Lets start by talking about how armor should perform in case of an impact:
• The time taken to reach peak force should be delayed as much as possible so the body has time to accommodate
• The force should be dissipated in a linear manner – not too fast
• The maximum residual force should be as low as possible
Let’s start by taking a look at hard shell protectors:
The force gradient of this hard shell protector (in this case from a major Italian brand) shows clearly the flat rising at the beginning. The reason is the comfort liner under the hard shell. At 4.0 milliseconds (ms) the soft foam is completely compressed without any noteworthy energy-reduction effect. At 4.0 ms the hard shell starts with damping – the effect is a buckle in the force gradient. In practice this effect will lead to broken bones. The maximum residual force of 34 kN in fact stays under the standard (EN 1621-1) limit of 35 kN, but in reality is way too high. Another unfavorable aspect of the force gradient is the extreme steep gradient of the curve. The occurred energy will be reduced within a very short time –bad for the human body! A leading criterion is the gradient of the curve in kN per millisecond. In this case the gradient in the important area (between 4.0 and 4.5 ms on the timeline) lies at about 50 kN/ms.
Lets also take a look at rubber-based protectors:
This force gradient is similar to the one of the hard shell model. In addition the protector of rubber material (from an established Scandinavian brand) with kind of lattice structure possesses a very high residual force – and reduces the energy in a too short time for the human body. The gradient between 3.5 and 4.3 ms represents over 30 kN/ms! The distinctive buckle is not as extreme as the one of the hard shell but however is still not optimal– and in practice the bones under this protector will concede as well.
Now lets take a look at Sas-Tec:
The protector from SAS-TEC shows an ideal force gradient. The residual force of only 11.7 kN is very low and the gradient is excellent as well. From the beginning at 2.0 ms up to the force maximum at 5.18 ms the curve shows an almost linear gradient with only 3.7 kN/ms! This is how you prevent the body from damage by perfect damping.
What’s the difference?
The comparison between a hard shell protector and the SAS-TEC protector allows a clear explanation of the other differences between these types. It’s not only the difference in maximum force reduction and gradients of the curves. The third aspect that plays an important role is the time that passes until the peak of the force. The longer this time – the better for the body. If the energy is reduced in a very short time (as with a hard shell protector) the exposure for the body is very high. But if the energy is reduced over a long span of time (as with the SAS-TEC model) then this exposure is significantly reduced.
What about the Sas-Tec Back Protectors:
According to the standard 1621-2 the peak of the residual forces of back-protectors may be 18 kN (level 1) respectively 9 kN (level 2). The SAS-TEC back-protector SC-1/12 (used in Teiz products) passes the higher Level 2 without any problems. With the maximum residual force of 7.1 kN (shown in the chart) the SC-1/12 remains clearly under the limit of level 2. More positive aspects besides the low residual forces are the almost linear force gradient with sensationally 1.85 kN/ms and the very long span of time until the force peak. From the first contact until the highest value the body has 3.84 seconds to accommodate.
One of the most comprehensive tests is the comparison done by Motorrad magazine. In the test, Sas-Tec based products ranked 2nd and 3rd. The APS (Deflexion) based Rukka armor ranked 6th where as d3o ranked 10th (one up from last). According to the article d30 armor actually exceeded the 35 kN limits during their testing.
The whole article is available online but is German. A scanned page (with the best bits) is provided below. The article was published in 6/11 so is fairly recent.