Crikey, you guys must live sheltered lives. I thought I was the only one who found tyres gripping. Sorry

, I had to get that one in.

Okay, but don?t blame me if you get bored!
During the '60s the tyre manufacturers were searching for ways to improve grip levels without sacrificing mileage. Nothing changes, huh?
The obvious way is to increase a tyre's width in order to increase contact patch size, after all, without changing the compound a bigger contact patch will give more grip.
The problem was that at the time all motorcycle tyres were imperial sized, e.g. 4.00-18, and tyres of this type are just as tall (from base of bead to top of crown) as they are wide.
So if you then make a 4.50-18 tyre it's going to be 4.50 inches tall. This has obvious implications for tyre clearance on full suspension compression. Sure, bike manufacturers can design bikes around ever taller tyres but there comes a point when a bike's performance would be compromised.
The other problem is that a 4.50-18 tyre would consist of a larger amount of material, which would increase cost and weight, adversely affecting handling and suspension response.
So the solution that Dunlop came up with was to design a tyre that was 90% as high as it was wide. This allowed them to increase a tyre?s width, and hence contact patch size, without increasing the tyre?s height. It also meant they could keep the cost and weight of extra materials down to a minimum.
To illustrate this let?s look at the common 3.50-18 size: the new-for-the-late-?60s ?90% Imperial? equivalent of 3.50-18 became 4.10-18, i.e. 4.10 inches wide with a height that is 90% of this width, or in other words 3.69 inches high. Okay, so a tyre that?s 3.69 inches tall isn?t exactly 3.50 inches but it?s so close that no one would notice.
These new sizes became what was commonly called ?low profile?. Although the word ?profile? actually refers to the curvature of the tread surface. The term ?lower aspect ratio? would be more correct. (More on aspect ratios in a moment).
So in the case of the 4.10-18 they now had a tyre that had roughly the same contact patch size as a 4.00-18 but the ride height of a 3.50-18. S2/S3 and KH250/400 riders in the 1970s rejoiced!
Other common sizes at the time were 3.10 (roughly the same as 2.75 inches), 3.60 (roughly 3.25) and 4.60 (roughly 4.00). So H1/H2 riders had cause for celebration as well.
The 4.25/85-18 Dunlop TT100 is a bit of a size anomaly in that it doesn?t really fit into traditional Imperial sizes or 90% Imperial sizes. It?s equivalent to a 4.00-18 and it?s in a size category of its own.
At this point I need to explain what an aspect ratio is. For those that already know, please excuse me but it?s worth explaining what it means for those that have never really been interested in tyres.
A tyre?s aspect ratio is the tyre?s height (from bead to crown) expressed as a percentage of its width. This is not something that we need to worry about with the ?90% Imperial? sizes talked about above. In those cases we just have to accept that they have a 90% aspect ratio. But aspect ratios play an important role in the way tyres sizes started to be expressed in the late ?70s.
What happened as the ?70s came to and end was the arrival in motorcycling of the Metric system of tyre sizing.
Let?s use 120/90-18 as an example. Expressing a tyre?s width inches became sooo last year.

120 referred to the tyre?s width being 120mm. And the tyre?s height was now included in the size; the number 90 meaning that the tyre was 90% as high as it was wide, or in this tyre?s case 108mm.
Of course, the metric take-over only went so far. Because imperial rim diameters were so well established they?re still expressed in inches.
This sexy new metric size system stopped the fledgling ?90% Imperial? system before it really got started and these days only one of the sizes, 4.10-18 is still in use on new small trail bikes and even that?s dying off slowly.
Sure, you can still buy tyres in these sizes, but they?re mostly tyres aimed at the classic bike and trail bike markets.
I?ve never managed to get to the bottom of why the tyre?s lower aspect ratio wasn?t included in the tyre?s size, even after research with some of the original designers of the TT100. I guess that at the time they were playing with something so new and didn?t think about where things would go next. You also have to wonder how they would have expressed the sizes of the lower aspect ratio tyres that came along in the early ?80s. It?s a good job that metric sizing arrived on the scene when it did.