Violent bouncing on launch.

[rfoll]

My car is a 78 Skylark, (essentially a Nova). I changed some things and now if I launch the car hard it goes through 2 or 3 big bounces in first gear. It can be so bad I have a hard time shifting to 2nd gear. Some history is in order that readers can follow the progression to where I am now. The car has been lightened to about 3200 lbs. The battery is in the trunk. I built sub-frame connectors and it has a 5 point roll bar. I have been running 400+ inch SBC engines with a Th 350. Until recently I used a 26X10 Hoosier slicks and a 3.73 gear in an 8.5 10 bolt. The springs are factory multi-leaf and have the factory clamps on them. As power increased I experienced problems with a bounce in low gear shortly after launch. I Installed CalTrac bars, added 4 degrees of pinion angle, (it currently measures 3 degrees), 90/10 and 50/50 shocks, and the problem mostly disappeared. I changed from a 4400 rpm converter to a 5100 and times improved. A new 415 engine dropped times int the high 10s at 120+ mph. The front tires would lift a few inches and 60' times were down around 1.5. I built a 4.10 axle for the car hoping for a better 60' and faster times. The car launched much cleaner. The front tires came up and settled down smoother. I only reduced ET by maybe a half tenth, possibly because the engine was turning 6800 rpm through the lights, somewhat above what I believe to be the usable power rpm. I liked the combo but the really poor plug welds on the center section gave up and destroyed the case. Back to the 3.73 gears and with a better burnout technique, I was running a consistent 10.90-10.93 et. The tires wore out. Some quick math said that a 29" tall tire and the new Eaton 4.56 axle I had would get me the same wheel speed as the 4.10 gear/26" tall tire. And that part of the story is true. But the gear change puts 20 percent more torque to the rear axle than the 26"/3.73 combo. The launch is violent and doesn't hook very well. I move the lower bar on the CalTrac to the bottom hole and it helped some. I removed the cheap 50/50 shocks and replaced them with Strange single adjustable units but the improvement is minor regardless of the firmness setting. The bounce feels like a typical spring wrap up, but I would thing the CalTrac bars would prevent this. There have to be a huge number of these GM suspension setups on drag cars and I'm hoping someone has been able to solve this and tell me what is going on. Thanks, Rick.

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[Rick!]

With the 1.5" increase in the slick, you changed the IC of the car. CalTracs don't change IC, just the amount of front spring "anti-wrap".
You may want to raise the front the same amount and see if the car is happy again.
Or, there are lotsa fast Novas on 26" slicks and leaf springs and even 3.73s running high 9s.
A search on google regarding leaf spring Novas and instant center will give plenty of reading material on what your next moves might be.
You also want to see if you are running out of travel on the rear shocks. SA shocks are typically only adjust rebound. If you have them really loose, go the opposite way and see if slowing down the extension calms things down. For sure, you need to video a few launches of each side of the car and analyze the tire movement, chassis rise or squat, and any leaf spring wrap up that may occur. iPhones have great slo-mo with 250fps so . Down load QuickTime player to view the video frame by frame and you can really see what is going on.
Here's a practice video so you can see what's possible with a leaf spring launch.
When you go through it frame by frame, there is some crazy stuff going on. Watch how far the rear axle extends without raising the car and watch both slicks - they do different things at the same time. Big Block Nova that runs 10 teens. Right click on the video and save it to your laptop to view it in Quicktime.

[rfoll]

It occurred to me the change in stance might have something to do with it. I can see where it might delay the rise of the front. My front tires are 26" tall MT Sportsmans, the 8 ply tire. They are heavy, but I started worrying about the speed rating of my front tires when the trap speeds exceeded 120 MPH. My front shocks are Competition products adjustable, but I don't think they are really very good. Raising the front of the car would not be easy or cheap. I can't lower the back without having a tire rub.

[1980RS]

My car is a 78 Skylark, (essentially a Nova). I changed some things and now if I launch the car hard it goes through 2 or 3 big bounces in first gear. It can be so bad I have a hard time shifting to 2nd gear. Some history is in order that readers can follow the progression to where I am now. The car has been lightened to about 3200 lbs. The battery is in the trunk. I built sub-frame connectors and it has a 5 point roll bar. I have been running 400+ inch SBC engines with a Th 350. Until recently I used a 26X10 Hoosier slicks and a 3.73 gear in an 8.5 10 bolt. The springs are factory multi-leaf and have the factory clamps on them. As power increased I experienced problems with a bounce in low gear shortly after launch. I Installed CalTrac bars, added 4 degrees of pinion angle, (it currently measures 3 degrees), 90/10 and 50/50 shocks, and the problem mostly disappeared. I changed from a 4400 rpm converter to a 5100 and times improved. A new 415 engine dropped times int the high 10s at 120+ mph. The front tires would lift a few inches and 60' times were down around 1.5. I built a 4.10 axle for the car hoping for a better 60' and faster times. The car launched much cleaner. The front tires came up and settled down smoother. I only reduced ET by maybe a half tenth, possibly because the engine was turning 6800 rpm through the lights, somewhat above what I believe to be the usable power rpm. I liked the combo but the really poor plug welds on the center section gave up and destroyed the case. Back to the 3.73 gears and with a better burnout technique, I was running a consistent 10.90-10.93 et. The tires wore out. Some quick math said that a 29" tall tire and the new Eaton 4.56 axle I had would get me the same wheel speed as the 4.10 gear/26" tall tire. And that part of the story is true. But the gear change puts 20 percent more torque to the rear axle than the 26"/3.73 combo. The launch is violent and doesn't hook very well. I move the lower bar on the CalTrac to the bottom hole and it helped some. I removed the cheap 50/50 shocks and replaced them with Strange single adjustable units but the improvement is minor regardless of the firmness setting. The bounce feels like a typical spring wrap up, but I would thing the CalTrac bars would prevent this. There have to be a huge number of these GM suspension setups on drag cars and I'm hoping someone has been able to solve this and tell me what is going on. Thanks, Rick.509.jpg

Your Skylark is the same chassis as my 80 Camaro. My car used to do that all the time until I got rid of the slapper bars and put in SSM lift bars (still using them) with CE 3 way shocks set on the 50/50 setting. In the front I used a set of Moroso trick springs from a Chevelle and had to make some locating cups to center the springs in the frame and lower control arms. Now here's where it get interesting, I ran my CE 3 ways in the front for years on the 80/20 setting and I decided to get some Calvert 90/10 front shocks. Well it turns out the part number and application is wrong for the chassis we have as the shocks are listed for 67-81 Camaro, but they are 1.5 inches too short for the 2nd gens. I called and got blown off Calvert saying I did not know what I was talking about then sent them a picture of the 2 brands of shocks side by side. Did not hear a peep out of them for 2 weeks and then got an email saying just use an shock extension (I already knew that) so I bought a pair and decided to try them on my old CE shocks 1st. Boy did that make those old shocks work better than they ever worked in the past.

My thinking is that by putting the piston further down in the shock itself made it work better and that's what I think happened. I sold the Calvert's and actually made money on them, but it I get some new shocks I will ask my good Stock, S/S friend Randy what to use as he rebuilds race shocks for people all over the US, but for a cheap fix on a the F and X chassis the launch works great. My car has super good 60ft times also with the new set up and with m 358 last fall will pull the wheels 6-8" and has never done that with the smaller engine in the past.

[rfoll]

Since my posts, I replaced the front tires with 1 1/2" taller Goodyear front runners. $100 new at the Portland Expo swap meet Also removed the Competition Products front shocks with Calvert front shocks, $40 new at the Portland Expo swap meet. I have not run it yet, aside from the rain, I discovered the trans case is cracked. Likely a result of the violent bounce.

[miniv8]

Your bigger rear tires will like less air pressure than the small ones for the same power applied.
It might be dead hooking on the bigger tire, bringing the wheel speed down prematurely after the initial hit.
The starting line ratio is aggressive with the th350. Have you tried running the 2step way lower or leaving off idle?

[rfoll]

This is a foot brake car. So far with the bigger tire it doesn't dead hook. If i leave with just enough rpm to clean up the engine it does not bounce so bad. I have not experimented with dropping air pressure. The tires are currently at 12.5 psi.

[jeff swisher]

I have had 3 75-78 Novas and a 78 Skylark, 72 Nova and many other Leaf sprung cars.
Some would pull 3 foot wheelies. Gets scary not seeing the pavement but my cars all went straight as an arrow.

Full spools in a 9" and I mover the rear leafs all forward.
I would have at least 3 leafs under the front spring eye or as far forward as I could go and still lay flat on the main leaf.

On the passenger side it got 1 extra leaf. I usually had Ford truck leafs I used..as those trucks donated their 9" for my GM cars.

I never ran cal tracks or traction bars just shift the long leafs forward and add another long leaf and then build a clamp for the front of the spring pack
from plate steel and 7/16 or 1/2" bolts to clamp the front of the spring pack together.

No spring wrap up No wheel hop.
I have used 90/10 shocks on the front and never anything special on the rear except a good working shock.
I have used Gabriels, Bilstein and air shocks on the rear and never any bouncing front end issues.

Stock rubber bushings in the front and I did NOT remove serrations from the bushings.
I have ran cars from 2870 lbs to 3800 lbs set up the same way.

I built a couple clamps for the front of the rear leaf springs on a buddies Ford Harley truck that kept wheel hopping and it eliminated that issue.

Make sure the slicks are not spinning on the rims also.
I think in the picture I see 1 rim screw so you may have that issue sorted out.

[af2]

Tire pressure is what I would also adjust. Think of a basket ball? lower pressure less bounce..

[rfoll]

i have given some thought to adding some spring on the front half. Unfortunately I discovered a crack in the bell housing above the cooler line boss running up towards the top . No racing for me until I build a new trans case.

[1980RS]

Since my posts, I replaced the front tires with 1 1/2" taller Goodyear front runners. $100 new at the Portland Expo swap meet Also removed the Competition Products front shocks with Calvert front shocks, $40 new at the Portland Expo swap meet. I have not run it yet, aside from the rain, I discovered the trans case is cracked. Likely a result of the violent bounce.

I hope you checked the length of the 2 brands of shocks before you put the Calvert's in? as what they list for the 70 and up X and F cars is not right (shows as 67-81) and they are too short and will hurt the launch of the car.

[rfoll]

They will extend more than the travel of the lower A arm and will compress more than required.

skylark sunset.jpg

[fordified]

I'm not sure of what you're asking. Squaring away the rear springs is pretty easy to do with a set of bars that prohibit the springs from rotating.

The rest comes down shocks. Double adjustables are always better than singles but properly valved singles can the get you there. I always tune with my shocks front and rear. The springs hold up the car. The shocks do everything else for the most part.

50/50 and 90/10s are 70's technology at best. Single adjustables are reasonably priced and will run circles around non-adjustable shock no matter what anyone tells you.

Call Randy Mans at Fastshocks to get you squared away. There are others who revalve shocks but he's just as good if not better than most. If you're going to buy used shocks talk to him first.

[rfoll]

I'm not sure of what you're asking. Squaring away the rear springs is pretty easy to do with a set of bars that prohibit the springs from rotating.

The rest comes down shocks. Double adjustables are always better than singles but properly valved singles can the get you there. I always tune with my shocks front and rear. The springs hold up the car. The shocks do everything else for the most part.

50/50 and 90/10s are 70's technology at best. Single adjustables are reasonably priced and will run circles around non-adjustable shock no matter what anyone tells you.

Call Randy Mans at Fastshocks to get you squared away. There are others who revalve shocks but he's just as good if not better than most. If you're going to buy used shocks talk to him first.

Basically, I had a well sorted out setup that went crazy when I changed tire diameter and gear ratio. I get the impression you did not read the entire post.

[fordified]

I'm not sure of what you're asking. Squaring away the rear springs is pretty easy to do with a set of bars that prohibit the springs from rotating.

The rest comes down shocks. Double adjustables are always better than singles but properly valved singles can the get you there. I always tune with my shocks front and rear. The springs hold up the car. The shocks do everything else for the most part.

50/50 and 90/10s are 70's technology at best. Single adjustables are reasonably priced and will run circles around non-adjustable shock no matter what anyone tells you.

Call Randy Mans at Fastshocks to get you squared away. There are others who revalve shocks but he's just as good if not better than most. If you're going to buy used shocks talk to him first.

Basically, I had a well sorted out setup that went crazy when I changed tire diameter and gear ratio. I get the impression you did not read the entire post.

I read the post but it was unclear if you’re getting wheel hop or a bouncing front end. Either way that’s typical shock settings.

Get someone with a slow motion camera and video the launch on both sides. Put a GoPro under the rear pointing at the suspension.

I change gear ratios in my transmission and rear end all the time. I always have to adjust my shocks and rarely my four link.