Axle Wrap - Tech

Tech Question - Axle Wrap

Mike,

I believe I am getting spring wrap on my 1930 Essex Sedan Street Rod, from an occasional heavy foot, and as a result I'm going through U-joints too frequently. The transmission shaft / pinion gear angle appear to be good. I have about a 380 HP 400 CID engine, 700R4 transmission, 3:25 rear end and 28" tires. My car weighs 3,200 Lbs. I believe the total weight on the rear springs is about 1,500 Lbs. typical. With two rear passengers and a full tank of gas it would be closer to 1,900 Lbs.

My current leaf springs, I believe came from a 65 - 73 Mustang. I mounted them backward (31" forward / 22" back) because of limited distance for rear spring perches (about 21-1/2" to 22-1/2" to the rear of the axle). The springs are currently almost perfectly flat when the car is empty. I have a 3" block between the spring and the axle to adjust the ride height. Ideally they car would sit about 1" higher as it did about 20 years ago.

My primary objective is to eliminate much of my spring wrap. I am currently thinking of relocating the front spring perches farther back and selecting different leaf springs. I could relocate the front perches to anywhere between 25" to 31" from the axle; the rear perch stills need to be about 22" back from the axle.

I notice that the 1955 Ford T-Bird springs are 25-1/2" x 22-1/2"x 2-1/2" and 1986-1997 Nissan Pickup springs are 25-1/8" x 22-1/8" x 60mm. But I have no specification on these other than length and axle location. Could you provide me specs on these two springs i.e. max load, arch, Lbs./in. deflection.

Any other suggestions would be appreciated also. I need Spring Guru help/ advice.

Thanks, Litch

Litch,

You do have a few problems here and you are on the right track as to fixing them. Let me start by explaining why the springs are winding up.

When measuring springs one measures from the center of the front bushing back to the center bolt and then from the center bolt to the rear bushing. Take both measurements following the curve of the spring. The front of the spring is called the Short End and the rear of the spring is called the Long End. They are called this because 99.99999% of the time the Short End goes to the front of the vehicle and the Long End goes to the rear of the vehicle.

Besides helping support the weight of the vehicle, the Short End of the spring acts as a trailing arm by locating the axle and eliminates or greatly reduces axle windup.

The Long End of the spring provides most of the support and cushion.

Look at the spacing between the end of the leaves. Notice how much closer the ends are to each other on the Short End vs. the Long End? This closeness is what gives the Short End more strength than the Long End. This spacing, a.k.a. stepping, is critical to the design of any spring.

When mounting springs backwards, that is the Long End to the front, just about no resistance to axle windup is offered because the Long End is designed to support weight and provide cushion. It is the cushion providing aspect that allows the spring to windup with much less force.

While mounting the springs backwards is the main cause of your problems, it is being compounded by the 3 inch lowering blocks. Even when the springs are mounted the correct way, lowering blocks increase the amount of axle windup because of the increased leverage. The amount of leverage being applied to the spring during acceleration and braking is increased the farther away the spring seat is from the axle. This increased leverage causes the springs to distort more which allows the axle to rotate with less force.

Your are on the right track by relocating the front hanger. Getting it back as far as you can will help immensely. However, neither the T-Bird nor Nissan spring will do the trick for you. Remember what I said above about the leaf stepping? The Long End stepping is much greater than the Short End.

While getting the short end shorter than it is now will help in solving your problem, it will require us to build you a custom spring to get all the aspects of the spring correct. Nothing "off the shelf" will do the job for you. Give me a call and we can get the project started.

To learn even more about springs and their design, go to our Tech Section and download our Spring Tech 101 and check out all the Tech Questions.

-Mike