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 Sprocket diameter size
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rayg50
Male Moderator
2082 Posts
[Mentor]


NYC, NY
USA

Honda

Shadow Spirit 750DC

Posted - 05/26/2013 :  8:39 AM                       Like
One of the mods that comes up periodically in a VT750DC forum is the lowering of the number of teeth on the rear sprocket. While I have read a bit on it to try to understand the effect I'm still not quite getting the how/why it works.

My understanding is that the lowering of the number of teeth (on the rear sprocket) will lower the speed at which you accelerate but will allow you to maintain the higher speeds at a lower rpm.

I've read a bit about gear ratio's and it makes my head hurt. The math is easy but it does not help my understanding of the why. The only way it makes sense to me is if fewer teeth means a reduction in the diameter of the sprocket. Maybe I've looked in the wrong place but I cannot find any reference as to the size (diameter) of the sprockets.

Does my thinking, that smaller tooth count corresponds to a smaller diameter on the sprocket, hold water?

greywolf
Male Moderator
1484 Posts
[Mentor]


Evanston, IL
USA

Suzuki

DL650AL2

Posted - 05/26/2013 :  10:05 AM
That's how it works. Changing the number of teeth changes the sprocket diameter and the final drive ratio. What is referred to as lower gearing has a higher numerical ratio and vice versa if you're looking for another reason to scratch your head.

A smaller front sprocket or larger rear sprocket, i.e. lower gearing, raises the rpms required to go at a chosen speed and gets into a higher torque and horsepower section of the engine's power band as the mechanical advantage of the rear drive increases. It also may increase fuel consumption, vibration, noise and engine wear plus limit the top speed.

A larger front sprocket or smaller rear sprocket, i.e. higher gearing, lowers the rpms required to go at a chosen speed and reverses the above changes.

Note the changes "may" be noticed. Changing the power band and shift points by a gearing change can do unexpected things. For example, a long distance tourer may try higher gearing for better fuel economy by lowering the engine rpms but find no improvement. The engine will probably have lower power and torque numbers at that lower rpm and require a larger throttle opening to compensate.

Chain drive bikes make sprocket changes cheaply and easily enough to encourage experimentation. I tried a larger front sprocket once and decided after one long trip that I didn't like it. The power reduction required using a lower gear in many situations than I was used to.

http://www.gearingcommander.com/ lets you plug in different gear sizes to see what rpm changes occur.

Edited by - greywolf on 05/26/2013 10:11 AM
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Alabusa
Male Senior Member
254 Posts


Muscle Shoals, Alabama
USA

Suzuki

Extreme

Posted - 05/26/2013 :  10:17 AM
Yes, it holds water.


CBR
Stock size ear

CBR 60 tooth

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rayg50
Male Moderator
2082 Posts
[Mentor]


NYC, NY
USA

Honda

Shadow Spirit 750DC

Posted - 05/26/2013 :  1:09 PM
quote:
... lets you plug in different gear sizes to see what rpm changes occur.
I've been to that sight before (I follow most links posted here) but after having done a little reading the sight makes more sense to me. Just to confirm that I am reading it right, when I plug in my bike it lists a final drive ratio of 17 / 42 which I take to mean 17 front teeth and 42 rear teeth.

When I replace 42 with 38 I see an across the board increase in mph. Am I safe in assuming that the acceleration drop is also across the board and not just at the lower speeds (gears)?

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greywolf
Male Moderator
1484 Posts
[Mentor]


Evanston, IL
USA

Suzuki

DL650AL2

Posted - 05/26/2013 :  2:44 PM
That's correct. A change in the rear drive affects all gears.
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Daddio
Male Advanced Member
775 Posts
[Mentor]


Calera, AL
USA

Suzuki

Bandit 1250

Posted - 05/26/2013 :  5:08 PM
I was planning on using a smaller rear sprocket to correct for my speedometer reading higher than my actual speed. Physically, the bike will be going faster, but the speedommeter will not know it. Currently, I believe I am nearly ten percent slower than my speedometer reads.

I understand greywolf's observation that I may not be getting better gas mileage or hurting it. I wonder if MPG was corrected to reflect the difference in actual miles read?
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greywolf
Male Moderator
1484 Posts
[Mentor]


Evanston, IL
USA

Suzuki

DL650AL2

Posted - 05/26/2013 :  6:32 PM
quote:
Originally posted by Daddio

I understand greywolf's observation that I may not be getting better gas mileage or hurting it. I wonder if MPG was corrected to reflect the difference in actual miles read?

The bikes used in my personal case and involving other inputs I studied were 2004-2011 Suzuki DL650s. The speedometer/odometer pickup is on the front wheel and is unaffected by a gearing change. My observation and those of other indicated those bikes do not get better fuel economy when changing the stock 15T front sprocket to a 16T or 17T. It stays about the same.

Bikes like the Bandit and my 2012 Dl650 that have the pickup on the front sprocket shaft will change the speedometer and odometer readings with a gearing change. 12oclocklabs makes a speedoDRD and Healtech makes a speedo healer. Either will plug into the bikes speedo wire connector and allow dialing in the correction factor needed to fix the speedo readout.

Be aware, however, the speedo and odo will both change by the amount selected by the device. My speedo read 7.2% fast but the odo was only 2.7% high. I corrected the speedo but now my odo is reading low. If I ever sell the bike, I'll need to declare a corrected mileage figure. A gearing change on a bike with a rear speedo pickup warrants the same action.
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Daddio
Male Advanced Member
775 Posts
[Mentor]


Calera, AL
USA

Suzuki

Bandit 1250

Posted - 05/27/2013 :  8:06 AM
quote:
from greywolf:
If I ever sell the bike, I'll need to declare a corrected mileage figure. A gearing change on a bike with a rear speedo pickup warrants the same action.

We accept inaccurate speedometers and odometers from the manufacturer's. When I asked my dealer about the inaccuracy, he explained that a reading within ten percent was industry standard and the warranty claim for correction would be denied. I also tried a different dealer and got the same answer.

Perhaps I am rationalizing my feeling on this, but as long as you have not disconnected or rolled back your odometer, I feel that in good faith that this would not be something that you would need to report on the official DMV documentation. I would disclose the situation verbally to the buyer of the bike and let them decide if they still wanted to purchase it.

Back on topic - While the tooth count is a valid way to determine the drive ratio, it is the actual diameters/circumference of the gears that is the deciding factor. You could theoretically replace your chain and sprockets with pulleys and a v-belt of the same diameters and get the same results. The teeth are there just to provide a positive, no slip connection.

edit - I also would have preferred to use a larger front sprocket but I was told that is not recommended because the casing may not allow the proper clearance for the larger diameter sprocket.

Edited by - Daddio on 05/27/2013 8:12 AM
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greywolf
Male Moderator
1484 Posts
[Mentor]


Evanston, IL
USA

Suzuki

DL650AL2

Posted - 05/27/2013 :  9:25 AM
A larger front sprocket is preferred over a smaller rear if there is room. The larger diameter is easier on the chain. I'm sure some other owners have looked into it. Check web sites for your model. One and maybe two teeth over can usually be accommodated. The tooth count gives an exact final drive ratio. Both chain and belt final drives have teeth.

The way I read the law, if an owner does something to cause the odometer to read low, that owner needs to correct the mileage before selling.
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scottrnelson
Advanced Member
6883 Posts
[Mentor]


Pleasanton, CA
USA

KTM

990 Adv, XR650L

Posted - 05/28/2013 :  9:19 AM
You guys need to spend more time riding bicycles in order to understand how sprocket size affects speed at a given RPM. My mountain bike has three different front sprockets and seven rear, and since I ride it quite a bit, I have a pretty good feel for which gears make it easier for me to go uphill or start out from a stop and which ones are best if I'm trying to go as fast as I can down a hill.

It works exactly the same way on a motorcycle. Lower gearing makes it easier to pull away from a stop, and I've lowered the gearing on all four of the Ducatis that I've owned, because they were all difficult to ride smoothly at slow speeds. Higher gearing helps you get better fuel economy and has the engine running slower at highway speeds.

As for how much change you'll get, it's a simple ratio. If I change my front sprocket from 15 teeth to 14 teeth, the change is 0.93333 or about 7%. It's 7% easier to pull away from a stop or the engine runs 7% faster at a given speed. Such a change feels like more than 7% on something like a Ducati Monster for how much easier it is to pull away from a stop.

I've also gone from a 42 to a 45 on the rear which just happens to be exactly the same change in ratio as the 15/14 change. The larger rear keeps the chain further away from the swingarm than the smaller front, which is why I usually prefer that.

On my Honda XR650L, I have 45 and 48-tooth rear sprockets and 13, 14, and 15 front. 15/45 is great on the street, but just too high off road. On tight stuff I probably want 14/48, which is a change of 0.875 or 12.5%. It makes it much easier to ride the bike through slow tight stuff without bogging down the engine, and makes it possible to use second gear in some places where I would have to use first with stock gearing.

A lot of people on the various motorcycle forums seem to have a real problem understanding gearing changes, but it's so simple. To figure the ratio change, just divide the old number of teeth by the new number of teeth. Switch the order if it didn't produce the number you like - for example, 0.9333 vs. 1.0704 to see that you have about a 7% change.

A lot of people also have a hard time understanding whether their chain will still work, but the amount of movement of the rear axle for a one-tooth change is almost exactly 1/4 of the distance between two pins on the chain. If you have that much adjustment room, you can use the same chain.
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rayg50
Male Moderator
2082 Posts
[Mentor]


NYC, NY
USA

Honda

Shadow Spirit 750DC

Posted - 05/28/2013 :  11:46 AM
quote:
A lot of people also have a hard time understanding whether their chain will still work, but the amount of movement of the rear axle for a one-tooth change is almost exactly 1/4 of the distance between two pins on the chain. If you have that much adjustment room, you can use the same chain.
You anticipated my follow up question, since I am equally unfamiliar with the chain / sprocket match up.

If a drop or increase in the number of pins results in the need for a change in the chain wouldn't that also cause a need to change the front sprocket?

BTW, I have a 3 speed 70's "racer" and a 21 speed mountain bike. Guess which one I ride 75% of the time. But then again I don't have any mountains in NYC, just a grandson who likes to ride in the woods.

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scottrnelson
Advanced Member
6883 Posts
[Mentor]


Pleasanton, CA
USA

KTM

990 Adv, XR650L

Posted - 05/28/2013 :  12:21 PM
quote:
Originally posted by rayg50

You anticipated my follow up question, since I am equally unfamiliar with the chain / sprocket match up.

If a drop or increase in the number of pins results in the need for a change in the chain wouldn't that also cause a need to change the front sprocket?
If you want maximum chain and sprocket life, you're supposed to change both sprockets and the chain at the same time. A new sprocket is not likely to increase wear on an old chain, but an old sprocket can definitely wear out a new chain more quickly.

In the case of my XR650L, I'm changing sprockets quite often and it doesn't bother me if I cut the life of the chain in half. But it's not quite that bad - I expect to lose a few thousand miles of potential chain life and that's okay with me.

I've had a 14/45 sprocket combination with my current chain, which puts it near the back of the adjuster range. I've also had a 15/48 sprocket combination (four more teeth) with the same chain, which puts it near the front of the adjustment range. The last time I was riding on the trails up at Georgetown, I might have been able to use 13/45, but there is a chance the chain would have been a bit loose still after adjusting it all the way to the back. I didn't think of that option when I was struggling with the bike on difficult trails with the street gearing of 15/45, but I had everything that I needed with me to have made the switch. And it definitely would have helped.

When I had the Ducati ST2, the standard sprocket size was 15/42. Due to the primary gears inside the engine, that was almost the same as 15/39 on the Monster. I knew that the Monster was much easier to live with when switched to 15/41 or 14/39, but thought that since the bike was a Sport TOURING motorcycle, higher gearing would be better. Only after riding it about 1000 miles did I realize that not only did the gearing feel too high at low speed, but 6th gear felt too high at highway speeds too. So I bought a 14-tooth front and was much happier with the change.

In a few thousand miles when it was time for a new chain, I went with the stock 15-tooth front but went up two teeth to 44 on the rear, which felt perfect to me for that bike. With the previous 14/42 the rear was an exact multiple of the front which produced some noticeable harmonics when riding. I would have had those same harmonics with 15/45, but not with 15/44.

I find it interesting all of the little things that make a difference overall in how a bike feels to the rider. And we should all learn to make the easy adjustment to make them work best for us.
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greywolf
Male Moderator
1484 Posts
[Mentor]


Evanston, IL
USA

Suzuki

DL650AL2

Posted - 05/28/2013 :  1:02 PM
In the more exotic area of theory, many prefer odd numbers of teeth. The idea is that varies which teeth contact which chain links. That spreads any wear tendencies over the entire chain. The worst scenario is even teeth numbers and the rear evenly divisible by the front. Each tooth meets the chain in exactly the same places each time around. A 14/42 would be a bad combination for example. Unless a chain has a so-called half link with bent side plates, chains always have even numbers of links.
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scottrnelson
Advanced Member
6883 Posts
[Mentor]


Pleasanton, CA
USA

KTM

990 Adv, XR650L

Posted - 05/28/2013 :  3:29 PM
quote:
Originally posted by greywolf

In the more exotic area of theory, many prefer odd numbers of teeth. The idea is that varies which teeth contact which chain links. That spreads any wear tendencies over the entire chain. The worst scenario is even teeth numbers and the rear evenly divisible by the front. Each tooth meets the chain in exactly the same places each time around.
Although the sprocket on the bottom had other problems, can you tell if it has an even or odd number of teeth?
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greywolf
Male Moderator
1484 Posts
[Mentor]


Evanston, IL
USA

Suzuki

DL650AL2

Posted - 05/28/2013 :  3:36 PM
quote:
Originally posted by scottrnelson

Although the sprocket on the bottom had other problems, can you tell if it has an even or odd number of teeth?


Even. The misalignment that caused side wear shows alternating curved wear on the narrow links and pointed wear on the wide links. An odd number of teeth would not show points.
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rayg50
Male Moderator
2082 Posts
[Mentor]


NYC, NY
USA

Honda

Shadow Spirit 750DC

Posted - 05/28/2013 :  10:00 PM
Very informative thread (with a few I should of known that) my thanks to all the contributors.

I've seen some chain sprocket kits but I gather they are intended to replace the OEM parts. How would you match up the rear, front sprockets and chain if you went with something other than OEM specs? For example if I went from a 42 rear to a 38.

From my bike specific site threads it would seem I could use the same chain and front sprocket but I am wondering how one would know.

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greywolf
Male Moderator
1484 Posts
[Mentor]


Evanston, IL
USA

Suzuki

DL650AL2

Posted - 05/28/2013 :  11:04 PM
Typically, replacement chains come with more links than needed and are cut to the length you need with a chain tool that anyone replacing a chain really ought to have.
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rkfire
Advanced Member
1688 Posts


Stratford, CT
USA

Suzuki

Bandit

Peer Review: Blocked

Posted - 05/29/2013 :  9:15 AM   Join poster on Facebook as Friend  
quote:
Originally posted by rayg50

Very informative thread (with a few I should of known that) my thanks to all the contributors.

I've seen some chain sprocket kits but I gather they are intended to replace the OEM parts. How would you match up the rear, front sprockets and chain if you went with something other than OEM specs? For example if I went from a 42 rear to a 38.

From my bike specific site threads it would seem I could use the same chain and front sprocket but I am wondering how one would know.





The replacement kits are stock size sprockets and chain.

You want to know what's stock sizes if you want to change gearing. You know that it has a 17 tooth front, and 42 tooth rear sprocket. Now you might want to know how many links a stock chain is for reference.

In your case, if you were to go from 42 to 38 at the rear, the smaller sprocket AND a stock chain length means you'll have a sufficent length of chain. Chains can be shortened if needed.

Unless you do the majority of riding on Interstates, and find the rpm's annoyingly high, I'd leave the gearing alone, or even go slightly larger rear sprocket IF I changed anything. Otherwise, it's a proven Honda engine, that should be reliable and long lasting as is. You're not prematurely wearing it out.

Your bike is about 550 lbs, with about 36 HP. My hunch is the last thing it needs is taller gearing. Just my 2 cents.
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Magnawing
Male Senior Member
281 Posts


The Woodlands, TX
USA

Honda

VF750C

Posted - 05/29/2013 :  10:38 AM Follow poster on Twitter
Not to argue but if you want to lower RPM, you need a smaller sprocket (fewer teeth) on the rear or a larger sprocket (more teeth) on the front. In my experience, changing the front sprocket is usually easier but, in reality, if you're changing one you should replace the other anyway to prevent uneven wear.
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scottrnelson
Advanced Member
6883 Posts
[Mentor]


Pleasanton, CA
USA

KTM

990 Adv, XR650L

Posted - 05/29/2013 :  11:24 AM
If you feel that the gearing is right for your bike, it's best to stick with the same gearing. My KTM seems fine, at least on the street, so I feel no need to change that. The XR650L is also decent on the street, but since it gets ridden off road, I have alternate lower gearing that makes it easier to ride there. All of the Ducatis that I've owned were easier to live with when the gearing was lowered.

I don't believe it is a good idea to change gearing if you don't know why you're changing the gearing. Change to solve some issue that you have to hopefully make the bike better.

It also should be noted that there are three different chain sizes: 520, 525, and 530. The larger numbers are wider with the '5' meaning 5/8" between pins. One popular change for sportbikes is to go from a 530 or 525 chain width to a 520, to supposedly save weight or something. They also occasionally end up with broken chains or sprockets on 150 hp bikes. Think very seriously before going to a narrower chain.

I went from a 525 to a 520 on my Ducati ST2, because I knew that the Monster and early Superbikes came stock with a 520, and the engine wasn't significantly more powerful than either of those. But if I still had that bike the next chain would be a 525.
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gymnast
Moderator
4260 Posts
[Mentor]


Meridian, Idaho
USA

Harley-Davidson

Sportster Sport

Posted - 05/29/2013 :  4:41 PM
"I don't believe it is a good idea to change gearing if you don't know why you're changing the gearing. Change to solve some issue that you have to hopefully make the bike better."

Right you are Scott. When I added a sidecar to my 78 Sportster I went down (correction, not up, thanks Pat) 3 teeth on the countershaft sprocket to pull the heavier load. This also resulted in lower top speed and slightly lower cruising speeds (to minimize engine wear and vibration).

For race bikes we had stacks of both final drive as well as countershaft sprockets in order to get the best set up (both acceleration and top speed, always a compromise)for different tracks and track conditions.

On my 97 Sportster (used as a street bike only), which has a belt drive, I have "left well enough alone" because the factory ratios seem to work just fine. If I need a bit lower gear ratio, I just downshift.

It is interesting to note however that the rear sprockets on the European delivery Sportsters have slightly smaller rear sprockets though the transmission ratios are the same as the US models. Perhaps this suits higher cruising speeds on the autobahns and such.

Edited by - gymnast on 05/29/2013 7:42 PM
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