FlatOut with C4 suspension build... Questions/Issues- HELP!!!

[gearhead2069]

So I am currently working on a 51 3100 and my plan is to install an 87 c4 front and rear suspension. After reading some other builds here, my confidence was built up enough to do it for myself. The problem is that I keep running into issues when it comes to the geometry and directions for the suspension hardware that I had sent to me. I need help.

So you can see the issues that I came into with the front suspension on my build thread. I was able to work around it and get it to work.

http://67-72chevytrucks.com/vboard/s...d.php?t=626397

So now for the issues with the rear suspension. I am calling on some assistance here because I really want to think that the issue is my own instead of it being Flatout's. I have completely followed the instructions that came with the hardware without fault. I keep going back and measuring everything to find the measurements are as described in the instructions. I started out to find my wheel centers, marked them were I thought they looked good then measured 6 1/4 inches back to find the positioning for the rear differential crossmember just as instructed. Followed the rest of the directions and tacked it in. Stood back to find that my wheels are now sitting way to far back (according to my liking).



So the only issue that I could imagine there being that would throw the wheel centers off is here where the half shafts do not appear to be straight from one end to the other. They are actually angled toward the back of the truck.



So the question is for those who have had experience with either this kit or with c4 suspension components. Is this the way that the IRS geometry is supposed to look or are the components somehow out of place? Have any of you who have taken the same route had any of the same issues that I have had with getting the FlatOut kit to fit?

I'm not an experienced builder by any means as this is my first but I have enough mechanical knowledge to get me into trouble. If any of you could help shed some light or offer advise, it would be greatly appreciated!

[1project2many]

The design of the suspension could allow the wheels to sit rearward of the diff but in OE application this isn't so. Have you managed to attach the trailing arms rearward of where there they need to be? Or do you have the suspension so low in the photos that the arms are not at the correct angles?


These images might help you work out where you need to make adjustments. Sorry the chassis line drawing is so small. I can't seem to find a larger version for ya.

[gearhead2069]

Originally Posted by 1project2many

The design of the suspension could allow the wheels to sit rearward of the diff but in OE application this isn't so. Have you managed to attach the trailing arms rearward of where there they need to be? Or do you have the suspension so low in the photos that the arms are not at the correct angles?


These images might help you work out where you need to make adjustments. Sorry the chassis line drawing is so small. I can't seem to find a larger version for ya.

Thank you for your response and the EXTREMELY helpful spec sheet!

I actually have not attached the trailing arms at all. When I mounted the diff in the frame the lower control arms actually pushed the half shafts rearward like you see them in the previous picture and I have a Lot of resistance when trying to pull them straight when the diff is mounted solid. That is the only reason that I was asking was to see if this was an OEM spec. The half shafts are sitting at a 2 degree downward angle as was explained in the FlatOut instructions.

[starterman99]

on my 1950 I have the rear axle centerline mocked up at 36 5/8 from rear of frame.the batwing crossmember is 6 inches to the rear edge of the crossmember from centerline.I have the trailing arms and all mocked up.

[1project2many]

Too much tow out (tie rods too short) would pull wheels to back.

There's different width C4 (early and late). Did you get all parts from same vendor as instructions? Wider diff and narrow instructions would cause wheels to want to to be off center. Or potential parts mismatch...

[gearhead2069]

Originally Posted by 1project2many

Too much tow out (tie rods too short) would pull wheels to back.

There's different width C4 (early and late). Did you get all parts from same vendor as instructions? Wider diff and narrow instructions would cause wheels to want to to be off center. Or potential parts mismatch...

Well, the instructions for the hardware came from FlatOut whereas the IRS was purchased used off of a what I am assuming was a wrecked car. I believe that the later models had a track width of around 1 inch wider. That was one of the questions that was asked when I bought the hardware. Thanks for the idea on the tow out. That will be the first thing I try next time I get some time off work. I'll keep you updated. I can't express my appreciation.

[gearhead2069]

Originally Posted by starterman99

on my 1950 I have the rear axle centerline mocked up at 36 5/8 from rear of frame.the batwing crossmember is 6 inches to the rear edge of the crossmember from centerline.I have the trailing arms and all mocked up.

If you don't mind me asking, what diff/year c4 parts are you using?

[starterman99]

I believe its 1985 c4 dana 36 I got them from a guy who parted out corvettes.that is what he told me it was from.

[starterman99]

this is my set up so far

[gearhead2069]

Originally Posted by starterman99

this is my set up so far

What did your pinion angle come out to be? If you used the FlatOut kit did you place it at the recommended 2-4 degrees or did you place it level? I only ask because obviously with the 2 degree pinion angle the rear diff crossmember that bolts the batwings to the crossmember could not be placed in level front to back. So my only thought is that I worry that this may cause the bushings to wear out prematurely. I don't know... Like I said before... Limited knowledge. So I am only going off of the speculation that more pressure will be placed on one side of the bushing wrather than it being distributed across the entire bushing. Of course, the front crossmember that bolts where the torque tube used to is level. What is your measurment between the two crossmembers if you don't mind me asking?

[1project2many]

Universal joints last longest if they rotate through an angle of 2-7 degrees. Additionally, the torque reaction from the diff causes it to try and rotate up every time the vehicle accelerates. It shouldn't be too hard to angle the rear mounts but if you install them at 0 deg the bushings on the batwing can handle it.

[gearhead2069]

Originally Posted by 1project2many

Universal joints last longest if they rotate through an angle of 2-7 degrees. Additionally, the torque reaction from the diff causes it to try and rotate up every time the vehicle accelerates. It shouldn't be too hard to angle the rear mounts but if you install them at 0 deg the bushings on the batwing can handle it.

Yeah, I set the pinion to a 2 degree with the crossmember bolted to the batwings when I was mocking placement. I am assuming the batwings are level with the pinion as when I measured the angle of the crossmember it too came out to be a 2 degree angle. I tacked it in at 2 degrees and decided to search the wealth of knowledge here to ensure it will work out alright. So, I guess it will be fine then.

Thank you for setting me straight.

[starterman99]

I Don't know about yours but on mine the crossbar that mounts to front of pinion,the torque tube is not straight.when they welded the differential mount to it the bar bent slightly and as it is offset to one side the measurements are not equal on both sides. I had to play with it to get it halfway straight.My pinion angle is set at 2 degrees.I had to set it all up and tack it in a couple of times to get it where I liked it.One thing I did was to replace the bushings in trailing arms and batwing before I started, because the bushings are worn and the bolt holes were not straight.That way I can get it all straight and true from the beginning.

[gearhead2069]

Well, I just wanted to thank you guys. First thing I did when I got off of work this morning was open the tie rods up a little and the half shafts straightened right up. I was sweating there for a minute. I guess when I removed the mono-leaf it allowed them to shift back towards the rear of the diff... Or at least that's what makes sense in my mind.





I am assuming that it is the trailing arms that will keep the half shafts straight once I start trying to set the correct toe?

[gearhead2069]

Originally Posted by starterman99

I Don't know about yours but on mine the crossbar that mounts to front of pinion,the torque tube is not straight.when they welded the differential mount to it the bar bent slightly and as it is offset to one side the measurements are not equal on both sides. I had to play with it to get it halfway straight.My pinion angle is set at 2 degrees.I had to set it all up and tack it in a couple of times to get it where I liked it.One thing I did was to replace the bushings in trailing arms and batwing before I started, because the bushings are worn and the bolt holes were not straight.That way I can get it all straight and true from the beginning.

Wow... I didn't notice that until you said something about it. That crossmember is sure enough bent. I guess I'll be pulling that back out and taking a torch to it.

[tmoble]

A month or so late, but when I did the research on my install I found a couple places that mentioned that the half-shafts angle forward a couple degrees. U-joints don't like to run straight, they need to have a little angle. u-joints also don't like going through an over-center type condition while stressed. I think GM engineered the deal so that the half-shafts are angled forward slightly and will move forward a little more under hard acceleration, avoiding the over-center deal.

[FarmerSid]

gearhead, did you find that the measurements from the instructions did indeed put the axle in the center of the wheel opening. In the instructions, where did it say to locate the two cross members?

[gearhead2069]

Oh, I completely forgot about this thread. So I continued to have problems with getting everything line up how I wanted. When I brought out the toe rods the axles would correct but then I would have issues with the toe being set way too far in. I ended up getting really frustrated with it because it just wasn't making sense in my mind so I went to a Corvette forum. One member there diagnosed that the bushings in the camber arms were probably worn out and causing the camber arms to pull the half shafts back. I still didn't understand how those bushings would cause an issue as their articulation is supposed to be up and down not side to side. Well, I unbolted the camber arms and sure enough it was like letting a spring loose. They snapped back and as I started looking, apparently the bushings have been breaking apart and the bits were collecting on one side of the joint causing them to pull backwards... Ridiculous!

FarmerSid, I'll have to look back at the instructions to see where they said to place the cross-members. I do remember that once the cross-member that the bat wings are bolted to was mounted, I bolted the front cross-member bracket to the diff, the front cross-member to the bracket and the measurement they said that I should have wasn't the measurement I was seeing between the front cross-member and the rear. (I want to say that they said it should be a meaurment at 7 inches between the two and once everything was positioned where it needed to be, my measurement was coming much different than that.) Like I said, I'll have to look back over it for you once I get off work in the morning.

[FarmerSid]

I didn't realize you had started this thread either. I know I will have to refer to it as I'm going to have to make my own cross members etc to install my C4 IRS in my stock frame. I will continue with questions I have for you in your build thread to keep things together.

Cheers!

[4544speed]

I sure wish that I had seen this post. I installed the Flat out kit on mine in 12-2013. I had plenty of problems. The instruction measurements were not correct in a few areas. Mine has been installed and I took it back apart for painting. I did not have the issue with the half shaft that you had. Keep this post updated if you will. Thanks

[gearhead2069]

Originally Posted by 4544speed

I sure wish that I had seen this post. I installed the Flat out kit on mine in 12-2013. I had plenty of problems. The instruction measurements were not correct in a few areas. Mine has been installed and I took it back apart for painting. I did not have the issue with the half shaft that you had. Keep this post updated if you will. Thanks

Yeah... I've pretty much continually had issues with the install. Keep finding little things here and there that I have had to remove and correct. Unfortunately I can't say that I am entirely impressed. Although the instructions for the front suspension was in no way correct, I was able to work around the inconsistencies and get things done. For the rear suspension... All I can say is that there was a lot of frustration. I am hoping that everything has been worked out at this point.

[Blue1952]

Installed one also, instructions aren't very clear and after many phone calls that were answered pretty quick, the calls started going to voice mail and not answered.

[4544speed]

They did the same to me. There are too many items that are questionable in the limited directions they give. Mine is not on the road yet as I am still working on replacing floors but I was not impressed with the directions or the answers they gave to questions. IT was like they had never actually installed one themselves.

[Blue1952]

That's what I was thinking, and when I asked tech questions about dimensions they couldn't give me answers, very frustrating to say the least for what they get on the pieces. My truck isn't on the road either, hopefully soon.