Pipedream Sirius S5 Review

Have you ever listened to an album or artist for the first time and hated it, only to later have it become one of your favorites? That was a regular experience for me throughout my 20s and 30s. I’d get turned on to a new record that by all accounts seemed perfect for my tastes—through word of mouth or on Pitchfork—only to immediately dislike it. To be certain, I’d give it a second and third spin, and during one of those listens, something would click. Then, before I knew it, I was in love. That epiphany-esque narrative closely parallels my relationship with the Pipedream Sirius S5. I got it back in late summer of 2022, and it took six or seven rides before I truly started to understand it. The S5 has since become one of my favorite bikes, and I’ve put countless miles on it for this detailed review. Read on for all the juicy details about this unexpected love affair.

• 
• 

What’s the Pipedream Sirius S5?

Before I explain how the magic happened, here’s a little background on what the S5 is and what makes it so interesting. As some of you may know, the Pipedream Sirius isn’t new. It was first released in 2007 and has been the Scottish company’s flagship platform ever since. It’s the longest-running frame in their catalog and now has a lineage that’s five generations deep, as its name implies. The fifth-gen frame that’s reviewed here was released in 2020 and designed by Pipedream Cycles founder and owner Alan Finlay, as were all the previous versions. It was formerly built for 27.5” wheels and tires, but Alan completely revamped the platform as a 29er for this iteration, and gave it a much more progressive geometry, which I’ll dig into later.

On paper, the Sirius S5 looks pretty straightforward. It’s a short-travel steel hardtail that’s optimized for 100-120mm 29er forks and designed to fit 29 x 2.6” or 27.5 x 2.8” tires. The S5 frame is made from a custom-butted, heat-treated CrMo 4130 tubeset that Alan specced himself. The commitment to the material is emphasized in the graphics that adorn the back of the seat tube, a series of eight symbols that represent the chemical composition of CrMo 4130—plus one “special element” called Moxie, which is denoted as Mx. The frame features a CNC-machined chainstay/bottom bracket yoke, a 44mm head tube, an M12 Boost axle, and +/-16mm sliding dropouts. It has a single set of bottle bosses on the downtube, external shifter and brake cable routing, and internal dropper routing on a seat tube that fits 31.6mm posts. However, the straightforwardness stops there.

Pipedream defines the S5 as something of an anomaly designed to “Go far, go fast, but most importantly, go for fun.” They even toss around the term ATB in its definition: Is this Downcountry? Is this XXXC? We don’t care, this is possibly the epitome of an ATB. I personally wouldn’t call the S5 an ATB. Literally, maybe, but I usually reserve that term for rigid bikes that fall somewhere in between gravel steeds and MTBs. Not to say I couldn’t put a rigid fork on the S5 and transform it into one hell of an interesting ATB, but I’d categorize the Sirius as a mountain bike. Then again, the S5 is unique and unlike any other mountain bike I’ve ridden, so it might just deserve its own classification.

One brazen idiosyncrasy with the S5 is its sizing. Pipedream completely nixed standard S/M/L options for the S5 and instead offer the frame in either Longish, Long, or Longer. All three have the same 105mm head tube length and 586mm stack height, and the latter two share the same 420mm seat tube length. This is part of their geometry schtick, a unique approach designed around dropper posts to accommodate most riders between 4’11” (1.50m) and 6’4” (1.93m) with the selection of the appropriate length post. Confused? I was too, at first, but I loved the fact that the Longer had such a significant reach, and I also prefer bikes with a short stack height. Being 6’0″ with a 34″ inseam and a relatively short torso, that’s what works for me. A shorter stack also allows me to toy with different riser bars without sacrificing fit. The more I rode the S5, the clearer Pipedream’s sizing approach became.

• 
• 
• 

A Complex System

The Sirius S5 is indeed an oddity, a characteristic that it shares with its namesake: the brightest star in the night sky and one of our solar system’s nearest neighbors, some 2.64 parsecs away. The word comes from the Greek Seirios, which means glowing or scorching. But behind that glow, Sirius the star isn’t exactly what it seems. It’s actually a binary system made up of two stars.

There’s an analogy nested in there that’s not too far of a stretch, so hear me out. The Sirius S5’s brilliance comes from the combination of two otherwise distinct elements: very progressive modern geometry and short-travel efficiency. By contrast, most progressive hardtails—such as the Kona Honzo ESD and Pipedream’s own Moxie—are designed for use with longer 140-160mm forks. The Sirius S5’s geometry is specced for a short 100mm fork with the option to run forks with only up to 120mm of travel.

Some folks might assume that 120mm “isn’t enough” for the type of riding that the S5’s 65° head tube angle, long front-center, and 77.5° seat tube suggests, but I think it makes a lot of sense. Forks perform better than ever, and a Pike or Fox 34 set up as with 120mm of travel is much more capable that the mountain bike industry lets on. Moreover, a short travel fork intrinsically moves less, which helps the bike maintain a more consistent geometry—whether it’s being aggressively ridden downhill, pedaling along on a bumpy trail, or climbing. That consistency provides benefits in both efficiency and ride feel. I don’t think this is a popular or widely understood combination, however. There aren’t many other short-travel hardtails that have such a modern geometry as the S5, really. As you’ll learn deeper in the review, I think this is a pretty magical and greatly underrepresented combination.

Build Kit

The S5 is only available as a frame, so that’s what I got for review. I built it using parts I had on hand, as well as a new wheelset from Logos. It’s a bit of a franken-build, as you can see in this full kit list:

• Frame Pipedream Sirius S5 (large)
• Fork Rockshox Pike Ultimate (120mm)
• Wheels Logos Eudae Carbon
• Front Tire Maxxis Assegai 29 x 2.6″, EXO
• Rear Tire Maxxis Rekon 29 x 2.6″, EXO
• Crankset SRAM Eagle X01 Carbon, Wolf Tooth Camo 30T chainring
• Derailleur Shimano XT 11-speed, Garbaruk Cage
• Shifter Shimano XTR 11-speed
• Cassette Garbaruk 11-speed, 11-50T
• Bottom Bracket SRAM DUB
• Handlebar Stooge Moto Bars, 800mm
• Grips Ergon GS1
• Headset Hope
• Brakes Hope Tech4 E4
• Saddle WTB Koda
• Seatpost OneUp V2 210mm

• 
• 

• 
• 
• 

First and Last Spin

One of the main reasons I wanted to try the S5 was that I loved the British-born Cotic SolarisMax, another relatively progressive short-travel hardtail. It was like a gateway drug that led me to feverishly search for other options from across the pond. A handful of UK brands have been prioritizing slacker head tube angles, steeper seat tubes, and added front-center length for a while, well before “long, low, and slack” became an industry-wide mantra. My theory was that since these same folks had been experimenting with these building blocks longer, perhaps their designs were more fine-tuned.

I ultimately believe that’s true, but I also quickly learned that not all UK-conceived steel hardtails are created equal. I was expecting the Sirius S5 to be something akin to the Cotic. The measurements aren’t too dissimilar, as you can see in the comparison above, but these two bikes are completely different animals. As mentioned in the introduction, I wasn’t really into the S5 out of the gate. It felt cumbersome to pedal and difficult to handle on climbs during my first couple of rides. The front end felt unwieldy when trying to get over technical bits, and it simply felt like too much. But, every time I rode it, bits of those early suppositions melted away. It’s worth adding that I often experience more leg fatigue with new review bikes. It’s as if unfamiliar geometry requires the use of different parts of leg muscles that have been dormant. I think that played a part in this bike feeling a little sluggish at first, since it no longer does, even after I ride another bike, such as the Cotic.

• 
• 

Climbing steep and technical bits took a little longer to click with the S5. I think it’s a fine climber now, but it required some adjustments to my riding technique for this to materialize. The S5’s long stance and slack front end require a forward attack position on the steeps. I also began to steer more with my feet and lean the bike more, relying less on the steering and more on feeling out the terrain. Riding this way also made me realize how well this bike is balanced. It’s one of those bikes that you can pause in the middle of a section of technical trail, adjust, and power through, even if you’re not good at that, which I’m not.

• 
• 

• 
• 
• 

My first and final impressions of the S5’s descending capabilities are another story altogether. I was immediately blown away by how well this bike handles when pointed down just about anything. Despite only having a 120mm fork, it was decidedly more confident and capable than any of the full-suspension bikes I’ve tested in the past five years, many of which were equipped with longer 140mm forks. The more I rode the S5, the more these impressions were galvanized. It simply goes down anything and maintains best-in-class traction while doing so. My riding evolved with this bike. The long front-center pushed me to align my body forward toward the handlebars, even more so than on other progressive bikes. The old school downhill mentality of straightening your arms and hanging off the rear simply doesn’t work with a bike like this.

Pipedream calls the formula responsible for this descending prowess Drop Optimized Geometry, which I hinted at earlier. Most of the bullet points behind this blueprint are familiar terms. I think the big differentiator is the slack 65° head tube angle and low 605mm stack, coupled with the super-steep 77.5° seat tube angle, all of which position your weight low over the front of the bike. It’s a recipe that rewards you for moving your weight forward, chin over stem, and trusting the S5 to do what it does best. And while all these measurements responsible for such magic may seem not too far off from those of the Cotic, they make a huge difference.

Pipedream Sirius vs. Cotic SolarisMax

Let me circle back for a quick comparison of these two bikes, which I’ll sum up in a paragraph. The Cotic is a better climber than the Pipedream, but not by leaps and bounds. I’d say it’s also a hair more comfortable for pedaling over a lengthy period of time—something I’d attribute to the longer chainstay, slacker seat tube, higher stack, and little more frame suppleness. However, I think both are quite similar in those categories, particularly the more you ride them and get used to them. The Sirius is a far better descender than the Cotic, by lightyears, or parsecs, if you will. As mentioned, it’s unlike anything else in that regard. A few more plusses for the S5 include better cable routing (the dropper routing, in particular), more tire clearance, and better versatility with sliding dropouts. The Cotic edges out the Sirius for frame bag space, for what it’s worth.

Is the Pipedream Sirius S5 a Bikepacking Bike?

The answer to this question is yes and no, but it also doesn’t really matter. From a ride quality perspective, the S5 gets better with luggage, as expected. This is the case with a lot of bikes, especially ones with more aggressive geometry. The load anchors the front end and tames the steering. So, the pedalability and ride qualities mentioned above only improved in that regard. It also gave the frame a softer feel. And a medium-light load didn’t seem to quell the responsiveness of the frame.

All that adds up to a really great riding bike that’s perfectly suitable for bikepacking, if you’re into this type of bike. It’s an excellent choice for routes that have a lot of singletrack or chunky rides where that added stability comes into play. It’s also a great option for those looking to forego full-suspension, but want a trail bike that’s up for a challenge and better suited for bikepacking.

• 
• 

• 
• 
• 

All that said, it doesn’t have any bikepacking-friendly features to speak of. It’s a hardtail, so the frame triangle is OK for an adequate frame bag, but with its low-slung stance and curved seat tube, it’s not exactly cavernous. To illustrate, the camo Bedrock frame bag pictured on it was made for a size medium Jamis Dragonslayer back in 2015. Seat tubes were longer back then, so it kind of makes sense, but I still think a centimeter or two higher seat tube would be proportionally accurate for the “Longer” size frame. I have long legs, but that’s a 210mm dropper and there’s still several centimeters of exposed post, as you can see in the photos. Additionally, I really like bikes with a low stack height, but I think the Sirius S5 could stand to have a slightly longer headtube too. A slight increase in both of those measurements—and perhaps a little less curve to the seat tube—could add a lot of space.

• 
• 

Enter the S6

Fortunately, Pipedream has already addressed several of these concerns, which is why many of the cons mentioned above don’t particularly matter. Better yet, Pipedream didn’t even need our review as a nudge to make these changes, as bike companies often do—a reminder of the importance of independent, detailed reviews. Pipedream will be releasing a sixth iteration soon. The S6 will come in both titanium and steel, both featuring a lot more bikepacking-friendly provisions that include rear rack mounts, a three-pack under the downtube, and five bosses on the upper side of the downtube. Additionally, they will have a pair of mounts on the rear of the top tube. The S6 will also have a vectored headtube, increasing in length as the frame size increases. We’re excited to get a hold of one of these beauties this spring.

• Model/Size Tested: Longer/Vintage Green
• Actual Weight (full build): 12.9kg (28.6 pounds)
• Actual Weight (frame): 2.64kg (5.83 pounds)
• Place of Manufacture: Taiwan
• Price: £699
• Manufacturer’s Details: PipedreamCycles.com

• 
• 
•