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 Kyosho Inferno ST US Sports
  Part #2: RTR-equipment and accessories 

 RTR = Ready to Run

Ready to Run has to be taken with a grain of salt: For obvious reasons there is no fuel supplied (Kyosho Germany recommends Morgan Fuels Sidewinder 25%) and neither are the batteries.  RTRs also differ from kit models by extra built-in R/C equipment, engine and GP accessories. With the Inferno ST-S US Sports Kyosho includes the following:

  • Engine: Kyoshos' engine series in big block-size: The GXR-28 has got 4,6ccm and is equipped with a pullstart.
  • Radio-system: The radio control is a (27 MHz AM) computer system built by Sanwa, according to Kyosho. A 'Perfex' mini receiver is preassembled into the truggy next to the servos.
  • Accessories: Kyosho hooks you up with the obligatory tools plus a glow plug heater and a fuel bottle. (250ccm)

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 GXR-28 take one:  big block power bite-sized.

It has become en vogue to equip RTR truggies with big block engines. These are engines the size of a .21 engine but they actually have a displacement of 4.1 to 5.2ccm.
This makes it possible to produce low-cost engines that are still relatively poweful given their greater cylinder capacity. In the .21 class this would only be possible at much higher costs.

Kyosho makes no exception here and powers the Inferno ST-S with a 4.6ccm engine.
Typically for Kyoshos' engine series is the bolted exhaust manifold (which is normally held in place by a sturdy spring) as well as the direct attachment to the chassis via four M3 screws.

Surely one can argue about the pros and cons of this type of mounting. But it's a fact that Kyosho spared themselves the usually necessary motor mounts that have to be acquired seperately in order to use other engines.

The carburetor of the GXR-28 features two controls (main needle and idle needle) and a throttle stop screw.
Laudably Kyosho is pretty keen on safety and therefore a tension spring that closes the carburetor automatically is attached to the slide valve.

The spring keeps the car (and its surroundings) from greater damage in case that a linkage loosens itself and the servo is no longer connected to the carburetor. Unlike the 'mechanical fail save' in Jamaras' 'Track Sau' the spring is not strong enough to close the carburetor plus a dead servo.

A really intriguing construction can be found at the rear side of the engine: The pull start that Kyosho thought up here is pretty much the only 'onboard' starting mechanism that could come close to a starterbox... Why is that?
  • The pull start does not use a one-way bearing (a well known wear and tear part with pull starts)
  • As soon as the engine is running the whole starting mechanism is completely decoupled so that sealing the backplate becomes an easier job.

The ingenious thing about it is that Kyoshos system is no more complicated than conventional pull starts, but let's see what goes on behind (or rather inside) it!

The picture on the right shows the components of the starting mechanism:  The starter shaft looks similar to the ones of other pullstarts - apart from a bore hole at its end...
The notch in the disc allows the starter shaft to 'hook' into the pivot.
The thing right next to it is the most important part of Kyoshos' starting system: The slotted steel bushing in connection with an actuating pin replaces the ordinary one-way bearing.

In order for the system to work the starter shaft has to be able to move in axial direction. (picture 1 below)

  • The rope drum engages with the actuator bushing and rotates clockwise when starting (see picture 2) The actuating pin glides along the slot and pushes the starter shaft against the pivot and engages the crankshaft. A form locking connection is only established when the actuating pin reaches the end of its slot (like in picture 2) and the starting disc hooks onto the pivot. Depending on the position of both points of engagement up to 10 centimetres of rope are needed until the engine is turned - but decently then, because the system uses no wearing parts. 
  • When the spring is winding up the rope like in picture 3 (or the engine is running faster than the pullstart!) the actuating pin again rides along its slot to the end point and pulls away the starter disc from the pivot. Pullstart and engine are now completely decoupled from each other until the next starting procedure.

From a mechanical point of view, the system could still be improved: no shims are used between the cast aluminum case and the starter bushing/shaft that could take axial forces and prevent abrasion. But it remains to be seen whether such shims are really necessary.
In any case Kyosho performed a masterstroke with this pull start!

Inside the crankcase, it's somewhat less spectacular.
The inner surfaces are not polished, the bottom bushing of the piston rod (which has a bit more play right from the start than most other engines) is drilled triply for better lubrication but the shape has not been optimized. The piston runs in a three-channel cylinder liner where I had to find some swarf from machining. The 14 millimetre crankshaft has a 'Standard' end. This means that the clutch bell needs an additional flywheel adaptor.

The clutch seems to fit the engine well: The two-piece clutch is completely released at idle speed and engages adequately: It neither exerts the engine nor is there a need for unnecessary high RPM to get the power down to the wheels.

The tuned pipe is a 'real' two chamber type with a surprisingly high amount of damping without taking engine power. (8 millimetres exhaust, 12 millimetres of cone opening, pressure fitting in the second chamber)


 Perfex RC-system

One of the obvious accessory-upgrades against the 'standard Sports' is the radio unit.
You get a computer system with twelve model memory slots and all the goodies like end point adjustment for throttle/brake and steering, expo and sub trim. Self evidently with those features you will also find basic functions like servo reverse and a steering throw limit.

Menu-wise some manufacturers could take a leaf out of Kyosho's book:
The whole radio gets by on just four buttons. Using the menu keys you scroll the parameters which then start to blink on the big, well-readable display. Setting is done by pressing '+/-' and then it's the menu key again: simple and effective!
There are additional rocker switches on the top side for trimming both channels and adjusting the steering throw without having to use the menu. The display shows trim positions, model memory and a battery indicator. Power is supplied by eight AA cells and - looking at other RTRs it's not too trivial to mention the fact that this transmitter is also featuring a charging jack. 

Top right to the wheel you can see a swich for an additional channel which is neither used with the Inferno ST nor supported by the mini-receiver. (35x25x16 mm)
The Perfex-transmitter can surely be counted to the RTR-premium class. It is only surpassed by very few kits with FM system. But those come in at a totally different price level.


Up to now the RTR-equipment made quite a decent impression but now, it's time for what I called a 'lack of consequency' in the first part of the review:

Right at the end Kyosho lives up to the RTR-cliché and throws at us some plastic geared and busehd servos with a nominal force of 5kg*cm and a retention force of 8kg*cm 
(80% throw, 5° deviation) at 6 volts.
Not even the loosened servo saver is able to conceal it - quite the contrary: Due to the twenty degrees of caster at the front, especially the high speed performance is afflicted.

A real pity - looking at the auspicious start of it all. And incomprehensible - given the fact that this is a truggy.

The additional costs for servos that match the other RTR-components would have been bearable due to the added value.


 "Me glow you long time ..."

Next to a fuel bottle, some Allen keys and a four-way socket wrench Kyosho also supplies an unconventional but interesting glow plug heater: The housing fits two fat D-cell batteries connected in parallel which allows for capacities up to 30Ah - more than three times of a conventional 2V lead battery and nearly ten times of a sub-C glower. 
With rechargeable batteries it's still up to 18Ah -  they only have to be taken out in order to be recharged because the heater is only pressed onto the glow plug and will not hold to a charging plug by itself.


Interim results - take two.

It's just like the feature-thing again - just not as harsh: A simple engine with a revolutionary pull start , great radio ... and servos that are no more than an also-ran.

Again you get the impression that Kyoshos way was paved with good intentions - but somewhere along they got more off-road than they should have. Would have been better to get a map  - meaning to raise the retail price in order to afford better servos.

Indeed the Inferno ST-S US Sports does have a very decent RTR equipment (compared to other RTRs) Due to the servos (like in lots of other cars) its suitability is somewhat limited: Even the beginner should at least exchange the steering servo for a truggy-adequate one. In our case it will be the Thunder Tiger/Ace RC DS1015 for roundabout € 85.

 Text and photography by Aaron Banovics, translation by Markus Simon
 This article has been published on www.offroad-cult.org on 20.12.2006.