Suzuki RMX450Z: Building Suzuki’s Misunderstood Off-Roader, Part 1

Apr. 26, 2011 By Dan Paris
The Suzuki RMX450Z

We’ve just started tearing into our new Suzuki RMX450Z. The RMX has seen mixed reviews in the press, mostly because the trail rider crowd finds them too ‘motocrossey.’ Suzuki took great effort to make the RMX more motocross-oriented than the usual Japanese offerings, but it doesn’t help that the tiny aluminum fuel tank (shared with the motocross RMZ450F) only holds enough fuel for about 40 miles before you start pushing. Somehow during the transformation from motocross bike to off-road bike, the RMX also gained nearly 25 pounds and lost much of that snappy RMZ power.

No, the RMX doesn’t sound like a winning combination, but to understand the magical draw of the RMX you need to ride one. The motocross feel, plus the famous cornering abilities of all Suzuki’s, made us like the RMX enough to buy one.

To be honest, the RMX is pretty close to being a winner right out of the box. Being essentially a ‘softened’ RMZ450F with a magic button, a spark arrestor and an 18-inch rear wheel, we saw through the fluff knew it wouldn’t take a whole lot to make the RMX shine as a closed-course-only race bike.

Our immediate goals are to improve overall engine and suspension performance, carve weight and increase fuel range. Once it’s dialled in we’ll have a closed-course-only RMX with performance levels greater than the RMZ … and hopefully a shelf full of trophies from everything from sprint-style enduro, obstacross, motocross and even ice racing.

The performance differences between the RMX and RMZ can be boiled down to a few categories:

1. The RMX uses a 2009 RMZ450F frame, with a 2010 RMZ450F swingarm and an 18-inch rear wheel.
2. The RMX engine has electric start, with the battery mounted up on top of a side access airbox.
3. The RMX engine is tuned, through lower compression and milder cams, for smoother low- to mid-range power delivery.
4. The RMX transmission has slightly lower first, second and third gear ratios and slightly higher fourth and fifth gears than the motocross version.
5. Spring rates and damping curves at both ends were massaged for off-road … but not by much. The RMX is actually pretty happy on a motocross track, but it is perhaps a bit stiff for small trail junk or at lower speeds.
6. Suzuki added the usual off-road stuff, like a plastic skid plate, coolant overflow tank, kickstand and a bunch of EPA neutering stuff to reduce noise and emissions.

We already knew the RMX was close to the mark as a racer; probably the closest any Japanese manufacturer has come with an off-road bike since the old two–stroke Suzuki RMX250s. With technical advice from Suzuki’s Rodney Smith and OTFFSS Suzuki Canada’s Steve Simms we’ll start making the RMX450Z into a racer.

RMX Throttle Stop

The first thing to do to the RMX for closed-course competition is to remove the throttle stop screw. It’s in an awkward location but if you squint and hold your tongue just right you can weasel the stop from the throttle body.

The RMX throttle ttop screw is removed.

Once this screw is removed you can do two things: Grind off the non-threaded end and reinstall, or if you want to be able to easily convert the RMX back to EPA legal status do what we did and grind a metric Hex bolt to length and use it to plug up the hole.

RMX plug hole in throttle body

The RMX airbox is severely choked off to reduce intake noise. Not only are the side inlets blocked off, the rubber snorkel is so restrictive it would look silly on a 100, let alone a 450!

Suzuki RMX airbox

By removing the seat and battery you can easily push the rubber snorkel down into the airbox to remove it.

RMX airbox snorkel removal

That’s only half the battle with the airbox however. Our future plans will involve opening up side vents in some way that will still prevent water from splashing inside. The battery also blocks much of the airflow through the top, and since it’s located so high we will eventually kill two birds with one stone by installing a lightweight mini-battery.

The RMX muffler plug

The muffler of the RMX isn’t much different than the RMZ model, other than a very restrictive butt plug. Like the airbox restriction, the size of the RMX outlet is so tiny it would be more at home on a 100.

Thankfully the RMX muffler restrictor can be removed in about five minutes without compromising the spark arrestor. Yes, the bike is louder but not obnoxious. If you go one step further and also remove the spark arrestor the bike becomes very obnoxious, and irresponsible to operate anywhere but on closed-course areas with no spark arrestor requirements. We’ll be addressing this in future instalments by trying a few different exhaust systems that will reduce weight and improve performance while meeting noise and spark arrestor regulations.

RMX EE Handguards

The RMX doesn’t come with handguards. Depending on where you ride you might want motocross-style brush guards or more sturdy wrap-around aluminum enduro-style guards. We love the Enduro Engineering Roost Deflectors because they are the perfect half step. Stiff enough to take a fairly solid glance off a tree, and they are also light and unobtrusive. The aluminum mount allows them to flex out of the way in a fall. In the near future we’ll be adding unbreakable levers and an aluminum throttle tube to further protect the controls from crash damage.

We bolted the RMX back together and went for our second ride. The bike now ran like a real 450! In stock EPA form you had to clutch the bike into third gear wheelies, it flamed out and stalled easily and wouldn’t give a motocross 250F a good race through the gears. But while the uncorked RMX ran stronger and was more satisfying to ride it still wasn’t perfect. It sounded lean and ratty and didn’t start as easily as we’d like.

RMX Yoshimura Cherry Bomb

Magic bullet! Yoshimura’s Cherry Bomb plugs into the RMX and allows the EFI system to take full advantage of the opened up airbox and exhaust, or to run aftermarket exhausts. It takes about two minutes to install and not only does it make the RMX run smoother, it also adds power to all points in the powerband. With the mods we’d done and the Yoshiumura Cherry Bomb we are quite happy with the rideability and overall performance of our RMX!

The unused parts and suspension from our project.

Back in the garage, with just over an hour of running time, we started getting ready for stage two of our RMX450Z build. We changed the engine oil and filter and checked the valve clearance, which was spot on. When we removed the suspension we serviced the swingarm linkage and shaft. They were bone dry, without so much as even a greasy fingerprint! You’ll definitely want to grease the steering head bearings as well. We took off items we wouldn’t need for racing, including the kickstand, coolant recovery tank, taillight assembly, the kickstart lever and assorted reflectors and brackets. That little box of stuff weighed over eight pounds!

For stage two we’ve enlisted Works Enduro Rider to set up the suspension to meet our unrealistic goal of soaking up trail junk acceptably while still being able to take on the big hits of outdoor motocross. We’ll be adding armor for Endurocross-style racing, we'll try a few different exhaust systems and install an oversized fuel tank. We’ll dig into other areas as well, like improving the ergonomics. We’ll even race the bike a few times!

In stage three we’ll start getting power-crazy by digging into the cylinder head to play with porting and camshafts. We’ll also be experimenting with EFI re-mapping, installing an automatic clutch and fine-tuning the RMX for closed-course racing! Building and racing our hot-rod RMX is going to be fun! Newsletter
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