Bikernet Blog Search Bikernet
Ride Forever -
Friday Edition


Bikernet Road Stories and Baker Transmission Tech

By Scooter Tramp Scotty

Share this story:

Today I’m gonna talk about the Baker 6 speed transmission gear set I recently installed into my old evo bagger. Since the quality of different aftermarket six speed manufacturers has already been covered quite completely in various other articles, I’m not going to get into that here. Instead I’ll try to answer those questions that I, and probably most of you who want one of these things, would like answers to. And I intend to do it using simple language rather than complex gear-head jargon.

My first questions was: Would my first five gears remain the same?

Not always. Many of these aftermarket trany manufacturers offer close ratio racing gears. This means they'll make the shift between your first five gears closer together so you can stay in the high rev power band while on the race track. This is often done by raising the first gear, which makes it more like your second gear, so they can push the rest of the gears closer together. Now I don't know how often you take your or everyday road bike out on the track, but I do know that when trying to maneuver a big-ass motorcycle at slow speeds through a tight parking lot a low first gear makes the task dramatically easier. I personally don't know a single person who wants close ratio racing gears in their everyday road bike. Most want wide ratio road gears. In fact, I know two guys who've taken their aftermarket six speeds out because of just this problem. The Baker gear set I bought retains your first five gears almost exactly and I notice no difference except that the extra gear I’ve always been looking for is now there.

My second question was: Will my 80inch evo engine have enough power?

Because Harley's six speed came out same year as their bigger 96inch engine, and unless you've bumped power considerably or installed a big engine, most of us who want to install an extra gear ride the 80inch evo's or 88inch twin cams that came from the factory with five gears. So do our little engines have enough power to pull a sixth gear? And why would we need more power in the first place?

To answer that second question I’ll need to explain the physics of speed; which I was told at Bonneville. These laws dictate that, if you're going 100mph and now wanna do 200mph you don't need double the horsepower—you need four times the power! And if you wanna go 300mph you'll need four times that! The fastest motorcycle in the world uses two radically souped up, turbo charged, alcohol burning, Hayabusa engines. The idea here is simple: it's like shifting a ten-speed bicycle into top gear; it gets real hard to turn those peddles. It's the same for an engine, and therefore high gears require more power—which is why most of us can climb any hill easily in any gear except our top one. So, for my steep investment into a six speed gear set, will I be able to use it? Well, with all the traveling I do, and having recently put a Baker DD6 gear set behind my little evo, I’ll answer that question by the end of this article.

My next question was: What's the difference between overdrive (OD6) and direct-drive (DD6) units?

Before I get into that let me point out that all stock HD transmissions—four speed, five speed, and modern six speeds—use a direct drive, or 1 to 1 ratio, top gear. Why? Because unlike the aftermarket overdrive units, direct-drives eliminate the need to run your engine's power through the transmission (which costs an estimated 8% of your horsepower) and instead runs that power almost directly from your engine to the rear wheel. How does it do this? For that you'll have to take note of the images I’ve included here. I’m going to explain this in the simplest and most mechanically un-inclined language possible (bear with me you gear-head guys).

Image 1: With the primary cover removed, you can see how the engine's only job is to turn that small sprocket on its left side. It's power is then pushed through that double row chain to next turn the clutch shown at right.

Image 2: With the clutch and engine sprocket removed you can see how power must first come from the engine's main shaft shown at the end of Big John's thumb, to the transmission's main shaft at the end of my finger. From there power goes into the transmission.

Image 3: Here you can see where the engine's power must first come in through the transmission's main shaft, which runs through the center of the pulley and into the transmission, before it's then pushed through a total of four internal transmission gears, two shafts, and four sets of bearings before being ultimately brought back to turn the belt pulley (shown with the belt still on it) where it's then taken to the back wheel. So you see, the friction of running the power through all those gears, shafts, and bearings ultimately saps some of it and costs a pony or two from making it to your back wheel.

Image 4: The transmission's job is to change the ratio between main shaft and drive pulley. For instance, I think my stock first gear ratio is 3.21 to 1. This simply means that the transmission's main shaft (shown at the end of my thumb), where power comes in from the engine, will spin three and twenty one one-hundredths of a turn for every time the belt pulley (shown at the end of my finger), where power is taken to the rear wheel, goes around once. Not really rocket science is it? Now, for a 1 to 1 (direct-drive) top gear the manufactures have a simple way of just locking these shafts together so both spin at the same speed. This system eliminates the need to run power through the transmission and, although the gears still spin inside it, with no power running through them almost none is lost. Easy-peezy right? Images 5 and 6 offer a super easy illustration of how it works...

Image 5: This is a shot of my old 5 speed out of the cases. The lines indicate how power goes through it and ultimately to the back wheel. On over drive units (OD6 and others) 6th gear's power also runs through the trany in this same way. On a direct drive units however (DD6 and stock), top gear's power is routed very differently, as illustrated in image 6.
Image 6: How all direct-drive top gears work. They stop power from traveling through the transmission and instead reroute it directly to your back wheel by simply locking the transmission's main shaft and belt pulley shaft together.

And that's how direct drive works. While overdrive units actually turn the belt pulley faster than the transmission's main shaft by running it through different ratio internal transmission gears where some power will be lost, direct drive eliminates the use of internal gears altogether therefore saving that power to drive the rear wheel instead. Although Baker does make an overdrive unit, they told me these use thicker/stronger gears and are recommended only for very high horsepower engines. But their direct-drive units are still considerably stronger than stock and probably the best choice for most of us.

While overdrive is fine for big engines, its slight power loss may hurt those of us without them. And this is why all factory tranys use a direct drive top gear. Well then, if all factory top gears are all the same ratio then how do they make that new sixth gear higher than the old direct drive four and five speeds? Easy, they simply install a bigger sprocket on the engine's main shaft then use a longer chain to attach it to the clutch. They change the ratio inside the primary rather than in the transmission. And, just like changing wheel chain sprockets on a dirt bike, this makes all the gears higher. Next they simply lower the transmission's first five gears and wallah, they've got a direct drive 6th gear that'll do an easy 90mph.

Image 7: Baker's direct-drive unit does exactly the same thing and an engine sprocket with four extra teeth and longer primary chain comes in the kit. This sprocket is also available with only three extra teeth; which offers the option of lowering all your gears, including sixth, just a little.

Baker told me that a bone stock evo will probably not pull sixth gear to well without a slight modification or two...

So, has my little 80 inch evo with only an EV27 bolt in cam, Mikuni carb, and cobbled together 2 into 1 exhaust made from old used parts and a torch, been able to pull this extra gear? Has this rather pricey investment paid off, or was it really just a waste of my money? Well, my original thoughts were that I'd probably only be able to use 6th under optimum conditions and across the plains and deserts that make up most of this country. I figured that any little hill would probably force a downshift rather quickly. But in truth the thing's worked considerably better than that expectation. Under optimum conditions sixth gear accelerates pretty damn well, and even while bucking a headwind or pulling a moderate hill I’m able to accelerate—although not like a rocket. So far this DD6 works very well and sixth gear just feels like it should always have been there.

Here are my top gear RPM averages:
At 70mph I’m turning about 2,800 RPM.
At 80 it's doing 3,100.
90's kicking out 3,500 RPMs.

On flat roads this gear will also work down to around 60mph at about 2,300 RPM. Although the bike would just assume I downshift, at this speed it still hums along fairly well and this ability is very convenient for those times when heavy traffic is perpetually speeding up then slowing down.

At the date of this writing I’ve only used this six speed in flat old Florida, and Georgia where there are no real hills to speak of. But I intend to put 20,000 or so miles on this DD6 then write another article on how it's been. And although Baker gave me a little discount on this trany for writing these articles, I still had to put out a lot of money. That being said, and as usual, these articles will not be a company sales pitch. Instead they'll be an honest evaluation coming from someone who travels aboard his bike almost constantly.

Truthfully though, so far, I really love this extra gear and hope to never own another five speed. 
Hit the logo below for more information on Baker Drivetrain's product line. 

Share this story:

Back to Tech

Reader Comments

I Think It Is A Great And True Article.

Sunday, October 23, 2016
Editor Response Everything we do is true or we don't do it....

Your thoughts on this article

Your Name
Anti-Spam Question:
Please enter the words you see in the box, in order and separated by a space. Doing so helps prevent automated programs from abusing this service.