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In any racing discipline, as competition increases so do the speeds. According to Factor’s engineering director, Graham Shrive, this increased competition in gravel racing is raising speeds to the point where aerodynamic optimisation can offer a real advantage. That’s an interesting tidbit from a well-qualified engineer who previously claimed that aerodynamics in gravel was overrated.
With these increasing speeds and renewed perspective, the premium carbon bike manufacturer has released the Ostro Gravel, an aero gravel race bike that’s been seen doing the rounds in big American events this year. And with Ostro in the name, it unsurprisingly looks like the company’s well-rounded aero road race bike.
Built to be fast in one-day races and a little versatile beyond that, the Ostro Gravel combines modern geometry with an aerodynamic design that’s said to be designed around the lower velocity of riding off-road. Add in clearance for 45 mm rubber, UCI certification for road racing use, and a surprisingly robust-but-light construction and this is one intriguing gravel bike (which could be mistaken for a road bike) that stands among a shortlist of roughly similar bikes such as the BMC Kaius, 3T Exploro RaceMax, Cervelo Aspero 5, and not many more.
Over the past few weeks, I’ve been putting some miles on the Ostro Gravel, on and off the road. This in-depth review covers what Factor has to say about the new Ostro Gravel and what I have to say about it, too.
[ct_story_highlights]What: An aerodynamically optimised gravel race bike. ||Key details: Integrated design with one-piece handlebar/stem, clearance for 700 x 45 mm rubber, T47 Assymetric bottom bracket, UCI certification. ||Weight: 913 g (painted 54 cm frame), 490 g (uncut fork), 8.05 kg (as tested, without pedals). ||Price: US$5,500 / €5,450 / £4,730 (frameset with handlebar and more), US$9,800 / €9,710 / £8,430 (complete, as tested). ||Highs:Impressive stiffness, good stability balanced with quick handling, solid feel, superb manufacturing quality and detail, well-considered headset assembly, lifetime-warrantied bearings.||Lows: Stiff seated ride, fiddly hose routing through the handlebar, blue bottom bracket, washer on thru-axles can fall off during wheel removal. [/ct_story_highlights]
Intentions of speed
The Ostro Gravel is a rather clear example of just how quickly the sport of gravel is progressing. It was barely two years ago that Shrive suggested the benefit of a fully aero gravel bike would be a fringe case, and so the company opted to focus on ride quality and low weight with the design of the LS gravel bike (a model that continues in the range).
The growing professionalism of gravel racing has certainly swayed Factor to reassess this point of view, but an increase in speed is not the only reason. “Tyre size has come down a bit from where the trend was pointed,” said Shrive about the commonly seen 38 and 40 mm tyres winning races. At one point, it looked like even wider rubber would take over in all facets of gravel. Meanwhile, recent travel limitations gave the former head of Cervelo’s engineering some free time to dive deep into research of tube shapes that still offer benefit in the lower-velocity demands of a gravel race bike.
“As soon as you put a (frame) tube behind a 40 mm tyre there’s really nothing you can do about the airflow, it’s just going to be a mess,” said Shrive about why Factor has instead focussed on optimising tube shapes that sit in uninterrupted places, specifically the handlebar, head tube, outsides of the fork, and the seatpost. These relatively deep tube shapes feature truncated airfoil profiles tailored to make slow speeds faster (well, more efficient).
The simple goal of these lower-velocity-focused tube shapes is to trip and re-curve passing air sooner. By doing so, Shrive said, the airflow can then re-attach along the rest of the tube for a longer period and therefore create less drag.
Meanwhile, the other tubes, such as the down tube, mimic those used on the Ostro road bike (although without the concave shaping). Factor’s CEO, Rob Gitelis, noted that the tubes placed in more turbulent areas are already well optimised for a balance between weight and aero, at least with a road tyre in place. Similarly, the somewhat unique “converging nozzle” of the Ostro carries over to the gravel version, something Shrive notes will work as intended by keeping air moving through the fork crown if the bike is used with road tyres.
Sitting at the front of the bike, and said to be one of the main speed benefactors, is a wholly new one-piece handlebar and stem. Named the HB02, this carbon number is said to save approximately 5-6 watts (at 48 km/h) compared to the original integrated handlebar of the Ostro road bike, and 9 W when compared to a traditional two-piece bar and stem. I’ll return to this bar shortly.
Certainly, Factor has put a whole lot of energy into the overall aerodynamic design of the Ostro Gravel, but the company makes no specific claims about watts or seconds saved. All design iterations were limited to digital modelling (with a deep base of prior knowledge). The company is booked in to validate its already-in-production design in a wind tunnel, but arguably such testing is likely only useful for future iterations.
In the meantime, Shrive speculated that with road tyres fitted, the Ostro Gravel’s increased tyre clearance and new handlebar are likely to best the road-focussed Ostro in the wind tunnel.
Strength and safety
The data may not confirm this, but it sure feels like product recalls have grown at a similar rate to the rapidly increasing number of hidden-cabled and one-piece-integrated bikes hitting the market. Factor itself had a public fail in the early days of the Ostro, and it’s almost daily that I hear from riders who lack trust in these sleek designs dominating new bike releases. And I truly don’t blame such consumer hesitation, given the legally required safety test standards are lagging behind these modern integrated designs.
In discussing this topic with Shrive and Gitelis, I learned that the company has taken its own testing path to best understand extreme failures that the existing ISO standards don’t test for. And notably, Factor has been doing loaded impact testing on wholly built front-ends, including the shifters/levers.
Shrive noted that he’s increasingly trying to change the conversation around this testing away from a static load and one to energy dissipation. “This is what we do with wheels, and we know when the wheels will fail based on a striker that predicts real-world outcomes,” Shrive said. “So we started to apply this to the rest of the bike, impacting all the different parts on the bike.”
And that brings me to the round-shaped steerer tube of the Ostro Gravel. This is a notable difference from the Ostro road bike, where that steerer is flattened on its face in order to squeeze the cables down the front of a narrower head tube. According to Gitelis, the design goal on the road is all about minimising the frontal profile, and such a design will continue to exist in future road bike iterations.
Meanwhile, the Ostro Gravel takes a simpler path with a concept similar to what the likes of FSA’s ACR and Deda’s DCR have already rolled out. It uses an oversized 1.5″ headset bearing top and bottom, and then runs a common 1 1/8″ round steerer within it (the steerer tapers to 1.5″ for the bottom bearing). Factor says the lower velocity of gravel riding allows for this wider shape to not introduce an aero disadvantage. What wasn’t said was that round steerers are potentially better suited to hammering across rocks and ruts for 200 miles (320 km) at a time.
Moving to a common oversized headset means the Ostro Gravel can be used with many other integrated headset and cockpit options. However, Factor (and sibling brand Black Inc) supplies its own. This includes the eye-wateringly expensive self-lubricating solid polymer SLT bearings from CeramicSpeed. Factor combines that with its unique long-fibre-injected carbon fibre compression ring that contacts the carbon steerer with a smooth and tall interface.
All told, it’s a system that Gitelis suggests should solve our common complaint of the difficulty around maintaining (or replacing) bearings within a hidden-cabled system.
From my point of view, I agree that those SLT bearings will likely last the life of the bike. Likewise, that uniquely long compression ring gives me faith that steerer damage is unlikely (through cutting or material depression). However, some level of maintenance, namely cleaning and greasing the outside interfaces of the bearings, will be needed eventually. Thankfully, this won’t require cutting brake hoses so long as the bike is set up with headset spacers and/or excess hose tucked within.
Jumping back to the topic of strength, nothing on this bike feels flimsy or underbuilt for off-road use. Similarly, the aero-profile tubes all feel ready to take a knock or two. To me, it all feels like Factor has turned a page and is producing bikes that may carry a few more grams, but in turn, will safely survive ongoing real-world usage. And beneath the surface, the Ostro Gravel continues with Factor’s use of TeXtreme at the base layer, and boron as a reinforcement in the seatpost clamp area.
That’s not to say the Ostro Gravel is heavy, it’s just evident that Factor isn’t trying to walk the tightrope between weight and reliability. The 54 cm painted frame (as tested) weighs a respectable 913 grams. At 490 grams (uncut), the fork isn’t on the edge. And it’s a similar story for the 40 cm-wide (43 cm at the drops) one-piece handlebar/stem that comes in at 368 grams – about 60 grams more than the Ostro road bike. Meanwhile, a 400 mm-length straight seatpost is a burly 262 grams, and you can feel that there’s no compression in the material when tightening the integrated seatpost wedge to its secure 8 Nm figure.
Geometry and fit adjustment
The geometry of the Ostro Gravel is a modern mix of performance race meets extended front centre, and it’s more aggressive in both handling and fit geometry when compared with the Factor LS gravel bike. There are six sizes, all of which feature long reach figures and performance-minded-but-not-overly-aggressive stack heights. Like the other brands doing similar, the goal is to put you more between the wheels, rather than over them.
Those seemingly scary reach figures are designed to be matched with slightly shorter stem lengths and are also based on the growing trend of forward-slammed saddles. Factor is using fairly steep seat angles to encourage a more forward position, but a choice of either a straight or 20 mm-setback seatpost won’t force you into adapting to a new position. Though, as someone with a cross-country mountain bike background who has long run a forward-positioned saddle, I’m all for the trend. Also, I don’t doubt that people used to shopping bike sizes based on reach figures will be thrown a real wobbly curve ball by the Ostro Gravel.
The one-piece HB02 handlebar is available in 20 mm increments ranging from 360 to 440 mm (measured at the tops). Each handlebar grows by 15 mm either side at the drops due to the 7° flare which occurs after the hoods (for example, the 40 cm bar tested measures at 43 cm in the drops.) Factor then offers stem lengths ranging from 80 mm to a surely-you-are-a-pro 140 mm length. And on top of that, Factor is currently finalising the design of an optional one-piece bar that will allow aero extensions to be fitted for all the gravel rebels.
Whatever size combo you choose, the bars offer an 80 mm reach, a compact 120 mm drop, and a 3° backsweep. The shape is rather unique and offers more grasping positions than usual. For example, the top’s cut corner lets you rest your palms more comfortably on the bend behind the hoods. And speaking of those hoods, the rear-set clamping position was made to offset the longer reach quietly introduced by current Shimano and SRAM hydraulic hoods.
Another neat detail is that the flat aero profile happens foward of the stem length. This means you won’t be hitting your knees on the backside of the bar like I’ve complained about with some other one-piece cockpits.
The wheelbase is certainly longer than a road bike, but the steering geometry is well aligned to one. Factor combines semi-steep head tubes angles with long offset forks which result in some fairly quick trail figures. In other words, the Ostro Gravel is quick-handling. Notably, those trail figures remain roughly the same throughout the size range due to two different fork offsets being used.
Factor’s reasoning for this quick handling is based on how tyre diameter directly impacts the trail figure. The smaller the tyre diameter, the shorter (and faster) the trail figure. The bigger the tyre, the longer the trail figure (slower handling). It’s an approach that the likes of Trek have long used and I’m glad to see it become increasingly popular.
The Ostro Gravel offers a performance-road-bike-friendly 58 mm trail figure with a 700 x 30 mm tyre fitted, while a 700 x 40 mm tyre sees that trail figure lengthen to a still-quick 61 mm. Max out the 45 mm tyre clearance of the bike (which allows 4 mm of surrounding clearance) and your trail figure will be 63 mm.
The bottom bracket height is neither CX-tall nor Trek Domane low, with the rear-centre length growing by a few millimetres as the sizes progress.
Assembly, part fitments, and bike options
The Ostro Gravel makes no excuses for being an integrated bike with several proprietary features. The seatpost is unique to the frame, and it’s held in by Factor’s integrated wedge clamp (one that is easy enough to access with common tools).
Furthering the narrowed focus of this bike is a lack of fender and rack mounts – a common omission on race bikes. You get provision for two bottle cages within the front triangle, a spot underneath the front triangle, and mounts for a bento-style bag on the top tube. And one tech detail easily overlooked is that all of those provisions aren’t your regular aluminium rivnuts but are classier-looking flush mount rivnuts.
This gravel steed can only be used with hydraulic disc brakes and electronic gearing. The way the brake hoses are routed through the gap in the headset compression ring leaves no room for mechanical cables; frankly, such cables don’t belong anywhere near a bike with wholly concealed cabling like this.
That concealed brake hose routing runs through Factor’s split headset spacers, into the stem, and then through the handlebar. The pinch point for me proved to be the rather small exit holes at the handlebar, something that bent the ends of my magnetic internal cable-routing tools and resulted in some naughty words. And I can assure you that those words would only continue if I’d needed to also run Di2 wires out the same holes.
I suspect (hope) Factor will fix this pain point in future production runs, especially since it makes swapping out a handlebar harder than it should be.
While a handlebar or stem length change will require re-routing those hoses, it’s far easier to adjust stem height. Factor supplies an aero preload topcap for a sleek setup and a round topcap if you’re unsure about the final steerer length. For example, I removed one of the aero split headset spacers from below the stem and ran regular round 1 1/8″ headset spacers above while I dialled in my fit. It’s a small detail but is something that a few popular integrated systems don’t allow for.
Another small detail is the supplied computer mount that bolts directly into the front of the handlebar (there’s a cover in place if you choose not to use it). It offers an adjustable angle and Factor even supplies a universal adapter so that you can add a light, camera, or both. According to Shrive, this computer mount was load tested with 800 grams on a wheel bump test for a hell of a long time.
Looking to the bottom bracket and Factor continues with T47 Asymmetrical, an offset iteration of T47 that I’ve yet to see another frame manufacturer adopt. The good news is that the threads match the T47 standard, the width between the bearings is normal, and regular cranks fit. In reality, nothing stops you from running any T47 Internal-style bottom bracket and then using a few spacers on the driveside.
The better news is that you probably won’t even need to seek out a bottom bracket as Factor supplies its bikes and framesets with a CeramicSpeed coated bottom bracket along with the needed covers and adapters to suit 30 mm, DUB, and 24 mm spindle cranksets. And like the SLT headset bearings, these bottom bracket bearings also include a lifetime warranty.
Another nice touch with the bottom bracket is the integrated backing to the tool interface, which stops your chosen tool from touching the paint when installing or removing this bottom bracket. That doesn’t sound like much, but Colnago C64s (and many other bikes with thin threaded bottom bracket cups) are floating around with chipped paint because the tool butts up against the frame. My only complaint is that the bottom bracket cups are anodised blue, and it’s unnecessary to draw so much attention to the bearings that sit between the crank. Factor, please make this part black.
Otherwise, the rest of the frameset is rather normal. You can run whatever road or gravel wheels you wish. The frame takes flat-mount brake calipers. And despite the chainstays not being dropped, there’s still plenty of chainring clearance. For example, you can run up to a 1x48T chainring if used on SRAM Wide or a 52/36T double crank with new Shimano 12-speed. There’s also a removable front derailleur tab, with a replacement frame cover provided should you not run a front derailleur.
Already in production, Factor is offering the Ostro Gravel as a frameset for US$5,500 / €5,450 / £4,730, including the headset, seatpost, bottom bracket, and Black Inc cockpit. There’s a rolling frameset for US$7,500 / €7,430 / £6,450, which gives you the frameset plus Black Inc’s first gravel wheelset. At 1,489 g paired, the Thirty-Four features a 34 mm depth, 25 mm-wide hookless rims, and rolls on Black Inc’s hubs with CeramicSpeed Coated bearings hidden inside.
We then get to the complete bike option, all built with SRAM 1x or 2x groupsets and starting from US$8,200 / €8,120 / £7,060 (SRAM Force AXS XPLR 1×12). I tested the near-top-tier complete bike, decked out with full SRAM Red AXS XPLR 1×12, a power meter, and Black Inc wheels. You can expect to pay US$9,800 / €9,710 / £8,430 for this one.
My 54 cm sample weighed 8.05 kg (17.74 lb), without pedals or bottle cages, but with a healthy dose of tyre sealant in the 40 mm Goodyear tyres.
Depending on where you live, Factor Bikes are either sold through a retailer, or you deal directly with the manufacturer. If you’re in Australia, the UK, or the USA, you’ll likely be dealing with Factor direct. And while the Ostro Gravel is far from being a cheap bike, the inclusion of high-end bearings and a few added extras offers some value compared to Trek, Specialized, Pinarello, and so on.
There’s a choice of two colours: a matte and raw “Naked Grunge” (as tested) and “White Grunge”. The White Grunge will likely carry an approximate 50 g weight penalty.
The Ostro Gravel as a gravel bike
Finally, I get to how the Ostro Gravel rides. And if I were to pick a single word to describe it, that word would be ‘stiff’.
It’s a rewarding feeling on the road or even hammering up a well-kept dirt road. The bike responds without hesitation, and there’s no vagueness between your hips and the wheels. In many ways, the Ostro Gravel responds like a top-level road bike. However, the obvious downside is that whatever vibrations, bumps, or impacts the tyres don’t absorb, your legs or ass will.
To return to an earlier part of this review, this is a limitation and a trade-off of the slippery aero shapes that were tested to take a beating. “When you get aero sections and you elevate the test standards up like we have for gravel bikes, you start to get really damn stiff shapes trying to resist those loads as the shape doesn’t lend itself to deflect,” said Shrive, all the while suggesting that they expect most of the bike’s ride comfort to come from the tyres.
The comfort from high volume tyres is something that many bike brands rely on, and while the tyre system may be the most efficient spring system in a rigid bike, it has its limits. With wide tubeless tyres run on modern-width rims at low pressures, the limits will keep you isolated from decently rough terrain. However, those limits end when the ground is littered with square-edge bumps, depressions in the ground, and relentless vibrations. And in these rough situations, the stiffness of the Black Inc wheels only increase the feedback felt.
We commonly complain about rigid gravel bikes being unbalanced front to rear. It’s relatively easy to make a seatpost flexible for comfort, but so often it’s rarer to find a front-end that’s equal in comfort. Oddly enough, I found the Ostro Gravel to be the reverse, with the one-piece handlebar flexing just enough to feel more muted. Meanwhile, that deep section and stiff seatpost offers little comfort from what the rear wheel fails to silence. The result is a bike that keeps your hands more comfortable and allows you to easily stay in control. However, staying seated through the same sections feels fatiguing for the rest of the body.
Certainly, bigger tyres will help to offset this stiff ride, and if it were my own I’d be putting in some tyre inserts to get the most out of the available tyre volume. That said, there’s little else you’ll be able to do with getting more comfort from this bike as the seatpost can’t be switched out for something more flexible.
The bike’s stiffness certainly shines through when participating in a game of under-biking. And while the front-centre length is extended, it’s not quite long enough to wholly avoid toe overlap (at least in the 54 cm size) in awkward technical circumstances.
It’s different when going flat-out on road or even family-car-friendly gravel roads. Here the tyres do plenty, while the longer wheelbase adds a healthy amount of stability and control. Such stability often comes at the expense of a bike’s ability to react quickly, but that simply isn’t the case here and it’s easy to adjust the direction of the Ostro Gravel.
The fit geometry of the Ostro Gravel is sure to leave many riders second-guessing what is best for them. At 170 cm (ish) I could have gone with either a 52 or 54 cm size. I opted for the larger with a shorter stem (90 mm) and narrower bar (400 mm). It certainly feels like a performance bike, but neither the reach nor stack are problematic for my currently poor flexibility.
One-piece handlebars can often make or break an otherwise good bike, and Factor has done its research here. No matter where I placed my hands along or around the bar, they always felt at home.The generous top surface gives you control with only a light hold, while the flare at the drops is enough to add plenty of wrist clearance without feeling like you’re holding onto an entirely different handlebar.
The Ostro Gravel as a road bike
Factor didn’t just design the Ostro Gravel to race gravel; it was built knowing that many will want to use it as a road bike, too. And Factor surprisingly took the extra step of putting it through the UCI approval for road race use.
As mentioned before, going to a road tyre has a direct impact on the handling of the bike and, indeed, the Ostro Gravel becomes more darty even with an all-road-friendly 32 mm tyre fitted. Here it barely takes a twitch to get the front wheel to dive into a corner, and the slower you go the more noticeable such twitchiness becomes.
However, changing to road tyres on the Ostro Gravel doesn’t quite transform it into the road racing machine you might hope for. With its extended front centre, the geometry weirded me out.
I was looking down at what seemed like a fully aero road race bike, which steered like a race bike, and which had stiff ride quality like an aero race bike, and yet, the front wheel was noticeably further out in front of me. I found myself having to correct for this, getting my weight more over the front of the bike for sharp corners. Slow climbs would reveal a feeling akin to a head tube that’s too slack (even though it isn’t) – a sense of wheel flop if you will.
Now none of that would stop me from using and enjoying the Ostro Gravel as a road bike. It also wouldn’t slow me down or hold me back in hanging in a group ride or racing mid-pack in a club race. But it’s still important to note that the allure of the unicorn race bike that’s as comfortable being a focussed road bike as a gravel shredder isn’t a reality – there are always trade-offs.
Furthering those compromises is the rather blatant issue of gearing. The 44T front chainring matched with the 10-44T XPLR cassette had me hunting for an easier gear when riding loose, rocky, and 20%-gradient off-road inclines. Meanwhile, swap the tyre to a road-friendly size and suddenly you’re left hunting for a harder gear.
Sadly there’s no easy answer to this problem, but the solution probably involves a front derailleur – something that Factor does offer within its bike build options.
To play the role of selfish writer for a moment, my personal dropbar bike journey has brought me to want a fast and light gravel bike that I can predominately leave in road mode with a different set of wheels. The Ostro Gravel is the bike I’ve been looking for in so many ways. And yet, so much of my local gravel errs to the side of wildly rough and technical, and as a result, the Ostro’s integrated approach and road race bike stiffness just aren’t ideally suited to these under-biking desires.
While it doesn’t suit my localised needs, there are surely many riders out there seeking a do-it-all dropbar steed for riding on the rivet across poorly kept sealed roads and better-kept dirt roads. And the Ostro Gravel is certainly a worthy contender for such riders.
Beyond the stiff ride quality, perhaps the only remaining sticking point is the lack of quantitative data for what a non-professional rider may gain from the aero design approach. I’d want to know that I’m saving a good amount of energy in my legs to overcome the increased fatigue of the stiff structure.
Of course, like anything, that answer will always depend on the day’s situation. A smoother gravel course will see faster speeds and less vibration fatigue on the body; conditions where the Ostro Gravel is likely to shine. Meanwhile, on a tortuous course, and therefore at slower speeds, I’m not quite convinced the Ostro Gravel will be the fastest bike of the day.
Factor is neither the first to bring aero to gravel nor will it be the last. The market for road-bike-lookalike gravel race steeds is a fast-growing one, and it’s because of the versatility of such bikes.