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Final Drive 101

Uncle Loomis - Merchandising

PUBLISHED: DEC 13, 2013

It's said that four wheeled apparatuses move the body, and two-wheeled ones move the soul. We're 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 can tailor the drivetrain to exploit your bike's traits 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 at a given RPM range - some spin faster or slower than others. Large V-twins, for instance, frequently 'redline' around six or seven thousand RPM's, whereas a modern sportbike might rev to nearly seventeen thousand RPM's. The transmission allows you to move from slowly to quickly, harnessing the power of the engine in the RPM range where it makes the best use of its power. Without the transmission, you would have an engine that was either fantastic at making quick jackrabbit starts, or one that could run at a hundred miles an hour (Or perhaps some compromise of the two)… but not all of them. The gears in the transmission allow you to both 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:

  • Driveshaft - 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 - they require very, very little attention typically, and their service is actually easier than a standard engine-oil change.
  • Belt - 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 a similar chain-driven bike. If one happens to break on the road, incidentally, a rider is often stranded. Repair kits exist, but their usefulness is dubious. They are a bit more difficult to change gear ratios on than a chain, but do allow the possibility.
  • Chain - the most common and oldest method of moving rear motorcycle wheels, 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 relubrication. On the upside, chains are one of the most efficient ways to move power - there are very little parasitic losses given up to the chain itself.

Cool. Um, why would I want to mess with changing 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 we'll work our way into the esoteric reasons.

  • You broke something! 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 appears to be '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. Assuming you stick with stock-spec components, here's the benefits you may see:
  • Beauty - aftermarket chains, sprockets, and pulleys are often better looking than their stock counterparts. Chains come with colored sideplates, and sprockets and pulleys are often cut with intricate designs and finishes ranging from stately to eye-popping.
  • Performance - Aftermarket rear sprockets are typically aluminum, which is lighter than the factory steel sprockets, reducing un-sprung weight (rotating weight - basically, stuff that's attached to the wheels is extra-important to keep light.) Factory HD pulleys are aluminum and steel, so the weight savings is not as great here.

That sounds great, but I am just getting a lighter sprocket/pulley? That seems like a bad reason to make the switch.

Well, in the scenario above we assumed you were going to use stock-spec components. However, there is no rule saying that you need to do this. Let's talk about two non-stock upgrades you could make to your bike and the associated benefits that come along with them.

Change your tooth count - switching up the size of your sprockets or pulleys is a good way to move your bike's power output into the range you're using it. Smaller front pulleys/sprockets and larger rear pulleys/sprockets make a bike accelerate more quickly at the expense of reduced terminal speed. Conversely, larger front sprockets/pulleys and smaller rear units will make your bike a little more sluggish off the line, but getting a little more mojo in top gear is the reward, along with slightly improved fuel mileage. You may notice more or less ''buzzing' from the motor through the frame; sometimes a gear change can shake or introduce a harmonic that is causing irritating interference in your riding style, like a vibrating clutch lever, or mirrors that just aren't clear.

If you're a track rider, maybe you run a really tight, twisty track and never get beyond fifth gear. High speed is not the issue here - but acceleration and throttle response definitely are. Another potential advantage to this scenario is a shorter or longer wheelbase - since chain-equipped bikes typically have some adjustment of the rear wheel in the swingarm, if you are careful with your measurements, you can often change the wheelbase of your bike a little to change its handling characteristics for a given riding environment.

Similarly, 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.

Change your chain - chains come in different widths, or pitches. Most sportbikes come with a 525 or 530 pitch chain. It is possible to bump down to a more slender chain - 520 is common. This reduces weight, but due to better materials and workmanship, it can be possible to achieve the same tensile strength in the narrower aftermarket chain that the OEM chain provides.

OK. I need a few more miles an hour for my long freeway cruises/the ability to smoke my sportbike buddy off the line on this dad-gum Harley of mine! What sort of parts should I be looking for?

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.

To order your belt, you'll need to know your year, make, and model. If you are changing pulley sizes, you may need a longer belt or find the 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.)

That's great, homie, but I ride a sportbike. This baby is straight chain driven! What sort of stuff am I going to 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 - the choice is yours! Hard-anodized aluminum attempts to capture the light weight of aluminum with the wear characteristics of steel. Similarly, some steel sprockets have tons 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. (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.

Connection styles can be one of two methods:

  • Master link - they're easy, included with many chains, and require no more than basic hand tools to install. They are easily repaired on the side of the road in a jiffy, and make chain removal for cleaning 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.
  • Riveted link - this joins an inside and outside 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 the tool that is needed to rivet and break the link.

Chains also come in a few flavors:

  • Non-sealed chain - 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 relubricated. It's also the chain with the least amount of frictional drag due to its lack of sealing surfaces.
  • 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 relubricated 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.

OK, I think I got this. Can I install it by 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 sportbike, some of your fairings may need to be removed. If you are going to use a riveted link, you will need a chain breaker/riveting link tool. The job should not be terrible - more difficult than an oil change, but less difficult than steering head bearings or fork seal replacement, we think.

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?)

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 - 877.792.9455!