MXA’S 2012 KTM 125SX MOTOCROSS TEST: AN OLD FRIEND RETURNS

February 27, 2012
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Q: FIRST AND FOREMOST, IS THE 2012 KTM 125SX BETTER THAN THE 2011 125SX?

A: We don’t know, because KTM stopped importing the 125SX to the USA four years ago. Still, we know that KTM has made quantum leaps forward since 2008.

Q: WHY IS THE KTM 125SX BACK?

A: When KTM debuted the original KTM 144SX in 2008, its sales quickly eclipsed those of the 2008 125SX. KTM took one look at the sales numbers and decided to cut their expenses by only importing the more popular 144SX, which was soon to be called the 150SX.

So, why is the 125SX back in the KTM lineup for 2012? First, many race organizations didn’t know what to do with the 150SX. While it was amateur legal against 250cc four-strokes, it wasn’t legal in 125cc two-stroke classes. Second, KTM thinks of the 2012 KTM 125SX as a stepping stone bike. Because it is more than 20 pounds lighter than the typical 250 four-stroke, it is the logical choice for young riders transferring out of the 85cc class. Finally, while the USA is wild about 150cc two-strokes, the rest of the world still demands 125cc two-strokes. Since KTM is making them for all the other countries, it seemed logical to load up a couple of containers and ship them here also.


Return to triumph: After being gone for four years, MXA test riders were glad to reunite with the 125SX. It has the same linkage and updates of the other KTM two-smokes with a personality and purpose of its own.

Q: WHAT’S THE DIFFERENCE BETWEEN A 125SX AND A 150SX?

A: The list of differences is relatively small.

Displacement. The 150SX is 143.6cc. The 125SX is 124.8cc.

Bore and stroke . The 150SX bore and stroke is 56mm by 58.4mm, while the 125SX is 54mm by 54.5mm.

Gearing.
More power can pull taller gearing, thus the 150SX has one more tooth on the countershaft sprocket.

Power valve. The power valve must be timed and optimized for each engine’s port timing and bore and stroke. The 125SX and 150SX have a power-valve height adjustment just below the expansion chamber.

Weight. Because the 150SX has a different bore and stroke, it is a little heavier than the 125SX.


Instant upgrade: The 38mm Keihin PKW feeds a Moto Tassinari VForce3 reed, which is standard on KTM SX bikes.

Spring ding: The KTM tiddler has a very soft 4.7 kg/mm shock spring, but a bit more versatility thanks to the new linkage.


Q: WHAT CHANGES DID KTM MAKE TO THE 125SX FOR 2012?

A: All the orange two-smokes get linkage this year. To accommodate the linkage, KTM naturally had to change the frame, swingarm, airbox, shock and shock tower. Since they were changing those parts, KTM decided to make some performance improvements in addition to the necessary alterations for the rising-rate linkage. Here are the updates:

Linkage. When only the four-strokes got linkage last year, the marketing team at KTM was quick to claim that PDS was better suited to two-strokes. Like two-strokes, the PDS system is lightweight and favors more ground clearance. Now they are telling us that linkage with a higher geometrical rising rate is the way to go.

Frame. The upper mount of the frame needed to be centered and repositioned for the linkage, because the shock has to be centered to drop into the hole in the swingarm. It also needed a bracket on the bottom for the linkage to bolt to. The rest of the frame geometry remains the same as last year.

Swingarm.
The swingarm has to have a place to bolt to. KTM took the opportunity to refine the design of its one-piece, cast-aluminum unit to get the right amount of rigidity in the right places.

Head stay.
There is a new cylinder head stay/engine bracket.

Suspension.
The WP forks and shock get new seals, bushings and settings. The new shock length is 490mm and the stroke is 130mm. The old PDS shock is 73mm shorter overall, and the stroke is 28mm less.

Air boot.
An advantage of the PDS system was that the shock could be offset to one side to make room for a straighter intake system. Because of the linkage, the 2012 shock has to be centered in the frame, so the intake had to be redesigned. KTM tried to compensate for this with careful performance tuning to the air boot’s aerodynamic shape.

Kickstarter. We can’t imagine breaking a 125 kick lever, but all the Katoom two-strokes get stronger kick-levers, courtesy of a reshaped casting (which was really done to accommodate the larger-displacement two-strokes).

Tires. The old Bridgestone tire combo has been replaced with Pirelli Scorpion Mid-Soft tires.

Bodywork.
KTM is quick to name-drop here. Apparently, GP legend Stefan Everts has a flare for style. The airbox and rear number plates are now white (they were black last year). The rear fender has been reinforced, and there is a new Renthal crossbar pad for protection.

Q: HOW DOES THE 2012 KTM 125SX RUN ON THE DYNO?

A: Peak horsepower on the 2012 KTM 125SX is 34.69 at 11,200 rpm. That is decent power for a 125cc two-stroke, but the real story for the 2012 125SX isn’t peak, which is only six-tenths of a horsepower more than a Yamaha YZ125. Instead, consider the low-to-mid transition compared to that of the YZ125. At 7000 rpm, the KTM 125SX makes three more horsepower than the YZ125; at 8000 rpm, it makes 2-1/2 horsepower more. From 9000 rpm to 10,600 rpm, the YZ125 and 125SX are a match?until the KTM jumps up at 11,200 to gap the Yamaha by six-tenths. The increased low-to-mid allows the KTM 125SX to get on top of each gear quicker, which makes the powerband feel crisper and snappier.

Strangely, KTM 125SXs used to make a lot more peak horsepower. Back in 2007 and 2008, the KTM 125SX was a power monger. How much more power did it make? Our 2007 KTM 125SX pumped out 37 horsepower (at 11,700 rpm), while the 2008 KTM 125SX hit 38.13 horsepower (at 11,400 rpm).

Q: HOW DOES THE KTM 125SX COMPARE TO THE 150SX ON THE DYNO?

A: There is no comparison. At no point on the dyno curve did the 125SX come within three horsepower of the 2012 KTM 150SX. The 125SX peaked at 34.66, while the 150SX hit 38.83. This power difference played out at every increment along the powerband. There was more than a four horsepower difference from 9000 rpm to 11,900 rpm.

Q: HOW FAST IS THE KTM 125SX ON THE TRACK?

A: Every MXA test rider immediately felt the improved bottom and midrange power of the 125SX. You didn’t even need to ride the bike to know that the power was situated in a lower rpm range. When someone went by on the bike, you could hear the engine running at a lower pitch. We also noticed this when we watched Ken Roczen race a KTM 125SX at the Fermo, Italy, 250 Grand Prix. He wasn’t going for the flat-out, high-rpm, screamer attack that is the bread and butter of most tiddler pilots. Originally, we thought he was riding it wrong, but now that we have been racing the KTM 125SX, we recognize that he was working in the sweet spot.

There are some upsides to having the usable power at a lower rpm. Keeping a 125 from falling off the pipe for an entire moto is very difficult, but at least the 2012 125SX doesn’t have as far to fall. The torquey powerband also makes it easy to transition from the brakes to the throttle in mid-turn. If a 125SX rider works hard, he can put in fast laps and stick with 250Fs. On straights and uphills, however, the 125SX pilot will be reminded that his engine is half the size of a 250F.

A low-to-mid powerband is best for a stepping-stone bike, because it makes the larger chassis and increased power easier to adapt to.

Q: HOW WAS THE GEARING?

A: When you’re working hard to wring out a two-stroke, it is common practice to add a tooth to the rear sprocket to get higher in the powerband quicker. With the strong low-to-mid of the 2012 125SX powerplant, the sweet spot of the powerband was always close at hand. We were happy with the stock gearing.


KTM’s silencer has a very unique and durable nylon bracket system.

Q: WHAT WERE OUR BEST JETTING SPECS?

A: Here’s what the MXA wrecking crew ran in the 125SX’s 38mm Keihin PWK carb.
   Mainjet
: 182
   Pilot
: 42
   Needle
: NOZI
   Clip
: Third clip from top
   Air screw
: 1-1/2 turns out
   Notes
: For soft terrain, fast riders, big hills or cold weather, we run a richer pilot and main. For comparison purposes, the 150SX comes with the same main, but a leaner 40 pilot. We, however, do not run the 182 mainjet on our 2012 KTM 150SX, preferring to go to a richer 185.

Q: HOW GOOD WERE THE WP FORKS?

A: They were on the soft side for race suspension, but that’s understandable when the target rider is just stepping up to his first full-size bike. MXA assigned the 2012 KTM 125SX to our lightest test riders, and they felt the suspension was in the ballpark for their weight. And since the only other KTM that we have to compare it to over the last four years is the 150SX, we can’t really comment on any suspension changes from model year to model year. We can say, however, that the 2012 has better suspension than the 2008 model.

Q: WHAT ARE MXA’S RECOMMENDED FORK SETTINGS?

A: The KTM 150SX and 125SX share the same suspension components and setup. Here’s what we ran in our 125SX. The stock settings are in parentheses:
   Spring rate
: 0.42 kg/mm
   Compression
: 6 clicks out (12 clicks out)
   Rebound
: 10 clicks out (12 clicks out)
   Oil height
: 350cc
   Fork-leg height
: 5mm up
   Note
: Bigger and faster riders should go to 0.44 or heavier springs.

Q: IS THE LINKAGE REAR SUSPENSION BETTER THAN PDS FOR THE 125SX?

A: Even if KTM’s linkage system were significantly better than KTM’s PDS no-link system, it is still a tough call to rate the linkage system over the PDS. Why? Three reasons:

(1) The linkage adds 5 pounds to a bike that used to be the lightest full-size bike on the planet.

(2) The linkage compromised the intake tract by forcing the air boot to go around the shock. This could well be where the missing horsepower from 2008 went.

(3)
Very light bikes with relatively low horsepower don’t put as much demand on shock performance as their big brothers. That being said, in many situations, MXA test riders were hard-pressed to note a major difference between the linkage bike and its PDS predecessor.

On the plus side of the linkage scale, there is a distinct improvement at high shock-shaft speeds (where the PDS had a tendency to kick). Additionally, the linkage system is easier to set up for a wider variety of riders. The PDS system often required different shock springs and more test time to get it dialed in.


Sky shot: The 2012 KTM 125SX takes the guesswork out of judging speed for most jumps. The best strategy is to leave it pinned.

Q: WHAT ARE OUR RECOMMENDED SHOCK SETTINGS?

A: Here is what we ran. The stock settings are in parentheses:
   Spring rate
: 4.7 kg/mm
   Hi-compression
: 2 turns out
   Lo-compression
: 15 clicks out
   Rebound
: 15 clicks out
   Race sag
: 100mm
   Notes
: With PDS, you have to have the right shock spring to get a good setup, but it’s not so critical with the linkage system.

Q: HOW DID THE 125SX HANDLE?

A: In a world of lumbering four-strokes, riding a quick-revving, lightweight two-stroke feels like playing a video game. On the one-two-five, you can be bolder than the competition. You can leave it pinned longer, flat-land jumps, wait until the bitter end to brake and get really squirrely in tricky sections. Not only can you ride like an idiot, but you can survive the experience because it’s easier to wrestle the lightweight bike around.

The KTM 125SX chassis is pretty close to neutral and doesn’t have a tendency to oversteer more than understeer?or vice versa. It does what you tell it to do. So while our four-stroke-only test riders drove the bike around by the front wheel, our experienced two-stroke holdouts let the rear wheel hang out for maximum speed. The common denominator was carrying speed and being aggressive.

Q: HOW GOOD WERE THE PIRELLI TIRES?

A: KTM equips their thumpers (SXF) with Dunlop Geomax MX51 tires and the two-strokes with Pirelli Scorpion Mid-Soft treads. MXA testers are more familiar with Dunlops than Pirellis; however, the Pirelli Mid-Soft tires, like the SX two-strokes, were a fun change of pace. Both the tires and bikes are lightweight and like loose dirt. For harder terrain, the Mid-Hard is a good choice. In any case, a rubber update was overdue. The previous models featured outdated Bridgestone M59/M70 tires.

New beast: The 2012 125 powerplant gained low-to-mid as a trade-off for the fire-breathing peak output of previous KTMs. It comes on earlier and likes to be shifted into the next gear. 

Q: WHAT DID WE HATE?

A: The hate list:

(1) Power. If we hadn’t looked up our old dyno charts, we would have said the KTM was an in-the-ballpark, competitive 125cc race bike. But, KTM’s previous engine casts a shadow over every product they make. They live by the horsepower sword?and they also die by the horsepower sword. The old-school KTM 125SX was a weird-handling bike with a rocket-ship engine. Now, they offer a great-handling bike with three less horsepower than it had when it handled poorly.

(2) Bars. You would think that smaller, 125-size test riders would like the low bar height of the KTM 125SX?surprise! They didn’t. Just like the tall test riders, they wanted taller handlebars or bar mounts.

(3) Weight
. Weren’t we just praising the bike for being so light? Yes. For a 125, it’s still very light, but the linkage was like a Thanksgiving dinner for the KTM. The Yamaha YZ125 is lighter. The KTM 125SX hits the scales at 202 pounds.

(4) Preload ring. The good news is that the linkage system isn’t quite as picky about spring rates and sag. The bad news is that the plastic preload ring is easy to mangle. With MXA’s constant rotation of test riders, the preload ring has a short life expectancy.

(5) Gas cap. Even KTM doesn’t like KTM’s gas caps. Ryan Dungey’s, Marvin Musquin’s, Antonio Cairoli’s, Max Nagl’s and Ken Roczen’s bikes have thread-on gas caps. Maybe we will too?in 2013.

Q: WHAT DID WE LIKE?

A: The like list:

(1) Grunt. We liked the low-to-mid power of the KTM 125SX. It made it easier to ride and was a great alternative to the more powerful KTM 150SX.

(2) Clutch. We love hydraulic clutches. And while we didn’t have to use the clutch as often on the new-style powerband as we did on the old screamer engine, when we did need it, the Magura system was awesome.

(3) Front brake.
The 260mm Braking rotor and Brembo caliper can bring the 450 to a screeching halt, so imagine what it can do for the 125?it can throw you over the bars.

(4) Hero factor.
On the 125SX, we were able to ride harder, longer, leave it pinned and pitch the bike into turns better than on any 250 four-stroke. It felt like we had an “S” on our chests.

Q: WHAT DO WE REALLY THINK?

A: The KTM 125SX isn’t just a watered-down version of the 150SX; it has a personality all its own. It doesn’t pump out the raw numbers that it did a few years ago, but from a marketing standpoint, the 125SX has a much more user-friendly engine than the old screamers. We think this bike will be a big success. Yes, Pro riders would love to have more top-end power. But, guess what? Pros don’t race the 125cc two-stroke class. Bringing back the 125SX was a smart choice for a company that has been making all the right moves.

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