nmcgrawj said:
I was thinking about beefing up my rear end with some 31 spline axles so i can launch a lot harder...how would that help in comparison to subframes or lower control arms?
How does swapping to 31 spline axles increase traction? Oh wait, it doesn't. Axles with a higher spline count is for strength only. A 31 spline axle will take more power before snapping. A 33 spline will take even more.
Right now, your traction limiter is at the contact patch (i.e the bottom of the tire). What you need to do is create a better platform for the tires to attach to. That means STABILITY! What are the causes on instability in the Mustang's rear suspension? Let's talk about a couple of them as they apply to your immediate situation.
Chassis flex:
The stock mustang chassis has the torsional rigidity of a wet noodle. When you launch, the rear axle is tryng to turn clockwise (the right rear going down and the left rear going up) as you look from the rear, but it's held in place by the control arms and springs. The engine is trying to twist the other direction but held in place by the motor mounts. So these opposing torsional forces are being transmitted to the chassis and trying to twist it up like a pretzel. The problem is, the chassis is so weak that it does a damn good job of it, especially with torquey motors.
So who cares if the chassis twists? Tires are on the ground right? Absolutely wrong, and in two ways. One, that chassis flex absorbs power that could be getting to the ground and getting you down that track faster. Two, if the chassis allows that flex to occur, less traction will be available at the left rear tire. Remember what I was saying in the other thread about the clutch packs in the axle? This is where it comes into play.
Thos clutch packs will hold a certain amount of torque but as soon as that limit is reached, they'll slip. T-loks are kinda crappy that way. Anyways, so if that left rear lifts too far and enough of the friction is lost, it will allow too much torque to transfer and the clutch packs will slip allowing the dreaded "one wheel peel."
So what do we do? Why we stiffen the chassis of course! There are numerous ways to do this but the best initial modification is subframe connecters (SFC's). They attach to the front and rear subframes, drastically improving torsional rigidity. There are lots of SFC's out there but here are the two major rules:
1) DO NOT USE BOLT-ON SFC'S! If you do use them, weld them in. Do not leave the bolts in as the only means of attacment. They will eventually egg out the bolt holes and tear through the subframes. This is very very bad!
2) Use full length SFC's regardless of the power level you are at. Remember, the Mustang chassis needs all the help it can get. Short or standard length SFC's are not as good as full length. Pay the couple extra dollars now. It's money well spent.
Control arm design:
The stock control arms are stamped steel. They're easy and cheap to make, but they flex like a yoga instructer. By swapping to a design of greater rigidity, increased stability of the axle is attained. Designs to look for are tubular in nature. I prefer steel for high power drag race applications but aluminum arms are installed on street/strip cars all over the place with no problems.
Control arm bushing material:
The stock arms use a rubber bushing design that absorbs vibrations and noise from the axle and road surface. They also allow the axle to flex about, limiting positive control. By swapping to bushings that use a harder material (like polyurethane, delrin, or even steel), even more control of the axle is achieved.
Unfortunately, the design of the 4-link rear end allows a great amount of bind to occur if the upper control arms bushings are not allowed to distort. For this reason, I NEVER suggest using any material other than rubber on the UCA's in a car that is street driven. If it's a drag car only, by all means swap all the bushings with steel. If it goes on the street, maintain the stock UCA bushings.
Matter of fact, the only time I recommend swapping the UCA's is when the bushings are worn and need replacing. Then you may want to consider swapping to the FRPP heavy duty UCA's and bushings. Of course, you could always ditch the UCA's entirely, but that's another topic.
So what does all that mean? Basically speaking, get some SFC's installed, get some decent LCA's installed and go from there. I wouldn't worry about spring rates just yet. Considering your comment about money, you've got some time to consider your options beyond those first two mods.
