Sprocket installations of small bikes

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Aside from changing your sprockets on a whim, just to change them,
or changing them by feel, to make your bike feel more or less tame when accelerating,
You can also change sprockets with a specific purpose in mind.
Pursuing that purpose with math, rather than by experience, can trade you precious Dollars to just a bit of your time researching, and going over the numbers.
Math can tell you a lot about what you need to know about riding with different sprockets, without having to cash up the cost of all these sprockets and installation time.
[SIZE=”4″][U]In the first example, we’re going to change the sprockets, with the purpose to tune the bike for top speed:[/U][/SIZE]

The procedure is very simple; and mainly meant for a 125 to 350cc motorcycle; since bigger bikes usually have different speeds to ride them at.

All you need is the bike, access to buying the correct sprockets (eg:online), the right HP/Torque graph, and access to GearingCommander.com .

It works for bikes with chain drive.
Belt, or shaft driven motorcycles don’t work like this.

First you’ll have to download a HP/Torque graph from the internet of your bike.
This is an example of a Honda Rebel 250’s HP and torque and HP curve:


What you see in the curve is pretty common for most sub 500cc engines.
They have a HP band, an area in which the bike performs at it’s peak performance.
In this case, the Honda Rebel has it’s powerband from 6600 to 8750 RPM.
As you can see from the graph, running the engine in RPMs lower or higher than this band, we will lower the HP output at those RPMs.

Since we’re focusing on top speed, we would like to gain this top speed, within the powerband of the bike, rather than at the redline of the bike, where it makes less power.
So, next we do, is ride the stock bike on the interstate, as fast as we can.
Sometimes it takes a good, full 2 minutes before top speed can be gotten, as the engine and engine oil needs to warm up.
A totally cold engine vs a totally hot engine could differ 20MPH in top speed easily!

Once you’re riding at top speed, you record the speed, and if possible also the RPM you’re getting.
In this case it would be 83MPH at ~9k RPM.

We can verify RPM if we don’t have a tach, with gearing commander:

Next, we go back to our HP curve, and notice that when we would gear our bike to do 83MPH at 7500RPM instead of at 9k RPM, that we can gain 1HP from the engine, and 2 LB ft of torque; power gained due to less friction losses, and less pressure losses on the oil pump.

We use gearing commander to get the right sprockets to match the top speed with the RPM we desire.
We also check our bike, to see if the sprockets would mechanically fit the bike, and order them.
Our rebel seems to host counter sprockets from 12t to 15t, with 14t being stock.
It also has 33T rear stock, and we can fluctuate from 25t all the way to about 50t I believe (when the chain guard is removed).

We fill out Gearing commander, 15/29t, and find that at 7500RPM the bike would be doing 84MPH.

We know that at 7500RPM the engine has more HP, and more Torque than at 9k rpm, which means it should go faster than 84MPH.
So we try a 28t instead, and test it out on the road, and find that the bike actually goes 87MPH top speed.
That would be 4MPH top speed gained, by the correct sprockets.

We now want to calculate our MPG gains.
If with our stock riding, we got 70MPG, mainly a mix of 3/4th city and suburbs, and 1/4th highway; our new MPG should come close to:
70MPG * 15/14 * 33/28 = 88MPG
The formula is derived from:
MPGstock * New front sprocket / stock front sprocket * stock rear sprocket / new rear sprocket)

We have gained an average of 18MPG compared to stock!!

You’ll notice the more you ride at low speeds, in final gear, the higher this actual number becomes; and the faster you ride, the lower the MPG difference becomes.

When this number becomes lower than a previous gearing, we speak of lugging. A lugging engine, is an engine that is taxed beyond it’s capabilities.
Mostly either at very LOW RPMs, or at top speeds; or, in the HP band but going up a hill and the bike is losing speed; can the engine start lugging, and might it be necessary to go into a lower gearing that can carry the load consistently.
We could go back to gearing commander, and try other sprocket combinations.
Suppose that a 27t or a 29t would have slower speed than a 28t; it would mean that the 15/28t is our optimal sprocket for top speed.

[SIZE=”4″][U]We now want to combine high top speed, with great MPG.[/U][/SIZE]
We can do this, by creating an extra overdrive; and by making our second to last gear, the same ratio as our last gear.
In the example above, our Honda Rebel’s 4th gear would need to have the same speed per RPM as our modified 5th gear.

To start working on this, we go to Gearingcommander again.
We try to get the same speed results in 4th gear at 7500RPM as we had before.
It turns out that we’d need a 15/24t or a 16/25t to do so.
This is mechanically impossible to fit on the Rebel.
But should it have been possible, then we’d be able to run top speed in 4th gear, while maintaining great MPGs in 5th gear from 35MPH (2500RPM) to 60MPH (4400RPM).
In our above example, we could not fit the sprocket on the Honda Rebel.
So we can not gear it for top speed in 4th and great MPG in 5th.
Aside from just equipping it with a 15/25t, which is the maximum gearing the Rebel allows,
[SIZE=”4″][U]we can use a 3rd method to calculate, or aim for a good low speed sprocket.[/U][/SIZE]

When we’re riding mostly in suburban roads, where the speed limit is 30-40MPH, our speed would be between 35-45MPH (since almost no one on a bike actually keeps the speed limit).
We will want to make sure that the engine will not be in a too low RPM range.
We ride with our current sprockets, in final gear, and slow down and accelerate, oscillating acceleration and deceleration at a constantly lower and lower RPM.
At a certain point in the RPM range, we find that the engine is no longer pulling the load very smoothly; say, 2500 RPM.
We now know we can’t go below 2.5k RPM in stock gearing; so the engine won’t start making odd noises.
We add 500RPM as a safety barrier, and change our gearing to suit our most optimal low RPM (3000RPM) at our most ridden speed (40MPH).

We use gearing commander again and notice that a 15/25t is getting pretty close to the gearing we desire!
It gives us 39 MPH at 3000 RPM.


We order the 25t sprocket, install it, and test ride it;
In this case, starts from a dead stop are a bit harder, but not impossible.
At 1400RPM, in 1st gear with the 15/25t sprocket setup, the speed is 5.9MPH.
At 1400RPM, in 2nd gear stock (14/33t), the speed is 6.6 MPH.
This means that our first gear is shorter than a stock 2nd gear, and if we can start the bike in second gear stock, we can much easier start it in 1st gear modified.

There’s really a lot, lot, and lot more that comes to play in selecting the right sprocket. HP and Torque needs to be looked at, as well as wind resistance, to see if the bike has enough acceleration for regular traffic.


Finding fuel efficient beginner motorcycles

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For future bike owners that are looking at fuel sippers,
On number 1 is Honda CBR250, with fuel injection gets between 80-100mpg.
If you’re not into sport bikes, know that less efficient body frames eat mpg.
I would say bike number 2 would be a Honda Rebel 250.
It gets between 66 and 80mpg (us, not imp) stock , and it can be raised to 100mpg with a sprocket swap.
Stock a Rebel has a 14/33t sprocket setup, hopelessly undergeared, and only good for either a mountain climber, or a hooligan wanting to look old school?
Pay $75 to buy a 15t sprocket front, and anywhere between a 30 to a 26t rear.
I tried all combinations, and found:
15/30t on a rebel is pretty neutral, boring gearing
15/28t for fastest acceleration (allows you to shift from the top of the powerband to the bottom of it in next gear, basically allowing you to constantly accelerate at the powerband), and highest top speed sitting upright of 80mph
15/27t highest top speed tucked of 87mph
15/26t highest top speed if you’re small, and light, and have feet on the passenger pegs and tucked forward, 90mph.

With a Honda Rebel, you have 65mph guaranteed (headwind of below 20-30mph), and 75mph wind still stock.

If you want a tad more power, a VStar 250 will do +3mph, but consumes more fuel. About 10mpg more on average.

If you want better fuel mileage, and less top speed, a Suzuki TU250X does 70mph windstill, and upto 80mph top speed with a sprocket mod.
It also sips 80mpg stock, and 100+mpg with the sprocket mod.
I personally would never take a Suzuki TU250X on the interstates, but it’s great for town, suburban, and highway.

If you’re mainly looking for a city commuter, a Sym Wolf 150, together with a Kawasaki Eliminator 125 are your best options. They get well above 80mpg stock, and can get 110 to 120mpg with sprocket modification.

A motorcycle’s mpg will drop to 90mpg tops at 60mph, 80mpg at 65mph, and can drop to 60mpg at 80-85mph. A 3/4 sized bike gets the best mpg.
Bikes in this class are:
Honda cbr250r/300r, CB300f, CBR300R, Kawasaki Ninja 250/300, Suzuki Boulevard S40, TU250X, Yamaha MT03, Vstar 250, KTM Duke 390, Rc390, and more….

Larger than 3/4 sized bikes, is linked to added wind resistance, thus lower mpg.
Sport bike fairing may reduce wind drag, and increase mpg by a few over cruiser/standard style bikes.

The most aerodynamic bikes are the 3/4 sports bikes.
Then the naked bikes
Then the standard bikes
The cruiser bikes, touring bikes, and dual sport bikes are the least aerodynamic.

On average,
Honda focuses on best mpg, and has smallest cc in category. They’re usually also the most reliable and most efficient engines around.
Yamaha usually beats the competition by upping the ante in the cc department.
Their bikes are good and reliable, almost honda quality, and in some ways even better.
Kawasaki is usually right in between Honda and Yamaha. It builds it’s engines around numbers. 300cc for kawasaki means 299cc. Not 286 like Honda, nor 324 like Yamaha.
Suzuki usually has the worst performing engines of them all.
They’re like the “Nissans” in cars of the motorcycles.

On the other hand, Honda makes the worst transmissions. They’re usually clunky and shift out of gear. Yamaha and Suzuki produce very smooth shifting transmissions.

Qua bike design, Honda bikes are lightest in weight with no frills.
Yamaha would be second in lightweight, and come with frills.
Kawasaki is a nice compromise.
Suzuki bottoms out usually with top heavy designs, as well as being no frill. Add that to a less good engine design, and which makes them hopelessly overpriced for what you get…

KTM doesn’t have a lot of beginner bikes, but the RC390/Duke 390 is right in the sweet spot power wise, and the weight is great too.
Body design is sublimal. KTM just has an older, ugly looking dash, a vibrating engine that together with the hard seat make the bike unsuited for the longer rides.
The stock brakes are also pretty bad, so not meant for track racing either…

A 250 is most at home at speeds of between 35 and 75mph, aka city and highway, or, the slower lanes on the interstates.

If you need to do frequent rides of 75mph plus, you’ll need to get larger ccs, starting from the “holy grail of motorcycles”, a 350cc.
A Honda CB300F with a 50cc bump would be it. Many people are asking for it.
Yamaha R3, and MT-03, and Kawasaki Ninja 300/Z300 may do 100mph, but only at peak engine rpm.
Personally, I’m not so much for hese type of engines (short stroke engines), and much more for a CB300F (which unfortunately has a tad too little power for interstates).

As far as the Suzuki Boulevard S40, and Yamaha SR400, both their top speed is low (85mph), and vibrate like crazy, they both are air cooled, which means lower compression, resulting in lower performance and worse mpg.
What’s worse, is the Boulevard S40 is a belt drive, so you can’t modify the gear ratios, and the conversion kits for sale on the S40 look mightily ugly!

The SR400 doesn’t have a starter engine, and costs way too much!

So if you’re still looking for a bike, to get high top speeds from, and good mpg,
A bike that doesn’t cost an arm and a leg,
Honda’s CB300F comes closest, with a CBR250R second, and a Honda Rebel 250 third.
I’m not a Honda guy, but Honda specializes in mpg, so it would be the no-brainer to get.
If you’ll never find yourself on the interstate, and wont surpass 60mph, Suzuki TU250X is the right one for you.

If speeds of over 100mph are necessary, then you’d have to step up to a 500cc class.

120mph, 650+cc sport bikes, or 900+cc cruisers


*Edit: As of 2016, Honda and Kawasaki have added a 125cc bike in their arsenal, which is a great alternative to the Wolf Sym for the city! Also definitely recommended to add +1 tooth to the front sprocket, and if possible -3t on the rear sprocket for better MPG, while still acceptable acceleration speeds in the city.