Sub frame alignment? - Lateral-g Forums

jlwdvm · #1 10-02-2015, 09:40 AM

Any good tricks out there to get the front sub on a 1st gen f-body squared up with the rear, setting wheel base, etc?

paulk68 · #2 10-02-2015, 10:11 AM

this guy makes it look pretty easy

https://www.youtube.com/watch?v=bmrrNXEbPKg

**Spiffav8** · #3 10-02-2015, 11:33 AM

Quote:

Originally Posted by paulk68

this guy makes it look pretty easy

https://www.youtube.com/watch?v=bmrrNXEbPKg

So easy your WIFE can do it! Yeah....that isn't happening. lol

**Sieg** · #4 10-02-2015, 12:42 PM

Good system........as long as the rear differential is square in the car.

The pinch weld is the most accurate reference on these cars and it's not exactly a fine line.

jlwdvm · #5 10-02-2015, 12:58 PM

I took about a week of measuring and tweeking the sub and rear end placement before I welded on my rear end tabs for my Ride Tech 4-link...everything was perfect. Then I decided to drop the front sub to pull the motor to install a Improved Racing oil pan baffle insert, and a few other things that were easier with the sub out. Everything is back together now and it is time to start tweeking again!

Ron in SoCal · #6 10-02-2015, 01:07 PM

Quote:

Originally Posted by Sieg

Good system........as long as the rear differential is square in the car.

The pinch weld is the most accurate reference on these cars and it's not exactly a fine line.

Not sure I'd trust the pinch weld Sieg, but your first point is key as a place to start. Measure the car twelve ways from Sunday and get that rear end centered as perfectly as possible. Then when placing the subframe underneath you get a reference point for a good X dimension from the front lower ball joints to a common reference point in the rear. If using rear shock mounts - or any other location on both sides such as axle tube ends - make sure they're square / centered as well!

The downside to this method is the the car may not be square to begin with, but it should track straight even if the body is a little off kilter.

There's more, but that's a good start.

EDIT: even after all that, I think mine was off by 3/16". Not sure if it was due to the subframe not being square (?), but that was as close as it would get.

paulk68 · #7 10-02-2015, 02:24 PM

Quote:

Originally Posted by jlwdvm

I took about a week of measuring and tweeking the sub and rear end placement before I welded on my rear end tabs for my Ride Tech 4-link...everything was perfect. Then I decided to drop the front sub to pull the motor to install a Improved Racing oil pan baffle insert, and a few other things that were easier with the sub out. Everything is back together now and it is time to start tweeking again!

So I am in the same boat. I have the front sub out of the car to paint and install the ridetech suspension and I have not even touched the rear ridetech bolt in 4 link heck i dont even have my rear end yet to weld the bracket too. Wich one should I put in first the rear or get the front as close as i can then do the rear?

jlwdvm · #8 10-02-2015, 04:08 PM

I squared the front up using measuring holes in the sub and the rear frame rails. After I got the front sub squared to the rear frame rails I squared the rear end to the front and set my wheel base as well. I have my car on a 2-post lift, so it is easy to take lots of measurements!

**GregWeld** · #9 10-02-2015, 06:01 PM

This is the reason for a frame table!! LOL

I'd set the car on leveled jack stands (you can use some thicknesses of steel sheet etc) - get it level fore and aft and across... I like a 6' level -- and use some lengths of steel tubing to bridge the gaps... (borrow these if you need to). Or get yourself a laser level so you have a fixed point to measure from - these have become quite inexpensive and can be used for lots of things.

Set the car down on them at the same points of contact side to side and front and back... See if the car sits level or does it rock etc. Then I'd drop some plumb lines down to tape on the floor. See if these points are "square"... same distance ACROSS in an X pattern (like squaring up anything)... and that they're the same width and distance front and back... if you can find that square.... Then connect the dots! From there you can pull a tape for take off measurements.... again using a plumb bob to a point on the floor.... The sub should be square to the other square plus or minus a 1/16th...

Using the taped points on the floor - you can pull repeatable measurements and you can reposition various parts (such as the sub) by simply realigning your floor points and the plumb bob...

Make sure you don't set up your jack stands in a place that will interfere with bolt holes etc that you'll need to access. Use tape to mark the floor where they sit so you can move them or sit them back in place if you need to reposition for some reason.

Remember that these bodies were done on an assembly line and the tolerances "then" weren't so great! Don't be surprised to find you body "off" - wheel openings side to side at different heights - or ground to various points off... These cars are designed with rubber bushings at the suspension points - and the subframe was on rubber bushings - and the body mount positions are "close enough"...

Your alignment is designed to be able to make adjustments... but you still want to start off with everything as tight of a spec as possible.

**GregWeld** · #10 10-02-2015, 06:14 PM

Quote:

Originally Posted by Sieg

Good system........as long as the rear differential is square in the car.

The pinch weld is the most accurate reference on these cars and it's not exactly a fine line.

I would agree with this point -- but most of these pinch welds have suffered some serious abuse over the years -- so it's not always a reliable point of reference. I'm seen some pretty jacked up pinch welds (literally!)