1971 Chevy C20 Trailing Arm Suspension

[Shaky]

Meh - split the difference. (215 + 145)/2 = 180.

[jbwolfe]

Originally Posted by Shaky

Meh - split the difference. (215 + 145)/2 = 180.

I don't know, there seems to be quite a bit of variation. I found this thread:

http://67-72chevytrucks.com/vboard/s...d.php?t=316066

[Shaky]

Well, as a mechanical engineer I'll say this: torque is not really a good way to preload a bolt.

What an engineer really wants is to stretch the bolt a certain amount. Too little and the fastener won't keep the parts together properly. Too much and you'll yield the bolt and it'll break very quickly (if it doesn't just break during torquing). This is why serious engine builders will measure the rod bolt stretch when building a high-performance engine, not torque.

In aerospace applications, where weight is critical, this becomes very important since you don't get the luxury of having a lot of extra margin on your fastener.

In automotive, especially older American trucks, there's a lot of extra margin. (Example: my buddy had a '67 C10 with rusted out lower front control arms. At highway speeds he hit a rim and tire that were lying in the road and it didn't even affect his alignment).

So, the general engineering rule of thumb for preload relative to torque is: for a given torque you will see +/- 30% scatter on preload! I would assume the GM engineer in 1971 knew this and chose his torque value accordingly.

What does all this mean to me? On a suspension fastener on a '67-'72 Chevy pickup I don't get too carried away on getting the exact torque listed in the manual. I make sure the threads in the nut and (in this case) the U-bolt look pretty good. I put them together dry (oiling the threads will change the amount of preload for the same torque) and torque it.

Do I use a torque wrench? Yes, I do. But if I'm a little under or over I don't sweat it because I think the GM engineer factored in a lot of allowable variation.

Now a nut being used inside a helicopter transmission may also be assembled using torque (not stretch) but that is done with a recently calibrated torque wrench, with close-tolerance rolled threads, in a temperature controlled environment, and by trained personnel. So the scatter is much smaller.

[jbwolfe]

Originally Posted by Shaky

Well, as a mechanical engineer I'll say this: torque is not really a good way to preload a bolt.

What an engineer really wants is to stretch the bolt a certain amount. Too little and the fastener won't keep the parts together properly. Too much and you'll yield the bolt and it'll break very quickly (if it doesn't just break during torquing). This is why serious engine builders will measure the rod bolt stretch when building a high-performance engine, not torque.

In aerospace applications, where weight is critical, this becomes very important since you don't get the luxury of having a lot of extra margin on your fastener.

In automotive, especially older American trucks, there's a lot of extra margin. (Example: my buddy had a '67 C10 with rusted out lower front control arms. At highway speeds he hit a rim and tire that were lying in the road and it didn't even affect his alignment).

So, the general engineering rule of thumb for preload relative to torque is: for a given torque you will see +/- 30% scatter on preload! I would assume the GM engineer in 1971 knew this and chose his torque value accordingly.

What does all this mean to me? On a suspension fastener on a '67-'72 Chevy pickup I don't get too carried away on getting the exact torque listed in the manual. I make sure the threads in the nut and (in this case) the U-bolt look pretty good. I put them together dry (oiling the threads will change the amount of preload for the same torque) and torque it.

Do I use a torque wrench? Yes, I do. But if I'm a little under or over I don't sweat it because I think the GM engineer factored in a lot of allowable variation.

Now a nut being used inside a helicopter transmission may also be assembled using torque (not stretch) but that is done with a recently calibrated torque wrench, with close-tolerance rolled threads, in a temperature controlled environment, and by trained personnel. So the scatter is much smaller.

I appreciate your input. I also talked to a friend of mine who is a mechanic, and basically said the same thing, that there is a lot of tolerance, and to not worry to much about it.

Given that they are 3/4" U-bolts, I think I may torque them to around 200 ft lbs and call it good.

[Shaky]

Originally Posted by jbwolfe

Given that they are 3/4" U-bolts, I think I may torque them to around 200 ft lbs and call it good.

I think that will work. Check 'em again after 500 miles and call it good.

[jbwolfe]

Originally Posted by Shaky

I think that will work. Check 'em again after 500 miles and call it good.

Sounds like a plan. Thanks for all your help Shaky!

[jbwolfe]

I finally finished installing the new control arms! I borrowed a click torque wrench and clicked everything down to the specs in the manual, except the u-bolt nuts which I torqued to 200 ft lbs. Everything went well, I ended up replacing the shocks as well. The old ones worked, but when I was putting them back on I realized that they didn't match. One was about a couple inches shorter, and used different bolts. So I bought a new pair from Napa.

When I put it back together, I used all new nuts and bolts and washers, new shock plates, new u-bolts and nuts and washers, new spring clamps. I think the only things I didn't replace were the springs.

The truck hadn't moved in over 8 months. Hadn't started it in 6 months. I put the jumper cables on it, checked all the fluids and added a quart of oil, pumped the gas a few times and it fired right up! Oil pressure went right up to normal immediately. Drove it up and down the road a bit. Sure feels great to have that thing back on the road! Just have to insure and get tabs. Gonna haul some scrap metal in this Tuesday.

Here are some pics of the final product. Thank you all for your help everyone! It seemed like an overwhelming project at first, but I got through it and feel more confident working on things now.

Next project is going to have to be wheel seals and breaks...

[Shaky]

Looks sweet. Nice truck!

[jbwolfe]

Thanks! It's been a learning project for sure. I'd like to learn to weld, and do some body work on it.

[68 TT]

Glad you got it back together and back on the road. Great work.

[jbwolfe]

Thanks! I've used it to haul two loads of scrap metal into town. First load weighed in at 2100lbs, second load weighed over 2800lbs. The truck was barely squatting, and drove great! I noticed inside the glove box that it was ordered with "Heavy duty rear coil springs".

[Tinkermc]

looks like 1 heck of a lot of work so far, newer 1s sure look nice

[tlyday]

Thank you so much for starting this post. I have been struggling with how to go about this same procedure for a few weeks now. My trailing arms arent in as bad of shape as yours were, but they still need to go. Bought a parts truck last night just for the trailing arm assembly and I am planning on getting started this evening. Your post has given me a good plan of action. THANKS

The Blue/Green 68 is my project.

The Primered pos is my parts truck.

Final pics are of my original tailing arms

[Shaky]

Originally Posted by tlyday

Final pics are of my original tailing arms

"Air-conditioned".

[Shaky]

Hey jbwolfe - time to re-torque your U-bolts!

[gale]

Originally Posted by notsolo

Nice used trailing arms show up here or on Ebay from time to time, I found a nice set of 3/4 ton from California a few years ago came off a 1964 3/4 ton....The difference between 1/2 and 3/4 ton is a flat strap welded top and bottom to reinforce for heavier load.

May depend on the year. My '67 C20 has a formed steel channel on both sides of each trailing arm. The two halves plus the two reinforcement channels are riveted together.

I bought a pair on this list and they look good but shipping will kill you.

The placement of the reinforcement from the factory surprises me because midspan is not where they usually give up.

Before I ran across the pair on this site I had planned to fabricate them from 3" channel (C3x4.1) but that is quite a bit of work.

[jbwolfe]

Originally Posted by tlyday

Thank you so much for starting this post. I have been struggling with how to go about this same procedure for a few weeks now. My trailing arms arent in as bad of shape as yours were, but they still need to go. Bought a parts truck last night just for the trailing arm assembly and I am planning on getting started this evening. Your post has given me a good plan of action. THANKS

The Blue/Green 68 is my project.

The Primered pos is my parts truck.

Final pics are of my original tailing arms

You're welcome, hope it helps. I know how you feel, my truck sat around for about six months before I finally decided to tackle it, and I'm glad I did, because I've gotten a ton of use out of it since I replaced the trailing arms.

One thing you might consider, don't bother trying to take off the old u bolts, get some cutting wheels for your angle grinder. I made the mistake of trying to use a small air cut off tool, and it worked, just took forever. Mine were rusted so bad that even after cutting them off I couldn't have pulled them out of the control arms to re-use the shock plates if I wanted to. Other than that, everything either came apart easily or broke off.

Nice truck! Looks like about the same color as mine. I love the 67-68 grille. Keep us up to date with how it goes.

Originally Posted by Shaky

Hey jbwolfe - time to re-torque your U-bolts!

Thanks for the reminder Shaky, been meaning to do it, but haven't gotten around to borrowing a torque wrench. I really just need to buy one.

[Motoxrrar]

I would personally convert to leaf!!!! Pretty easy. Mostly it's all bolt in stuff. If your junk yard has a bunch of leaf trucks you could probably get away with same or less cost and have a better modern setup. The trailing arms are nice for hotrods or drag trucks but not the best for a work truck. I've had my leaf for years. 15+ hauled well over its max before never had a problem.

[jbwolfe]

Originally Posted by Motoxrrar

I would personally convert to leaf!!!! Pretty easy. Mostly it's all bolt in stuff. If your junk yard has a bunch of leaf trucks you could probably get away with same or less cost and have a better modern setup. The trailing arms are nice for hotrods or drag trucks but not the best for a work truck. I've had my leaf for years. 15+ hauled well over its max before never had a problem.

Thats an option, but have you ever hauled a load in a truck with trailing arms and good coil springs? In my experience, its actually a better setup than leaf springs. The trailing arms seem to distribute the weight more evenly between the front and rear of the truck. The trailing arms also eliminate wheel hop. I've hauled a lot of weight in my truck with the new trailing arms. I've come out of the gravel pit with other 4,000 pounds of gravel in the back, and more than a few times I've had over a cord and a half of green firewood in the back, which I estimate weighing close to 6,000 pounds. I've never bottomed it out, it handles the weight quite well going down the road, and from looking at it loaded you'd never know there was that much weight in it.

[Motoxrrar]

Actually I have had both trucks. Trailing arms and leaf. My current leaf I have loaded close to 12,000 lbs on it. It did squat down level with that much weight. But pulling that I had no problems, never had a problem with wheel hop. That trip was a good 800 miles I drove with that load. I prefer my leaf over the trailing arms based on my experience with both trucks. If I were to build a hot rod though I would go with trailing arms. But a work truck the ride seems to be nicer with leaf overall. My opinion.

[jbwolfe]

Originally Posted by Motoxrrar

Actually I have had both trucks. Trailing arms and leaf. My current leaf I have loaded close to 12,000 lbs on it. It did squat down level with that much weight. But pulling that I had no problems, never had a problem with wheel hop. That trip was a good 800 miles I drove with that load. I prefer my leaf over the trailing arms based on my experience with both trucks. If I were to build a hot rod though I would go with trailing arms. But a work truck the ride seems to be nicer with leaf overall. My opinion.

Wow thats a lot of weight, what on earth were you hauling? What truck do you have? Do you have the stock leaf springs? I've only had problems with wheel hop in my half ton ford with a 2.75 axle, and that's only if I'm doing city driving with no load. I'm going to put 3/4 ton leaf springs in my ford though and that should help if not eliminate the problem.

[Motoxrrar]

I have a gmc Sierra grande came with a camper originally. Stock suspension for the truck. I was hualing stud corral panels stack on a flatbed semi trailer it was probably a good stack about 15ft high. Wasn't my first choice but it was the only solution at the time. My dad had a 2005 ford f250 at the time with a v10 I wAs able to out pull him going uphill with my 383 I build for the truck at the time.

[jbwolfe]

Originally Posted by Motoxrrar

I have a gmc Sierra grande came with a camper originally. Stock suspension for the truck. I was hualing stud corral panels stack on a flatbed semi trailer it was probably a good stack about 15ft high. Wasn't my first choice but it was the only solution at the time. My dad had a 2005 ford f250 at the time with a v10 I wAs able to out pull him going uphill with my 383 I build for the truck at the time.

What year? 3/4 ton? So the 12,000 pounds was being pulled on a trailer? Yeah I've never understood the idea of the ford V10, seems like a built V8 or a diesel would be better.

[Motoxrrar]

Yeah it was on a trailer had the bed loaded down too with hay. 3/4 ton

[jbwolfe]

Originally Posted by Motoxrrar

Yeah it was on a trailer had the bed loaded down too with hay. 3/4 ton

Wow that would make a good Chevy commercial. I wonder though how much weight was actually sitting in the bed of the truck. Thats were you really test the suspension.