Coil Spring and Truck Bars for a Square Body

[thompson335]

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Been working on adding some truck bars and coil spring rear suspension to my 1980 square body.

in an effort to get a better ride and hopefully some better handling I took an old 72 rear end and suspension and adapted it to my square body.

I have not been able to find much information out there on how to accomplish this, but this is what I have accomplished so far. Hopping that some of you guys that are much more accomplished on suspension than I am will chime in.

located axle position on frame prior to disassembly
Removed rear end and leaf springs
added entire rear end, truck bars, and cross member to square body frame in original axle position.

• installed cross member with brackets up and truck bars contacted frame before axle did when I moved it thru full travel, also had drive shaft clearance issues along with cab.
• installed cross member with brackets down like factory, truck bars still contacted frame before axle did (still had all the same clearance issues)

On my third try I dropped the cross member down 2.5 inches. This corrected all of my clearance issues and where it gets fuzzy for me. Do I have enough ant-squat? The bars have a 5 degree up angle on them at ride height.

Is this enough, how much higher should the front mounting point be than the rear?

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[bucket]

I don't have an answer for you, but I like what you are doing. I've always been curious about adapting the stock parts.

 

[TubeTruck]

Heres a section from a Herb Adams book

Anti-squat is not difficult to calculate, however certain values on the vehicle must be known, such as the center of gravity between the front and rear of the car, and the height of the center of gravity . With this information, a scaled drawing of the tires and this information can be made. Once a drawing is made, then the rods of the 4 bar system can be plotted onto the drawing. According to Adams, “The instant center is that point around which the linkage can be assumed to react” . To determine the instant center, the paths of the 4 bar rods are extended forward to find their intersection point. This is the point around which the rear axle would rotate if the 4 bar connected to it.


The anti-squat value is based on where the instant center falls. Using the same drawing as before, a line is drawn from where the rear tire touches the ground; to a point at which a vertical line is drawn through the front axle and the center of gravity meet. If the instant center is on this line, then there is 100% anti-squat in the rear suspension. Remember from earlier, more or less than 100% and the vehicle will rise or squat, respectively. This amount of rise or squat is relative to the instant center’s height in relation to the “100% line”.

Hope this helps. I know it confuses me a bit but I haven't done it before.

On another note I want to see how this turns out. I'm loving the concept.

 

[thompson335]

Yea I have read that before. Two things in it are causing me confusion. It is LS swapped and I have know idea where the center of gravity is. Best guess would be the cam shaft. The second is the two intersecting lines of the 4 link. With truck bars you do not have that upper link position to set up your upper line on the drawing.

 

[Craig 85]

I just watched a show the other day and on an LS motor they used the crank for center line. You may contact these guys with regards to the rear suspension. They specialize in truck arm suspensions.

Hotrods to Hell
http://hotrodstohell.net/truckarm/truckarm_index.htm

http://hotrodstohell.net/in_the_works/hillbilly.htm

 

[Catbox]

I just watched a show the other day and on an LS motor they used the crank for center line. You may contact these guys with regards to the rear suspension. They specialize in truck arm suspensions.

Hotrods to Hell
http://hotrodstohell.net/truckarm/truckarm_index.htm

http://hotrodstohell.net/in_the_works/hillbilly.htm

That was what I was just going to find.

Cool idea sir!

 

[thompson335]

looks like it hangs down a bit lower than mine. I went with what I had. I can not raise the brackets any higher because the arms will hit the frame at full compression. I measured the angle on a couple stk trucks and it was about 3.5 degrees, and mine is 5 at ride height. should be in the ball park.

That hotrods to hell truck must be a long bed because my bars go up under the cab about a foot.

 

[thompson335]

Made some notches cut the frame and fitted them. Need to start welding up tomorrow. Should give me another inch or so of travel.

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[bluex]

I have them on my truck but I used a kit from porterbuilt that he no longer makes.

Im doing an 82 now with a kit from CPP. If you need to you can call them an buy the cross member only. Look it up an you can find the install article hot rod did. So far I have the cross member an c-notch/spring mounts installed. You will probably have driveshaft clearance issues if you don't modify the stock 67-72 cross member.

On my truck an this 82 the trailing arm cross member is nearly up against the rear cab mount brackets.

The ride quality an handling are vastly improved over the leaf spring setup. You will be very happy with it imo. It's one of the best things I've done to my truck.

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[bluex]

If you're interested in my install it starts near the bottom of this page in my build thread.

https://www.gmsquarebody.com/threads/78-gmc-forever-build.16379/page-4

 

[thompson335]

very nice!
the cross member looks to be mounted very similar. so far I have no drive shaft clearance issues issues at full up or down. I do think I may have a very small clearance issue with the cab. we will see when I get it back on

Very funny, I have the very same camaro wheels on my truck, no clearance issues though. I used 1.75 adapters up front and 2 inch in the rear. we will see how that works out when I put the camaro disk brake setup on the rear.

 

[thompson335]

Here is a picture of mine before I tore it all apart

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[F-64]

I did a quick search, but I'm running out of steam.
Truck arm rear design is used by NASCAR. You should be able to find some info on the web regarding setups.
I'm sure I have a bunch of articles in my Circle Track, Grassroots Motorsports and Racecar Engineering magazines that describe moment centers, roll centers, cog, etc. If I remember correctly, you may be able to find COG by weighing your car than re-weighing again with the rear elevated.

https://suspensionsecrets.co.uk/how-to-calculate-centre-of-gravity-position/

 

[WesN]

I drag raced a 67 shorty for about 12 years and I made crossmember brackets so the front of the bars could be adjusted up or down. I used the Alston manual of that time period and did adjustments as a ladder bar. Ended up going about 2” down on front and with a 454 th400 and slicks it would carry the left front tire. Check pinion angle you can get wedge shims to go in between diff and bar mounts if adjustment is needed.

 

[bucket]

I drag raced a 67 shorty for about 12 years and I made crossmember brackets so the front of the bars could be adjusted up or down. I used the Alston manual of that time period and did adjustments as a ladder bar. Ended up going about 2” down on front and with a 454 th400 and slicks it would carry the left front tire. Check pinion angle you can get wedge shims to go in between diff and bar mounts if adjustment is needed.

This kind of info is simply awesome