Are carbon wheels safe and a good choice?

For the last 6 years or so, days of the week have meant almost nothing to me. If it was nice on Wednesday and mountain biking was better than working, I'd just catch work up on Saturday and that's that. Now, in a funny inversion, because of this every weekday posting convention, I'm like the only person I know who has any TGIF sentiment left. 

Curt, a longtime customer who lives in OR, is getting a replacement frame that's causing him to switch from rim brakes to disc brakes. And I'll let him take it from here: "My long term wheel debate is whether to get a carbon wheelset.  I've enjoyed your blog posts the past month.  Any chance you'd think of doing one on the durability, strength, and safety of carbon rims vs. aluminum?  I think I remember seeing too many crunched carbon rims from their early years..."

This is a great question, and though it's got a lot of ground we've covered in the past, addressing things from different angles teases out additional shading. 

To start with, it's sort of hard to screw aluminum up. This isn't to say that all aluminum rims or parts are the same, as they absolutely aren't. Different alloys are more expensive or demand less process/quality control or are more difficult to extrude or finish or whatever, and there are tradeoffs at every price point. The aluminum rims on a $200 bike in the big box store are miles and miles away from the aluminum rims we use. And you can make rims too light, as aluminum rims cracking at spoke holes is not uncommon, although I'll never be convinced that some of the issues we've seen are just weight/"not enough material there" issues. Anyhow, in the broad sense, once you know what you're doing with aluminum, it's fairly easy.

Carbon is more demanding of process control than aluminum is, at any level. It's easier to screw composites up, both in engineering and fabrication. Aluminum is isotropic, which means it's got the same strength in every axis. Carbon is anisotropic, which means that it's wicked strong in some directions and wicked not strong in others. I'll use a sail picture to illustrate this concept - the dark fibers are the reinforcement that take the load. The translucent film is there just to hold the fibers in place in the correct shape and prevent the air from whistling through.

Obviously you can get away with a lot less overall reinforcement here, because you're very effectively putting it where it's needed and leaving it out where it's not. But if you get it wrong? Whoops. The sail splits in half. Or the rim splits open.

So, short story long, if you screw up where/how you think the loads are running, big problem. And then there's the process of making the stuff, which has to be tightly controlled. You can't just go down to the hardware store and get a gallon of Country Joe's Fantastic Epoxy and mix it up with Discount House of Carbon's Famous Carbon Tows and make wheels. There's a bit more engineering that goes into it. And you have to handle all the stuff carefully along the way. 

You need to have the right amount of the right kind of resin impregnated into the carbon, and then you need to store and handle the prepreg material a certain way, or it all gets screwed up. (cue ominous minor chord music...)

Trying to manage rim brake heat really threw a spanner into the works on this one. Basically everything you do to make resins more heat resistant adds complexity and tradeoffs. The carbon fibers themselves are basically completely heat proof, so you're not worried about that, it's the plastic that's the issue. And whether you're doing elaborate curing schemes or adding silicon into the resin or whatever has been done to make things more heat resistant, you're making it way harder to cook the recipe right, and the finished pieces that come out are more brittle.  

Taking the brakes off of the rims unwinds all of that mess. You really don't want to use heat resistant resins when you don't have to. Every single carbon expert I've ever talked with about this says that you want to cure at the minimum possible temperature you can, that that will make the process work best and have the best resulting parts.

There was a time when we saw a lot of problems with bead hooks in carbon rims, to the point where I swore off using hooked carbon rims ever again. The problem there is getting the whole structure to heat super precisely in a mold - delivering the same exact precise heat profile to every little nook and cranny of a complexly molded rim. You don't need that precision with lower temp resins. 

So we went through this period where aluminum plodded along being aluminum with its pros and cons, and carbon went from magical new whizbangery to "holy Hannah what hath we wrought?" with all sorts of issues. I mean, look at how much print and server space was used in talking about this rim or that's heat resistance and would they work and blah blah blah. 

And now we've reached a point where carbon rims, having removed the necessity of heat resistance, have kind of become what they should be. They can improve, for sure, both in process reliability and moving the bar up in terms of weight and strength and other metrics, but just unsaddling them from needed to withstand brake heat was enormous.

So that's a long way of saying that carbon rims these days are by and large quite good. Into our third year of being back in the carbon rim game, we've had two warranty requests on carbon rims. One rim sort of buckled in transit to the customer, the other had a failure on the laminate of what would have been the brake track. Both were covered, though the second I'm not sure there wasn't something else going on, but they were both covered. That's a lower failure rate than our moving average on aluminum rims is (which is entirely encompassed in spoke holes cracking). 

In both carbon and aluminum, you're going to et a rim that works to within about a six sigma failure rate. But the important question still to answer is, are the tradeoffs worth it?

These are actually Altamont Lites - we need to get the camera out more.

Let's take two wheel sets and compare. In corner we've got the aluminum Boyd Altamont disc, and in the other we have the Cafe Racer 46. We'll build both with Industry Nine hubs just for the sake of argument. The Boyd rims are 30mm deep and 20mm wide inside/24mm wide outside, at a weight of 500g/rim. Cafe Racer rims are 46mm deep and 21.5mm wide inside/28.5mm wide outside, at a weight of 440g/rim. Both are strong strong rims. We're fine building a 24/24 Altamont set for a rider up to like 180+ pounds, and I don't often meet people I wouldn't build a 24/24 set of Cafe Racers for. 

The Cafe Racers cost $290 more. They have a little added width inside and quite a bit of added width outside. They're obviously 50% deeper, and the rims are 12% lighter. We expect both to last darn near forever (obviously things wear out but lifespan is long here) without the brakes wearing them down. We expect the "didn't get a good one" rate to be about the same on both. If you're super concerned about aerodynamics, the Cafe Racers are going to win that one, especially if you put a slightly wider tire on them. Cafe Racers are going to withstand a harder whack than Altamonts are, but when they meet the whack that beats them, they're more expensive and there's no "well just try bending it back into shape" - carbon don't bend.

Removing the cost part of the equation, there's no contest, you'd pick the Cafe Racers. So it boils down to cost. You can choose to pay more for the attributes that are in favor of the Cafe Racers and account it that way, or you can choose not to. You can choose to amortize that cost difference over now a longer life span, or you can choose not to.

As we've said many times in the past, we've set ourselves up to be agnostic on which wheels people buy - we don't benefit by pushing anything. When you get a recommendation from us, it's what we think will suit your stated purpose and parameters best. Sometimes (actually often) the first parameter is "I want carbon" or "I don't want carbon," and that takes care of that. But really it just boils down to are these attributes worth extra cost to you over your planned and expected time frame. 

The weather dial here in southern New England has finally been set to "not suck" so should get some good riding in, hopefully you can too. 


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  • Dave on

    Both of these are more spoke count questions than rim material questions. For riders over 180 (or somewhere around there, depending on some other factors) you simply bump up the spoke count for the aluminum build. I chose Altamont for this comparison because it’s the most stout aluminum rim we use, but if we were talking about Altamont Lite then a 180# rider would already be in a 24/28 build. And a 200# rider on Altamonts would be on 24/28 or 28/28 lacing. Cafe Racer would still be on 24/24.

    We figure most people are on relatively light bikes in the sort of 18# or less range. If your bike is much more than that, then it’s helpful to know. If you commute with heavy bags or you want to do bike packing with 20# of gear, then that’s helpful to know as well, and we can design around that.

  • James on

    Not to confuse the matter even more…how about the rider who is a solid 200#, rides strong but is gentle on their equipment? Tire choice will be 32mm or larger.

  • Mick Lovell on

    Good writing…..again!

    So what about the aluminum vs. carbon comparison for the > 180lb rider? Say a rider is 190 lbs. Are all of these statements still true? Who would you NOT build a set of 24/24 Cafe Racers for? Do you factor in riding style, bike weight, etc? Say a 22lb gravel tourer vs. a 16lb climbing bike?

    Thanks!


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