71 Cheyenne suspension/wheel info

[spdrcr29]

Ok, so this is my first post, and I'm already asking questions that require alot of work from those who respond.
I have a 71 Cheyenne (my third 71-72, but the first with leafs), and I plan to run 20" rims, and possibly a very mild drop. Tire size is my problem. Currently, the size is 235/75/15. The "plus 5" size is 275/45/20, but those tires are almost a full inch taller according to everything I've read. I was looking at 255/35/20 and 275/40/20 as potential sizes, but am being told by a pair of local shops that the 255 is a "car tire", and the difference between the 275/45 and 275/40 don't actually amount to an inch when mounted on a 8.5 inch rim.

I'd really appreciate anyone with pics of any of the above sizes on their trucks to post a pic with the size/drop info.

The other question is: Are shackles going to give me an 1.5 or 2" drop without sacrificing alot of ride quality? Or, should I use re-arched leafs, or the front hangers? I'm not trying to get a wild drop, because I'll be towing a boat and a bike trailer a couple times a year, but I would like it to be about 2 shorter in the back, and about 1" shorter in the front (simple... coils). The truck was optioned with rear leaf springs.

Thanks,
Ray



1971 Cheyenne, LS-9 pkg, factory air, 350, factory "posi", etc, and it was the previous owner that forgot about the factory paint combo and doused it in red.

[70Longhorn]

I don't have any pictures for you, but here's some tire and shackle info I hope will help.

The 275/40R20s are the closest height (based on advertised dimensions, not actual tire measurement) at about .2" shorter. The difference between the 275/45 and 275/40 is almost exactly an inch based on the same calculations.

The reason the 255 can be considered a "car tire" is because of the short sidewall. A good rule of thumb is that as sidewall gets shorter, load capacity goes down. A 35 or 40 series tire will have a very low load rating.

Since you want to tow, here are the advantages to the 275/45R20 tire. Higher load rating, almost negligible size change (about 3%), and the taller tire will fill the fender opening better with less drop appear more dropped than it is.

As to the shackles, ride quality *may* change because it changes certain suspension geometries, but it's probably not noticable. You'll have pretty much the same carrying capacity, but less wheel travel to the jounce bumpers. If you have room in the fenders with those new meats you can shave the bumpers a bit and get some travel back. The only critical thing you'll need to do is check pinion angle relative to the transmission output shaft before the drop, then use shims between the springs and axle to match it up afterwards.

I'd like to see the truck when it's dropped!

[spdrcr29]

Thanks for the info,
The 45 series are gonna be the route I take....
Soooo, how um...important is it to have the correct pinion angle if I'm doing the 2 inch drop. The truck already has a 2 piece drive shaft with carrier.... I figured that alone would kill the whole shimming thing with a mild drop.

Now.... where do I get the shackles.... Does anybody know if they're the same as the 73-87 shackles????

[spdrcr29]

OK.... Now some more questions.... More of a preference issue, but I'm gonna ask anyway.....

What's gonna be better on a daily driver..... Edelbrock/Carter 600cfm or the original quadrajet rebuilt?

Second..... Can somebody send me some side pics of their bench seats (near the arm that holds the back and bottom together, and maybe the mounts)...... Just trying to cypher whether or not I have an original 71-72 seat under the reupholstered velour that was put in sometime in the 80's....

[70Longhorn]

Not sure about the shackles. Probably similar, but most likely not the same.

As for the pinion angle, I'd personally go with what feels ok. If you don't feel a vibration then it's probably good enough.

The deal is that u-joints need to run either with no angle change between the shafts in normal condition (ride height, no load, not moving), or opposing angles to cancel out vibrations. In the case of a 2-piece driveline, at least one of the three joints would need to have no angle change. I've never measured, but it's possible that the shaft at the tranny (primary) is mostly in line with the engine. In that case you could match the pinion angle to the primary to put the 2nd and 3rd joints in phase. That would probably mean shimming the pinion up, oddly enough. Shimming the carrier bearing up would work too, and in that case you'd try to get the primary and secondary shafts in line and match pinion angle to trans angle.

I hope I'm using terminology that makes sense. And sorry for the slow reply! You've probably finished it already. If so, I'd still like to see the pics!

Originally Posted by spdrcr29

Thanks for the info,
The 45 series are gonna be the route I take....
Soooo, how um...important is it to have the correct pinion angle if I'm doing the 2 inch drop. The truck already has a 2 piece drive shaft with carrier.... I figured that alone would kill the whole shimming thing with a mild drop.

Now.... where do I get the shackles.... Does anybody know if they're the same as the 73-87 shackles????