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Discussion Starter #1
hi,
i have 4 speed bars on my street strip '65 GTO...these are the stamped steel units that connect the rear lower and upper control arms. i have stiffened mine. i run a turbo 400 and my 60 ft is typically a low 1.70- high 1.60. i am wondering if i add aftermarket rear upper control arms, or some other modification, if i can ditch these bars. i thought i heard once they help protect the bulkhead above and in front of the rear end from twisting from the abuse of hard launches...i have also heard that they keep the rear end square in the car under hard launches, but i am not sure what they do. i am considering installing a rear sway bar for more uniform bite from both tires. i run a 12 polt posi...any thoughts? thanks
 

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Do not ditch the bars. They tie the upper and lower control arms together to add strength to the rear suspension as well as stiffen the frame and keep it from cracking. I like and used the UMI in my '68 Lemans build which is still under construction. They are heavy duty, pre-bent to fit under the floor pan, adjustable, and come with all the hardware to install. 1968-1972 GM A-Body Control Arm Reinforcements/Frame Braces [4028] - $129.99 : UMI Performance, Inc.

A rear sway bar may help the car launch straight and minimize rear axle twist. You may also want to consider the air lift drag bags. These are used to preload the suspension by adding a little more air to the right side bag to counter rear end twist. Air Lift Drag Bag Kits for Performance Vehicles | JEGS

Adjustable upper control arms can be used to adjust pinion angle. UMI and Spohn sell them. I went with Spohn on my build. Some drag racers will alter the pinion angle to preload it to counter the rear axle twist on launch. It is something you can play with and adjust as you go to see if it makes any difference.

If you are having any issues with wheel hop, you can change the height of the upper control arm attachment point at the rear end. QA1 Anti-hop bars, or Lakewood no-hop bars will raise the upper control arm to change geometry and typically eliminate wheel hop on hard take off's. If you don't have any issues with wheel hop, then you don't need them. https://www.summitracing.com/search/product-line/qa1-anti-hop-bars

Rear shocks are important as well, you can get adjustable drag shocks and try the different settings to see how the rear suspension reacts.

So, many options with some of them being trial & error. Keep the control arms braces and adding a rear sway bar would be my first thing to do.

Next I would go with the drag air bags. If I had any wheel hop, then the adjustable upper control arms to adjust pinion angle settings to see if this helped any & then the anti-hop bars.

I also suggest this book by Dick Miller. Lot of info and he also sells suspension parts to get your car to hook: Suspension Technical Information.
 

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The frame crossmember stiffening bars are critical to rear frame crossmember life under a performance '64-67 A-body. The stamped & rivetted design of the early frame crossmembers often created cracks on the passenger side of the rear crossmember near where the shock mounts. Pontiac issued a service bulletin on this problem, towards the end of '65 production, with dealer installed stiffening bars prescribed that were designed to merely tie the frame together. The original stamped design worked very well & is avail in reproduction formfrom Ames & other catalogue vendors. Over the years, have pulled the original frames out from under literally hundreds of '64-72 GM Abodys, as well as as installing performance built rears in many dozens more. The only frames that I've come across that were damaged in the rear shock crossmember area were early cars which had previously seen a ton of drag strip action in the late 60's & '70's. None of these damaged rear crossmember frames had evidence of originally having the stiffening bars in place.

Under the '68-72 GM A-bodys, besides factory 4 spd cars, the rear frame stiffener bars made it under, as well, numerous GM division models with automatic transmissions. At least in the '70-72 years, all 442's received the rear stiffeners. These latter body style stamped steel bars do their job very well, & they're avail, clean used, for $45-60 a pair. Have been under many high 9 sec, low 10 sec '68-72 A body bracket cars, cars with clean frames, that have had the stock frame stiffeners under them for decades & have observed no damage to the rear frame crossmember. Would strongly suggest, on anything more performance oriented than a putt-a-round cruiser, adding the stamped steel stiffeners to one's A-body as well as boxing the upper control & lwr control arms when replacing bushings. A good percentage of early 442's, at least the '66 & '67's, had boxed upper control arms which numerous early GTO racers bought over counter & added to their cars. Have a pair of these factory boxed uppers, will get a pic.

On the lower arms, mid season intro '64 442's were the first to offer the 7/8" rear sway bar. These were installed for '64 & '65 with partially boxed lower control arms. The partially boxed lowers were also used on the 201 built '65 Z16 Chevelles. The problem with the early style partially boxed lowers is the U shaped lower arm eventually cracked at the point the short reinforcements stopped. This "shortee" style boxing was replaced by fully boxed insert lower arms for '66 models. Have yet to run across such damage with clean factory style boxed '66-72 lower arms. Have what's left of a rusty pair of partially boxed lowers out of a '64 442 Coupe I drug home years ago, will get some pics, one next to never runs across these.
 

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Discussion Starter #5
re 4 speed bars--more info on my combo

Hi Jim, thank you for your detailed, insightful; and thorough response. and thanks for the helpful links. after some more research on other sites i have discovered these bars go by different names, OPG calls them trailing arm mount braces and your link calls them GM A-Body Control Arm Reinforcements/Frame Braces. We always called them 4 speed bars because they came on 4 speed cars. i will continue to use my stiffened stock 4 speed bars.

i have boxed my lower control arms and i have run airbags inside my coils for years, experimenting with different pressures. they are responsible for lowering the 60 ft to its current low 1.7s -- high 1.6s. i believe now though i have maxed out that combo. i have drag shocks front and rear, the front adjusted to let the front come up, the back shocks adjusted to squat, but the car does not raise the front much nor does it plant the tires hard upon launch. i believe the air bags were interfering with this motion preventing the rear end from squatting. acting on this i removed the air bags on both sides. before i could get valid tests of this theory, i shelled the rear end. so while it is out getting rebuilt, i am using the opportunity to examine the rear suspension. i have urethane bushings in the front and rear suspension and have been told, i need to "loosen up" the front to let it rise. i understand this to mean to loosen all suspension bolts in the front so there is no bind in the movement.

i also have considered drag coil springs front and rear. presently i have NAPA replacement springs front and rear. i do not know what my pinion angle is, but once the rebuilt rear end is re installed i will check that.

i do not have wheel hop, thank goodness.

my next move is a rear sway bar, though i hate adding weight to the car.

traction at the strip is a challenge. i run hoosier quick time pros, DOTs, 11.50 X 15, 26 inch tall on weld 8" X 15" wheels, which is about all i can get under the car, as, you no doubt know, there is precious little room under the rear of early A-bodies. i have about 10 inches of rubber on the ground.

anyway, thanks again Jim, for your detailed response, and if anyone else out there has suggestions, i would love to hear them...
thanks,
best, john
 

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Discussion Starter #6
Thanks!

thanks GTO Junior for the advice and fotos. they clearly show the bars in question. i have responded in detail to PontiacJim. you have an OHC Sprint? Too cool!!!!
 

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Discussion Starter #7
Hi Pinion Head...thank you for the in depth info on these 4 speed bars and their function...please read my previous post for more info on my combo. my lower arms are boxed, and i have stiffened the 4 speed bars, as mentioned above. i have not boxed the upper control arms. I need to check the pinion angle when the fresh 12 bolt goes back in the car and may install some after market adjustable upper arms to set pinion angle.

i am looking for a rear sway bar. again this is for straight line launch, not cornering. what is the lightest size i can go with and still get the function i am looking for on my early a-body ?
thank you,
best, john
 

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Def wouldn't ditch the frame reinforcements. Running a stock hsg 12 bolt & you used a short tail T400 (not the long tail T400 out of a big car), your pinion angle is most likely fine. On the rear sway bars, we've always run the stock 7/8" diam bars, they bolt right on with a few thin shims & are bent correctly & Ive never ran across a lighter rear bar for an A-body. This style bar is avail in repro form for $90-110. Some of the aftermarket bars are bent funny where they kick back from the rear of the boxed arms and below the center, the latter in order to allow rearends with different shaped center hsgs.
 

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Discussion Starter #9
OPG, among others, has the rear sway bar for around 100 bucks, thanks for the info...
 
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