Best Type of Rear Suspension ???

[qmaster]

I am trying to learn more about the different types of rear suspensions available for a Cobra roadster.

It seems that the most expensive but suspension that provides the softest ride is an independent suspension. But there is also a straight axle and a four-link suspension too, right?

What are the differences and what are the pros and cons of each type?

Thanks.

Jason

[COBRANIP]

How do you plan to use your Cobra? GENERALLY SPEAKING......
.."Softness" is more associated with spring rates and tire size than suspension type;
..In a drag race environment the straight axel is probably a better choice;
..On a bumpy road or track the Independent suspension is generally superior;
..On a smooth road or track there is not much, if any, difference (ON A PROPERLY DESIGNED SYSTEM).
This sort of learning is half the fun of building a Cobra!
Good luck.

[RICK LAKE]

qmaster Jason It depends on what cobra kit you buy. The other thing is what you are going to do with the car and how MUCH HORSEPOWER are you dropping in this car. Jags rearends are nice, good for about 450hp and torque. This is with the 27 spline rearend. The ratio of the rearend also has alot to do with max HP in the car. The small the rearend ratio the less power I would put through it. You can offset this with what trans you are going to run, and what ratios are in it. Automatic IMO are easier on the drivetrain than a speeder. 30 spline Jag rearend with chromemoly shafts and carrier support are good for about 600 HP and torque. Some cobras use the 8.8 IRS from ford. They are good for about 500 hp and torque with the stock output shafts. There has been some twisting of the CV joints on these rearends with high hp motors or dragracing. Alot also depends on what tires you are running on the car. Autocross or tires with 140 wear indicators are more sticky to the road and give better traction, they are also harder on the drivetrain. If you are going to dragrace the car, a 9" ford rearend is the way to go with a 5 link suspension setup. The car will take alot of abuse with this rearend. If you have big bucks, a franklin quick change rearend is good for 750 HP and multi gear changes in 20 minutes. The internals are 9" ford gears and the ratios are from 2.40 to 6.50. IMO a IRS rearend will handle a little better than a solid axle. There would you have the rear brakes? inside or outters. 4 Shocks or 2 for the rear end. Jason we need a little more info on who and what you are looking for. This will get you started on options. My last suggestion is to OVER ENGINEER the car and try and make it bullet proof. If you want to talk, call me in the evening. I'm in NJ 732-254-3536 I will give you the best info I have on this. 8 years racing a jag rearend with 3.31 gears. No problem yet to date. Have gone for 368hp to about 460. Will know on Sat the final numbers on the 482 motor. Torque is the important number more than HP.

[john chesnut]

Jason, Rick is a great source, knows his stuff. Call him if you have questions. I have built 7 cars in the past 7 years and have built them with both live axles and independent rears. Also, with different types of front independent suspensions too. Books have been written on the subject. It would be worth while to read a couple. A pretty imformative one was written by Herb Adams. He was a racer and a GM chassis engineer. It was called, I think, Chassis Engineering. Should be able to find it by doing an internet search.
The purpose of any performance suspension is to keep the tires in as much contact with the road surface through out its full travel as possible. And to transfer as much power to the road surface. The first for handling, the second for traction. More traction, faster acceleration.
A live axle is simple, reliable, and cheap. The way it is mounted to the chassis has a great effect on handling and traction. The mounts on the axle matter as much as the way it is mounted to the frame. There are many configurations of control arms that can be used. All have advantages and disadvantages. All suspension components travel in an arc. This is true for side to side movement and front and back movement. Side movement results in camber change, front and back in a change in a change in wheelbase. These movements can be set up for specific applications. A set up for ultimate cornering will not be the same for ultimate traction. The rule of thumb is to try and make the arc as large as possible. The larger the arc the smoother the changes will be. The smoother the changes, the more predictable the handling. The length of the arc is directly dependent on the length of the control arms. The Cobra has the seats mounted as close to the rear tire as possible. This results in very short control arms and therefor erratic handling.
An independent rear suspension, in theory allows for better handling. This is because each wheel moves independently. So, the tires should be able to maintain better contact with the pavement. You will not get as much traction with it. A live axles control arms can actually be arranged so that they act as a big lever under acceleration and in effect transfers the weight of the car down onto the rear tires. Most independent rear suspensions transfer little if any of the vehicles weight to the tires.
The best independent rear suspension consists of upper and lower aframes. The axle is splined so that it can change in length as the suspension moves up and down. If the lower control arms are angled up in the front and the differential is mounted in the frame in way that the mounting component acts as a lever it is possible to transfer a good amount of the cars weight to the rear tires. This lever effect occurrs because as power is applied to the diff it wants to rotate up. This motion can be used to transfer the vehicles weight.
Most vehicles under acceleration squat down in the rear. With an independent rear suspension this causes the wheels to go into negative camber. This results in a loss of tire contact and therefore traction. Modern cars, such as the Corvette are designed with anti squat.
The jag and the C3 and C4 Vette use the rear axles as the lower control arms. They are fixed in length and use ujoints to attach the axles to the hub and diff. This type of suspension is a modified swing arm. Because the axle cannot change length the tire moves in an arc and dramatically changes camber and therefor tire patch contact when it moves. The other problem is that as the ujoints and diff components wear there is excess play and this causes a change in lenght. Again not great for handling.
The Vette and I beleive the Jag too attach the hub to the chassis with control arms that are mounted parallel to the frame. These control arms must rotate as the wheel mounts up and down. When using any type of bushing except for heim joints there is a bind condition that occurs. None of these things are serious if the car is used as a road car. But for ultimate handling they create problems. I don't like to use heim joints for a street car because they are noisy, wear out fast, and create a harsh ride.
I believe that the T bird IRS uses an upper and lower aframe with a CV axle. So, in theory it is the better way to go. I don't have any experience with them.
The best production rear suspension in my opinion is the C5 and C6 Corvette. It is an all aluminum unit that has anti squat built in and is a true double aframe system. The axle is splined. I am just finishing a new scratch built Cobra and am using the diff and suspension from the C5. I built a protype chassis with these components last year and the handling was unbelievably great. The Vette uses a transaxle and a torque tube that ties the engine to the transaxle. This is how they transfer weight for traction. The transaxle will not work in a Cobra as the trans tunnel would have to be too wide. I engineered and had made a conversion for the diff made so that it would accept a driveshaft. I also built the transmission tunnel as a structural member of the space frame. The diff bolts into the tunnel. this "torque" tunnel acts as my torque tube.
I know that I am long winded. All of this is great fun. Decide what you are really going to use the car for and then built it with the best suspension for that purpose. Have fun!!! john
I prefer the Vette rear to the Tbird because the Vette is all aluminum and has a separate control arm for toe adjustments. Also, the older jags and Vette rears suffered from a high roll center which compromises cornering and stability. The new vette has a low roll center. There are differences in scrub radius's too. It never ends, bye!!!