Updated: Feb 23
The big difference is in the gas volume and as you can see one has a bulb and one does not. The most common explanation to this has some truth but is not the real reason for the change. The most common answer to this question is less rod force which increase feel in the car, and this is true but not the real reason for the increase in feel. We feel like the decrease in gas pressure due the larger volume of gas help the shock piston process the oil better without bypassing oil around the main piston which increases the grip at the tire making more feel.
First let us understand what we have going on in the gas part, of a shock. There is a gas volume that is determined by the builder (divider height) which is important to the outcome of the shock. On a BSB we set on a 1.000 tall post which sets the gas volume to .600 inch at full in compression. This allows for a min rod force change from full out to full in, while the gas volume changes over the full stroke from 1.000 to .600 inch on a 12 Series 6” shock. What is happening is the divider is moving up in the tube as the shaft is moving into the tube, this is what allows the shock to function with out locking up. As this is going on the gas volume is getting smaller and the rod force is increasing at the same time (common gas law), the other thing that is going on is the gas pressure is increasing and forcing oil to move at a faster rate than the piston is moving.
The pressure increases are determined by 2 things, the amount of gas volume to start and the amount of gas volume change (shaft size). Let us look at the different shafts length and gas volume in a 12 Series BSB shock.
· .472 (12mm) /2 = .236
· .236 x R2 = .0556
· .0556 x 3.1415 = .1749
· A = .1749
· V = .1749 x length of shaft 6” = 1.049 7” = 1.2243
Body size: 12 Series
· 1.500 dia. of body / 2 = .750
· .750 x R2 = .5625
· .5625 x 3.1415 = 1.7671
· A = 1.7671
Divider movement in tube:
· 6” shaft 1.049 / 1.7671 = .593 movement
· 7” shaft 1.224 / 1.7671 = .692 movement
Gas volume in a 13 Series shock (2.272) which is 33% larger than a 12 Series shock when given the same 100 psi of holding pressure will see a decrease in total gain over the 12 Series shock. When this pressure is pushing on the oil column it will increase the flow of oil that travels through the shock piston. The increase in pressure in the air volume (common gas law) will also be less in the 13 Series at full compressed than the 12 Series. We will do a gas pressure test to determine this later. In a 20 Series and a 25 Series the gas pressure increase is 50 psi over a 9" stroke. This means in a 20 Series shock with 100 psi the gas pressure increase from 100 psi to 175 psi in a nine inch stroke. On a 25 Series shock the gas pressure increase is from 100 psi to 130 psi over a 9 inch stroke. Less gas pressure can do 2 different things but one of them is to increase the oils ability to flow through the piston.
Advantage 13 Series Shocks:
· Better oil control that makes more grip.
· Can add Base Valve to shock, makes more grip.
· Can add SRT to both 12 and 13 Series shock, makes more grip.
· Better feel for driver.
Base valve vs, non-base valve:
The main reason we like to us a base valve shock is because it makes the shock work better, it makes the oil flow function like it is intended to do. When this happens, the shock creates grip because the shock is processing oil correctly. For this to happen the base valve must be built correct and this is where we see the most problems with oil flow or return to the main camber. The lack of oil on the return stroke cause the shock to cavitation around the main piston which causes a lack of valving at this point, to a driver, the car just sucks when the track is kind of rough or slick. The upside to a BSB base valve is the improved traction in the rough and in the slick along with feel for the driver, but mostly in the rough. What I like about the BV shock in the rough is m When in the rough this allows you a place that you never been able to run a BV before and a condition where the car lacks grip. This is when the shock needs it most help and most people will not or cannot increase the gas pressure to make it better. So, the BV at this point is ideal to make the maximum amount of traction, and because of how we build it that allows for better tractions in all conditions. Here is what it looks like when it is in the tube.
Rev-a: 1-7-2021, fixed header.
Rev-B: 2-23-2021, changed air volume to a 12 series shock.