# Sizing Sprockets

## Recommended Posts

Does it matter which sprockets you change out? I mean would you be better off making adjustments to the rear vs the front or both? I have some thoughts on this matter. Some will say it doesn't, but does it?

##### Share on other sites

Does it matter which sprockets you change out? I mean would you be better off making adjustments to the rear vs the front or both? I have some thoughts on this matter. Some will say it doesn't, but does it?

Fossil;

I have been told to calculate the ratio by using the rear sprocket tooth count as the numerator and the front sprocket tooth count as the denominator. The schedule below shows the shortest to tallest ratios derived from a combination of two front sprockets and four rear. This suggests to me that the change to the front has a bigger impact but YRMV.

36/15 = 2.40

37/15 = 2.47

38/15 = 2.53

36/14 = 2.57

39/15 = 2.60

37/14 = 2.64

38/14 = 2.71

39/14 = 2.79

Mika

##### Share on other sites

If you are concerned about chain wear and efficiency, you should use the largest possible front sprocket and alter gearing/ratio with the rear sprocket. This is because it takes more energy for the chain to make a sharp turn around a tiny sprocket than a wider arc around a bigger sprocket. Also, the less the chain have to change direction, the less it will need to bend, the less it will wear and the less it will waste energy as heat. And with a larger sprocket, the force is spread over more teeth, reducing sprocket wear.

The only downside with larger sprockets is increased weight, some of it unsprung, as both chain and sprockets will be some ounces heavier.

##### Share on other sites

Yes Eirik,you are correct but is there anything else? What forces are acting on the countershaft sprocket when downshifting. Does it take more engine rpm to match up gears on the blip? The counter shaft is rotating faster, correct?

##### Share on other sites

If you keep the rear sprocket and fit a smaller front sprocket, the output axle will indeed turn faster for any given road speed. The difference between the gears will, however, be the same; let's say that 12000 rpm in first = 8000 rpm in 2nd gear and so on, they will not change. But since the speed gaps will be smaller between the gears, revs could drop further than before during braking, but hardly enough for it to matter. You will, however, change down - and up - at lower speeds than before.

Let's say that with the stock gearing your bike would do 70 mph @ 12000 rpm in 1st and, naturally, turn 8000 rpm at the same speed in 2nd gear re example above, and you go from 17 to 15 teeth on the front sprocket, your new speed would be ~62 mph at 12000 and 8000 rpm respectively. So if you used to change down at 70 mph and still do, you will now need 13500 rpm to compensate for the lower gearing. So you either wait longer before changing down or rev it higher.

The side loads on the output shaft will be less with a smaller sprocket by pulling the forces closer to the centre of the axle, but again this hardly matters in real life.

Not sure if you got any wiser

##### Share on other sites

So If I am going into a turn at 70 mph with a 17 tooth front sprocket, It will be much easier to match engine and transmission rpm's than a 15 tooth sprocket. I will also be less likely to over rev the engine on down shift?

Also, the density of nitrogen is less than air. If I filled my tires with nitrogen would I have less rotating mass?

##### Share on other sites

So If I am going into a turn at 70 mph with a 17 tooth front sprocket, It will be much easier to match engine and transmission rpm's than a 15 tooth sprocket. I will also be less likely to over rev the engine on down shift?

Also, the density of nitrogen is less than air. If I filled my tires with nitrogen would I have less rotating mass?

But there are more to it. Perhaps using the lower gearing you'll have enough power in 2nd to get out of the corner instead of having to rev its tits off in 1st to get the drive, making it simpler to match rpm. I honestly do not think it matters at all when it comes to matching rpm what overall gearing you choose as the individual gaps between gears when it comes to rpm changes will stay identical - you only vary the speed where it happens. So it's better to find the best gearing for the track you are going to ride, usually what allows you the least amount of gearshifts, than to worry about matching rpm. Also worth considering is that the lower the gearing, the better the acceleration.

Some swear to nitrogen and say it makes a difference in performance and also ride quality, others will claim it's all in their heads. As I understand it, pressure should differ less with temperature when running nitrogen, which must be considered as well.

##### Share on other sites

Just if you're not aware of it, there's a little tool that can help somewhat understand gearing changes.

http://www.gearingcommander.com

##### Share on other sites

Thanks guys. I was picking Eirik's larger than normal brain but for the most part breaking it down a little to understand some particular problems that might occur with downsizing the front sprocket. I ran a 15 tooth front for years on my old CBR which is 16 tooth stock changed between a 44, 45, and 46 tooth rear depending on the track...I was doing to badly but was missing shifts and getting false neutrals. I started looking into what the fast guys were using and found that for the faster tracks a 16/44 and 16/46 were the common sizes thay were using. I switched and now can communicate better on where the faster guys down shift, upshift and what gear for what corner. I have a reference. I also don't have the shifting problems I used to...That is basically why I brought it up.

##### Share on other sites

Also, the density of nitrogen is less than air. If I filled my tires with nitrogen would I have less rotating mass?

Some swear to nitrogen and say it makes a difference in performance and also ride quality, others will claim it's all in their heads. As I understand it, pressure should differ less with temperature when running nitrogen, which must be considered as well.

As far as I understand it, the primary reason to run with nitrogen it to get rid of the water moisture in atmospheric air, which (apparently) has some ill effect on the pressure change. Maybe Steve can help out here in the Tire forum.

Oh, and never ever replace nitrogen for oxygen in a shock in case you don't have nitrogen handy.

It had never occurred to me that the number of teeth on the front sprocket could have an effect on missed gearshifts and false neutrals. I've dropped a tooth in front and haven't seen this on my R6, even though the Yamaha gearbox isn't exactly known for its smoothness and positive changes from 1->N->2 and back.

Kai

##### Share on other sites

It had never occurred to me that the number of teeth on the front sprocket could have an effect on missed gearshifts and false neutrals. I've dropped a tooth in front and haven't seen this on my R6, even though the Yamaha gearbox isn't exactly known for its smoothness and positive changes from 1->N->2 and back.

Kai

Kai;

I have been alternating between a 14 and 15 tooth front sprocket (and corresponding rear sprocket change) and found that I was finding false neutrals on more than one occassion at the end of long straights were I needed to go from 6th to 3rd in a hurry. It was very unnerving to rail into a corner without a gear selected knowing the load I was adding to the front contact patch. It never occurred to me until tonight that it only happened when I was running the smaller front gear; coincidence - maybe and then again, maybe not.

Mika

##### Share on other sites

I don't think it's a coincidence on the missed shifts. I think there is a threshold we have for the blip, a small window for getting in the gear. If the countershaft sprocket is smaller, more rpm's are needed to match the two and that there is more load created at the initial engagement...I've had more than a couple of missed shifts and one that casued me to run off the track during a race. I have run the new gearing at Road A and Barber recently, decreased my lap times to "very quick" and had zero issues with missed shifts. Im staying with the stock countershaft sprocket.

##### Share on other sites

I don't think it's a coincidence on the missed shifts. I think there is a threshold we have for the blip, a small window for getting in the gear. If the countershaft sprocket is smaller, more rpm's are needed to match the two and that there is more load created at the initial engagement...I've had more than a couple of missed shifts and one that casued me to run off the track during a race. I have run the new gearing at Road A and Barber recently, decreased my lap times to "very quick" and had zero issues with missed shifts. Im staying with the stock countershaft sprocket.

I have to ask; do you think it is a direct result of the smaller front sprocket, or because of the overall gearing (that results from a smaller front sprocket)?

##### Share on other sites

Eirik,

I think it is a direct result of the change in countershaft sprocket.

##### Share on other sites

I would be very surprised if that's the case, although I cannot rule it out. When you changed from 15 to 16 teeth up front, did you add 3 at the back also to maintain the same overall gearing as before? Personally, I would think overall gearing is the cause and not the size of the sprocket.

##### Share on other sites

I would be very surprised if that's the case, although I cannot rule it out. When you changed from 15 to 16 teeth up front, did you add 3 at the back also to maintain the same overall gearing as before? Personally, I would think overall gearing is the cause and not the size of the sprocket.

Eirik,

You are probably.....most likely correct. Then it might be safe to assume that there is a threshold for gearing that should not be crossed with stock transmission gearing ratios. Regardless of front sprocket size do not go over a certain ratio?

##### Share on other sites

Most of us are stuck with the stock internal ratios, but race teams and dedicated amateurs will also change individual gearing to get as close to perfect as possible for any given track. But let's work with the stock ratios and look what they do

Typically, a modern race rep have a ratio of 1.8-2.0, depending on model and size. That means that 1st gear gives you 1.8 to 2 times as much rpm as 6th gear for any given speed. The ratio is completely unaffected by final gearing, the only thing available for us to experiment with.

For simplicity's sake, let's go with round numbers and a ratio of 2.0 with 75 mph available in 1st gear and 150 in sixt at its 15000 rpm redline with an even 15 mph gap between each gear. Rpm in the next higher will be, when shifted at redline; 2nd: 12500, 3rd: 12857, 4th: 13125, 5th: 13333, 6th: 13500 rpm. In real life,gearboxes tend to be a little more spread out in the lower gears and a little closer between the higher gears, but for out example this works well enough. As you can see, rpm will drop less the higher the gears. It also works the other way; when changing down you need to add more rpm for each gear you go lower. Note that the changes in rpm would again be identical regardless of final drive ratio.

When choosing your final gearing, you must do this so that you avoid having to change gear midcorner as much as possible. If, for instance, you reach a maximum of 110 mph on a straight just before you reach a corner with this gearing, you either have to overrev your engine in 3rd or grab 4th for a split second. If this is the only corner not having a good match around a track, you're fine. But if you have several places were one gear is too tall and the other is too low, you may want to look into a change of final drive gearing. If the track has a very low speed corner that is too slow even for first gear, you may want to lower your gearing by 8-9 % so that you just about redline 4th at 110 mph instead of at 120. This will lower all gears an equal amount, also first gear. This will also give you 8-9 % more torque in each and every gear for any given speed.

OTOH, if you have a corner where 1st is just a tad too low and 2nd a bit too high, you may want to raise your gearing about 5 % so that you can reach 110 in 3rd and make 1st more useful where it used to be just too low.

In the end, you'll end up with a compromise. Unless you are very, very skilled, you are better off running gearing a bit tall, which means revs lower for any given gear and speed. Maybe plan your shifting points at 12500 rpm, leaving you with 2500 rpm left for overrev in case you have an odd corner or two that doesn't quite fit or where you don't want to have to shift. Less rpm makes it easier to shift gears and also makes the bike less nervous. In addition, less rpm makes it easier to change direction due to the lesser gyroscopic effect from the crank.

Mere mortals would do good trying to minimize the number of gearchanges required around a race track. Use a gearing that keeps the engine above its midrange torque dip and below the rev limited in as few gears as possible. Not having to match rpm or being afraid of the high rpm hit that comes with a low gear when feeding on the throttle frees up brain capacity that can better be used to find the best line and braking point. But if you are starting to win races, you need to look for fractions, and having as close to the perfect rpm as possible at every acceleration point becomes very important.

One more thing to consider; when the engine operates on falling torque, that is at revs above peak torque, it's easier to spin the rear tyre without so much risk of spinning out and getting highsided. Good to know when you start to go really fast

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.