Feb 18, 2006

Upsizing motorcycle tyres

I was running my motorcycle, the Bajaj Pulsar 180 DTSi with an extra wide rear tyre. Instead of the standard 100/90-17, I put in a 120/90-17. For two reasons. First, that's the size I got (I would have preferred a far lower profile, say a 60 or 70). Second, I've always wanted to try it. All the questions in motorcycle magazines elicit pretty standard responses, so I wanted to know that the fuss is all about.

The responses take two lines. The first is: 'We do not recommend upsizing the rear tyre. Because it will require a rim upgrade, which is too expensive to bother with. Also, the extra mass the larger tyre will slow the machine down. The second is: 'We do not recommend this. It only makes the bike feel heavier. Plus, the manufacturer decides the tyre size after a variety of test across conditions and gives the tyre best suited to the motorcycle. And conversely, adjusts the suspension according the properties of that tyre. Why meddle with tested equations.'

The truth, not surprisingly, is pretty close. On adding the tyre, the bike seemed a bit tipped on to its nose. Which is a strange feeling. The front end turns pretty sharply, but just as sharp is the feeling that the fat rear tyre resists the change in direction. The on-the-nose feeling also makes the bike buck a little when you use the front brakes sharply without the stabilising effect of the rear brake. It also causes a noticeable change in the way you get feedback. A manufacturer tuned handling package usually (should, at the very least) has a well-balanced front-rear feedback ratio. It's a bit of an intangible, but the rear tyre, as its size grows begins to yell when it is required to merely speak. Not only is this unsettling, it also tends to 'hide' the feel the front end. Upsizing the tyre does have a few benefits though. Grip rises, no question. With it, it brings its friends, mid-corner stability, poor road tractability and in specific brand/model cases, wet weather stick.

Now, I reverted to the old 100/90-17 tyre today, and my word, what a difference. The motorcycle seemed able to change direction much, much faster. The return of balanced feedback has raised my confidence in the machine, and the under braking stability has returned. Maybe the best change would have been to find another tyre in the same size, but stickier.


Anonymous said...

Hey, i realise this post is a couple years old now... but I was just recently thinking about doing the same, and found your entry very informing. Thanks. helped me out alot.

Anonymous said...

To upsize, you have to do both tyres together, at the same ratio.

100/90 to 110/80
140/70 wd go to 150/60

Only go up 1 size from standard.
100/90 - think high mpg
110/80 - think high grip
120/70 - worse than the above 2 sizes
120/90 - worse than the above 3 sizes