» TESTS » TRUGGIES offroad-CULT: die CommunityTestberichte über Buggies, Monster Trucks und RC-EquipmentFachartikel zu interessanten Themen sowie EinsteigerhilfenQuicktips für den Offroader!zur Linkseite von offroad-CULT
 

» Deutsche Fassung: hier klicken!

 Kyosho Inferno ST US Sports
   Part #3: Test drive and final verdict
 
 Final preps
 

Normally I'd show you some tricks before we're really getting down to it. But this time - there aren't really any tricks to be played! 
As mentioned before Kyoshos RTR truggy is so thoroughly built that there's nothing left to add. The truggy bodyshell covers engine and fuel tank completely, so there is neither a need for a heat sink guard (made from zip ties) nor for a 'refuel rip cord' - again using a zip tie to fasten it to the tank cap. (see picture below)

Starting without a starterbox can become a bit of a pain because the engine has to be primed somehow. You either pull the starter for a couple of times -while having the tuned pipe stopped up; or you have to blow into the pressure line yourself in order to let the fuel flow.

Or - for about € 6 QWorld will send you a handy little fuel pump plus mounting - which has been secured onto the splash guard in our case. For cars without fuel filters this pump doubles as a large stone filter.


All hyperlinks marked like this will be opened in a new browser window!

  
 GXR-28 take two: Break-in
 

The manual suggests about two or three tanks to break the engine in. A bit more than a quarter-litre seemed not quite enough for me - given the measured compression torque: 90 Ncm is quite impressive for a pull-start engine. Normally I use up to one litre to run in my engines.
Naturally I started out with a rich setting - which is three turns on the main needle and four on the idle with the GXR-28. Kyosho Germany recommends 25-percent-nitro Morgan Fuels Sidewinder - so this is what I used. 

The first fire was a quick and easy one - thanks to the additional fuel pump. Starting can feel a bit weird in the beginning because you have to pull out some inches of rope until the starter plate couples - if it couples at all (rare slipping is possible - but given the starting mechanism there should be no damage, apart from a little wear of the starter plate)

After a couple of minutes on idle rpm, I realized that the carb is really tricky on the needle positions. Especially the margin between rich and too rich but also lean and too lean is very narrow. With a mill so jumpy it can be quite difficult to reproduce carb settings.
 
The top right picture shows the idle needle of an  Axial .28 Spec 2 and of the GXR-28 below. You can clearly distinguish the spheroidal cut of the Axial needle from the conical cut of the GXR needle. The Axial needle starts with a diameter of 1.4 millimetres, the one of the GXR with 1.5 millimetres. Both needles go into a 1.6 millimetre bore.

Therefore the needle of the Axial .28 can increase the nozzle cross section continually up to 24 percent before opening it completely while the GXR-28 needle will work only up to 12 percent.
This means a much harder transition into the mid -range for the engine because the fuel flow increases abruptly.
Obviously this cannot be compensated by the distinct conical cut of the GXR-needle because the transition mentioned before turned out to be a major problem in practice. The engine -more so than others- tended to be overly rich, although the carb was set according to the manual and the engine performed good under idle and high rpm conditions.

Other possibilities like pressure increase in the fuel tank or fuel holdup in the pump were ruled out, performance did not really change by using different glow plugs either (factory, Rossi Nr. 3, Rossi Nr. 4, OS R5 in identical setup.)
 

 

The GXR-28 also surprised me temperature-wise: It broke the 100°C barrier on its second tank! A richer setting was not possible because the engine wouldn't have been able to run without the glow plug heater. This left me with decreasing the idle-RPM which was no problem thanks to the massive 44 millimetre flywheel.

 
 Lights, camera, a-and action!
 

When it comes to carb settings you can doubtlessly find more docile mills. Especially the tendency to be simply flooded in the critical rpm band where the clutch fully engages obliged me to turn the engine leaner than necessary. Temperature problems? The German manual suggests an optimum of 120°C, the English manual specifies this to the bottom of the heatsink.
At this time of the year you can go easy with things but still we'll have an eye on the engine's temperature. 

Having arrived at our test track the GXR-28 had to be brought to life. With temperatures close to freezing point a high-RPM starterbox can really save your day - but in the end we managed to fire up the GXR-28 with the pull-start.
 
The first few minutes were easy-going: On the one hand the engine had to be set correctly, on the other hand the steering had to be trimmed before we could really hit it.                                                   But come on, honestly, a wide truggy suspension and the big tires can be really seductive, can't they? ...

As expected, this is no big deal for Kyoshos ST US Sports. Nothing broke thanks to the tough plastic material, nor did the suspension float increase. The body is strong enough to withstand impacts and guard the heatsink in rollovers.
The short joint cup mentioned in the first part (see picture) caused no problems, the torque rod is strong enough to keep the dogbone in place – all as right as rain!
And in case there are still problems caused by extreme, overpowered driving the longer joint cup FM-185 (2 pcs; € 13.90 Kyosho Germany retail price) will do just fine.

Engine performance is where you'd expect to find it with a rather simple built three-port mill. There is no way for the GXR-28 to come near to engines in a comparable price range like the Axial .28 Spec2 (4.6ccm, 8 ports) or SH-Engines X-4 Blue (4.1ccm, 5 ports, turbo ported crankshaft)
But the sturdy pullstart is to the GXRs' credit - and this is more important to the novice than sheer engine power. Pullstart-wise Kyosho's GXR-28 excels all the pull-start engines listed above. (and all others that I know of)
But beginner-friendliness is put into perspective by both the tricky carb settings and the extensive drift of the fuel-mixture generation against carb temperature.
While testing this meant that the engine (cooled down to 20°C) was nearly impossible to restart with the fuel-mixture set to optimum working temperature. The idle had to be set much leaner (too lean for operating temperature) to allow for a decent start.
The idle-RPM acted just like that - it drifted way more than with other engines over the temperature range. But this was less critical due to the late coupling-point. Leak air could be ruled out as a possibility of the tricky engine performance afterwards.

A few words on the kit-setup: By and large it is okay, but the suspension is in desperate need of some setup tuning. The following pictures show the suspension at moderate acceleration, light braking and rather slow turns.
In all three situations you can clearly see the early and exaggerated response. This causes imprecise steering at higher speeds. On shifts in direction the Inferno ST reacts much slower than (could be) expected. Acceleration comes with lots of understeering while braking causes a hardly controllable rear end - both because of the huge weight transfers that are caused by the soft suspension.

This is sufficiently sorted out by putting harder springs onto the front and rear end - like Kyoshos' white ones (IF-336W) or springs with a similar rate from other manufacturers. You should also increase the viscosity of the oil to improve the handling of the truggy on bumpy tracks, in case it becomes too jumpy with the harder springs.

By the way, harder springs will eliminate another problem: With the stock springs the adjustable range of the knurled nut is not enough to set the ride height to a reasonable level. (suspension arms parallel to the ground)
 

Unfortunately the maximum steering throw of the Inferno ST is just below 30 degrees for the inner wheel so the turn radius is pretty large even when driving slowly.
The culprit is quickly found: As you can see in the picture on the right the c-hub comes with a generously sized lug. By trimming this down you can increase the steering angle up to about 35 degrees.
But now the dogbones run the risk of sliding too deep into the axle shaft joints and therefore bind under maximum steering throw.
This can be avoided by upholstering the axle shaft joints with foam rubber or buying prop shafts  IFW-163.
To make use of the increased steering throw, the servo - servo saver linkage has to be relocated to the outside, since the ball joint would hit the upper suspension arm mount in its stock position.

The c-hubs of the Inferno ST-R (IF-304B, IF-305B) come without these lugs.


 
 Checkpoint!
 

Meanwhile it should be clear that 'features' do not count to the strong points of Kyosho's Inferno ST US Sports. But not a lock of them is the problem here - more so the reduction of costs that led to some compromises that are not on par with the class standard. In combination with the GXR-28 it all doesn't really come together. Not even the excellent transmitter and the ingenuous pull-start can help it.
I'd like to stress, that for the manufacturer, 'RTR' is always a compromise between adequate equipment and  possible equipment given the planned retail price. So I'll take both aspects into account.
The ratio of price and equipment can only be compared to equal packages of other manufacturers.

But Kyoshos' truggy stands out when it comes to the assembly: I have never seen an RTR car that cleanly built. Combined with the known Kyosho-quality of parts this can only mean the maximum rating. 
It's the same with the rigidity: it is based on the high quality of parts but also on the tradition of the Inferno MP series which features well-dimensioned parts without 'teething troubles'. (the missing spiders are counted to the equipment section because the diffs are built for four bevel gears!)

The stock setup came as a surprise for me - it felt 'just right' on the workbench, and to be honest, I would have done it likewise with my buggy-based experience.
However the long suspension arms are not to be forgotten - they put much higher loads onto the shock absorbers. The long suspension travel then leads to dramatic weight transfers - with all their consequences.
Apart from the shock setup the factory set suspension geometry is quite a good foundation for further tuning - which depends on the actual track anyway.

Finding hop-up parts is no issue with Kyoshos' beginner truggy: You can indulge yourself in factory parts of the Inferno MP 7.5 buggy and the ST-R truggy. Furthermore the MP 7.5 buggy is widely supported by aftermarket manufacturers. The only disadvantage? The parts are costly - but which tuning isn't?
The suspension can be set up by adjusting shock geometry, camber, toe-in (but without turnbuckles) and steering. (Ackermann geometry) Blocks for anti-dive/anti squat and toe-in settings for the rear suspension have to be purchased separately, just like the c-hubs for more caster.

Conclusion: Being a technically adept hobbyist I really liked the Inferno ST US Sports and its clever details right from the beginning. But from the beginners' point of view the rookie-factor is affected by the tricky engine.
Kyoshos' Inferno ST would have been spared such a down-rating of the features (which is the only real weak point) by raising the retail price to about € 599.
With this budget at least one of the three shortcomings (Steering setup/2 spider diffs/engine mounts, servos and engine) could have been eliminated to gain at least one more point.

The competition is really tough - and this is not going to change any time soon as the truggy market booms.
Long feature-lists are doubtlessly good sales arguments but it should be mentioned that not only features but also quality will affect the cost/performance ratio. Weighing both factors can only be a personal opinion.

Those intereseted in Kyoshos' truggy should really consider to take a look at the ST-R model.  The race version leaves no room for compromises in the feature-department. Should you already have decent R/C equipment the additional charge for the pro model is put into perspective because the engine and servos of the RTR-version don't really offer good medium term value.


Special thanks to Kyosho-Germany!
  

 
 Text by Aaron Banovics, photography by Aaron Banovics and Gerhard Banovics, translation by Markus Simon
 This article has been published on www.offroad-cult.org on 22.12.2006.