This was a 1 day build when I was bored at home.

I wanted a Radio controlled vehicle with 6 wheels, and as cheap as possible. So I bought 2 cars sawed one i half solvent welded the back end to the intact car.

The parts:

2 cheap rc cars(mine was Topraiders Tiger(Bionic) )
Some legos
2 li-po cells
Soldering iron
Wires
(some duct tape.......)
Acetone

Step 1: Adding Extra Juice to the Car

I had some li-po cells laying around from an old laptop battery. The car takes 4 AAA(1.5V) batteries in series which equals to 6 volts, and a top speed of 12km/h(7.5 mph).

2 li-po(3.7V) cells in series gives you 7.4 Volts and therefore extra speed out of the motors and the benefit of rechargeability. (I charge mine using some alligator clips and a 5v cheap battery charger)

Solder on the 2 li-po cells with some wires, connect one end to the negative terminal of one cell to the positive terminal of the other cell. Then add a red and black cable out ( I added a snap-in-clip-thing for easier removal)
Take out the AAA-batteries and find the positive/negative terminal from the old battery pack which leads to the PCB. Solder a new set of wires to the li-po pack.

Step 2: Joining the Cars

I took the soldering iron and removed the motor wires from the PCB of one car, then sawed-off that car in the middle so the whole back-suspension system came out as one piece(ワンピース ;). That's because the hinge should still be in use so both back-ends(motors) is held in suspension.

I gladly found out that the cars chassis was made out of ABS plastic so I could solvent weld them with Acetone.

I took some legos that seemed suitable as a linkage between the two, guesstimated the fit and fastened the legos to the cars with acetone, aim for as much surface area as possible so the solvent weld can make a strong joint. I used the battery hatch from the sawed up car as re-enforcement at the bottom och the joint.
Take the connections, of the motor that is attached to the end of the 6 wheeler, and solder them to the PCB pads shown in the picture.

Step 3: Test Drive!

Make sure that your new vehicle is strong enough for the first test run, if you feel like the joint of the new back-end isn't gonna hold up, add some more legos(ABS pieces) with acetone to strengthen the cars rigidity.