the rear brake and both throttles. The kit servo tray was cut in half on the milling machine to allow repositioning of the servos on the chassis. There is only just the space to fit the 2 servos and the extra engine.

Stage 3 - electrics

I fitted a Futaba S148 for the throttle and rear brake (currently the only brake) and a Cirrus clear-case CS-60 2BBMG for the steering. I got a bunch of these cheap went I went to America on holiday. They are a quarter the price of an equivalent medium-torque servo here. Hopefully I will be able to get some better servos soon, but for now, the �800+ I have spent on this machine has cleaned me out. The radio receiver is placed on the fuel splash guard, I will have to take extra care to avoid dousing it in fuel! The trouble is, there is very little space on the chassis for things like this. The battery pack is a home-made 5 cell 650mAh AA pack. UPDATE! I now have a Hitec HS605BB Hi torque steering servo, way better. It has 93oz.in of torque.

Stage 4 - air box and filter

This was made as in the engines section. It is designed to take as much restriction off the air intakes as possible. With this setup, the engines will run practically as if they had no air filters fitted - although they will always be sucking dirt-free air. The air box is mounted up at the front bulkhead, and is big enough to cover the whole of the front steering assembly. The intake is huge, and is positioned right under the bonnet of the truck shell. Depending on the proximity to the lexan, an air scoop hole may have to be cut from the shell.