Porpoising on launch help

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TerryN
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Porpoising on launch help

#1 Post by TerryN » Mon Jul 16, 2018 12:43 pm

Hi all, I have been fighting with this issue for about a year, and I'm about at my wits end. I know there are a lot of threads on here, and I think I've read all of them. I've tried so many things I've lost track, but there has been nothing yet that has cured the bouncing on launch. the car usually smashes the tires really hard then bounces up and get about 100 mph wheel speed (maybe slightly exaggerated) and then does it again. The car is a 2700 lbs (51% on rear), 4 link, about 800 RWHP, 60' is normally 1.27ish, with my best run being 9.06 @ 8200 DA I just changed all the spring, 150s front and 110s back (no change). I have double adjustable Strange shocks on the back and have gone from one extreme to the other with no help. Single adjustable Strange struts on the front set to full loose. I've tried every suggested IC point that people have told me to try (no change). The car always goes perfectly straight and handles good on the top end with no issues. There is obviously something I haven't tried yet, just don't know what it is. Bar spread? IC location? Tires?........ I included a link to a video, hopefully it will work. Thanks in advance for any help.

Terry

https://drive.google.com/file/d/11irTbW ... p=drivesdk
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John_Heard
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Re: Porpoising on launch help

#2 Post by John_Heard » Mon Jul 16, 2018 11:07 pm

I watched the video in slow speed and I'm seeing very little if any rear extension on the shocks. You said you've tried different settings, do they all act like that? Were they full tight extension in that video?

Front doesn't look like it has very much travel in it. I doubt you want full loose, do you have travel stops on the front? Looks like it's got maybe a couple inches in it? I'd try a tighter front but rear really looks like the problem area. The porpoising is all from the tire loading and unloading, it's not the shocks they don't appear to be moving in the rear.

TerryN
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Re: Porpoising on launch help

#3 Post by TerryN » Tue Jul 17, 2018 12:17 am

Hi John, thank you for the reply. I'm not sure where the shocks were set in that particular video, but everything I have tried has ended up with very similar results. You are correct about the front struts not having much travel (2.5") When i say the front struts are all the way loose they are about 50/50. The only adjustment I can make on them it to tighten up the extension. As far as the rear shocks, I talked to a guy at Strange and he suggested 7 clicks from full loose on the extension and 4 clicks from full loose on the compression. I tried that this past Friday. Same results. I'm pretty sure that part of my problem is my 4 link brackets on the chassis side are too low in the car creating too much down angle on the top bar? But short of cutting the car apart, not much I can do. I was thinking about raising the ride height a little to help, but can't go too far or the tires will be hitting the bottom corners of my quarters.

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Re: Porpoising on launch help

#4 Post by John_Heard » Tue Jul 17, 2018 8:11 am

First thing, set the rear shocks to full loose on both extension and compression. Can you push down on the rear of the car and make them move easily? You should be able to pull up on the car as well and see them move a bit. If you can't, there's a problem with the shocks that needs addressed. You say you've tried all kinda of adjustments on them and nothing changes, well that's a big red flag there's an issue with them. Unless these are brand new shocks on it, you should thing seriously about sending them in to get checked out, or replace them.

I'm not sure about your bar angles, but everything you're saying is screaming there's a shock problem.

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Re: Porpoising on launch help

#5 Post by TerryN » Tue Jul 17, 2018 8:44 am

Actually the double adjustable shocks in the back are only 2 weeks old. I swapped out the single adjustable shocks trying to fix this. The back of the car still seems pretty stiff. The second guy I talked to at Strange said he would have recommended 95 springs after the first guy sold me the 110s. Do I still have too much spring?

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Re: Porpoising on launch help

#6 Post by John_Heard » Tue Jul 17, 2018 8:57 am

I don't know if that's too much spring or not. Try adjusting the shocks like I suggested and see if you can move the car. If you can't, and those shocks are new, it sounds like something is bound up.

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Re: Porpoising on launch help

#7 Post by TerryN » Tue Jul 17, 2018 9:42 am

Again, thank you for your input. I will take a look tonight after work and let you know what I find.

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Re: Porpoising on launch help

#8 Post by TerryN » Tue Jul 17, 2018 10:46 pm

Well, I feel really dumb. LOL I found that the wheelie bars were the problem. Normally when I'm crawling around under the car I have them removed I didn't notice that the re was not clearance between them and the frame. When they are adjusted all the way to the ground there was only about 1" of clearance. So I'm guessing that when the car launched the frame was hitting the wheelie bars and unloading everything. So I guess I'm racing Friday without the wheelie bars. We'll see what happens, but I'm guessing it will be a whole different monster. Thanks again for you input. Much appreciated! I'll let you know how it goes.

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Re: Porpoising on launch help

#9 Post by John_Heard » Tue Jul 17, 2018 11:05 pm

Glad you found something. With the wheelie bars off were you able to make the suspension move with shocks loose?

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Re: Porpoising on launch help

#10 Post by TerryN » Wed Jul 18, 2018 8:27 am

Yep, it feels like it should now

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Re: Porpoising on launch help

#11 Post by TerryN » Sun Jul 22, 2018 4:41 pm

Okay, got to make a few runs Friday. Definitely not as violent in the car, but still bouncing. I put some zip ties on the shock pistons, they are definitely bottoming out. The car never seems to settle down going down the track. Feels like an old Cadillac floating down the highway. I also seems like the 110 springs have a hard time returning the car to ride height. Was thinking I need to put a stiffer spring on now and increase the shock ride height for more compression travel? Thoughts? The link should still work to see the new videos. Thanks again.

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Re: Porpoising on launch help

#12 Post by John_Heard » Mon Jul 23, 2018 7:25 am

Video was the old one you put up.

I think I'd work on fixing the shock travel issue first before thinking about changing springs. The springs aren't so soft they are stacking up / binding are they?

If the suspension isn't extending during the run maybe you don't have enough anti-squat in it.

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Re: Porpoising on launch help

#13 Post by TerryN » Mon Jul 23, 2018 9:59 am

https://drive.google.com/file/d/1-N74Zg ... p=drivesdk

Try this link.

Good point about the anti-squat. I will re-measure everything and see where I'm at since I adjusted ride height and changed springs. Do I need to be close to the neutral line? about 100% ?

I'm pretty sure the springs aren't binding, but I'll take a look at that again.

this 4 link stuff is all new to me, so my thought process doesn't go in the right direction sometimes, So I really appreciate your input.

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Re: Porpoising on launch help

#14 Post by John_Heard » Mon Jul 23, 2018 12:47 pm

I have a leaf spring car, so yeah I don't play much with 4 links either haha.

Yeah I think you want to be in the 100% anti-squat or more, and your car doesn't appear to be extending the shocks so that might be something to work on.

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Re: Porpoising on launch help

#15 Post by TerryN » Tue Jul 24, 2018 10:45 am

Did some measuring last night. This is what I came up with. Thoughts? I can also get to the same IC point moving the bottom bar up one hole front and back if it is better to have the bottom bar closer to the housing, or does it even makes a difference.
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