MATTER: Columbus Futura Cross, Adjustable Rake, and Steering Stability
‘A fork is a fork is a fork,’ some might say.
You wouldn't expect me to make such a statement, would you? No, because, as ever, the devil is in the details. But which details matter? Suspension forks dazzle us with features and metrics, fodder for comparison and deliberation. Details are excluded from the frame when features abound. Oh, could I tell stories about suspension fork devilry.....
If you don't care about the details, don't have time for them, find them distractingly confusing, and/or came here for a simple answer to the question, 'Is the Columbus Futura Cross fork a quality option, and good for gravel riding across a spectrum of wheel sizes and configurations?', I will spare you the time required to read on: yes.
Yes, the Futura Cross meets all my expectations and requirements for all-road and gravel riding, and I highly recommend it.
This piece was never going to be simple or short. It’s one thing to convey whether the Futura Cross does fork things well. It’s another thing to dig into the reasons behind Columbus committing any time, money, and energy into developing and manufacturing the fork. And it’s yet another thing to formulate recommendations for you, dear reader. This piece is about a lot more than a fork.
If you’ve been struggling to get your head around why certain cyclocross and gravel bike front-end configurations have felt normal, good, and the opposite, you’re thinking about making changes to your set-up, commission a custom bike, or purchase a new one off-the-shelf, what follows is for you. It’s dense, and it’s not as well organized as I might wish it to be. I could apologize for this, but I’m literally writing these words early in the morning before starting my actual job, so I won’t. I hope it helps, despite any blemishes.
Details that MATTER
If you’ve followed my writing on ‘gravel bike stuff’ for years, you’ve probably seen me wish for a good gravel/cyclocross fork with adjustable rake. This goes back more than 5 years, and I wrote about it in ‘How to choose a gravel bike Part-2: geometry’ piece, which is the most-read item on the site. As soon as I had disc brakes on my cyclocross bike and was able to mount 650b Woven wheels, I wanted to test different fork rakes. But there wasn't a ready way to do so.
In response to a reader’s question about gravel bike ride quality, I wrote the following in MATTER of FACT: Gravel Compliance and Comfort:
Fork: this is the hardest one to tune, because very few people have opportunity to compare forks on the same test bike. Unless you do that, and keep all other variables constant, it's hard to parse out specific fork characteristics. So a lot of this is anecdotal and circumstantial.
Now that all the tuning variables are elucidated, I suggest the fork is the one you want to stress about the least.
Prioritize fork length (axle to crown' being what the frame is designed to run), then stick with the 50mm rake they want on there. That's a great geometry with a 72 degree Head-tube angle (HTA).
If you'd like to have the option of tweaking your trail (front end steering geo) based on wheel diameter and tire size and/or front load for touring, I'd point you toward the Columbus Futura Cross fork, with adjustable rake (as discussed in my gravel geometry page).
Knowing Columbus, I suspect the Futura Cross strikes a nice balance in terms of steering precision and resilience, and the adjustability is great for future-proofing the bike. In the 52mm rake setting it would be great with the big tires, and in the 47, just like a nice CX bike with the smaller tires. What I don’t know, but would like to explore, is how the Futura Cross and Cross Plus compare in terms of stiffness, given the latter is intended for some additional load. I’m working on acquiring a sample of at least one of the models to get some practical experience with its ride quality so I can speak to it in greater detail.
Columbus was kind enough to provide me a Futura Cross sample to carry out the evaluation I had in mind. Going into the test - which has extended 12 months - I was confident I’d be able to easily discern the impacts of rake change on my bike’s handling, but I was less certain about being able to discern compliance subtleties.
How Did We Get Here?
MISSY GIOVE'S 1993 YETI ARC ASLT - I measured this HTA at 72 degrees. Photo: The Pro’s Closet
The relevant historical context around adjustable fork rake is that quality dropbar bikes used to always have specific fork rakes to work in harmony with each frame’s geometry. This norm carried through to how the first mountain bikes were designed and built. These bikes used rather wide handlebars, which influenced front-end geometry, and as racing took off and rapidly transformed from long-distance enduro-style events to more intense cross-country, more road racers entered the fray. 'Roadies' on MTBs pushed their bodies into more laid-out positions with longer stems, and reduced their handlebar width so they could still steer. Frame and fork geometries were tweaked in parallel (at least on the better bikes), and suspension forks were soon part of the equation. As front suspension gained popularity and was proven at the highest levels of the sport, the realities of mass-manufacturing drove 'standardization' of fork rakes. Gary Fisher speaks about this in detail in this fantastic podcast conversation.
While road bikes rolling on consistently small tires continued to be built with harmonized frame and fork geometries, keeping the concept of 'trail' alive within the road scene, it became a background issue, at most, for the MTB community. Trail couldn't be manipulated with both key variables - head-tube angle (HTA) and fork rake - because fork rake was locked-in. The only way to influence the handling characteristics of a given frame was to vary HTA, tire size, tread, pressure, stem length, and handlebar width. If a given HTA and fork rake combination was ill-matched, the other variables at play both clouded the geometry dynamic, and offered options for tweaking it. However, this was in the context of MTB riders not knowing what an excellent mountain bike could actually feel like, and the extreme variability of off-road terrain and surfaces obscured handling nuances.
Back in the mid 1990s, some of us started using 'downhill bars' on our XC hardtails in an effort to gain more leverage and control over a front-end that was far more similar to a road bike than a modern MTB, with front wheels positioned too close to our feet. With 120 and 130mm stems, there was a lot of torque for our hands and arms to handle when our front wheels tucked under. We started looking for frames that would 'rake-out' our forks, slackening our HTAs, but as we did this with fork rakes that were 'locked,' we experienced new handling quirks. This was especially true when we raked-out our bikes by extending fork travel. At the time, none of us seemed to know anything about fork rake, so we couldn't even know what we were missing.
Companies like Cannondale, who developed their own forks, could have varied their HTAs per frame size and tweaked their fork rakes for each size to match. But they didn't, because there wasn't enough to gain in terms of performance against the added cost of producing multiple fork geometries per fork model. They went with the same HTA per frame size, while road bikes continued to vary HTAs per size to balance variables like toe-overlap on smaller sizes (slacker HTAs, more fork rake), and weight distribution on larger frames (steeper HTAs, less fork rake). If you peruse current MTB geometry charts, you'll see that the common practice is to run the same HTA across all sizes for a given model. The reason rake and trail are variables that come up in MTB geometry discussion these days is that fork travels are adjustable, and two rim sizes are being used in a multitude of configurations. 27.5 and 29" wheels with 'normal' or 'plus' sized tires will work best with different fork rakes. Mullet set-ups, with 27.5 wheels on the back, 29" on the front, benefit from tweaks in fork rake. Thus, many suspension fork manufacturers are offering their forks in two rake options.
Adjustable Fork Rake: Why?
Given the fact that trail has always been relevant in the road cycling domain, and it is now firmly established in the MTB domain, the scene is set for adjustable rake to enable proper function of a diversity of gravel and all-road configurations. The primary factor driving this adjustability is tire choice. Ultimately, this is about maintaining a configuration that supports fun riding, and minimizes the likelihood of scary experiences occurring.
Road bikes constrain tire volume and run relatively high pressures: up to about 32mm, 50psi and up. Between 23 and 32mm, there's enough of a change in tire dynamics to make a rake adjustment worthwhile. However, a rider accustomed to a 23mm swapping to 32s will not likely be disturbed by the handling of a 32mm tire, because such a change will make a road bike handle 'slower' and less 'nervously'. This tire size isn't large enough to generate 'oversteer' on a road bike, which is the 'scary' trait that can be experienced with higher volume tires on 'wrong' front-end geometries.
In contrast, 'gravel bikes' are essentially where MTBs were in the 1990s, or perhaps early 2000s in terms of consensus around handling requirements. In other words, there is no consensus. The same dynamic played out with MTBs, and led to a low-key ludicrous proliferation of categories. WTF is 'down-country,' anyhow? Seriously.
Some gravel bikes will benefit from adjustable rake forks because they are gravel bikes. Huh? I'll explain.
Fat bikes with 4" and larger tires run pressures at and lower than 8psi (I'm not joking, below 7 is where they are run for snow riding, which feels fantastic on trails. However, at these pressures, fat bikes feel INSANE on pavement or packed dirt. On these high-grip surfaces, the front wheel's rim moves within the tire casing as the tire rolls across the ground. This creates oversteer that feels horrendous. However, fat bikes are NOT being reconfigured to improve this handling 'quirk', because they SUCK for hard surface riding no matter what you do with geometry. If you improve a fat bike's geometry for pavement, it will not perform well on snow. So don't do that.
Contrast the fat bike against a gravel bike. A gravel bike should feel good on any surface drop bars are suited to, including hard-packed snow (ATMO). If we wanted to optimize handling for soft terrain, we could do that. If we wanted to optimize for pavement, we could do that (wait, it's done, 'road bikes'). We're trying to optimize for both surfaces, often within one ride. Modern MTBs are NOT at all trying to do this. They are optimized for technical trails. Therefore they tend to suck on pavement. Thus, there's a big market for gravel bikes. And when I say 'optimize,' what we should really have in mind is a focus on finding an 'optimal compromise.'
Making ‘Optimal Compromise’ Real
Changing tire diameter, volume, tread, construction, pressure, and format (tubed, tubeless, tire insert), along with rim depth, influence how your bike steers. While most road bikes are ridden with the same tire set-up all season (and increasingly so, as gravel bikes off-load non-paved and poorly paved road bike use), gravel bikes are generally used for a broad spectrum of riding across four seasons. They are also increasingly being pressed into 'Jack-of-all-trades' use-cases, shod with road tires for sporadic road rides. Most riders will choose their gravel bike over their road bike for travel, and it's gravel bikes that tend to be pressed into bikepacking use over road bikes. Consequently, a gravel bike might see itself shod with anything from a slick 700c x 26mm road tire (this is literally my fast-road setup I use on my fast-gravel bike) to a 700c x 55mm knobby. The outer diameter of these wheels differs drastically, and a gravel bike like my Brodie Romax Carbon isn't designed to handle well with either of them. As I've covered previously, there's a strong rationale for dropping from 700c diameter wheels to the smaller 650b option when increasing tire volume beyond around 38-42mm on a given gravel bike (depending on geometry). This decision revolves around ride height preferences and steering requirements for a given use-case.
Regardless of rationale for manipulating tire and rim specs, every tweak we make yields effects. Within a tight range of tweaking, changes in handling nuances will be 'absorbed' by the rider, unconsciously. The degree to which small changes are absorbed depends on the rider's sensitivity : experience ratio. Riders who've spent countless hours riding different types and configurations of bikes over weeks and seasons will tend to rapidly adjust to handling quirks. Those who have tended to ride one bike setup alone for long periods of time will struggle with configuration changes, especially if they are sensitive to feedback from the bike. Sensitivity to feedback, while perhaps unnerving, is actually quite beneficial, however, as it informs the rider immediately when something about their bike's handling is now different: ‘be careful.’ For those who are insensitive to bike feedback, changes in configuration that create handling quirks are more likely to not realize they need to adjust their riding technique until they are in a situation where things are going wrong, and it's too late. They crash, don't really know why, and become afraid of their bike.
This is what I'm trying to help avoid by writing this.
Fork Stability and Compliance
One of the reasons there was pushback against the introduction of disc brakes to dropbar bikes was compliance, which is a way of saying ‘passive suspension.’ With rim brakes, it was relatively straighforward to build the desired amount of compliance / flex / comfort / suspension into rigid frame and fork assemblies. For forks, this was accomplished through careful choice of tubing (material, thickness, diameter, taper), crown, and fork shape. Similarly, chainstays dominate the degree to which a frame absorbs and transmits road feedback, so seat-stay-mounted caliper brakes didn't really influence a frame's comfort/compliance to a significant degree. Disc brakes asymmetrically load forks and frames, and at the same time bring focused load to fork and frame materials. This introduces demand for reinforcements to ensure stable steering under braking load, and maintain structural integrity throughout the product's lifecycle. As disc brakes were becoming available for dropbar bikes, the concern was that adding material to support discs would reduce the frame and fork's ability to provide sufficient passive suspension, which translates into comfort, stability, and efficiency.
I believe these concerns were and remain well-founded, and the impact of disc brakes on compliance has been part of the transition we've seen to increasingly voluminous tires on dropbar bikes. Not only do disc brakes allow larger tires to be fitted, the also require them to be fitted. On the balance, I'm happier with disc brakes than I was with rim brakes, especially when talking about cyclocross bikes and the woes of brake chatter and finicky linear-pull brakes. I already wanted to use higher volume tires than the norm before disc brakes were incorporated.
Increasing tire volume masks nuances between rigid forks with regard to compliance (comfort) and stability. The more conforming a tire is, the harder it is to discern fork flex, be it 'good' or 'bad.'
'Good' fork compliance is about isolating the rider from road vibrations and impacts. Vibrations 'suck' energy from our bodies, wearing us down over time, while higher amplitude impacts can hurt our hands, wrists, and backsides. Rigid forks are designed to function the same way as suspension forks, but at a micro scale. The idea is not to simply absorb energy in the form of vibrations and impacts, but to return that energy to the tire on the 'backside' of the input.
With suspension, this dynamic is easy to understand. A fork allows the front axle to travel upwards while the fork crown remains at its vertical position (theoretically). As the axle travels upwards, 'damping' controls the speed at which the fork's lower assembly moves, then the speed at which the assembly returns the axle to its original position. When the fork's tuning is well balanced, it 'pushes' the wheel down the backside of rocks and roots, which helps propel the bike forward. When the fork's rebound damping is too slow, that potential energy is effectively wasted. It doesn't help move the bike forward.
Rigid forks do the same thing, again, at a micro scale. Carbon fiber affords the greatest degree of 'tuning' available, when compared against steel, aluminum, and titanium, because the way each fork's carbon fibers are laid over each other, in combination with the 'glue' that is used to bond the assembly together, determines the structure's vibration damping performance and ability to take high-energy impacts and return to original form without sustaining damage.
Cheap forks can accomplish strength by overbuilding, with the obvious result: very rigid, shock-transmitting ride-quality. To balance strength stability, and compliance, is hard. It also brings to mind cycling's 'Golden Ratio': LIGHT : STRONG : CHEAP. We can only ever have two of these elements at once. As carbon products push the Golden Ratio's limits, the margins of error narrow. Designing a light and strong carbon fork that also provides a compliant ride quality demands a great deal of expertise and care go into the selection of raw materials, the 'layup schedule' for the carbon fiber, and precisely repeatable construction methods. As margins of error narrow, everything gets more expensive. At the same time, the engineering and quality control that go into building light carbon forks also enables compliant ride quality. This means the money that goes into saving weight on the bike also goes into improving the bike's performance over everything but the smoothest pavement. The same can't be said for dropping weight from a crankarm, a stem, or a thru-axle. These components are not intended to provide beneficial compliance, and shedding weight from them doesn't impact how our bodies feel on our bikes.
RESULTS: A Year on the Futura Cross
Which Bike? Why?
I chose to install the Futura Cross on my most ‘aggressive’ gravel bike, my custom T-Lab X3. This bike has the slackest HTA of my CX and gravel bikes - 71.5 - and was built with a 50mm rake fork. The bike was set up for the roughest riding I did at the time, since I still didn’t have a mountain bike rolling. The biggest tires the bike fits are 650b x 48mm, which I tended to run most of the time. The bike was meant to serve as a second cyclocross bike for fall, which would see it run 700c x 33mm tires. The outer diameter of these two set-ups is essentially the same, but they don’t handle the same. With the 71.5 HTA, I wanted to achieve handling that was more stable than steeper HTAs when the bike’s tires were side-loaded: off-cambers, the inside edges of ruts, turns. Essentially, I wanted the bike’s geometry to nudge it toward ‘standing up’ when the front wheel slides, versus ‘turning in’.
For the T-Lab, I’d likely ride 700c x 35m Rene Herse Bon Jons sometimes, but would rarely have cause to use anything smaller than 33mm, which is my preferred size for snirt riding. This bike was well-suited to that sort of riding, particularly because of its trail. On this bike, a 47mm fork, on paper, would be ‘bad’ for pretty much every tire size and format I wanted to actually ride. But what would it feel like? I wanted to know. And the truth was a lot of folks write me about custom bikes they’re commissioning, and they often talk about mostly wanting to ride something like a 35 or 38mm tire, and sometimes bump up to 42 or a 650b x 48. They are often offered 47mm rake forks, and want to know whether they’ll be happy with the result. At a common 72-degree HTA, trail already starts to creep out of the ‘sweet-spot range I indicated back in 2017: 55-64mm. A 38mm tire pushes to 65mm trail, and it only gets worse from there. A 72 / 47mm bike can only really feel good beyond 700c x 35mm if you bump down to 650b and run a 42mm tire for 60mm trail, or a 48mm tire for 62mm trail. I’ve used both these extensively on bikes with this geo, and confirm that the numbers don’t just look good on paper. At the same time, I’ve never liked 38mm slicks on that geometry, while 38mm and 42mm knobbies were fine. This is because knobbies dull steering dynamics and feedbacks.
At 71.5-degrees HTA, I wanted to know whether 5mm fork rake could really be felt; was it obvious? Would it matter? Would I wind up using one setting and forget about it?
First Impressions
Setting up the Futura Cross was mostly like any other fork install, with the exception of putting the modular dropout together. Small details like hidden fender mounts are very nicely executed, and the expander plug provided is excellent; it’s plenty long enough to reinforces the steerer-tube when spacers are placed above and below the stem. Many expander plugs are too short, and don’t sufficiently support the steerer tube, which can lead to material break-down and potential failure. The included instructions are VERY good in showing how the rider must ensure the plug is located within the stem's clamping area. This is something riders get wrong often. The fork also comes with a stem-cap and bolt, ready to go. The raw carbon on the steerer-tube showed no signs of voids or problematic defects.
As you can see in the photos, the multi-rake assembly uses a concentric recess for the ‘flip-chips’. This design translates into non-reliance on the small screws used to connect the assembly parts. The axle’s tension pulls everything tight; the small screws’ job is primarily to keep the two sides of the assembly in place while the front wheel is out. The diagram shows a design that would have been less secure than what Columbus is actually producing.
Riding Results
Overall, I’m quite happy with it the Futura Cross. The fork has never felt ‘harsh’ on the bike; comfort feels ‘‘neutral’. I can feel the difference between handlebar flex more than I can feel the difference between this fork and most others I’ve ridden. I only have one carbon fork that feels discernably ‘off’, the one on my Brodie Romax Carbon flexes under high out-of-the-saddle torque more than I want.
I'm able to produce enough torsional flex out of the Futura Cross to get some brake rub here and there, but I consider this an acceptable compromise for its compliance. The drop-out chips have been fairly secure, but I have learned that they do need periodic checks to ensure they are tight, as heavy braking can cause axle loosening. Related, if these screws are loose, mounting the wheel can position the wheel a bit out of alignment, then shift after the first heavy braking occurs. There’s no ‘problem’ here; just check the screws are snug periodically.
I enquired with the Columbus team about whether their Futura Cross + fork, which has mounts for attaching load along the sides of its legs, was built more robustly than the Futura Cross. It would stand to reason that they’d do this, right? Right, the Futura Cross + is indeed comparatively ‘overbuilt,’ and will not have the same degree of compliance/comfort/passive suspension as the Futura Cross. If you want to maximize comfort, grip, and energy conservation, the Futura Cross is optimal. I’d recommend against going with a Futura Cross + just because you might do some bikepacking.
I’m entirely happy with the fork’s ride quality and perceived strength and durability. I trust the fork, and would be confident riding any other fork Columbus makes.
Adjustable Rake: Worth It?
Worth it? Sure could be! What you must want to know is: can you feel 5mm difference in fork rake, back to back?
Yes, easily, and unequivocally. The difference is stark, and can allow you to go from terrible handling to excellent handling. For example, 71.5 / 47mm is a bad configuration with virtually any tire set-up you’d actually want to ride, with the exception of 650b x 42mm. That’s actually only a size I’d use to get more volume out of a bike with geo that doesn’t allow for use of a 650b x 48 or 700c x 42. At 64mm trail, this 650b x 42 is at the upper limit of the good-handling range. With a 650b x 48mm tire a slick tread would likely feel a little floppy (oversteer), but a knobby tire works fine.
I didn’t crunch steering geometry numbers before mounting the Futura Cross to my T-Lab (71.5-degree HTA), because I didn’t want to bias my perceptions. When designing the bike, I figured I’d ultimately put the Futura Cross on, and probably use it in 52mm rake most of the time. As I mention above, 71.5 is the slackest drop-bar bike HTA I have, and I had no experience on this geometry before the T-Lab.
I created the chart below to illustrate how a realistic range of tire set-ups translate into trail for the three fork rakes I’ve been riding across three bikes.
Futura Cross at 47/52mm on T-Lab X3 @ 71.5 HTA
TRP OE 47mm fork on aluminum Brodie Romax @ 72 HTA
Carbon OE 50mm fork on carbon Brodie Romax @72.5 HTA
I generated all these values with this calculator, and what you see is an extract; I also produced numbers down to 28mm tires, 700c x 48mm, and 650b x 42mm; these are all ‘outlier’ sizes, however, IMO. The range I show here covers a practical use-range, from a 32mm ‘road tire’ to a 33mm cyclocross tire, through to a 42mm slick or knobby. Having tested 700c x 48mm knobbies, I still advocate dropping to the 650b diameter beyond 42mm. This is a matter of ride-height and trail, and you can infer from the chart that some combinations will allow you to run a 700c x 48mm tire at a good trail value. If you want a really stable-feeling bike at speed, and accept a drop in agility, the size could be for you.
Defining Limits
The chart backs up my perception that 71.5 HTA is slack enough to render any rake lower than 52mm undesirable. The red cells in the chart specify trail values that don’t ride well with slick tires on pavement. Bear in mind that knobby tires drastically reduce the severity of wheel flop and oversteer.
I maintain my threshold of 64mm at the upper end of ‘good trail’ values.
The question of whether adjustable rake is valuable is separate from which specific settings are deployed. 52mm rake is essentially the lowest a 71.5-degree HTA bike should have, so the Futura Cross’s 47mm setting is effectively ‘useless.’ But my T-Lab’s geometry wasn’t common when Columbus developed this fork, and it still isn’t common. So the question of adjustable rake is less about my idiosyncratic bike, and more about other bikes out there, which have steeper HTAs.
At 72.5, both Futura Cross settings are useful. A 50mm rake fork, as I have, also covers my desirable tire range well. I actually go as low as 26mm tires on my carbon Romax, which puts me at 55mm rake; on the low side. I would run that tire size at 47mm if I could. On the other end of the tire volume spectrum, take a look at the 72.5 bike with 42mm tires. At 60mm rake, this size feels quite neutral. The Futura Cross on this bike would offer two interesting options here, to contrast 58mm and 63mm trail. 63 would likely feel like the bike ‘carves’ turns more, and reacts more to input from the hips. The 58mm trail setting would yield a lighter steering feel in the hands, and the bike would be easier to countersteer. The front wheel would also be more prone to buffeting in cross-winds.
If you’re on a 72 HTA bike with a 47mm rake fork, you can improve your situation by swapping to a 50 or 52mm rake fork, one way or another. The Futura Cross’s benefit would depend on whether you’d use both wheel diameters, and the sort of handling dynamics you’re after. For a bike as slack 71.5 or 71, 52mm is the bottom-limit of useful rake, and 57mm could be the ideal alternate setting.
But why?
You’ve read a lot by this point, and you might be wondering: ‘Why is Matt so picky about all this?’ The reason is that I’ve experienced many crashes I think could have been avoided if my bikes had better steering geometry. Most ‘bad experiences’ in this domain occurred on cyclocross bikes adapted for ‘gravel’ races with tires larger than 33mm. Some of them occurred on snow and ice during winter rides. I’ve been ‘railed’ into trees by ruts, I’ve had my bike do the wrong thing on off-cambers, and all sorts of other incidents. As a MTB rider ‘by training,’ a lot of the steering dynamics I’ve experienced on drop-bar bikes have felt very wrong, and I’ve been working to understand and rectify these issues for more than a decade.
I wanted to run a slacker head-tube angle on my T-Lab to improve steering stability through ruts and off-camber surfaces. Consider the 71.5/47mm/32mm configuration in the chart: trail is 67. While this might feel ok with a cyclocross knobby on pavement, it won’t work well on a cyclocross course, because the front wheel will oversteer when the tire looses traction in a lean. This amplifies traction loss. On the opposite end of the spectrum, for contrast, I raced a steel Pinarello cyclocross bike for a year or two, with a 74 degree HTA, 35mm rake. I’m not kidding. This bike had trail at 63mm, the same as my Romax with 47mm rake, right? Yes, with flop at 2mm lower. The Pinarello handled like a scalpel on a CX course, cutting left and right easily; ‘sharp handling.’ On off-camber turns it consistently tore grass out of the ground, since it placed so much weight on the tire’s contact patch. On dirt/gravel roads, it was THE MOST TERRIFYING bike I’ve ever ridden downhill. The front wheel would lose traction easily when leaned; it was awful. That bike was fascinating, as I could tell that it could excel on certain types of cyclocross track, but was a VERY specific tool. I came to realize that a lot of riders were having bad experiences on cyclocross and gravel bikes as they were changing their wheel and tire configurations, so I was very motivated to get to figure it out. Here we are.
Flop and Oversteer
A slack HTA with enough rake makes the bike ‘want’ to keep steering straight when leaned over. This is a matter of opposing forces.
Trail acts like lever that causes the ground to push the wheel behind the steering axis. With 0 trail, this doesn’t happen. Both of these have the effect that more relative lean to the force vector (vertical gravity when not turning) the more turn-in, and this is probably related to the idea of weight-shift responsiveness. Note that these phenomenon have to do with lean, it’s not just gravity working to destabilize steering while caster effect stabilizes steering on a vertical bicycle as per the wheel flop explanation. These are not identical effects so even when you compensate for one with the other, handing will likely feel somewhat different. - Kuromori Cycles
Cannondale’s Super 6 is an example of a good trail execution. At 61 degrees, the bike uses a 55mm fork rake. This yields 62mm trail with a 33mm tire. This works well because the weight over the tire’s contact patch is balanced against the distance the front hub lowers as the bike is leaned. The more the hub lowers (which happens increasingly as HTA slackens), the more flop/oversteer we have to contend with at the handlebars. Increasing rake changes how much the hub lowers when we steer and lean the bike; it decreases hub lowering. This can be felt most strongly when countersteering down a descent, with the back wheel sliding out to a side.
Imagine the Super 6 rolling along an off-camber / side-hill. There’s little traction, and the idea is to just go straight until a turn 30m ahead. You position your weight on the outside pedal, which loads the bottom-bracket, translating weight into the head-tube, which translates weight down the fork legs. The weight is behind the axle and tire’s contact patch.
In this scenario, when the front wheel loses traction the rider will need to shift weight loading the bottom bracket to influence the head-tube in relation to the tire’s contact patch. You can’t just ‘steer’ out of these slides with your hands; the hips are the dominant influence, as the centre of mass, essentially.
When a bike’s geometry is ‘right’, hip/body force will swing the head tube into the hill, and countersteer the front wheel back to upright. In contrast, when body input doesn’t/can’t drive this ‘swing,’ the front wheel oversteers/flops quickly, and crosses the threshold of salvageability. When it crosses over this threshold, the tire goes from rolling forward to rolling BACKWARD. This is when your hands are pulled away from you, your inside foot goes down, but there’s no hope.
On pavement, the ‘flop factor’ or ‘oversteer’ becomes increasingly noticeable as tire diameter and volume increase. Inertia is implicated, but my suspicion is that this phenomenon is primarily driven by the difference between riding the tire’s ‘crown’ - centre of the tread - while riding in a straight line, and the tire’s ‘shoulder’ - the useable profile of the tire we transition to when leaning the bike through turns. Consider a 700c x 25mm tire for a moment. As you steer left and right, does your tire transition from riding ‘high’ on the centre of its tread to significantly lower when you turn or angle the wheel? The tire’s profile drops away, of course, but it can only drop so much, a few mm. In contrast, a 700c x 42mm tire leaned the same amount, moves the front axle down significantly MORE, which can initiate oversteer. Again this has to be countered with fork rake, so that the head-tube’s load (read: load you out into your fork with your feet, legs, hips) can counteract the tire’s 'desire’ to turn in. The magic of a great handling bike is the balance of flop/oversteer and countersteer. If we can’t influence the head-tube with our bodies to manage what our tire is doing on the ground as we turn, we have a bike that either handles really slow and sluggish (like a truck) or scary and unmanageable.
With ‘small tires’ - let’s say <42mm - on pavement, air pressure doesn’t allow for a great deal of deformation as we turn, and grip isn’t particularly high. The same is true for knobbies on hard surfaces. The handling quirks / badness we can experience from mismatches in wheel setup and steering geos occurs primarily on pavement, where grip is high WITH ‘slick tires.’ Knobby tires in the same casing construction and size DO NOT adhere to the pavement as much and can even have a ‘self-righting’ characteristic, where their knobs ‘want to’ stand the tire up. For example, the RH Juniper Ridge in 650b x 48mm does NOT oversteer on a 71.5 / 47mm set-up, while the slick Endurance tire in the same size does. The same slick in Extra-light casing DOES NOT. The phenomenon of the ‘ground pushing the wheel behind the steering axis’ is what we experience as flop/oversteer. Air pressure, tread, and casing construction influence this phenomenon at the upper end of the rideable trail range (I’m not sure about the lower end of the range). MY speculation, in the case of the 650b x 48 Rene Herse tires in supple and stiff casing variants, is that the stiff casings don’t deform as much as the supple ones do as the bike leans, so the ‘pushing’ of the ground against the contact patch can’t be absorbed / muted enough. 66mm trail is already ‘too-high,’ but it’s the stiffer tire construction that renders the geometry ‘terrible.’
Conclusions
You might wonder: how much difference does 1/2 a degree here or there matter? How much does 1mm this or that way matter? The truth is, seemingly minute differences are either ‘absorbed’ by rider (which depends on many variables), or, 1/2 degree here and mm there can be amplified by specific wheel configurations and body positions.
Here are the ‘key insights’ I’ve extrapolated from my experience.
Tire Construction
Tire construction influences steering dynamics more ‘strongly’ as volume increases. You WON’T see this jump out from the numbers, and this phenomenon, which has pushed folks away from 650b wheels/tires, is not widely understood.
I was surprised to experience pronounced steering ‘quirks’ when running the fork in the 47mm rake setting, which helped me understand why some of my friends have reported bad experiences with 650b x 48mm tires in the past. I’ve ridden Rene Herse’s very compliant Extralight Switchback Hill tires in 650b x 48mm over numerous years, on many bikes with 47mm rake forks, and they’ve always felt good, sort of like a 33mm 700c tire except with way more volume. In contrast, the same tread (close to slick) on an Endurance casing was verging on unrideable on the same wheel and fork setting. The stiffer tire, at any useable pressure, generates scary oversteer. The bike needs to be micro-managed. In contrast, the knobby Juniper Ridge tire in the same stiff casing does not handle different from the EL model. Both are good with 47mm fork rake; 52 doesn’t yield a noticeable improvement.
I also compared the 700c x 42 Snoqualmie Pass in EL, my favourite all-rounder tire, against the Endurance casing version. At 47mm rake, BOTH tires oversteer; this tire size, with high-grip generated by ‘total’ tread contact with the ground, ‘wants’ to turn in when leaned. This is oversteer. In the 52mm setting, neither oversteer; the stiffer tire just rolls slower.
Tire Tread
What about 700c x 48mm knobbies; what do those do? I rode the Oracle Ridge in EL casing with my 650b x 48 Juniper ridge on the back, since the 700c doesn’t fit there. I also tested the 700c x 48mm pair on another bike, with 50mm rake. The tire handled slow/very stable on both bikes, and didn’t hint at oversteer. I don’t think it would oversteer at all in 47mm rake either, as knobs on large tires don’t seem able to generate the grip required to create oversteer. It could also be about the knobs ‘pushing back’ to vertical as the front wheel leans, at the micro level. I can’t say for sure how the physics are working, but I know how the tires feel.
The two rake settings have been as valuable as I suspected over a broad range of tire sizes. As an ambassador for Rene Herse tires, I’ve been able to test apples-to-apples, using tires from 35mm to 48mm with the Futura Cross. I’ve been able to test Extralight casing tires in 700c x 35mm, 42mm, and 48mm. I’ve tested 650b x 48mm EL ‘slicks’ against the same tire in a heavier ‘Endurance’ casing. I’ve also tested Rene Herse’s knobby 650b x 48s, in EL and Endurance casings. Outside the Rene Herse range, I’ve also used a variety of cyclocross tires in 33mm, and 38mm Panaracer Gravel King SKs.
What about using ‘small tires’ on a gravel bike?
Most riders won’t have much experience riding ‘small tires’ on forks with rakes greater than 50mm. Many modern gravel bikes are being sold with forks with >50mm rake, and marketed as totally capable of riding anything from 25mm to 50mm tires. This is true, they are ‘capable,’ but what does it feel like to use small tires on such a format?
The strange thing is that low trail doesn’t equate to twitchy handling. While it’s necessary to increase trail as we increase tire volume - which balances against how the tire’s footprint changes and interacts with the ground - the result is more about making the steering more ‘direct’ than ‘faster’. Small tires - 32mm, for example - are not very stable in cross-winds at 52mm rake, as the trail is too low. Wind buffeting can be felt in the hands, particularly in windy seasons like spring and fall. It’s a minor thing, in my experience. The main thing you can expect to feel when riding a small tire on a long rake is a mismatch in what you expect - aggressive handling - and what you get: not that. The bike’s low trail will preclude it from ‘diving into turns’, because its flop-factor will be low. Realistically, this is probably a characteristic that will be welcome to many riders, because it will tend to balance against reduced traction.
Next?
‘Standard’ HTAs, which are referred to as ‘steep’ in the context of gravel and MTB, work well across a broad range of tire sizes with standard fork rakes =/< 47mm. A 73 degree HTA, for example, is effectively ‘optimized’ for a 33mm tire with 47mm rake: trail = 57mm, flop = 16mm.
On loose surfaces, there are disadvantages to ‘steep’ HTAs, however, which are not evident in the numbers. While it’s not difficult to achieve secure handling dynamics on paved surfaces with steep HTAs and ‘normal’ fork rakes, gravel and cyclocross bikes need to handle well on loose and slippery surfaces. Steep HTAs do not offer sufficient stability when leaned and breaking traction; slacker HTAs and longer fork rakes offer this stability.
Conceivably, a 52mm / 58mm pairing for a next-generation Futura Cross would offer a more useful trail range for gravel bikes pushing into the ‘modern’ end of the HTA spectrum. While at the time of writing most HTAs on gravel bikes are still steeper than 61.5, and the 47 / 52 is well-suited to these bikes. 72-degree HTA is where we get into a really compelling argument for running the Futura Cross in its current form.
Questions? Gaps? Comments?
Thanks for reading all the way through this brick of a piece, which was one of the most difficult to write of any; it took about 30 hours to get here, and I’m still wishing I was able to be more organized and clear! I’m writing with riding as my starting point, while the numbers are a tool; translating sensations on the bike is a difficult task, but I love trying.
Please let me know how what I’ve written resonates with you, whether I’ve left any gaps, and pose any and all questions that might come up. If I have anything wrong here, I want to correct it; don’t hesitate to challenge me! My primary goal is to help ensure more riders have good experiences on bikes, and there’s always more to learn and communicate better.
