It holds! And will it last? Or saddle stress tests
After a few months of testing under cycling butts and positive feedback from riders, we moved with the saddle to the test laboratory in Liberec VÚTS (specifically to the Department of Calculations and Modeling). Mechanical tests of the saddle were on the agenda.
While in KOA we verified, whether we could correctly model and manufacture the upper part of the saddle, in VÚTS we focused on the carbon shell and rails, respectively on the connection of these two parts of the saddle. To make everything as it should, mechanical tests were carried out according to the ISO standard. It determines 4 load conditions that the saddle must withstand.
From the left: Martin, Václav and Jirka analyze the course of the mechanical tests.
Joints are a critical point of structures and during development, we first had to figure out which joint to choose for rails and shell. Normally, the rails are attached to the shell with screws or glue, or it is possible to laminate the rails directly to the shell. During the development of the saddle, we tried and tested all these variants. In the end, the glue wins in the speed of assembly, and it is a cleaner and lighter solution.
Carbon shell with rails on the scale. When we say that our saddle is ultralight, we mean it.
When we knew how to connect the parts, it was necessary to figure out what. The connection between the shell and the rails is quite complicated, there is tension and peeling. In addition, both parts have a specific shape. So we had to find a glue that is strong enough and flexible. We selected three types of glue with different ratios of strength and elasticity, with which we glued the saddles for mechanical tests.
Saddle fixed in the jerk under load at the back of the shell.
It is time for mechanical tests. We attached the saddle to the seatpost, just like on a bike, and we fixed it in a jerk (a device for measuring mechanical properties). Mechanical tests were carried out by vertical loading upwards in the front and rear parts of the shell (in the diagram below). According to the standard, the saddle loaded in this way must withstand a force of 400 Newtons, which corresponds to a weight of 40 kg. This type of loading does not simulate cycling, but rather handling the saddle. For example, when you adjust the saddle or if you lift the bike by the saddle.
Drawing of the seat attachment and vertical load in the front and rear.
On the saddle we have glued joints two, front and rear. More critical is the front, since the glued area here is much smaller. This means that the joint is subjected to more stress under the same load.
During testing, we achieved strength values of 260 N, 370 N and 650 N for individual adhesives. From the measured values it is clear at first glance that the first two adhesives do not form an adequately strong bond and therefore do not have sufficient properties for bonding the saddle.
Martin and Václav analyze the damage on the carbon shell and rails of the saddle after the load in the jerk.
The third glue, on the other hand, reached 160% of the required value (400 N), so it is perfectly suited to the glued joint. In addition, the break did not occur directly in the joint, but the carbon fibers were torn from the supporting structure. This means that even a stronger adhesive would no longer be able to increase the strength of the bonded joint. In the next round of mechanical testing, we will have a cyclic stress test simulating the load on the saddle while riding a bicycle. But we'll talk about that sometime next time.
So it sticks and endures!
From the left: Martin, Jirka and Václav take out the third sample with a smile, which successfully passed the mechanical tests.