Dear Welder Series… RPU Coilover Mounting Tech Question

Dear Welder Series…
Here’s a question for you A guys and specifically the RPU guys. I just pulled off one of my rear coilovers to find out the spring rate. The coilovers are TCI All American’s and they have a progressive rate spring. The rate of the coilover is 300 lbs. according to the chart in the Yogi’s catalogue I have.

What spring rate are you running in your vehicle?

My shocks were mounted on the bottom bolt hole and I have been bottoming out for a while now (too often anyway), in fact, I now have an additional problem, Because of my bottoming out, rear axle to the frame, I have now broken my rear tab on my frame where the rear box mount is. Wondering why the truck was a little more rattlely than ever before, now I know.

Dear Mr. RPU…
Regarding coil-over set-up, check http://104.154.121.234/tech/tech-sharing/coil-over-mounting-offset/. The only reason to adjust the lower threaded collar is to achieve the “designed ride height” for your coil-over, which is compressed 1/3 of the stroke from full open. You should establish this designed ride height for your coil-overs. One coil-over will have to be removed to do this, unless you can get open and closed specs from TCI. The rear ride height of your RPU gets adjusted by choosing a hole in the lower bracket that gives you clearance to the frame and the stance that you want. When a custom frame is being built, we use(d) a bar with holes at the coil-over designed ride height. Then brackets would be built/installed to have the frame sit the way we wanted. A Model ‘A’, ’32 Ford, etc, have some generally accepted brackets that give a generally acceptable stance. Your TCI frame has their brackets and I think your RPU sits great! Bump stops are a great idea… I got some Energy Suspension #99103G cone-shaped stops from Paul Barber. (But I haven’t installed them yet. DW probably will want to figure out some trick way to do it!)

The springs sound heavy to me, but the way to really tell is to check the ride height. Can you achieve the designed ride height with that spring by adjusting the lower collar? Let’s use a designed ride height (DRH) of 13″ in this example. TCI steel coil-overs were 13″ DRH, but they have some new ones now and I don’t want to “assume”.  With the truck sitting on the ground, measure the coil-over center-to-center. Use the threaded collar to get the DRH. This will be a 1:1 function. (If the coil-over measures 13-1/2″, lower the collar 1/2″ to get to 13″.) Because of the shock valving and other resistance in the suspension, you should bounce the rear of the frame to get things to settle. If you are not able to get to the DRH because you run out of thread on the shock, the spring is definitely too stiff/strong/heavy. Within the threaded collar range, you can probably use any one of 3 different spring rates and still get the DRH. (The threaded collar will be higher for lighter rates and lower for heavier rates.) This lets you dial in a rate that gives a “half-ride” that you like. The other “half” will be dialed in through shock valve adjustment. Coil-overs with a dial-in shock valve adjustment give a wide range of ride quality, so be sure to test through a range. (With our ’32’s ShockWaves, we started at mid-way, or 8, but have settled on 3.) Usually counter-clockwise is softer and called 0 or 1, clockwise is stiffer. I may add to this as thoughts come to me, but this should give you something to work with for some information.

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