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 who's, what's and where's. How many people are behind Jezken and where are you guys based out of?Initial inception of the idea was mine (Jeremy Sherwill) whilst I was studying Mechanical Engineering at university in Melbourne. One of my good buddies, Kendal Judd, came on board and we set about designing the bike for our final year project. Like a lot of companies do, we just used our names to come up with a brand - Jezken. We have a couple more people giving input to the project now, but it is definitely still my baby. We don't have a fixed location at the moment, but most of the work is done in a little town called Albury/Wodonga which lies on the New South Wales and Victorian state borders in Australia. I worked for an engineering company called Omega Technical Engineering which gave me access to some pretty expensive CNC machines to get the job done.I'm not going to lie, I've not heard of Albury or Wodonga. Good riding nearby?This area is pretty important to me, my parents live there and my Dad, who is also an engineer, has become more involved as the project has progressed. Our team riders are also nearby. There are quite a few good tracks around the area we test on. Most of the riding gets done up at Mt. Beauty, an Australian Nationals track that has a really good mix of terrain to ensure everything is working smoothly.It sounds like somewhere I should escape to during our wet and gloomy B.C. Winters. Back on topic though, why design your own bike?The initial idea came from having some time on my hands, I like to work on something in my spare time. I did a lot of reading and researching of what is out there and how it could be done better. I know that what we have, even as a prototype, is a whole lot better than a lot of bikes out there. We are both passionate about riding and being able to work on a project that you are really interested in makes it so much more rewarding. So far Jezken has been a side project to work and university, but we would love to make it a full time profitable venture. Luckily I had the opportunity at school and work to have access to a CNC machine and taught myself to use it. Apart from welding and heat treating, we designed and manufactured everything - from manufacturing jigs to the actual frames. We have learned a lot from the first few revisions and the time required to make new bikes should go down quite a bit.
|The Jezken looking ready for action.|I know that it's still the early days, but the bike looks great. What are the details?We are up to the 3rd revision of the frame and the crucial details of the latest bike are:
|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.
How many prototypes have there been so far?This season's race bikes are the 3rd revision, our first prototype was completed in August of last year, but due to an unfortunate accident (more on that below) we weren't able to test or race until the start of the following year. The main changes so far have been optimization of the swingarm and geometry. The first bike worked exactly how we expected so it was just a matter of building on that. The Jezken is designed around a geabox. What advantages does this give the bike?The main advantages as I mentioned early is low unsprung weight and centralized mass, but there are additional advantages that make gearbox bikes really unique. Low maintenance is another big plus, as well as no exposed drivetrain to get ripped off and spoil your day. Gear changes are instantaneous and silent, with or without pedaling. And the disadvantages...The main perceived disadvantage of gearboxes by the general mountain bike community is weight, however the bike's performance and race results so far tell quite a different story. Using a gearbox usually means that your hands are tied when it comes to location of the main pivot, although chain growth is not an issue due to the pivot's location around the drive cog on the gearbox. Was this an issue at all during the design process?We are actually quite happy with the bottom bracket/pivot relationship as it allowed for a slightly forward and reasonably high pivot which produces rearward axle path, but without the negative chain growth associated with most equivalent designs. As a lot of riders know, a rearward axle path allows for the wheel to move back and over square edged hits efficiently so the whole system works very well. Which gearbox did you choose to use?For the first few prototypes we have used a Universal Transmissions Gboxx2. From the beginning we have planned to make our own gearboxes, but it is a matter of time and money at the moment. We have dealt with Nicolai directly to order the gearboxes. As some people are aware, they have decided to stop making the Gboxx2, so that gives us plenty of motivation to get our own going. The gearboxes bolt to the frame with 8 bolts via male-female mounts (To understand this it's best to look at a picture of it). It has seven speeds and uses a twist shifter at this point in time. Care to share some details of your own gearbox design?Our own gearbox is really in the early stages with general layout and ideas are still being thrown around, there isn't a whole lot to talk about there. You're a rock! Spill the beans for us...Sorry! We have some pretty unique plans for the next revision of the swingarm as well, but can't let the cat out of the bag with that one either! You'll just have to wait...It seems that gearbox bikes come and go without many gaining any traction with most riders. Why do you think this is? That is a difficult question to put an exact answer to, but I think the bicycle industry is very ingrained with tradition and the derailleur has been around for quite a while now. An example of this would be carbon fiber frames in DH. As most engineers and technical minded people already know, carbon is an ideal material for DH, but it has taken one of the big players, Santa Cruz, to prove this to the masses. I think it is just a matter of time before gearboxes are common place in downhill racing. The advantages are so clear and we are definitely out to show how well these bikes can go!
|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.|I've heard that you have some fast Aussies on your bike... Yeah, we certainly do! Liam and Joel Panozzo live in Mt. Beauty which is just up the road. I met Liam through riding and we were both pretty keen for him to race on a Jezken. He ended up winning the first race on the bike in the 09/10 Victorian State Series and went on to win the overall series title. He was pretty excited the first time he rode it. This season both Liam and Joel are riding Jezken and we are excited about how things will go. Getting feedback from those two must be invaluable as well? For sure. Joel has had nothing but good things to say about the bike and that means a lot coming from someone who has had a hand in developing a lot of successful bikes. Liam had quite a bit of input in refinements to this season's race bikes and it makes the process a whole lot easier. We are really stoked to keep working together, getting results, and making bikes.
|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.|You had a major setback that involved a kangaroo on the highway. What happened? Yeah, old skippy thought it would be a good idea to jump out in front of us last year as we were driving back from the World Champs in Canberra... unfortunately we had the newly finished Jezken Mk1 on the back of the car. The bike's main frame was ripped in two, but amazingly we were able to still use the swingarm and some other components on the next prototype. However in the big scheme of things surviving a multi roll over at 110km/hr is a pretty sweet feeling. My buddies got out with a few cuts and I ended up with a broken arm and a pretty sore neck, as for the car... well, the pictures tell the story. At the time it was tough because all that work got destroyed without getting to enjoy it. That's all behind us now and now we don't swerve for critters!
|The Jezken's swingarm in the early stages of life.|Do you have plans to bring the bike to full production? At the moment it is going to be a little while before going to production, it all comes down to money. I'd be lying if I said I wasn't keen to get something going. When we were drawing up concepts 2 years ago our goal was to make the bike out of carbon fiber, but time and money didn't allow us to do this straight away. This remains a big focus and I hope to get a small production run (20 or so bikes) for the next model. This would be for people I know, our race team, and still a controlled testing program. At this point I can't really comment on price for the bikes - that said, I think they will be good value for the performance you get! Thanks for your time, Jeremy! Any shout outs to those who have given you a helping hand in seeing the Jezken come to life? Of course! There have been many, but a short list would include my old boss John Hinrichs and Cheyne Forbes of Omega Technical Engineering, Rob Chaplin from RC Metalcraft, Geoff Sherwill for putting in plenty of hours to help make it happen, and Ken from Tekin Suspension. Thanks guys!
|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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