need help with my suspension 1970 gmc

[bigboybodry]

I need some help with my suspension guys. My truck is a 1970 gmc bbc truck th400 12 bolt rear end. I have no idea what it weighs and the chassis guys around here do not want anything to do with it. I took it to the track last friday night for the first time with the new engine/setup and it isn't even trying to transfer weight to the rear tires. I have the qa1 stocker star shocks on the front with the upper shock mount moved to the top of the frame. moroso front spings but I don't remember the part number. the rear has strange single adjustable shocks and moroso springs and again I don't know the part number. the battery and 20 gallon cpp fuel tank are mounted behind the rear axle. I called moroso and they want to know the front and rear weights etc....... also I have heard about lowering the trailing arms on the crossmember whats to this? There is a video of the run at the track in my sig. I did a burn out and then they put some vht down and still spun broke loose when I bumped second also. Thanks for any input

[Marv D]

The truck appears to pitch rotate at the hit,,, but the tire looses bite and lets it fall. I'd experiment with tire before going too deep into chassis changes. Lots of trucks kicking booty with the stock suspension. No reason you can't

I didn't see any flex in the sidewall,, what tire is that and how much air you running

[bigboybodry]

mt stiff side wall 31 10.5w on a 16" wide wheel and 18psi of pressure

[BigDan3131]

Umm if the tires are 31x10.5w why are you using a 16" wide rim for a tire with only 10.8" of tread?

[RatPwrd72]

The tire has to work and I'd say you don't have enough hit to work a stiff sidewall slick. More HP, a transbrake or a manual trans would warrant the stiff tire.

IMO, you've changed too much of the stock pieces that work really well. Good luck.

[Super73]

Watching the video, it looks like something in the front suspension to me. Looks like you are hooked initially and then loose traction 2-3 feet out.


Just a few questions on the chassis,

How much upward front travel do you have before you pick the tires up? Truck looks fairly low so I would assume a good amount.

You mention moving the front shock mounting location, is this causing any binding through out the travel of the control arm? Best way to find out is rest the chassis in the air of jack stands, pull the springs and try to run the control arm / spindles through their full motion (they need to be assembled like they would be when driving).

How easy are both the lower and upper control arms to move with out the spindle attached?

Are you running a front sway bar?

What setting did you have the front shock in?


If there is any bind or lack of front suspension travel, you could hook initially but have a problem hooking 2-3 feet out. As the front rises, your truck is going to want to move out. If the front binds and or has no more travel, then you are now trying to pick up the full weight of the shocks, springs, control arms, spindles, brakes, wheels and tires. If you don't have enough hit and power to pick all that up the weight goes in the opposite direction, back down. When that happens you take weight off the rear tire and spin.

The front sway bar, if still on, will allow some travel, but it wants keep everything even between the two control arms, slows down front suspension reaction and can bind things up with the more travel you ask of it.



Do you have any kind of sway bar / antiroll bar in the back? If not, I would suggest getting one, or if it is the factory one, put a better one on it.


Truck sounds great. Listening to it though out the pass I think it could use a bigger converter, at least if you are going to keep it NA.

[bigboybodry]

I will check the upward front travel tonight
the front shocks were all the way down
the shock mount on the front is from cpp and the angle is the same as stock
the arms felt good when I assembled the front end
no front sway bar
I have a bickel anti-roll waiting on the chassis guy to get around to installing it.
the converter is a nos converter built for this engine and truck so should come on on nos
I know on my camaro with the big block in it you car grab the front fender and pick the front end up some on the truck you cannot I feel like its a spring issue.
I do have travel lemiters on the front but they don't come into the factor until the shock is almost fully extended.
how would I know what spring to order without having some Idea what the front weighs

[bigboybodry]

Originally Posted by RatPwrd72

The tire has to work and I'd say you don't have enough hit to work a stiff sidewall slick. More HP, a transbrake or a manual trans would warrant the stiff tire.

IMO, you've changed too much of the stock pieces that work really well. Good luck.

the motor put down 819hp and 756 ft so I know the power is there and its a nos motor
I think the truck should have more upward travel on motor than it does using the foot brake. It use to 1.49 60 foot with a cast headed 461 in it and now it has a little lighter front end,alot more motor and aluminum heads(lighter).
I know its all about dialing it in just going to take some time and help from you guys to point me in the right direction.

[Marv D]

I think I this can explain what were saying about the tire not 'working' ... HOOK requires the tire to work. Your's isnt. Shauns points about the front suspension ,,all I can say is DITTO. If the front sin't working the back ain't gonna either.


and speaking of the front working,, here's my old 12 second combination. Look close, you can see the sway bar! Stock GM front suspension and I relicated the shock mounts to give a set of CE cheepie adjustable front shocks LESS effect. There on 60/40 in this pic. It would dead hook with a little 480HP motor and a 4500 stall Midwest converter. But the rear suspension worked,, allowed the front to work

WOW,, huge picture,, try a link
http://www.small-block-chevy.com/truck9.JPG

[GMC70c20]

I would say there's a trick or 2 left to do with the front end before you start working with rear end geometry. I'm assuming the front suspension is stock minus the shocks, it heavy plus throw a bbc and a turbo 400 at it and it gets really heavy. I would try reducing the front shocks to a 90/10 setting or something close then stuffing the lightest spring in the front end that will fit and not bind at ride height. This will put some pre-load in the front end so it will transfer weight easier. As for the rear the reason people say lower the front of the control arm is using "resultant vectors" the suspension will push in a line more direct to the center of gravity of the vehicle. Essentially what is happening is you are not transferring enough weight to the rear tires and the torque the engine is applying to them is more the the tractional force that the tires can create with the specified vertical load. You should try to get your hands on a copy of the book "door slammers" it has ALLOT of good info in it.

[bigboybodry]

Thanks for the info I'll order the book today and a set of scales
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[Super73]

In addition to Morgans book door slammers, he has a 5 DVD set of one of his seminars. Worth every penny.

Dave posts from time to time on the dragstuff web site (.com or .net). He has said that the icg distances published in the book, to add I think 4" to them.
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[bigboybodry]

in the vedio the front shocks are turned all the way down and it still doesn't try to transfer.

[bigboybodry]

marv d what size whee and tire are you running on your truck side wall etc..?

[Marv D]

Ain't no secrets here, Motor made 666HP at 6400,<588 tq @ 4900> trans is a Hughes TH400 with a brake, Converter is a Midwest 4800 stall. 9" with 4.30 gear, Launch at 4500 on a rev limiter wired in with the brake. Tire is a Hoosier 33x17 QT Pros on a 15x15 rim @ 10.75 psi, 60' is 'usually' in the mid 1.4's,, on a loose track 1.5's. Going to get axle weights and verify total weight this week I hope.

[fatgump]

Originally Posted by Marv D

Ain't no secrets here, Motor made 666HP at 6400,<588 tq @ 4900> trans is a Hughes TH400 with a brake, Converter is a Midwest 4800 stall. 9" with 4.30 gear, Launch at 4500 on a rev limiter wired in with the brake. Tire is a Hoosier 33x17 QT Pros on a 15x15 rim @ 10.75 psi, 60' is 'usually' in the mid 1.4's,, on a loose track 1.5's. Going to get axle weights and verify total weight this week I hope.

666HP in Hell's training ground. Is there something your not telling us?

[Super73]

Here is a link to the Dave Morgan's input on 4-link's that I mentioned: http://www.dragstuff.com/forum/viewt...971&hilit=link

It's about 3/4 the way down the page.

[Marv D]

Originally Posted by fatgump

666HP in Hell's training ground. Is there something your not telling us?

I keep telling people that motor is posessed,,, It's taken different chassis anywhere from 11.50 to 9.98 with not that much weight/combination change. When it's happy it will bring home a win lite, when it's not.....

[bigboybodry]

ok guys I picked up a set of scales tonight so what do I need to look for my chassis books/videos haven't showed up yet. by the way I only have these for the weekend. Thanks Daniel

[bigboybodry]

the front ride height is 27 3/8" from the ground to the top of the wheel opening at rest and just before it picks the tires of the ground it hits the bump stops at 30 3/4". the spring measures at 11" at rest/ride height.

[bigboybodry]

heavy scale results

[bigboybodry]

so what front spring would I need to order to keep about the same ride height?

[Marv D]

?,, is that with you in the seat?

I'm no suspension expert for sure, especially with the leverages and way the stock suspension works. But the bias on roughly 55/45 is better than I would hope for. She's 'portly' for sure at 3950 but ought to be able to get it rotating to put that weight on the rear!

I doubt this helps, but I ran a 160"# spring and it was too soft and mushy for street driving even with a front sway bar. I found a set of 210"# springs and cut them down to the ride height I wanted. With the sway bar in urathane bushings with lots of lube, and the soft CE shocks,,, it works for my pile,, but were not talking apples and apples here either (4link and less power than you)

[bigboybodry]

Originally Posted by Marv D

?,, is that with you in the seat?

I'm no suspension expert for sure, especially with the leverages and way the stock suspension works. But the bias on roughly 55/45 is better than I would hope for. She's 'portly' for sure at 3950 but ought to be able to get it rotating to put that weight on the rear!

I doubt this helps, but I ran a 160"# spring and it was too soft and mushy for street driving even with a front sway bar. I found a set of 210"# springs and cut them down to the ride height I wanted. With the sway bar in urathane bushings with lots of lube, and the soft CE shocks,,, it works for my pile,, but were not talking apples and apples here either (4link and less power than you)

Yes that is with me in the seat 2 full nos bottles and a full tank of fuel. I even put the rear anti-roll in the bead above the rear axle because I know it wil be on before I take it back out.

[bigboybodry]

So if I did the math correct I need a 1908.041666666667lb spring at 11" per side?