Do you ever think about building the ultimate cut-and-paste motocross bike? This is the dream where you take the chassis from the lightest bike, the motor from the fastest bike and the suspension from the best-handling bike. In theory, you could have the best motocross bike on earth, but it would cost as much as four bikes, and you would have a bunch of really crummy parts left over.
This year, however, it might be easier. You may be able to get all the parts from just one bike. The 2017 KTM 450SX-F will most likely be the fastest, the lightest and the most advanced 450cc motocross bike you can buy. And, it’s already assembled.

Last year the KTM got a new motor and frame. KTM was on a mission to find and eliminate excess weight. The motor remained a single-overhead-cam, four-valve, electric-start five-speed, but it was made more compact in all dimensions and got a new throttle body and EFI system. The bodywork, the linkage and the frame were all new and all lighter. The WP 4CS fork, on the other hand, saw only a few changes, as it has each year since it was first introduced in 2013.
When the smoke cleared, the 2016 KTM 450SX-F was the fastest and lightest 450cc MX bike on the market, and it was the only one with electric start. It was an amazing feat. The only reason it didn’t take over the world was the fork. The 4CS was basically an off-road fork that had been modified and recast as a motocross fork. Then, at the mid-year point, KTM came out with a limited run of Factory Editions that tipped the company’s hand for 2017. The WP AER 48 was standard up front. This was an air fork that shared virtually nothing with the old 4CS. New game.

Now the 2017 450SX-F has an even newer version of the AER 48 fork, plus an interesting list of other changes. The bike now features what KTM calls traction control, which can be activated through a handlebar switch. This isn’t the existing “launch control” that is designed to be used only for the start; it’s an embedded program designed to prevent excess wheelspin throughout an entire race. It [isn’t] traction control as defined by the AMA Pro Racing rulebook, which prohibits traction control that compares front-wheel speed and rear-wheel speed. The KTM uses no wheel sensors.
The rear suspension valving and spring are new and designed to work with the new fork. The top triple clamp is different, and the handlebar clamps feature a separated lower clamp and bridge-type upper clamp for more torsional stiffness. The rear brake pedal is 10mm long, and less aggressive brake pads are used. The aluminum head stays are 60 grams lighter than the steel ones they replace.

At the risk of reporting news that isn’t news at all, the KTM motor is an absolute beast. Aside from the Husky 450, which shares the exact same motor, it was the most powerful motocross bike of 2016 by a significant margin. Unless there are unreported surprises still coming, it will be the most powerful motor of 2017. It wins in low-rpm torque, the midrange and on top. The fact that the motor is so powerful throughout the rpm range is what makes it manageable. If the power were all on top, it would take a Dungey-level training program just to hang on. But, the KTM builds predictably to a climax in the mid-9000-rpm range, which is very high for a 450. The long, smooth power delivery makes it easy to upshift early if your arms can’t take it.
Apparently, the people at KTM understand that this kind of horsepower might feel good in the short run, but it’s a little demanding over the course of a long race. The new handlebar switch makes it easy to switch between EFI maps. You have a choice between the standard map and a second one that can be preselected before the race. You can either dumb down the motor a little or, if you’ve really been eating your Wheaties, you can go for a more aggressive map. Then there’s the traction control, which can be activated by pressing a second button on the same control unit. At first most of our test riders thought the traction control felt just like another, slightly milder ignition map. But when the track got rough—really rough—it was apparent there was more to it. Most riders reported that the bike actually tracked straighter through harsh chop. KTM claims that the feature works when the computer recognizes wheelspin through sudden rpm spikes and instantly retards the ignition. Honda employed a similar system over 10 years ago on the CR250R two-stroke and the Husaberg 501 four-stroke. The big difference with the KTM is that you can take it or leave it. If you choose to leave the traction control button alone, you’ll quickly learn to do the same thing with your own right wrist.

We already have enough time with the WP AER 48 fork to know that we like it. It defeats the most common argument against air forks because it’s not difficult to set up. There’s one Schrader valve, one rebound clicker and one compression clicker. This is in stark contrast to the Showa TAC fork, which has three air chambers, and the KYB PSF2, which has two air chambers and six clickers. The WP fork actually does have an internal balance chamber that eliminates the need for a rebound spring, but it’s maintenance-free. The chamber charges itself on every stroke. It’s simple, and it works.
In action, the new fork is excellent. Compared to the 4CS it replaces, there’s no contest. The biggest improvement is on small, sharp-edged bumps. The old fork did well on big impacts, but it had an abysmal comfort rating. The air fork is much more well rounded. Most riders felt it was too stiff with the standard air pressure (157 psi or 10.8 bar), so, as stipulated in the unofficial lore of air-fork tuning, we first tried reducing compression damping. In this case, though, the best solution was to reduce air pressure a full 10 pounds. We’re sure the stock setting works for someone, but it isn’t us.
Do we like them better than our favorite forks from KYB and Showa? Last year the Yamaha spring KYB fork on the Yamaha and the Showa triple air on the Honda CRF250R got our top ratings. As we get more setup time on the AER 48, we’ll find out if it’s up to that level. We already know it’s in the same league, and that’s saying a lot.

There’s more to the KTM 450SX-F than a motor and fork. It’s all good stuff. The bike’s cornering manners have improved every year, and now we have to rate it near the top of the handling heap. The fact that it lost even more weight this year has something to do with it. Fork springs are apparently very heavy. The 2017 450SX-F weighs in at 223 pounds without fuel. That’s 3 pounds lighter than last year’s and around 15 pounds lighter than most other 450s.
The electric starter is a bonus. We used to be nervous about having no kick-start lever for backup, but we’re over it. The lithium battery is a little cold-blooded, but it always works. The KTM 450 also offers great brakes, clutch feel and overall comfort. The bike scores well across the board. Does that mean it’s the ultimate motocross bike? Not necessarily, but it’s a very good place to start.
• Improved fork
• Lightest in class
• Electric start
• Excellent power
• Excellent brakes
• Hydraulic clutch

• Air forks still require more attention than springs
• Cold-blooded battery
• Plastic shock preload adjusters
Engine type … SOHC, electric-start, 4-valve 4-stroke
Displacement … 450cc
Bore & stroke … 95.0mm x 63.4m
Fuel delivery … Keihin EFI, 44mm
Fuel tank capacity … 1.8 gal. (7 l)
Lighting … No
Spark arrester … No
EPA legal … No
Running weight, no fuel … 224 lb.
Wheelbase … 58.5″ (1485mm)
Ground clearance … 14.6″ (370mm)
Seat height … 37.8″ (960mm)
Tire size & type:
Front … 90/90-21 Dunlop MX3SF
Rear … 120/80-19 Dunlop MX3S
Front …. WP AER 48, adj. reb./comp., 11.8″
(300mm) travel
Rear … WP aluminum piggyback, adj. prld, hi & lo comp., reb., 11.8″ (300mm) travel
Country of origin … Austria
Suggested retail price … $9399
Manufacturer … www.ktm.com

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