Why do our rims have 21mm and 25mm inner widths?

Our philosophy is that when designing a product, it is important to follow what works, rather than what is trending. One of the latest trends has been an ever-changing rim internal rim width. It seems like every couple of years, internal rim widths have been increasing. In this article, we will explain a bit about rim and tire widths and why we chose 21mm and 25mm internal rim widths for our road and gravel wheels.

What is the point of wider rims?

Many years ago, both road bike and MTB internal rim widths were significantly narrower. At the same time, so were the tires that were trending. In the case of road bike wheels, rim brake bikes were the majority. Rim brake calipers in general do not support wide tires and rims. V brakes were the exception, but they are typically only found on lower-end bikes. MTB wheels also switched do disc brakes, but at a much earlier time.

A note about compatibility

If one wants to use a wider tire, it makes sense to choose a wider rim within reason. There are of course exceptions, as some tire-rim combinations are not ideal and actually unsafe. At Particle, we like to stay on the safe side and strictly follow the recommendations of the ETRTO. While some may recommend rim-tire combinations that fall out of this range, we do not recommend such combinations due to safety concerns and performance losses.

Why we choose 21 and 25mm internal widths

Are 23mm internal rim width wheelsets better? 

One of the most common questions we get is "Why don't your road rims have 23mm inner widths?" This is actually a really good question, because 23mm internal rim widths are quickly becoming the norm, so there must be a reason for this, right? 

While we initially considered offering wider internal rim widths due to the trend of 23mm inner widths, we decided to find a credible independent testing source before making a decision one way or another

Unbiased Independent Testing 

Bicycle Rolling Resistance independently tests the rolling resistance as well as many other metrics, such as wet grip, puncture protection, and much more. They are the foremost authority on rolling resistance. In addition to just testing tires, they have "Specials," which are articles that explore other topics. The article for relevant to rim-tire combinations for performance is this article titled "The Rim Width Test: Road Bike, CX/Gravel, and MTB Tires on 18, 22, and 26 mm Rims."

Their Conclusion

Their main takeaway about road wheels is "Our conclusion for road bike tires is that there is nothing to gain in rolling resistance when moving to rims wider than 72% of the (specified) tire width." What this means for 28c tires, perhaps the most common tire used on road bikes is that there is no performance gain when the internal rim width increases beyond 20mm. From a performance and comfort standpoint, it is unnecessary to "upgrade" from 21mm internal rim widths to 23mm internal rim widths.

The following is the data from the intependent tests run by Bicycle Rolling Resistance. These are the recommendations from Bicycle Rolling Resistance: "For 25 mm wide road bike tires that would come down to 17-18C rims which is what most road bikes already come with these days. For 28 mm wide tires, moving up to 19-20C rims would hit the sweet spot."

So why are rims getting wider?

While there is a trend for wider tires which can certainly offer more comfort, and rims have gotten wider as well, 21mm internal widths already offer the best performance for 28c and even 30c tires. In our opinion, there are two different reasons for the move to wider rims when not necessary, 

1. Independent testing data that is not readily accessible to cyclists
2. The desire for brands to sell more products 

Why choose 21mm rim widths then?

Rolling Resistance and Compatibility

The truth is that while 19-20 internal rim widths are good enough for the most popular tire width, 28c, our design choices are not limited to rolling resistance, we are also interested in aerodynamic performance, durability, and compatibility. 

21mm internal width rims allows for increased compatibility, as 23mm internal width rims cannot safely be used with 25c tires.

The RCX series 31mm external rim width paired with the 21mm internal rim width offers extra impact protection as opposed to a similar rim but with a 23mm internal rim width. Our RCX33 Ultralight can survive 3x the UCI vertical impact test standard.

What manufacturers often neglect to mention is another very important factor, the external rim width!

Tire-Rim Combination Aerodynamics

One of the most important considerations we recommend when choosing a wheelset is your rim-tire combination. While the craze is about inner widths, for performance, we place a much larger emphasis on outer widths. The reason is that a tire that, when inflated, is wider than the external rim width of your rim, will significantly impact its aerodynamic performance. This is due to the "Rule of 105." The aerodynamics of the front wheel especially important, as it is is the first part of the bike that disturbs airflow when riding.

Independent Testing

The following information is referencing these two articles from Silca about Tire Pressure and Aerodynamics and Choosing the Right Road Tire. Silca, well-known for their independent testing. We recommend reading both articles if you would like to know about aerodynamics and wheels. The key takeaway is:

"The optimal aerodynamic relationship between tire and rim is that the rim should be 105% as wide as the tire.  If your tire is 25mm wide, you would want to have a rim that is at least 26.25mm wide.  This allows the system to keep airflow attached to the wheel as long as possible.  When the rim is too narrow and the tire too wide, air will hit the tire and immediately detach causing an aerodynamic penalty."

What this means is that for a 28c tire, the ideal rim width is at least 29.4mm. However, inflated tires are often wider than their stated width. For this reason, we have decided to use rims with a 31mm outer width to stay on the safe side.

What about 25mm internal width wheels?

What applies to the RCX series also applies to rims with wider internal widths like the GCX series. GCX series wheels such as the the GCX Ultralight are designed for both road wheels and gravel wheels. Its 25mm inner width can support 30c tires and above, which are becoming the norm, even in professional road racing. It is aerodynamically optimized for 30-32c tires according to the "Rule of 105" due to the 33-36.5mm outer widths depending on the model. In terms of rolling resistance, this is what the independent tester, Bicycle Rolling Resistance, has to say:

"For CX/Gravel tires, we conclude that the performance delta between 18C, 22C, and 26C rims is very small. As most modern CX and Gravel bikes already come with rims with a width somewhere between 19 and 25 mm, there is little to be gained."

While there is also the same trend of gravel wheels getting wider, according to Bicycle Rolling Resistance, there is little to be gained by going wider. GCX rims are already sufficiently wide, and going wider would restrict tire choices when following the ETRTO standards. The GCX series wheels are designed for both wider road tires and gravel, making it a perfect for those who like both.