With all the money spent on engine performance items like exhausts, ECU fuel-mapping boxes, cams, pistons, etc., it's amazing that so many riders overlook a simple upgrade that can boost their bike's acceleration numbers for minimal dollars: gearing. For a fraction of the cost of a quality exhaust system, your roll-on acceleration from various speeds and in all gears will increase significantly. And there is potential to drop almost half a second from the quarter-mile ET and possibly add some mph to boot. All this from changing your sprockets.
Final gearing selection by the manufacturer for a stock motorcycle is a tricky deal, especially with some powerbands getting narrower and engine rpm capabilities soaring ever higher with each successive model year. The gearbox ratios must be close enough to keep the engine within the meat of its powerband with each shift, but not so close as to require an additional gear, which would add weight. The final drive gearing (sprockets) should be short enough to help acceleration, but tall enough to maintain some civility on the street; these are streetbikes, after all. There are fuel economy and comfort concerns; if your sportbike is spinning at high rpm while cruising on the highway, you're using more fuel and possibly running at the rpm range where engine imbalance is the greatest, causing excessive vibration. Emissions and noise regulations can also play a role in the final gearing chosen by the factory for a particular model.
Although lower gearing paid...
Although lower gearing paid huge acceleration dividends with the Honda RC51, it also increased the bike's wheelie tendencies in the lower gears, killing the quarter-mile times. In tighter corners, this could be a liability; choose gearing with caution.
Maximizing the performance of your bike by choosing the proper gearing has its limitations, of course. You can't make your otherwise stock 600 turn eight-second quarter-miles, or gain 30 mph in top speed. But if you're willing to slightly compromise the bike's overall street manners in return for increased performance, you can gain a little extra speed for minimal cash outlay. We decided to see just how much acceleration could be gained by changing the stock gearing on a couple of popular sportbikes.
Even if you decide to stay...
Even if you decide to stay with the stock 530 chain on the RC51, aluminum sprockets like this Renthal unit will still save you 11 ounces of unsprung weight, an incremental but crucial gain.
BENEFITS AND DRAWBACKS
The benefits are obvious: increased acceleration without any other modifications to the engine or otherwise. There are a few drawbacks, depending on how much you change the overall gearing. If the gearing is altered substantially, your bike will probably lose some speed off the top-but then again, when was the last time you took your bike to the salt flats? Then there's the aforementioned changes to engine character at highway cruising speeds: turning higher rpm will put many bikes in a range where vibration increases significantly. You also will be shifting more frequently than before due to the shorter gearing, and your fuel consumption may suffer incrementally.
Switching to a 520-size chain...
Switching to a 520-size chain and sprocket combo can get you major weight savings, even on a 600. The RK 520 Race Conversion kit (available through FTM Enterprises; 760/732-3161, www.ftmbiz.com) is over a pound lighter than the stock GSX-R600 setup. The Vortex aluminum sprocket included with the kit (on the right) weighs a pound less than the stock Suzuki sprocket, a huge gain in unsprung weight.
The biggest drawback for most riders will most likely be speedometer error. Since many sportbikes today use a sensor that measures the rotation of the countershaft sprocket or shaft to determine road speed, changing your sprockets will make your speedometer reading even more inaccurate than it already is. Changing the countershaft or rear sprockets can increase speedometer error by an additional few percent. During our testing, one bike with stock gearing and tires was indicating 64 mph at a radar-gun-measured 60 mph. Fitting a one-tooth smaller countershaft sprocket increased the inaccuracy to an indicated 68 mph while reaching an actual 60 mph. Replacing the stock rear sprocket with a unit two teeth larger increased the necessary indicated speed to 69 mph. Altering your stock gearing will most likely require some recalibration of your speedometer readings (whether in your head, or electronically).
Even a modern 600 benefits...
Even a modern 600 benefits from gearing optimization, as our GSX-R600 made some major gains in acceleration.
THE TEST MULES
We chose the Honda RC51 and Suzuki GSX-R600 as our test mules for a couple of reasons. All stock bikes can benefit from optimizing their gearing, but there are a few that can realize substantial gains from lowering the overall ratio. The most obvious candidates for improvement are V-twins; virtually every V-twin sportbike is equipped with incredibly tall gearing from the factory, and the RC51 is no exception. Cruising at an indicated 70 mph has the engine loafing along at a tick under 4000 rpm, far too low in the powerband to be responsive in a roll-on acceleration situation.
The Suzuki GSX-R600-while a highly capable middleweight supersport bike in its own right-suffers from a dearth of midrange acceleration. We wanted to see how much we could pep up the middle portion of the Suzuki's powerband, without drastically effecting the top end, or forcing the rider to tap dance on the shift lever even more than what is usually necessary on a top-shelf 600.
In fact, we were wondering if shortening the gearing on a modern 600 would even be worthwhile. As 600 powerbands march inexorably to stratospheric rpm levels over the years, the engines' overall gear ratios have become shorter and shorter in order to enable access to that increasingly exclusive power. Take a look at the following transmission primary gear and gearbox ratio listings for Honda's CBR600, from its first iteration as the infamous Hurricane, to the latest take-no-prisoners CBR600RR version. Note how the primary reduction ratio is getting shorter (higher ratio numbers) along with the overall transmission ratios, and how the gear ratios are becoming closer to one another.
1987 Honda CBR600 Hurricane
Primary Reduction 1.775:1
1998 Honda CBR600F3
Primary Reduction 1.863:1
2003 Honda CBR600RR
Primary Reduction 2.111:1
This aluminum RRP rear sprocket...
This aluminum RRP rear sprocket on the left (available through Sprocket Center; 888/265-2141, www.sprocketcenter.com) only weighs five ounces, compared to 20 ounces for the stocker on the right. Note that the stock RC51 chain is only gold-plated on the left side, only where visible; the rest of the chain is unfinished.
When you've got a powerband that basically starts at 9000 rpm, you need to get the engine rpm spun up quickly, and the current ultra-short-stroke motors lack the low-end grunt to accelerate quickly through the first 5000 rpm. That means short gearbox ratios, and short final drive gearing (sprockets). With gearing this short in the current 600 class, could we possibly extract any more performance by lowering it even more?
ROLL-ONS AND QUARTER-MILE ACCELERATION
The roll-on acceleration test consisted of running the bikes up to 60 mph (measured with a radar gun, due to the speedometer error incurred by changing sprockets), then accelerating for an eighth-mile in both top gear and fourth gear, measuring trap speed at the end of the eighth-mile run. Quarter-mile was your standard run-and-gun NHRA-length dragstrip at LACR (Los Angeles County Raceway) in Palmdale, California.
We made the usual runs with the stock gearing to establish a baseline, then changed the countershaft sprocket on both bikes to a one-tooth smaller unit (15-tooth on both the Honda and the GSX-R). We then lowered the gearing further by fitting a 47-tooth on the Suzuki (stock is 45-tooth), and a 43-tooth on the Honda (stock is 40-tooth).
The results in the roll-ons were predictable. Major gains were apparent in both gears, with mph picking up drastically in fourth gear roll-ons. In fact, check out how the little GSX-R came very close to nipping at the RC51's heels with just the smaller countershaft sprocket. If you can put up with increased rpm at cruising speeds on the highway, there's some major performance gains to be had with revised gearing.
The quarter-mile runs were far less conclusive, which we can attribute to one thing: wheelies. Even the relatively midrange-weak GSX-R600 suddenly turned into a sky-shot monster with the shorter gearing, and the Honda was virtually impossible to launch using full throttle in first gear. Since the bikes weren't lowered, their high center of gravity made them extremely wheelie-prone, which killed their ETs. However, note that the mph trap speeds increased with each gearing change, a sure sign that acceleration was much quicker. We are confident that taking the wheelie tendency out of the picture could shave up to a half-second from the ET. A quarter-mile run, unfortunately, requires that all 1320 feet are traversed as quickly as possible.
Switching to a 520 chain setup...
Switching to a 520 chain setup on the RC51 saves almost two pounds, with much of it unsprung weight. We used an RK GB520GXW chain and RRP sprockets on the Honda.
IS IT WORTH IT?
There are other considerations in upgrading to aftermarket gearing on your bike. Switching to a smaller-size chain (from a standard 530 to a 520, the popular racing size), along with the matching sprockets, can drop some weight in both rotating and unsprung masses.
But there's no doubt that the major appeal lies in the fact that, from a performance per dollar ratio, it's hard to beat upgraded gearing. It's about as simple to install as an aftermarket exhaust, yet isn't as readily visible. Literally free horsepower for the taking.
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 to displace water and stop 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. -A.T.
This article originally appeared in the June, 2003, issue of Sport Rider.