TECH TIPS: Gearing for the real world

Posted  in Technical tips

It’s a controversial topic in this country, but most of us know that on a world scale, New Zealand’s open road speed limit is pretty slow.  Most motorcycles in the mid to large size range (500cc and up) are designed with the European market in mind and that means Germany’s speed limit free Autobahns…For NZ road (and off-road) riding, most production bikes are geared a bit too high as standard.  Having a superbike that will do 100mph in 1st gear sounds impressive, but it’s just not practical when going for a fast jaunt through your favourite twisty roads will see you spending most of your time in 2nd gear.  What’s the point of the other 4?  Well, there is something you can do about it; it’s ok to change your gearing to make it better.

Typically, a good gear ratio will have your bike sitting in the rev sweet spot at open road cruising speed in top gear.  A gearing ratio like this will also mean that getting off the mark is easier and you’re more likely to have power on tap and a range of gears to choose from at the speed you need it.  That sweet spot is when the engine is relaxed and you can get along without needing to use too much throttle, but not so low that it feels bogged down.  Obviously this will vary from bike to bike.  For example, something like a 1700cc Triumph Storm will be happy cruising at around 2,000rpm, whereas a 750ss Ducati prefers to be closer to the 4,500rpm mark.  Don’t worry, you’ll have plenty left to be able to pass safely and still lose your licence if you want.  It’s a judgement you’ll have to make yourself, depending on your bike and where or how you like to ride it most.

So, how do you change the gearing on your bike I hear you ask?  Well, without stripping the gearbox down and changing the internal gears, which is a very serious undertaking, the simplest thing to do is to change the size of the sprockets.  If your bike has a shaft or belt drive, then this exercise might not be quite so easy, but for most bikes it’s a relatively simple task.

Figure 1: Sprocket ratio

If you take a look at the diagram in figure 1, you’ll see the bike’s final drive ratio is to do with the circumference of the arcs the chain follows as it goes around the sprocket.  Imagine a point on the arc on each sprocket.  The smaller circumference on the front sprocket means it will need to make close to three turns for its point to travel the same distance as one turn for the point on the rear sprocket.  The size of these arcs is measured by the pitch circle diameter, or PCD.  Given this PCD is directly proportional to the number of teeth on the sprocket, the gearing is simply a ratio of the number of teeth between the two sprockets.  So, if you take a typical rear/front sprocket size of 36 and 14 teeth, then the final drive gearing ratio will be 36/14 = 2.57.  To make the bike rev higher at a given speed, you need to increase the ratio.  So, adding one tooth to the rear sprocket will give a ratio of 37/14 = 2.64.  Alternatively, you could decrease the size of the front sprocket whereby 36/13 = 2.77.  As you can see, changing the front sprocket gives a greater difference.  Roughly speaking, changing 1 tooth at the front is equivalent to nearly 3 teeth on the back.

Those of you who are clever will have realised that all this changing of teeth numbers means the chain tension needs to be adjusted each time to accommodate the increase or decrease in circumferential distance around the sprockets.  Sometimes you may even need to add or remove a link on your chain.  If either your chain or sprockets are worn, it’s a good idea to replace both at the same time anyway.  Unless you have a stock of different sized sprockets and chains to play with, it can be a bit of a pain trying to guess what you will need.  A good bike shop should be able to give you the right advice though.  If you are unsure, order a chain that’s a little too long as it’s pretty easy to take a link or two off with a chain breaker.

"Typically, a good gear ratio will have your bike sitting in the rev sweet spot at open road cruising speed in top gear."

 

So, where to start?  First stop is your local bike shop.  They’re usually pretty helpful and should know a bit about how your model of bike performs on NZ roads.  Another good place to look is the internet. Do a bit of research to see if anyone else has gone down the same route and recommended a particular sized sprocket for your bike.  Alternatively you could rely on your own judgement and guess.  Once you’ve decided on what you want to try, it’s a simple task of making the changes and going for a ride.  If you don’t have the tools or the know-how to make the changes yourself, then any bike shop will happily do it for you and it shouldn’t cost too much in labour.  Don’t forget, if you don’t like it you can always try another ratio or go back to where you started.  In most cases though, you’ll likely find the bike will be easier to control, more responsive and more practical for day to day riding.

'Gearing for the real world' was written by Rodney O'Connor of Eurobike Wholesale.