In a previous post, I was advised on raising the rear end on my '70 Mach 1....

[machoneman]

for better really high speed steering control, now with a front-down angle on the whole car.

But today, looking over my stock OEM height rear shackles, it appears that adding the universal 3-4 hole longer aftermarket shackles will have them clip the rear under-the-bumper valance. And not by a little either! 

Have those doing so on a '69-'70 Stang had to trim said valance for clearance?

[Rory428]

Not really sure how raising the rear of the car is supposed to help high speed stability. Raising the rear reduces front suspension caster, which should tend to make the steering effort lighter, but more prone to wander, not really a desirable trait in my opinion.

[machoneman]

Here's my original question Rory and the replies, starting about 1/2 way down the page.

http://fepower.net/simplemachinesforum/index.php?topic=5280.15

The car has all the suspension tricks but I can't lower the front anymore (ground clearance) save maybe 1/2", tops. The idea was/is to raise the back a tad and kill the aero lift at way over 130 w/o adding a pavement-dragging, Bud Moore Trans-Am style, front spoiler. One thing I didn't explicitly mention is the all-aluminum 351W engine (save the block) water pump, radiator, p/s delete and more removed weight. It is really a lightweight in the nose, perhaps an issue compared to heavier engine combos. 

[babybolt]

So you already have a plastic factory front spoiler and the car is lowered in the front, does it have a rake when view from the side?  I think the factory Trans Am cars had a 1.5 degree rake, but just going by memory.  You could start blocking off some of the front gaps and grille opening.

[machoneman]

Thanks babybolt. Did not know of the rake angle. With a spirit level in the door sill, the car sits dead level, maybe 1/2 of one degree down. And yes, the OEM plastic spoiler.

But, my question still remains. It looks like bolting on taller rear spring shackles to raise the rear maybe 1" or so looks to have them hit the rear lower valance. Have you done so w/o interference?

[Heo]

I would lower the front if possible but if you want to raise
the rear end. Re arch the rearsprings some

[rcodecj]

Thanks babybolt. Did not know of the rake angle. With a spirit level in the door sill, the car sits dead level, maybe 1/2 of one degree down. And yes, the OEM plastic spoiler.

But, my question still remains. It looks like bolting on taller rear spring shackles to raise the rear maybe 1" or so looks to have them hit the rear lower valance. Have you done so w/o interference?

I have taken off quite a few of the longer shackles that were on cars when I bought them.
None of them ever had an issue with hitting the rear valence.
Some of them were raised up massively and still did not hit the valence.
I can't see it being a problem.
I would re arch the rear springs though instead, it will look a lot better in my opinion.

[machoneman]

Thx!

[scott foxwell]

Adding length to the rear shackles helps level the springs and reduced rear bump and roll steer. You can do the same by raising the front of the spring by reversing the eye.

[machoneman]

Thanks Scott I forgot about the reverse eye deal!

[machoneman]

Update!

Bought and installed Scott Drake's 1" taller shackles and the car now has an approx. 1.25 degree down angle. Quite happy with the nifty and well made & designed SS shackles that almost look too good to be hidden underneath! Actually got a 1 1/8th inch rise but this may settle down to 1" with some driving. Too damn cold lately but should take a run this Thursday. 

Not cheap at $60 a pair but well worth it when compared with the mere steel rail-type shackles (as I call them) that don't have formed pockets for the bushings to nest into. Highly recommended if you only need a slight rise.   

[Falcon67]

If this is drag racing, the front end caster has more to do with high speed handling than anything in the back end.  We run 2 1/2 minimum, 3+ preferrred.  My dragster has a hell of a caster angle, never measured it.

Note that moving a leaf spring up " one inch " get's you 1/2" because the spring will absorb about half the move up the shackle.

[machoneman]

If this is drag racing, the front end caster has more to do with high speed handling than anything in the back end.  We run 2 1/2 minimum, 3+ preferrred.  My dragster has a hell of a caster angle, never measured it.

Note that moving a leaf spring up " one inch " get's you 1/2" because the spring will absorb about half the move up the shackle.

THX! Poor weather hasn't allowed a drive yet but i did get a measured 1 1/8" gain, at the rear bumper, after adding the Drake shackles. I'm sure after a drive it'll settle down a tad more as my experience is like yours, about 1/2 the gain.

[TomP]

Caster... you need caster, forget about anything at the back end. You need to crank the strut rods as far as they go and shim the front upper control arm bolts and get the caster 3 degrees or more. The more the merrier, 10 is not too much but you can't get that without big changes.

Dragster is probably 30 degrees.

[machoneman]

Caster... you need caster, forget about anything at the back end. You need to crank the strut rods as far as they go and shim the front upper control arm bolts and get the caster 3 degrees or more. The more the merrier, 10 is not too much but you can't get that without big changes.

Dragster is probably 30 degrees.

Thanks! This Mustang though has a road-race set-up. Having owned and driven long ago front engined dragsters, your 30 degrees and more is what those spindly SPE-type torsion bar front suspensions had and they were  a dream at the top end. But one had to be careful backing up after a burnout as the wheels would easily flop over!

The Stang has IIRC (been a while) about 2.5 degrees caster. Yet it was all fine until I hit 130 or so, then the front got scary light. The down angle the whole car now has should help to prevent high speed lift. But the local Chicago weather has turned way too cold to test same so it all may need to await Spring.

I've also thought about adding essentially an aluminum extension to the front lower valance OE-type plastic spoiler to kill more airflow going under the car. That or fabricate a all-aluminum spoiler (not easy either to make up) that's about twice as tall as the plastic one.  More fun experimenting I guess!