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Technical Transverse spring preload/setup question

Discussion in 'Traditional Hot Rods' started by mechanic58, Sep 7, 2013.

  1. 27troadster
    Joined: Mar 14, 2013
    Posts: 105


    Pasadenahotrod has the key to all of this. Bottom line, the more the shackles are parallel to the ground, when the suspension is fully loaded, the less side to side motion there will be. The more up and down the shackles are, the more side to side motion.

    Now the tricky part is guessing how much the final weight of the car will be and therefore how much will the spring deflect under load. If you have access to a press with a gage, then you can put the spring in it, compress the spring to what you think the weight will be and measure the distance between the spring eyes, then add the center-to-center length of the shakles divided by 1.414, times 2 for 2 shackles and put your perches at that spacing. For example, if the spring eye-to-eye when loaded is 38" and the shackles are 2" then: 38"+(2"/1.414)*2 = 40.82" would be the perch spacing required to attain 45 degree shackles.

    Now once the car is all assembled, there are some things you can do if the shackles are not between 30 and 45 degrees. If the shackles are too up and down (greater than 45 degrees) you can add a panhard rod. Also you can re-arch the spring with a press and a piece of 4" channel. Or remove / add leafs.

    The other thing that will make your ride terrible is splitting the radius rods to the rails. This will cause suspension binding and a stiff ride. Basically, when one wheel moves up and down, that end of the axle must twist as the radius rod moves through it's arch. However, the end of the axle at the other wheel will not twist because it is attached to the other radius rod. For the suspension to work, the axle would have to twist. This is fine for an I-beam front axle, but that rear axle will not twist. This is also why parallel bars are used on tube axle front ends and people don't run split bones or radius rods with tubular front axles. Now, on light rods you can get away with it but the ride WILL be harsh. I set my T (in my avatar) up that way the first time and snapped the welds were the radius rods met the rear axle. For version 2, I made the rear suspension into a 4-bar set up where the radius rods that are split to the rails are the lower links. The radius rods have tie rod ends at the frame mount point and at the rear axle mount point. Then two rods are located between the rails and use tie rod ends to mount to the frame and to the top of the axle. I made the upper rods angle in to control side to side motion.

    After setting up the rear suspension correctly, the ride became much, much better, like going from a buckboard to a Cadillac.

    There are pics of my 4-bar at:

    Hudson48, pls post some pics, someone here may see something that was overlooked that is causing problems.

    -Brent- likes this.
  2. hudson48
    Joined: Oct 16, 2007
    Posts: 2,769


    P1020279 (Medium).JPG The car is built and has been on the road for some time. I have always been unhappy with the rear ride but put it down to being a low channeled car with transverse spring. From info here it looks like I may be able to get improvement on the ride comfort in the rear. Maybe not a Cadillac but better than what I have.We are going to install longer perch spacing next week as my car man said when they were reassembling the spring last time it was a difficult job to get the spring across to fit the 40mm spacing. He made up some dummy 50mm spacers and it was much easier so maybe he should have left them in!! The perch angle is spot on 45%. Could only get one picture of RHS as I can't get it on the hoist at them moment with the Hudson up there.
  3. hudson48
    Joined: Oct 16, 2007
    Posts: 2,769


    Did the perch changeover with longer ones and it did make some difference but still very stiff. I am using the SoCal 32 spring and it has the main leaf plus EIGHT others of reducing length down to a really small one at the top that is more of a spacer than a functioning leaf. I think it is time to remove one or two more leaves to reduce the stiffness.
    I have attached a photo of the spring perch we took out and you can see how it had spread under load. I will now follow the advice from another poster and remove shocks and check spring and remove one leaf and see how it goes.
    Question is what leaf do I start with?
    P1020280 (Medium).JPG
    Last edited: Dec 2, 2015
    Joined: Feb 21, 2010
    Posts: 340


    Hudson, you ever get a handle on your stiff ride? I have a similar issue with my 34

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