While there are some great production bikes out there, sometimes the most interesting machines are made on a small budget and by only a few people. Australians Jeremy Sherwill and Kendal Judd are a shining example of what happens when you mix together equal parts determination, vision, and know-how. The result is their Jezken gearbox DH bike.
Inside you can check out photos of the bike and read a great interview with Jeremy about the design.
Jezken Prototype Gearbox DH bike |
| The Jezken looking ready for action. |
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| The Jezken uses a low and centrally mounted gearbox that allows the bike to have as little unsprung mass as possible which lets the rear wheel respond quicker to the terrain. The bike sports 8" of rear wheel travel from a standard 3" stroke shock and features geometry that allows head angle adjustments from 63 degrees to 65 degrees as the course demands. |
• 8" of rear wheel travel
• An adjustable head angle from 63-65 degrees. The bottom bracket height is nice and low, but the actual specs for that are under wraps at the moment
• We decided on a linkage driven single pivot from the start as it allows a lot of adjustment of the leverage ratio to fine tune suspension performance and works well to incorporate a gearbox
• We are using a 3" stroke, 9.5" eye-to-eye damper. Providing an overall leverage ratio of 2.6, however the leverage ratio changes through the travel due to the linkage
• We have been developing the bike with Fox suspension, both the DHX 5.0 and RC4 dampers have been working really well - it's all in the shock setup
• You will notice that the chain actually runs through the swingarm, the design just ended up going this way purely for structural efficiency. It works well as the chain is tensioned and makes for a neat and unique looking rear end. Gearbox bikes are known for generally being a bit weighty. How much does the Jezken weigh?Internet forum weight weenies aside, performance is really what matters in the real world of racing. The overall package weighed 19.5kg (43lbs) the last time we checked. But there is a lot more to a bike than just its total weight...Low weight was definitely one of the objectives, but the biggest issue for performance is unsprung to sprung weight ratios and center of mass. This is where the Jezken really excels and anyone who has ridden the bike can really vouch for this. Without the weight of a cassette, freewheel and derailleur hanging off the rear wheel, the bike is able to track like nothing else. Our team riders have been amazed how well the bike works. Also, the majority of the weight is sitting in the gearbox which is central and low - this allows the bike to be moved around really easily.  |
| The driveside view of the Jezken makes it look more MotoGP and motocross than mountain bike. Have a look at how the chain passes through the massive swingarm on its way to the rear hub. Although it looks like it would make a fair bit of noise, it is taut and doesn't slap the swingarm at all. The bike is said to be incredibly silent. Pivoting rear dropouts allow the chain to be tensioned by loosening the three bolts and rotating the entire unit back. |
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| The boys proudly showing off their hard work. It looks like someone spent their day at the polishing wheel! Take note of the dialed internal cable routing on this version as well. |
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| The Jezken's swingarm in the early stages of life. |
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| This is the car that had the run-in with the Kangaroo. Hard to believe that anyone could survive this, let alone escape with mostly minor injuries. |
You can keep tabs on the Jezken bike by
checking out their blog.
All photos courtesy of JezkenThe Jezken machine looks like a very sorted bike, simple and with the most weighty bits put where they make the most sense. Let's hear from all of the internet armchair engineers out there, what do you make of the bike?
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