Thursday, September 29, 2011

Squared Off Motorcycle Tires

The Lost Motorcyclist has now put 37,000 km on the second rear tire of the Vulcan 900, and it's still going. This is a record for me, my typical longest wearing rear tire up to now has been about 18,000. I am surprised to find this tire doubling the usual life of a tire. It is a Metzeler Marathon ME880 180/70x15B, bias ply, tubeless, made in Brazil. It cost $200 for the tire, $60 installation, and $18.99 for a new inner tube.

Some owners get long wear from the tires by running them until they are bald or down to the cords. Well, I have done that in the past but still not exceeded much more than 20,000 km. These tires never did have a tread in the centre, so it's hard to tell how much wear there really is. The tread at the outside is still deep, and that's because most of the wear takes place in the centre of the tire (actually a little offset to the left). I can see they are slightly squared off, meaning that the centre part of the tread is flat.

This tire has lasted a long time because of several factors. First, it is an extremely wide tire, and so compared to skinny tires, it can put more rubber on the road. Not only is the contact patch bigger, but there is more actual rubber on the tread. Second, it is a fairly large diameter tire (15 inch rim plus the height of the sidewall), which also puts more rubber on the road, and keeps the number of revolutions down. Third, I would guess it had a hard rubber compound. I can feel that the tire occasionally slips on the road under acceleration going around a corner. Soft rubber would stick better, but not last as long.

I usually change the tire when the handling of the "squared off" cross section becomes too annoying. Squared off tires have a peculiar dynamic compared to new round section tires treads. The difference is in the location of the contact patch, which on a new, round section tire, is always in the center. With a squared off tire, when you lean to the right, the contact patch also moves to the right, and this makes the motorcycle feel funny - usually you have to apply counter steering pressure all the way through a curve. With a new tire, the motorcycle can corner almost without handlebar pressure. Another place that squared off tires feel funny is in a straight line when the road surface is uneven or slanted to the side. The contact patch will move back and forth depending on where the high point of the road is, and this makes the steering feel vague and forces you to grab the handlebars tighter.

There is no way I know of to correct a squared off tire. I suppose driving on twisty roads and scraping footpegs for 20,000 km might work, but around here there is about 0.01 km of curving road every 50.0 km of straight, so do the math. On these roads, the squaring off is only going to get worse. Need I say that shaving off the corners with a cheese grater is a very bad idea?

Some riders have given up on round profile tires altogether, and use car tires. This trend is called "going to the dark side" in a reference to either Star Wars or Dick Cheney, and the fact that car tires are not legal on motorcycles. There is a huge amount of information on this on the Internet. There are plenty of online debates, with entries often started with the words "You, sir, are an idiot!". There are also some videos of tires in action, some accounts of 160,000 km experience in using car tires with no trouble at all. I have never used a car tire on a motorcycle, but I do not have any fundamental objection to it one day. To me, it seems to be simply an extreme case of squared off motorcycle tire, but with plenty of tread in the middle contact area. Now that motorcycles are using these very wide motorcycle tires, the differences do not appear too great. I would worry however, about a speed wobble. Speed wobbles used to kill a lot of motorcyclists, but is seems the bikes (or tires) of today are not as prone to speed wobbles. Unfortunately, the wobbles happen suddenly and car tires have not been scientifically tested the way motorcycle tires have been.

In any case, the appeal of the car tire is mostly the cheapness and long wear. With the new motorcycle tires that seem to get extraordinarily long life, I do not really need the hassle of driving on an illegal car tire the next time I get safety checked on the way to Port Dover for Friday 13th.

Picture: That's my tire, this afternoon. 55,000 km on the Vulcan.

1 comment:

  1. Drat! That reminds me ... gotta get my tires replaced this winter ;-)