There appears to be a never ending discussion between riders about what tyre to fit to their motorcycle. As there are many elements to this, we thought we’d break it down into 3 different components:
- Sizes & Profiles
- Compounds & Construction
- Category & Application.
So let’s start at the very beginning…it’s a very good place to start (to be read to the tune of “Do Ra Mi”)…
Firstly, let’s discuss what those numbers on the side of your tyres actually mean. The first number indicates the width of the tyre from edge to edge and is measured in millimetres. In the examples below, we have two different sizes, a 180 rear and a 120 front. Please note that this is a fitted width. Sometimes different model tyres appear to be different widths when displayed in the rack (prior to fitting), but this is more from the way they have been transported or stored and have little or nothing to do with the fitted width. The second number you see, which is straight after the forward slash (/) is what is known as the aspect ratio of the tyre. The number represented is not in millimetres, but rather a percentage of the width of the tyre. In the examples shown, that would mean that the 180/55 has a width of 180mm and a height of 99mm (or 55% of the width), which is measured from the bead to the top (or crown) of the tyre. Similarly, the front tyre has a width of 120mm and a height of 84mm (or 70% of the width). Still with me? Good. The final number, which is 17 in both these examples, simply indicates the size of the rim that these tyres will fit.
Now that we’ve got that out of the way, how do you decide what size tyre will best fit your motorcycle? Generally speaking, the best size tyre to fit to your bike is the size that the manufacturer recommends. As boring as this might sound, you have to consider that in developing your motorcycle a team of engineers put their collective heads together and came up with that particular wheel (and therefore tyre) size for your bike. Now, I’m not doubting your intelligence, but do you really know more than a team of engineers whose profession and sole purpose is to determine how your motorcycle will function at its best? Unlikely!
So what is the result if you do decide to fit a different size to your bike? The most common thing of course is to over tyre the bike by fitting a larger tyre…usually a wider rear. Why? Because bigger has to be better right? And…doesn’t it look fully sick? Well, sometimes, and sometimes not. Although a wider rear tyre might (in your opinion at least) enhance the aesthetics of your motorcycle, a wider tyre than your bike is designed for can alter the handling significantly. The most common results of over-tyring the rear is for the bike to; resist steering or holding a line; resist steering initially then feel like it’s going to fall into the turn; make the front feel less planted or more vague; physically “push” the front or cause it to lose traction earlier than it should. Another aspect of this which is a fairly recent phenomena, is that an incorrect tyre size can interfere with traction control systems and other electronic gizmos found on modern motorcycles. Do these sound like good enough reasons not to mess around with tyre sizes for your bike? I would hope so! On occasion a tyre manufacturer will produce a tyre that is larger than the motorcycle manufacturer determined was ideal for that wheel (or bike), but in these instances the manufacturer alters the construction of the tyre to fit the narrower rim (this is not common and is generally limited to track speciality tyres).
The one exception to this is on the aspect ratio. Commonly this is in the 190 sizes, where both 50 & 55 are both available. If you consider the fact that this number is a percentage of the width, the difference between these tyres would be approximately 9.5mm, which is less than a centimetre. Now, if you are one of the 3 x Spanish or 1 x Italian riders that currently dominate the MotoGP Championship, you might notice this difference, but for us mere mortals? Probably not! So don’t stress too much over that particular number, but the width for your bike? Absolutely!
Below are some diagrams to hopefully help you understand these points more completely. Stay tuned for episodes 2 & 3 of this topic!
Any feedback on this blog, or is there another topic you’d like to know about? Send an email to firstname.lastname@example.org and we’ll see what we can do!