rear shock setting

[JOE B]

will running a stiffer rebound setting on the rear shocks let the car rotate more up on the rear tire. i.e slow down body seperation.

[JOE B]

fore got it a leaf sring car >>>>>>

[Bill Harris]

Here is a long winded answer to a seemingly simple question.

The answer is yes, and here is why.

When you launch the rear axle housing tries to rotate such that the pinion yoke tries to move toward the floor of the car. The leaf springs (and traction bars) transfer that force to the front spring mount which is attached to the chassis/unibody. At the front spring mount the rotational force is redirected such that is is essentially vertical, which tries to lift the chassis with respect to the axle housing, what is usually called "separation".

So why control the separation? The lifting of the chassis (and often the entire front end of the car in a wheelstand) takes energy. Most of the energy is stored as potential energy in that the chassis lifts in relationship to the rear axle, as well lifting the front end as the chassis rotates around the axle. The rest is stored as elastic deformation of the chassis and rear springs (bending). Since the motion takes place over a period of time, power is used to create the motion (power is energy applied over time). The only source of energy is the torque that is applied to the rear axle through the drivetrain.

Recall that: "For every action there is an equal and opposite reaction", Newton's third law of motion. In order for the power (torque applied over time) to be applied to the tires and move the car forward, the axle housing has to push against something. If that something is extremely rigid, then all of the torque is applied to the tires as it is supplied by the drivetrain (think hard-tail dragster). However, if the axle housing is pushing against something that is compliant, or soft, then some of the drivetrain energy is used to move the housing against it's resistance and less torque is applied to the tires. The nature of the resistance that is seen by the axle housing has a direct affect on the amount and rate that torque is applied to the tires. In other words, by controlling the "softness" of what the housing pushes against, you control the power that is applied to the tires for a period of time (until all the softness or compliance is taken up).

Without shock absorbers, the only thing that resists the separation would be the springs themselves. When you lift the chassis away from the axle you are UN-loading the springs, and they don't provide much resistance to that force. If you add the shock absorbers between the axle and chassis, they provide additional resistance to the lifting force depending on how hard it is to extend them. The spring itself is very important of course, and may times multileaf springs are clamped together to increase their stiffness which absorbs power through frictional losses.

Since it takes time and energy to lift the chassis and front end, that power is not available to the tires. If you had perfect traction then all of that power would be wasted and the car will not be as quick as it could be. However, since traction is usually a major limitation on a car equipped with leaf springs (like a stocker with 9" tires) the control of how much power is applied to the tires is very important. Rear shock absorbers that have adjustable extension rates are a tuning method to control the resistance to the axle housing rotation. A shock that is easily extended will present a softer resistance to the axle housing than a shock that is hard to extend. The body will separate more with the soft setting and more energy will be stored in the chassis than a stiff extension setting. The more energy stored, and the longer it takes to store it, the less power is applied to the tires. This is often referred to as the amount of "hit" on the tires. A very rigid rear suspension with a stiff shock extension will result in a harder "hit", since more of the torque is getting to the tires and less is being used to power the chassis movement.

The "separation" is a visual indication of how much energy is being stored in the chassis. The amount of front end lift is another indication. If you watch a pro-stock car you will see very little movement in either the front end or the rear end. The sophisticated suspensions and chassis on these cars allow nearly all the available power to be applied to the tires to move the car forward. Power application is controlled more by clutch slippage than by trying to store excess power in the chassis. Stockers with automatic trans, lots of torque and limited tire size can maximize performance by redirecting excess power to the chassis according to how good the track is hooking.

[JOE B]

thanks very much bill . now i have a better understanding of what is going on and how to work with it . again thanks. joe b

[Todd Geisler]

Originally Posted by Bill Harris

A shock that is easily extended will present a softer resistance to the axle housing than a shock that is hard to extend. The body will separate more with the soft setting and more energy will be stored in the chassis than a stiff extension setting. The more energy stored, and the longer it takes to store it, the less power is applied to the tires. This is often referred to as the amount of "hit" on the tires. A very rigid rear suspension with a stiff shock extension will result in a harder "hit", since more of the torque is getting to the tires and less is being used to power the chassis movement.

I mean no disrespect, but that statement contradicts what I've understood about suspension function/reaction. What I feel is contradictory is the fact that a stiff rebound setting is a harder hit to the tire and a softer rebound setting is a softer hit to the tire. Now granted, I run a GM tringulated 4 link style suspension (G-body), but I think many of the same theories still apply.

Here's my thinking on the subject.

The first reaction of the suspension at WOT is the rear suspension separates to some degree. It's this separation that pushes the tire away from the body and down to the track. The rear shock compression and rebound settings control the velocity of this movement to control the "hit" to the tire.

Now it's my understanding that a stiff rebound setting creates less hit to the tire and is used most often on days when the track is real good and less "bite" is needed out of the chassis to maintain traction. Less "hit" to the tire means your using more energy to go forward and less driving the tire into the ground. My own car I've got a real good baseline shock setting for real good track days, but the car won't go down the track when it's hot/greasy out as the shock settings are pretty firm.

On days when it's really hot out, 130-140 degree track temps, and just goey/greasy, you loosen up the rebound softer to allow the chassis separation to hit the tire harder and plant the tire harder. This may give up some ET as your putting more energy into planting the tire and less into moving the car forward.

I mean no disrespect, just wanted to hear more on differing opinions on this subject.

I run a DOT D/R tire which is similar to a D/R slick, but is shorter at 27 3/4" tall and thus a slightly stiffer sidewall due to less height of the sidewall. As I said, the car hooks very well on good track days, but had problems in the heat. I wound up buying a second set of rims and put a pair of Hoosier 10.5 x 28 stiff sidewall slicks (CO7 compound) on to use till I am able to dial in a shock tune for the hot/greasy track days for the DOT D/R tires.

[Myron Piatek]

What the heck! I'm going to take a shot at this.
It's been shown to me that a stiffer rebound will hit the tire harder. At least it helped 60's for me!

Imagine you are doing a bench press. As you push up, the bar lifts off of the supports and pushes you down, to an extent. (Soft rebound that "gives".) The energy is disapated up and down.

Now imagine doing a bench press with the bar secured to the supports. You push up, trying to lift it, but it doesn't "give". (Stiffer rebound.) The harder you push, the more energy is directed down, pushing you (rear end & tires) down harder into the bench (track).

But everything from the engine, to converter, to gears to shocks and springs has to be "tuned" to work together for optimum results.

Maybe leaf springs and coil spring suspensions react a little differently?

I hope that's a good analogy. It's been a rough week already!

[Todd Geisler]

Ok, for the sake of arguement.

On a really good track, where do you set the rear shock rebound? soft or firm?

On a poor/marginal/hot/greasy track, where do you set the rear shock rebound? soft or firm?

[Bill Harris]

Hey, no disrepect taken! I'm no suspension expert. I just decribed the operation of a leaf spring setup as I understand it from experience and a little physics. I'm not sure how the shock settings affect a four-link style suspension. A four link is a much more complicated (and sophisticated) suspension than a leaf spring. The fact that there are upper and lower control arms that are transferring the energy to the chassis in a push/pull manner is a completely different process than a leaf spring. It may very well be that the shock setting affects the launch differently with that geometry than the leaf spring.

I like Myron's analogy and his experience is the same as mine with leaf springs and traction bars. I always soften my shock settings on loose tracks and tighten them on tracks with good bite and the car responds accordingly. My analysis seems to explain the action from an simplistic energy perspective. If the car is doing anything other than moving forward, then energy has to be going somewhere other than to the tires. The greater the motion in a direction other than forward, and the longer that motion takes place, the less energy is being applied to the tire/track interface.

I'll have to pay attention to the four-link style suspended cars on launch to educate myself. Do they have a tendency is to squat on launch?

[Todd Geisler]

Originally Posted by Bill Harris

Hey, no disrepect taken! I'm no suspension expert. I just decribed the operation of a leaf spring setup as I understand it from experience and a little physics. I'm not sure how the shock settings affect a four-link style suspension. A four link is a much more complicated (and sophisticated) suspension than a leaf spring. The fact that there are upper and lower control arms that are transferring the energy to the chassis in a push/pull manner is a completely different process than a leaf spring. It may very well be that the shock setting affects the launch differently with that geometry than the leaf spring.

I like Myron's analogy and his experience is the same as mine with leaf springs and traction bars. I always soften my shock settings on loose tracks and tighten them on tracks with good bite and the car responds accordingly. My analysis seems to explain the action from an simplistic energy perspective. If the car is doing anything other than moving forward, then energy has to be going somewhere other than to the tires. The greater the motion in a direction other than forward, and the longer that motion takes place, the less energy is being applied to the tire/track interface.

I'll have to pay attention to the four-link style suspended cars on launch to educate myself. Do they have a tendency is to squat on launch?

With the stock style GM triangulated 4 link suspension, it depends on how the suspension is setup to whether it separates or squats. In stock form the angle of the bars have negative anti-squat which means they squat in the rear when power is applied and this essentially pulls the tire up in the chassis reducing loading on the tire. My car has the upper bar angles altered to change the instant center length & height which also alters the anti-squat values to around 115% which creates a moderate separation, but not near as violent as say the old style Southside Machine bars were (around 175% A/S as I remember).

Apparently we cannot compare hit to the tire between leaf and coil spring style suspensions. I understand the coil spring stuff fairly well, but never had the opportunity to work with tuning a leaf spring car.

[JOE B]

i will give a little input on to what my trouble has been . both foot braking and trans braking launch it seams that the car goes allmost straight up with a lot of body separation. its does have full frontend travel .

a link to some launch photos.
http://www.gallery.smithspeed.com/th...s.php?album=22

its the orange pinto .