Pete's Place

IronButt Association rides, reports, and product evaluations.

Wednesday, October 1, 2014

Darkside on the Triumph Trophy

UPDATE 05/19/2020
My conclusions below were BEFORE I got the right tire. I subsequently ended up putting over 40,000 miles on the CT and had no regrets at all. I rode many rallies including the '15 Iron Butt Rally. The right tire for me was a Vredestein Sportrac3. It had a symmetrical tread pattern and nice rounded corners. I ran it at about 25PSI. Tire life was 18,000 to 20,000. I did have make my center stand a bit wider to clear the tire.

Darkside is the term used to describe putting a car tire (CT) on a motorcycle.

This has always had some appeal to me since a large part of my travels are on highways and the savings can be significant. Based on user experiences car tire life on a motorcycle will significantly exceed that of a motorcycle tire, with reports of mileage in excess of 25,000. Consider that on my K1200LT, the best mileage I could get was around 12,000 miles. Since I put over 160,000 miles on this bike I estimate that I purchased at least 15 tires at an average cost of $200 for a total of $3000. Could I have run car tires, I would have expected my tire cost to be less than $1000.

The controversy over running a CT on a bike is one of the most fervent discussions you can find on the motorcycle forums. The naysayers are convinced that putting a CT on a bike will result in immediate death and/or dismemberment of the rider. The proponents are equally rabid in their assertion that CTs are the best thing since sliced bread. I have researched both sides and have yet to find any hard science from either side.

On the naysayer side, there is a lengthy tome on the Goldwing forum  that is chock full of scientific data. But, as I gave it a critical read, I found that it was sorely lacking in "information". In my 30 years of experience as a technical writer, I have learned to recognize when a writer is cutting and pasting content with little to no understanding of said content. This is very apparent in this write up. There was nothing but opinion to substantiate the purported risks of putting a CT on a bike.

I was cautioned about the following but was not able to confirm any of the following:
  • crashes (regretfully, accident reports don't include any information as to the tire type)
  • insurance cancellation 
  • ticketing for not having the correct tire
  • voiding the warranty (this might be possible if your dealer is a jerk but nobody reported this happening)
On the proponent side, there is no scientific data that I could find, just anecdotal information about CT experiences. While the lack of scientific data appears damning, the experience can't be discounted. There have been thousands of riders, racking up millions of miles, over many decadewebsites, with no evidence of negative consequences, let alone dire consequences. The Darkside web is an excellent resource.

 I determined that the only way I would get an answer that would satisfy my curiosity would be to conduct my own trials.

When I acquired the Triumph Trophy SE, I began looking into putting a car tire on it. I acquired a spare rear wheel so I could easily swap things around. I'd had the bike for over a year before I finally got around to tackling this project. Following are my trials and observations.

First off I had to find a tire that would fit. The stock MC tire is a 190/60 on a 17 inch wheel. I needed to determine the optimal tire size and found a Tire Size Calculator to help me out. There was very little excess space between the stock tire and the swing arm but with careful measurements I determined that a 205 width tire would probably fit. Searching for a specific tire size is virtually impossible, I finally located a site that was immensely helpful, allow you to find tires using specific criteria

The 17" wheel proved to be extremely problematic. In the North American market there are very few tires options in this size wheel. This was the largest diameter tire I could find in a 205 width, which would change my speedometer and gearing as follows:

While I would have preferred a narrower tire the only tires available tire reduced the tire diameter so much I was concerned it would make it too hard to get onto the center stand. 

Finding a shop that will mount a CT on a motorcycle wheel was the next challenge. After stopping at several places I finally found a tire store that would take on the job AND they had used tires. This was great! I'd be able to verify that a 205/55 would fit since they would take a return. They didn't have a 205/55 so I took the next closest match,  a NEXEN 205/50. I was out the door with a good used tire, mounted and balanced for $30. I started out with 40 psi and, when I mentioned that it was pretty rough some fellow darksiders said that most folks ran low pressure. I dropped it to 32 psi which helped considerably.

I had seen videos of CTs on bikes but none had a good comparison. My first experiment was to shoot some video to see what was going on with this tire. Here are two videos, the first is a side by side comparison, the second is a side view under mid and high speed turns.

Following are my observations and opinions of putting a CT on this bike.
I have ridden it about 5000 miles over a wide variety of roads and road conditions.


  • I put about 5000 miles on this tire and it showed very little wear. I didn't measure tread depth before I started but comparing my before and after photos it looks like 1 to 2/32s of wear. I can certainly see a new tire going in excess of 20,000 miles.
  • Once speeds exceed 30mph there is almost no difference in handling beyond the extra effort required for turns (see con's for lower speed behavior).
  • Long stretches of straight road are easier to handle.
  • Gravel/dirt roads are less stressful since the tire provides more stability.
  • Rain was less stressful since there is more tire on the road.
  • On a 2500 mile round trip consisting of mostly interstate and primary highways I was very pleased with the CT. I didn't have any problem navigating twisty mountain highways. (see cons for related opinion). Tire pressure on this trip was 32 PSI


  • The ride of the CT seems to be harsher (you can see this in the first video to some degree)
  • It is more difficult to initiate a turn. 
  • Once in the turn it requires more effort to maintain the turn since the tire is trying to go back to the flat.
  • Low speed maneuvers can be frightening since the tire wants to stay flat and continue going straight.
    At low speed any variation in the pavement can cause the bike to lean. As a radical example consider a pavement transition from low to high, if this is encountered at an angle the CT will tip to the low side of the angle. This can cause the bike to tip over if you are unprepared for this behavior. I almost lost is as I was backing up to a curb (California law on motorcycle parking). When the tire touched the curb, the bike lurched, and I barely was able to hold it up. 
  • At highway speeds it requires more effort to maintain a track and requires experience to compensate for this behavior. If you are drifting out of your desired track you must immediately apply enough force to regain the track or you might find yourself in the wrong lane (as I did on more than one occasion).
  • On a rally in northern California last month I rode over at 12 hour period I navigated 350 miles of extremely technical mountain highways. That 30mph average should give you an idea of the complexity of these roads. By the end of the day I absolutely hated the CT and regretted that I had decided to use it on the rally. The extra effort required for turning becomes quite tiresome and can certainly raise your fatigue level beyond what most long distance riders would want. Tire pressure had been raised on this rally to 35 PSI. 


 I think that the folks that love the CT on their bikes are very conservative riders and are likely on a cruiser or heavy touring bike. The CT is NOT a sport bike tire, the handling characteristics are quite a bit different and take some time to get used to. While I was able to maintain my spirited canyon carving riding style (dragging the footpegs), the additional effort required to initiate and maintain a turn pretty much spoils the fun we expect from a sport bike.

If I had to pick one or the other I'd stay with the motorcycle tire. Thankfully, I have two wheels so I can swap. 

If I can predict that the riding will be similar to my first trip I would definitely mount the CT. The performance under non-sport conditions is perfectly acceptable and the improved tire life and dirt/gravel handling more than offset the negatives.

BUT, if the road conditions and riding are anything like what I encountered on the rally, NO WAY! There were not nearly enough pro's to offset the fatigue level.
Bottom line is that if I didn't have the spare wheel the CT would be history.

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