TECH: RIM WIDTH
WORDS AND PHOTOS BY CHAD DE ALVA
STABILITY VERSUS PERFORMANCE EXPLAINED
Street bikes and dirt bikes have totally different designs that reflect the two very different types of riding these bikes are made for. Adventure bikes bridge the gap between dirt and street so their design has to enable the bike to do two completely different things, which is why many riders consider adventure bikes an exercise in compromise. Adventure bike manufactures must make their bikes safe to use on the road, so when it comes to choosing the width of the stock rims, manufactures lean toward rim widths that provide on-road stability and performance. Riders who want to make their adventure bikes perform better off-road will either change the rim widths of their stock wheelset, or they will pick up an off-road specific wheel set. Narrower rims on an adventure bike can provide serious performance benefits in certain areas, but they are not without their drawbacks. To understand how rim width impacts adventure bike performance, I reached out to bike manufactures, wheel manufacturers, tire manufactures, and very experienced ADV riders to compile all of their knowledge and experience into one article. Let’s geek out.
It’s imperative to consume this information with an open mind, which is a polite way to say that we all need to check our egos so we can take a critical look at what makes the most sense for ourselves as unique riders. Running an off-road wheelset and dirt bike knobby tires on your ADV bike will look cool, but if you never ever actually ride in the dirt, your wheel and tire choice is costing you serious performance on-road. So set your bike up to perform best for how you ride – not what looks good on the ‘Gram. You will be happier in the long run if you can be honest with yourself when it comes to making wheel and tire choices.
Tires and wheels work together as a system, and there are limits as to what size tire can safely be installed on a given rim width. So no, you shouldn’t try to install your favorite 120/100-18 dirt bike knobby tire on the stock rear wheel of your KTM adventure bike which measures 4.5-inches wide. If you want to run a 120 rear on your ADV bike, you need to have the correct rim width, and a popular ADV rim width that works with 120-sized tires is a 2.5-inch wide rear rim. Narrower rims open the door to more aggressive off-road tire options, make for a quicker handling bike, and narrower rims are better protected from impacts as we will see in a minute. Yet, you can’t get something for nothing, and the trade off for more off-road performance with aggressive tires is a bike that can be scary on the pavement. Narrow rims and off-road tires can make a bike squirrelly (compared to stock), and this wheel and tire combination will provide less grip on-road than wider rims with on-road / off-road (not dirt bike knobby) tires would.
Conversely, the stock width KTM adventure wheels work well on road, and they can certainly be used off-road, but they’re easy to damage off-road if you aren’t careful. Their width means that the rims are more exposed, so when you smack into a rock there is less tire available to absorb that impact which means more force from the impact is transferred to the rim – and that’s how flat spots or other rim damage happens.
To put this in perspective, let’s take a look at how rim width changes the profile of a tire. I mounted a Dunlop D908RR in size 150/70-18 on three rims that cover the range of applicable rims, as per the Dunlop website. I used a 2.15-inch, 3.5-inch, and a 4.5-inch rim. Look closely at how the height, width, and overall profile of the tire changes as a function of the rim it is on. The tire’s overall width changes by 0.625-inches going from the 2.15-inch rim to the 4.5-inch rim. Yet the change in tire profile is more telling than overall width. On the 4.5-inch rim, the tire’s sidewall is the widest point of the tire. Now look at the 2.15-inch rim where the tread blocks are the widest point on the tire. When measured to the centerline of the tire, the profile is more than a quarter inch taller on the 2.15-inch and 3.5-inch rims. Note that the 3.5-inch rim creates nearly the same tread profile as the 2.15, while the increase in rim width will lend to better sidewall support and provide better high grip (on-road) cornering performance.
Now let’s take a look at a front tire where we can see how different rim widths are much more exposed or protected when the same size tire is mounted. The photo of the two cross sections uses the same 90/90-21 tire mounted on a 1.6-inch and a 2.5-inch front rim. For the record, this isn’t a perfect analogue of the tire profile as the tire cross section isn’t inflated, but it still gets the point across. Notice how much more tucked under the tire and protected the 1.6-inch rim is when compared to the 2.5-inch rim. Consider smashing both of these cross-sections into a square edged rock ledge – the 2.5 inch rim would be more likely to suffer damage from the impact. Now if we consider which tire would be less likely to deform under cornering loads, the 2.5-inch rim will perform better in high speed, high grip cornering situations. The point here is that rim width comes with trade-offs, so you need to take a realistic look at what is going to make the most sense for how you like to ride. Thankfully, there is a robust wheel aftermarket, and companies like Dubya USA can help you determine what rim widths make the most sense for how you like to ride.
There are two main paths to choose from when it comes to adventure bike rims. You can have your stock wheels re-laced with new rims, or you can pick up a second wheelset. Either way, you’re going to gain performance and durability, so the question is what option makes the most sense? A complete wheelset does cost more than replacing rims, but it adds a serious amount of function to your ADV bike in that you can have an off-road wheel set and a more road-focused wheelset ready to go.
This is a great option for riders who want maximum off-road performance when they’re doing off-road rides, but also want the bike to perform on road with a quick wheelset change. Re-lacing the stock wheels with aftermarket rims will give you a much stronger, more rigid wheel than stock. Dubya can lace up almost any rim width you want to the stock hubs, so if you’ve smashed your stock rims or are ready to unlock more tire options, this can be a great way to go.
Adventure bikes are a compromise. But they’re a compromise that happens to be a ton of fun to ride both in the dirt and on the street. I’ll tell you right now, doing only one type of riding with an ADV bike is leaving fun on the table. As adventure bike riders, we all ride our bikes in different ways, so there is no such thing as the best rim width for every ADV bike rider out there. The way to find the best rim widths for you as a unique rider is to be honest with yourself when it comes time to pick up new rims or wheels. The guys at Dubya USA are super knowledgeable, and no matter how you ride or where you ride, they can help you determine what rims will support the type of riding you like to do best. From riders who avoid the street unless there is no other choice, to 50/50 riders, and riders who are just starting to dabble in the dirt, Dubya can set you up with rims or complete wheels that will help make your bike perform the best for how you like to use it. Adventure bikes are so much fun for so many different types of riding, so make rim width choices that will help you enjoy your bike, because that is ultimately what this whole thing is about.
For more information on rims or wheels, please reach out to us directly. We are now a DUBYA USA dealer and can help you get the wheels that will best suit your needs. UPSHIFT STORE - Wheels
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This story was originally published in Issue 64
