Technical FAQ: Wide rims, light bulbs and quonset huts

Dear Lennard,
I just wanted to check in with you regarding your Q&A last week. As you know, the proper brake pad choice is critical when optimizing a carbon fiber wheel’s performance and maximizing its reliability, and I think it’s important to take any guess-work (on the part of the consumer) out of play. Reynolds has literally spent years and thousands of hours engineering and testing our CTg system for safety and reliability. I would discourage any cyclist from using any pad, for any reason that is not recommended by the wheel manufacturer.

The Reynolds engineering and design team simultaneously engineered the brake pads and brake track to be an integrated system, working together to effectively minimize heat buildup from pad-to-rim friction and provide the best possible performance. The Reynolds system is called CTg (Cryo-Glass Transition) and the chemistry of the Cryo-Blue brake pad is engineered as a complimentary component of the resin system in the carbon fiber laminate in the wheel’s brake track. In the case of a warranty claim, following the manufacturer’s instructions to a “T” is always in the customer’s best interest, and using anything other than a Cryo-Blue brake pad can void the warranty. Reynolds is not alone in recommending proprietary pads with our wheels, as Zipp, Easton and a few other well-known brands do the same.

Because of the nature of carbon fiber, it is critical that all cyclists follow their particular wheel brand’s recommendations to avoid problems that some of your readers have experienced. While another pad might appear to provide acceptable braking performance, it may not dissipate heat as effectively and could cause other unseen problems with the rim. We would encourage your readers to follow the manufacturer’s instructions with regards to brake pad selection — all of us on the brand side only have cyclists’ best interests at heart in making these determinations.
— Paul Lew
Director of Technology and Innovation
Reynolds Cycling

Dear Paul,
That’s a very good point, and I appreciate your feedback very much. It’s possible to imagine a scenario in which a rider’s safety could be compromised by “switching different brands of carbon pads all around.”

I ought to clarify my statement, though, since I didn’t word what I had intended very well. Indeed, on the road, I always use the brake pads that come with the rims until I have a problem with them. Then I check with the wheel maker for a recommendation on another pad to try. I ride in the mountains almost daily, and I’ve seen enough pads, tires, and rims fail on carbon wheels that I am quite cautious about pad choice on my road bikes.

In answering that question, I was specifically thinking about cyclocross, however, as that’s the instance where I “switch different brands of carbon pads all around.” In cyclocross, the braking is so brief, the speeds are so low, and the temperatures are also usually so low that rim heating is not an issue. But squealing, grabbing, and even chattering can be. So for cyclocross, I do indeed switch pads around if I’m scaring other riders with the screeching noise or scaring myself with brake grabbiness.
— Lennard

Dear Lennard,
I’m curious about the various tire/rim combinations and their effect on traction. One of the things I’ve read about wider rims is that they change the shape of the tire from light-bulb-shaped to a rounder profile, improving traction while cornering. If this is the case, does running a wider tire on a 23mm rim negate this benefit, since presumably the wider tire would go back to being light-bulb-shaped? If my primary concern is grip in corners, which is better, a 23mm tire on a wide rim or a 25mm tire on a regular narrow rim? How does a 25mm tire on a wide rim match up, and would an even bigger tire improve grip further, or would the loss of the rounder profile decrease grip while cornering?

I guess the key question is this: is the improved grip from a wider rim simply a result of the larger contact patch, or does the light-bulb shape of a tire on a narrower rim tend to flop over, as some proponents of wider rims claim?
— Dennis

Dear Dennis,
Here is an answer from Jake Pantone, marketing and sponsorship director for ENVE:

Cornering feel and sensation can certainly be improved with wider rims as long as the tire size is properly mated to the rim. In the case of our Smart ENVE System (SES) wheels the front rim is 26mm wide. The extra width came into existence strictly based on aerodynamic data, but in the process you also provide a wider platform for the tire to mate with. Essentially you are broadening the base of the triangle, making the tire’s sidewalls spread out more, and eliminating what is commonly referred to as “tire-roll,” which is essentially lateral flex in the tire’s sidewall. As the tire’s width gets larger than the rim’s tire interface you begin opening the door for the tire to begin flexing laterally again. Another factor in the equation is tire pressure. In order to reduce excessive tire roll, say with a 20mm-wide rim and 23mm or 25mm tire, you have to run higher (generally accepted as normal) pressure. With a wider rim you can run less pressure (10-20psi less) because the added support is mechanically preventing the tire from being able to roll or flex, thus improving traction.

There are also factors such as the tire’s actual contact patch. With our SES rims specifically, a 25mm tire in most cases will provide a very favorable result to the customer who is primary concern is cornering and traction. If aerodynamics/stability is the area the customer wishes to optimize then our data shows a 23mm tire to be the best choice with our SES rims.

I have attached two pictures of the tire and rim interface between our standard 65 clincher and our SES 3.4 clincher. – Jake

And here is an answer from Joshua Poertner, technical director for Zipp Speed Weaponry:

For starters, it is probably a bit misleading to say that the wider bead seat increases traction or grip. I think that we have proven that the wider bead seat yields a wider/shorter contact patch than a narrow bead seat, and in theory this might provide some rolling resistance benefit, and also makes it easier for the tire to conform to rough surfaces, which might lead to improved traction in some conditions, but there is little to no data showing either improved traction or decreased Crr, so we hesitate to really make claims to either of those as benefits here.

The difference between the light-bulb shaped tire Dennis talks about and the geometry of a tire on a wide bead seat rim primarily affects the lateral deflection of the tire when cornering. I too have heard the “flop over” description used, but basically as the tire rotates down to contact the road, it is laterally deflected by the cornering forces, this results in a deformation that leaves the tire not actually going in exactly the direction it is pointed. Technically this is called “slip angle” and there is a lot of great stuff on the Internet about it. Less slip angle yields more responsive handling, higher slip angles can feel vague or even sketchy; taken to extreme examples, we’ve all had slow front leak that we thought we could get home on and found that a front road tire with 20psi can be turned quite noticeably before actually changing the direction of the bike.

It is nearly impossible to say that this geometry difference increases traction/grip at a given pressure (there is some theory here that it might do this, but no good testing or data to support it, and even the theory predicts very small improvements on certain surfaces), but it most certainly is felt by the rider as improved turning responsiveness and better line holding under high cornering loads. This wider base also yields more air volume between the tire and the road, which allows the rider to run lower pressures with similar pinch flat resistance. 

This takes us to the real driver of grip, and that is pressure; contact patch size for a given pressure will be the same across tire sizes, but lowering pressure puts more rubber on the road surface and can directly improve traction/grip. Simply using wider tires at lower pressures can also increase traction/grip without increasing pinch flat likelihood, and does it with the added benefit of additional radial height between the rim edge and pavement, but this setup will have higher lateral squirm under cornering loads. Choosing between small tire size changes will yield pretty nuanced results, comparing a 23mm to a 25mm is very difficult as they feel so similar. I have 32mm tires on my commuter and I can run them all the way down to 70psi before I don’t like the handling, but the grip down there is phenomenal and I hardly notice road seams! You are likely to feel as much change in tire squirm under hard cornering due to tire pressure as you are due to the additional width, if anything, the 25’s will be a bit more comfortable but might feel less “racy” when pushed. 

As with many things, there is a LOT of personal preference involved here, and the real benefit of the wide rims is that they allow for more options. Generally the peak traction/grip point for the tire of your choice is most likely at a pressure below what you are using; wide rims allow you to go lower with fewer consequences. I always recommend trying out different combinations to see what you like best; road surface conditions and even weather can change things, so don’t be afraid to experiment. – Josh

ENVE and Zipp are among those wheel manufacturers that have embraced the wider profile, and it’s interesting to hear slightly different nuances of perspective from them.
― Lennard

Dear Lennard,
Is a wider tire (25mm) the functional equivalent, other than weight, of a wider rim?
— Michael

Dear Michael,
No, it is not. The wider rim allows the tire to spread out more (like a Quonset hut rather than like a light bulb), so there is a reduced tendency for pinch flats. Other than the larger tire keeping the bike higher up off of hard impacts and thus reducing pinch flats, the narrow rim with wider tire increases the tendency for pinching the tube at lower pressures, because its width overhangs the rim more and thus can more easily catch some inner tube between the rim and the tire.

The wider rim reduces the tendency for the tire to flex laterally when cornering, as it triangulates the tire more by widening its base and making it less susceptible to flex due to side forces. The wider tire has the opposite effect.

From an aerodynamic perspective, the wider rim smoothes the air flowing past the tire over the rim by getting the tire/rim combination to be shaped more like the leading edge of an airplane wing. The wide tire on a narrow rim, on the other hand, is worse aerodynamically, as it deviates even further from that perfect airfoil shape than it would with a narrow tire.
― Lennard