Ben Meir
The Vulcan Explained: Pt1, Structure
- April 15, 2021
- , 3:55 pm
The REAP Vulcan’s radical, attention-grabbing design prompts a lot of questions. In this article, we’re going to focus on why we chose this shape and how we netted all of its benefits while avoiding all of the potential compromises. We’ll explore the aerodynamics in detail with our next article.
Why this shape?
The Vulcan uses a beam frame for two main reasons: aerodynamics and comfort. Both contribute to REAP’s guiding principle of energy efficiency. Every REAP, now and in the future, is designed to ensure your energy is spent on going forwards – faster or longer or both – and not wasted on aero drag, unwanted flex or sapping discomfort.
Secondarily to that, the beam frame also aligns with REAP’s other principle of striking and disruptive design. Simply, it was an opportunity to build a stunning bike with superior performance, avoiding potential compromises through our advanced and unique engineering.
The Vulcan achieves its class-leading aerodynamics through the elimination of the seat-tube and seatstays and their associated drag, and through the highly refined shaping of the beam frame. We’ll cover aero in detail next time.
The comfort function of the beam type toptube may look obvious, but it isn’t a soft-tail and doesn’t flex conspicuously. For instance, you can’t push down on the saddle and feel it move like the suspension of a mountain bike. If it did that, you’d feel it bob as you pedalled, which is clearly undesirable and inefficient. Rather, the top beam provides a level of vibration damping and bump absorption that transforms the ride experience and saves energy without acting like a pogo stick.
The Challenges
The beam frame has a number of difficult, inherent engineering challenges that must be overcome. Eliminating the seat-tube and seat-stays places huge stress on the toptube and the chainstays as they are now unsupported and their loads are carried at their end points – the seatpost clamp and the rear axle – which creates high leverage. The first challenge is supporting the vertical load of the rider’s weight and the extra forces created while riding, such as by bumps and compressions in the road.
The second challenge is managing the torsional (twisting) loads in the frame created by steering and by pedalling, especially climbing out of the saddle. A seat-tube completes the front triangle of a conventional frame, bracing it against these forces, and seatstays brace the rear axle from twisting in the chainstays. Without these elements, there’s a lot more work for the rest of the frame to do.
You might think that a triathlon bike doesn’t need crisp handling and high stiffness under power to the same extent as a road bike, but we disagree because efficiency is our obsession. We were not willing to accept any flex under power because a frame that flexes tangibly when you’re out of the saddle is also flexing in smaller degrees when you’re in the saddle, and while you may not be able to feel it, it’s still wasted energy. The Vulcan is much stiffer than most triathlon bikes and it feels so much better as a result.
Handling is an especially under-rated metric for tri bikes. We set the goal for the Vulcan to handle like a great road bike because, very simply, precise handling equals confidence and confidence creates not only a more enjoyable ride but a faster one.
Think about racing through a roundabout. On an average tri bike, it’s necessary to break position and move to the base bar to feel confident as you make the rapid steering inputs. A great handling tri bike like the Vulcan gives you the confidence to hold your aero position, take the turns on the extensions, carry your speed, and avoid wasting energy by having to accelerate again.
The Solutions
In order to make a beam frame stiff enough, there are two possible solutions. The first, and the cheapest, is to add lots of material to increase strength, but this would ruin the compliance and add ‘parasitic’ weight – mass that doesn’t contribute to performance but is made necessary by poor design. The second option is to use advanced materials and methods, and that’s what REAP is all about.
Carbon fibre comes in many specifications and, as with everything, the best fibres are more expensive. At REAP, all materials are selected by engineers, not accountants, and both the Vulcan and Vekta are made with a blend that is over 80% ultra-high modulus Toray M43J unidirectional fibre. That’s a vastly higher percentage and grade than most manufacturers use in their top models.
Expensive materials alone don’t make a great bike, though. Precise deployment is essential. Carbon fibre is most effective in tension (against pulling forces), so the engineering task is to orient the fibres with the loads to maximise their benefit. Once we understand where the forces are going, the carbon fibre lay-up is intuitive to us. That’s 25 years of experience paying off.
The Vulcan uses massive box-section chainstays to achieve its strength and stiffness at the rear end. Their shape creates a much larger, therefore stronger, junction area with the bottom bracket shell and a greatly improved structural efficiency. Continuous unidirectional fibres run along the full length of the stays, providing immense resistance against flex. When a force tries to flex the chainstay what it is doing is attempting to stretch the fibres to the outside, and because we use continuous ultra-high modulus (high tensile strength) unidirectional fibres the chainstays are massively strong while remaining compliant.
The toptube works in a similar way. The huge junction area with the headtube and downtube creates strength, stability and a brace against which the toptube is cantilevered. Continuous fibres are run the full length of the toptube and around the headtube for support, and the performance of the carbon fibre lay-up is boosted significantly by our trade secret manufacturing method, a hugely effective innovation that’s unique to REAP. The frame’s structure is optimised for our methods and materials, rather than being a conventional twin-triangle design made with carbon fibre tubes in place of metal, and it retains the vibration damping and energy-saving pliancy that’s inherent to both the material and the frame concept.
The Process
Development of the Vulcan began with understanding the loads acting on the bike. We used our contacts in the automotive industry to lend cutting edge Finite Element Analysis (FEA) to set the start points. Digital design in CAD (Computer Aided Design) was carried out in-house and we soon had the first prototype ready for testing.
When we say REAP Bikes are ‘Made in Britain, for a reason’, this is what we mean. We were able to iterate our prototypes rapidly because our entire operation is under one roof. Our small team packs a lot of cycling experience and, even more importantly, has the legs and skills to push bikes hard, and sets very high goals for every aspect of ride quality.
While aero work was carried out first in Computational Fluid Dynamics (CFD) and then a wind tunnel, the ride and handling were refined by road testing, comparing notes, iterating the carbon fibre lay-up, and making another prototype. And repeat until completely satisfied. Our deep understanding of the materials is key to this process; once we decide that we need to make an adjustment, we know exactly what to change – whether it’s fibre type, orientation, size or thickness – to achieve the desired effect. That’s why we call it hand-crafted carbon fibre.
