final drive ratio

[Cobra]

Alright, I know there is soneone on this web that can help me. I am thinking of changing my primary sprocket on my Harley to lower the final drive to get more pickup.



My stock sprocket is 25 tooth, the rear wheel is 70 and the final drive in fifth gear is 3. 15



If I change my primary to a 21 tooth sprocket what would my final drive be in fifth gear?



What is the equation?





Thanks



Bill

 

[Vaughn MacKenzie]

Cobra, I think the easiest way to compute your final drive ratio is:



Original Ratio x old sprocket number divided by new sprocket number:



3. 15x25 = 78. 75

78. 75/21 = 3. 75.



Your new Final drive ration in 5th will be 3. 75 or about 20% faster engine speed. If you redline at say 120mph your new top speed will be 101mph.



I don't know much about Harleys but you might get what you are after by going with a 22 or 23 tooth drive sprocket:



22 tooth = 3. 58 Final

23 tooth = 3. 42 Final



Vaughn

 

[Steve St.Laurent]

I came up with the same number as Vaughn. Something to possibly keep in mind is that the smaller the front sprocket the more the belt will have to bend around the sprocket potentially increasing belt wear. I'm not really familiar with Harley belt drives so I don't know if that's a significant factor or not. I know on most chain driven bikes the rule of thumb is that a 1 tooth change on a front sprocket equates to about a 3 tooth change on the rear and going down on the front sprocket will definately increase chain wear. Just something to consider.

 

[Cobra]

Vaughn and Steve,



Thanks for the info. The belt should not be in question. The primary sprocket is ahead of the belt and turns a chain. The only primary sprocket that Harley offers for my bike is the 21 tooth, over the stock 25 tooth.



Bill

 

[Cobra]

Steve,

I should have gotten out my shop manual before my last post. The primary sprocket is the first one in line and is run off the engine. That turns a chain which goes rearward but still in the primary chain case to the trans sprocket. The trans sprocket has the forward part of the belt connected to it. From there it goes to the rear wheel.



Thanks for the info.



Bill

 

[Steve St.Laurent]

Here you go - http://www.nightrider.com/biketech/calc_gearratios.htm . With your shop manual you should be able to calculate anything you want from there.

 

[Cobra]

Steve,

Thanks a million for the web link and the info. Using my shop manual I did the caluations before I got the link. Then I used the web link to verify. All the caluations were the same, including the one provided by Vaughn, which was the shortest. I can't thank you guys enough for the help.



Now, my problem is a mental one. Do I spend the money and time to change the primary sprocket which lowers 5th gear to 3. 75, my existing stock 4th gear is 3. 86 which means that I would nearly cruising in 4th gear going cross country. Or do I keep what I got and just down shift to 4th when a challenge occurs.



Guess you can't help with that one.



Bill

 

[Steve St.Laurent]

Or get some real power with a 4 cyl muscle bike! #ad
Sorry, couldn't resist .



#ad

 

[Peter Campbell]

Hey no getting on someones religious beliefs. (LOL), a picture of a rice rocket to a harley owner- utter blashphemy

 

[Tim1]

DON'T

Hey bill, that's way way too much change! On my bagger (85) I changed the rear sprocket and belt and went to the international gearing to get taller gears and I down shift when needed. Believe me, if you make this change your moter will be screaming and Harley engines don't like to run that way. Just like the cummins. Just my 2 cents. Tim

 

[Cobra]

Tim, yeah I hear you. I pretty much decided to stay stock and reach for lower gears when needed. There is a longstory behind all of this which I won't bore you all with. But, I traded a 93 FLHTC for my new ride. The 93 was built to the hilt and the guys I ride with couldn't touch me. Top end or from dead stop. Now they blow me away.



Of course if I had STEVE'S bike they wouldn't even try becuase they would know better. I'm a Harley man but I love speed which equals crotchrocket big thumbs up to you Steve.



If you aren't living on the edge you are taking up too much space.



Bill