Darkside: Using a car tire on the rear wheel of a motorcycle.
Double Darkside: Using a car tire on the rear wheel of a motorcycle and a rear motorcycle tire on the front wheel of the motorcycle.
Among long distance motorcyclists, Darksiding is one topic sure to make riders take sides for and against, with little hope of reconciling in the middle. Yet some riders have run a car tire on a motorcycle successfully for tens, if not hundreds of thousands of miles.
Why do it? because car tires are generally much cheaper than motorcycle tires and the car tires last a LOT longer, especially on the rear wheel of the motorcycle. I go through a rear tire on the ST1300 about every 12,000 miles and a front tire about every 20,000 miles. Texas roads will eat my rear tire in less than 10,000 miles. At $200.00 per rear tire, that gets expensive. I changed out 5 tires in 2019 and 30,000 miles.
It seems counterintuitive that a car tire would work on a motorcycle, but it can. Tire choice is important. Motorcycle rims are generally much narrower than car tire rims, so you end up spooning a wider tire on a narrower rim, which can be difficult. Some car tires have more rounded edges, especially sport tires, which makes them sticky in turns. I’m told it takes a couple thousand miles to get used to the somewhat different feel, and while really aggressive cornering on a mountain road is probably best left to a sport bike tire, a car tire can be pushed quite hard. Mileage is the main advantage. One can get 20 to 30 thousand miles out of a car tire rear tire.
Hancook Ventus V2 car tires are a popular choice for some bikes. My motorcycle rear tire size is a 170/60R17. There are no car tire choices that size. The nearest is a 205/50R17, a much wider tire. But, with the rounded edges and the narrower rim, the tire ends up fairly rounded, especially at the edges when mounted on the motorcycle rim. It’s not easy getting it on the rim with tire irons, but I managed with just a little extra effort, four tire irons, a custom 2×4 frame, and a couple of bungee cords.
I now have the tire mounted and installed on the bike. Unfortunately, it rubs a bit on the right side of the swingarm. There seems to be some casting differences from year to year on the ST1300 and 2007 may have some extra material at the top of the swingarm where the casting seam is.
I have taken about 3mm of material off the right side swingarm with a flap disc on a grinder. Comes off fast, so watch what you are doing. The tire no longer rubs.
Unfortunately, a snowstorm blew in as I was putting the bike back together, so ride update coming…
April 2021 update:
In 5 months I have ridden 8000 miles on the darkside, Hankook V2 Ventus 205/50ZR17 car tire. It’s taken longer than I anticipated getting used to the differences between a car tire and a motorcycle tire. A lot of it is simply building up trust that the tire isn’t going to slip.
I have tried many different tire pressures. Online suggestions go from a fairly low 25psi to a high of about 40psi. I found the lower pressures to be too bouncy, giving a ‘squishy’ feeling when cornering. Once, the rear end bounced off the ground while turning when I hit a bump. It tracked well, but was rather surprising. Experimenting, I have found that 32psi works pretty well for me with 42psi on the front motorcycle tire. I’m running a Michelin PR4 GT on the front.
I can see no discernable wear in 8000 miles. I have not noticed a change in mileage because of the rear tire. Side to side transitions are just as quick as the motorcycle tire. Emergency maneuvers are easy and solid. You do have to keep a bit of pressure on the bar to sustain a turn – something you do not have to do on this bike with motorcycle tires both ends – but it’s a minor inconvenience. Braking is solid. For the most part transitions are smooth. Sometimes I can feel a bit of a ‘ridge’ when turning at slow speeds. I’ve learned to trust the tire and just push over it. The only time it feels unsettled is when you hit a ridge or hole in the road surface just as you are transitioning that ‘ridge’. I haven’t run in the rain yet, so no opinions on that.
I have run several miles of dirt. On dirt, the front seems to run lighter and wander more than when I had a motorcycle tire on the rear. However the rear Hankook seems planted and has good grip. I haven’t run in deep sand yet as I try to avoid that on a 700 pound motorcycle. Grooved pavement causes the bike to wander more than with a motorcycle tire front and rear.
Overall, I’m happy with my choice. I have purchased a second Hankook to install just before the Iron Butt Rally, regardless of how much tread is left on this one. I will also run a new Michelin PR4GT on the front for the rally.