Monster engines and wide tires are a hot ticket, not to mention the dream of many bike nuts. However, stout engines and brawny, road-hugging tires both create unique yet similar drive-train challenges that the bike builder must address to achieve reliability and a balanced ride. Fortunately, the crew at JIMS USA has realized the popularity of this excess. With that in mind, they have developed two transmissions that support high power and wide tires: the Fat 5 overdrive and right-side-drive six-speed. Both transmissions are American made. Gearsets are precision CNC-ground from 9310-AQ material and finished with an AGMA-12 surface. Moreover, the transmissions have been tested with up to 800 lb-ft of torque.
Fat 5 OverdriveTen years ago, 96- to 106-cubic-inch street engines were considered big; today, they are only average at best. Today's typical big-street engine falls into the 120-something cubic-inch neighborhood, with some engines ranging up to 155 inches and even larger. Monster mills easily double a stock engine's power and torque, placing enormous stress on a bike's drivetrain. When ground-pounding power is hooked up to the pavement through high-torque clutches and sticky tires, peak loads rise above what stock transmission gears can handle. JIMS designed the Fat 5 overdrive gearbox to handle such loads.
The Fat 5 overdrive transmission fits both Evo and Twin Cam chassis and is available as a complete transmission or replacement Superkit gearset, which includes gears and shafts assembled to a billet trapdoor. JIMS' compact shifter technology uses sliding shift rings for gear engagement, which results in quicker and smoother shifting. To support high-power loads from big engines and massive tires, JIMS took advantage of the compact shifter design and re-designed every gear to increase width and strength. The resulting benefits are increases in gear width from 18 to 53 percent, depending on the loads subjected to each gear. Due to the thicker gears, the transmission is 0.870-inch wider on the right side, which may cause interference with the stock exhaust or other components.
The Fat 5 also uses the same "shift ring" technology that is utilized in the '06 Dyna and all '07 factory transmissions. Shift-ring technology provides 100-percent tooth-to-tooth contact, resulting in quick, smooth, and positive shifts.
JIMS' Fat 5 transmission is actually a five-speed gearbox, with Fifth gear being an overdrive. An overdrive transmission offers the benefit of rpm reduction when in top gear without affecting the ratios of the intermediate gears (First through Fourth). The Fat 5's internal gear ratios are as follows: close-ratio 2.91 First, 1.93 Second, 1.31 Third, 1.00 Fourth, and 0.886 Fifth. Fourth gear results in a direct-drive 1:1 ratio between the engine and rear wheel, while Fifth gear provides a true overdrive ratio, which overdrives the rear wheel compared to the engine. Essentially, the Fat 5 concept takes the ratios of a five-speed gearbox and spreads them equally across four gears, with Fifth gear acting as an overdrive. This is the hot setup for high-torque big-tire applications, because four gears are sufficient for quick acceleration, while the overdrive Fifth gear offers the benefit of rpm reduction for smooth highway cruising.
Right-Side-Drive Six-SpeedBuilding a wide-tire bike is one way to make an attention-grabbing statement. As such, it's not surprising that fat-tire bikes are so popular these days. But installing a muscular rear tire in a left-side-drive chassis requires a powertrain offset, because the engine and transmission must be moved to the left-or the primary drive spaced leftward (or a combination of the two)-to allow clearance between the drive beltchain and rear tire.
To put this into perspective, consider that a stock Big Twin 130-series tire is about 5.2 inches wide. Installing a 240-series tire (9.6 inches wide) requires about a 2.2-inch offset, and a 300-series tire (12 inches wide) a 3.4-inch offset, depending on the width of the drivebelt or chain. However, such large offsets can create a significant left-side weight bias, resulting in poor balance and ridability. Furthermore, this shift creates less ground clearance on the left side of the bike by reducing lean angle. Generally, the Big Twin can handle about a 200-series tire when using a 1-1/2-inch belt or a 240-series tire with a narrow belt or chain before left-side weight bias becomes an issue with a stock-style tranny. However, if you want optimum balance, a right-side drive is the only way to go, especially if you're thinking in the 300-series-tire range.
To maintain a balanced motorcycle with a monster-tire application, JIMS introduced its right-side-drive six-speed overdrive transmission, which eliminates the need to offset the drivetrain. The right-side-drive transmission can handle a 280/300-series tire and includes critical design improvements for improved street use and reliability.
For example, standard right-side-drive transmission designs use a longer mainshaft than stock. The longer shaft, combined with high-torque loads from big engines and fat tires, causes mainshaft flex, which can result in premature part failure. To minimize flexing of the mainshaft, which is common in high-horsepower and wide-tire applications, JIMS' right-side-drive design incorporates an outboard support bearing for mainshaft integrity. A built-in pump pressure-feeds lubrication to the support bearing through an oil galley in the trap door and pulley cover.
The right-side-drive gearbox includes six speeds, with top gear being an overdrive. The internal gear ratios are as follows: close-ratio 2.91 First, 2.19 Second, 1.59 Third, 1.22 Fourth, 1.10 Fifth, and 0.87 Sixth. Gear ratios from First through Fifth are spaced slightly closer together than the Fat 5's intermediate ratios.
JIMS' right-side-drive transmissions are available for Evo- and Twin Cam-style transmission mounts and will bolt into most RSD-specific frames with no frame modifications. All case covers are CNC-machined from billet aluminum, then polished and chrome-plated. Black, silver, and polished cases are available. Additionally, the gearbox can be ordered with either hydraulic or cable clutch actuation.
If you're building a high-torque engine or wide-tire bike, JIMS' Fat 5 overdrive and right-side-drive six-speed transmissions may just be your ticket to hooking up ground-stumping power and a smooth, balanced ride.
We wanted to see for ourselves what the JIMS Fat 5 overdrive and right-side-drive six-speed transmissions look like, so we decided to disassemble each one. Follow along and see for yourself. We'll start with the Fat 5.
Here's a side-by-side profile of the two JIMS transmissions: RSD six-speed (bottom) and Fat 5 Overdrive (top). Both trannys include a Neutral indicator switch, electronic speed-sensor mounting, and stout gearsets. If you have a big-power mill and need a brawny gearbox with overdrive for reduced highway-cruising rpm, consider the JIMS Fat 5 overdrive transmission. For those with a monster engine and wide-tire bike in mind, look at the JIMS RSD six-speed for strength and reliability in addition to smooth, balanced cruising.