In racing lightness is everything, and that was the main objective when designing the 2001 Suzuki RM250 motocrosser, the subject of this week’s Church feature, the first one of 2016. Yep, we’re going back 15 years and checking out what MX’ers were looking as the hot ticket. Every facet of the RM250 was approached with optimum racing performance in mind, and according to author Mark Kariya, the Suzuki two-stroke didn’t disappoint. Read on to see what the RM250 was all about.
Torrance, California, August 09, 2000 — Thin is in these days; at least from the standpoint of most super-models and other revered objects of physical beauty.Thin generally means light, but to an engineer who is designing a racing machine, the two characteristics (thin and light) are not just gauche reasons why a young man’s respiration quickens when viewing images of super-models. Thin and light are directives from upper management responding to race team requests; and they’re to be incorporated into next year’s machines.
Now, if they went through all the trouble to shed weight here and there yet ended up with a bike that’s only two pounds lighter than the current model, what’s the point? Good question.
The total number of pounds needs to meet regulations for various racing organizations like the AMA, which stipulates that 250cc motocrossers must weigh a minimum of 215 pounds sans fluids. Add a glide plate and, perhaps, a gusset or two for strength, and you’ve got it. Of course, most amateur racers won’t have to bother with meeting minimum weight rules so, to them, it’s only a bragging point.
Just as, if not more important than, total weight is where that weight is concentrated. That’s when things get a bit more complicated. That’s when things like weight distribution, center of gravity and mass centralization take the forefront and the engineers’ efforts begin to make more sense.
First, let’s take a look at just a few of the places where Suzuki made the ’01 RM250 lighter than its predecessor: The frame has been completely redesigned with a focus on greater rigidity as well as improved geometry. Though it retains the semi-double cradle design, the rectangle-section front downtube is made of smaller-section tubing, 40x 45mm instead of 50 x 45mm. In addition, the rear cradle tubing resembles a KX-style perimeter frame with stamped 22.9 x 40.0mm rectangular sections replacing the tubing previously employed. The main frame is 580 grams lighter than the Year 2000 while the removable, aluminum subframe is 120 grams lighter (about a pound and a half altogether).
Geometry-wise, the new 250 is shorter and more compact. It’s a half inch shorter between the axles, now measuring 57.9 inches. Rake and trail is up to 27.5 degrees and 115mm (4.5 inches) from 27.0 degrees and 111mm (4.4 inches) to retain decent straight-line stability despite the shorter wheelbase. In addition, the pegs have been moved up and back by 9mm (0.4 inches) each way to improve rider positioning.
Improved rider positioning through lighter and more compact parts extends to the new fuel tank (8.0-liter/2.1-gallon capacity instead of 8.5/2.2), seat, radiator shrouds, side panels and rear fender (which now has weight-saving cutaways for easier grabbing to hoist the bike around). Redesigning the front number plate and front fender (which now has air scoops along the rear edges to increase the radiator’s efficiency without sacrificing mud protection) saved just over 30 grams (about an ounce) for each part.
Eliminating the front brake disc cover saved a good three ounces, but don’t worry; there’s a small brake hose cover to provide some protection. Using 1mm thinner material and a different design for the brake pedal saved another 90 grams (three ounces). We’ll save time and mind-numbing detail by saying that practically every part of the chassis received careful redesign in the quest for less weight.
And the same thorough search for lightness was extended to the engine. Here, Suzuki not only desired a lighter and more compact powerplant, but it also expected more output, especially in the low to mid range. To that end, the engineers responded with a smaller-diameter head, thinner cylinder walls where possible, shorter cylinder stud bolts and flanges, different primary drive gear shape, tubular shift lever and numerous other updates. A geared shift mechanism replaces the 2000’s link-style to improve the RM’s already excellent shift feel, though it is no lighter.
Other enhancements include a two-stage, two-piece power valve replacing the one-piece exhaust valve and machined side exhaust valves. The two-stage valves begin opening sooner and are thus able to open over a wider rpm range than the old one-piece valve. In addition, the side exhaust valves (now spinning on needle bearings instead of balls) open the differently shaped sub-exhaust ports independent of the main valve, further optimizing exhaust port timing. Combined with the thinner (thus, lighter), reshaped expansion chamber, 38mm Keihin PWK with Throttle Position Sensor carburetor and 3-D ignition map, they help produce added torque and smooth revving. A smaller-diameter magneto reduces weight by a couple ounces, but there’s been no change in flywheel effect because Suzuki compensated with added crank inertia.
In total, the new RM250 produces greater torque with a flatter curve than before, while the horsepower chart sees gains from the beginning and makes peak power earlier.
Besides the lighter parts, the engine is also shorter and more compact. Part of this is due to moving the water pump from the front right inside the cases to externally, just in front of the clutch cover. That helps reduce front-to-back length by nearly an inch.
In the suspension department, Suzuki opted to purchase from Kayaba. The inverted fork sports 46mm inner tubes instead of the beefier 49s of the 2000. Apparently, Suzuki felt the added frame rigidity permitted use of the smaller and lighter (by 800 grams or one pound, 12 ounces) cartridge fork.
But another feature Suzuki touts is how the new fork separates the upper portions of the top tubes into two air/oil chambers. All forks have air above the oil level, of course, and this definitely affects the suspension action.
The fork springs help hold the fork up in the stroke and provide some bottoming resistance, but when you land off those big jumps or hit square-edged holes at speed, the air inside the fork tubes plays a greater role. That’s why raising the oil level (reducing volume of air) helps resist bottoming. This is a function of what Suzuki calls the air spring load being dependent upon damper stroke or position.
What Suzuki and Kayaba have done with the new fork is make the air spring load dependent upon damper speed as well as position. Separating the upper portion of each fork leg into two air/oil chambers (but not completely sealing them apart from each other) gives them that feature.
At low fork stroke speeds (such as small, rolling bumps), oil is able to flow from section to section easily so the combined air volume of air/oil chambers one and two are used, thus providing normal bottoming resistance.
However, at high fork stroke speeds (landing off big jumps, hitting square edges), oil cannot flow from chamber one to two because oil cannot flow fast enough through the holes and edges of the separating check plate. In this case, air/oil chamber number one alone is employed. Since it has substantially less volume by itself than when combined with air/oil chamber two, it provides greater bottoming resistance. At least that’s the theory.
In back, the new piggyback-reservoir Kayaba shock is smaller at 46mm, partially because the former 50mm-diameter shock wouldn’t fit. But the new, 260-gram-lighter (nine ounces) unit also incorporates both high- and low-speed compression damping adjustment, plus the upper mount now rides in roller bearings for improved feel. There are also changes to the linkage that results in lighter pieces and a different leverage ratio. One of the benefits is claimed to be improved traction due partially to suspension responsiveness from the roller bearings, especially on washboard surfaces.
With that fairly detailed technical briefing out of the way, we were more than ready to ride the ’01 RM250. Suzuki invited the enthusiast press to a new practice track called Third Gear Pinned in Beaumont, California, roughly halfway between L.A. and Palm Springs. Since it was July, we expected it to be hot-and weren’t disappointed in that regard. (Glancing at a digital thermometer on top of a bank while driving back in mid-afternoon showed a fairly impressive 106 degrees!)
But neither were we disappointed with the new Suzuki. Third Gear Pinned is an outdoor-style track with some good-sized table-tops and drop-aways, as well as off-cambers and a variety of turns. Though not rough enough for a hyper-critical suspension test, given our level of motocross ability, the track seemed fun and pretty safe-and that’s more important the older we get.
The new 250 immediately impressed us as being easy to ride, comforting to our novice-level abilities. In fact, the revised motor was almost thumper-like in its ability to pull us around the track; we could’ve ridden the entire thing in third gear with only some clutch slipping out of the tightest, most technical corners. Yet, when called upon in the straights, it willingly spits you to the following corner post haste. The RM pulled cleanly off the bottom, ran through a healthy midrange and kept driving, falling off just a bit on top. Shifting? Never gave it a thought so it obviously didn’t bother us.
The thinner (rear), lighter rotors worked well when it came time to scrub speed or adjust the angle of attack with a little brake slide to set up for turns. Modulation felt good, and neither end required undue effort.
Cornering appeared to be a particular strength, with the Suzuki willingly diving to the very insides of any turn if we even thought “I’ll bet that inside line is quicker” or “Gee, that looks like a nice, smooth line I’d like to try.” Powering through faster turns and outside lines proved equally confidence-inspiring, partly because the bike was so easy to move around on for those critical weight shifts that make or break cornering success. In addition, it felt quite light and flickable without seeming too small like a minibike, at least for 5-foot-10, average-sized riders.
Yet on faster, cobbly sections of track (Third Gear Pinned offered softball-sized dirt clods in several sections for such evaluation), the RM250 didn’t display any tendency toward headshake-again, comforting to those of us lacking the full-time, full-throttle verve of Greg Albertyn and Damon Huffman who rode the new bikes obligingly (yet at almost bored pace for them) for action photography.
As we went out for session after session on the RM, we grew more confident in our abilities, so that strongly says something positive about the bike. While we didn’t progress to supercross stars, the suspension did seem compliant enough. We did end up overjumping a drop-away once, but neither end bottomed with a painful metal-to-metal clash. The Kayabas at both ends seemed to gobble up the smaller irregularities without transmitting harsh feedback.
We haven’t spent extended time on any of the recent 250s, but the ’01 RM250 certainly seems to reinforce the lighter is better concept. As the first shot fired in the 250cc motocrosser wars, the Suzuki is definitely on target. How it compares to the rest of the class shall have to wait until we get all the bikes together in the future.