Well, I’m not sure what I’ve got myself into, but this is the second in a series of technical interviews with engineers in our industry. Not everything is, “cut it out, try it and if it breaks make it thicker,” in our world. Many companies have engineers studying and experimenting in controlled environments to bring to the market products which are safe, excellent performers using the latest in technological advances. Recently, I interviewed Jeff Johnston of Metzeler tires to ask all the questions I could think of about motorcycle tires.

Mark Barnett: Jeff, first of all, we all read the maximum pressure on the sidewalls of our tires but hear that we should be running lower under most circumstances. Do the tire manufacturers give out recommended pressures or just the bike manufacturers? And what else do we need to know about inflation pressures?

Jeff Johnston: The carcass of the tire dictates how much air you can put into it. What we always like to say about air pressures is that we have a minimum and then we tell the customer, the minimums will give you the best possible performance but not the best possible mileage. If you load the bike, you need to increase the pressure, up to the maximum listed on the sidewall. The maximum by law has to be posted on the tire. We list the minimums on our website. For example, on a standard Harley-Davidson we recommend 38 minimum on the front and 44 on the rear. We know that’s the best starting pressure. But we tell the customers if you run 40 to 42 on the front and 48- 50 on the rear, the bike will handle fine, no problems with grip or wet weather capabilities and you will get the best possible mileage. The difference in pressures is only about ten percent but that can mean an increase of three, four, five, or even six thousand miles in the life of the tire.

MB: Your wide tires are pretty much all steel belted radials, what does the steel belt add to the performance of the tire?

JJ: We can use that steel cord to help the performance of the tire. The steel cord is very thin, a bundle of three is twisted together with a couple of other bundles and the end result allows us to give the bike bump absorption and grip that no one else can do. The steel belted tire has a little suspension built into it; it helps maintain the contact patch better. When you push on the tire it quickly comes back to its original shape with the steel pushing on it. Another thing the steel belting that goes from one edge of the tread to the other offers is that it acts like a giant heat sink. As one side of the tire gets warm, the steel starts transferring the heat across the tire so you don’t get hot spots. You even out the heat of the tire, it is virtually impossible to overheat a steel belted radial tire the way we build them. We can use a softer compound of rubber for better grip and performance with the steel belt without the fear of overheating. Our patented steel belted radial construction is very different from automobile radials.

MB: I notice that all the Metzeler tires have a distinct profile, that they appear rounder than others when viewed from the back. How does this affect steering and straight line performance?

JJ: We are a technology driven company who produce products for riders. We would never build a tire for posing, our tires have to do what they are intended to do. With the big tires, you have to have a nice rounded profile, we are famous for that. Whether for a stock Harley or on our 240, 260 and larger tires, you need the rounded profile for cornering. Some people call it power steering when you compare other tires to the replacement sized Metzelers. The tires steer better and there is no negative affect on straight line performance. We compared our 260 to two of our competitors’ tires with flatter profiles in straight line performance. Because of the steel cord construction and our rubber compounding, we put more power to the ground. So a rounded construction that gives better steering doesn’t have to take away from straight line performance. Our tire is much lighter than others which helps it as well.

MB: There are lots of tire and wheel combinations out there, how can a builder find out which rim width is appropriate for each tire?

JJ: Our full line brochure shows the permitted rim widths and the favorite rim for each tire model. Most tire manufacturers have this on their website. Our information is on www.us.metzelermoto.com. We give as much information as possible to the customer.

MB: It seems everyone has a machine cutting wheels out of aluminum. How can a buyer know that the rim was well designed, that it will properly bead up to the tire?

JJ: The big thing about a wheel is that it follows the set standards in the industry. In the USA, the Tire and Rim Manufacturers Association sets the standards. For a tube type wheel or a tubeless type wheel there is a certain shape and design to the flange where the tire seats against the wheel. If a company doesn’t follow this standard, they will have trouble getting liability insurance. So this isn’t much of a problem. In the past, some companies used to cut down auto wheels and not change the flange. There wasn’t enough of a contact patch and this could have been a serious problem. Overall, wheel companies should have stress tests they can give to consumers to show that their designs are proven. Their insurance company should force them to have testing to prove the reliability of their product.

MB: In the Harley world, not much attention is paid to the weight of the bike, the wheels or the tires. What advantages are there to having lighter wheels and tires?

JJ: Tire and wheel assemblies have a lot of inertia and are unsprung weight. If you can lessen the unsprung weight, the motorcycle will perform better in almost every way: accelerate and stop faster, go over bumps better and turn better. The steel used in the radial tires is very thin but hugely strong. You don’t have to use two or three layers of rubber coated fabric to build a tire, hence the radials are usually much lighter.

MB: Thanks, Jeff. Many custom bike builders just worry about how the tire makes the back end of their bike look, but it is good to know that so much time and thought goes into making the tires work right for the folks who still actually ride their bikes.