When we started developing the Evolve, one thing was clear: It needs to be light to be relevant. So we started to look into how we could cut the weight without compromising performance or strength.
ARC8's very first product was the Essential, a trail bike frame. It was never intended to be a lightweight product, but ended up being competitively light for XC use. Despite that, it was mainly used with 140 mm forks, and we saw a very low (knock on wood) warranty rate. It showed very good stiffness in independent testing, and it was overall a very well received bike. Therefore we knew we have a design and manufacturing partner with the potential to go lighter. So everything we did we benchmarked against this frame.
To make a frame lighter, you have three possibilities: Reduce the surface area, use thinner wall thickness, or a material of lower density. We have yet to find a material that is suitable for frame production and has a lower density than carbon fiber composites, and we do not want paper thin walls on a mountainbike. So our first focus was to create a design that has a reduced surface area compared to our benchmark.
We came up with a design that uses 11% less surface area on the front triangle, and 22 % less on the swingarm. Based on the Essential, we expected a weight reduction of 253 g in carbon material and paint alone.
The easiest way to reduce weight is by eliminating parts altogether. The ten bearings used in the Essential accumulate 134 g of weight. We reduced that to just two bearings or 30 g. And yes, we did not go down the path of using underdimensioned bearings, it is still nice and strong 6902 (15 mm ID, 28 mm OD) type bearings. The slider that replaces the link will get a blog for itself and has more advantages than just weight savings, but it is lighter than a linkage too. Overall, we could reduce the weight of all hardware by 118 g.
The Essential is a light frame, but already visually you can see that it uses siginificantly bigger cross sections, and a lot more bearings.
When producing a carbon frame, there are two basic ways of forming used: Internal pressure coming from a bladder, or external pressure coming from the mold. Usually, both technologies are used in a frame, internal pressure for areas that can be accessed by the bladder, and external pressure for areas that need to be solid, cannot be accessed by a bladder or need defined inner and outer surfaces (e.g. the bearing seats for the headset). Generally speaking, internal pressure creates lighter products with better performance and less chance of manufacturing errors. Areas formed by external pressure usually accumulate more material than really needed, just for process reasons, so we cut down on them. There are two main areas where we did that: The main pivot and the brake mount.
The main pivot area of the front triangle is entirely formed by internal pressure. After molding, the window through which the swingarm attaches is cut out, and the mounting surfaces CNC machined. This allowed us to get rid of a hefty chunk of material in this area, without loosing anything.
The postmount standard that we use these days was never intended to be used on frames, it was originally designed for forks. Instead there was a postmount standard for the rear (ironically not much different to what we know now as flatmount), but the industry decided that IS was a better standard than postmount, only to eliminate IS a few years later. So we ended up with a standard developed for forks on the rear of our frames. And it shows, because it gives us very limited space for a threaded insert and a mounting point that is in no man's land, usually requiring molding by external pressure. Thankfully, this changed with flatmount, which is in a reasonable place, can be formed by internal pressure, does not require inserts and therefore is lighter.
Despite the weight reductions, we succesfully retained the stiffness of the frame. It took a few tricks to do so. We took full advantage of the fact that we could design this frame for single chainring cranks with a 55 mm chainline, which allowed us to make the main pivot 60 % wider than on the Essential. This resulted in a massive boost in stiffness.
One of the advantages of the slider is that is stiffer than a linkage system.
We were careful in selecting how an where we change cross sections to reduce the surface area. The biggest reduction in surface area came from the seat stays, which see very little bending forces. But the bigger learning from developing the Evolve was, the popular opinion of bigger tubes equal higher stiffness is not universally true. Linear statics tell you that, but with lightweight frames we are getting beyond the limits of this. We have to take effects like buckling and change of shape under load into consideration, which means linear behaviour does not apply. So against general wisdom, a smaller cross section with bigger wall thickness can result in better stiffness and strength.
We reduced the weight of an already light frame by 35% without sacrificing strength or stiffness. This was possible due to a combination of measures described above. in no way this would have been possible with just one of them.