School of Drivetrain

We manufacture 2 types of 6-speeds for Big Twins; the OD6 (overdrive 6-Speed) and the DD6 (Direct Drive 6-Speed).  Both 6-speed configurations achieve a significant RPM reduction on the highway without compromising around town performance.  In other words, you get the five gear ratios you currently have with a 5-speed plus that extra gear you're always looking for on the highway.  To determine which 6-speed is right for your application you must look at your engine output, riding style, bike gearing, GVW, and performance priorities. 

 

 

 BAKER Overdrive Six Speed      BAKER DD6

 

 

 

 

 

Generally, the DD6 is designed for touring riders who put the miles on, and the OD6 is for the speed freaks and custom bikes.  Study the information below or consult our tech department to make the decision which 6-speed is right for your application.

 

The OD6 offers more gear ratio choices than the DD6.  These ratio choices are tailored towards the more aggressive rider with more aggressive engines.  As engine output increases, the utility of the stock 1st gear ratio diminishes.  In other words, a stock 1st gear will only get you partially across the intersection from a dead stop, before you have to shift into 2nd gear.  Precisely the reason why we developed R-ratio 1st and 2nd gears for OD6.  Also, bigger engines can pull a sleeper 6th gear ratio and this is why we developed the deep overdrive for OD6.  The 6th gear ratio in the DD6 is fixed at 1:1 by design and can't be changed.

 

The undercutting on the side of the dog teeth (highlighted with red line on image below) is an important distinction between DD6 and OD6.  The OD6 undercut angle is 4 degrees, the DD6 angle is 1 degree.  The function of the undercut is to hold the transmission in gear, along with the shift system detent.  4 degree undercut angles, as compared to 1 degree angles, have slightly increased shift effort and slightly compromised shift quality but are necessary with aggressive engine tuning.  For reference, some pro-stock V-twin applications run undercut angles as high as 10 degrees.  In contrast, Japanese motorcycles usually have undercut angles close to 0 degrees but they can get away with that because their engines lack the violent power pulses of a 45 degree V-twin.

 

 Dog Tooth Angle

What Does Overdrive Mean?

 

 

Conventional Power Flow
Conventional Overdrive Power Flow: 92% Efficient

 

5th gear in our OD6 has a 1:1 ratio.  All ratios greater than 1:1 are underdriven like 4th gear which is 1.23:1.  Ratios less than 1:1 are overdriven like 6th gear which is .86:1 and gives the resultant RPM reduction on the highway.

 

OD6 Technical Highlights

 

- 165 ft-lb torque capacity
- Many gear ratio choices for performance applications
- Some light case modification may be required due to casting variation from the factory
  
OD6 Application Guidelines

 

- 1990-99 Evo engines producing more than 100ft-lbs of torque
- 1999-2006 Twin Cam engines producing more than 100 ft-lbs of torque
- American Ironhorse, Bourget, Titan, Gilroy, Indian, and other 45 degree V-twin American Motorcycles
- Any Harley or 45 degree V-twin American Motorcycle with an open belt
- All 1986-89 Harley's with original tapered mainshaft clutch interface. 
  

What Does Direct Drive Mean?

 

 

Direct Drive Powerflow
Direct Drive Power Flow: 99% Efficient.  Waste Less Horsepower

 

6th gear in the DD6 has a 1:1 ratio; the same as 5th gear in a 5-speed.  The 1:1 gear is THE direct drive gear.  With the DD6 the 'overdrive' effect on the highway is achieved by 'overdriving the primary with a 28 tooth compensator sprocket.

 

DD6 Technical Highlights

 

- 120 ft-lb torque capacity
- No exhaust clearance issues due to door width and profile
- Minor case modification may be required due to casting variations
- Combination of helical and spur gears strikes a balance between strength in the lower gears and silent operation in the top gears
- 28 tooth compensator sprocket, supplied with kit or assembly, also gives 14% more starter torque
Best achievable 6-speed fuel economy, period
  
DD6 Application Guidelines

 

- Most 1999-2006 Twin Cam Harley applications
- Riders who demand a 6-speed with the smoothest shift quality available
- Touring riders who put on serious miles and spend a majority of the time on the road in 6th gear