Changing sprockets on a Honda CBR250RR MC22: gearing for torque
For those of you who appreciate the fundamental aspects of changing the gearing of your motorbike through altering sprocket sizes, skip down to the heading ‘Results of the sprocket change’, to read about how we changed the rear sprocket on our CBR250RR MC22 from a 52T steel sprocket (standard size) to an alloy 55T sprocket, and our thoughts on the result.
Fundamentals of gearing
Like all things in life, deciding what gearing to go for on your bike is an exercise in compromise. While you cannot change the ratios of each individual gear in most motorbikes (without significant time and expense) you can change your overall gearing, to an extent, by changing your front and rear sprockets. By altering the sizes of your sprockets, you can influence how ‘short’ or ‘long’ your overall gearing is. Making your gearing ‘shorter’ means that you are increasing the torque multiplication that is occurring through gearing, and also the RPM which the engine will be doing at any given speed. This also has the effect of reducing the theoretical top speed of your bike, because it will hit maximum RPM at a lower speed.
One of the primary reasons for shortening your gearing is that it proportionally increases your rear wheel torque at all RPM. For example, if your engine makes 20NM of torque at the crank when doing 2,000RPM, with an overall gearing multiplication of 10x, then your effective wheel torque is 200NM at 2,000RPM. If that same motor makes 30NM of crank torque at 4,000rpm, its wheel torque will be 300NM, and so on. By installing a rear sprocket that is 5% bigger, I will have an overall gearing multiplication of 10.05x and consequently, my wheel torque at 2,000RPM is now 210NM and at 4,000RPM it is 315NM.
Another aspect of shortening gearing is that at any given speed, the RPM of the motor will be proportionally higher. If your engine originally spun at 4,000RPM at 60kph in 6th gear and you consequently increase the gearing multiplication by 5%, your engine will now do 4,200RPM at 60kph in 6th gear. This effect brings with it one primary benefit and a number of undesirable characteristics. The benefit stems from the higher RPM of the engine will mean that you will likely have more power and torque available from the engine (until either or both of these things is past their peak). However, this increase in RPM also increases engine noise at any given speed and more often than not, fuel consumption.
By contrast, making your gearing ‘longer’, typically has the inverse effects of shortening your gearing, including: reducing your overall wheel torque at all RPM; reducing your RPM at any given speed; improving fuel economy at higher constant speeds (this depends).
Another thing to note is that on bikes whose speedometers are connected to the front sprocket, changing gearing will also change the speedometer reading. Lowering gearing will tend to make your speedo overestimate (or ‘over-read’) your speed, while lengthening your gearing will make your speedo underestimate your speed. This will also affect the distance travelled, measured on your odometer, which relies on the speedo to determine distance. For instance, if you lowered your gearing by 5%, for every 100km you travel, your odometer will read out 105km travelled. This can be useful to keep in mind for those of you who rely on the odometer to determine when they should refill. It is also important to note that prior to altering gearing, the speed displayed by your speedo may already either under or overestimate the speed you are doing, although the latter is more common.
As you can see, the effects of altering your gearing are all interlinked. Whether you should alter the overall gearing by changing your sprockets will depend on what you intend to use the bike for, and your tolerance to the side effects of the changes you’re making. For instance, if you want to increase acceleration, then you may want to increase the multiplication factor through sprocket changes, but how big a change you wish to make will depend on what increase in fuel consumption you are willing to live with, alongside the increase in engine RPM (and noise) and decreased top speed.
Which sprocket should I change?
Most motorbikes have a front and a rear sprocket. Typically, people start with changing the rear sprocket, rather than the front. One benefit of altering the rear sprocket is that, as they are larger than front sprockets, you have more granularity with the size of the changes you make to gearing. For example, on a rear sprocket with 40 teeth, increasing it by one tooth will increase torque by 2.5%. By contrast, if that same bike has a 15T front sprocket, and you decrease it by 1 tooth, you increase torque by 6.66%. Thus, changing the rear sprocket allows you to alter gearing in smaller increments. A significant downside to swapping to a smaller front sprocket is also that you experience a decrease in the longevity of that sprocket, as you are putting the same force onto a smaller surface area, due to it having less teeth. However, if you want to make a significant change to gearing then it can be beneficial to change both sprockets, which will also minimise the likelihood that you will need a longer chain.
When changing the size of your sprockets, you may also need to lengthen or shorten your chain, depending on how much adjustment is available through your rear swingarm and how big a change you are making to the sprocket sizes. Going with a larger rear sprocket will mean that you may need to make your chain longer, though adding links or adjusting the rear wheel backwards through the swingarm adjustment, while a smaller front sprocket may require a shorter chain, or for you to bring the rear wheel in through the adjuster.
Results of the sprocket change
When we changed the tyres on our bike we ultimately went for Michelin Road 5’s on the front and rear. Honda specifies a rear tyre that is 140/60/R17. The closest size we could get in the Road 5 is a 140/70/R17. With the Road 5 our wheel torque decreased by 4.7%, being the difference in the tyre sizes. Accordingly, we wanted to offset the reduction in rear wheel torque by increasing the size of the rear sprocket. Let’s face it, the CBR250RR MC22 has a paltry amount of torque to begin with, so we didn’t want to lose any more torque if we could avoid it.
Since changing the tyre, we not only noticed an overall decrease in power and torque, but also found the bike to be a bit sluggish during slow riding in second in the city, where it was previously fine.
| Rear sprocket size | % change from stock size |
|---|---|
| 53T | +1.92% |
| 54T | +3.84% |
| 55T | +5.76% |
| 56T | +7.69% |
While we can do the maths to determine the impact of various sprocket options on the bike, we found a website called Gearing Commander that makes this very easy, and already has all the gearing data for a ton of models. One of your constraints when deciding how many teeth to increase or decrease your sprockets by, will be the availability of your desired sprocket to purchase. The percentage change in gearing and consequently wheel torque is represented in the below table. As you will see, the percentage increase with each additional tooth is 1.92%.
We decided that a 55T would be best, providing a net torque increase of 1.06%, compared to using a standard size rear tyre and standard rear sprocket. The next challenge was finding someone who would stock that size for the MC22. Preferably the sprocket would be alloy, to decrease rotational and unsprung weight.
Ultimately, we bought an X.A.M sprocket through WeBike. That sprocket ended up taking about three months to arrive, as it appeared to be a ‘made to order’ product. It’s hard to complain though, we hadn’t identified any alternatives that were both alloy and that amount of teeth, and the fit and finish of the sprocket is fantastic, with the price being reasonable. The XAM alloy sprocket weighed in at 397 grams, while the standard steel 52T sprocket weighed 692 grams! That means the bigger alloy sprocket was 42% lighter than the stock sprocket, that’s a whopping difference.
We are happy to report that we immediately noticed a difference when riding the bike with the new sprocket. The difference was noticeable and, dare we say it significant, although not drastic. The bike now pulls more cleanly at lower RPM in second gear, while slow riding. When sitting in 6th gear cruising at 70kph, the bike also has more urge to accelerate while remaining in 6th gear. The bike also doesn’t require as much throttle (or dropping gears) when going up inclines. For those of you wondering whether a 5.76% increase in torque is perceptible or worth it, we consider that on both counts it is.
We have kept records of the bikes fuel consumption prior to the sprocket change, and will be keeping a record post sprocket change, and will update this blog post in due course. While more aggressive gearing generally increases fuel consumption, we are curious to see whether it does in circumstances where the slight bump in torque allows you to ride this bike in higher gears than you otherwise would, and to accelerate up to the speed limit that bit quicker than before, when you have the throttle open at the same angle. We suspect that fuel economy around the city will marginally improve, while economy on the motorway will slightly decrease. In our eyes, that would be a fair compromise.