Originally Posted by chuyler1
The springs I found that fit are MOOG CC250
. Which have a rate of 412 and a load of 786. Free height is 13.44" and installed height is 11.50". The only info I have on the OEM REPU springs is that they are 11 7/8" tall with a spring rate of 512 lbs.
Excellent info, Chris.
First, for those not familiar with springs...
- The rate of 412 will be in pounds per inch, so each 412 pounds of load causes an inch of additional compression in length.
- The free length/height is with no force on them.
- The installed height is the length when carrying the designed load (the truck, sitting empty and stationary).
- Compressing a 412 lb/in spring from the free height of 13.44" to the installed height of 11.50" (1.94 inches difference) should take 799 pounds... but the "load" value is the force at the installed height, and is slightly lower (786 lb) - explanation to follow. The total load for two springs as installed (1572 lb) should be substantially more than the empty weight on the pickup's front axle, because the suspension arms are levers and the spring location is only partway out them.
Originally Posted by chuyler1
When I couldn't get the first set in, I cut them down from 13.44" to 12". This produced an acceptable ride height but very soft front end that would dip around corners and there wasn't enough clearance to turn the wheel full lock without rubbing the fenders. I thought like you that cutting the spring would increase the rate and make it closer to the OEM rate but that's not how it felt.
If the Moog springs were constant-rate, then cutting one down from the original free length of 13.44" to 12" would make it 1.12 times as stiff (12% stiffer, 11% shorter). That's 461 lb/in, still softer than the stock 512 lb/in spring. So cutting did make them stiffer, but apparently not enough.
The complication is that the Moog springs are not constant-rate - they're variable-rate. This is also called "rising rate" because as the spring compresses the more closely spaced turns at one end close up solid so it is as if they are no longer spring, but just a rigid spacer, leaving a shorter effective spring and thus a higher rate (stiffer spring). If the wider-spaced end was cut off, 11% of the length would be less than 11% of the spring. If the photo on the linked page is correct (it is actual part, not just a typical example) it has 6.5 free turns of coil, and cutting one off has the same effect on the starting spring rate whether it comes from the wider-spaced end or the tighter-spaced end... one turn would make it 18% stiffer than stock (487 lb/in). Also, the quoted rate is probably an average, so the actual rate at full length is lower (see the load value note above), and at the installed height the first coil might already be closing up.
The Moog springs were probably good before cutting, because they are intended for use at higher loads, and in that case they would be compressed enough to be in their stiffer range... they are probably supposed to carry a heavier load at stock height, not a stock load at raised height... or a reduced load at raised height. Of course, Moog doesn't list them for the REPU at all - they're for the Courier and other Mazdas, so they fit the suspension but maybe are not appropriate for the loading.
Also, those springs are square on the low-rate end
and the spec says tangential on the other end (square means the last turn is closed up so that it meets the next turn - hard to see in the photo). Some coils have ends ground flat so they work on flat mounts, but these are the more typical version that needs a seat shaped to hold the coil end. If the square end is just cut off it becomes tangential, and some force will be required to collapse that first turn down to match what the factory-prepared end always looks like.
The way to change height without changing the rate of the springs is with spacers
... but they take up space and thus may limit compression travel. Would the stock spring fit with a spacer, or would it close up entire and block further movement before it should?
Originally Posted by chuyler1
I'm thinking the WD hitch will pull the front down to a safe height.
WD would work, but it just seems like a lot of hardware and operational hassle to fix a problem with wouldn't even exist with the front suspension at its stock height.
Back to the fundamental nature of the situation...
The longer springs are in there to raise the ride height
moderately from stock. Doing this solely by a spring change - any kind of spring change - means moving the normal ride point higher in the same range of travel, and that means giving up droop travel - that's a problematic combination with reduced front axle
load, even though the load is still above that of an empty truck.
The solution for raised trucks is to not just extend the spring, but relocate the suspension mounting points downward - or from the other perspective, lifting the truck off of the suspension. I doubt that's worthwhile in this case. The slickest systems lower the bottom control arm, leave the top control arm where it was, and connect them with a longer spindle carrier;however, there is not going to be an extended spindle carrier for a REPU... or anything from Mazda, or anything from 1975.
Next not-as-wild idea: use the stock springs, add cheap air bags (Airlift 1000 or Firestone Coil-Rite) inside them, and pump them up to add just an inch or so of ride height. These bags are not intended for lifting, but presumably by staying within the stock suspension travel range they would be acceptable. They can be let down (to low but not zero pressure) to correct ride height when towing without WD (or for the original "low rider" attitude). Firestone doesn't list applications this old and obscure, so a suitable match would need to be found based on spring inner diameter and length.
One last techie note: the solution in oval-track "stock" cars to adjusting coil-spring rear suspension is to put the top mount on a screw jack, and turn to desired height. This doesn't change the rate, but does change the height for a given load... it's like a quickly adjustable spacer. I don't think I would try to fit one those into that spring mount area, especially with the droop stop for the upper arm.