THE GRUDGEBOX Baker Drivetrain

The GrudgeBox is an all spur 6-speed overdrive transmission that delivers uncompromised performance in all gears and significantly more torque capacity than the stock unit. The primary impetus for the design was performance, but the GrudgeBox was also designed for those who spend a lot of time in the saddle looking for that extra gear on the highway. 6th gear is a true overdrive that is numerically mild to yield a 250 RPM drop as compared to the stock unit. The gearset fits into the stock transmission case with no modifications. It has earned its BAKER pedigree with extensive highway testing and many merciless passes at the dragstrip.


Grudge racing is street racing, often done illegally, in which real dollars are bet on a single race. As such, the internal configuration of the engine and drivetrain are a closely guarded secret to gain an advantage over the competitors. The GrudgeBox was conceived to be a high- performance transmission with performance inspired gear ratios but no outward appearance to give away the fact that a thoroughbred torque multiplying machine occupies the inner walls of the transmission case.

The GrudgeBox is available with different bearing door finishes: machined billet, chrome, polish, powdercoat with highlights, and Sleeper. The Sleeper door is available to achieve the aforementioned anonymity. To make the Sleeper door we apply mechanical taxidermy to a stock factory door by milling out all anatomy and bone structure on the stock door to leave a .125” thick shell with stock factory powdercoat. The shell then fits over a billet sub-door, like a 3D puzzle, to yield the Sleeper door which fathered the name GrudgeBox (figure 1). Externally, the GrudgeBox with the Sleeper door gives no indication that a drag racing transmission lives underneath and that may be the difference between winning and losing.


Three different shift drum configurations are offered; standard pattern (1-N-2-3-4-5-6),

N1 pattern (N-1-2-3-4-5-6), and reverse N1 pattern kill (6-5-4-3-2-1-N).

The N1 drum is similar to the standard pattern drum, but neutral is relocated from the standard position between 1st and 2nd to the position below 1st gear. The N1 drum makes it effortless/mindless to find neutral. Racers, those who live in mountainous regions, and people with restricted foot and leg mobility rely on the N1 pattern to find neutral every time without having to think about it.

Reverse N1 pattern kill drums are for the advanced disciples of the quarter-mile club. As the name implies, reverse N1 pattern is just like the aforementioned N1 pattern but reversed. To upshift, you stomp down on the lever. The “kill” part relates to repurposing the neutral switch to allow for clutchless upshifts and downshifts.


The GrudgeBox is the most innovative, well executed, robust transmission we have ever endeavored to design and manufacture. Significant GrudgeBox features include:

1. 6-Speed overdrive design. The 5th gear ratio of the GrudgeBox is 1:1 which is equivalent to the stock transmission 6th gear ratio. The overdriven 6th gear ratio offers a 250 RPM reduction for highway cruising.

2. Straight cut (spur) gears. Gear School 101. Helical gears are used in most motorcycles and cars these days because they are quieter than spur gears. However, there’s a cost for noise reduction because helical gears give up horsepower. The amount of horsepower given up is proportional to the angle of the helix on a given gear pair. The stock transmission has helical gears in 2nd through 6th. The GrudgeBox is configured with all spur gears and no sacrificed horsepower.

3. Gear width. Stock transmission gear engagement is typically .500”, GrudgeBox is .700” in 1st through 4th with nearly twice the circumferential tooth thickness. Generally speaking, torque capacity of a transmission is the product of the gearset center distance, gear engagement width, and circumferential tooth thickness at the pitch line.

4. Tapered roller main drive gear bearing. We replaced the problematic self-aligning stock main drive gear bearing with a tried-n-true opposing tapered roller bearing pair; patent pending. We did this because any transmission is only as strong as its weakest link.

5. Dog tooth engagement. The dog teeth on the stock 3rd and 4th gears have roughly .200” axial engagement with a less-than 1° undercut. This is a formula for gear hop-out. For the GrudgeBox we chose 4° undercuts with .250” axial engagement in all positions to guarantee NO gear hop-out or missed shifts, which translates into NO lost revenue if you are a Grudge hustler.

6. Direct acting shifter pawl. Upshifting with the stock 07-later shifter pawl is much like pushing on a rope. To make upshifts crisp and precise, we developed a direct acting pawl that engages the drum pins with negligible free play and no ropes.



Stock H-D

1st – 3.20
1st – 3.34
2nd – 2.21
2nd – 2.31
3rd – 1.61
3rd – 1.72
4th – 1.27
4th – 1.39
5th – 1.00
5th – 1.19
6th – 0.92
6th – 1.00