56 rear wheel placement.

[rustay56]

Hi All, I recently painted a car for a customer and as part payment we agreed that he would set up and weld in my Morrison 4 bar rear end, Williams engine mounts and Progressive Tubular X Member into my 56 sports coupe chassis. Since he was a Engineer and had done two rear ends already I thought this was a sound plan and would speed things up for me.(you guy's know where this is going don't you). The car was painted and paid for,however my chassis keep dragging on with the usual excuses and some I hadn't heard before. He finally announced after 15 months that he couldn't finish it and I could have it back. The 4 bar is tacked in place,the upper control arm mounts on the diff housing have been cut on the inside (diff center side) and rewelded to reduce the top bar angle. (something about them hitting the inside body mount). I'm going to reassemble the whole rear end assembly next week and start measuring everything and go from there. I have a S.B.C and T700 and some front engine mounts to use as a guide to set up the side mounts.Now finally my question.:) I remember something about moving your rearend back 1/2-3/4 in when running larger tyres to centralize your wheels in the opening.I plan to run 28's,have I got this correct or not.I was also told to set the trans pan parrallel with the bottom of the chassis rails and then fab up the trans mount.Is this also correct? Sorry about the long post but I thought I may as make my first one a good one;) Thanks in advance for any reply's.
P.S I will take pic's (I know the unofficial forum rules).:)
Regards Andrew.

[chevynut]

I remember something about moving your rearend back 1/2-3/4 in when running larger tyres to centralize your wheels in the opening.I plan to run 28's,have I got this correct or not.

I don't know about "centralizing" the tires in the opening, because the opening isn't round it's teardrop shaped. The stock tires are about 28" in diameter. Some people with wide tires in the 30" range do move the rear axle back to allow more clearance between the front of the tire and the rocker. Only you can decide if it's something you want to do. I would drop the body on the frame, install the tires, and see if you like where they sit.


I was also told to set the trans pan parrallel with the bottom of the chassis rails and then fab up the trans mount.Is this also correct?

The engine and transmission should be set up at about 4 degrees down in back, which is the stock angle. Once you locate the engine mounts, set the engine at the 4 degrees and install the transmission mount. Also, the pinion angle should be set to match the engine angle under load. I personally would set it at about 1/2 degree or so less than the engine angle with a 4-link.

[Rick_L]

Just wanted to reinforce what Cnut said. There's no need to move the axle back until you get to a tire greater than 29" tall. And if you have a proper axle width, you don't need to with a 29"+ tire if you have a big enough wheel tub. 29" tire and a stock width axle is the limit on clearing the outer body work (front part of the wheel cutout). With a narrower axle you clear the outer body, but over 30" or so you need a "race tub" not a stock one.

To me, moving the wheel (axle) back in the wheel opening doesn't look right. I guess that's subjective, but my perspective is that the Chevy stylists got that right, don't change it.

Another possible unintended consequence of moving the axle back is possible clearance issues with the fuel tank, etc., but it's probably not a problem if you only move it 3/4" or so unless it's a wagon with a stock tank and a non-stock axle.

On the driveline angle, put it down 4 degrees in back. This is the crankshaft/transmission shaft centerline. Don't depend on the bottom of the trans pan to be parallel to the transmission centerline unless you verify that it is.

Hope you get the rest of your money from the derelict.

[rustay56]

Thank You Rick and Lazlo for your prompt reply,s.
Lazlo, The body is at my workshop and the chassis is 600 miles away at a friends house.That's as close to home as he could derelect could deliver it.(but that,s another story).The front suspension has been removed and he hasn,t marked the original rear axle center line onto the chassis as a reference,so I have no idea what my wheelbase is.My friend that now has my chassis has a 57 chassis with a LS1 4L60 conversion in his shed that we can use as a reference.I have down loaded the Morrison 4 bar installation instructions and will take them with me. Im abit concerned that he has cut the upper control arm mounts and changed the angle inwards.Triangulated 4 bar is a compromise at best without narrowing the angle and making it worse.
Rick L , I didn,t realise you could get a 29in tire in a standard wheel tub so if I need to go abit larger I can, I have 4:11 gears already setup.My axle width is 56in face to face.I will take your advice and leave the axle in the standard position provided the cross member has been placed correctly?.As far as getting some money back,I believe in karma, that shit will come back on him.I can let it eat me up or just move on and dont give it another thought,I think I,ll do that,I,ve got a 56 to build,let the fun begin :)
Regards Andrew.

[chevynut]

If the front suspension has been removed it's really easy to find your correct wheelbase. Under the front upper a-arms there are holes that are the "datum" for the frame. All measurements are taken from the center of that hole.

http://www.55-57chevys.com/index.html

If you look at the GM assembly manual in the link above (section 2 sheet 17) you will see all these measurements. The front spindle centerline is 2" back from the center of that hole, and the rear axle centerline is 117" back from the center of that hole (115" wheelbase). Doing a little math you can also see that the rear axle centerline is 44.62" forward of the center of the rearmost bumper bolt hole.

I would draw a vertical line on the frame hump where the axle centerline is supposed to be and drop a plumb bob down from there to see where your axle actually is. You sure don't want it too far forward.

Hope that helps.

[rustay56]

Thanks Lazlo. That's exactly what I need.

Regards, Andrew