First Gen Herb Adams Mod - Did it

[First Bird]

Hey all,
The Herb Adams Modification is a popular mod for 2nd Gen F-Body cars that raises the front mount location of the leaf spring. This increases anti-squat and reduces roll oversteer.

I searched the internet for information on modifying the First Gen F-Body. I only found a few comments that the floor would have to be modified, but not how. This is how I did the Herb Adams Mod for my 1968 Firebird.

Remove the rear axle and springs. You'll be working under the car so, yeah, make sure it's well supported. Remove the front leaf spring mounts - 3 bolts. Often the cage nut breaks. I lucked out prepping the bolts with PB Blaster and using an air impact wrench and all came out easy.

Measure and mark the center of the new hole 3/4" from the center of the old hole.

[/URL]https://static1.pt-content.com/images/pt/2010/07/th_HAmodDrillPrep-1.jpg (http://i736.photobucket.com/albums/xx3/Bigwalltom/Herb%20Adams%20Mod%20-%20First%20Gen/HAmodDrillPrep.jpg)

To get the 1/2" hole right I center punched the mark, drilled a 3/16" hole, then finished with the 1/2" hole.

https://static1.pt-content.com/images/pt/2010/07/th_HAmodNewHoles-1.jpg (http://i736.photobucket.com/albums/xx3/Bigwalltom/Herb%20Adams%20Mod%20-%20First%20Gen/HAmodNewHoles.jpg)

I used an air cut-off tool to cut a hole for the leaf spring. On my first cut, I took too much front to back and not enough on the sides . The side cuts must remove any curve. The metal must come straight up from the bolt hole or it will interfere with bushing.

https://static1.pt-content.com/images/pt/2010/07/th_HAmodSpringClearance-1.jpg (http://i736.photobucket.com/albums/xx3/Bigwalltom/Herb%20Adams%20Mod%20-%20First%20Gen/HAmodSpringClearance.jpg)

https://static1.pt-content.com/images/pt/2010/07/th_HAmodMountCut-1.jpg (http://i736.photobucket.com/albums/xx3/Bigwalltom/Herb%20Adams%20Mod%20-%20First%20Gen/HAmodMountCut.jpg)

This part is why First Gens require floor mod: Distance from the new hole to the floor is 1-1/8". Distance from new hole to top of spring is 1-5/8". At least 1/2" of new space has to be made above the spring.

https://static1.pt-content.com/images/pt/2010/07/th_HAmodMeasureFloor-1.jpg (http://i736.photobucket.com/albums/xx3/Bigwalltom/Herb%20Adams%20Mod%20-%20First%20Gen/HAmodMeasureFloor.jpg)

I wanted at least 3/4" above old floor surface. I drilled holes from below so I'd have landmarks to cut floor from above. I used 1/2" drill thinking I could use a jigsaw. Jigsaw didn't work because of the double floor in that area and obstacles on top around hole. Again I used a cut-off tool to cut the floor. After cutting, I realized I did not have to remove as much floor in the right side of the cut. The right side in the picture is outboard on driver side.

https://static1.pt-content.com/images/pt/2010/07/th_HAmodFloorCut-1.jpg (http://i736.photobucket.com/albums/xx3/Bigwalltom/Herb%20Adams%20Mod%20-%20First%20Gen/HAmodFloorCut.jpg)

A word about the double floor. At first I worried that the gap I cut in the double floor needed to be welded back together for strength and to keep out the elements. I concluded it does not because there are other holes in the double floor, notably the cage nuts, where elements can enter from below. As far as strength goes, I'm going to weld in DSE SFC's so I'm not worried. If you're doing this mod and not doing SFC's you'd probably want to weld the double floor again.

I left a 3/4" curve on each side so I could bend them up to form an arch. Grind the arches for welding BEFORE you bend them up. I didn't.

https://static1.pt-content.com/images/pt/2010/07/th_HAmodHolePrep-1.jpg (http://s736.photobucket.com/albums/xx3/Bigwalltom/Herb%20Adams%20Mod%20-%20First%20Gen/?action=view&current=HAmodHolePrep.jpg)

Using my MacGyver amateur metal working skills, I made a patch from 18 gauge sheet metal.

https://static1.pt-content.com/images/pt/2010/07/th_HAmodPatch-1.jpg (http://i736.photobucket.com/albums/xx3/Bigwalltom/Herb%20Adams%20Mod%20-%20First%20Gen/HAmodPatch.jpg)

[U]https://static1.pt-content.com/images/pt/2010/07/th_HAmodPatchFit-1.jpg (http://s736.photobucket.com/albums/xx3/Bigwalltom/Herb%20Adams%20Mod%20-%20First%20Gen/?action=view&current=HAmodPatchFit.jpg)

I prepped all welding surfaces with copper weld-thru primer. I ground paint off the rear seat mount next to the hole to maximize grounding. Then I MIG welded the patch to the floor by tacking around to minimize heat. This picture is before grinding. After grinding there were some gaps that needed another tack or two. Then I grinded again. After it cooled I brushed it clean and hit it with enamel until I can epoxy primer the whole interior later.

https://static1.pt-content.com/images/pt/2010/07/th_HAmodWeld-1.jpg (http://s736.photobucket.com/albums/xx3/Bigwalltom/Herb%20Adams%20Mod%20-%20First%20Gen/?action=view&current=HAmodWeld.jpg)

[MrQuick]

nice work....we did a few in the past but built a new box for the front mount. We found that after you chop all that metal out the stock perch for proper leaf eye location it tends to be very flimsy. After time it with flex, bend and create a failure point.


vince

[First Bird]

Thanks Vince. I'll take your advice and build a box for the front mount.

[MrQuick]

no problem, it also makes a great roll cage tie in.

vince

[First Bird]

Hey Vince,
I'm planning the box now. Could you tell me more about how you did yours? What thickness metal? 1/8"? Also, did you use stock bolting or weld in the box? I hope to not redo the floor welding I did. Did the boxes you made have a lid? By that I mean, did you enclose the top like the stock mounting bracket before it is cut? Appreciate any help.
Regards,
Tom

[Skip Fix]

I remember seeing an article in Circle Track years ago where Herb Adam's son built a box on a second gen to allow multiple holes for placement and they tied the roll bar to it. Maybe a Street Stock class car.

[MrQuick]

Hey Vince,
I'm planning the box now. Could you tell me more about how you did yours? What thickness metal? 1/8"? Also, did you use stock bolting or weld in the box? I hope to not redo the floor welding I did. Did the boxes you made have a lid? By that I mean, did you enclose the top like the stock mounting bracket before it is cut? Appreciate any help.
Regards,
Tom

I like to work with 3/16" and 1/4" for most suspension bracketry. 3/16" should be fine in this case.

Since you have a hole you can make a drop down box that ties into the inner rocker panel and top of the frame rail. Box is welded together out of car then clips into the hole you trim. In my case its tied into the inner rocker, top of the frame rail and incorporated into the subframe conector.
You will find that the factory floor to rocker tack welds are less than satifactory. So I would remove most of it where your box ties into. Just remove the floor tack section, not the inner rocker. You have to be carefull of the inner rocker spot welds. Try not to damage the inner rocker.
Its ok to redo the welds you did. Its part of the job. Plus, you can always use more practice. :)
Vince

[First Bird]

Vince,
Thanks for posting those pictures. That is a better way. I like that alot. I wouldn't have thought to take it over to the rocker. I definitely can use more practice welding. Hah! I appreciate your time.
Tom

[Lowend]

Nicely done...
It only took someone 40 years to figure this out :)

[bryster]

Great write up...was thinking about doing the same thing, No I am armed with more info...Thanks

[John Wright]

I welded/plated the sides of my spring perches with 1/8" material, stiffens them up pretty nice.

https://static1.pt-content.com/images/pt/2010/09/Platesforstiffeningbrackets-1.jpg

[First Bird]

Inspired by fellow Forum member John Wright, I beefed up the front leaf spring hangers before mounting them back in their stock location.

I cut three pieces, 1/8" steel, to go around the hanger. I got the curve by heating with a MAPP torch until it just began to glow, then beat it with a hammer over the anvil. I let the pieces air cool. It sure was fun getting the steel bar to match the curve of the stamped hanger. Not being sarcastic either - it was fun.
https://static1.pt-content.com/images/pt/2010/11/th_HangerFitment1-1.jpg (http://s736.photobucket.com/albums/xx3/Bigwalltom/Herb%20Adams%20Mod%20-%20First%20Gen/?action=view&current=HangerFitment1.jpg)

I tacked them together on the hanger. I kept the bushing bolted into the holes to make sure they stayed lined up during welding. I took the tacked piece off and welded the inside joint. Then bolted it back on and finished the outside joint.
https://static1.pt-content.com/images/pt/2010/11/th_HangerWeld1-1.jpg (http://s736.photobucket.com/albums/xx3/Bigwalltom/Herb%20Adams%20Mod%20-%20First%20Gen/?action=view&current=HangerWeld1.jpg)

Now that the three pieces were one, I welded it to the hanger 1 inch at a time on the highest setting.
https://static1.pt-content.com/images/pt/2010/11/th_HangerFitment2-1.jpg (http://s736.photobucket.com/albums/xx3/Bigwalltom/Herb%20Adams%20Mod%20-%20First%20Gen/?action=view&current=HangerFitment2.jpg)

The finished pieces fit in the stock location after massaging a little more room in the frame - 1/2" up and 1/8" in - for the bolt head and nut to move up higher. Lock washers don't fit now and are not needed because the nut physically cannot come off when the hanger is in place. I'll loctite them anyway though.
https://static1.pt-content.com/images/pt/2010/11/th_HangerFinished1-1.jpg (http://s736.photobucket.com/albums/xx3/Bigwalltom/Herb%20Adams%20Mod%20-%20First%20Gen/?action=view&current=HangerFinished1.jpg)