1973 F250 2wd suspension-

25

Asked by Dec 16, 2010 at 01:17 PM about the 1973 Ford F-250

Question type: Car Customization

if i replace the front coil springs will it clear 235/85R16 and if i replace the rear leafspring, will it clear 305/70R16? any help is appreciated

1 Answer

If the truck is equipped with the twin I beam front suspension system, be very careful when changing anything pertaining to ride height. Ford used that system from the 60's though the early 2000's on their full size 2wd trucks. (to the best of my recollection) Since it doesn't utilize an independant A-arm design, moving the truck up just an inch for clearancing purposes will affect camber, making the wheel lean outwards towards the top, creating extreme wear on the outside edge of the tire as well as affecting caster making the forward leaning axis of the knuckle fall back to zero or possibly even a negative degree. This would disrupt the truck's self straightening capabilities as you drive down the road forcing you to make constant steering adjustments, and creating bump steer. Sounds crazy, seeing as how Ford used the same design for 40+ years, but, as it is a good heavy duty setup, it's one of the worst designs to modify without re-engineering the entire system. The reason that these things are so greatly impacted by such small changes is because the driver side wheel is supported by the spring, (obviously) the main I beam that runs from the passenger side framerail to a fixed, non-flexible knucke, and a trailing arm that mounts near the primary cab mount. If the I beam moves down, the angle of the knucke changes side to side. If the trailing arm moves down, the angle changes front to back. An independant setup allows the knuckle to stay at virtually the same angle, lifted or slammed because it utilizes 2 essentially parallel control arms that move together at the same rate, (no major change in camber) and doesn't require a trailing arm because usually they're triangulated arms (A-arms) so there's no caster changes up or down. That's probably unneccesarily long, but be advised of those possibilities. As far as wheel and tire size, any local tire shop can tell you exact dimensions of a wheel/tire combination, or you can calculate it yourself. 235/85R16 = 235mm wide, with the sidewall being 85% of that width (x2 for top and bottom sidewall height) plus 16 inches for the wheel size @ the rim. Coil springs may be as easy to lengthen as adding a polyurethane lift puck (maybe $10 at a local off road shop) and rear leafs could be lifed an inch or 2 with a longer shacke or a longer U-bolts and a plate between the housing and the spring. (On a spring-under axle setup) Or a longer coil, stiffer coil, or even a new coil on a '73 would push the front up while there's always similar options with leafs for the rear. Leafs can be re-arched too, if you just need a bit more height. Hope that helps, sorry it's so long.

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