Drag race rear suspension advice needed!

johnnyriviera

johnnyriviera

Well Known Member
Hello all!

Having trouble getting my 62 to stay hooked up. It grips at the hit, then spins a couple car lengths out.

My setup : 4 speed, 12 bolt rear, second upper arm, boxed lowers, Global West upper arm mount braces (crossmember to frame), QA-1 double adjustable rear shocks, Hoosier 28/9.0 slicks. Car sits a little lower than stock.

Car has run a best of 11.69 with a 1.70-1.71 60' WHEN it stays hooked, but it usually doesn't.

My car squats a lot at launch, and I have been tuning that out with the shocks, but it does not seem to be helping the problem much. I was wondering if lowering the front mounts of the upper arms via the Global West "anti squat brackets" would be the direction to go. Or is there other things I'm missing here?

There is so much knowledge here, and I am quickly exhausting what chassis smarts I have.

Thanks in advance,

John
 
jim_ss409

jim_ss409

Well Known Member
Supporting Member 5
Generally speaking, having the car squat on the launch is not a good thing.
It kind of looks fast because it looks like you're transferring weight to the rear suspension,,, and in fact you are transferring weight but the problem is that the suspension is absorbing the weight rather than transferring the weight directly to the tires.
As the suspension squats the springs and shocks do eventually plant the tires, but the problem is that by the time that happens, the tires are already spinning.

Imagine if you could stand on a scale while holding the car up by the rear bumper. Lets say the scale would read 2,000 lbs.
Now if you suddenly lifted the car, the scale would read much higher for about a second or so.
That's the effect you get when the body lifts instead of squatting. You hit the tires much harder for the first few feet.

Again, "generally speaking" you don't want the body to lift too much either because that can also create problems.
As a starting point, you usually don't want the car to squat nor lift very much.

I think the instant center on these X frame cars tends to be too low and too far forward for best results in drag racing, and I'm not sure, but I think having the car sit lower than stock height tends to make the position of the instant center worse.
It's kind of hard to accurately see where the instant center is on these cars but as a ballpark setting to aim for, I think it should be somewhere in the vicinity of the center "U" joint.

Setting the instant center on a regular four link is pretty easy but it would be harder on the factory setup. You might try checking it and then try jacking the car up a little to see if it improves.

If you can get it close, it should give you a better launch.

Here's an article about instant center settings,,, I'm sure there's plenty of others out there but this is the first one I saw...
http://www.how-to-build-hotrods.com/tuning-4-link.html

Hopefully others might chime in with some real world experience.
 
Tom Miller

Tom Miller

Well Known Member
I know you asked about rear suspension, but curious what your front suspension consists of?
If the front isn't doing the right thing, it makes tuning the rear impossible.IMHO

I've been racing in the Pure Stock Muscle Car Drags the last 20 years. We have to run the factory size street tire on all four corners.
Last few cars I've raced had G70-14 redline reproduction Firestone Wide Ovals.
Last car I raced went 12.89@103 with a 1.79 60ft on these tires.
If your front rises too fast it will transfer weight too fast to the rear and bounce/unload the rear tires.
Make sure your front suspension moves easily, maybe some upper ball joint spacers or extended length upper joints, make sure your front suspension hangs before front shocks run out of travel. And of course check things the guys listed above.
This is just my experience what works on a 68-72 GM A body. I know the x frame is a different animal, but the front HAS to work right to do the magic.
 
Tom Miller

Tom Miller

Well Known Member
Here is car I described above.
69 Buick special Deluxe 350 Buick/280HP
TH350 auto 4.10 gear.
My kid couldn't seem to time the photos at the right time, but the second pic from the rear, on this day at Osceola Dragway, I was getting daylight under the driver front.
 

Attachments

  • 13568775_1375678335791936_6052541660434081980_o.jpg
    13568775_1375678335791936_6052541660434081980_o.jpg
    72.5 KB · Views: 32
  • osceola launch.jpg
    osceola launch.jpg
    70.7 KB · Views: 20
Last edited:
Tom Miller

Tom Miller

Well Known Member
Pass rear shock hard to plant pass rear, driver soft compress on my stuff.
Cars don't squat more on pass side because it's transferring weight to that corner, it's planting the driver rear so hard it's trying to lift the pass rear which compresses and causes the pass squat.
stiffer rear spring pass side, air bag, tighter shock,etc whatever combination of those 3 or all of them, you have to plant the pass rear.
Slicks and sticky tires will mask a suspension issue. Sometimes you can make a suspension work "good enough" with sticky tires that you can get away with problems. Making a car "work" on street tires is a whole nother realm .
 
Last edited:
La Hot Rods

La Hot Rods

Well Seasoned Member
Supporting Member 15
Tom Miller said:
Pass rear shock hard to plant pass rear, driver soft compress on my stuff.
Cars don't squat more on pass side because it's transferring weight to that corner, it's planting the driver rear so hard it's trying to lift the pass rear which compresses and causes the pass squat.
stiffer rear spring pass side, air bag, tighter shock,etc whatever combination of those 3 or all of them, you have to plant the pass rear.
Click to expand...

You are correct,Tom.
I did get that backwards. :facepalm
That's why those wagons work so good with the fuel tank in the left rear quarter.
 
johnnyriviera

johnnyriviera

Well Known Member
Thanks for the all the great advice everyone!

I am currently looking at the front, I have adjustability on the rear, but none on the front. I talked to Global west, and they actually talked me out of the anti squat brackets at this time until I get the front dialed in. They thought that now that I am stiffening up the rear, that my front may not be extending fully due to the fact that I don't have that massive rear squat motion.

That said, adjustable fronts are on the way!

I have been making small adjustments in the rear, and I think I currently have both shocks on 7 clicks of rebound stiffness (out of 18) and compression is 6 on the driver side and 7 on passenger (again out of 18)

I would have to check my notes to be sure.

attached is a recent launch pic. It does roll on to the passenger side a bit.
 

Attachments

  • launch.JPG
    launch.JPG
    85.2 KB · Views: 54
R

RussC

Well Known Member
jim_ss409 said:
Generally speaking, having the car squat on the launch is not a good thing.
It kind of looks fast because it looks like you're transferring weight to the rear suspension,,, and in fact you are transferring weight but the problem is that the suspension is absorbing the weight rather than transferring the weight directly to the tires.
As the suspension squats the springs and shocks do eventually plant the tires, but the problem is that by the time that happens, the tires are already spinning.

Imagine if you could stand on a scale while holding the car up by the rear bumper. Lets say the scale would read 2,000 lbs.
Now if you suddenly lifted the car, the scale would read much higher for about a second or so.
That's the effect you get when the body lifts instead of squatting. You hit the tires much harder for the first few feet.

Again, "generally speaking" you don't want the body to lift too much either because that can also create problems.
As a starting point, you usually don't want the car to squat nor lift very much.

I think the instant center on these X frame cars tends to be too low and too far forward for best results in drag racing, and I'm not sure, but I think having the car sit lower than stock height tends to make the position of the instant center worse.
It's kind of hard to accurately see where the instant center is on these cars but as a ballpark setting to aim for, I think it should be somewhere in the vicinity of the center "U" joint.

Setting the instant center on a regular four link is pretty easy but it would be harder on the factory setup. You might try checking it and then try jacking the car up a little to see if it improves.

If you can get it close, it should give you a better launch.

Here's an article about instant center settings,,, I'm sure there's plenty of others out there but this is the first one I saw...
http://www.how-to-build-hotrods.com/tuning-4-link.html

Hopefully others might chime in with some real world experience.
Click to expand...

Pay attention to Jim's guidance on instant center. X frame cars also have a lot of flex... that's not good... flex is controlled with a proper cage. The gem here is the advice on instant center
 
Ishiftem

Ishiftem

Well Known Member
Supporting Member 2
Good advice in this thread. Cars like yours like more front end travel and a relatively quick rise. A longer lower rate spring helps in this department. If you watched a slow motion video of your launch, the first movement you would want to see is the rear end housing separate from the body. Then as the car starts to move you want to see the body come down slightly over the wheel as the weight transfers. If the first thing that happens is the rear squatting, that is bad. Instant center is a large change, shocks are a small change. My advice would be make a pass with the shocks full loose on compression and extension. Then each pass thereafter increase extension by two clicks till the 60 foot falls off. Then go to compression and do the same thing. Then back to extension to fine tune. If it likes a very soft extension setting then try a shorter and/or higher instant center. If it likes a ver stiff extension setting, Lengthen and/or lower the instant center.
 
You must log in or register to reply here.
Top