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Thursday, November 19, 2009

Big sprocket equals more speed? Maybe

Want to make your Royal Enfield Bullet go faster? Why not just increase the size of the front sprocket? That means that, without increasing engine rotations per minute, the rear wheel will turn faster. And away we go! Maybe.

"Andy" in Callahan Florida happens to be selling a 2002 Royal Enfield and notes in his ad that it has a 22-tooth front sprocket. That's the biggest they make. I asked him what it's like.

"I find the 22-tooth is better for the top end (but) you have to rev up a little more to start off. I have no power problems 'at all,' possibly since Florida is flat.

"I still have the same problem as before though: it only revs so high with the stock carb so top speed is still limited but about at 77 mph instead of 68ish. I weigh 190 lbs and I usually carry about 5 to 10 pounds in my side bags. As far as shifting down (for hills, in traffic), no! It didn't make that much of a difference. It made a difference but not a considerable amount.

"I actually wish it did take me to the point where third was more cruisable but I still pass it by and ride in fourth going 40ish and it pulls real good and maybe even better than the stock gear when riding with the big dogs; they actually can't pull away from me 'til I top out, then I am toast."

He adds:

"If they made a 25 or 26 tooth I would buy it to try out and cruise third on the steep bridges and hit forth on the down hills and flats to push 85-90."

Like many owners of original Bullets, I have increased the size of my own front sprocket.

My 1999 Bullet 500 came with a 17-tooth front sprocket and 38-tooth rear. Using the Dropbears.com Sprocket Ratio Chart, my original ratio was 2.24. The change to an 18-tooth front sprocket brought the ratio down to 2.11. That's a change of just about 6 per cent. Sounds modest.

My wife added the discarded 17-tooth sprocket to a very nice shadow box she made for me.

On the road, the improvement was phenomenal. There was no noticeable difference in the way the motorcycle accelerated from a standing stop. Presumably, my educated left fist was smart enough to release the clutch just the right amount to compensate.

What was different was that I no longer needed to shift to second half way through the intersection. I could actually hold first long enough to get across, building up some real speed. The time saved by not having to shift gave me a bit of a jump on traffic.

Second lasted longer, too, and third, previously a milquetoast gear, was now a tiger, good for all sorts of situations that previously would have had me lumping along in fourth. The 18-tooth front sprocket was such a good idea, and so popular, that Royal Enfield eventually made it standard on the 500.

The only problem in the old four-speed bike was the huge jump up to fourth, which now felt like overdrive. This is where you confront the fact that you don't really have much power. Fourth was a real show stopper for my unmodified Bullet but — oh well — it was good for cruising.

"Top Speed" on my Enfield is a concept more influenced by wind direction than gearing. But I will say that casual cruising speed seemed to bump up a bit, from 38 mph (indicated) to 41 mph.

Now let's use the Dropbears chart to try out a 22-tooth front sprocket. The ratio is now 1.73! That's a 23 per cent change from my original 17-tooth wheel and fully 18 per cent from even my upgraded 18-tooth sprocket.

It's hard to imagine. But I would love to give it a try.

10 comments:

  1. This is an excellent article David. I really enjoy the technical tips here because they're easy to understand and have practical application. The photos are helpful too.

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  2. As I am not in the bike owner fold yet, is the sprocket mode expensive and can you do it yourself?

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  3. On the old four-speed Bullet, getting to the front sprocket was a messy, time consuming job I was happy to watch a trained mechanic perform. I don't know what is involved on the five-speed or UCE bikes. But these already have more appropriate gearing, so there should be no crying need to modify them. I certainly wouldn't do it until the warranty is expired. In fact, I waited until the 17-tooth front sprocket was worn out to get mine done!

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  4. Well seeing that my quest is for a newer classic motorcycle, ie UCE model. I should not have to get me hands dirty at least for a while. Interesting point my local dealership siad if and when I buy the bike that I can spend sometime with the mechanic and they will show me how to service my own bike. I asked about the effect on the warranty and said that this would not be a problem. Mmmmm I wonder. They have not seen me what I can do with a screwdriver and a hammer. ;-o

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  5. From my experience with low powered engines ( 125cc 2 stroke ) a difference of 1 or 2 teeth in the front sprocket can be appreciated and helpful, but other small modifications should come together.
    When I'll get my UCE ( hopefully soon ) what i'd like to modify is : silencer, sprocket ( if necessary, need to try before) a different mapping ( if possible ) , tires, and the "pedestrian slicer".
    Yes tires, they look in my opinion a bit too small, and the rear tire itself if substituted with a bigger size, will have the same effect as the front sprocket and a bigger tire might make the bike even more comfortable on bad roads.

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  6. hello !
    i have a Enfield Robin Diesel !!
    and i would like to bye a bigger frontsprocket ,maby 25 T ,to this bike !
    where can i find this ??

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  7. Alan, I believe sprockets that size are available from Classic Motorworks, www.enfieldmotorcycles.com
    You can shop on line there. We'd love to hear more about your diesel motorcycle. What's it like?

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  8. Hi David,

    I wish to increase the initial power of my RE Electra 5s (iron cast) as i need more pick up speeds for a uphill race coming up soon !! Can you please give me the accurate counts of teeth's for the sprockets !!!! Much thanks !! You could email me the details at ***** mihirpadia@in.com *****

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  9. hi, im dr. sushant rahal from india.i purchased the first export model launched in india under the name of bullet machismo (export model) in 1998. i had gone through certain modifications. the engine doesnt run without battery and i have increased the battery life by changing the wiring of the bike and by replacing dynamo. moreover the previous sprocket was of size 15 or 16 teeth which i replaced with 18 teeth. i have also changed the normal 3 clutch plates to 4. results r good. the only difference it makes is on steep roads where bike even stops sometimes.
    but this problem is alleviated in new models with increased number of clutch plates and twin spark plug mechanism.
    one can easily go for size 22 in newer versions of 500cc bikes.
    you will simply just love it.

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  10. Hi David,

    I wish to increase the initial power of my BULLET STD 350 CC , 1990 modal. as i need more pick up speeds for a uphill race coming up soon !! Can you please give me the accurate counts of teeth's for the sprockets !!!! Much thanks !! You could email me the details at joyalphilip@yahoo.com. please i am waiting for you are reply thanku

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