» 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 #1: Overview and Construction

 Once upon a time ...

"Inferno-ST" - with some old stagers this name might ring a bell. Right, Kyosho offered an "ST" model in the age of the Turbo-Inferno. Based on an already ten year old buggy chassis, with bigger tires, adjusted gear reduction and a truck-bodyshell Kyosho's first ST model was virtually the ancestor of the now popular truggy class.
The truggy of today is designed around a buggy chassis just as well. It is therefore stretched and equipped with longer suspension arms and shock absorbers. The bigger tires (about 140 mm in diameter) are compensated with a bigger spur gear in order no to overexhaust the engine.
Like that, truggies are nearly all-purpose models: Thanks to the buggy-like suspension they cut a fine figure on the race track, but the massive tires together with a vehicle-width of clearly over 400 mm and long suspension travel allow full blown bash heats just as well.
Whatever use the truggy is put to - the buggy chassis and the drivetrain normally stick it out unscathedly. 1/8th scale buggies got their reputation of being the toughest and sturdiest offroaders for a reason.
So it was just a matter of time until Kyosho would enter the new scene with a truggy model based on their pretty successful Inferno MP series. Now there are already three versions to choose from:

  • Inferno ST Sports: Based on the Inferno MP 7.5 this is an RTR-truggy - the beginner version so to speak.

  • Inferno ST US Sports: The US version (RTR as well) of the Sports is valorised equipment-wise but similarly built upon the modified MP 7.5 chassis.
  • Inferno ST-R: The racer of the pack comes without R/C and engine. The base is provided by the current MP777-SP2 buggy including its extensive tuning-equipment.

For this review Kyosho-Germany provided us with the Inferno ST US Sports which will possibly replace the Sports as a beginner model.
The following remarks therefore refer to the test model and cannot be unrestrictedly generalized e.g. due to serial dispersion, especially with the measured values.

Although "RTR" might sound like "unwrap and go" such models should always be disassembled and checked. Gas powered models, especially of this size, can be dangerous when out of control due to a loose bolt. You also get to know the vehicle along the way - sort of dry practice for the real thing, in case that maintenance work and repairs have to be done.

Length:      500mm
Wheelbase:   350mm
Width:       415mm
Heigth:      150mm
Weight:     3450g
(manufacturers' data)

27MHz transmitter,
servos, receiver
4.6ccm engine
fuel bottle
glow plug heater

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

 A look under the hood - and a quick overview
Without tires and body the Inferno ST could nearly pass for a buggy. On the second glance the longer suspension arms stand out - and - for the connoisseur of the MP 7.5 buggy chassis - the chassis plate that has been stretched to either end.
The layout feels pretty clean. At the moment Kyosho is the only manufacturer that does not take advantage of a laydown steering servo. By this they fail to take the "important" opportunity to lower the center of gravity by 0.2 millimetres; instead Kyosho seems to concentrate on the nitty-gritty: an upright steering servo with a good linkage lineup causes linear steering action and is exposed to less transverse force at maximum throw than a laydown steering servo.
The radio box can be numbered among the more spacious ones in the buggy/truggy class and offers seperate compartments for receiver and battery (up to five AA cells can be fitted in easily).
The fuel tank is located to the left of the center diff and equipped with an overfill fuel channel and an internal stone fuel filter (to avoid air bubbles in the fuel line and ensure stable engine operation even if the Inferno ST takes a longer spread-eagled rest on its roof after a rollover). Power is delivered by a 4.6ccm GXR-28 pull start engine connected to a two-piece aluminum tuned pipe.


 Appraisal: driveline and suspension

Once again we start our review where there is not much left to disassemble: The differential gears. As usual, the Inferno ST US Sports is using three of them, the centre differential balancing differences in rotational speed between front and rear suspension and thusly allowing to adjust the braking balance.
The differentials of the Kyosho truggy are built like one would expect them to be for an 1/8th scale off-roader: rugged o-ring sealed plastic body, steel crown gear and solid outdrives. But two juicy details are hidden inside the diffs of the ST US.

To withstand the greater forces that would come up in a truggy, Kyosho decided to use spiral gears for the differential. In comparison to straight cut bevel gears this type has a bigger contact surface and a better meshing  - in short the spiral gears run smoother and can take greater forces.

Although the crown gear is cast steel the (sharpened) macro view shows an amazingly high surface quality. The bevel gear is made of machined steel and hardened afterwards. 

A type of precision rather uncommon for this price range becomes apparent here: With 7.98 millimetres the bevel gear shaft clears the bearing for only two hundredths of a millimetre - a sensible measure given the precision gears. Normally the clearance ratio of the bevel gear would be four hundredths of one millimetre.

Now, if the intricate gears should enhance the stability of the differential then the things that lurk inside it can (not even) be understood as a paradox:
The differential is ready for a four spider setup but with the notorious SPORTS-series traditionally only two spiders will manage to cheat their way into the diff case.
Bevel gears (IF-102) and satellite shafts (BS-107) are needed for upgrading two differentials. To rebuild the center diff as well, a second IF-102 bevel gear set becomes due. Four bevel gears will not only enhance load transmission but also stabilize the outdrives.
The upgrade will cost you €20 (Kyosho Germany retail price)

Realistically speaking the stripped down diffs will bear up for quite some time. Comparable amounts of torque are transferred upon the crown gear and (with two spiders) to every bevel gear due to the different distances of the gears from the rotational axis of the diff.
Nonetheless four spiders will stabilize the outdrives in two planes (up/down - left/right) under load condition while two bevel gears can only support them in one plane. As a consequence greater stress is put upon the friction-type bearings of the outdrives. If this caused bearing play the meshing between sun- and spider gears would be impaired - this increases diff wear and the bevel gears will be ruined due to excess bearing  clearance.

Anyway, the high grade of the assembly has to be appreciated: the diffs are well shimmed, run smoothly and are filled with silicone oils of different viscosity. (front:medium, center:hard, rear:soft)

The gear boxes are of the 'one-chunk wonder' type; meaning that they will not only encase the diff and bevel gear but also hold the caster blocks and the shock towers. Therfore the gearboxes have to be built sturdier than the elastically supported boxes of the Robitronic Mantis Truggy for example. (click for a picture)
Both gearbox halves feature a sealing lip and fit together without any play. The differentials are factory shimmed - and dead right they are. After putting together both gearbox halves I can say that in my whole hobby career I have never experienced a bevel gear that would run so smoothly and nearly noiseless. No sticking, no tight spots, no rattle, no tedious fiddling around with shims... the gears mesh perfectly without even having been run in - it's just wonderful! For this reason these gears can be counted to the high-grade ones on the current buggy/truggy market. Enough grease has been supplied (which was just removed in order to take some photos) so there are no complaints about the assembly of this critical component - when counting the economised bevel gears to the equipment section.

With "e-clipped" hinge pins the suspension is connected to the gearboxes. It can only be distinguished from a buggy suspension when compared in length. Dirt-shielded c-hubs remain in the realms of the ST-R.

The wheel adaptors are also different from the ST-R and the buggy standard: instead of an aluminum hex with M10 fine thread the ST feels more at home with the 1/10th scale standard: The wheel is secured to the axle by an M6 locknut. The 14 millimetre metal hex is grown to the standard 17mm size with the help of a plastic adaptor. With the 14 millimetre hexes some monster wheels can be fit on too - provided that they got an inner rim diameter of at least 77 millimetres and 36mm maximum offset. 
The whole concept may look a bit funny to 1/8th buggy drivers and is probably the result of budgetary cuttings when compared to the ST-R. But thanks to the chequered locknuts the system provides a reliable wheel mounting.
The Inferno ST uses e-clips on nearly all suspension parts.
The more elegant screw-pins are only sporadically used at the shocks and the upper suspension arms.
Luckily the plastic parts are generously sized so that there are one or two spots left where an M3x5mm grub screw can be put in to secure the hinge pins, witch need some notching with the dremel where they meet with the grub screw. The forward pointing e-clip should remain on the hinge pin.

No set screws are needed at the front suspension arms because the front bumper keeps the hinge pins in position! Therefore the e-clips on the back side can also be kept as spares.

Kyosho goes plastic with the shock absorbers.                            On both front- and rear suspension they are 116 millimetres long and can only be distinguished by the minor spring preload at the front.

The preload (and therefore the ground clearance) can be easily set by a knurled nut so there is no need for plastic clips.
From the outside the shocks give a good impression. Even though they are not as perfectly filled as the Robitronic Protos shocks they feature an excellent smoothness not often found with RTR-models. The shock absorber on the top right picture has been quickly compressed for a couple of times. However the photo shows absolutely no leaking of oil.
Perfectly recognizable though is the absence of a securing clip. That means the sealing o-rings are installed into the shock body from the inside - not so great. The shocks generally don't appear maintenance-friendly at all because the damper cap is likely to cant while screwing down and can only be tightened with the guiding help of the knurled nut. (in case it remained on the shock body in the disassembly process)
The first rebuild will not be too far away because there is no way to fix any shock boots to the smooth body.
Shock absorbers of equal quality like from the Robitronic Protos or the Sports buggy series (MP 7.5 Sports) are favourable here.
If the rigidity of the plastic shock body is adequate -especially with the almost lay-down front suspension- remains to be seen.

The shock towers are taken directly from the Inferno MP 7.5 buggy. Unlike the ST-R the ST has a more of a laydown front shock geometry due to a smaller shock tower.
A thickness of 2.9 millimetres may not be too overwhelming but the low-mounted body posts and the truggy bodyshell should present an appropriate protection for the shock towers so no bent parts have to be feared of.

The shock mount turned out nice: At the suspension arm screw-pins are used - unlike the more complicated type on the ST-R which employs grub screw-secured rods.
At the shock tower the steel ball is held in place by an Allen screw and a locknut on the rear side. The chequered locknut is pressed against the shock tower when tightened and thusly prevents twisting. This type of mounting reminds a bit of the Lunsford Racing quick change system.
Equipped only with an Allen wrench and a Phillips screwdriver shock positions can be changed quickly. In order to take advantage of this feature the ST offers five mounting holes on each suspension arm and even more on the shock towers. (four at the front, six at the rear)


Power transfer is done without universal shafts but the suspension is built so that the dogbones will stay put - without the need of lugs - over the whole suspension travel. Furthermore there is enough play so that the dogbones will not bind up or fall out, even when facing flex which is generally inherent with suspension arms of this length - nice job!

 Chassis details

Because Kyosho's Inferno ST follows the classic buggy concept there are no real surprises in between the front and the rear end.
But it should not remain unmentioned that assembly- and material-quality of the (plastic-) parts is as excellent as the assembly of the RTR-model.

Eight bolts (and the two servo screws) have to be loosened in order to take out the R/C unit of the Inferno ST.
As mentioned in the beginning, Kyosho laudably employs servos mounted upright. Hence the radio plate is exclusively held by four plastic posts. (and the fourfoldly attached radio box)
In comparison to their MP6 predecessors these posts have become way sturdier so that upgrading them with aluminum posts (IFW-113) does not seem necessary even with stronger servos.
The servos that come with the kit are both labeled Perfex KS-1401. The plastic gear servo leaves a bit of a strange impression but more on that in the second part!

The radio box is of the "bigger is better" kind - which is always good thing to be!
The battery tray will accept nearly any four- or five cell battery pack. (Here: 5x Sanyo AA 2500 mAh NiMH - in GP cars only soldered packs should be used!)

The compartment next to it is a wee bit smaller. But also here it is enough for a standard receiver, (42x35x20 mm) thoroughly wrapped into sponge rubber.
An on/off switch is hidden at the rear side of the radio box, protected from dust and grime by a rubber sleeve.
To give the radio plate a more "racy" look it is equipped with a transponder mount. The position is rather badly chosen (due to the missing sideguards) because the transponder dangles more or less freely in the open.

Between the front and rear suspension, erverything is aligned purposefully around the centre differential block.
Latter is equipped with (seperately adjustable) disc brakes.
In order to prevent canting of the vented steel brake discs a pair of brake guides is mounted to the chassis plate. Each guiding thingy can take up to two discs. A four-disc brake will not only need further brake calipers but also longer pad mounting screws.
Talking about the brake calipers: These are self-releasing, meaning that they do not need any spring to push them apart after braking. The calipers have enough play so that they can be realigned solely by the brake disc - which works smoothly with the Inferno ST.
To increase the durability of the brake pads they should be glued firmly onto their steel shoes!

The front brake is equipped with a fuel splash guard. This should prevent soaking the brake disc in the course of over-enthusiastic refueling maneuvres and therefore reducing brake power caused by the residual oil. In order not to get the chassis all manky with dirt the tank features an overfill fuel channel that will accept excess fuel and guide it through the chassis to the ground.

As we're talking about fuel: As shown in the picture below the engine is mounted directly onto the chassis in good old Sports-fashion, although (compared to the GS-21R) the crankcase already has 'proper' mounting lugs. Luckily the chassis accepts motor mounts but the fact that they are nowhere to be found in this kit (also facing equally priced truggy-competitors) is a clear 'no-go' equipment-wise.
If you wanted to provide your GXR-28 with a more stable base or race other engines as well you would have to pay about € 20 (Kyosho-Germany retail price) for the engine mounts. (IF-107 and IF-108 are needed)

To support the long truggy chassis both gearboxes are equipped with torque rods, the Kyosho-like design consisting of two ball ends and a threaded rod.
There are numerous 'hop-up rods' by aftermarket suppliers that are milled out of an aluminum block and promise even more rigidity but I have had positive experiences with the ball end thingy that can give a little in an emergency situation. And there always remains the question of personal taste - whether you like a 'harder' or a more flexible chassis. 
A harder chassis provides a more precise handling but setup mistakes are more likely to cause the truggy behave badly.

The chassis' flex could cause problems for totally different reasons. I praised the suspension because it would -not nearly- let the dogbones bind up or fall out over the full suspension travel but the rear part of the chassis might not come out that praiseworthy.
The photo down to the left shows the dogbone to the rear gearbox. There is not much play due to corresponding lugs inside the centre diff outdrives that could give way (eg. after a harder langing) without risking to bend the dogbone.
At the same time there is not much left that could keep the dogbone from falling out if the chassis flexed too much. There seems to be no reason to use the short joint cup especially when a longer version actually exists - looking at the front gearbox in the picture down on the right.
For fairness reasons it has to be admitted that the rear part of the chassis is really firmly built which makes it virtually impossible to 'tease' the dogbone out with your bare hands.
Anyway, as an off roader you know the forces that are at work when you're proverbially off the beaten track  - so only pracitcal experience will show whether the short joint cup is enough or not.

Last but not least comes the steering. Tried and tested parts of the eigth-scale buggy-world are put to use. Hollow 6-millimetre steering posts reinforce the front gearbox - it's just the kind of steering setup we like!
The beefy steering arms seem to be made of a harder type of material than the other plastic parts. The adjustable servo saver features a heavy spring with a wire gauge of 1.5 millimetres. Out of the box it is set as soft as possible - supposedly to go easy on the servo gears.
All in all it's a solid piece of work that will also get along with stronger servos. (plus 10kg*cm would be recommendable for the steering)
Something about this steering should look a bit odd to the observant reader who has already had contact  any 1/8th offroad car 

... the last hint is provided by the picture on the right - before the 'puzzle' is solved.



"In dubio pro reo" - and Jamara proved it with the 'Track-Sau' that it is indeed possible that a cropped 2 millimetre steering link can work with a plus 4kg car.
The only thing is: Jamaras' link was two centimetres short, absolutely parallel to the push/pull direction and connected to the steering arm by a five millimetre double-sided adaptor. (see picture)

Kyosho's version is about three times as long, not parallel to the push/pull direction (because the linkage declines a bit towards the steering) and is connected to the steering arm by a three millimetre single adaptor. Lacking a counter bearing this induces more play than all the other connections of the steering altogether.

The good news: Even with a supply voltage of 6 volts the 'bent wire' remains pretty unimpressed by the (servo saver-alleviated) steering power of the servo.
The bad news: It needs a much stronger servo and a way harder servo saver to tap the full potential of the Inferno ST.

The Tragedy behind it? For just € 10 you could have an 'offroad-proof' luxury linkage using a stainless steel turnbuckle and six millimetre ball ends free from float!
It remains unclear, why Kyosho decided to give up the perfectly able linkage of the MP-6 Sports Buggy and use the linkage shown above.       


 Steering - reloaded!

OK, let's assume that you overlooked the last couple of pictures ...
Then here's how to assemble the steering linkage of Kyosho's RTR-truggy! ;-)

First some thoughts:

  • In principle a linkage could be everything stable enough and with the possibility to mount it onto the servo saver with an M3 screw. Depending on the construction the servo saver might have to face "outwards" because the linkage could touch the reinforcement plate which connects the steering posts to the gearbox. I'd like to avoid the outward steering link, the inward-type looks classier and is better protected.

  • For Kyosho is using an upright steering servo against current (promotional) opinions the merits of this kind of setup shold be taken advantage of. This implies a parallel linkage lineup from steering arm to servo saver to avoid transverse force.

It can be difficult to get the linkage "high enough" on the servo side.
Especially when the servos were mounted without vibration-reducing rubber pads.
For cases like that Academy or HPI offer threaded 5.8 millimetre ball studs with a low installation height. A ball stud like that is attached to the servo arm with a locknut.
Another ball stud is used at the servo saver. It should be one of the kind that have to be secured with a screw from below - don't forget your threadlock!
A turnbuckle is used to connect both ball ends - it should be between 50 and 55 millimetres long.

The pictures above show that I've worked ahead a little bit and got rid of the original servo in favour of Thunder Tigers' DS1015. The torque readings in part two will show why.

Another hint: The Inferno ST US's steering uses only friction bearings but still it's not a bad idea to prevent dust from getting into them. Before the steering arms are set onto the posts a piece of foam rummer should be put on. This helps reducing the steering float and furthermore no (astonishingly fast) grinding dust can settle.
Foam rubber is also vital should you choose to upgrade a ball-raced steering: The small ball bearings are pretty damageable!




Interim results - truly not easy!
Kyosho's newest RTR-Truggy is a sophisticated concept. But in practice it seems rather rough - perhaps an allusion to the preferential field of application of the Inferno ST?

First example: On the one hand the US-Sports excels with luxury gears - but then the budgetary red pencil gets in the way of a consequent continuance that would allow for four bevel gears. (at least for the front and rear differential)
We'll have some other examples coming along until the final verdict.

To push this in perspective: It's laudable when a manufacturer like Kyosho takes the meaning of "RTR" serious and presents an excellently assembled car - quality control will have its share at the € 559 retal.
And apart form the example listed above - plus the missing engine mounts and the absurd steering linkage - at least the rest of the provided equipment is about average.
A well equipped fuel tank (stone filter, overfill protection and good fittings), steel gears, well glued high-grade tires with foams and the big separated radio box millitate in favour of Kyoshos' truggy.
Only: They can not really drown the bitter smack that is left by the shortcomings that are way below class standard.

 Text and photography by Aaron Banovics, translation by Markus Simon
 This article has been published on www.offroad-cult.org on 12.12.2006.