Why is a base valve shock better or is it? As you know, I have always been for it in ours shocks, and when I say ours, I mean BSB. As I have always said, not all base valves are the same, and they are not. We are not going to cover that, just want to make that statement to clear that up for 1st time readers.
The Base Valve, what is this thing we talk so much about and what is it doing. Well, here is a picture of it in a tube so let’s explain it, and its function and why it is so good to the car.
This is the base valve; it is a piston that is fixed in the tube (solid) as not to move up and down. It has shims on each side of it and they regulate pressure that is created in the tube by the main piston. It also converts the high pressure made by the piston into low pressure allowing the divider piston to handle it with less gas pressure.
Now to understand the base valve better we must understand what is going on in the shock and what we want to happen in the shock. We will look at these items to understand what is happening.
· Activity in the compression stroke:
· Divider piston function without a base valve:
· Pressure in the oil column:
· How it effects the car:
Let us start with Pressure in the oil column and why it is there. The pressure is made from the main piston as it controls the force that is being applied to it. This pressure is on both the compression and rebound side of the piston. The rebound side pressure is absorbed by the seal head (seal head is the object that the shaft comes out of), this allows all of it to be directed back to the piston and forced. Some oil may bypass the piston, but the rebound stroke is strong because of the seal head. The pressure on the compression stroke is being held in place with the divider piston and gas pressure, as you increase the gas the divider will work better in holding the oil column in place. The divider has a tuff job to do and fails at doing it in many ways, therefore we have shock problems that are hard to determine the origin of the problem. The divider is holding the oil column in place as it is slowly moving in the opposite direction of the pressure being applied to it. This is often when the failure happens, as the car hits a bump on the track it overloads the divider with pressure that the piston cannot process thru the shim stack and pushes the oil column up the tube.
More info on base valves.
Added note: All twin tube shocks have a base valve in them, this is what allows them to work under pressure, it is also what makes them fail. Although they work on the street they are not the best for the race track. Maybe in a different world the twin tube shock would be good to race on but until than we will not.
Divider holding oil column in place during compression stroke. The divider moves up and down in the tube because of displacement of the shaft. As the shaft goes inside the tube it displace oil volume and this makes the divider move up and down. The shaft diameter and the bore of the shock will determine the amount of movement in the divider piston.
Figure 2 Stroke line with activity.
Let’s look at a stroke in compression without a big bump on the race track and say it is moving 3 inches in travel. This is not one solid move in that direction but a bunch of little moves back and forward as it moves in the compression stroke. This is what we call activity in the stroke and the divider must deal with it flawlessly to make grip at the tire. This is a hard task to mange and where most failure start and grows from. With out this task being performed the shock goes into cavitation and the load at the tire is wiped away at this point. This has been seen here lately in a soft spring setup, as the cars have more grip in them because the spring adds activity to the shock because of its lighter rate.
Okay so we have covered Pressure, divider function, and activity, so now we need to look at how the base valve when in its proper stage will increase the grip at the tire. The Base valve in its fixed position will hold the oil column in place, much like the seal head, this will force the oil to be process thru the main piston increasing grip at the tire. It will decrease the need for the divider to super sensitive to small moves and make the main piston active in its function. The base valve also performs another function as it decrease the internal pressure made by the main piston turning it into low pressure. This allows the gas behind the divider to be lower in pressure because it is now not controlling as high of pressure as it was. Most people consider this to be the best function of a base valve as lower gas help increase feel for the driver, but the improved base function is responsible for the biggest increase in feel and grip at the tire.
Remember this, that all base valves are not created equally so the function of them will also differ. I feel that our base valve performs very well in all conditions (smooth to rough) allowing the car to perform. This is our main goal, to increase grip at the tire and we have seen that the base valve is one of the main items in doing this in our shock. We also have other items we use to increase grip levels but our goal here is to understand better the function of a base valve and how this tool works in conjunction with our car.
Gofast and God Bless: Jay Neal