flipped front axle on 39 plymouth truck

[kyotb8]

Hey everyone, I need some advice. I want to put the front axle on top of the springs to lower my truck. How much clearance do I need from the top of the axle to the frame ? Some say 4 inches, but I see lots like this with maybe 2 or 3 inches clearance before it hits.
I've got about 5 1/2 inches clearance now from the bottom of the frame to the top of the spring pack where the axle will go if I flip it. the axle is 2 1/2 tall there, so I will have 3 inches or less travel before bottoming after a flip. Is a c notch ok for the front if done securely ? I see them over the rear axle all the time.

Also, will I need to make shims to set the correct caster of 5-6 degrees ? I plan to weld a plate to the bottom of the axle to help secure it. Any input is very appreciated, I'm trying to save the cost of a dropped axle if I can.

 

[donsrods]

You might get by with 3 inches but if you hit a particularly hard bump....... We had to notch the frame on my Sons RPU for a little more clearance and we shot for 4 inches.

Don

 

[kyotb8]

Thanks for the input, Don. 4 inches seems to be safe. Any concerns over the caster settings on the one you did ?
Thanks again for answering.
Jeff

 

[zzrodder]

I did the axle flip on my 55 Chev pickup some 30 + years ago and also did a C notch on the frame to maintain about 3 to 4" clearance, worked out great, never had frame contact though I did also put in some bumpstops just in case.
What I did have an issue with was massive bumpsteer and torque steer because the drag link now angled up to the steering arm instead of being parallel to the ground. I solved the problem by raising the steering box the same amount as the axle was raised, 4".

 

[kyotb8]

Thanks for the input, ZZ. I've been doing a lot of reading and asking all the questions I can so I hope to get it right the first time...or at least the SECOND time !!! I'm planning a phord box with the pitman pointing up, or a mopar box with the pitman down if I have room. Trying to keep the old rig as mopar as I can. Thanks for your input. But what about the caster angle? Am I too concerned ? I see some rods with the kinpin angle tipped way back, and others not too much. Did you shim or somehow make the caster angle correct when you flipped your axle ? Thanks again for taking the time to answer, I've rebuilt a few rides, but always just went back stock, so this kind of thing is new !

 

[zzrodder]

In my case, I welded tabs to the bottom side of the axle to mirror the original mounts on the top side and ran the U-bolts right through. Since the axle sat on the springs in pretty much the same position as originally, I don't think the caster was changed at all from stock. I seem to recall in those trucks the axle didn't have much caster anyway, at least compared to a lot of rods which are commonly set at about 5 degrees back. In some cases, if there is not enough caster, the steering will not automatically return to centre or will wander when you take your hands off the wheel.
To play it safe, I'd shoot for 5 degrees anyway with the mount positions.
Something else to keep in mind is that these old trucks were never expected to drive on todays roads at todays speeds so if the axle was mounted straight up, it would have made it easier to turn the wheel at low speed, they never dreamed of them going down a freeway at 70 mph.

 

[kyotb8]

Thanks ZZ. I'll shoot for 5 degrees or a little more. Sounds like a good plan to put some new pads on the bottom of the axle. I will do some highway driving ! Thanks again !