Ride Height:
Is the height of your car when it is sitting and the suspension is at the optimum setting (location). I can not stress how important setting ride height is on a chassis.
First, you need to pick out the wheels and tires you will be using in the front and in the rear. The diameter of the tire determines the ground clearance, frame angle, engine angle, shock length and correct angle to weld the front suspension.
After establishing the diameter of the tires, find the front spindle location based on the manufacturer's specifications. The car manufacturers all have specifications on this. If you plan to install a front end kit, the manufacturer should also be able to provide you with this important information.
Take the radius of the tire diameter and use these measurements to set the distance from the ground. In the front measure from the center of the spindle and in the rear measure from the center of the axle. You now have "ride height" and everything should be relative to the ground. Of course the ground or suspension jig should be perfectly level as all your measurements will be made from this origin.
No chassis builder can build a chassis that is tuned with optimum suspension without knowing proper "ride height". How did he know your "ride height" without asking you what wheels and tires you are going to use? Maybe he does not want to know so when your car does not handle correctly he can sell you some additional parts. Remember to ask a salesperson these questions when you are buying. Do they sound credible or just out to make a buck?
Now you are ready to set the engine and pinion angle.
Engine and Pinion Angle
Engine and Pinion Angle: There has been numerous "hear says" about what the pinion angle should or should not be. Hopefully this will clear up a lot of misconceptions. One of the biggest misconceptions is the engine angle is it is relative to the frame. Engine, pinion angle and ride height is all relative to the ground.
Most engines are set at 3 degrees, when you look at the side of the intake manifold on a carburetor engine you will notice the manifold has a wedge in it. This wedge is to allow the carburetor to sit level. The main purpose of having an engine angle is to allow for more room in the passenger compartment by having a smaller tunnel. That is why the car manufacturers build front engine cars with the engine at an angle. Drag race cars have a 2 to 3 degree negative engine angle, the engine will tip down in front. Having this negative angle allow for better weight transfer to the rear of the chassis. The 3 degrees you keep hearing about is the recommended angle for carburetor cars with intake manifolds set at 3 degrees. With the engine and pinion set at 3 degrees they do not line up directly and requires you to make them parallel. This misalignment allow the u-joints to rotate on a street car.
There is a misconception regarding the pinion angle on drag cars. I have seen many people advise that the pinion needs to be 2 - 7 degrees down. In relation to what? I disagree with this theory and set-up. The pinion needs to be "parallel" to the engine angle or in a straight line for the least amount of horse power lost.
This following is a quote from Mark William's web site,
"There is a general misunderstanding bout "dropping the pinion down" several degrees. This is a practice that could be applied only to leaf spring cars without any traction control devices where springs can “wrap” and change pinion angle. This practice would not apply to 4-link, ladder bar or torque arm equipped cars. Failure to maintain matched and minimum operating angles increase erratic non-uniform output velocity from the drive shaft to the differential."
You can set up the engine angle at almost any degree just as long as the transmission is perfectly in line with the pinion yoke. However, when the engine and pinion yoke is in direct line, it does not allow for the rotation of the u-joints. This straight alignment is mostly used on drag race cars.
If you would crawl under your car and look at the pinion angle you will notice it is the same angle as the engine/transmission (they are parallel). A good example of having negative engine/pinion angle is when you lower a 1 ton crew cab where you lower the front more than the rear and there is quite a "rake". In this case you have to put a wedge on the back side of the saddles to tip the pinion angle down.
If you are building a chassis you need to first determine ride height. This is the most important thing you need to do. Decide what wheels and tires you want to use before you pick up the welder. If a chassis builder or a salesman does not ask you what tires you are going to use do not buy from him. How can he build your chassis without know the tire diameters? How can he set up the front end and engine angle? What they do is build a chassis based on what they want or think you should have and not what you want.
Now determine the ground clearance you want for your oil pan. Set your motor mounts based on this and the angle to best fits your car. Try to set the transmission directly in line with pinion yoke and if that is not possible set the pinion parallel to the engine angle. Before you weld the brackets on the rear end housing be sure that the rear end was set at "ride height". Ride height is where the rear end would be with the car done.
If you find a chassis builder or salesman that says you have to have the engine angle at 3 degrees and there is no other angle you can set if at, find another chassis. He apparently does not understand the principles of setting correct engine and pinion angle. Also no chassis builder can build you a chassis with optimum suspension without knowing proper "ride height".
Coil Over Shocks
There is a misconception about setting up coil overs. The correct way to determine what length you need to to first determine "ride height". After you have your chassis at "ride height" measure the distance between mounting point of the shock. If you have a stud mound measure from the hole to the eyelet mount on the other end. Ideally you want 66-67% compression and 33-34% rebound.
Example: The measurement between mounts is 12". and you have a shock that travels 3" total. Multiply 33% x 3" = 1" this is the rebound measurement. Add the rebound measurement to your "ride height" measurement of 12", thereby 12" + 1" = 13" extended and 10" compressed.
Coil spring rate is determined by the manufacturers chart. They engineer this based on the material type of material is use in making the springs, coils per 1" and diameter of the coil. Use the chart and recommended spring rate. Just remember the manufacturer does not know what angle the shocks are mounted at, so you will have to factor this into the spring rate. Here is a chart for spring rate corrections:
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|
Front Sping Rate |
Vehicle Wt on Front Wheels |
250
|
Under 1200 lbs
|
350
|
1200 - 1500 lbs
|
450 |
1600 - 2000 lbs |
550 |
2000 - 2400 lbs |
650 |
2400 + lbs |
Rear Spring Rate |
Vehicle Wt on Rear Wheels |
80 |
Under 1000 lbs |
95 |
1000 - 1100 lbs |
110 |
1100 - 1200 lbs |
130
|
1200 - 1300 lbs |
150 |
1300 - 1400 lbs |
170 |
1400 - 1500 lbs |
200 |
1500 - 1700 lbs |
225 |
1700 - 2000 lbs |
250 |
2000 - 2400 lbs |
300 |
2400 + lbs |
Example: Recommend straight mount weight = 300 pounds; Shock is mounted at 30°, therefore 300 ÷ 0.75 = 400 pounds. You need a 400 pound coil spring.
This formula will not work on an IRS set-up. IRS's are based on the motion ratio at the tire. You will need to calculate two distances, 1. distance from the center of pivot point of the lower trailing arm to the lower shock mount (distance D1); 2. distance from the center of pivot point of the lower trailing arm to the end of trailing arm (distance D2). The Force Ratio is (D1 ÷ D2) = Fr
Force Ratio (Fr) = D1/D2
Weight (W) your car to determine the weight on the wheel. Divide the weight of the wheel by FR to determine the fore required at the spring or the spring ratio (Sr)
Spring Ratio (Sr) = W/Fr
If your spring is mounted at an angle you will need to use the same Angle Correction Factor (ACf) as shown in the chart.
Spring Ratio (Sr) divided by Angle Correction Factor (ACf) = Adjusted Spring Force (ASf).
Spring Ratio (Sr)/ACf=ASf
The Adjusted Spring Force (ASf) is not the spring rate. The Adjusted Spring Force (ASf) can now be used to select the proper spring rate for your application. .
Normally shocks are set up Static Compression Ratio (SCr) of 33%; Adjusted Spring Force (ASf) x Static Compression Ratio (SCr) = Static Compression Force (SCf).
Adjusted Spring Force (ASf) x Static Compressison Ratio (SCr) = Static Compressin Force (SCf)
The required spring force can be obtained several different ways. A lighter rate spring with more preload or a stiffer rate spring with less preload will gernerate the same spring force. The softer rate will generate a smoother ride while the stiffer spring will result in a harsher ride. The typical vehical will have a Ride Frequency Ratio (RFr) of 1.0 to 1.5, where the 1.5 being like a truck. You will need to consider these options when you are selecting the proper spring rate fro your application.
Static Compression Force (SCf) x Ride Frequency Ratio (RFr) = Adjusted Spring Force (ASf)
Example: Jag rear suspension in 32 Ford.
D1 = 11", D2 = 17" or D1/D2= .647 Force Ratio (FR);
Rear 1500 lbs with unsprung weight of 100 lbs/wheel or 1200 lbs sprung weight. 1200 (W)/.647 (Fr) = 1855 (Sr);
Shocks mounted at 15 degrees or Angle Correction Factor (ACf) = .93;
1855 (Sr)/.93 (ACf) = 1995 Adjusted Spring Force (ASf) for both wheels or 997.5 (ASf) per wheel;
997.5 (ASf) x 33% (SCr) = 329 (SCf);
329 (SCf) x 80% (RFr) = 263 (ASf) or a 275 pound spring.
When you mount the shocks at an angle this is the hypotenuse (longest length) of a triangle. The advantage of mounting the shock at an angle is there is more vertical movement for your suspension. The forum la for a triangle is a² + b² = c². a = base; b = height or vertical movement of your suspension and c = the travel of the shock. You will need to know 2 of the factors to calculate the vertical movement. To find the vertical movement the formula is b = a x √c
Example: The travel of the shock is 3" the base is 3" so, 3" x √3" (or 1.732") = 5.196" of travel. Yes, you get 5.196" inches of vertical travel with a 3" shock. This is how important the shock angle is. However, you must remember to factor the angle ratio when figuring your spring weight.
QA1 is only one manufacturer that makes adjustable shock bodies that adjust both the compression and rebound "simultaneously". All other shock bodes adjust only the rebound or compression with a single adjustable shock body. What this means is you get a double adjustable shock body for the same price as single. You would have to pay twice as much for this same type of adjustment in another brand. In the fall of 2010 all the new shock bodies now have a 18 point adjustment and will adapt to a remote adjustable controller.
I learned about chassis from two men, Russ Meeks and Jack Slavik. These two forgot more than most people know. Do you remember the 30 rear engine roadster built in 1971 and the Revellution Funny Car? Meeks built the roadster and Slavik built Art Whipple & Ed McCulloch's funny car. Meeks built my 30 roadster in 73 with a Morris rack and pinion mounted on the axle, front 4 link and rear 3 link with a drive line traction bar. Meeks was cutting two spindles and welding them together in the 60's, Super Bell called him and asked him how to make them. They still use Slavik's 2 degree negative engine angle when building funny car chassis. These men were the pioneers of suspension technology.
