This article is about wet weather performance of motorcycle tires. Weather can be unpredictable, especially when taking long trips, so knowing how your tires perform in the rain / wet is extremely important.
If you’ve ever seen a Moto GP or Superbike race during wet conditions, you’ve witnessed first-hand that wet weather performance can vary dramatically from dry weather performance. Let’s examine the graphic below. The grooves circled show the fundamental component of a tire’s wet weather performance. It’s “negative ratio”.
The negative ratio is the first component to understanding the ability of a motorcycle tire to evacuate water. So, what is the negative ratio of a tire?
Imagine the tire’s imprint if you rolled it across sand, that is to say the tire contact patch (the only point of contact between your tire and the ground). All sections that don’t leave an imprint (the grooves) make up the negative ratio.
It’s measured as a percentage. The lower this percentage is, the less negative ratio the tire has. This sounds like a double-negative I know and Sister Mary Knuckle-Cracker would have me in the dunce cap if this were six grade English, but luckily I matriculated from six grade to seven grade (barely), so hang with me while I attempt to explain using some photos and graphics.
If we talk about a 0% negative ratio motorcycle tire, we are talking about a totally smooth tire like a road racing slick. Believe me… you do not want to be on a set of slicks in the rain!
Motorcycle Racing Slicks = 0% Negative Ratio
If we are in the 50% range or higher, we are talking about a knobby tire like an aggressive adventure tire.
Adventure Tire = 50+% Negative Ratio
We generally don’t discuss motocross tires in terms of negative ratio, but if we were to include them just for fun, they would be in the 70+% negative ratio range.
Off Road Tire = 70+% Negative Ratio
To take the example to the ridiculous extreme, we cannot have a tire with a 100% negative ratio because that tire would be 100% empty space and therefore the tire would not exist. Maybe someday there will be hoovercycles with no tires at all, but for now let’s stick to the ground the old fashioned way with sticky icky gooey tires.
In summary, the lower the negative ratio is, the more grip it has in dry conditions (road racing slick). The higher the negative ratio, the more drainage it has in wet conditions.
If you want to get “tire geeky” (and we know you do) we can go one level deeper (pun intended) than the negative ratio of the tire. After all the negative ratio is only a two-dimensional concept. Let’s go 3D and do a mathematical calculation to determine the “water channel” as shown in the geometric diagram below.
The negative ration is two-dimensional (Width x Length) of the negative space. The water channel is a three-dimensional concept which adds tread depth to the mix (Width x Height x Length). When the water under the contact patch exceeds the volume of the water channel to evacuate water from beneath the motorcycle tire, hydroplaning occurs.
What effect does speed have on hydroplaning? Simple really… as the motorcycle’s speed increases, the tires’ water channels are trying to evacuate a given amount of water in a shorter amount of time (i.e. your motorcycle is covering ground faster). That’s why in the rain you would be able to go faster using Sport Touring Tires instead of Sport Tires and you’d never want to be on slicks.
Keep in mind that the wear of your tires is the most important factor for your safety. This is particularly true in wet riding conditions. A worn tire has a smaller water channel and will channel less water than a new tire because the depth of the water channel is decreased. Check your tires often and always think about total water channel versus remaining mileage when preparing for a long ride / trip. When you compare the cost of a new tire to the total cost of your road trip it will probably be a small percentage, so I suggest taking the safe route and when in doubt install a new tire.
Also keep in mind that many sport / hypersport tires are essential slicks on the sides of the tire (0% negative ratio at high lean angles), so if you get caught in the rain on this type motorcycle tire, limit your lean angle to less than you are used to on dry pavement.
This article is not addressing rubber compounds, air pressure, tire temperature or ambient temperature. Those factors all effect traction as well, but to cover everything in one article would be… well a book and not an article. Please check out the other articles in this section for info on these other factors. There are literally hundreds of combinations and if you’d like to discuss something specific, please give us a call. The 8 Ball Motorcycle Tires team is ready and willing to answer all your questions.
Finally as with all things motorcycle related, practice makes perfect. When you are on a ride in dry conditions, pay attention to the feel for your tires. Then do the same when you are in wet conditions. Knowing your tire handling characteristics, wear condition and your riding abilities will lead to many miles of enjoyable riding. Now get out there and wear those tires out!
Founder & CEO
8 Ball Motorcycle Tires