frame twisted or trailer arm?

[keh27]

installed drive shaft and found this problem, rear end is centered from frame side to side with adjustable track bar. engine is centered in frame. But I still have the drive shaft angled toward the right side. I'm guess maybe something moved when I cut out the rear cross member to install drop member? Any one ever seen this? i'm think time to go to frame shop. dang It

[davepl]

It's hard to tell from photos. It's looks wrong in many ways but most of that is camera angles, because it looks different from one photo to the next.

I think your best bet is to get a yellow legal pad and a yellow tape measure and start measuring corner to corner on everything you can find. You'll find out what's wrong soon enough. Anything within 1/16" is "perfect". Anything within 1/8" is darn close. If you start getting 1/4" difference from opposing corners I'd really scrutinize those dimensions.

Sometimes it's just bad luck of a lot of in-tolerance things adding up to out-of-tolerance, but I'm going to guess you have a more serious misalignment.

Edit: Those are just my own personal benchmarks for placement accuracy. Not from the assembly manual or anything official.

[67ChevyRedneck]

Stupid question... is your new cross member upside down?

[Kennuck]

Originally Posted by 67ChevyRedneck

Stupid question... is your new cross member upside down?

what he said?

[hugger6933]

Look up some of my old threads I did some in the paint and body section on extensive measuring. It went in depth on how to measure the frame for square, the to make sure the cab was sitting square on the frame so that the sheet metal would fit. If the cab isn't really square on the cab when you hang the sheet metal you are just moving your problems farther out and making them bigger as you go. I also in those threads there are several they tell how to measure the front and beds as well for better gaps. I know this isn't helping you now but the measurement principle is the same. If you can't find them looking under the search X measurements. Jim

[vidman]

When I purchased my fab 9" from Moser I was talking to the sales-tech people and they told me that the stock chevy truck 12 bolt is offset 1/2 inch to the passenger side. Make sure that both rear wheels have the same backspace and measure from your backing plates to frame to get center. Make sure that your crossmember is bolted in the center of frame.

[keh27]

Originally Posted by 67ChevyRedneck

Stupid question... is your new cross member upside down?

No Sir it isn't .... but I did make sure, haha

[Fitz]

Originally Posted by 67ChevyRedneck

Stupid question... is your new cross member upside down?

Not so stupid a question because that sucker is upside down!

[mechanicalman]

Originally Posted by Fitz

Not so stupid a question because that sucker is upside down!

That's why I'm trying to get him to check his pinion angle, I know a drop member hangs the trailing arms differently to off-set lowering effect on said angle, so I was thinking he might want to know his angle and be sure if it's upside down or not (if it's upside down it will be off a country mile).

And, all the drive-shafts I've seen seem to be centered enough in the loop to not look askew. I think we have established that the rear ends are not on a perfect center from the factory, maybe the cross-members are a little off-center as well to keep it in the middle?

Thus, if the cross-member is upside down, and they actually ARE compensated on the center loop (not saying they are, I don't really know), that would compound the problem he created when he centered his rear end and hence the off-center effect between the shaft and the loop.

I think a driveline needs to be off-set at least a degree on both the vertical plane and the horizontal plane to minimize vibration, so that might be why the rear-ends are not centered from the factory.

[Bowed]

More pictures would help .

[Kennuck]

Did the old cross member have this problem? I highly doubt you bent your frame installing the drop member. If the old cross member didn't have this problem then the new cross member is where the problem lies.

[Bagd72Chevy]

The pinion on the 12 bolt is offset to the passenger side. GM almost always uses the same length axle shaft on both sides so the carrier is centered and pinion is off to the side. If the rear axle is centered in the frame then the driveshaft will be closer to the passenger side of the crossmember. And it is more than 1/2" offset. Between 1 and 2 inches.

I just discovered this on my 67 GMC (leafs). At somepoint someone changed the 12 bolt axle and welded the perches on centered off the pinion. my entire axle is over to the driver side by ~2".

Solution: Cheat the axle over 1/2" or so with the track bar or clearance the crossmember if you're worried about it hitting. I would have no less than 1/4" clearance.

[keh27]

Originally Posted by Bagd72Chevy

The pinion on the 12 bolt is offset to the passenger side. GM almost always uses the same length axle shaft on both sides so the carrier is centered and pinion is off to the side. If the rear axle is centered in the frame then the driveshaft will be closer to the passenger side of the crossmember. And it is more than 1/2" offset. Between 1 and 2 inches.

I just discovered this on my 67 GMC (leafs). At somepoint someone changed the 12 bolt axle and welded the perches on centered off the pinion. my entire axle is over to the driver side by ~2".

Solution: Cheat the axle over 1/2" or so with the track bar or clearance the crossmember if you're worried about it hitting. I would have no less than 1/4" clearance.

This is correct, after checking with a buddy and looking his truck the rear end is off to the right side he has the two piece drive shaft. The one piece drive shaft tents to make it look worse. the engine wasn't centered in the pic so it is better than the pic, I will look into again once the body is back on. I will post more pics at that tome. The X measurements are around 1/8 inch of each other . So I will let everyone know how it turns out. Thanks everyone. for all the help

[mechanicalman]

Originally Posted by keh27

This is correct, after checking with a buddy and looking his truck the rear end is off to the right side he has the two piece drive shaft. The one piece drive shaft tents to make it look worse. the engine wasn't centered in the pic so it is better than the pic, I will look into again once the body is back on. I will post more pics at that tome. The X measurements are around 1/8 inch of each other . So I will let everyone know how it turns out. Thanks everyone. for all the help

Sounds like the "X" measurements are really good.

Curious about the drop member and your pinion angle vs your output shaft angle. Is it parallel coming straight out the trans and straight out the pinion, on the same plane?

[davepl]

Yep. On my 12-bolt Camaro ('69) the fact that they used the same axle length but an offset for the pinion actually caused tire clearance issues on that one side only.

So, I bought a wheel with 1/2" more offset (inset?) for that side. Problem solved. Sold original wheel on eBay to help recover the cost.

Then converted to a Currie 9" a couple of years later. So now I have one wheel with 1/2" too much offset. Live and learn. And never sell anything, it seems.

My Currie "Crate 9" came as a box of parts. I had to press on the bearings, pretty much do everything. I have no idea what made it a "Crate" anything. Except the parts came in a crate. And this was from Summit, not craigslist.

[Lee H]

Once you get the cab, bed, and everything else installed you will need to recenter the diff and it will be a whole lot closer.

[keh27]

thanks for the comment and help so far.

[mechanicalman]

Originally Posted by keh27

installed drive shaft and found this problem, rear end is centered from frame side to side with adjustable track bar. engine is centered in frame. But I still have the drive shaft angled toward the right side. I'm guess maybe something moved when I cut out the rear cross member to install drop member? Any one ever seen this? i'm think time to go to frame shop. dang It

Good advice from others to "X" measure the frame.

Cut the cross member to install the drop member? You mean, removed the rivets? Very possible the frame sprang when you removed it, especially if it has been bent and straightened before.

Has the frame been cut or shortened?

Crazy idea, just for grins and giggles: Measure the loop area of the cross-member to see if the loop is in the exact center of the frame, or if it's off-set, and if so to which side it is off-set.

Read into the following what you will.
RULE 1: UNIVERSAL JOINT OPERATING ANGLES AT EACH END OF A DRIVESHAFT SHOULD ALWAYS BE AT LEAST 1 DEGREE.
http://www2.dana.com/pdf/J3311-1-HVTSS.PDF

Keep in mind the dynamics of a driveline operate on both the vertical plane and the horizontal plane, so you actually have 2 driveline angles to consider when you by-pass the factory engineering and install the adjustable pan-hard bar. It would be interesting if you could re-install your stock pan-hard bar and see where that might lead you.

[keh27]

Originally Posted by mechanicalman

Good advice from others to "X" measure the frame.

Cut the cross member to install the drop member? You mean, removed the rivets? Very possible the frame sprang when you removed it, especially if it has been bent and straightened before.

Has the frame been cut or shortened?

Crazy idea, just for grins and giggles: Measure the loop area of the cross-member to see if the loop is in the exact center of the frame, or if it's off-set, and if so to which side it is off-set.

Read into the following what you will.
RULE 1: UNIVERSAL JOINT OPERATING ANGLES AT EACH END OF A DRIVESHAFT SHOULD ALWAYS BE AT LEAST 1 DEGREE.
http://www2.dana.com/pdf/J3311-1-HVTSS.PDF

Keep in mind the dynamics of a driveline operate on both the vertical plane and the horizontal plane, so you actually have 2 driveline angles to consider when you by-pass the factory engineering and install the adjustable pan-hard bar. It would be interesting if you could re-install your stock pan-hard bar and see where that might lead you.

I cut the crossmember in half after removing rivets. I am wondering if it was ever straightened. no it wasn't shortened or cut...thanks

[mechanicalman]

Originally Posted by keh27

I cut the crossmember in half after removing rivets. I am wondering if it was ever straightened. no it wasn't shortened or cut...thanks

So, measure the new one to see if it's centered in the frame. Keep us posted, many minds are better than one.

[Bagd72Chevy]

His crossmember is not upside down guys. The drop crossmembers are used when more than 5" of drop is desired in the rear and they mount the trailing arms higher to correct the pinion angle. You know it is not upside down because the holes for the parking brake cables are on the bottom. Whether or not his pinion angle is correct doesnt matter for this discussion. Adjusting pinion angle will not move the driveshaft side to side.

By chance is that a larger diameter driveshaft than stock?

[Lee H]

In order to correctly measure and adjust pinion angle the vehicle needs to be fully assembled, or weighted down front and rear, and at the desired ride height. That's also the right time to center the differential.

[Fitz]

Originally Posted by Lee H

In order to correctly measure and adjust pinion angle the vehicle needs to be fully assembled, or weighted down front and rear, and at the desired ride height. That's also the right time to center the differential.

Couldn't agree more. Also, pinion angle will need to be rechecked once the suspension "settles". I put a 4-6 drop on my truck along with replacing every bushing in both front and rear. When I put it on the ground I could put my closed fist between the top of the tire and the lip of the fenders but after 3 weeks of driving and debugging everything settled and I barely had an inch between the top of the tires and the fender lip. That made my pinion angle change dramatically.

Left photo is the day the assembly was finished, right side is three weeks later. The only difference is the gas tank is full in the photo on the right and the exhaust cutouts are connected.

[no1udknow]

A simple way to see it all is to get yourself a spool of thick string and some duct tape. lay out strings laterally from matching points on the frame such as rivets, holes etc, then run a string from the rear most centerline to the centerline of the front x-member. then lay out some diagonal strings from matching points, stand back and have a look, it makes it very visual and easy to see if there is a problem with the frame.

I learned it from a thread here on the board,(http://67-72chevytrucks.com/vboard/s...d.php?t=627050 ) very helpful...

[keh27]

Thanks for all the good info guys. I WILL GIVE A PINION ANGLE READING AFTER ALL IS COMPLETED