Examining the “Dark Side”

Not open for further replies.


Staff member
Premier Member
IBA Member
IBR Finisher
IBR Staff
Examining the “Dark Side” - Car Tires on Motorcycles

By Tom Austin from the 2015 Fall/Winter issue of Iron Butt Magazine

Photos Courtesy of Greg Marbach and Tobie Stevens

According to the Motorcycle Industry Council Tire Guide: “Because a motorcycle is a single-track vehicle and leans as it turns, motorcycle tires are quite different than car tires. Whereas car tires have a fairly flat profile and a contact patch that varies little in size or shape, motorcycle tires have a U-shaped profile and a contact patch that changes size and shape during cornering.” Despite the obvious difference between tires designed for passenger cars and tires designed for motorcycles, there apparently are several thousand riders, who refer to themselves as “Darksiders,” who are using a passenger car tire on the rear of their motorcycles. The benefits claimed by riders who use car tires are primarily longer tread life and lower tire cost.

This article examines the arguments for and against the use of car tires on the rear wheel of a motorcycle.

The Arguments Against Car Tires

Previously published comments on the use of car tires on motorcycles include the following:

“…never mount a passenger car tire on a motorcycle rim; the flat profile of a car tire is incompatible with the dynamics of a vehicle that leans as it corners, and the section of the tire in contact with the rim (the “bead”) is incompatible with motorcycle rims.”
(Motorcycle Industry Council Tire Guide, http://www.mic.org/downloads/MIC_Tire_Guide_2012V1.pdf)

“It is unsafe to install passenger tires on motorcycle rims. Motorcycle tires and corresponding motorcycle rims (typically marked with “M/C” and/or “MT”) are designed with a different bead seat diameter and flange contour than passenger car tires/rims. Consequently, any attempt to mount a passenger car tire on a motorcycle rim may cause inflation pressure loss or the beads to break with explosive force resulting in possible serious physical injury or death.”
(Rubber Manufacturers Association, Tire Information Service Bulletin, Vol. 28, No. 3, 2012.)

“The use of passenger car tires is not recommended for motorcycles. Tires designed for passenger cars can be dangerous when used on motorcycles and motorcycles with sidecars, as bead seat diameters of motorcycle rims and passenger rims are different.”

“Car tires are fundamentally unsuited for use on motorcycles. Moreover, that's not just my opinion but also the opinion of every tire engineer I've ever discussed this with.”
(Mark Zimmerman, July 1, 2009 http://www.motorcyclecruiser.com/mounting-car-tires-motorcycles-shop-tal)

My personal communications with a number of riders who use car tires indicate they all recognize the obvious differences in tread profile, but most are unaware of the design differences that exist in the bead area between tires and wheels designed for motorcycles and cars. The Tire and Rim Association, Inc. (TRA) is the standardizing body for the tire, rim, valve and allied parts industry for the United States (http://www.us-tra.org/). Founded in 1903, TRA establishes and publishes detailed specifications for tires and wheel rims. The figure below shows just two of the most significant differences between motorcycle and passenger car rims based on TRA specifications.

As the figure shows, the height of the bead flange and width of the bead seat are significantly less for a motorcycle rim than for a passenger car rim. Other differences not specifically labeled in the figure include the curvature of the bead flanges and the height and shape of the bead hump. Even the diameter of the rims is slightly different. A 15” motorcycle rim is about 0.1” greater in diameter than a 15” passenger car rim. A 17” motorcycle rim is about 0.1” smaller in diameter than a 17” passenger car rim. Only 16” rims have diameter specifications within 0.01” of each other.

The next figure shows how the beads of motorcycle tires and car tires mate with a motorcycle rim. As expected, the bead of the motorcycle tire fits perfectly. The bead of the passenger car tire does not make full contact with the bead flange and does not sit flat on the bottom of the bead seat area. The tire sits up on the bead hump, thereby increasing the risk that the bead will come unseated.

There are a number of differences in the design of motorcycle and car tires that can affect the performance of the tire, but the most obvious is tread profile. Because of the difference in tread profile, a motorcycle doesn’t handle the same way with a car tire mounted on the rear. The flat tread of the car tire makes the bike want to stand back up after being leaned into a turn. More pressure on the handlebars is required to keep the bike at the proper lean angle. In addition, depending on the shape of the transition between the flat part of the tread and the sidewall, it can be more difficult to make smooth transitions in lean angle as road curvature changes.

Another handling-related issue with a car tire is that it requires noticeably more effort to hold the motorcycle upright when stopped on a road surface that slopes to the side. If the road surface is dropping to the right, the motorcycle wants to fall to the right. The “U” profile of a conventional motorcycle tire prevents this from happening.

Added to these handling issues is the possibility of an accident being caused by the tire suddenly slipping out of the bead seat in the middle of a turn as a result of the incompatible tire and rim profiles illustrated in the second figure.

For the reasons described above, the use of car tires on motorcycles appears to be an incredibly bad idea.

The Arguments For the Use Of Car Tires

Arguments in support of using car tires on motorcycles can be found on a number of internet forums. For example, there is special section of a popular forum for Gold Wing riders at

More generic forums dedicated to riders’ experience with car tires can be found at

http://mcdarksiders.forumotion.com/ and http://forums.delphiforums.com/DarkSiding/start.

On these forums, and as noted in the introductory paragraph to this article, the benefits claimed by those who use car tires are primarily longer tread life and lower tire cost. Indeed, longer tread wear is generally available with a car tire that costs about half of what an OEM tire costs. For example, a 180/60R16 Dunlop Elite 3 for a Gold Wing costs about $200 mail order while a 195/55R16 car tire that is almost exactly the same diameter can be purchased for less than $100 and will usually provide longer tread life. Depending on the particular OEM motorcycle tire, certain car tires are reported to provide about double the tread life. Others report more modest gains, but some riders report achieving four times the tread life with a car tire.

In spite of the lower cost and longer tread life, car tires would still seem to be a poor choice if they compromise handling and safety to the extent that the critics claim. The reason so many riders use car tires in spite of the warnings is that there is a lot of anecdotal evidence that certain car tires perform fairly well on certain motorcycles. Testimonials on the above-referenced forums are one source of such data. Most forum members report relatively minor changes in handling that they find easy to get used to. Based on what is reported on the forums, the frequency of tire failures is no greater than is reported for motorcycle tires.

Although many forum participants are vociferous in their support of car tires, their arguments are based on personal experience rather than from a full range of rigorous engineering tests subjecting the tire/rim combination to virtually all conditions expected in actual use. These posters are also quick to point out that most people who are critical of the use of car tires are basing their opinions on “theoretical” concerns rather than hard data.

As an automotive engineer, I don’t have a problem with making decisions based on “theoretical” concerns. I can appreciate that the use of SAE sockets on metric nuts is generally a bad idea. However, I also recognize that there are situations in which an SAE socket will work with a metric nut even though it wasn’t designed to do so. Likewise, I’m prepared to be convinced that a tire and rim that weren’t designed to be used together might actually work. The problem is that the tire/rim compatibility question is enormously more complicated than the socket/nut compatibility question. There will never be the rigorous laboratory and track testing done for car tire/motorcycle rim combinations that is done for motorcycle tire/motorcycle rim combinations because no motorcycle or tire manufacturer has the incentive to do the testing. All we have to go on is the personal experience reported by riders who have tried various tire/rim combinations.

In addition to the anecdotal data available on internet forums, I personally know several long-distance riders who report only minor handling issues with car tires and zero tire failures over tens of thousands of miles. In contrast, I know a long-distance rider who experienced a catastrophic failure of an OEM motorcycle rear tire on a Gold Wing while riding 2-up in hot weather.

My own experience with car tires is directly related to the catastrophic failure of an OEM tire on a Gold Wing mentioned above. The tire temperatures being reported by the SmarTire monitoring system installed on my own Gold Wing showed that I was at risk of a similar failure with a motorcycle tire specifically designed for the bike. Running 80 mph during 100°F ambient temperature, I was getting high temperature warnings from the SmarTire system and I was seeing rear tire temperature readings in excess of 190°F. Based on the personal experience of a fellow long-distance rider, I decided to try a Kumho 195/55R16 ECSTA SPT XRP (a “Run Flat” car tire with very stiff sidewalls).

After switching to the car tire, my rear tire temperatures dropped 20-25°F, back into the safe range. Handling of the bike was compromised in that more pressure on the handlebars was required to turn and the transition between lean angles as road curvature changes wasn’t quite as smooth. However, it didn’t feel much different than when riding on a worn motorcycle tire that has become flat in the center of the tread.

One of the reasons the Kumho 195/55R16 Run Flat provides reasonable handling is that the edge of the tread is rounded, keeping the contact patch off of the sidewall at a lean angle of more than 30°. As can be seen in this YouTube video qoNDo7o1d6Q :

the transition from upright to leaned-over is fairly smooth. The screen shot from the video above shows that there seems to be a reasonably sized contact patch with a lean angle of more than 30°.

In spite of the lack of problems many riders report with various car tire/motorcycle rim combinations, the fundamental incompatibility between the design of a car tire bead and the bead area of a motorcycle rim is an obvious concern, at least to me. Many riders don’t worry about this issue because, as the old saying goes, ignorance is bliss. However, based on my own experience and the experience of other long-distance riders, I believe I am reasonably safe running a Kumho 195/55R16 Run Flat on my Gold Wing, especially considering the lower tire temperature that it gives me during hot weather. Although I know the tire bead doesn’t fit the rim the way it should, it appears to be good enough. Under pressure, the car tire bead is staying in place.

A described above, my personal experience with car tires on motorcycles is limited to one particular combination. For that combination, my experience is quite similar to what is reported on the Internet forums mentioned above. A list of other motorcycle model/passenger car tire combinations that have been “successfully” used is available at http://darkside.nwff.info/?p=tires. Note, however, that there does not appear to be any kind of screening process for adding combinations to this list.

Installation Issues

If you don’t have your own tire mounting machine, it can be difficult to find a shop to mount a car tire on a motorcycle rim. Many dealers don’t want the liability associated with mounting a tire that is clearly not approved for use on a motorcycle. However, a list of dealers reportedly willing to install car tires on motorcycle rims is available at http://darkside.nwff.info/?p=installersList .

If you do have your own tire mounting machine, mounting a car tire can be significantly more difficult than mounting a motorcycle tire. Because of the stiff sidewall, mounting a Run Flat tire can be particularly difficult. The Kumho 195/55R16 is much harder to mount on a Gold Wing rim than the Dunlop Elite 3, which is hard enough.

So Which Arguments Prevail?

The arguments against the use of car tires on motorcycles are more compelling than the arguments for their use. The positive results reported by most participants in Dark Side forums have to be tempered by the fact that preconceptions about the effect of a tire change are affecting the results they report. Most people who believe a particular change will be beneficial will routinely report a benefit even when they are evaluating a placebo. That’s a scientifically proven fact.

Even taking the reports on Dark Side forums at face value, handling is compromised when using a car tire on a motorcycle. In addition, there are safety risks associated with the fact that passenger car tires do not mate properly with motorcycle rims. In the case of the Gold Wing, however, using a specific passenger car tire has been shown to reduce tire temperatures from the unsafe levels that can occur using the OEM motorcycle tire at high speed during high ambient temperatures. For this reason, combined with the lack of data indicating a greater rate of tire failures, riders are currently allowed to use car tires in the Iron Butt Rally, although the IBR doesn’t recommend their use. IBR Rule II.B.2.f only requires that motorcycles shall be equipped with tires meeting or exceeding OEM load rating specifications and that any motorcycle equipped with a “run flat” type of tire must be equipped with a functioning tire pressure monitoring system that will provide the rider a visual indication of low pressure when the motorcycle is being ridden during daylight hours and at night. That provision covers the situation where a Run Flat car tire loses air pressure that could lead to overheating and catastrophic failure (because the rider is unable to detect the loss in air pressure).

Despite my successful personal experience, I think car tires are best avoided. Because of the fundamental incompatibility between car tires and motorcycle rims, I would never experiment with a tire/rim combination that hasn’t been successfully used by a number of other people that I know to be aggressive riders. Because of liability concerns, I would never recommend that someone switch from a motorcycle tire to a car tire even if it were a tire that worked for me. By mounting the tire myself, I know that the tire bead was not damaged during the install, but I also know the risk of bead damage is higher. Someone else installing the same tire that worked for me could damage the tire during mounting and that damage could contribute to a catastrophic failure.

At the next tire change on my Gold Wing, I’m going back to a motorcycle tire. I plan to try the latest version of the 180/60R16 Dunlop Elite 3. Since I last used this tire, Dunlop has changed to what they call “Multi-Tread,” with a harder compound at the center of the tread. Combined with this tire’s 992-pound load rating, I expect the harder compound to experience lower temperatures. If my rear tire temperature problem is solved, I won’t be going back to the Dark Side. If it isn’t solved, I may just slow down a bit during really hot weather.

Tom Austin

For a highly technical article on car tire use on motorcycles, visit:
Scroll down to: Darkside Tech: Using Car Tires on a Motorcycle Rim - this is an extremely long technical article with numerous diagrams and interesting photos highlighting the difference in motorcycle tires vs car tires. If you are not into the tech, it is worth scrolling to the end of the article to view cutaways of car and motorcycle tires and rims. Mike


Even though car tires are currently allowed in the Iron Butt Rally, it should be noted that a car tire isn’t necessary to complete the rally without a tire change. There are conventional motorcycle tires capable of going the distance. Due to the way it was designed, the 2011 Iron Butt Rally enabled the highest mileages ever. In that rally, Jim Frens set a new record by running 14,185 miles on the radial Metzeler ME880s he ran on his Gold Wing. In that same rally, second place finisher Eric Jewell logged 13,162 miles running Michelin Pilot Roads on his Honda ST1300. 2011 Iron Butt Rally winner Peter Behm also ran the entire rally (13,544 miles) on one set of Pilot Roads on his Yamaha FJR1300.
Not open for further replies.