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TOPIC: Understanding Sprocket Sizes and Power Delivery

Understanding Sprocket Sizes and Power Delivery 2 years 1 month ago #104280

  • SafferCA
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I have read a few comments and threads this week regarding sprocket sizes. You're almost left with the notion that it is some kind of voodoo. Of course, it isn't and takes a little understanding to grasp what is happening.

To give you a simple illustration. When I'm not on my KLR I'm riding my other bike. It is a fixie, meaning it only has one gear. I don't have the luxury of multiple gears.

When you set it up you either opt for traffic light pull away which means lower top speed, or poor pull away and higher top speed; or you try for something in the middle.

I have a ratio of 2.93, which means for every one rotation of the pedals the rear wheel rotates 2.93 times. Assuming I can only pedal at a certain maximum, in order to have a higher top end I would want to increase the ratio, but that would mean struggling to get going at the traffic lights, and vice versa.

A similar process applies with your motorcycle except in the reverse because the sprockets are the other way around (large at the rear and small at the front), and the maths is different.

Granted, you have a gearbox, but this is fixed, as is the available power. In order to simulate the effects of different gear ratios and/or more power which translates into more pull away torque or more top end speed, the easiest and most commonly adjusted item is the ratio of front to rear sprocket.

The standard sprocket sizes (and this may vary with your bike) on the KLR650 is 15T front and 43T rear. This gives you a magic ratio of 2.87, which means for every single rotation of the rear wheel the front sprocket (countershaft) rotates 2.87 times, which, by the way, is why the front sprocket wears out quicker than the rear. Now refer to the table below.

What other combination would give us a similar ratio to 2.87? You could use a 14T/40T, 16T/46T, 13T/37T, and so on. But all of these combinations will feel the same except you would have to replace and/or adjust the chain accordingly. What would be the point? This is just to illustrate that a magical combination without understanding the inter-connectedness will deliver nothing.

Let's say you want to increase your torque or pull away speed. This means you want to reach the power band sooner, in other words you want to increase the engine revolutions for a given speed. This means increasing the front sprocket rotation relative to the rear sprocket. It seems counter-intuitive but it is a torque multiplier. For a single rear wheel rotation the front sprocket now rotates faster. This means you want to INCREASE the ratio. To achieve this you could adjust either the front or rear sprocket, or both. The table will give you an idea as to how much/little the ratio will change for various combinations.

Now, let's say you want to improve your highway speed or top end. This means you want the rear wheel to rotate a lot faster relative to the front sprocket rotation, in other words, you want more cruising speed for a given level of engine revs. This means you want to DECREASE the ratio. Again, the table will give you an idea as to how much/little the ratio will change for various combinations.

Now here's where the voodoo comes in. Sprocket combinations are personal choices and it all depends on what you are trying to achieve. Subtle changes usually involve adjusting the rear sprocket and vice versa. Bear in mind that the chain may need to be replaced/adjusted as well.

Finally, you are not creating new power, despite it feeling like that; that is a myth and a dyno test would prove it. It's just an illusion and it comes with a trade-off, namely an increase in the one (pull away speed) will always mean a reduction in the other (top end speed), everything else being equal.

Hope this helps some of you. :)
Last Edit: 2 years 1 month ago by SafferCA.
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Understanding Sprocket Sizes and Power Delivery 2 weeks 3 days ago #117803

  • JPMV
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Excellent Article!
Thanks a lot!
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Re:Understanding Sprocket Sizes and Power Delivery 2 weeks 3 days ago #117805

  • Psycho Porra
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Thanks Saffer. This will help alot of KLR riders.

I did hear somewhere that changing the standard 15 front to a 16 is the equivalent of changing the standard 43 back 2 to 3 sizes smaller. Your table just confirms that??
"The Brave do not Live Forever, but the Cautious do not Live at all"
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Understanding Sprocket Sizes and Power Delivery 2 weeks 2 days ago #117809

  • Dieseldawie
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What impresses me is that a newbie actually used the search function rather than asked a silly question. Well done JPMV...
I like to drink lots of beer??
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