If you’re into racing dirt bikes then Thanksgiving in Southern California means one thing: Troy Lee Designs and Elrod Racing’s A Day in the Dirt Motocross Grand Prix. For the second year in a row MotoUSA participated in the annual competition, this time at Pala Raceway aboard a pair of 2011 KTM
SX-F motocrossers. We chose the 450 and 350 SX-F in an effort to compare and contrast the differences in a racing situation where both man and machine are put to task. We’re splitting these race tests into two individual reviews by model.
2011 KTM 450 SX-F
With a price tag of $8799 the 450 SX-F continues to be KTM’s top-level motocross bike. As its nomenclature implies it is designed to compete in the 450-class which is reserved for motocross bikes with the highest capacity, most powerful production engines available.
Compared to your typical jump-laden mx races, A Day in the Dirt utilizes a grand prix-style formula. This usually equates to a long and relatively high-speed course with a reduced number of big jumps and technically challenging obstacles. This type of layout generally favors bikes that put lots of power to the ground and this is an area where KTM’s big-bore Single won’t disappoint.
450 SX-F Jetting Specs:
Clip Position: 6
Fuel Screw: 1/2 turn
Leak Jet: 50
Despite employing a conventional 41mm FCR flat-side Keihin carburetor the 450 engine runs well when its warm even compared to fuel-injected dirt bikes like its sibling 350. From bottom-to-top the engine pumps-out a linear and tractable spread of power which makes it easy to manage regardless if you’re on the first or last lap of a moto. I recall how its friendly powerband didn’t overly fatigue me during my 40-minute Novice Vet Classic GP which I finished fourth out of 26 riders.
Obviously since it has an extra 100cc’s of displacement on the 350, it’s no surprise that the 450 feels faster—especially from low-to-mid-rpm. However get the engine spooling at high revs and the difference in top-end power becomes far less noticeable despite what the dyno chart shows.
“It feels like a big ol’ tractor and is really easy to ride,” commented Intermediate-level Nick Thiel who rode it to a 15th place finish out of 57 riders in Sunday’s Open GP. “The engine works really well on slick terrain and flat corners and the rear tire hooks up and resists spinning excessively out of corners even on a worn tire. It flattens a little bit on top but it makes great power everywhere else that all you need to do is grab another gear. The five-speed tranny is also a big improvement over the four-speed they used in the past.”
The 450’s drivetrain delivered mixed results of us. While final drive gearing was spot on perfect for a fast GP, it might be too tall for most motocross tracks. Though we didn’t encounter any problems with the transmission the clutch did fade early on in our motos. This made it difficult on Saturday and impossible on Sunday to control power when entering a slow corner. It is important to note that clutch performance degraded as the engine oil accumulated more run time so it’s critical to use a high-quality lubricant (we used Motorex 10W-50 engine oil) and change it along with the filter often (at least every 5 hours) to reduce the chance of it fading.
While I never encountered any engine hesitation at my pace, Thiel did when he plowed over an obstacle in a high gear while simultaneously slamming open the throttle. He said it wasn’t bad but still is one area that could be improved upon. One feature that we unanimously loved is the push-button electric start which completely eliminates the worry of getting the engine re-lit after a crash.
Although there were a fair number of wide-open straightaways there were also many slow speed corners and chicanes. This is one of the few areas that the big 450 struggled. And it wasn’t due to its brakes. The Brembo calipers and stainless-steel lines work fantastic, delivering strong and fade-free performance. Instead, it was because it feels more cumbersome. While it weighs only 4 pounds more than the 350 (250 lbs. vs. 246 lbs. with a full 1.98-gallon tank of fuel) in motion the difference feels much more substantial with it requiring more steering effort.
Part of the problem can be attributed to the softly sprung front end. In stock form the front suspension doesn’t offer enough pitch control especially during aggressive corner entry. Adding compression damping did slow down weight transfer but never to a satisfactory level. This made the bike more prone to head shake over braking bumps and therefore more challenging to control when entering a corner.
“It got really nervous at high-speeds and felt like it would pack up,” explains Thiel. “I think it had a really soft fork spring (stock is 0.50 kg/mm). It’s hard to tell—I think the valving was soft too—especially around the mid-stroke but the spring ratio had more to do with it.”
We also had damping complaints with the rear end too. While the rear suspension was resistant to shock fade even during long motos and it performed great in flat corners or on the power around berms and outside corners, when high speed whoops or large bumps are encountered it had a tendency to swap—especially off throttle. It is important to note that our test bike was fitted with a heavier shock spring (6.0 kg/mm compared to the stock 5.7 coil). We addressed the problem by first confirming race sag (between 100 and 103mm) and then reducing both low and high-speed compression as well as slowing rebound damping slightly. While these changes helped we still felt that the balance of the bike was off.
“Basically the fork contradicted the shock and vice versa,” noted Thiel. “The shock was stiffer than the fork and it wasn’t well balanced. If it had a more balanced feel or a stiffer front end, I think it would work a lot better.”
Since the fork was so soft, body position became a key method of reducing the load on the front end. Fortunately, the KTM features well thought out ergonomics that allow average and above sized riders to maneuver their body around the motorcycle effectively which can help mask handling woes. One thing we did notice however is that the handlebar bend felt like it had more rearward sweep as compared to bar employed on the 350 which made us feel a tad bit less comfortable.
Although we weren’t overly impressed with the balance of the chassis, with a little help from the aftermarket suspension world we think we could get KTM’s 450 SX-F dialed in to our liking. Sure it doesn’t feel quite as nimble as the 350 for the most part the ergonomics and powerful brakes feel about the same and the suspension damping was fade-free. And who can forget about that engine! Not only does it pump out a forceful spread of power at all rpm, it produces it in such a way that it isn’t challenging to exploit and doesn’t fatigue the rider.