What’s in a rigid fork?
At face value, there isn’t much that separates one rigid fork from another, especially where road bikes are concerned, so it shouldn’t be hard to replace one at short notice, right? In the past, this might have been true, but rigid forks have changed a lot in recent years, and now, consumers must grapple with a much wider range of specifications to find a perfect match.
This is a topic that we first delved into in 2012, and now, Matt Wikstrom re-visits it to explore all of the details that are important to a fork, including current design strategies and the pressing issue of safety.
Rigid bike forks have come a long way in the last 30 years. Back then, classic steel forks with slender curved legs were commonplace, even for MTB, and perhaps the most pressing issue for any rider was deciding between paint or chrome for the final finish.
A revolution in fork design was underway, though. Gary Fisher had already started experimenting with larger steerer tube diameters for MTB, Paul Turner was busy creating his first RockShox suspension fork, and John Rader was hoping that somebody would take notice of his threadless headset.
The road market, meanwhile, was more intent on saving weight, hence the early appeal of alloy forks, but it was Kestrel’s first carbon fork that proved a turning point for the industry. In time, larger steerer tubes and threadless headsets would become just as important to this market, but that wouldn’t happen until the end of the ‘90s.
Y2K ushered in a new era of bike design as carbon composites came of age. There was a period where the material was eyed with suspicion, or simply dismissed as far too expensive, but it didn’t last long. Manufacturers moved from piecemeal construction to sophisticated all-carbon creations that could be sculpted and fettled to achieve a diverse range of design goals, including improvements in aerodynamics. All of this consolidated the position of composites as the material of choice for rigid fork construction, and now, most riders wouldn’t consider anything else.
While the choice of material hasn’t changed in recent years, rigid forks have still been evolving to accommodate new brake fittings (direct mount rim brakes and disc brakes), thru-axles, and wider tyres. Tapered fork steerers have also become more common with a trend towards larger fork crown diameters, adding further to the amount of diversity — and incompatibility — in fork designs.
As a result, there is no longer any such thing as a standard fork, road or otherwise. Instead, consumers must pay attention to a variety of specifications in order to find a compatible product when selecting, replacing, or upgrading a fork.
The fit of a fork
Every rigid fork must satisfy three important fit criteria: first, the steerer must fit within the head tube of the frame; second, the fork legs must accommodate the front wheel, including the width of the tyre and the type of axle; and third, there must be suitable fittings for the front brake. This is something that we have discussed previously in our look at how to handle a custom bike build, so I will only touch upon the most important issues here.
Fork steerer diameter: There are just three fork steerer sizes to contend with: 1in/25.4mm, 11⁄8in/28.6mm, and 11⁄4in/31.8mm, which are usually (but not always) matched to a head tube with an internal diameter of 30mm, 34mm, and 37mm, respectively. All three can be threaded or unthreaded, though the former was essentially phased out at the turn of the century. In some cases, it may be possible to fit each of the steerer sizes to larger head tubes, but that will depend upon suitable bearings and cups.
A tapered steerer uses a larger diameter for the lower headset bearing, either 11⁄4in/31.8mm or 11⁄2in/38.1mm, but it must be matched to a larger head tube diameter. Some frame manufacturers make use of a head tube with an internal diameter of 44mm to accommodate this increase, while others will also use a tapered head tube. As above, the frame plays a critical role in determining what size fork steerer can be installed.
Fork steerer length: The length of the fork steerer must always be greater than the length of the head tube. In the case of a threaded steerer, an extra 30-40mm will be required for the headset, while threadless steerers will require at least 50mm to accommodate the stem as well. For this reason, aftermarket forks are often supplied with very long steerers so as to accommodate all frame sizes, with the excess removed after the stem is installed.
It is important to note that there is a limit to how long the steerer can be left in order to raise the height of the stem and bars. In the case of threadless carbon forks, this usually amounts to no more than 30-50mm of spacers, though the exact amount varies from manufacturer to manufacturer, and is detailed in the installation instructions.
Fork leg length: The length of the fork legs dictates the size of the wheel (and tyre) that can be used with the fork, but rather than provide measurements, most manufacturers simply list a recommended maximum. This number can be exceeded in most instances, but only by a little, and can be subject to other limitations, such as those imposed by a rim brake calliper.
Axle and brake fittings: In most instances, a fork is dedicated to one type of brake (single-bolt rim; direct mount rim; post mount disc; flat mount disc) and one type of axle (quick release skewer; 12mm thru-axle; 15mm thru-axle; 20mm thru-axle), and in most instances, there is no way to convert from one to another.
Satisfying each of these criteria will ensure that a fork will physically fit any given bike, but there is more to consider, since the amount of rake (or offset) and the axle-crown length will have an effect on the geometry of the front end of the bike, and therefore, steering and handling.
Fork rake affects steering and handling
Almost all forks incorporate some offset for the front axle, which pushes the front wheel ahead of the steerer tube axis and away from the frame. When combined with the head angle of the bike, it determines the amount of trail for the front wheel, which in turn, has an effect on steering, as detailed in our previous article on the geometry of bike handling.
A large amount of trail encourages the front wheel to remain centred and adds stability to a bike, especially at high speeds; smaller amounts make for lighter steering and greater maneuverability, but the bike can become unstable at high speeds. For this reason, fork rake is normally matched to the head angle of any bike to achieve a suitable amount of trail.
“There’s an asymmetrical loss function with trail,” explained Damon Rinard, the engineering manager for road bikes at the Cycling Sports Group. “If you have too much trail, then the bike wants to go straighter than you intend when you go around a corner. That can be dangerous if there is an oncoming car, for example, but it is not inherently dangerous. If you have too little trail and go really fast, then the bike can get away from you, and when you try to correct, the reflexes of the human body are not as fast as the shimmy frequency of a bike that’s out of control. So it is better to err on the side of too much trail.”
The amount of rake for a road fork typically ranges 40-50mm, and when combined with a head angle of 71-74°, this can produce 45-75mm of trail (depending on the width of the tyre). 55-60mm of trail is often regarded as the sweet spot for a road bike, however Rinard has found that an extra 5mm in either direction will often work well for a lot of riders. In addition, small changes (e.g. 2mm) in fork rake and trail are unlikely to be noticed.
That should simplify things for those that are replacing an existing fork, though it’s still worth taking notice of just how much rake there is to start with. In this regard, this is not a specification that is normally displayed on a fork, and it can be difficult to measure accurately, especially when the fork is fitted to a bike. Some manufacturers may list it in their geometry charts; otherwise, a reasonably sound measure can be made by determining the distance between the front axle and the bottom bracket shell with the fork facing forwards, then backwards, and halving the difference between the two.
One other reason for taking notice of fork rake is that it has an effect on toe overlap, albeit a small one. A fork with less rake will bring the front wheel closer to the frame and the pedals, increasing the risk of toe overlap. Once again, a small reduction (2mm) is likely to go unnoticed, while a bigger reduction may create a problem, especially for small frames.
While all road forks are designed to accommodate a 700c wheel, the length of the legs, or more specifically, the axle-to-crown length, can vary, often according to the maximum tyre size that can be used. For example, a standard aftermarket road fork (~28mm maximum tyre size) has an axle-to-crown measurement of 365-370mm; that number increases to 380-385mm for an all-road disc fork (~35mm maximum tyre size) and 395mm for a dedicated gravel fork (~50mm maximum tyre size).
It is important to pay attention to axle-to-crown length when replacing a fork because any difference will raise or lower the front end of the bike. Thus, fitting a fork with longer legs will raise the front end of the bike, slackening the head tube angle and increasing the amount of trail, which will slow the steering of the bike. Shorter legs will have the opposite effect, resulting in quicker steering.
If the difference in axle-crown length is just a few millimetres, then any effect is likely to go unnoticed (Figure 1). A larger change (10mm or more) shouldn’t be ignored, especially if it is a reduction, because it may suddenly render the bike unstable at high speeds because of the increase in head angle and reduction in trail.
The axle-to-crown measurement is rarely, if ever, recorded on most modern road forks, and like rake, it can be difficult to measure accurately. That’s because it is measured along the steering axis of the fork. The actual axle-to-crown length, measured along the centreline of the fork leg, will always be a little longer, but if the rake is known, then the axle-crown measurement can be calculated using the Pythagorean theorem.
It should also be remembered that any change in the axle-to-crown measurement will also have an effect on the stack of the frame, so the position of the stem may need to be adjusted after the new fork is fitted. In the case where a large number of spacers (40mm) is already required, then a shorter fork should be avoided so that there is no risk of exceeding any limit on the number of spacers that can safely be used with the new fork.
This list of additional options for a fork is small, starting with fender mounts. In the past, these took the form of eyelets at the dropout; now they can be hidden in the lower leg and crown of a carbon fork. Shoppers need to keep in mind that the maximum tyre size that can be fitted with the fender will generally be less than the maximum tyre size recommended for the fork.
Rack mounts are another option for a fork, and these can take a couple of different forms with fittings on the front or side of each leg. Some will suit pannier racks while others are designed for cargo cages. The former offers more carrying capacity but will add more weight to the fork to slow the steering of the bike.
Finally, some forks offer support for front hub dynamos in the form of internal wire routing and a light mount at the crown. For those that want to take this integration a step further by taking advantage of SON’s self-connecting SL system (where the electrical contacts for the hub are incorporated into the dropouts), they will have to source a custom-made steel fork.
What about stiffness?
As the sole structure responsible for mating the front wheel with the rest of the bike, a fork has a lot of work to do. It must contend with lateral, longitudinal, and vertical forces as the bike is moved from side-to-side, and in and out of corners, over all sorts of terrain at high and low speeds, with and without braking.
Throughout it all, the fork is constantly flexing.
“They’re all going to flex,” explained Mark Hester, from Prova Cycles. “Even the stiffest fork is going to flex when you brake. They kind of have to. It’s like an aeroplane wing; it has to deform, otherwise it will snap off.”
There is a limit to how much flex is acceptable, though. If the fork cannot resist the forces of braking, for example, then the rider will experience shudder, and in the case of disc brakes, it can also cause the bike to steer to the left.
“The fork can’t be infinitely flexible,” said Kevin Nelson, Enve’s chief engineer. “It’s about finding the right balance for that part. Figuring out that sweet spot is an iterative process, and we rely heavily on ride testing for everything we do. We frequently qualify multiple laminates for production and then we do ride testing to decide what direction we want to go.”
Rinard also agrees on the importance of carefully tuning the stiffness of a fork.
“During the development of a fork, the layup is tuned during a make-it/break-it cycle, where we do a bunch of tests with different samples of the forks. We change the layup until it is the right strength — not too little, not too much — and it is the right stiffness. We know that lateral stiffness is important — a laterally flexible fork can be squirrelly in a corner — so we tune the lateral stiffness so it’s stiff enough. Extra stiffness is just extra, so we don’t aim for exceedingly stiff, but enough.”
This kind of thinking appears to permeate the industry because when the lateral and longitudinal stiffness is compared for a range of manufacturers, there is a surprisingly modest amount of variation in fork stiffness (Figure 2). The sweet spot for lateral stiffness appears to be around 50N/mm, while the amount of longitudinal stiffness is typically twice that for any given fork.
This kind of data is rarely, if ever, included in the specifications for a fork, so it’s not something the shoppers can easily assess or compare. With that said, there may not be any great need for it.
“There are a lot of rigid forks that ride really well,” said Rinard, “and there are a lot of flexible forks that ride okay, but I’m hesitant to go too much into the fork stiffness thing because it just doesn’t matter much. The lateral stiffness can matter, but fore-aft [longitudinal] stiffness is not that significant until it becomes too flexible. Then, braking shudders are no fun, especially with disc brakes.”
If nothing else, the near-homogeneity in fork stiffness in the current market suggests that any difference in steering and/or ride quality is going to be a matter of nuance. Moreover, in the absence of methodical testing, few riders will ever understand if they prefer a fork that is more or less stiff, or, if they will even notice a difference in lateral or longitudinal stiffness.
Can a rigid fork enhance the comfort of a bike?
In absolute terms, a rigid fork does not have a lot to offer in terms of comfort and compliance simply because it is not designed to compress like true suspension. Thus, when it comes to soaking up sharp hits, the only way that a rigid fork can absorb any of this shock is by flexing.
In this regard, steel forks with slender curved legs can be quite effective. Jan Heine found that a lightweight steel fork could match the performance of a short travel suspension fork on a corrugated surface through visible flexion of the lower legs. This not only alleviated sensations at the handlebars, it also provided a significant reduction in the amount of power that was wasted to suspension losses when maintaining the speed of the bike on the corrugations.
These differences were enough to convince Heine that a longitudinally flexible fork may have more to offer road cyclists than a stiff fork, or at least, that a stiff fork was not a strict necessity. Tour Magazin tends to agree with this notion. It awards higher points for comfort to those forks that exhibit more longitudinal flex.
Rinard is not convinced that this kind of fore/aft flex has much to offer road cyclists, though.
“You can target flex areas in a frame,” he said, “but for a fork, you’re always going to have so much stiffness that you can’t really say that one fork is more comfortable than another. The differences among forks are pretty minor, and you don’t necessarily want to have the flex in one place and not another place. So don’t look for comfort in a fork; instead, look for it in tyre pressure.”
If nothing else, consumers will find it much easier to adjust tyre pressure than find information on the relative compliance for any given fork. There will always be a limit to the amount of comfort that can be achieved for any bike that has a rigid fork. So, for those riders that have lowered their front tyre pressure and are still looking for more compliance, some kind of suspension system will be necessary, which may be as simple as swapping to a new stem.
There is no need to explain the importance of a safe fork to anybody because the repercussions for any kind of failure are as obvious as they are potentially catastrophic. Needless to say, the bike industry has a healthy regard for the issue along with decades of experience with various materials and a sophisticated understanding of the engineering requirements for a safe and durable fork.
Any fork that enters the market, OEM or otherwise, generally has to satisfy ISO 4210 (or the local equivalent), which outlines the minimum requirements for strength, durability, and impact resistance. While the testing conditions for this standard are admittedly quite limited and only go so far towards replicating real-world use, they do something to ensure a minimum standard of fork construction.
“The ISO test standards have generally proven to make, more or less, reliable forks, and the whole industry agrees to do them,” explained Rinard. “But almost every company that I’m aware of, and every company I’ve worked for, exceeds those standards. Internally, the lawyers go, ‘Yeah, it meets the minimum, but what if a fork breaks?’ And so we go higher: 20 percent above the legal minimum requirement, sometimes 200 percent above it. We have an internal requirement for multiple fork samples to pass the fatigue test — all of them — and to see the number of cycles that they go to. Sometimes it’s double, triple, quadruple. I don’t know any bike engineers that don’t respect the risk of a fork that is a little weak.”
Enve is equally rigorous in its approach to testing forks, which includes a careful study of what it takes to make them fail. “One of the things that Enve has always done is we’ve taken failure mode really seriously,” said Nelson. “We have a battery of tests that we go through and one of them is to basically bend the fork in that direction until it breaks, and then we have energy and deflection requirements associated with that.”
To this end, carbon composites have proven almost ideal for creating a strong and resilient fork because of the material’s exceptional fatigue resistance. Recent advances in resin technology and a move towards one-piece construction have only improved the quality of rigid forks. Newer technologies are also in development to further enhance the safety of carbon fibre bicycle forks.
Nevertheless, forks continue to fail (albeit very rarely) and/or trigger product recalls, not because of any shortcomings in the material, but because of defects that arise during manufacturing.
“In the case of an early failure,” said Rinard, “there could be variations, which are unfortunately inevitable in modern manufacturing. Metals are not perfect, welds are not perfect, composites are not perfect, which is why we do these tests on more than just one sample. I know it’s no solace if you’ve broken a fork and suffered from it, and I don’t want to sound callous, but forks, in general, are pretty safe. The engineers that I know have a really, really healthy respect for the possible injuries that could result from a fork that might fail.”
While most major manufacturers depend upon a quality control program for monitoring defects, Canyon goes one step further by conducting a CT scan of all of its rigid forks. The equipment involved is very expensive, and experienced operators are crucial for this effort, but it offers the company a near foolproof strategy for identifying a defective product before it reaches the customer.
There are other signs that the industry is intent on improving the safety of carbon forks by attending to aspects that are overlooked by the current standards. Here, the top of the fork steerer is the most obvious, which can be damaged through misuse, or, during shipping. “What we’ve done at Cannondale since I arrived is we’ve made our carbon steerer tube wall thicker,” said Rinard. “It was 2.5mm thick and we went up to 3mm. There was no test that the 2.5mm wall failed, but we still had failures in the field, perhaps due to misuse or abuse, but we didn’t want those failures. It added 9g to the weight of the fork and it is there to protect against that stuff.”
For Nelson, and Enve, he sees the scope of fork design and engineering expanding to take in more of the unexpected.
“There’s always a downward push on weight of a product,” he said, “but I think what’s coming is an expectation that the product is going to last longer through the things that are really difficult to test for, the sort of strange things that happen, like falling over in the garage or a rock strike. You see that on some of the product design in MTB, and I think we’re going to see more of that.”
While these measures promise to extend the service life of the fork, there is still no easy answer to the question of how long a fork can be safely used. No amount of testing can ever fully anticipate what can happen in the real world, and when the span of time stretches to many years, there are all sorts of variables that can influence the lifespan of a fork, both positively and negatively. As a result, riders must exercise their own judgement, though it is possible to engage an x-ray/ultrasound service to make a thorough (i.e. sub-surface) assessment of any fork to help with this decision.
Summary and final thoughts
Rigid forks have come a long way in the last twenty years. Thanks to the rise of carbon composites, forks are now lighter, yet arguably safer, and in some cases, stiffer and/or more aerodynamic. There is more variety in specifications, too, due to the introduction of new brake fittings, front axles, and larger tyre sizes. While this provides consumers with much more variety in products, it also generates a range of incompatibilities that can confound the uninitiated.
By contrast, when it comes to function, there is much less variety to contend with. Indeed, any differences in weight, stiffness/compliance, and aerodynamics will be a matter of nuance, so there is really no point in dwelling upon these aspects when shopping for a fork. What is more important is the geometry of the fork — specifically, its rake and axle-crown length — and whether it will complement the geometry of the frame. Altering one or both of these aspects is more likely to have a noticeable effect on the behaviour of the bike, and therefore, this is where shoppers should direct all of their attention.