First Look: Contra Bikes' Prototype 2.0 is a Steel, High Virtual Pivot Enduro Bike
Contra bikes is the brainchild of Evan Turpen, former downhill racer, mechanic and self-taught engineer. After being the lead mechanic at a local shop, he took out a loan and learned to use engineering software from scratch in order to develop his own bike. His first prototype was a high-single-pivot steel bike which was a thoroughly impressive-looking first attempt. You can read more about that bike and the man behind it in this deep dive by Dan Roberts.
In that interview, Evan suggested the next step was to go aluminum, but after investigating that route he decided to stick with steel. Instead, he's changed the suspension design to a high virtual pivot for his second prototype, which is nearly ready for production. I called Evan to find out why.
The new bike has come a long way from the first prototype (left).
Details
Wheel Size: 29"
Travel: 164mm (vertical), 170mm fork
Suspension System: High virtual pivot with idler pulley and counter-rotating links
Claimed weight: 37.2 lbs with pedals
Materials: Chromoly steel tubes with machined aluminium links
Website: contrabikes.com
Wheel Size: 29"
Travel: 164mm (vertical), 170mm fork
Suspension System: High virtual pivot with idler pulley and counter-rotating links
Claimed weight: 37.2 lbs with pedals
Materials: Chromoly steel tubes with machined aluminium links
Website: contrabikes.com
Suspension design
Evan says his interest in high pivot suspension was piqued when he got a chance to try out a Commencal Supreme SX. "There were a lot of problems with it (high pivot forces, manufacturing issues, lack of progression, idler drag, funky pedalling and braking characteristics).... but I was blown away with how good it was in the rough. It was better than any downhill bike I had ridden at the time. With Contra, I wanted to improve all of the negative aspects of high-pivot bikes while keeping the positive characteristics intact." This is what inspired Evan's first high single-pivot bike. For version 2, the idler and the rearward axle path remain but the layout has changed.
It uses two short, counter-rotating links like a VPP design; if viewed from the drive side, the lower link turns clockwise and the top link turns anticlockwise as it compresses. Compared to a single-pivot, this reduces the anti-rise, which Evan wanted to prevent it from feeling like it was squatting into its travel under braking. The virtual pivot point (centre of curvature) remains high above the BB, like the physical pivot in the first prototype, creating a similarly rearward axle path to the single-pivot bike.
While most short-link bikes use some sort of strut(s) to connect the chainstay and seatstay, forming a solid rear triangle, here a machined aluminum rod connects the pivot points of the two links together. "It's a kind of a hybrid swingarm in a sense," Evan explains. "Instead of an asymmetric strut connecting the chainstay and seatstay just on one side, which can create a torque on the links, I have a symmetrical aluminum part I call the tie-bar... The tolerance on that can be really good because it's a machined part."
A lot of high pivot bikes mount the idler on the swingarm such that it moves as the suspension compresses, affecting the kinematics. I asked Turpen why his idler is attached to the frame. "I looked at having it mounted on the link because that was actually easier to package, but with my design, I couldn't get as good characteristics as mounting it to the frame. Also, the anti-squat isn't affected by chainring size with this design."
Instead, it is possible to adjust the anti-squat by changing the size of the idler pulley - a smaller pulley creates a lower chain line which increases anti-squat. Whereas if you want to change the chainring size, that will change the gearing only and not the kinematics. Evan says the position of the pulley will change with the frame size, so taller riders on bigger frames should get similar pedalling characteristics to smaller riders on the smallest size (assuming riders are roughly the intended height for the frame size.)
A lot of high pivot bikes mount the idler on the swingarm such that it moves as the suspension compresses, affecting the kinematics. I asked Turpen why his idler is attached to the frame. "I looked at having it mounted on the link because that was actually easier to package, but with my design, I couldn't get as good characteristics as mounting it to the frame. Also, the anti-squat isn't affected by chainring size with this design."
Instead, it is possible to adjust the anti-squat by changing the size of the idler pulley - a smaller pulley creates a lower chain line which increases anti-squat. Whereas if you want to change the chainring size, that will change the gearing only and not the kinematics. Evan says the position of the pulley will change with the frame size, so taller riders on bigger frames should get similar pedalling characteristics to smaller riders on the smallest size (assuming riders are roughly the intended height for the frame size.)
The bike is designed around a 24 tooth idler (a 22 tooth is shown in the pictures). This is bigger than most idlers and is intended to reduce drag. As the chain bends around the idler, the links have to articulate under tension, then straighten out again as they head for the chainring. The bigger the idler, the smaller the articulation angle and so the less power is lost.
Similarly, the idler is positioned further forwards than most. This gives a longer chain span between the cassette and the idler, which reduces the angle that the chain has to move laterally to reach the extremes of the cassette. This should also help reduce drivetrain losses when pedalling in the biggest and smallest sprockets.
The idler position goes with an effective pivot point which is quite far forwards, which gives the axle path a slightly longer radius of curvature than most designs. This allows the axle path to move backwards throughout almost all of the travel, without moving too far forwards at the end or moving back too fast at the beginning of the travel. The axle path shape is similar to the single-pivot first prototype, which had a physical pivot point far in front of the bottom bracket, although the counter-rotating links do result in a shortening radius of curvature towards bottom-out, which causes it to come forwards slightly further at the end.
The axle path is rearward throughout most of the travel, with a very slight forwards movement at the end. Note the scale is very squashed - it moves backwards 22mm over the 164mm of travel. The Leverage curve is progressive throughout, with a coil-friendly 24% progression.
Antisquat is calculated at a touch over 100% at sag in all gears. Anti-rise is on the high side, but lower than most high single pivots and drops off steeply through the travel.
Construction
The steel frame is welded locally by John Caletti. Evan investigated making the second bike out of aluminum for weight savings, but as you can see, chose another path. "I really have nothing against steel," he explains. "I was able to hit a stiffness level, at least in FEA (Finite Element Analysis), that was comparable if not slightly stiffer than my really heavy prototype swingarm I had, and the weight is 160g less than the aluminum swingarm I had designed so I was just like 'okay I've got to do this'. It's more affordable and probably more durable because the fatigue life of steel is higher than aluminum."
Evan was most concerned with saving unsprung weight from the swingarm for the benefit of suspension performance. The bearing housings and dropouts are designed to be as minimal as possible (which Evan admits makes the welding trickier) and the wall thickness is different for the chainstays and seatstays to improve strength where it's needed. Eliminating the two links used to drive the shock on the previous bike saved a lot of weight too.
The new virtual pivot design saves a lot of weight over the old single pivot with its shock link by the BB.
Though the current dropout and hanger are designed to be as light as possible, Turpen intends to redesign the dropout to make use of a Universal Derailleur Hanger, thereby improving repairability. "Part of the point of the bike being steel is that people could have it for fifteen or twenty years."
As for the total weight, Evan says the frame is about three pounds (1.36Kg) lighter than the first prototype, and the new bike as you see it - including DoubleDown tires, a tool in the stem and pedals - is 37.2 lbs (16.9Kg). He says that with EXO tires, Fox 36, Float X shock and without pedals, you could get it below 35 lbs (15.9Kg).
Geometry
The geometry numbers aren't set in stone as far as the production bikes go, but the chart opposite shows the proportions of the bike pictured that Evan is riding, which corresponds to a size Large. For production, Evan will produce a Medium and an XL too, although the sizes may not be called that. For reference, Evan is 5’ 9.5" / 177cm tall.
The geometry numbers aren't intended to rewrite the rules and are broadly in line with other modern bikes. Evan thinks the geometry trends we've seen over recent years won't go much further (*Levy spits out his coffee*) so the numbers won't look dated in many years time. The head angle of 64-degrees is a touch steeper than some brands are going these days, but Evan points out that the bike's high anti-rise figures mean the rear suspension won't rise up as much when on the brakes. This means that relative to other bikes, the dynamic head angle when braking will be a touch slacker than the static figure would suggest.
If you did want it slacker, Evan says there's nothing to stop you from putting a 180mm fork on it. On the other hand, you could go down in fork and shock travel if you wanted more of a trail bike.
The geometry numbers aren't intended to rewrite the rules and are broadly in line with other modern bikes. Evan thinks the geometry trends we've seen over recent years won't go much further (*Levy spits out his coffee*) so the numbers won't look dated in many years time. The head angle of 64-degrees is a touch steeper than some brands are going these days, but Evan points out that the bike's high anti-rise figures mean the rear suspension won't rise up as much when on the brakes. This means that relative to other bikes, the dynamic head angle when braking will be a touch slacker than the static figure would suggest.
If you did want it slacker, Evan says there's nothing to stop you from putting a 180mm fork on it. On the other hand, you could go down in fork and shock travel if you wanted more of a trail bike.
What's next?
Evan is hoping to launch the bike in spring with around thirty bikes ready for sale in three sizes. There won't be any pre-orders. Evan wants the bikes to be ready to buy when they're up for sale so he isn't hanging onto customers' money for an unknown amount of time. After that, the aim is to eventually make up 150 frames a year. There'll be an air and a coil shock option, and Evan says it will be comparable in price to a carbon frame and it'll be US-made and designed to last a long time.
Evan added that, according to FEA, "this bike *should* be able to pass the Tri-Test DH certification (at least virtually until I can get one there!) I want to get it certified for downhill and I have a swingarm that will give it 200mm travel so you could run it as a downhill bike with a dual-crown fork."
Further in the future, Evan is investigating getting a carbon version made in North America, provided doing so would save a significant amount of weight.
We're looking forward to testing the steel version in 2022.
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#29/29isDead
But for real, that bike is stunning and I would trade a kidney to have one. Best of luck to Evan!
www.pinkbike.com/photo/21564472
Engineering using CAD in quite different?
www.vitalmtb.com/features/Steel-American-Made-High-Pivot-Enduro-Bike-Evan-Turpen-Contra-Bikes-The-Inside-Line,3309
So i don't think it's as much of an issue as you think. You tip in to the corner, shove it into its travel and suddenly becomes super stable,
Any plans to do custom sizing aka Kavenz-style (i.e. 4 reach, head tube length, seat tube length and rear center length options with all other numbers being shared)? Current options especially in carbon really limit size range, or suitability for XS or XXL riders. Having a way to do some level of customization would be a huge plus of working in steel domestically.
cheers!
Thanks!
B, rear derailleurs were (and it won't change) designed for chains that come horizontally.
Why suspension designers havevn't tried rotating the derailuer hanger counterclockwise? To bring the situation of lower pulley back to original layout.
Saw the bike. Stopped laughing. Seriously nice work.
Looks mint though!
When I red how that large radius make the idler less of a drag, and the forward position give a adjustable anti-squat value separate from chainring size, I am sold! Ill get used to the looks thankyou.