RC51 - 15/41 sprocket ratios ? - Sportbike Forums : Sportbikes Motorcycle Racing Forum
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post #1 of 17 (permalink) Old 08-14-2004, 12:39 PM Thread Starter
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RC51 - 15/41 sprocket ratios ?

I replaced my chain and sprockets on Thursday (thanks South for all the info

After initially deciding to go fo the standard 16/40 sprockets, at the last minute I thought I would go with the Dan Kyle reccomendations and instead put the 15/41 sprockets on.

Man, the bike is lethal. I cannot open the throttle full in 1st gear as the damn front wheel suddenly gets the urge to relocate itself behind the bike :.

Second gear is almost as bad, especially if it is a slightly bumpy surface.

We are at 5500feet altitude in Johannesburg, plus I need new air filters (have a lousy flat spot between 4 and 5k rpm) AND my bike is standard..... I would HATE to see what happens down at the coast with a properly tuned motor with a set of Sato's and PCIII :

Anyway, I found that for town riding, the new ratios work extremely well, but I rode the bike on the freeway today for the first time since replacing the chain/sprockets, and it was horrible.

What I would like to know is if anyone else has tried any other combinations of sprocket "ratios" for instance 16/41 or 15/40 ? I have to do a fair amount of freeway travelling too, so I am seriously considering going back to the standard sprocket sizes unless there is another combination which will give me the best of both worlds.

Steve


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post #2 of 17 (permalink) Old 08-14-2004, 02:13 PM
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Re: RC51 - 15/41 sprocket ratios ?

Hey, no problem :. If your old sprockets are still (somewhat) serviceable, maybe you could experiment with them. Going -1 in the front roughly equates to +3 (2.6 or something) in the rear. So, I think you've got the right idea: for a little more zip from stock, try the stock front sprocket with the +1 in the rear, or for a slightly less radical set-up from what you've now got, use the -1 front with the stock rear.

Personally, I can't imagine a -1/+1 set-up on a torquey liter V-twin. :
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post #3 of 17 (permalink) Old 08-14-2004, 04:13 PM
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Re: RC51 - 15/41 sprocket ratios ?

As noted in the Rogue site www.RC51.org the most common are 16/42 and 15/41....sounds like you'd like the 16/42 better. Here's what the site says:

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 <font color="red"> 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</font>. 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 &amp; simply swap out your sprockets &amp; use your stock chain (530) &amp; when it finally comes time to replace the chain then swap to the 520 conversion.

Dan Kyle 831-394-1330 has kits in stock &amp; more than fair prices.

BTW the Sprocket Specialist Titan Tough (hard anodised) last longer than the Renthal Hard Anodised sprockets that I had defended &amp; 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.




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post #4 of 17 (permalink) Old 08-14-2004, 06:10 PM
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Re: RC51 - 15/41 sprocket ratios ?

Unfortunately, SprocketSpecialists don't carry a 42 steel in the 530 width. Only 41,43 and up... :

I wondered if the 15/41 would be a little too agressive. Thanks for sharing your experience(s) Audix.



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post #5 of 17 (permalink) Old 08-14-2004, 06:59 PM
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Re: RC51 - 15/41 sprocket ratios ?

Hmmm, I was contemplating going 15/41 also in the future...but since we have to ride freeways often in SoCal, maybe the 16/42 would be nicer...I don't mind going the hard anodized AFAM route if there are no steel 42s available.


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post #6 of 17 (permalink) Old 08-14-2004, 09:09 PM
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Re: RC51 - 15/41 sprocket ratios ?

I run the 15/42 on my RC and I love it. Makes the bike handle lots better around town and still works well at freeway speeds. All it cost me was some top-end; but then, can't safely do 175 anywhere anyway. Also, switched to a 520 chain. Works well for me.
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post #7 of 17 (permalink) Old 08-15-2004, 09:04 AM Thread Starter
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Re: RC51 - 15/41 sprocket ratios ?

The best thing I can think of to describe what the RC feels like with the 15/41's is like my old 350 2 stroke. I am literally riding it now the same way as I used to ride the RZ.

I am now extremely careful coming out of corners and the same with acceleration from stand still. With the 15/41 I only open the bike up to full throttle from 3rd gear, anything below that I need to request take off clearance from the local airport :.

It's insane ...but great fun though ::

To give a little more detail ...the power comes on in a rush if you open the throttle hard and fast, and if in first, it is an instant 90deg vertical wheelie :-).

If I gradually open up (reasonably fast though) to full in 2nd from about 4000 rpm, the front starts hopping immediately and then starts 'climbing' when I get up above 6000. I have not given it too much of a chance to do that, so exactly how far it will come up I cannot say. I also have not tried "snapping" the throttle open in 2nd, but I can tell you it will wheelie instantly if I do :

Bearing in mind that I am extreme altitude and running a stock RC, I can guarantee that for those of you in lower lying areas with any mods will have the same problem in 2nd and possibly in 3rd that I have in first up here.

What I am going to try is putting a 16 tooth front sprocket with the 41 rear. This will be better than standard and nowhere near as radical as what I have now.

Soon as I can get the sprocket and try it out, I'll post the results :



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post #8 of 17 (permalink) Old 08-15-2004, 05:22 PM
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Re: RC51 - 15/41 sprocket ratios ?

You dont need new air filters as the stockers flow better then any aftermarket filter. Your flat spot is either the flapper valve or a map for your exhaust. Dont waste your money on the filters....
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post #9 of 17 (permalink) Old 08-15-2004, 09:03 PM Thread Starter
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Re: RC51 - 15/41 sprocket ratios ?

[ QUOTE ]
You dont need new air filters as the stockers flow better then any aftermarket filter. Your flat spot is either the flapper valve or a map for your exhaust. Dont waste your money on the filters....

[/ QUOTE ]

Thx for the info, I have read all of the write ups at Rogue so I won't waste any money on 3rd party filters.

As far as your suggestions to the other potential causes of the flat spot are concerned, I have done the flapper valve mod as per Rogue instructions and I also still have the standard exhausts without any PCIII or similar device for changing the stock fuel mappings.

I really do need new filters though ...not BMC's or K&amp;N's ...standard filters. I have not replaced them since I have had the bike, only blown them out with the compressor. I know that the flat spot is caused by the filters because the last time I blew them out, the flat spot disappeared completely and only returned about a month later.



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post #10 of 17 (permalink) Old 08-16-2004, 06:53 PM
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Re: RC51 - 15/41 sprocket ratios ?

I just put on a JT steel 42 sprocket on my RC-51, and it works great. I also think I'm now getting better mileage as I can now hit 6th gear without lugging the engine a less than 80 mph.
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