Bike Check & Interview: Evan Turpen's High Pivot Steel Beauty

Despite the flurry of home-crafted style bikes recently, we're still as keen as ever to see more. So, when Evan Turpen got in touch with details of his project we were of course interested.

But after only a couple of emails back and forth there were mentions of patents, performance drivers, high pivots and steel, which got our ears really pricked up. And with a visual taster of the bike it was clear that this was a home build on a slightly different level.

Delving deeper into the bike provided a cauldron full of information and insight into how this ex-racer took a simple note on a phone and made it a reality of a rideable bike, with all the challenges along the way.

This is a closer look at Evan Turpen's custom high-pivot steel beauty.

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History

When did you first get into bikes?

2000. I was inspired by a friend who had recently gotten into mountain biking. He had a beautiful Santa Cruz Bullit that I couldn't stop lusting after, especially after riding it around for the first time. As a money-less, job-less teenager there was no way I could afford such a nice bike, so instead I decided to dust off an oldie but goodie that had been sitting dormant for years. It was a Red Cannondale CAAD3 hardtail that was a Christmas present from my parents. About as far as you could get from a Bullit with its cantilever rim brakes, flat bars, and triple chainring, but I proceeded to ride the crap out of that thing! From that day on, mountain biking was all I wanted to do, and it was all that I did. I was obsessed.

When did you first start racing? And what discipline?

In 2001 I started racing downhill at a local race series. Somehow I managed to win the first race I entered as a junior and I was hooked on racing from that point on.

How long did you race for? Which disciplines did you race throughout your career and why?

Racing downhill, dual slalom, and the occasional 4-cross race was my life from 2001 to 2011. I raced because it gave me meaning and focus and despite the stress it was a hell of a lot of fun! As a privateer I never made any money racing bikes and struggled with coming up short results and sponsorship-wise. In 2012 I ran out of steam and took a break from racing to work on a small lift-accessed bike park in the Sierra Mountains. While building and maintaining trails at the park I decided to give enduro racing a go and signed up to race the Trans Provence later that year. I had no idea what I was getting into, but the concept of blind racing in epic locations was rad. Since then I've raced quite a few more enduro races and I will continue to race enduro for as long as I can. Don't get me wrong though, I still have a soft spot for downhill and dual slalom. I'm turning 35 this year, but I wouldn't say I won't race a downhill bike ever again.

How did your riding and racing background influence you into creating your own bike?

Through racing I was always very focused on trying to have the best equipment possible. Ultimately, the realization came to me that nobody makes the perfect bike. Most companies are limited by none other than themselves. Engineers have to make products fit into a particular box for marketing and sales reasons and therefore are unable to think outside the box. The design of this bike wasn't limited. It was a clean slate aimed at getting the most performance possible out of a bike with absolutely no obligations to anyone but myself.

What do you do aside from bike riding and making bikes? What’s your current profession?

My current professional title is "unemployed". I am actually a professional bike mechanic by trade. Up until October of last year I was the lead mechanic at a local shop. When I left the shop, I took out a $20,000 loan to help cover living expenses and give me the time I needed to focus on the bike design and fabrication. It was a bit of a risk for me as I had never made a bike before and didn't know if it would pay off, but after riding the first prototype I am 100% confident that this was the right decision.

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Development

When did you first start developing this bike?

The bike started as a note in my cell phone in August of 2018. It was basically a wish list of characteristics and features that I thought would make the "perfect" bike.

You mentioned that you’re not an engineer. Which hurdles did you have to overcome and which areas do you feel you've learned the most in to get to the end goal of a physical bike?

The first major hurdle was designing the bike in 3-D since I didn't have access to or the skills to use 3-D design software. I eventually managed to get a student license of Creo Parametric 3-D design software. I took an online Creo class and within 3 weeks had learned enough to have a very rough 3-D assembly. It was a proud moment to actually cycle through the travel to check clearances on the frame in the computer.

The next big hurdle was my lack of engineering experience as it relates to design. I shared the design of the bike with a friend who's an engineer and he suggested that I look at the forces generated on the links and frame using FEA (Finite Element Analysis). Up to that point I had been designing the bike by the seat of my pants based off what I thought a strong bike looked like. Learning and using FEA changed my life. I tested my links and my frame and I found that some aspects of my initial designs would have failed under hard bottom-out situations. My bike would have actually bent. Luckily using FEA I was able to come up with solutions that exceeded my requirements for strength and avoid these major headaches.

The last major hurdle was learning how to fabricate metal parts using a manual mill, lathe, and hand tools. Learning how to hit very precise tolerances over and over is very difficult, especially when the machines are cheaper Chinese ones! I generally try to hit a tolerance on critical parts of plus or minus 0.01mm. That's a full range of less than one one-thousandth of an inch. The equivalent of 5 times smaller than the thickness of an average human hair. I would say that I've definitely come the furthest with my fabrication skills. Something that is extremely valuable going forwards with developing the next version.

What factors did you give high priority in your development? You mentioned you wanted the fastest bike possible for its intended use.

Everything. But the main focus was always on speed and control, so priority was given to the kinematics and geometry: axle path, leverage ratio, anti-rise and anti-squat had a heavy focus. Carrying speed, predictability and generating grip are all really high on the list. I'm particularly proud of the anti-squat I achieved with this design. To the best of my knowledge no rear derailleur driven high-pivot bike has achieved what this one does.

How, from all the available choices, did you narrow down to this suspension layout and design?

I like simple things and I like smart things and I really like simple and smart things. When exploring all the different ideas that I had this was the simplest layout that consistently achieved the highest number of goals I had set for the bike. To the untrained eye and even to the trained eye it looks like a simple linkage driven high single pivot. And that is technically correct, but what it achieves goes well beyond the preconceived notions for that style of bike.

Were there any avenues that you went down, only to find you needed to backtrack and find another?

For sure! The first design that I thought was the end-all be-all had extremely high pivot forces. It would have tried to rip itself to shreds. Multiple iterations leading up to the current design had issues too. I would get really far along in the design only to find some death-blow clearance or strength issue that I couldn't work around. So many disappointing failures, but you learn from each one and hopefully don't repeat them ever again.

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The Bike

What are some specific details on travel, geometry, suspension characteristics?

I designed the bike with two travel settings so that I could have two bikes to test in one. It has a 140mm setting and a 164mm setting that both use the appropriately sized shock for that travel. I am about 5 foot 9-1/2 inches tall (177cm) and the frame has a reach of 480mm which is just perfect for me. The seat tube angle is plenty steep for climbing too. At my saddle height the effective seat tube angle is 77.9 degrees. Head angle is set to 64 degrees and I am running an extra short 37mm fork offset (something that I had been testing on my prior bike). BB height is 341mm in the 140 and 350mm in the 164. Reach, stack, headtube and seat tube angle remain identical in both travel settings. Chain stay length is 435mm in the 140 and 432.4mm in the 164. Because of the rearward axle path both bikes sit around 445-448mm chain stays at sag.




Kinematically the anti-squat is 100% at sag in every gear combination and is extremely stable even if you venture outside the sag zone. It pedals amazing. Anti-rise is 100% at sag. This bike is super stable under braking. Watch a Formula 1 car braking hard for a corner...It barely moves, it's just stable and planted and allows for the best control under braking. That's what this bike does. The axle path is also 100% rearward, but isn't so extreme that it creates awkward handling. This really helps the bike carry speed well through rough sections. The other benefit of the rearward axle path is increased front end grip from the bike naturally loading the front tire as you load the suspension into a corner. The bike really does create an "All Wheel Drive" feeling of traction and control for you. The leverage rate is also progressive and designed for increased small bump sensitivity and great bottom-out resistance. It is both coil shock and air shock compatible and you don't have to stuff air shocks completely full of volume spacers to get enough ramp up.

Who built the frame?

I built the frame, but I hired John Caletti of Caletti Cycles to TIG weld it. He's a local high-end frame builder who specializes in titanium and steel frames. He is an absolute wizard with a welding torch and there's no way I would have trusted the frame if I had welded it. I also hired local machinist bad-ass Dave Mather of Mather Machining to CNC machine my links and idler pulleys. He did a tremendous job of nailing the tolerances I specified and I will definitely work with him again. I bought my headtube, dropouts, brake mount, cable guides and seat tube sleeve from Paragon Machine Works. All other parts you see on the frame were machined by me on a manual mill and lathe. And some parts were made with saws, files and a power sander with a careful eye.

Why steel for the first frame?

Because it looks bad-ass! And I grew up during the heyday of Brooklyn Machine Works. Well, actually this was a decision I made in the first few months of researching fabrication techniques. It also came down to the fact that Santa Cruz County has some really amazing custom frame builders that work in steel and titanium. Aluminum has a couple extra steps required as well. After welding an aluminum frame it has to be baked in an oven until it is in an "annealed" state. This makes the frame very soft so that you can properly align everything (bend it all back into shape) after the distortion from welding. After it is aligned, it has to be heat treated which is another trip to a special oven in order to bring it back up to its original strength. If you don't take these additional steps, then your frame will be much weaker and likely fail.

Generally welding 4130 chromoly steel doesn't require post welding heat treatment to be strong enough to ride. Also, my welder, John, did an amazing job keeping the frame straight and minimizing distortion from welding. This frame is straighter than most production welded bikes and my geometry and kinematics were maintained really well because of this. 4130 tubing is also stiffer than aluminum in a more compact package so it allowed me more clearance to work with while maintaining a great level of stiffness. Going forwards though, if done right, aluminum can save quite a bit of weight over steel and maintain a good level of stiffness if tubing size goes up.

What challenges did you find from going from a computer model to a physical bike?

Too many to list. The biggest challenge was jig design and figuring out the best way to accurately hold everything in place so the frame was actually what I designed when it was done. This is probably the most important step when making a frame. That and mitering the tubes to fit as tight and straight as possible.

Did you need to go backwards and forwards between the computer and manufacturing a lot or did you know exactly how you wanted to make the bike while you were in development?

Sometimes. A lot of times when I would have an issue figuring out how to accurately make something I would improvise and create a one-off special tool just to perform one single task. If it allowed me to make the machines do the dirty work in making the part I was all for the increased time and effort it took.

When did you realize you could apply for a patent?

Pretty early on in designing the bike I realized the uniqueness of what I was able to achieve with anti-squat, anti-rise, and axle-path and I knew I needed to apply for a patent on the design. The anti-squat is the most unique feature of them all. To the best of my knowledge no current or past high pivot bike with a rear derailleur based drivetrain achieves 100% anti-squat at sag in every gear combination. With a gearbox and single final gear this is easy to accomplish.

How has the bike been received by your peers? What’s the feedback?

Well...so far, the only one to ride it properly has been me. First ride on it was one-week ago. I am really looking forward to getting as many people on the bike to offer their opinions, feedback and experiences with how it rides in the near future. I'm not afraid to see what other people think. And I'm really looking forward to pitting it against the clock through racing and back-to-back comparison testing. The clock doesn't lie!

Is the adjustability in the bike for a specific geometry or kinematic purpose or just for you to play around with things and learn?

The adjustability is simply there so I can test this suspension design in two major different categories without having to make two bikes. The shorter travel is more of an aggressive trail/all-mountain setup and the long travel is a full on big-hitting park/enduro bike. Both configurations weigh pretty much the same, so it's really only testing suspension performance and how that relates to the experience on the trail.

There have been a few emerging high pivot bikes recently, what do you say this bike has over its competition?

Everything at the moment. Well, almost everything... it's not carbon fiber so it is heavy. I wanted to take the performance benefits of a high pivot bike and eliminate the negative traits I've experienced while riding them in the past. Kinematics were huge, but also drivetrain efficiency and noise. My idler is 22-teeth for better durability since the loads are distributed over a larger surface area and the angle change that each link has to make as it enters and exits the idler is less. It also utilizes a really nice bearing that can more than handle the loads that it sees without being damaged. With my idler location being so far forwards the angle that the chain sees at the extreme ends of the cassette is noticeably less too. This gives you less efficiency losses in your climbing gears where you spend the majority of your time. And also, better efficiency further down the cassette when you are going really fast! I'm excited to eventually perform scientific efficiency tests on this design compared to a "normal" drivetrain and post the results. Judging drivetrain efficiency by feel is great, but you can't argue with actual numbers.

The devil is in the details, as you say. Which is your favourite detail? Is there a story behind that particular one?

One little detail that I really like as a bike mechanic is the lack of spacers required for the BB and cranks. This bike uses Super-Boost Shimano XT 12-speed cranks paired with a DH 157mm rear hub. The cranks are designed to be installed on a 73mm shell frame with 3 different spacers in various locations. I utilized all the space that would have been taken up by spacers to widen my BB shell as much as possible for an even wider and stiffer BB pivot and widest stance possible for the BB bearings. This also made it so there were no spacers to mess around with when installing the cranks. It's nice being able to do whatever I wanted since this was a one-off bike made just for me.

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The Next Bike

You’ve mentioned you’ve learrned a lot from getting to this bike. What were the biggest things that you’ll take into the next bike?

So many things! Namely jig design to improve weldability, lowering the forces acting on the frame so it can be made lighter and stronger, and designing the bike smarter to make it easier, and less expensive to fabricate. Stuff doesn't have to cost more to be better. Smart design can make products less expensive, lighter, and stronger.

Have you now invested more into the next bike with the move to an aluminum construction?

I haven't invested anything yet. I don't have any money to invest! I'm actually going to fully run out of funds by August of this year. The goal is to plan out the next steps and secure enough funding to keep moving forwards. If I have to go back to working as a bike mechanic full-time my progress on this bike will slow drastically.

Is this looking like a venture to selling the bikes at some point?

100%. Yes. The plan is to start small with a batch of at least 25 to 50 high-quality frames and then go forwards from there. They will be aluminum versions to save weight and be built entirely here in the U.S. Doing so helps ensure the best quality product, a timely development cycle, and a very enjoyable experience for those that choose to buy one of my future bikes. Carbon is something that I would like to do eventually too, but just doesn't make sense right now with such small initial numbers.

When are you planning to have version two?

The goal is to push hard to have version two ready to ride sometime in the fall of this year using everything that I've learned from this bike and more. It will also be my first foray into building an aluminum frame that will be much more representative of a production bike.



A big thank you to Evan for all the information and sharing his bike with us

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Reviews and Tech Bike Checks Enduro Bikes Evan Turpen