The Homer: A Steel, Single-Pivot Gearbox Driven Enduro Bike with 27.5 “Wheels

A special Irish bloke named Rory Beirne designed the contraption you see before you – a steel, single-pivot enduro bike with 180mm of travel that’s propelled by a belt-driven gearbox. There’s also an inverted dual crown fork, and 27.5 ”wheels at either end because that’s exactly what he wants. It’s not just leftover parts that were floating around the garage.

Rory is no slouch when it comes to riding anything with two wheels. He has ridden and raced all sorts of bikes, some with motors and some with tiny wheels and no gears. After spending years on a BMX, which takes the utmost precision to ride, Rory wanted a seriously capable mountain bike to deliver the same response and durability as his 20 “bike. As a self-announced critical person and structural engineer, it was time to put his money where his mouth is and depart from the norm.

With the help of his cousin, Stephen Beirne, they set out to build the perfect bike that did not exist anywhere else. Stephen owns and operates a steel brazing frame company in Dublin, Ireland, called The Freedom Machine, where he sat down after normal business hours to manufacture and assemble the frame.

The “Homer” creation is not something as mainstream as a Specialized Stumpjumper, a bike which Rory previously owned. Every single aspect of this bike, from the components and geometry, to the tubing and shock placement was meticulously questioned. His demands were quick handling and response from body movements, yet it needed stability and traction too.

That usually means 29 ”wheels, long chainstays, plenty of sag and progression in the suspension, and soft tires to grip the ground. What does Rory prefer? Nothing less than 30 psi in both of the 27.5 ”DH tires and firm suspension. 29ers bored him and “killed the fun”. He drew an excellent parallel for me: 29ers are to 27.5 ”what dirt jumping a 26” bike versus a BMX is like. “Horses for courses,” he added.

Standing at 184cm and weighing 72kg, he chose somewhat conventional numbers like a 475mm reach, 1250mm wheelbase, 79º seat tube angle, 30mm of BB drop, and chainstays that are evenly slotted for three positions between 420-445mm that grow by roughly 9mm under sag . What this offers him is a bike that is planted when resting, but has the ability to snap to life when told to leave the ground.

The seemingly narrow 135×12 mm Hope Single-speed hub flanges are actually almost 10 mm wider than the popular 148 Boost hubs, plus it uses male axle bolts where the washers key into circular slots to prevent the wheel from slipping in the dropouts. One small afterthought was the sliding dropout, yet it had a fixed brake mount. Compensating for the brake pad and rotor overlap with the adjustable chainstay length meant that to overcome this they would need to weld the caliper mounts in the center slot and use beveled washers were used to make up the height difference of the caliper clocking.


Moving onto the suspension setup is where the finer details get interesting. Even with that much pressure in the tires typically, Rory rides a 525 lb spring that only squats to 25% sag on the single pivot frame design. The single pivot isn’t totally linear and gains 8% in progression, due to the angled shock placement. That’s enough to mitigate wallowing in the middle of the 180mm of travel, 14 of which moves the axle rearward, while the EXT Storia’s hydraulic bottom out resistance takes care of the heaviest landings.

A ten year-old Manitou Dorado has been loved and lowered to 180mm travel. The main and ramp chambers are set to 72/130 PSI which a 35mm length stem if fitted to hold a high-rise bar. The offset is stock at 49mm, but there are some shorter, custom offset crowns in the works too.


One worry that does not cross Rory’s mind is flex. The downtube is a Reynolds 851, 1.2mm thick straight gauge in a 38mm diameter, and lighter 35mm top tube produce a solid, yet vibration absorbing front end. Out back, the 19mm chainstays and 16mm seatstays are 0.9mm thick 4130 for that compliant ride quality. All of the pivot hardware is CNC machined out of 4140, the dropouts are 316 stainless steel and TIG welded to 4130 hoods. On the drivetrain, the idler adapter and belt tensioner are machined from 6082 T6 aluminum by Steamer Industries.

By raising and shaping the rear triangle into a wider bridge, he was able to counteract too much twisting, but noted that he may add a seatstay bridge if less compliance is desired. In fact, he preaches that the USD fork chassis and spherical bushings in shock eyelets are what allow the suspension to actively track across cambers and chattery trail tread. The forgiveness in the frame and suspension could be just one reason why Rory can run such high tire pressure and firm suspension. Serious bike handling skills probably do not hurt either.

A mid-high pivot provides 114% anti-squat throughout the travel due to the idler location that also reduces the amount of belt growth. There is a notion that anti-rise on the high side of 100% can feel harsh while braking, since this can compress the suspension. The Homer’s anti-rise begins at 118% (110% at sag) and only falls to 88% at full travel, helping to preserve the bike’s geometry under braking – a theory adopted from Moto GP. Rory says he’s never experienced negative performance from that characteristic and chooses his braking points on smooth pieces of trail when he can.


Silence is another attribute that is appreciated after riding a BMX bike. A tight chain and less moving parts not only means there is less to go wrong, but nothing bounces around on the bike either. One way to take care of all that noise on a mountain bike is to first eliminate the derailleur, ultimately lowering the unsprung mass by moving the weight from the swingarm to the mainframe. Secondly, is swapping the chain for a belt. Rory was so sick of tuning twelve-speed derailleurs all of the time due to the accuracy needed for them to run perfectly that he caved and bit the bullet of increased drag that comes with a gearbox.

At the heart of the beast is a Pinion C1.9XR unit that draws a 568% range from nine gears through a twist-grip shifter – just another unconventional part on the Homer. The rear cog is the Pinion Gates 39T stainless steel opinion paired with a 19-tooth cog placed just below the swingarm pivot. By brazing the axle for the idler directly to the frame, there were less alignment issues to worry about and Rory hasn’t seen the need for a belt guide at either end yet.

Another bonus of the gearbox is being able to shift without pedaling and because the freehub is located on the mainframe, Rory says this works similarly to an O-Chain to alleviate pedal kickback.

Rory said he surprised himself by how high he can hop the 180mm travel bike, which isn’t a featherweight and attributes that leap to the balanced front to rear wheel unsprung weight ratios, in addition to the short chainstays too. It’s not the most efficient pedaling bike with the heavy tires holding it back, but he’s not racing or hunting Strava KOMs either. The focus is on the descent and being in total control.

So what’s next for, visually, the most unlikely mountain bike for a BMXer to ride, let alone build from the ground up? The stock 49mm offset fork crowns will be replaced by a much shorter, custom set and the same goes for the cranks. Those long 175mm Pinion crankarms will be replaced with 155mm to compensate for the low BB height. And if I had to make further guesses, Rory will continue to tweak Homer and send massive gaps. Somehow, I think he will stay away from 29 ”wheels and derailleurs though.