PIPEDREAM SIRIUS 4G 27.5+
£549 (frame) Versatile big-rubber steel trail bomber
UK firm Pipedream Cycles have been developing their Sirius steel hardtails since 2005 and in 2010 they were one of the first brands to o er a 650b-compatible frame. Their latest Sirius literally expands to take big 29er or mid-size 650b+ tyres for rock-shrugging fun.
The frame
While the fourth-generation Sirius chassis is expensive for unbranded steel, there’s plenty going on to compensate. The metal head badge sits on a ring-reinforced 44mm head tube, and the chunky, bent and butted down tube starts o round (38x38mm) and becomes oval (31x43mm) as it nears Pipedream’s unique bottom bracket assembly.
This is created by sliding the BB shell through circular holes in the end of the two chain stay end-plates and welding it in place, together with a curved brace-plate ahead of the rear tyre. It’s not a respaced chainring/148mm rear axle Boost set-up though, so while 27.5x2.8in tyres fit, 3.0in rubber is a squeeze.
The bolted rear dropouts slide back and forth to change geometry or adjust singlespeed chain tension, and can be swapped for 135mm quick-release versions. There’s semi-internal routing for a dropper post and room for a single bottle cage on the down tube.
The kit
Our Sirius was built up as a FUNN components/Zelvy carbon wheels demo bike. Pipedream don’t o er complete builds (although they will quote you for a full parts package if you send them your wishlist) but every frame comes with a FUNN Descend headset and Frodon seat clamp. You can also add a DVO Emerald fork for £579, which is £270 o the RRP.
The ride
The Sirius’s geometry was designed around a 110mm-travel, 515mm-long (axle to crown) Fox 34 fork, so the 140mm/555m X-Fusion McQueen that came with our test sample knocked the head angle back to a super-slack 64.5 degrees. Fortunately, the big frame tubes are bu enough to let you point the front wheel exactly where you want and the impeccably-damped fork can front out a lot of trouble.
At the rear end, the steel stays with long plate sections add to the soft, impact-swallowing feel of the plus-size tyres. That means you’re not getting bitten from behind anything like as badly as on a conventional-tyred hardtail, so you can push the front end harder. There’s still enough compliance in the frame to squeeze the traction of the Maxxis Ikon+ rubber to its easy-drifting limit on slippery, rooty, leafy surfaces too.
While tilting the frame back with the longer fork makes it feel good in slow, steep, tight situations, it reduces e ective reach and raises the BB noticeably, so the Sirius isn’t as stable in fast, carving turns as a bike designed around that length of fork as standard. Even with relatively light carbon wheels the rear end lacks the whip or obvious buoyancy that some riders might be after in a steel and/or plus frame, too. Muted power transfer also means that transitional trails can become a trudge, despite the fast-rolling tyres. If you can overlook these niggles, the frame will work as a sturdy, shock-shrugging base for a wide range of builds.