Dr Jay‚€™s Tire Blog
Revised July 27th 2011

Isn't the Internet a wonderful thing? You're probably reading this on the internet. You can literally find ANYTHING on the 'net. The Internet has it's problems though. Predators, scams, viruses and the worst of all, people who think they know what they're talking about post some bullshit and then everyone reads it and thinks it's for real! Sort of like these tech topics huh? :)

One recent load of crap came to our attention and compelled me to spend some time on the subject of tires. Motorcycle tires are really very technically advanced pieces of engineering. I mean, stop and think about it, they have to provide great reliable traction in straight lines in all conditions but also at lean angles up to 70 degrees! The tires must go from lightly loaded like an accelerating R1's front tire to supporting 600+ pounds of static load on that same R1 front tire while braking hard with the front brakes. Further, we expect great traction, stability AND great life (mileage). Truly amazing and a LONG way from tire designs from just 5 years ago. To understand some of these principles, we must understand how a motorcycle turns.

As most of you probably already know, in order to turn a motorcycle, it must lean into the turn. The faster you're travelling, the deeper the required lean angle to insure the motorcycle stays on it's intended path. The reason for this is a simple force referred to as "Gyroscopic Force". It is one of the fundamental laws of nature and works heavily on our motorcycles. It is what keeps us upright while travelling down the road and also what enables us to turn corners. Our wheels on our motorcycles are Gyroscopes when spinning and follow the same laws of Gyroscopic Motion and Preservation of Angular Momentum.

The behavior of a gyroscope can be most easily envisioned by considering the front wheel of a motorcycle. If the wheel is leaned away from the vertical so that the top of the wheel moves to the left, the forward edge of the wheel also turns to the left. In other words, rotation on one axis of the turning wheel produces rotation of the third axis. You can try this experiment your self. One way is to hold the wheel of a bicycle in your hands by the axle and have someone give it a spin. now while it is spinning, lean the top of the wheel to the right and you will feel a powerful force trying to turn the wheel to the right. Another way to feel Gyroscopic force is while riding your motorcycle down the road, push forward on the left handlebar and notice how there is an almost immediate response of the bike to want to lean to the left. The point is a Motorcycle turns by leaning it in the direction you want to turn. You cannot turn a Motorcycle without leaning it. The faster you go, the more you need to lean it to turn on the same radius.

Now you're wondering how this relates to the crap people post on the internet? It has been posted on the internet that you can use a car tire on the back of your Yamaha Cruiser to get longer mileage from the tire. Longer mileage but at what price, possible catastrophic failure and death. Hum...seems cheap to me....

First let me say that car wheels and their tires do NOT lean when they turn. In fact automotive tires actually must stay relatively flat in relation to the road in order to develop a maximized foot print for the best possible traction. We spend endless hours on our race cars with suspension geometry trying to keep the tires flat relative to the surface under dynamic loads. Ever seen the weird camber angles on the wheels when a stock car is not racing, the wheels are all at weird angles anticipating what the tire needs to do while the car is at speed, under load in the turns. Leaning them on to their edge is not intended or recommended and is actually counterproductive. Despite reducing the footprint to nearly nothing (no traction) when leaned, because of the flat shape of the car tires carcass it actually causes a jacking effect in the motorcycles suspension which upsets all the other geometries in the motorcycles chassis. It is extremely difficult to ride and highly DANGEROUS! Imagine having to make a quick reaction avoidance turn and losing all traction on the rear tire. Worse than that, they can only be described as "hazardous to your health". None of these "car tire" manufacturers would ever recommend their tires for motorcycle use for one simple reason, they are NOT designed for motorcycles- no one would ever design such a tire for motorcycle use as it is completely contrary to how motorcycles work. Even worse, Imagine, after an accident, they determined you were using a tire that was not DOT approved for use on your motorcycle. Do you think the insurance company might use that to deny your claim? Yep Motorcycle tires must be DOT approved for motorcycle use. (Standard 119 and 120 of the Federal Motor Vehicle Safety Standards)

Lately this is becoming a huge topic around the cruiser circles. I had one rider tell me that they are running these car tires underinflated (below the tire manufacturers recommended operating pressure) because "the sidewall distortion when they run them underinflated makes them feel less weird in the turns". (Those of you that know me can see me shaking my head) Never mind the fact that running tires underinflated causes a serious amount of heat build up and a potential for catastrophic side wall and tread surface failure (ply or tread separation) and even worse could suffer a bead failure and instantly lose all air pressure as tires REQUIRE an engineered amount of tire pressure to hold their shape and keep the beads seated.

It reminds me of an ad from a helmet manufacturer in the early '80's which read
"If you have a $10.00 head, wear a $10.00 Helmet". In this case it would be
"If you have a $70.00 Motorcycle, use a $70.00 car tire". There is absolutely no excuse for playing engineer and deciding that what you are doing is OK just because every one else is doing it. Lemmings follow each other off of cliffs too....

Ride Safe!

Dr Jay