Bore, Stroke and Capacity (Part 2)

KTM RC8 engine

KTM RC8 engine Source :

In Part 1, I shared my learnings on what bore and stroke meant and also showed how to use these to calculate engine capacity. From this table, the bikes have different bore and stroke combinations – some have larger bore vs stroke and vice versa. So what does that mean for the bike and the rider?

For bikes with similar displacement, a larger bore than stroke (bore stroke ratio > 1) means that the bike is able to rev higher, thus producing more HP. On the other hand, a larger stroke than bore (bore stroke ratio < 1) means that the bike will have more low end torque, or have a more ‘grunty’ feel.

Now, it is important to compare bikes with similar displacement when using this rule of thumb, because once engine displacement changes, many other factors affecting power and torque come into play.

Engines with a larger bore than stroke are called – oversquare or short stroke engines. Conversely, engines with a smaller bore than stroke are called undersquare or long stroke engines.

I’ve added bore-stroke ratios to the table below.

bore stroke table

bore stroke table

So what does these bore stroke combinations mean for the characteristic of a bike?

The KTM Duke and the Honda CBR have oversquare engines, this gives them more revvy characteristics, but likely lower overall torque. As a Duke rider, I can attest to that, it likes nothing better than to be revved hard on roads, and going up to 10,000 rpm is no problem. But at lower RPM’s, it does feel sluggish as it doesn’t have the torque to pull away from slower vehicles. This requires me to change gears more frequently to make up for the lack of torque.

Compare this to the KTM 125 EXC which is an enduro style bike, obviously a more square engine would give it better low end grunt, making it more suitable for the nature of dirt tracks and off-roading. The Yamaha YBR is also perfectly square, and this makes it perfect for the start, stop nature of city riding.

Now, if you were reading up to here, I imagine your question would be ‘Isn’t the YZF considered a sport bike? Why does it have the longest stroke?’

Good question!

To be honest, I haven’t quite figured this one out yet, so some more research is in order. Any YZF 125 owners want to shed some light on this?

Questions and comments below!


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