I’ve been working on race cars for almost thirty years and was sitting in the stands since I was in diapers.
This past season I didn’t go to many races after June. I needed the time away to rejuvenate. I was still involved every week because it is my full time job, but weekends were for exploring things I never got to enjoy.
As many know, if you’re involved in racing, you will miss weddings, parties, festivals, and other cool goings on because of our devotion to the sport. If you dig in too hard, at some point in time you will reach burnout and need a break.
The good thing about burnout is when you’re back, things are usually better than ever. Ideas come together better. Things flow better. Everything just seems to gel.
I started thinking this week about shocks.
I remember the first time I got my hands on some shock dyno graphs for a car I was working on. I felt somehow I was holding a pot of gold; if only I could understand how to understand what I was holding and how I could turn it into a faster race car.
I know there are many others who look at these graphs and, even if they can understand them, can’t quite figure out if what they are looking at is right or wrong.
To tell you the truth it is all speculation, a little guess work, and maybe you’ll stumble on something that just plain works without understanding how.
I’m going to give you some of my opinions on what I look for when looking at shocks for each corner of the car.
I can’t give out actual numbers, but I’ll give you some general guidelines. You’ll need to figure out your own numbers.
Let’s start with some general rules of thumb, if something like this even exists.
There is usually more rebound than compression. If you look at a linear piston, they are directional. The port for the compression side is usually larger than the port for the rebound side.
I’ve even heard a rule of thumb for the right rear having two times the amount of rebound than the compression.
Think about the compression of the suspension and the spring. The spring needs to be soft enough to compress the wheel into the chassis without upsetting the body.
After the wheel reaches the top of the bump it needs to return to the ground without the tire bouncing at the bottom of the bump. The spring has stored energy and wants to push a relatively light amount of unsprung weight (the wheel and part of the suspension) back to the ground.
If the rebound is too soft we will get what we call overshoot and the wheel will bounce on the tire side wall.
We are encroaching into the area of critical dampening and the true purpose of shocks, but we will try to keep it simple.
Critical dampening is when there is no bounce.
Now, of course, rules of thumb don’t always hold up, especially when you start tuning at different speeds.
I’ve seen and put shocks on cars where the rebound is close to the same as compression, when measured by a standard crank dyno up to 10 in/second.
But, rebound is usually always more than compression. Without being able to accelerate shocks up to racing speeds we really have to guess a little as to what will work at higher speeds.
I think the right rear corner is the only one that speed becomes a problem. I’ve never seen any data on shock speeds myself, but I’ve heard other people say they have seen them up to 40 in/sec.
This is mainly because of the motion ratio of the shock mount on a right rear birdcage. The shock will index in as the axle is compressed. This makes the shock shaft travel at a higher speed than the wheel.
If you look at your low speed numbers as well as the directional response area on the graph, this will give you a pretty good idea on how much bleed is in the shock.
The bleed is the amount of area that fluid can pass through from side to side without obstruction.
This is usually accomplished by putting slotted shims over the ports or by drilling bleed holes from one side of the piston to the other.
Here is what bleed does to a shock.
The more bleed, the less control. With too much bleed you can either get wheel hop or in the worst cases the car will bounce uncontrollably.
Bleed is a double edge sword.
It also adds traction because it allows the shock to change direction at the top of the bump easily.
Too little bleed and traction can suffer. Too much bleed and the car will bounce like a basket ball.
There are other things you can do.
This is where the art of shock valving comes in.
You can cut bleed to the point where you get really good response then install a soft cover plate shim to at least give the shock some control at lower speeds.
By the way, the cover plate shim is the first full shim against the piston. This act as a seal against the piston face.
I think now we are getting a bit too technical for the scope of this article. So, I’m going to ratchet it back a little bit.
One thing I always recommend to people getting shocks dynode is to get force vs. displacement graphs. Most shock rebuilders use a force vs. velocity.
That will give you some numbers, but it really doesn’t tell a complete enough story about the shock.
A shock histogram basically shows you how many much time a shock spends at a particular velocity.
For instance, if a car does a lap in 14 seconds, the shock will bounce around at different velocities throughout that lap. A histogram shows how much time a shock spends at each velocity for a given lap.
I’ve never seen a histogram where zero inches per second isn’t the largest number. Meaning a shock shaft is idle more than any other speed. As a matter of fact, all the histograms I’ve seen are shaped like a pyramid. Rebound on one side and compression on the other.
This also means that a shock spends more time at the lower speeds than the higher speeds. So, I equate that to meaning that the lower shock speed should be concentrated on a little more than high speeds.
This is where the bleed and response come in to play. This is also why I prefer force vs. displacement graphs. The response of the shock is easily seen and easily compared to other shocks.
I would also like to endorse Accuforce Shock Dynos. I’m not getting paid for it, but Keith has built one of the easiest to use, racer friendly dynes on the market. I’ve used one at work for years to test shocks. It’s a great and durable piece of equipment. And, it’s by far the easiest dyno to measure that I’ve seen. (Except for an EMA, but then we’re talking $50,000)
Although, If Keith wanted to throw one my way for my own personal use, I wouldn’t argue with him.
I think that about wraps it up for me today.