Like everywhere else in the world, there is a constant battle over places to ride in Southern California. Whether it’s public land, private property, closed track or open trails, encroaching developments threaten riding areas. Noise is often the primary complaint from those opposed to offroad riding. Sound issues are closely tied to the availability of places to ride. Even though most of us die-hard motocross racers don’t want to neuter our race bikes with super-quiet mufflers, it’s a small price to pay to keep our riding areas open. The problem begs the question: Can a quiet bike still be fast?
Michael “The Rock” Rigdon of Rock’s Racing has carved a niche as an anti-sound crusader and runs www.soundoffmc.com. He wants to help ensure the future of motocross by making bikes quieter. But quiet is a double-edged sword, because quiet is often a code word for slow. The Rock wants to prove that bikes can be quiet and fast (at the same time). Last year, Rigdon approached the MXA test crew about testing his Yamaha YZ450F Stealth project bike. He claimed that it would produce competitive horsepower and come in under the FIM/AMA 94dB sound limit. The Rock’s first attempt at such an endeavor proved to be a steep learning curve for him. Bad luck and bad decisions plagued the YZ450F Stealth bike project, and by the time MXA got to test the bike (March 2009 issue) a full year had passed from start to finish. Not only was the bike not all that quiet (although it was FIM/AMA legal), but it wasn’t very fast—certainly not fast enough for MXA’s pro-level test riders (although the lower class riders thought it was okay)..
THE ROCK’S END GOAL FOR THE STEALTH PRO PROJECT BIKE WAS
TO CREATE A FULL PRO-LEVEL PACKAGE. MOST PROS WILL TELL YOU THAT THE
SUSPENSION IS AS EQUALLY OR MORE IMPORTANT THAN THE ENGINE. FOX RACING
SHOX WAS IN CHARGE OF SUSPENDING THE BIKE. FOX EQUIPPED IT WITH THEIR
NEW PODIUM SHOCK AND REVALVED THE FORKS.
Michael Rigdon took the disappointment of his original Stealth project and chalked it up as a learning experience. Amazingly, he didn’t give up. Instead, he readjusted his goals and vowed to build another bike that would be quieter than 92dB and be competitive at the AMA professional level. The MXA wrecking crew didn’t laugh at him, but we giggled amongst ourselves after he walked away. The task of building a full-race YZ250F that’s competitive at the pro level is very difficult by itself, let alone building one with a major sound handicap.
Would it surprise you to discover that we agreed to test Rock’s Racing’s Yamaha YZ250 Stealth Pro out of pity? It’s true, we felt bad about lambasting his first attempt and wanted to give him a second chance—although we never thought he could do it.
SHOP TALK: WHAT’S UNDER THE HOOD?
To make a bike quiet and fast, the engine has to be very potent. Rock’s Racing chose C4 and Eleven10 Mods for the task of making the Yamaha mill as powerful as possible. First, the Rock gave the YZ250F to C4, who disassembled the engine and shipped it out to Chad Sanner of Eleven10 Mods. Chad went whole hog with it. Chad started by flowing the head and installing valve spring retainers, valves, valve seats and a high-compression JE piston (made to Eleven10’s specifications). Chad also selected Webcams cams with their own timing specification. The degree of the lobes of the intake cams are lower, which helps lower the sound level.
On the bottom-end, Eleven10 Mods converted the YZ to a wet sump engine (which was good for a two pound weight savings and a little more engine efficiency). Also, Eleven10 Mods’ Accelerated Surface Finish was done to the transmission and the entire bottom-end to reduce friction, improve shifting and, according to Chad, a reduce internal engine noise of the YZ. Eleven10 Mods’ crankshaft mod includes reducing its weight and balancing it with the piston while eliminating the counter-balancer to help the engine rev freely. Eleven10 Mods utilized a Vortex ignition and selected a bigger 38mm carburetor (with a Honda float bowl) and modified it for improved throttle response.
C4 handled the reassembly and then performed dyno runs to help ensure proper performance. The exhaust system of choice was an FMF MegaBomb header and Powercore muffler with the smallest Quiet Core and Bristol Core packing.
The Rock’s end goal for the Stealth Pro project bike was to create a full pro-level package. Most pros will tell you that the suspension is as equally or more important than the engine. Fox Racing Shox was in charge of suspending the bike. Fox equipped it with their new Podium shock and revalved the forks.
Santa’s elves must have had to work overtime to fill the Rock’s Christmas list on this bike. The list includes Dunlop’s new MX51 tires, Hinson clutch assembly, Napalm Racing hour meter/tachometer with gas tank mounting kit, DeCal Works graphics (premounted on UFO plastics), Nuetech Tubliss tires, ARC clutch perch and front brake lever assembly, an SDG seat, Loudmouth MX air filter, Stomp frame gripping material, MotoMaster brake pads and rotors, Renthal handlebars, chains and sprockets, Boyesen SuperCooler kit and XTRIG’s adjustable offset triple clamps and PHDS (Progressive Handlebar Dampening System) mounts.
To complete the look, TCR modified the stock wheels by turning down the hubs and black anodizing the rims. Works Connection provided protection, with their skid plate, water pump guard, rear brake master cylinder guard and rear brake caliper guard. Last, but not least, the bike was equipped with Ride Engineering’s braided brake lines, engine plugs, rear brake billet clevis, axle guides, and holeshot starting device.
TEST RIDE: HANG ON AND PRAY
Before getting Rock’s Racing’s Stealth Pro YZ250F out onto the racetrack, the first order of business was to sound test the bike (after 30 minutes of break-in for both the engine and the muffler). We conducted the test multiple times in accordance with AMA procedures to get a completely accurate reading. The bike was 91.0 dB! That is whisper-quiet, especially considering that a bike can be 95.9 dB and still pass the AMA sound test.
The high-idle, 5000 rpm conditions of an AMA 250 sound test is quite a bit different than the way bikes are actually run, but the Stealth Pro was also very quiet on the track. The sneaky Stealth Pro is difficult for other riders to hear over the roar of their own engines. In a race, by the time someone heard the Stealth Pro coming, it was too late to block the pass. As an added bonus, the heavily restricted bike didn’t sound like an enduro bike or a step-thru scooter. The exhaust note maintained a racy quality uncommon with most plugged-up bikes.
IN THE MIDRANGE, THE STEALTH PRO STARTED TO MAKE DECENT
POWER, BUT NOT ENOUGH TO DO MUCH GOOD IN A RACE. GOING FAST ON THE
STEALTH PRO REQUIRED LIVING IN THE HIGH RPM RANGE OF THE
POWERBAND—WHERE IT HIT HARD AND PULLED FAR.
Every MXA test rider agreed that the bike was quiet—but was it fast? Generally, really muffling the exhaust ruins the bottom-end power of a bike, and the Stealth Pro was no exception. In fact, our biggest complaint about the bike was a consistent blubber right at throttle initiation. After the initial crack of the throttle the engine ran smoothly. In the midrange, the Stealth Pro started to make decent power, but not enough to do much good in a race. Going fast on the Stealth Pro required living in the high rpm range of the powerband—where it hit hard and pulled far.
Test riders were rewarded by holding the power on and waiting a little longer to shift. If it wasn’t so quiet, we’d call the Stealth Pro a shrieker. We don’t want to give the Stealth Pro any left-hand compliments. It was fast when it got going (most definitely faster than the stock YZ250F we tested it against). It was not AMA National class fast—how do we know? Go back on the MXA website and read our test of Tyla Rattray’s KX250F National bike. We tested both Rattray’s bike and the Stealth bike on the same tracks at the same time. Sadly, the Stealth YZ250F didn’t have enough juice to overcome the 3 or 4 dB disadvantage and keep up with an AMA pro’s 95.9 dB machine.
While the bike could be ridden with the burble off the bottom, the Rigdon wanted the Stealth Pro to run without any hiccups. He took the bike back and consulted with his team of builders. They determined that the bigger 38mm (2mm bigger) carburetor wasn’t compatible with such a small insert in the muffler. Additionally, they rejetted the carb to run on pump gas.That eliminated the blubber.
The Fox Podium shock was very tunable and worked well with the variety of tracks and testers after a few clicker adjustments. The Fox revalved forks had a stiffer, race-appropriate platform and complemented the shock to balance the bike. The MotoMaster brakes offered plenty of braking power but were grabby and tough to modulate. On surfaces with transitions from loam to hardpack, handling was sketchy. The combination of the MotoMaster front brake and the MX51 front tire wasn’t a favorite of any MXA test rider. Overall, there were a lot of quality aftermarket parts on the Stealth Pro that work well individually, but altogether they made the bike feel a little bit hodgepodge.
CONCLUSION: WHAT DO WE REALLY THINK?
We admire the persistence and determination of Rock Racing’s Michael Rigdon. If at first he doesn’t succeed, he tries again. His Stealth Pro project YZ250F went much smoother than his previous Stealth project (we only had to send the bike back once to get the bugs out).
His achievement may not have met our acid test, but it was admirable in its own right. For his efforts, the Rock achieved his goal of a very quiet bike that is competitive for the average 250F rider. Considering its 91 dB sound rating, there’s no shame in coming up a bit short of AMA Pro-level speed. We would love to see what Eleven10 Mods (724) 562-8877 and C4 (661) 267-9966 could do without the big sound disadvantage. Still, if the neighbors start to complain about noisy dirt bikes, we would be happy to ride the Stealth Pro YZ250F.
