Technical FAQ with Lennard Zinn: Carbon Forks

Carbon road forks:
I get so many questions about carbon forks that I have decided to focus this column entirely on questions related to them.

Question:
I’m not big in a height sense, but am very stocky. I’m about 5’8″, 240 lbs. I was wondering if I could get your thoughts on steel v. carbon forks for bigger, heavier riders. What are the benefits of carbon forks and are they durable enough for bigger people (especially the full carbon steerers)? Are their damping properties less of a benefit to larger riders? Also, I’ve heard some argue that the light weight of carbon forks give a less ‘solid’ feel on the front end. In any case, any advice would be greatly appreciated!–Oscar

Answer:
I think carbon forks work great for heavy riders. While the ride and strength of steel forks is great, there is no market for them now and full-carbon forks is the trend on almost all high-end road bikes now. Occasionally, we build a frame for a very small person that requires adjusting both he fork rake and head angle in order to prevent contact of the toe with the front tire when turning, and carbon forks do not afford that option.

The majority of our customers are around your weight or heavier. We do get extra-thick steering tubes from True Temper for heavy guys.

There is plenty of “solid feel” with a carbon fork, and the ride is very nice.
–Lennard Question:
What is the state of thinking on the life-span of carbon forks?

Will they last forever, show signs of wear or should they be replaced after some span of years/use?

I’ve had a Time fork in my old Merlin since 1998. It’s lived through a few race crashes and seems as good as ever, but I still have trouble regarding it as something as long-lasting as a metal component. Thoughts?–Jon

Answer:
We cyclists have an opportunity unique in the wheeled racing world, namely the option to purchase the exact same equipment that the world’s top racers use. This is not possible with Formula 1 or Indy car racing, for instance. Parts used in those cars are the lightest, trickest ones the engineers could come up with, but nobody is taking on the liability of having regular consumers out using them.

With bicycles, we can buy that superlight racing stuff that Armstrong or Cipollini uses. As such we are asking it far more of it than to just carry a racer to victory in a single race, after which it could be replaced (heck, many Formula 1 parts are replaced during the race–they aren’t even intended to last the entire event). Once we buy it off the shelf, we expect that it should last for years and years, even more so because it was so expensive. We don’t consider the stresses that kind of repeated work puts on a light part, not to mention the effect of prolonged exposure to the elements, relative to what it would need to withstand in order to win a single important race. If we weigh twice as much as Armstrong, we still might buy his same bicycle or wheels and expect, nay, demand, that it hold up for years under us.

Nowhere is this more apparent than with carbon forks. We all know intuitively that the breakage of a fork while riding is too catastrophic to even consider. All cyclists cringe at the mere thought. It is great that you pose the question of what we should expect.

I put this question out to a number of carbon fork makers and got some very interesting answers. See below.
–Lennard

From True Temper:
We are confident on the long-term durability of our forks because we test far beyond the ASTM test standards for fatigue life on forks.

We have two types of fatigue tests:

1) ASTM

2) Ramped load testing

ASTM standards call for a load of 170 lbs. applied perpendicular to the steering axis, both pushing and pulling for 50,000 cycles without failure.

At True Temper, every Alpha Q model is tested to 250,000 without failure before a design is considered acceptable. Also production models are tested periodically for quality control.

True Temper’s own test is also used on every new model and in routine quality checks. Our test is a ramped load, meaning the load is increased periodically until failure occurs. Starting at 180lbs, the load is increased 45 lbs. every 5000 cycles. Every fork will eventually break. Strong forks will last more than 10,000 cycles with a load of 270 lb. But our minimum standard begins at over 15,000 at 315 lbs. for road forks and 18,000 for cross forks and tandem. But our production forks are stronger than that, often going into the 20-25K range and beyond at loads 0f 360-405 lbs.

Obviously, crashes are uncontrolled events and it is not easy to guess what loading was applied to a component by the speed or violence of the crash. After any crash it is important to thoroughly inspect the frame and components for visible cracks, dents, and bends. An Alpha Q fork that has been damaged (usually evident as a crack) should be replaced.
–Bert Hull
True Temper Sports

From Deda:
Carbon lasts longer than metal.Only love is stronger than carbon.Bonding is a different story.I believe that a good glue (epoxy) can last for 2000 hours of work, or about 800 days, not in continuous daylight, and below 35 Celsius.Whenever a carbon “part” has crashed, even if you cannot see a failure, if there is any reasonable doubt about having surpassed the elongation limit, the part must be replaced.
–Fulvio Acquati
Deda Elementi

From Kestrel:
On the lifespan issue, of course the person should contact the manufacturer regarding specifics on the product in question. For carbon forks in general, there should not be any limited life span, as carbon composites themselves are not subject to fatigue failures as metals are. So the fatigue life of a properly made carbon composite is “infinite”. Example, in Kestrel’s case, our forks (as with all our carbon products) have a lifetime warranty and are designed and tested to last a “lifetime” of use for the given product.

What should be more of a concern, again as it would be with a metal structure, is the kind of abuse and/or damage the fork has seen. It’s the old “inspect in case of crash, impact or other suspected damage” deal. If any fork is known to have been crashed or impacted I’d look it over real close and, if any damage is present or suspected, I would play it safe and assume the worst. Get it checked out immediately by the manufacturer.

If a carbon part – in this case a fork – is properly designed, properly manufactured and has not been damaged due to crash/impact/abuse, there is no reason it should not last just as long (or longer) than a metal component.
–Preston Sandusky
Sand Point Design (Kestrel Bicycles)

From Columbus:
Carbon forks can be long lasting but one has to take care of other aspects than metal ones.

A fiber composite material has higher mechanical characteristics than metals (higher tensile strength, higher fatigue life, higher stiffness) but it has even different behavior due to its “non-homogeneous nature”. This means that the fork project is really a “critical point” for life span, moreover the final user has to check the composite fork with attention to different aspects compared to metal one. In fact, for composite material the areas of coupling with other parts are critical (headset bearings, stem clamp, gap cap),composite has different impact behavior so for each shock is important to check fork, finally composite is sensitive to ambient agents (solvents, temperature over 90°C etc).
–Riccardo Carpinacci
R&D department, Columbus, Cinelli and 3T

From Look:
There is no limitation because carbon has a natural flexibility. It can be used a hundred years while maintaining the same stiffness.
–Ming Tan
Look Bicycles

From Reynolds:
After considerable testing and thought on the matter we find the question of “fork life” in terms of time to be a tough question to answer. Our fatigue testing would indicate that well built composite forks are far superior to metal forks with cycle counts running hundreds of thousands of cycles rather than tens of thousands. These tests are also run at much higher loads than metal forks can withstand further demonstrating the durability of composite materials. Based simply on fatigue life a well made and properly installed composite fork should last virtually indefinitely if the bike is not crashed or otherwise abused.
Mike Lopez
Reynolds Composites

From Easton:
There are two failure modes that could cause a fork to fail, fatigue or impact. Questions about life span are really questions about fatigue life. How many cycles can a fork survive before it is tired and worn-out? The good news is the fatigue life of carbon fiber is immensely more than that of metals. While the writer expresses concern about his carbon fork lasting as long as a metal component, there is nothing to worry about in terms of fatigue life on a composite fork.

The most likely cause of failure for a composite fork would be impact damage sustained from crashing. Most of the time any damage to a fork from a crash will be visible. Cracks can be seen. We would recommend that the fork be periodically inspected visually at the drop out area and along the fork legs to look for cracks or depressions in the material. Any fork that shows signs of cracking should not be ridden and replaced immediately.

In general terms, a component made from carbon fiber will far out-last a component made from metal.
John Harrington
Easton Sports

Question:
There were two things I wanted to ask.

Is there any kind of recommended /not recommended weight limits on the different carbon forks available, in particular the Reynolds, Time, and Easton forks? Is it possible that a new fork, carbon or not, with a different fork rake may help me with some “shimmy” problems I’ve experienced with my big Schwinn Paramount 62cm frame, or is this mostly a function of my frame’s tubeset?

I’m 6’4″ 183lbs and ride this great Schwinn made of Reynolds 853 tubing, and have ridden bikes consistently for 30 years.

As you stated in your experienceswith ‘shimmy’ most big frames are made with the same tubing as their smaller counterparts, and this isn’t so ideal for bigger riders. Sometimes this can probably cause these lower speed wobbles. I am looking to give my bike a little more resiliency and a smoother ride up front than I currently experience with the OEM Schwinn Paramount straight-blade fork, and at the same time lighten the bike a bit and maybe alleviate some shimmy problems if possible. Regardless of slight no hands speed wobbles at low speed, I’d still like to know if carbon would be recommended for riders my size. And if so, with carbon, aluminum, or steel steerers?

What fork rake would be ideal (40/43/45mm) for a frame that’s approximately 74/72 degree angles, or does this just depend on how I want the bike to handle?There are many Carbon forks to choose from and I get a different opinion from everyone, but I need an opinion from someone who’s a bigger rider with good experience.
–Todd

Answer:
The answer to your first question is below in the manufacturers’ responses. In general, fork manufacturers do not specify weight limits.

To your second question, you would need to try it. Sometimes, I have cured mild shimmy in a frame by changing out the fork, but many times, it has not worked. As for rake, the steeper the head angle, the less rake you want, and vice versa. Consider 74 degrees or more to be a steep head angle and 72 or less to be a shallow head angle.

As for material of the steering tube, as long as you cut it properly and do not crush it tightening down your stem, I recommend a carbon steering tube. Again, for our big bikes we use forks made by True Temper with an extra thick (and extra long) carbon steering tube meant for a tandem. See manufacturer answers below.
–Lennard

From True Temper (Alpha Q forks):
No, we don’t have a published weight limit on the pro fork or the big guy fork.The SUB 3 has a printed weight limit of 200 lbs (rider) but in testing it is stronger than many forks without limits.

Part of that is for marketing. People want the lightest equipment they can get away with.

Our Pro does not have a limit set for it.

The failure mode is not catastrophic. A broken fork will crack near the crown and allow more movement. It will feel “soft” because it will flex more under braking. The rider will see cracks on the outsides of the crown. The rider can ride home and remove the fork without danger. This failure mode is preferred to forks that can break and cause a wreck.
–Bert Hull
True Temper Sports

From Deda:
We do not have a limit for the rider weight.
–Fulvio Acquati
Deda Elementi

From Look:
No weight limit on any Look forks, frames or pedals (including the new Ti spindled pedals).
–Ming Tan
Product Manager – Look – CicloSport

From Reynolds:
At this point, Reynolds does not have any size or weight limits associated with our products.
–Mike Lopez
Reynolds Composites

From Easton:
Lennard, I assume you want me to answer the fork question. If you want my opinion on the frame shimmy, I would say that the shimmy is not a function of frame angles or fork offset. The stiffness of the frame and the rider’s position on the frame would be the areas I would look to improve. Bigger diameter tubes would increase the stiffness and a stiffer fork would help.

Now on to the fork. The rider’s weight is not an issue. 185 lbs. is no big deal. 200 lbs. plus is no big deal. The strength properties of the carbon fiber materials are equal to the task of any of the steel or aluminum forks on the market. However, given the size of the frame and the riders concern with shimmy, I would recommend the stiffest of these forks, which is the Reynolds.
John Harrington
Easton Sports

From Time:
Time does not have any rider weight limit restrictions. I am 200 lbs and have never had problems with steel or aluminum steerers or a full carbon steerer.
–Kurt Stockton
Time Sport International

From Columbus:
We haven’t any rider weight limit in our manuals, but we suggest a “reasonable limit” only for Muscle and Super Muscle fork: 80 kg (176 pounds).
–Riccardo Carpinacci
Columbus R&D department

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VeloNews technical writer Lennard Zinn is a frame builder, a former U.S. national team rider and author of several books including the pair of successful maintenance guides “Zinn& the Art of Mountain Bike Maintenance” and “Zinn & the Art of Road Bike Maintenance.”