Free Horsepower For The Taking! | Sport Rider

Free Horsepower For The Taking!

Gearing your bike for better acceleration and speed

There's much more to your bike's driveline than just delivering power to the rear wheel via a chain and a couple of sprockets; gearing selection can affect acceleration, top speed and even handling. Savvy racers know that choosing the correct gearing for a given track is crucial to a good setup, and can find major benefits from a minute change in final drive ratio.

By changing front and rear sprockets (or pulleys for belt drive; ring and pinion gears for shaft) you can alter your bike's final drive ratio, which in part determines wheel speed for a given rpm. Gearing ratio refers to the ratio of rear to front sprockets. For example, a stock Honda CBR600F4 has a 16 tooth front sprocket, and a 45 tooth rear, for a ratio of 45/16, or 2.81. Substituting a larger front or smaller rear sprocket lowers the ratio (sometimes called "taller" gearing), resulting in more speed for a given engine rpm. Likewise, a smaller front or larger rear sprocket gives less speed for a given rpm ("shorter" gearing).

But...But...

This begs some obvious questions: "Why can't I put a 20 tooth sprocket on the front and go 200mph," or "With a 13 tooth front sprocket will my 600 do eight-second quarters?" Performance numbers can be enhanced by sprocket selection to a certain extent, but the overriding factor is your engine's power and its characteristics. Choosing the correct gearing optimizes your powerband usage, maximizing power delivered to your rear wheel for the given conditions.

For starters, a basic gearing chart is essential. It's much easier to think in terms of speed as opposed to gear ratios. Speed for a given set of sprockets is determined as follows:

Speed = (rpm X rear tire circumference X front sprocket) / (primary gear ratio X sixth gear ratio X rear sprocket X 1056)

where rpm is engine speed (12,500 for the CBR), and tire circumference is in inches (80). Substituting numbers for our CBR600 example and some different sprockets, our chart looks like this:

Most of the chart usage is relative, so if you don't have all the specifications for your bike the formula can be reduced significantly to an approximation of:

Speed = (460 X front sprocket) / (rear sprocket)

Stock bikes are generally geared to reach their top speeds at just below their power peaks in top gear (12,000 rpm for our CBR's top speed of 157 mph, with power peaking at 12,500 rpm) so as to maximize speed and allow for things such as going downhill and wind. Changing to something like 17/44 on our CBR600 would actually reduce top speed, because in sixth gear the engine would spin much slower (11,000 rpm at 157 mph) and not make enough power to pull its top speed. Shortening the gearing to say 15/47 would have the CBR passing its power peak early, also resulting in a slower top speed. However, this is where sprocket selection can be advantageous: at a given road speed, and in a given gear, shorter gearing will have the motor spinning faster, where more power is available.

Gear for the Track

As a starting point for any racetrack, gearing should be selected so that on the longest straight, your bike revs just past its power peak in top gear-around 13,000 rpm in our example. Say our stock geared CBR revs to only 12,000 rpm, or 92 percent of its 13,000 rpm maximum at the end of a straight. Using the chart can eliminate guesswork in choosing the correct sprockets. We would need gearing that results in a speed of 92 percent of that for stock gearing (151 mph), plus a bit extra because closer to redline there would be more power to pull more speed. In this case, 15/45 gearing gives a speed of 153.3 mph, and would be a good choice.

Choosing sprockets to maximize revs gives you full use of your engine's powerband. It's usually best to gear as short as possible and use the higher gears, as these are generally spaced closer together. This means rpm drops off less each time you shift, keeping the engine more in its powerband. The key to gearing is to keep your engine as close to optimum rpm as possible everywhere on the track. In addition, using the shortest gearing possible will get you off the line quicker-an important consideration for short sprint races.

Once you're in the ballpark for revs on the longest straight, you can fine-tune your gearing for corner exits. Ideally, your motor should be spinning in the meat of its powerband exiting turns. Too slow, you're losing ground waiting for the revs to come up; too fast, you'll have to shift before the exit of a turn. Decide which are the most important turns and if they are worth tailoring your gearing to optimize those corner exits. Sometimes, it's worth letting the motor over-rev on the straight if you get a better drive from a crucial fast turn.

Note that even though the chart is based on an engine speed of 12,500 rpm, dealing in percentages and relative speeds enables you to use it for any rpm-the numbers are somewhat arbitrary and simply make it easy to evaluate the consequences of changing sprockets. Consider a second example of exiting a corner just below the powerband. You're determined to use the same gear, but are willing to change overall gearing to bump the revs up a bit. Knowing your engine is spinning below the powerband by a certain percentage, you can select gearing that is the same percentage shorter and exit a turn in your bike's powerband.

If you're lucky, you've got a cassette gearbox and can change the internal ratios to suit various turns. Using a spreadsheet program, you can expand the above gearing chart to include speed in each gear, along with each alternate ratio, and use this to help you decide how to tailor your gearing.

The Subtleties

Now for the fun stuff. Notice on the chart how 15/44 is virtually identical to 16/47 gearing...or is it? One obvious difference is that with larger sprockets, 16/47 gearing will give a shorter wheelbase than with 15/44, giving your bike a slightly more rearward weight bias. Or, maybe it's enough of a change that you could add a link to the chain and lengthen the wheelbase. Given sprocket sizes and chain length, comparative wheelbase numbers can be calculated and added to the chart, eliminating any guesswork.

A front sprocket with fewer teeth works the chain harder and robs power, as the chain has to curl more to match the smaller diameter. In general, a 14 tooth front sprocket is the smallest advisable, with 13 being used in extreme cases.

One thing to watch is clearance between the swingarm and chain. Smaller sprockets can result in the chain laying on the swingarm, and actually working the suspension when power is applied. One notable example is the early Kawasaki ZX-7R, on which many racers had to have the swingarm relieved for chain clearance. With high-horsepower machines, chain pull and its effect can be altered using sprocket selection. Anti-squat torque-which extends the rear suspension under power-is determined by swingarm angle and the distance between the swingarm pivot and top chain run. Even if the chain doesn't touch the swingarm, running it close to the pivot will enhance the anti-squat torque, much in the same way raising the swingarm pivot does.

The Strip

Like gearing for the straight on a roadrace course, a drag bike needs to be geared so that it reaches peak rpm just at the end of the strip. Usually, drag racers will use only the first three or four gears of their bikes' transmissions, as each shift costs some time; the savings at the start from shorter gearing is more than offset by the extra shifts that would be necessary. To maximize the amount of power put to the ground over the course of a run, the engine should be spinning at or close to redline at the precise moment your bike goes through the timing lights, as this eliminates any unnecessary shifts and gives maximum power near the end of the run.

Gearing for Success

If you're going racing, you'll need a selection of front and rear sprockets that will allow you to gear your bike for the different tracks you plan to attend. The greater your selection, the more chance you'll have of obtaining the optimum gearing-you just can't buy a 45 1/2 tooth sprocket. If it's time for a new chain and sprockets for your streetbike, take advantage of a gearing change depending on the type of riding you do. For highway droning, taller gearing will have your engine revving lower, making for a less buzzy ride. Or if you do a lot of canyon strafing, shorter gearing may give you that extra zip exiting corners.

Chain and Sprockets

When choosing a new chain and sprockets for your scoot, there are a number of things to take into consideration. A popular modification for racers is to replace a stock machine's 530 chain with a thinner and lighter 520 series chain and matching sprockets. Generally, roller chain is denoted as follows: the first digit specifies the chain's pitch, in 1_8 inch increments. The remaining digits correspond to the width of the chain in 1_80 of an inch. A 530 chain, then, has a pitch of 5_8 inch, and a width of 30_80 inch, or 3_8 inch. A 520 chain is 1_4 inch wide, correspondingly lighter, but also somewhat weaker than a 530 chain. There are some exceptions to this; For instance, 125GP machines utilize a 428 chain, which is a 425-sized chain with slightly thicker sideplates. Front sprockets are usually steel, but rear sprockets are available in either steel-which is somewhat heavy but lasts a long time-or aluminum (lighter, but less wear resistant).

Most stock bikes are equipped with an O-ring chain, and are endless-there is no link to split the chain for removal, and it has to be "broken." Replacement chains generally utilize either a rivet-type link, which must have the pin ends peened for assembly, or a clip-type link. A rivet link is generally stronger and less likely to come apart on its own, whereas a clip link is easier to assemble/disassemble but may come undone. If you use a clip link, safety wire around the sideplate and clip to hold it on, or use a dab of silicone sealant on the clip.

An O-ring chain employs tiny rubber seals between each plate and roller to keep the chain permanently lubricated. While it does add weight, this style of chain lasts longer and requires less maintenance than a non O-ring chain. The small seals may seem to add friction also, but once in use the grease thins out due to the induced heat, and an O-ring chain may spin as freely as a non O-ring unit.

Both styles, however, need periodic lubrication. The non O-ring variety more so-the O-ring type only displaces water and stops the outer surfaces from rusting. Use the appropriate type of lube-some spray-on types may damage the rubber O-rings-and oil your chain immediately following a ride. This will give the maximum amount of time for the oil to soak in. Always apply the lubricant from the inside of the chain, because centrifugal force will help the oil penetrate, rather than fling it on your tire.

This article was originally published in the August 2000 issue of Sport Rider.

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