Changing the Gearing
The RC51 has some extremely tall gearing, according to calculations & actual tests it is capable of speeds close to 75mph in 1st gear. A quick & easy mod is to replace the OEM 40 tooth rear steel sprocket with an aluminum 42 tooth sprocket.
By lowering the overall gearing with the taller rear sprocket you lose some topend speed in favor of quicker acceleration which is perfect for the RC51 as no one needs a 172mph top speed on a street bike anyway. It's much more beneficial to have the quicker acceleration & it's much easier on the clutch when taking off from a standing start as the now lower gearing requires less clutch slippage & lower engine revs to get the bike rolling.
For those of you that are curious here is an Excel Spreadsheet RC51 Gearing Calculator that can be used to calculate gearing changes on the RC51
The gearing can be lowered even further by going down 1 tooth on the front sprocket which is roughly equivalent to going up 3 teeth on the rear sprocket or you can even do both (go down on the front & up on the rear), but the bike tends to wheelie way to much to get the most out of the motor as you are forced to short shift in the lower gears to keep the front end planted instead of using the motor to it fullest potential to keep you going forward. When lowering the gearing you will also have to contend with increased compression braking from the motor which is already quite a problem on any large displacement V-twin. Too much compression braking combined with an aggressive clutch release will have the rear wheel locking up as you are trying to enter 1st & 2nd gear corners at optimum speed & trust me when I say it's no fun to have your rear wheel hopping all over the road when you are trying to set up for the corner on the other hand do it right & you can learn the technique of "backing it in"
All things considered the 42 tooth rear sprocket is the simplest way to lower the gearing as it requires the least amount of effort & expense ($50-$70). The stock chain will accommodate the 42 rear & also allows you to shorten the wheel base a little bit on the bike as you will have to push the rear wheel forward to compensate for the larger sprocket when setting chain slack. It should also be pointed out that with the stock chain & a 42 tooth rear sprocket that if you choose to run a 180 series tire that you might have clearance problems with any hugger you may have fitted or the tire might even expand at high speeds & literally rub the swingarm. The shorter wheel base makes the bike handle slightly quicker, but also seems to cause some rear grip issues when really picking up the pace In this case the 15/41 combination can be a great asset as it actually allows the rear wheel to be placed further back in the swingarm. Steel rear sprockets are available, but aluminum is lighter & is the preferred version. More to the point "Hard Anodized" aluminum is the way to go as the hard anodized coating while only costing about $10 more noticeably extends the life of the rear sprocket.
Naturally if you are racing the RC51 then your gearing may need to be much lower than discussed above, but for the average street rider the 42 tooth rear seems to be the easiest gearing change, but the 15/41 combination seems to be the most popular. Many owners are switching to a 520 chain conversion as well. Personally I am of the opinion that if your bike is new, save the money & simply swap out your sprockets & use your stock chain (530) & when it finally comes time to replace the chain then swap to the 520 conversion.
Dan Kyle 831-394-1330 has kits in stock & more than fair prices.
BTW the Sprocket Specialist Titan Tough (hard anodised) last longer than the Renthal Hard Anodised sprockets that I had defended & used for years as my sprocket of choice, but the AFAM's I have been using for the last couple years are wearing better than any sprocket I have ever tried.
