STARLING CYCLES SPUR GEARBOX PROTOTYPE REVIEW
THE FUTURE OF SELF-POWERED BIKES?
Words by Robert Johnston
Photos by Adam Lievesley
Never content on sitting back and letting his bikes sell themselves, Joe McEwan of Starling Cycles is constantly looking to find performance gains through prototyping interesting machines, like the High Jack Pivot Staer we tested back in 2019. The conception of the Staer came about as an accident of sorts, as the mis-machining of some frame components for a gearbox prototype left Starling with some expensive steel bits laying around. This was rectified the next time round, and so the Starling Spur gearbox prototype was born. I just had to get some time on it, so I was thankful that Joe was keen to let me put it to work for a couple months to produce this feature.
The Starling Spur is Joe’s take on the ultimate enduro bike, with 170mm travel at both ends, modern enduro-ready geometry; and of course, a different approach to gear shifting than a conventional bike. At its heart is the striking machined casing of the Effigear gearbox, which sits neatly within the signature Starling steel frame tubing thanks to some custom machined steel plates.
The Effigear gearbox sees the shifting duties relocated to within the front triangle of the frame, where there is a series of cogs within the gearbox housing. This housing contains an oil bath, like the typical gearbox on your car, giving the cogs the necessary lubrication to allow them to run for thousands of miles without a need to go inside. A single cog within the gearbox sits on the input shaft to which the cranks are mounted and drives a second cluster of cogs on the secondary shaft. These cogs are constantly meshed to their counterparts on the third (output) shaft, and each forms a distinct gear ratio based on the ratio between their teeth. The shifter selects the cog on the secondary shaft that is to be engaged, whilst the remainder are free to spin. The engaged cog then transfers the power to its mating cog on the output shaft, which drives the cog on the outside of the gearbox and transmits the force through the chain and into the rear wheel. Simple, right? As hard it is to describe concisely, the operation of the gearbox is based on time-served technologies from the automotive world, so you can rest assured it will stand up to the abuse of a mountain bike over time.
What does the gearbox seek to achieve? Well, the most obvious difference between this bike and a ‘conventional’ bike is the lack of a rear derailleur and cassette on the back wheel. This should provide numerous benefits out on the trail. Firstly, ripping off derailleurs or bending hangers is a thing of the past, as they simply do not exist. Combining this with a narrower rear hub (due to less width taken up by the singular rear cog vs a cassette) affords a narrower rear end, letting the bike squeeze through tighter ruts and gaps in the trail without issue. This lack of derailleur and extra cogs leads to a significant saving of weight on the rear wheel, which should allow the wheel to move out of the way of obstacles easier due to its lower inertia. The weight of these cogs and derailleur (plus a bit extra due to extra shafts, the oil bath, and the need to create a structural shell to contain it all that is damage resilient enough for typical mountain bike abuse), is then contained low and centrally in the frame, sitting around the area that the front chainring would usually occupy. This alters the sprung:unsprung mass ratio considerably in the positive direction (higher sprung, lower unsprung). If you have ridden an eMTB you may already be aware of the benefit of this altered ratio, which yields improved chassis stability and a more reactive feeling rear end. All of these should add up to improved suspension performance, letting the wheel track the ground better and ultimately increasing the amount of grip available.
Shifting the Effigear gearbox is more “natural” than the likes of the gearbox competitor Pinion, thanks to its standard trigger shifter on the bar instead of relying on a grip-shift. This is made possible thanks to a spring-tensioned cable, which pulls the cable taught from the opposite side of the gearbox selector shaft than the shift cable inlet. The unit fitted to the Spur on test was a 6-speed unit with a large spread of gears that was equivalent to a 12spd setup, but of course with large gaps between each ratio. This 6spd gearbox was an experiment by Starling to determine if there was any drag to be saved, but production models will ship with the equally efficient 9spd unit instead. Shifting to a harder gear is as simple as hitting the shifter, with no let-off of power required. Shifting to an easier ratio requires a slight technique adjustment however, with the need to let off the power entirely to allow the gear to pop into place. After hitting the downshift paddle, you can continue to pedal to your heart’s content, but will only receive the easier ratio once you perform the necessary let-off. When coasting, shifting either direction requires no pedaling input, which allows for useful pre-emptive shifting.
With the Effigear’s output cog designed to sit concentrically to the suspension pivot, the chain does not see any growth throughout the travel, negating the need for any form of sprung tensioner. Starling’s solution to chain tension comes in the form of an eccentric dropout, which allows the distance between the output cog and rear axle to be altered with just an Allen key. This comes with an additional benefit of an adjustable geometry – the wheel will usually have two positions that create the same effective chain length, producing a “high” and “low” setting for the bb with a slight effect on the head angle and seat angle also. The choice of hardcore Winter riders, single speed drivetrain setups are considerably easier to maintain; last longer due to a constantly straight chainline without shifting causing side-forces on the chain links; and potentially even stay cleaner due to the extra ground clearance of the chain compared with a dangling derailleur. This aligns well with Starling’s long-running mantra of reducing time spent in the garage performing maintenance in order to maximize time out on the trails.
The orientation of the Effigear box has a direct effect on the suspension pivot locations and subsequent characteristics of the rear end. With Joe’s selected position, the single pivot rear end sits slightly below and forward of where a regular chainring would sit. This leads to a long effective lever around which the wheel pivots, which produces a relatively straight and primarily vertical axle path. As the front cog and suspension pivot sit concentrically, and there is only a single rear cog, the pedaling characteristics remain constant regardless of the gear selected. Modifying the size of the two external cogs allows for some tuning of this, with a larger rear cog or smaller front cog offering increased pedaling support but reducing the gear ratios and potentially causing issues with spin-out in the hardest gear. The supplied 15t front cog paired with a 25t rear cog produces low anti-squat values, creating a bike with limited pedaling support but an accompanying high level of grip when pedaling. Thankfully, the Spur’s EXT Storia LOK V3 shock features an easily accessible climb switch, firming up the rear end to make the less technical climbs more pleasant.
As with the Staer tested previously, geometry of this Spur prototype was in keeping with the times, without going extreme. A 64-degree head angle, 73.8-degree effective seat angle, 475mm reach and 450mm stays combine to produce a stable package that matches the intentions of the gearbox-equipped 170mm travel bruiser. There is a sliding mount on the front end of the shock, which would allow for some geometry adjustment, but this was left in its standard setting throughout the test as the gearbox was the primary focus. The production Spur will feature a steeper 77-degree seat tube angle and slightly longer reach number, bringing the numbers in line with the 2021 expectations for a bike of this kind. Cable routing is the mechanic-friendly external setup, save for the last section of the dropper cable entering the bottom of the seat tube. With the gearbox there is no mech hanger nor bottom bracket to worry about, and the rear brake mount relies on an IS mount setup that requires the appropriate adaptor for the rotor size you desire. A slide-in bottle such as the Fabric or Fidlock system will squeeze into the underside of the top tube above the shock for the hydration lovers.
The supplied build was no-nonsense, with coil sprung suspension on both ends thanks to the Öhlins RXF 36 fork paired with the aforementioned EXT Storia LOK V3 rear shock. Magura MT Trail brakes with a 203mm front and 180mm rear rotor (due to availability) were tasked with stopping duties. A custom Project 321 single speed rear hub laced to a Mavic XM481 rim handled the abuse out back, with a Hunt Endurowide front wheel called upon. Relatively burly Kenda tires were fitted due to my familiarity and trust in them, with the ATC casing Hellkat on the front paired with the sturdy AGC Pinner with a Rimpact insert in the rear to ensure the hard charging would be supported. A Funn 35mm cockpit and saddle and Bikeyoke revive dropper rounded out the build, totaling up to a weight of 40.8lbs (18.5kg). Not a light bike, then, but there’s certainly room to shave off a good bit of weight for those so inclined. The tire setup alone could stand to lose up to a kilo, with Joe suggesting the suspension performance can negate the need to run such a burly tire setup.
Starling hopes to open the books for orders on the Spur from late summer and expects the retail to be in the region of £3k+ for frame with gearbox.