When Wide Is Too Wide

Rim widths have sure changed a lot in the past ten years. I've noted this in several earlier articles on this site. A good historical reference point can be seen HERE for instance. 

That article was written exactly six years ago now. So, even since 2020 things have radically changed in terms of what people are thinking about for inner rim widths. I'm speaking to what gravel riders use here. So, while references to mountain biking will be made, I am primarily thinking about gravel/all-roads riding. 

Why Is This Important?

The most influential facet of a bicycle ride - besides the all-important rider - is the wheel. Wheels have obvious influences over your riding experience, but I want to focus in on how inner rim width works in concert with tire volume and width. 

What happens with this relationship can greatly affect your comfort, efficiency, and overall experience on a bicycle. Take a look above at the image above. I was trying to visually show what happens to a tire when the beads of the tire are set further apart by the inner rim width when it is arguably too wide for the tire being mounted on that rim. In this example I have a 700 x 42mm WTB Resolute mounted on a Spank rim with an inner width of 30.5mm. 

The 30.5mm inner width with 2.8" tires

The profile of the Resolute gets flattened out. This might seem to be a good thing on the surface of it. However; what happens in reality is that the more tire you get to hit the surface laterally makes rolling resistance increase. Furthermore; if the casing gets too squared off you will start to notice a self-steering effect which is not desirable. This increases the rider's efforts to steer the bicycle through corners and in difficult terrain. 

I allude to some of this in my 2021 article on the Spank rims and those Resolute tires HERE. What I did not show there, or talk about much, is how having the beads of the tire set too widely apart from each other affects ride quality. 

There is an optimal shape tire manufacturers design to which allows what they have designed for to be fully expressed during a ride. These qualities are many. What we feel most is influenced by how the tire casing works against the surface you are riding on and the results of the steering/riding inputs the rider exerts on those tires. 

What happens when the beads of a tire are too far apart usually affects how well the tire casing can conform to the riding surface. Generally speaking, a tire sidewall should bow outward, not being a straightened out line from surface to rim bead seat. The straighter the tire sidewall gets the worse that tire works at doing its job of conforming to the surface. It affects negatively how the tire responds to rider input. Combine this with a flattened out tread area and the problems get more magnified. 

Of course, I am describing a worst case scenario, but this happens incrementally when we go from 25mm wide internal widths to anything around 30mm with gravel tires in the 45mm - 50mm range, which is arguably the sweet spot for tire width in gravel now.  Most gravel racing rims are not 30mm wide internally either, so why the worry?

Image courtesy of Continental Tire

 Aero vs Comfort & Handling:

The problem is aero related and how an aero focus in racing influences what you and I get to ride.  Most road cyclists who follow Pro racing know that aero is everything these days. Teams spend gobs of cash trying to find every advantage in aero to make their riders more efficient and faster. Of course, this has spilled over to gravel racing as well. 

Pro gravel riders are pushing the limits on tire widths. We are seeing wider than 2" tires being used in some cases. Aerodynamics dictate that a smoother airflow is achieved when a tire side wall seamlessly transitions off of the rim side wall on a deep section rim. Just like you see on skinnier road racing aero set ups. This affects choices in rim width.

The accompanying image here is from Continental showing the 700 x 50mm Dubnital on a wide, deep section rim. You can see the smoother transition from tire to rim clearly. This is all driven by aero concerns. 

Of course, this causes the tire to have some undesirable effects, as outlined above. In every gain in one area you often get compromise in another area. Racing isn't about comfort, at least not in a top-of-mind design sense. It matters, but these days, aero is king. 

I feel the wider tire thing is going to pull back a little bit from the excesses we see now. I also feel that rider comfort and handling will start to become more important in gravel racing as courses world-wide start to reflect a more MTB-XC feel than a true gravel road type experience. This is because, well, what I see most being called "gravel" isn't gravel at all. 

But this is a subject for another article. 

For now I think it is fair to say that the outlandishly wide inner rim widths and hyper-focus on aero for gravel will relax in favor of better handling and rider comfort leading to faster racing in the long run. I could be wrong, but this is my opinion. Got any thoughts? Hit me up in the comments section.