Technical FAQ

Technical FAQ: The lifespan of your carbon-fiber racing wheels

In this week's column, a reader wants to know whether there is any data on the life expectancy of carbon-fiber racing rims.

Dear Lennard,
I have been riding ENVE carbon rims on my two (one cross country and one trail riding) MTBs since about 2010. I examine them for cracks regularly, as does the bike shop I use. Is there any data or recommendation on life expectancy for carbon rims? I am considering replacing them simply based on age, but is that a waste of money?
— Robert

Dear Robert,
Since carbon fiber essentially does not fatigue as long as the carbon-and-resin matrix is not damaged, age alone is not much of a guide. What matters are impacts big enough to delaminate or crack some of fibers.
Here are some answers to your question, one from the company that made your rims, and the other from a company that repairs damaged carbon-fiber bike parts.
― Lennard

From ENVE:
This is a good question and one that merits explanation, given that many of us grew up on a 3-5 year lifespan expectation from alloy. The short answer is that a carbon rim is expected to last until it is damaged in a way that compromises the bond between resin and carbon fibers. One of the many cool traits inherent to carbon is that it doesn’t have a fatigue memory like aluminum. This long-term durability really comes down to the quality of the rim’s construction, and the amount of abuse from the rider. Not all carbon rims are created equal, but as long as the rim’s strength surpasses the rider’s ability to destroy it, the rim could last a lifetime. However, while Robert’s wheels are likely to keep rolling strong for another decade, it may be time to upgrade, given that carbon mountain wheel technology has come a very long way since 2010. Upgrading while not required, will net him stronger, lighter, smoother riding wheels… and a better ride experience.
— Jake Pantone
ENVE VP Product and Consumer Experience

From Ruckus Composites:
Fatigue life on carbon mountain bike wheels is a tough topic to answer. In a perfect world, carbon fiber can have an infinite fatigue life. But reality is much more complicated. The spoke tensions, riding styles, resin composition, and terrain can play a huge factor into the lifespan of a wheel set. Also, the constant loading and unloading onto car racks or the tailgate of a pickup can cause many interesting structural outcomes. I don’t know of any available datasets regarding MTB wheels and fatigue life. I am working on characterizing our own data; but at this time our datasets have an insignificant sample population size for a useful Confidence Level.

Here is what I do on all my carbon wheels as part of my normal maintenance routine as I am still riding an older pair of carbon rims. I grab a small spot flashlight and give them a quick look over to look for anything suspicious. The main areas I look at are the spoke holes, spoke tension and the rim wall/hook area. Ensuring that your spokes are evenly and correctly tensioned by a professional wheel builder is a great place to start. The spoke hole will show cracks that emanate from the spoke hole if a nipple is trying to pull through or has exerted excessive stress on that area. The rim wall/hook will show damage from impact with the ground or various objects.

I have not tried some new tubeless foam insert systems that now exist to protect the rim bed, but that might be something to consider using.
— Shawn Small
Ruckus Composites Owner/Engineer

Dear readers,
As long as we are on the subject of carbon wheels and their construction and lifetime, I thought I’d insert this interesting feedback from a wheel maker about an article I wrote for the magazine over a year ago regarding the engineering of deep-section carbon wheels. Heading into a new decade, I thought you might find the discussion thought-provoking of spoke patterns and the relative symmetry or asymmetry of the carbon layup at each spoke.

Note that the paired-spoke design to which he refers is one in which each pair of spokes coming from opposite sides of the hub meet at (almost) the same point on the rim. The spoke holes are drilled in pairs adjacent to each other, followed by a large spacing to the next pair of holes. This is in contrast to most wheels, in which all spoke holes on the rim are spaced uniformly, resulting in uniform spacing between every spoke.

Rather than tacking my comments below, I’ll mention now that I don’t see how it could be possible that the width of his clincher rim does not increase with increasing tire pressure, as he claims near the end of his letter; all clincher rim walls bulge outward as the pressure between them increases. I also do not see why the paired-spoke design would not be subject to the same “hinging” that I describe in the magazine piece that deep-section wheels are subject to when laterally loaded (as in sprinting out of the saddle).
― Lennard

Dear Lennard,
In the July 2018 issue of VELONEWS, you presented some “DEEP THOUGHTS” regarding deep section wheels. For your consideration, I’m attaching two pictures of an ultimately-deep rim at 60MM with 10 radially-laced Sapim CX-Ray spokes.

Your piece was instructive and useful to the general reader. Respectfully, I’d like to add some comments. In summary, you made a number of points as follows: “deep section rims are 1) vertically rigid, requiring few spokes but are 2) laterally flexible, often 3) rubbing the brake pads, and 4) often becoming slightly out of round with inflation”.

Regarding point 1), I think you should have shown a wheel accompanying the piece with fewer than 24 spokes. Nowadays, the vast majority of TdF team front wheels have 16 spokes, even with relatively shallow rims, and in our catalog, you can find 12-spoke wheels with deep section rims.

On point 2), it might have been valuable to mention some lateral-deflection numbers, which must be available in your data base through all these years of your wheel testing. Lateral deflection can certainly cause brake pad contact, but I can tell you that on the 10-spoker depicted below, I run my brake pads at 1 MM and have no pad contact in my riding (mostly moderate) style. In the truing stand with the 23 MM tire inflated to 100 PSI, lateral deflection measures 0.007″. At the rim, vertical deflection measures 0.008″ with no tire installed, and at the tire running surface with 100 PSI inflation, the deflection is 0.015″ on the Continental Grand Prix 4000 S tire. The ROLF Prima HOP & WOBBLE specifications for all wheels are 0.020″ (1/2MM), and the excellent trueness of these deep rims is partially due to the high quality material and the correct and consistent placement/assembly of the individually-cut, unidirectional carbon fiber arc segment sets. In the wheel pictured below, I’ve marked the end points of the individual arc segments, and you can see that the spokes are attached uniformly at the center of each arc segment. This assures that spoke tension variation through wheel rotation under load is uniform, not as you say, “not uniform because the layup of arc-shaped fabric pieces is not the same at every spoke”.

The second close-up picture shows the “seam” between two arc-shaped pieces and further, if you look closely, the fiber orientation of the unidirectional component pieces. The angle between the spokes and the individual fibers for each of the 10 segments is uniform, as is the angle between the fibers from one adjoining segment to the next. This, with the considerable vertical stiffness of the deep rim structure, assures a phenomenally true wheel.

High spoke tension assures a reduced load cycle for each spoke under load, and thus the radius decline between the hub and the road at the bottom-pointed spoke is very much smaller under load than in the conventionally-laced, 24-spoke wheel you show in the article. Generally, for a given build-up wheel, the total inward forces of all the spokes on the rim is constant, regardless of the spoke count. The tensions per spoke on the 24-spoker depicted by you can only be about 40 % of the tensions on my 10-spoker, if both rims have the same total compressive load.

Add to this that, at any moment during rotation, TWO spokes are bottom-pointed (over the Road Contact Point – RCP). In the paired design, the spoke tension reduction by percent at the bottom is considerably smaller, due to the much greater initial static tension than with conventional designs, again cut in 1/2 over that of the conventional design by TWO spokes located at any time over the RCP.

This huge reduction of the spoke load cycle in this 10-spoke wheel explains the survival of the very light Sapim CX-Ray spokes for the last THREE years I’ve been riding the 10-spoke mate to the depicted wheel.

These facts, along with SEPARATION of the HOP and WOBBLE deflection with spoke tensions with the paired design, allows phenomenally true builds that stay true. In the conventional lacing pattern, if a given spoke is tightened, the rim is drawn DOWN toward the hub as well as OUT toward the flange. Thus, both HOP and WOBBLE are simultaneously affected, and only a compromise can be achieved. Correcting one condition affects the other, so it becomes impossible to achieve perfect trueness.

By contrast, in the paired ROLF pattern, if a HOP has to be corrected, BOTH spokes at a defect spot are adjusted, either tightened or loosened simultaneously, with no LATERAL movement. If a WOBBLE correction is desired, one spoke is tightened, and its mate is loosened, with no change in the HOP direction. This yields perfectly true wheels in both HOP and WOBBLE.

This phenomenon is also present in use, when the bottom-pointed spoke pair is de-tensioned at the RCP, with none of the resultant lateral deflection that occurs with conventional lacing. With conventional lacing, at any given time only ONE spoke is de-tensioned at the RCP, which reduces its lateral force component at the rim, pulling the rim in the opposite direction of the affected spoke, since the single spokes ahead and behind it pull in the opposite direction of the de-tensioned RCP spoke. Thus, a conventionally-laced, low-spoke-count wheel traces a “sine wave” at the RCP, as demonstrated on page 352 of BICYCLES and TRICYCLES, the 1896 comprehensive treatment of Bicycle Design and Construction by Archibald Sharp, available from MIT Press.

To restate your points, “deep section rims are vertically rigid…requiring few spokes” – agreed, and I so demonstrate in the pictures below.

“Most people realize that deep section wheels are laterally flexible…often rubbing brake pads…and become slightly out of round when the tire is inflated.” Not agreed for my 10-spoker below.

Pre-preg “fabric pieces are machine cut”. Not agreed. Nowadays, lasers are used, resulting in much cleaner cuts!

“As pressure in a tire on a wheel increases, the rim width also increases …while the spoke tension decreases”. Not agreed to in my HIGH-quality, uniformly-layed-up 60MM deep carbon clincher rim.

“If the carbon layup were uniform throughout the wheel, the decrease in spoke tension would also be uniform”. Agreed; in ROLF wheels, the layup is uniform.

“On most deep wheels, the tension drop is not uniform, because the layup of arc shaped fabric pieces is not the same at every spoke”. Probably true for most wheels but certainly NOT true for the ROLF 10-spoker shown below, as demonstrated by its phenomenal trueness at 100 PSI and outstanding durability and ride characteristics.
— Rolf Dietrich
Founder, Rolf Prima


Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder (www.zinncycles.com) and purveyor of non-custom huge bikes (bikeclydesdale.com), a former U.S. national team rider, co-author of “The Haywire Heart,” and author of many bicycle books including “Zinn and the Art of Road Bike Maintenance,” “DVD, as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”
He holds a bachelor’s in physics from Colorado College.

Follow @lennardzinn