Suspension 101

[Super73]

Wow, what an overwhelming topic.. I am no expert and there is always room to learn new things, but I am going to try and delve in to the basics of suspension and give some recommendations as far as reading material. I want to use this thread as a general overview that can lead to more specific questions and in-depth answers.

Chapter 1 - Front Suspension:

Now, the trick to getting your truck to hook is putting the weight on the rear tires. The way to do this is to store as much energy as possible in the front end. To do this, you want to use the longest/lightest spring possible. This makes the front end very easy to lift when you need to transfer the weight off the line.

Most of our trucks are independent fronts based on 1960’s technology. From what I understand 1960-1983 front cross members for the most part are interchangeable. So this is going to cover them.

When figuring out the right spring rate and length you must know axle weights, unsprung weight and leverage ratio.

Unsprung weight is the weight of the:
Rotor
Caliper
Spindle
Rim/tire
½ of the shock
½ the spring
½ of the upper and lower control arms.


Here are some weights I have figured out:
Rotor: 26lbs
Caliper: 9lbs
Spindle: 27lbs
Lower control arm: 22lbs
Upper control arm: 10lbs
Shock 10lbs
Spring 15lbs
Rim/tire 30lbs (This is a guess)

So based on this, unsprung weight would be 121.5lbs * 2 = 243lbs. So if your front axle weight is (2260lbs), you will need to figure out springs on 2,260lbs – 243lbs = 2,017lbs and leverage ratio.


I have also figured out leverage ratio on our trucks to be 1.91-1. This is the leverage put to the spring due to control arm length. If the center of the spring was at the ball joint, the ratio would be 1-1 since the spring would compress 1” if the ball joint moved 1”. Leverage ratio 2-1 would mean the ball joint would move 2” for every 1” of spring. With a leverage ratio of 2-1, the spring is caring 2 times the sprung weight it would if the LR was 1-1. Our LCA is 17.25” from the shaft to the ball joint and our spring center is at 9” (9 / 17.25 = 1.91xx)..

So 2,017lbs of sprung weight with a leverage ratio of 1.91 would be equal to 3,866lbs on the springs. This number needs to be divided by 2 since there are 2 springs that support the front ends weight. Your front spring needs to be able to support 1,933lbs each at you desired ride height. If your truck sits where you want, take a tape measure and climb underneath the front. You should be able to slide the tape up the hole in the lower control arm where the spring sits. Put the tape up as far as it will go and write down the measurement to the LCA where the spring is. I like where my truck sits height wise and mine measures 9”. This measurement I am going to refer to as compressed ride height. If you desire a 9” compressed ride height, you need a spring that supports 1,933lbs each side at 9” compressed height.

Our front ends use a 5” spring. Below are some Moroso springs and the details for them:
Spring - Length - Rate - Coil bind (compressed)
47180 -- 19.500 -- 212 -- 7"
47140 -- 17.375 -- 240 -- 7"
47195 -- 18.000 -- 250 -- 6.875"
47170 -- 18.625 -- 270 -- 7.25"

Spring rate is how much weight it takes to compress a spring 1”. With that being said spring 47140 with 1,933lbs on it would compress 8.05” making the compressed ride height 9.32” (17.375” – 8.05” = 9.32”). That’s .32” more than your desired compressed ride height and with a leverage ratio of 1.91-1 it equates to .62” higher than what you were looking for. Keep in mind though, springs will settle some and eventually lower the stance a bit on their own.

Here is a link to an excel spreadsheet I put together as a tool for myself. It will do all auto calculating, so only enter info in the colored boxes.
Yellow boxes are for you to change.
Red boxes should remain unchanged unless your suspension is different or you have weighed your own parts and they are different than what I have in there.
Green boxes will reflect the data you need based on the info you put in to the yellow boxes.

http://73-ls1.com/67trucksetlist/Dat...g_sheet_v2.xls






Chapter 2 – Rear Suspension
Focused on the 60-72 rear suspensions but theory’s should apply to all ladder bars in general.
Squat/Anti Squat
Springs for the rear
Antiroll bar
Pinion angle, what happens under squat and antisquat




How fast can I go on the stock rear suspension?

I have a 60-72 Chevy pickup and I want it to hook. Should take the trailing arms out and run a ladder bar?

These seem to be common questions around here. What does our truck have from the factory? To tell you the truth, we have a great rear suspension platform to build from. Our trucks are blessed essentially a very long ladder bar. 52” to be exact. We are going to go over what or how we can make this set up work for drag racing.

What causes squat and antisquat? How does it help/hurt in situations?
Simply put squat is when the rear axle goes up in to the body of the vehicle causing the rear to “lower” under acceleration. Antisquat is just the opposite. The rear axle actually is forced in to the ground as the chassis separates from the rear axle. Squatting the rear suspension allows energy to be absorbed in to the chassis, sometimes a good thing. Antisquat puts that energy in to the tires forcing them in to the ground and sometimes adding traction.

How to figure antisquat out.

You can figure out antisquat fairly easily. It just takes a little leg work. You need to know where you center of gravity is both front to back and height and where your instant center of gravity is. To determine center of gravity front to back you need to get axle weights. If you have 60% in the front and 40% in the back, then take your wheel base and divide it by that amount. 100” wheel base would put you at 40” from the front axle. The height is a little more difficult.

For height you still need to put your vehicle on scales. You will end up putting your front axle on the scales. You must fix your front suspension from moving as we are going to jack up the rear axle at least 10 inches. If the front isn’t fixed, it is going to compress the front suspension and you won’t get a true reading. You will notice that your front axle weight will go up as you raise the back. Make sure to write down how many inches you raised the vehicle and what the front weight went up to. To make it even more accurate, take 3 readings (10” 12” and 14”)

Here is a link that you can put your data in to to give you the info you need: http://www.longacreracing.com/articles/art.asp?ARTID=22

Now, to know where your instant center or gravity is. From a side view, you would theoretically draw a line from your rear tire contact patch through the mounting hole for your truck arm. If your instant center line goes straight through your center of gravity, you will have 100% antisquat. Don’t let the percentage fool you. 100% antisquat means you will not squat the rear nor have any antisquat. But anything below 100% will squat and anything above 100% will induce antisquat.

Hopefully this crude drawing helps you understand what I am talking about.


Springs for the rear are very similar for springs for the front. Again, a light spring rate and long spring will give you more flexibility. The leverage ratio works the same here. You can use the previous spreadsheet for the rear too. If you moved your rear spring, you will have to adjust for that, also unsprung weight most likely will be different.

One thing that needs to be considered in the rear is shock location. Is your shock better off in front or behind the axle? What’s the difference? Well, if the shock is right at the axle, there is no leverage ratio applied. For every 1” of travel, the shock will move 1”. If the shock is behind the axle, for every 1” the axle moves, the rear shock will extend more than that. If it’s in front of the axle it will move less than that 1”. With a 52” bar, if your shock is at 26” (26/52=.5) your shock will move .5” for every 1” your rear moves. If it’s at 65” then (65/52=1.25) so your shock would extend/compress 1.25” for every 1” of axle movement. You will ultimately have more control if the shock is behind the axle. With an adjustable shock, every change to the valving will make a bigger impact due to the stroke of the shock being more. For an overly stiff and short shock, in front of the axle might be the way to go.

Antiroll bars in the rear help axle rotation and aid in keeping your vehicle level during launch. Your rear axle wants to rotate the same direction as your drive shaft under acceleration (Passenger side wants to go up in to the body while the driver wants to go down in to the ground). When this happens, the rear axle is physically rotating under you vehicle making your vehicle “twist”. An Anti roll bar will help fix that. When the axle rotates, it is also causing your lower control arm lengths to change. When this happens, your axle is no longer straight. It will cause the driver side wheel base to differ from the passenger side slightly causing the axle to no longer be aligned straight and the result is you go sideways.

Black voodoo, “Pinion Angle”. Let me start by providing a link to CarCrafts site that I feel does a pretty good job explaining setting pinion angle: http://www.carcraft.com/howto/91758/index.html

Now, with our trucks, I have figured for every 1” of axle change (up or down) the pinion angle will change aprox 1*. With our set up, you won’t see much pinion wrap due to it being a ladder bar by design. There might be a little do to the material used in the front mounting holes in the lower control arm, but the rear is fixed to these arms. The arms would have to bend which I guess is possible if you are making a ton of power, but at that point, I think you have other issues.

When the rear squats, the pinion will want to point down. When the rear has antisquat, the pinion will start to point up. This is something to consider when setting up your rears pinion angle. To get best results, I feel under load you should have 0 pinion angle. This means you need to know how far you are squatting or rising. This is where video is key. If you squat 2” set it up to compensate for that amount.





To come..

Chapter 3 – Shocks
Why you want adjustable shocks


Chapter 4 - Suspension Travel
How to increase front travel
How to make the back and front work together based on changing track conditions
Suspension limiter, why they might be used and their downfall


Chapter 5 – Trouble Shooting and tuning suggestions
It doesn’t hook right out of the hole, why?
Why does it hook for 3 feet and then blow the tires off?
It leaves sideways..


Chapter 6 - How/Why Video helps
Where to set the camera up
Helpfull hints to aid getting the most data for review during recording
What to look for