Effects on Chain Geometry: Decide on whether to have a smaller front sprocket or to have a bigger rear sprocket

When changing the final ratio, you can go 3 ways:

1) change all 3 parts: front and rear-sprocket and chain (always best !)
2) change rear sprocket and if necessary the chain (cheap)
3) change only front sprocket (cheapest)

It is a lot cheaper to buy a smaller front sprocket and keep your current chain and rear sprocket than it is to buy a bigger rear sprocket and possibly have to buy a longer chain. That is why people are always wonder how much the front must be smaller to have the same effect as making the rear 2 or 3 teeth bigger.

Of course there is more to it then just money. A smaller front sprocket will put more strain on the chain (and thus wear it sooner) and might even cause the chain to scrape the swing arm and damage it !
But a temporary solution might be to buy a smaller front sprocket, use it till the current chain wears out and then replace all by a bigger (normal) front sprocket and a bigger rear sprocket and a new matching chain.

Besides having impact on chain wear, changing the sprockets also has an impact on the chain length needed or in other words, on the sprocket center distance. By entering both options (front-smaller and rear-bigger) you can see the effect they have on your chain geometry and depending on your current options for still moving your rear wheel forwards or backwards, decide on one of both.

Let's assume you had a stock setup of 15-36, then changed it to currently 14-36 but now chain and sprockets are worn out and you want to replace all. Since you like the current final ratio but would like to return to a stock front sprocket, you need to choose need determine the size of the rear sprocket. So the question is what size rear sprocket will give you the same final ratio you have now (in Current setup) when using a (larger stock) 15 teeth front sprocket?

To find out, you will have to experiment by trying different values. Fill in all the know sizes for Stock and Current. Then in Custom setup fill in the new 15 front sprocket. Now you have to choose bigger rear, lets start with 39:

As a result  you can see that difference between Custom and Current is +1.1 % (more RPM at same speed). So this is not exactly the same as you have currently. Maybe a smaller rear sprocket will get the same result as the Current setup. Now enter a 38 rear sprocket and check the differences:

Now the difference between Custom and Current is - 1.49 % (less RPM at same speed). This is even a bit more (and in the other direction) off as compared to the current setup. Now it's up to you which version you prefer, the setup with 38 teeth (- 1.49% less RMP at same speed) or the one with 39 teeth (1.1% more RPM at same speed). So there seems to be no exact match for the 14-36 ratio but both 15-38 and 15-39 come close.

As your Currernt setup shows you wanted more RPM (7.14%  more torque and less top-speed), in this case I would suggest to go to 15/39 wich has the same effect but slightly more ( 8.33% more torque and less top-speed)

You can also use the "Final Drive Ratio Graph" which shows the custom final drive ratio in a graph and 48 other ratios based on it. There you can easily see which other sprocket combination comes close to the one you are looking for.

Next: Chain & sprocket wear influences

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