You hit the nail on the head. Calculating for the springs has been the biggest unknown factor for me. Looking at chassis diagrams and spring specifications can only get you so far. It's hard to guess how much the springs will compress in actual use.

When I built the frame I designed in a 1.5 degree rake which is typical of most trucks (P.S I hate leveling kits). Currently the frame has a 3 degree rake but the rear suspension is not really compressed at the moment. To get my ride height to about where I want it, the front needs to come down 1" and the rear 2.5-3".

Now to balance payload capacity and ride height. For comparison the stock leafs on the 1951 M-3 were 1025 lbs front and 3000 lbs rear each. I know leafs and coils compress differently.

Currently I have 1700 lb springs in the front which is about the lightest spring Ford made for the HD suspension. Aftermarket replacements come in at 2000 lbs since they are made for a F-350 chassis cab. Neat fact the passenger coil is rated higher than the drivers side due to the engine being offset. In 1980 Ford changed the height of the coil bucket on the passenger side to use the same springs on both sides. The nice thing about the 1973-79 HD suspension coil buckets is that later model springs fit no problem. It's just that they are shorter than the 1973-79 ones so I would need a spacer to use them.

Rear springs are for a 1992-2014 Econoline. Currently in the frame are OEM Ford but do to a ordering mix up they are for a E-150 which I didn't find out till later but I accidentally nicked one with the angle grinder so its scrap. I have a pair of new Moog E-350 springs to go it. These are rated at 1700 lb ea vs the 1450 lb of the E-150. The Moog ones are variable rate which throws another wrench into the works. Uncompressed they measure 16" and installed height is 13" in its original application.

Factory payload on the M-3 was 2800 lbs and being me I want to meet or exceed all factory specifications. So if you run the numbers:
3400 lbs
- 500 lb pickup box
- 115 lb of fuel
- 60 lb trailer hitch
- 150 lb of misc for good measure
= approx 2575 lbs of payload

I know I won't be able to reach that amount of payload without stressing my rear coil spring mounts. Although I have 2775 lb tires on 3500 lb rims attached to a 7500 lb rear axle. I do hope if I hook up a trailer with a 500 lb tongue weight it doesn't sag it out too much.

Now I did try to match the tailgate loading height. Stock height according to the body diagram is 26.77". If I can hit the magic 13" compressed height I should end up about 35.25" which is a far cry but comparably a brand new F-150 is 35.75".



I already have the rear coil spring mounts mounted behind the axle to drop the height and simplify mounting. Dropping it much more I'll run into clearance issues with the bed floor on full compression. If its not one thing, its another...

 

Awesome frame build! It's great to see just how much is really involved

 

Got some more progress done over the long weekend:





The steering box is now fully mounted. It actually fits pretty well since the front suspension was never designed for front steer.



I switched from a yolk to a companion flange because I couldn't find any new u bolts that fit. Everything was too wide. Surprisingly the flange was cheaper from Ford than the aftermarket. The other half is from a 2005 Explorer Sport Trac that I kept and used the same 1330 ujoint as my driveshaft.









The headers are finally built. They aren't perfect but it was also my first attempt. All the spark plugs are easily removable, lots of clearance for the wires and bolts plus the starter is still able to be changed no problem.





It was a bit tight getting the y pipe to clear and line up to where the header needed to be. I was hoping to fit in another resonator but didn't have enough length. I hope it won't be too loud as its 3.5" pipe. It's all 304 stainless from the headers back.

Till next time...