Do Not Underestimate the Power of the Darkside
Daily Slideshow: Putting a car tire on the back of a motorcycle is called going to the "Darkside." Yoda said about it "once you start down the dark path, forever will it dominate your destiny," which may not be true, but it may dominate your discussions with other riders you meet.
Improved Ride Quality
One of the main reasons to choose a Harley-Davidson touring bike is the ability to ride long distances in comfort. While the ride quality is good out of the box, and suspension upgrades can further improve comfort, tire choice has a huge impact on how a motorcycle behaves. If you swap to car rubber you may notice an improvement in ride quality, if forum posts are to be believed. In comparison to a motorcycles tire, most car tires have a significantly taller sidewall that absorbs bumps and provides a cushioning effect over road irregularities. Some darkside riders, as they are called, over-inflate the tire to give it more of a round profile, which may lessen the ride quality improvements, but others run them at lower pressures so they flex more, which will improve it.
Braking Perfomance and Wet Weather Grip
People who have gone to the "darkside" claim it provides a significant increase in braking performance, especially in wet weather conditions. Supposedly, this is due to the flatter profile and broader contact patch of the car tire has in comparison with a motorcycle tire, with its rounded, or triangular shape. Although a motorcycle tire will flatten to a certain extent under braking, it cannot match the cross-section width of a car tire. When riding in the wet, a car tire puts more rubber on the road for better grip in normal driving, but the pointy shape of a motorcycle tire makes it less likely to hydroplane in deep puddles than the broad flat car tire.
Increased Load Rating
The allure of the touring model Harley-Davidsons is being able to pack the saddlebags full of necessities, throw a partner on the back, and ride into the sunset. Unfortunately, the most touring bikes weigh in at a hefty 800+ pounds wet, leaving a set of OEM, or equivalent, motorcycle tires with little more than 500 pounds of payload capacity. In many instances, a bike with two passengers, gear, and a full tank of gas exceeds the tires load rating, causing accelerated wear, or in severe cases, a tire blowout. A "darkside" rider who chooses a 205/55/16 car tire with a load index rating of 90 will be able to support roughly 1,300 pounds on the rear tire alone. Of course, a bike like the Road Glide only has a GVWR of 1360lbs, with just 927lbs supposed to be on the rear tire, so if you are going to the "darkside" because of the weight rating, you should consider packing lighter or buy a trike.
Longevity and Cost
Possibly the top reason owners claim when switching to car tires is the significantly longer tread life, in a tire that cost significantly less. A typical rear touring motorcycle tire can last between 7,000-15,000 miles depending on the bike and usage. In contrast, owners have reported upwards of 40,000 miles when using a rear car tire. This is especially impressive considering many of the bikes equipped with car tires see 2-up cross-country riding with the bike loaded to the hilt. While the list price for an OEM rear tire for a touring model Harley is more than $300 (retail is still over $200), and aftermarket tires start at about $150, a car tire in the equivalent size often sells for less than $100.
The Square Shouldered Profile
While using a car tire on the back of your touring bike may have some benefits, there are plenty of drawbacks one should consider prior to going to the "darkside." The number one reason to avoid using car tires is that square shouldered profile, when compared to a motorcycle tire, leads to reduced maneuverability and handling. The bike can still be leaned into a turn, but it requires more effort to get the tire up onto the edge, and then you have less rubber on the road.
Mounting Issues and General Persecution
The second big drawback is getting the tire mounted because most bike shops won't do it, and many car shops will refuse; expect to sign a release of liability when you do find a shop to do it. The 16" car and bike wheels have a different profile shape in cross-section, and the beads of the tires may not seat correctly, which is part of the reason most shops won't do it. The third drawback is that car tires behave differently when riding along grooves in the road, or crossing railroad tracks, causing the bike to follow them, or jump over bumps, unlike when equipped with proper motorcycle tires. Crowned roads can also be a bit tricky as the bike will try to drift to the low side of the road.
You may also be shunned and ridiculed by other riders you meet. "Darkside" riders have their own section of the HDForums (and on other forums), as well as their own forums, for true believers where you are not allowed to criticize the practice, because they seem to get persecuted by the general online motorcycle community.
If you are looking for advice on how to do things properly, like tire changes, or even diagnose engine trouble, click over to the How-to section of HDForums.com