No, this has nothing to do with Star Wars, or that TV horror series from the 1980s. The “Darkside” is a term that has come to be used for the practice of fitting car tyres to motorcycles. If you haven’t heard about this already you probably soon will, because it's a practice that is growing in popularity. Why would people do that? Well, the obvious answer is cost. As you no doubt have noticed, motorcycles tyres are very expensive! My current rear tyre (admittedly a reasonably up-market thing) cost about the same as my current set of (admittedly reasonable-quality budget brand) tyres for the family car. So there is obviously a huge saving to be had!
But would it work? Cars are very different to bikes and the tyres for cars are very different to the tyres for bikes! So I would’ve said no; putting a car tyre on a bike would be stupid! But there enough people doing it now to indicate that maybe it’s not so stupid after all. A very experienced rider (who is far from being stupid!) who is the chairman of the Christian Motorcyclist’s Association of NSW, Ian Dodd, has just fitted one to his Suzuki cruiser. And he kindly sent me this story of his journey from sceptic to believer. But before getting to Ian’s story, let’s look at some of the arguments for and against this.
Now, a couple of points need clarifying here. Firstly, we’re talking about rear tyres, not front ones. The other thing is that we’re talking about cruisers, not sports-bikes. I don’t think anyone has seriously suggested putting a car tyre on the back of an R1 and then taking it for a few laps of Phillip Island!
There’s been much writing on this, both for and against, but I’ll mention just a couple of main points.

The most obvious thing is the different profile. Bike tyres are rounded, while car tyres are flat. If you’ve ridden on a worn back tyre you’ll know the potential problem here. Tyres tend to wear flat in the middle because that’s where the tyre spends most of its time – while the bike is upright. It wears flat and tends to have a shoulder before the curve down the sides. If you’ve ridden on a tyre that has worn like this (I think most of us would have!), then you’ll know how this affects the bike’s handling. The back kind of climbs up on the shoulder and makes the bike feel stiff and reluctant to turn. It can also affect stability. On my current bike, when a previous tyre became well worn it produced a wobble at the bars at a certain speed while decelerating. So a car tyre, which is an extreme case of being flat in the middle and having a shoulder, would be terrible!
Traction – or lack thereof – is another problem. The theory here is that there is less down-force on the tyre. Think about it; the weight of the bike is being distributed over a wider contact-patch, so the down-force is less. And less down-force means less traction, allowing more chance of wheelspin.

Those in favour of using car tyres claim that, as the bike leans, the tyre distorts, creating a flat section that results in a reasonable-sized contact-patch with the road.
The diagram on the left shows how this is supposed to work; the arrows obviously pointing to the flat contact-patch section. Good in theory, but does it work? Well, the photo at the top next to it seems to indicate that it does. I don’t know what type of bike this is, but you can see the tyre having flattened out on the edge. In the image below that you’ll see a comparison between a bike tyre and car tyre. The light-red represents an area of firm contact, with maximum down-force, while the dark-red (on the car tyre) is where the down-force is less because of the tyre being unnaturally distorted in that area.
Okay, but does it work? Well, let’s get to Ian’s story.

So, do you ride less than 10,000km a year, appreciate the finer handling aspects of your machine, corner aggressively, and only ride the open road? If the answer to any of these is “yes” then this may not be the article for you! “Darksiding” is the practice of fitting a car tyre to your motorcycle and in my opinion is only done by penny-pinching misers like me. Sure, there are those that will tell you they do it for the handling, the smoothness of the ride, the reliability of the tyre – but at the end of the day I can bet it was a cost-based decision. So let’s look at the journey so far, where I am at and how I got there.

The Early Days:
For the second time that year I had to put a rear tyre on the Boulevard, another $300 that I couldn’t really afford at that time – this was getting more expensive than the car. I started researching via the forums what tyres others were using, and what their longevity was. Imagine my surprise when I found out I was one of the luckier ones and just how much poorer I’d be if I was replacing them at their intervals. Reading on I discovered that there were some folks who, to my horror, were using car tyres. Never work I thought, the profile’s all wrong, the compounds aren’t soft enough etc, etc. The more I read the more I thought to myself – mad Americans (because it was mainly them that were doing it), get real and don’t put yourself at risk. But slowly over time I saw that there were no reports of mishaps, no sliding out on the bends and I couldn’t find anyone that had fitted a car tyre only to rip it straight back off or even return to using a bike tyre.
I spoke to fellow riders (I was probably one of them! Elwyn) to canvass their thoughts and ideas – they nearly all said don’t do it but they were coming from the same closed minded opinion I originally had. I listened and assessed what they had to say and nearly closed the book on the whole concept. In the meantime the Boulevard had started to show signs of having been ridden too many k’s and was replaced with a ’99 VL1500 Intruder. Can I just add here, I am extremely grateful for those motorcyclists who prefer owning to riding, ensuring that people like me end up with a bike that has less than 1000km per year clocked up on it!

The Here & Now:
Last Tuesday I tried for a pink slip – the mechanic said my bald head had more tread than my rear tyre so come back after you get a new one fitted. The next day I rang around and on Thursday trotted up to the bike shop with a Kuhmo on the back. The young bloke was scornful, “handle like *&^% you know, you won’t be able to corner, you’ll regret it”. I asked if he knew anyone that had done this? Silence tells a lot and off he went to start the job. That afternoon the rain and I arrived at his shop together; so here am I about to hop on a whole new feel and the road is getting slippery. I cautiously rode the 10km and waited ‘til the weekend.
Saturday morning finally got here – fine, dry roads ... if only! What better introduction to the darkside than wet, grimy, diesel-tainted roads! And here I was heading up Bells Line of Road and out past Orange.
I was somewhat surprised at the positive feedback I felt when applying throttle – power transfer to the rear wheel seemed more efficient, and obviously it was as I ended up 2.5km / l better off. Not once did the wheel spin taking off from the lights – not even on the white lines. Hmm, I am beginning to enjoy this! Wonder what the bends will be like?
Street / Slow Cornering – okay, this is where I had issues, understeering fairly often until I changed my style.
Fast Cornering – just a little more counter-steer to tip in, hold the angle and scoot around the bend.
Open Road – wow! Smooth, steady, great!
The day did fine up once I got over the Blue Mountains and headed out towards Orange, and after 300km I felt very comfortable and had been cornering at the same speeds as when I had a bike tyre on the rear. The next day was the return journey – dry but very windy. Little bit of understeer until I re-trained myself then back to normal cornering. Hit the base of the mountains and the rain set in, 35kph hairpins all good, ride stuck well across water, mud, gravel etc on the road.
Since then I have started my weekday commuting routine and as of day 2 had forgotten that I had a car tyre and was cornering, doing roundabouts etc the same as before.

The Pillions Perspective:
Hmmm – not much to report on here as it has only been one trip with a passenger but they did feel the ride was smoother and they were bounced out of the seat less.

The Final Word:
So it appears that the darkside is gaining another devotee – would I recommend it for you? No – this is a decision you have to reach on your own and it is based on your experience, bike, riding style and your insurer, yes some firms will renege on a claim if they spot a car tyre on the bike. Do your research, ask questions, wait a while, redo your research and then make up your own mind. The next hurdle you will face is actually finding someone willing to fit the tyre.

Thanks for the great story and evaluation, Ian. Certainly something to consider! Maybe not for a sportsbike, but for cruisers, well, Ian’s experience over a wide range of riding conditions shows that it might well be worth considering.
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