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 Piranha P2
   Do-it-Yourself Buggy

 Lean mean killing machine ...

Looking at the variety of currently avaliable 2wd EP buggies ranging from low price rookie model up to high end team kit you'd nearly have to justify yourself for bringing up your own thing. Alas, lots of interesting cars will only be offered as RTR-packages but still you can find hobbyists out there who enjoy building their whips clearly as much as racing them. Markus Simon ("madmax" in the Forum) proves to be one of those modelers. His "Piranha" buggy does it again, the "P2" alluding to the second edition. Let's see if it can keep up with its namesake!

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 Rounding up

The Piranha buggy concept is meant to combine ideas and technologies currently used in all different sections of R/C car racing.

  • Lithium-polymer batteries: The newest generation excels the proven nickel metal hydride batteries in power density at a much lower weight. A Li-Po pack with only marginal voltage drop under load condititon and a capacity of 3200 mAh weighs just about 200 grams and is lots smaller than an equivalent pack of older generation cells.
    The Piranha P2 employs two Kokam 3200HD cells providing 7.4 volts when connected in series.
    Most lithium-polymer batteries are only covered in a thin kind of foil which makes them light but also vulnerable to mechanical damage. So in the Piranha P2 part of the chassis' job is to protect the batteries effectively.
    Lithium polymer cells need chargers that are suitable for this kind of battery type. Like their nickel buddies they're not too keen on overcharging hence lipos mustn't be connected to any conventional nicad/NIMH charger! (also see battery-FAQ concerning this!)

  • „Mid motor“: Any usual 2WD buggy will have its motor located behind the rear suspension. This moves the center of gravity backwards and thus helps the rear end to gain more traction. The downside of having the motor mass out of the center is a certain amount of instability: When cornering, the car will -overstatedly- act like a pendulum.
    With the battery of the Piranha P2 only weighing half of any conventional nicad/NIMH pack the COG issue is solved, so there are no objections against moving the motor to the inside of the car - except one's really into making wheelies.. Hence the mid motor concept.

  • „Push-rod“-linkage: Horizontal shock absorbers became famous with the Tenth-Technology „Predator“ buggy. Inherent hassle with track setup and complex linkage geometry kept them from being more commonly used.
    On the other hand low profile shocks would definitely assist a cuneiform 2WD buggy outline with an emphasis on rear end grip. In times when additional front end grip is needed another wing in the nose area can be attached effortlessly - so let's have a go!

Robitronics' ICube ESC is one of the first lipo-compatible speed controllers for brushed motors. It will supply the Peak Racing Vantage motor with the energy provided by 2s Kokam cells.

Looking at this setup, it doesnt't take a genius to figure out that the Piranha P2 will never be race-legal; but nevertheless: This is about the joy of building plus testing and about innovations!
But there remains a question that might irritate some readers ready to experiment: How on earth should complex parts like suspension arms, hub carriers or even gear parts be produced? Not at all, for the best of it - because building those spare parts would cost way too much money and time!

The thoughtful hobbyist is likely to start out with a complete donor car and therefore prevent himself from facing total darkness when it comes to more delicate details like suspension geometry. A Team Losi XXX Kinwald Edition had been the favourized candidate for the Piranha P2. As the P2 would somehow use both front and rear suspension all the holes have to be copied to the new chassis accordingly. To accomodate the mid motor setup while efficiently protecting the lipo pack at the same time a tub chassis has to be built. 

Laminating 101
The chassis of the Piranha consists of a hand laminated u-profile which is cut and drilled after moulding. In order to save expenses and to facilitate machining it is designed as a sandwich construction. This means that only the first and last layer will be carbon fibre and provide most of the laminates' stability.
In between glass fibre cloth is used as an easy to handle and light weight coupling layer. The core is made of Kevlar 49 high module fibres being an optimum material due to their low specific weight and even more so enhancing the chassis' impact strenght.
A planed block of wood is used as a positive mould - either end being about five centimetres longer than the reinforcement layers of the chassis. The radii of the longitudinal edges have to be adequately shaped to avoid residual stress. Furthermore the mould has to be higher than the actual chassis because cut-off has to be kept in mind. The inner width has to be in accordance with the lipo pack that will be used.

To avoid the complicated treatment of the wooden block with mould release agent it has to be completely wrapped in PET foil. Upon this the epoxy impregnated layers of reinforcement fibre have to be arranged.
When the chassis reaches a thickness of about 3-4 millimetres it is covered in another piece of foil. Now the excess resin has to be removed from the laminate. In order to do this it has to be pressed between panels, held together by c-clamps. (First from the top, then from the sides) To ensure a constant laminate thickness the panels have to be shimmed with small aluminum plates - 3mm for the bottom of the chassis, 2mm for the sides.

Laminating uses hazardous materials, machining will produce particulate matter that may be harmful to your health - hence it is advisable to wear proper protection masks and gloves!



 It's taking shape ...

Some work has still to be done on the Losi-compatible profile before it starts resembling a buggy. The chassis has to be stiffened towards the gearbox at the rear and the bulkhead at the front. Due to their complex shape these reinforcement parts are modeled out of cardboard and then adapted to the chassis.
All drilling has to be done with similarly high precision, especially when it comes to the mounting holes of suspension-relevant parts like the rear pivot block.
At the front end the kickplate of the XXX is ground down to fit the slimmer chassis. Instead of attaching it with four counter-sunk screws it is roughened up and extensively glued into place with epoxy. This kind of assembly is likely to take much more abuse than a bolted connexion due to the better transmission of force. (Given that it has been done with care)
The steering servo needs gluing too: It's the only way to fix a 22mm high "low profile"-servo after it has been deprived of its mounting links. At least it is glued in a forward-thinking fashion so that it can still be opened to replace gears or bearings when required.
At the rear end it goes on with the gearbox. It is simply turned around together with the shock tower and mounted onto the chassis. That's why the rotating direction of the motor has to be reversed. This means not only a simple change of the motor wires but also adjusting the timing respectively. Turning the endbell 180 degrees would have the same effect but you'd lose the timing marks for orientation.
Back at the front end the Piranha P2 already starts taking the shape of a buggy. Corresponding holes have to be drilled into the chassis to mount the shock absorbers. The rocker pins are bolted into the Losi bulkhead. The bell-crank levers are ball raced to provide a smoothly working suspension. Additional rubber pads prevent the pins from breaking in rough conditions. Some plastic rod ends, threaded bolts and Tamiyas' "mini CVA shock unit II" get the front suspension up and running. Front buggy springs from Team Associated are used to find a reasonable setup. The rear end suspension uses conventional 1 inch Losi shocks. Two things have to be paid attention to: The center of gravity  is a little bit more towards the front, so a softer rear end will be needed. Then again an overall softer setup is required due to the considerably lower vehicular weight of 1200 grams.


 Access all areas: a closer look
Transmission powered by Team Losi:              All the gears are taken fom the first XXX-series providing an internal gear ratio of 2.56:1. The ball differential has been equipped with a ceramic thrust bearing from Corally because the thin thrust washers of the original diff are likely to run in and break apart when confronted with higher moments of torque.

Because the gearbox is turned to fit into the Piranha P2 the rotating direction of the motor has to be reversed too. Consequently, only timeable motors (or brushless motors, which are timed by the controller) can be utilised in the P2. The preferred rotating direction has to be set by adjusting the timing.

Both front and rear end are taken from the Team Losi XXX-Kinwald. The front suspension uses thread end bolts instead of the rather frail hinge pins and e-clips. Thus play is reduced and the suspension gains more reliability and overall strenght. Also the rear end has been deprived of all its e-clips: a set screw in the rear hub is holding the hinge pin in place.
The rear arms are reversed left to right in order to draw the rear axle nearer to the motor and therefore move the centre of gravity a bit backwards.

No more has the front shock setup anything in common with the stock Losi XXX. The push rod suspension features ball raced rubber padded rockers and Tamiya's "short shock-unit II". The aluminum parts on the main chassis act both as a shock mount and battery strap holder at the same time.

Due to the mid motor setup and the slim chassis  ridiculously little space is left for the steering parts. Hence the original servo mounting had to be left behind.
A 22 millimetres high "Low-Profile" Servo (GWS IQ 500-MG) is used instead. Cut down as far as possible it has to be CA'ed into the chassis. Furthermore, the connection cable to the receiver was given a new exit on the broadside of the servo. All the internals can be reached from the top side - so there is no need to detach the whole thing any time soon.                                                    The steering mechanism itself is a modified Losi-design built to collaborate with the new steering servo position.

"Economy" is the thought that comes to mind: space is pretty limited in the race - ready chassis. All components have to be crammed together, the ESC even using what little vertical space is left. But still everything can be exchanged with no more difficulties than in any other 2WD buggy.


 Cheers! From bottle to body...

Empty 2.5 litre PET bottles come in handy whenever you feel the need for building your own bodies or rear wings. (CocaCola called it the "shape bottle" for a reason!) Acting like heat shrink tubing when treated with a hot air gun they wrap tightly around the Piranha body mould carefully shaped out of balsa wood.

A PET body like that comes out extremely light weight - without losing any strenght when compared to polycarbonate shells. The body can be painted with your usual acrylic based polycarbonate paint.
A detailed how-to on moulding your own parts using PET bottles can be found here!



How's it gonna drive? Was it worth the effort? Please let that chassis hold!

… Thoughts like these cross your mind when the battery is connected for the first time and the control-LED of the speed controller lights up. Driving a car that took way more time and commitment than a kit or even a ready-to-run for the very first time triggers totally different emotions. Just like launching a new untested sailplane. Well, the Piranha P2 was spared any flights at the beginning so the tension loosens as the sleek buggy rolls off unhurriedly. The P2 cannot be compared with its aquatic namesakes yet - all the better for trimming the steering.
Some cautious cornering, then comes the straight; the first opportunity to kick it in the guts: Theoretically, the Piranha accelerates about 20 per cent faster than its XXX predecessor. In practice the P2 is simply as swift as an arrow - so fast that the next corner is taken off-the-road for safety reasons. It soon becomes clear that the Piranha P2 has a noticeable tendency towards understeering - given the absolutely stable handling performance this is an extremely positive sign for a rear wheel driven mid motor car that woulld be expected to show a greater oversteering tendency. Understeering can be traced back to the facts that the front suspension setup is rather hard and the steering throw was reduced to a mere 70 per cent. Latter had to be done because the the steering linkage did not have the optimum lenght. So it's confidence and a good feeling on the way back to the workbench to prepare the Piranha for the nex test session. By the way - the reversed motor could do with a little more timing ... !

There are many ways to R/C car racing. Being engineer, test pilot and race driver at the same time is definitely one of the most entertaining ones. It needn't automatically be a high end racer or even a world championship car - those models are mostly built by renown manufacturers with years of racing experience.
But also as a layman one can be proud of one's work - and getting a whole lot of background information about setup, suspension dynamics and materials can be listed as positive side effects.
A second hand car is easy to get and helps making your first steps towards the DIY buggy!

More infos about a new CR-based Piranha at our messageboards!


 Text and photography by Aaron Banovics, translation by Markus Simon
 This article has been published on on 05-31-2007.