The Stovebolt Site Forums The Shop Area Driveline Frame Notch Strength

A big drawback of the pre 1960s NAPCO trucks is that the frame was designed for 2WD front suspension. When the 4wd NAPCO axle is installed: the front suspension loses about 1.5" of up travel due to the leaf spring plate/bumper on top of the axle. Combine this with saggy old springs and you're left will less than 1" of suspension travel. Not cool for washboards or any type bumpy driving.

So, I'm thinking about notching my frame by 1.5-2" and have done a lot of thinking about how to reinforce the notch.

Geek alert! ... the computer is helping me think
Attached pictures show the two strategies I'm considering:
[img]https://imgur.com/nZhKwa4[/img]
[img]https://imgur.com/etd3Odp[/img]
[img]https://imgur.com/3nA5uDX[/img]

It turns out that the 'arch' design with reinforcement on top is stronger for suspension bottoming (on a bumper in the notch) yet weaker under normal conditions.
The 'fishplate' design without upper reinforcing is stronger for normal conditions, but is much more stressed during suspension bottom out.

I'm leaning towards the 'arch' design since I think the loads will be much harsher in the bottom out condition.
Note that the stress numbers and loading is purely comparative between notched and stock chassis an aren't totally realistic.
However I think its a reasonable way to compare the chassis reaction to the different load situations.

Let me know what you guys think!

Replace the springs and leave the frame alone.

It looks like you are able to pull this frame modification off, but that doesn't mean that is a good idea. There are bound to be unseen unintended consequences. Something along the lines of "no free lunch".
What do you plan on doing with this truck?
A friend of mine had a '57 NAPCO which has the riveted on crossmember for the transfer case, indicating that it was set up by Chevrolet and not an add on kit. He drove it like he stole it. He went through several transfer cases and a transmission, but he did not have any issue with the front axle bottoming out. His truck came with extra leaves in the spring packs front and back.

I converted 1956 GMC Carryall to 4X4. Can be seen on google.

WOW! that is a beautiful suburban... I'd love to see pictures of the suspension/underbody.


Sounds like your buddy put a ton of miles on that thing, I'm hoping to do the same!
My mounts are also riveted (except for the front left leaf spring hanger), and I've got a 44x2" leaf spring with 6 leaves plus a tiny 7" long leaf that doesn't really count. The specs say it's supposed to be 7 leaf on 4wd, but 6 on 2wd.

So far I've used the truck on a few roadtrip/offroad adventures around new mexico & colorado. Planning to do more of that in the future! It is extremely capable at low speed 4wd obstacles and has put a new Tacoma or two to shame! However, it's downfall is more average dirt roads with occasional rocks or washboards. This is where most of the non-highway driving occurs. I have to slow down to 10-20mph on roads where you'd normally drive 30 or 40. Airing down would help but I don't want to do that just to cruise some 2wd road.

You guys are right about beefing up the springs it would be way easier, and that's how the truck was original. However, I'm spoiled enough to have a garage, and the problem is that even with saggy springs I still scrape the roof a little when I pull in without airing down. The burb is so ridiculously tall already that I'd hate to look down even farther at those other lifted bro-dozers, not to mention I'm also a knuckle brain once I get an idea.

The consequences I've thought of so far are:
1) Fender rubbing... however the measurements indicate this won't be an issue.
2) Oil pan clearance... I only have 2" down to the steering link. However I can get a 1956 oil pan where the sump is a little farther back. With a little hammer massage I should get another 2" no problem.
3) I did a little more research and I think the 'motion' ratio for my axle is about 2:1 for a single wheel bump... so I'll be pleased if I can increase my leaf spring compression travel from 1" up to 2 or 2.5" (ie. 4 to 5" wheel up-travel) so I'll be shooting for a notch between 1 and 1.5"

What year is your Suburban?

How bad will the bump steer be with the extra travel?

Maybe you just need new springs and better shocks?

Mike B

Garages are relatively easy to lift, add one more layer of concrete blocks, etc., are not too heavy, so easy to do one corner or side at a time, might have to add one more door segment. Probably easier than modifying frame. Of course if attached to house, more difficult.

Ed

Normally I'm all for mods to trucks to make them better, but you just need to replace you springs man, or sell the truck and get a 1960 or later that will give you a lower overall height and more spring travel

Good comment! Bump steer is something i didn't consider. I think the link angles down slightly on it's way from pitman arm to knuckle. I'll have to check next time I have the axle on... attached picture of someone else's steering
link. It seems fairly civil with all the driving I've done so far. It's a '59 3100 [img]https://i.imgur.com/nF56dHc.jpg?1[/img]

Thanks for the input everyone! I'll think about it.

Looking at your picture, the U-joints are showing an extreme amount of threads showing at the top. If these u-joints are original to your NAPCO axle, it is telling me that you don't have NAPCO leaf springs to go with your NAPCO axle. I am pretty sure that NAPCO springs have additional leafs in the stack compared to 2-wheel drive trucks.
Carl

Carl, you are correct...the 1959 3100 4x4 trucks got one extra leaf...7 on the Front and 8 on the rear, 2-wheel drive trucks were 6 and 7 respectively.

Mike B

I say never ruin an old frame, they aren't made anymore. There is no reason to drill, weld or what ever, always a plate or bracket can be designed that uses the original holes on the frame. Same goes for in cabs, like under the dash installs.