The Homer: A steel, single-pivot gearbox powered Enduro bike with 27.5 “wheels

A special Irish guy named Rory Beirne designed the device you see in front of you – a steel, single-pivot 180mm travel bike driven by a belt-driven gearbox. There’s also an inverted double crown fork, and 27.5 “wheels at both ends, because that’s exactly what he wants. It’s not just remaining parts that float around the garage.

Rory is no slouch when it comes to riding everything with two wheels. He has ridden and ridden all kinds of bikes, some with motors and some with small wheels and no gears. After spending years on a BMX that takes the utmost precision to ride, Rory wanted a seriously capable mountain bike to deliver the same responsiveness and durability as his 20 “bike. As a self-proclaimed critical person and structural engineer, it’s time to put his money where his mouth is and go away 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 solder frame company in Dublin, Ireland called The Freedom Machine, where he sat after normal working hours to make and assemble the frame.

The creation of “Homer” is not as mainstream as a Specialized Stumpjumper, a bike that Rory previously owned. Every aspect of this bike, from the components and geometry, to the tubes and shock placement was carefully questioned. Its requirements were rapid handling and response of body movements, but it also needed stability and traction.

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

Standing at 184cm and weighing 72kg, he chose some conventional numbers such as a range of 475mm, 1250mm wheelbase, 79º seat tube angle, 30mm BB drop, and chain tags that are evenly spaced for three positions between 420-445 mm which grow by below sag 9 mm. What this offers him is a bike that is planted at rest but has the ability to snap life when it is said 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 in circular slots to prevent the wheel from slipping in. and dropouts. One small caveat 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 have to weld the caliper mounts into the center slot and use beveled washers were used to make up the height difference of the caliper clocking.

SUSPENSION SETUP

Moving to the suspension setup is where the finer details become interesting. Even with so much pressure in the tires typically, Rory drives a 525 lb spring that only drops squats to 25% on the design of one pivot frame. The single pivot is not completely linear and gains 8% in progression, due to the angle shock placement. That’s enough to reduce embankments in the middle of the 180mm of travel, 14 of which move the axle backwards, while the hydraulic resistance of the bottom of the EXT Storia ensures the heaviest landings.

A ten year old Manitou Dorado is lovely and reduced to 180mm travel. The main and disaster chambers are set at 72/130 PSI, which is a 35mm long stem as equipped to hold a high bar. The offset is stock at 49mm, but there are also some shorter, custom offset crowns in the works.

STEEL IS REAL

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

By lifting and shaping the rear triangle into a wider bridge, he was able to resist overturning, but found that he could add a sitting bridge if less compliance was desired. In fact, he preaches that the USD front chassis and spherical bushings in shock eyelets are what allow the suspension to actively track over cambers and chattery trail tread. The forgiveness in the frame and suspension can only be one reason why Rory can run such high tire pressure and sturdy suspension. Serious cycling skills probably don’t hurt either.

A mid-height pivot provides 114% anti-squat during the trip through the low location which also reduces the amount of belt growth. There is an idea that anti-rise on the high side of 100% can feel hard when braking, as this can compress the suspension. Homer’s anti-rise starts at 118% (110% at sag) and drops only to 88% at full travel, helping to keep the bike’s geometry under brakes – a theory adopted by Moto GP. Rory says he has never experienced negative performance from that characteristic and chooses to put his brake points on slippery tracks if he can.

THE SOUND OF SILENCE

Silence is another attribute that is appreciated after riding a BMX bike. A tight chain and less moving parts not only mean that there is less to go wrong, but nothing bumps on the bike either. One way to take care of all that noise on a mountain bike is to first eliminate the derailleur, eventually lowering the unsprung mass by moving the weight of the swingarm to the mainframe. Second, is changing the chain for a belt. Rory was so sick of tuning twelve-speed derailleurs all the time because of the accuracy needed for her to run perfectly that he bites the bullet of increased drag that comes with a gearbox.

At the heart of the beast is a Pinion C1.9XR unit that pulls a range of 568% from nine gears via a twist-grip shifter – just another unconventional part on the Homer. The rear sprocket is the Pinion Gates 39T stainless steel mount coupled with a 19-tooth sprocket located just below the swingarm. By soldering the axle for the idler directly to the frame, there were fewer tuning problems to worry about and Rory has not yet seen the need for a belt guide at both ends.

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 in the same way as an O-Chain to lower pedal kickback.

Rory said he was amazed at how high he can hop the 180mm touring bike, which is not fair weight and attributes that jump to the balanced weight ratios from front to rear wheel, in addition to the short chain tags as well. It’s not the most efficient pedal bike with the heavy tires that hold it back, but it’s also not racing or chasing Strava KOMs. The focus is on the decline and 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 are replaced by a much shorter, custom set and the same goes for the cranks. Those long 175mm Pinion crank arms are replaced with 155mm to compensate for the low BB height. And if I had to make further guesses, Rory would continue to adjust the Homer and send huge holes. Somehow I think he will want to stay away from 29 ”wheels and derailleurs.



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