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Final Drive 101


It's said that four-wheeled apparatuses move the body, and two-wheeled ones move the soul. I'm not inclined to disagree, but it does bring about a question: What moves the two-wheeled apparatus?

A number of things help perform that function, but the one we are interested in here is the final drive. Effectively, that means everything aft of the transmission that helps to spin the rear wheel. Depending on what type of bike you have, you may be able to tailor the drivetrain to exploit your bike's traits or to better suit your riding style and environment.

Let's talk about how the power is moved through the bike. The engine produces power, and spins in a given rpm range. Some spin faster or slower than others. Large V-twins, for instance, frequently redline around 6,000 or 7,000 rpm, whereas a modern sport bike might rev to nearly 17,000 rpm. The transmission allows you to keep the engine in the rpm range where it works best while having a wider range of speeds. Riding without the transmission would be like riding in the same gear all the time. You might be able to make quick, jackrabbit starts, or run 120 mph, but not both. The gears in the transmission allow you to accelerate rapidly and cruise at nice, low, fuel-conserving engine revs.

From the transmission, power moves to the rear wheel through the final drive. There are three common ways to move that power in use on modern motorcycles:

  • shaft driveDriveshaft: Often referred to lovingly as "shafties," these bikes move move power to the rear wheel via a driven shaft. Power arrives at the rear wheel through a ring and pinion gear that is not dissimilar to the units found in older RWD cars. Because changing the gearing is labor-intensive and aftermarket options are nearly unavailable, riders of shafties have limited options when it comes to changing final drive components. Shafts are frequently seen on commuter bikes or long-distance touring machines because they typically require very little attention and their service is actually easier than a standard engine-oil change.
  • belt driveBelt: Many motorcycles, notably late-model Harley-Davidson products (and some metrics, as well) use a belt final drive. Some of the advantages of a belt-driven bike are low maintenance, low noise, no "fling" of chain lubricants, and long life. They also are not particularly prone to damaging the bike or hurting the rider if they snap. However, because they are "endless," installation is typically more difficult than on a similar chain-driven bike. If one happens to break on the road, the rider is usually stranded. Repair kits exist, but their usefulness is dubious. Belts also make it a bit more difficult to change gear ratios than a chain, but do allow the possibility.
  • chain driveChain: The most common and oldest method of moving the rear wheel of a motorcycle, the chain has a lot going for it. Chains are strong and affordable. They are time-tested, and install easily due to the fact that a chain can be broken and mended fairly easily. Chains do require more maintenance than the other methods of driving a rear wheel, and can also cause severe bike damage and rider injury if they are not tended to carefully. Chains require frequent cleaning and lubrication. On the upside, chains are one of the most efficient ways to move power. Very little power is given up to parasitic loss.

Cool. Um, why would I want to change my final drive?

Oh, I love when people ask this. Nothing makes me happier than describing all the ways you could be happier riding your bike by trashing OEM components for purpose-built, go-fast goodies!

There are about eleventy-four billion reasons to switch up your driveline pieces, so let's start with the more common reasons and work our way into the esoteric ones.

Something broke

Belts, chains, and sprockets are all wear items. Maybe you got a stone lodged between your belt and pulley, or perhaps your chain has worn beyond specifications. (A lot of people call this "stretch." You should know that chains don't really stretch. The pins and plates wear, and the chain gets longer due to this wear. Belts don't stretch much at all, either.) Regardless of what happened, if your OEM components are damaged or just plumb worn-out, you need to replace them. You can often get replacement parts that are of higher quality than OEM pieces for less money and there may be other benefits.

The appearance of aftermarket chains, sprockets, and pulleys is often better than their stock counterparts. Chains are available with colored sideplates, and sprockets and pulleys are often cut with intricate designs and finishes ranging from stately to eye-popping. You can also gain performance. Aftermarket rear sprockets are typically aluminum, which is lighter than the factory steel sprockets, reducing unsprung, rotating weight (basically, weight savings in anything attached to the wheels pays extra dividends in performance).

You want to modify your bike

Aside from parts wearing out, you may want to make changes to your final drive to make your motorcycle's performance better fit your needs. Changing the size of your sprockets or pulleys is a good way to move your bike's power output into the range where you use it.

A smaller front pulley or sprocket combined with a larger rear pulley or sprocket makes a bike accelerate more quickly at the expense of reduced terminal speed. Conversely, larger in the front and smaller in the rear will make your bike a little more sluggish off the line, but you'll get a little more mojo in top gear as the reward, along with slightly improved fuel mileage. You may notice more or less "buzzing." Let's say your motorcycle's engine has an annoying vibration in top gear at the exact speed you ride to work on the highway every morning. A different size sprocket can move you out of the vibration zone and eliminate that daily annoyance at a low cost.

If you're a track rider, maybe you run a really tight, twisty track and never get beyond fifth gear. High speed is not an issue, but acceleration and throttle response definitely are. Different sprockets can make the bike better suited to a given track — and the change can easily be reversed.

On the other side of that coin, maybe you have an old Shovel that you ride around with your buddies who have more modern bikes. If you want a little more top end so your old cone motor's not screaming trying to keep up, a different pulley might get you the results you're looking for.

Sport bike riders in search of every ounce of performance and weight savings sometimes change to a different size of chain. Most sport bikes come with a 525 or 530 pitch chain. It is possible to bump down to a more slender chain — 520 is a common conversion. This reduces weight, but due to better materials and workmanship, it can be possible to achieve the same tensile strength with the narrower aftermarket chain.

I need a new belt, but which one?

Whether you're replacing a damaged or worn belt or changing to a different setup to gain a few more miles per hour on your freeway cruise or to try to smoke your sport bike buddy off the line on that dad-gum Harley of yours, you'll need to know your year, make, and model to order the right belt. If you are changing pulley sizes, you may need a longer belt or may even need to modify your swingarm to gain additional adjustability. (Belt-driven bikes typically do not have much room for adjustment, as belts do not stretch.)

Belts can be ordered with different reinforcing fibers, including carbon fiber, aramid, Kevlar, fiberglass, or polymer. Truth be told, there is lots of argument on the subject, and each material has its defenders. Although almost no belt carries a warranty, 60,000 miles on a single belt is not at all a strange proposition.

Your front pulley material will be steel, and your rear will be aluminum.

That's great, homie, but I ride a sport bike — what do I order?

Front sprockets will be made of steel, and rear sprockets can be had in either steel, aluminum, or hard-anodized aluminum. Steel will wear better, but aluminum is lighter. Hard-anodized aluminum attempts to capture the light weight of aluminum with the wear characteristics of steel. Similarly, some steel sprockets have a lot of material removed to offer strength without the weight penalty.

To order your chain, you'll need to have a rough idea of your length. If you are unsure, order long, and you can always cut it back with a die grinder or a chain tool. (Remember kids, we measure twice, and cut once!) You'll also need to know how it needs to go back together, and what style of sealing ring (if any) you want on your chain.

The two ways to connect a new chain are a clip-style master link and a riveted link.

A clip-style master link can be installed with basic hand tools. They are easily repaired on the side of the road in a jiffy, and make chain removal a snap! The downside is that some folks think they are weaker than riveted links. Additionally, many track sanctioning bodies specify a chain connected with a riveted link.

A riveted link joins an inside and outside plates of the link with a rivet pin. It is safe, secure, and how factory links are attached. Its downsides are its ease of removal (or, more specifically, lack thereof) and you have to have the tool needed to rivet and break the link.

Chains also come in a few flavors.

A non-sealed chain is the most affordable and most labor-intensive of the chains. Non-sealed chains have no seal between the inner and outer plates. This allows dirt in easily, but it also allows cleaner and lubricant in, too. It's the easiest to get dirty, but also the easiest to get clean and lubricated. It's also the chain with the least amount of frictional drag, due to its lack of sealing surfaces, which is why it is sometimes used in racing applications.

O-ring chains use small rubber O-rings around the pins on each link of chain. O-rings provide one sealing surface to keep dirt and grit out and lubricant in. These chains typically cost a bit more than their non-sealed counterparts. They need to be cleaned carefully and lubricated from time to time, but do not require the same level of attention that a non-sealed chain demands.

X-ring chains are similar to O-ring units, but use seals with an X-shaped cross section. This gives each ring twice as many sealing surfaces as an O-ring chain. These chains are the costliest, and even if a seal breaks, the other section of the seal should theoretically keep dirt out, and lubricant in.

Is this something I can do myself?

If you're riding a bike with a chain... probably! If you change your own tires, you are definitely able to do it! You'll need a way to elevate your rear wheel (but not by the wheel, as it needs to be removed). Basic hand tools are a must — your standard assortment of ratchets, sockets, and wrenches. If you have a sport bike, some of your fairings may need to be removed. If you are going to use a riveted link, you will need a chain breaker and riveting link tool. The job is more difficult than an oil change, but less difficult than steering head bearings or fork seal replacement.

If you are riding a bike with a belt — well, things are a bit more difficult. On a Harley, you are looking at removing the bags and rear shocks, and sometimes the swingarm. Next, the outer primary and clutch are removed, followed by the inner primary. The wheel is then taken off, and at this point, the belt may be removed and replaced, as can the pulleys. This job is not for the faint of heart. It's not difficult, but it is time-consuming and requires a weekend afternoon for most folks and some pre-planning. (Did you buy new primary gaskets and fluid? What about the stator sealing O-ring? Do you want to do your clutch while you're in there?)

If you have a shaft drive, then most of the above does not apply. With a shaft, you can't change gear ratios like you can with a chain or belt by swapping sprockets or pulleys. You're stuck with what the factory gave you. The tradeoff is lower maintenance. Just check your manual to see what periodic maintenance is required in the form of lubrication (usually, not much) and you're good to go. You'll be happily motoring down the road while your riding buddies are cleaning and adjusting chains.

Thus completes Final Drive 101! If you have further questions, feel free to get in touch with a Gear Geek via email (cs@revzilla.com) or give us a call at (877) 792-9455!