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SCrazy
09-27-2014, 08:27 PM
The attached video was taken from under my car today launching at New England Dragway, this particular launch was a 1.47 60'.


https://www.youtube.com/watch?v=cRBzHqjvlyM&feature=youtu.be

I have no idea what I'm doing when it comes to adjusting my new rear end but here is what I see.

At the hit the rear end separates but then immediately collapses and the tires loose grip.
The tires then start to grab and the rear end separates again and the car accelerates.

The shocks were set pretty loose on extension and at the mid point on compression.

Common sense says I need to increase the compression setting but what do I know????

What's funny is that with the solid axle rear I don't feel the loss of traction the way I used to with the IRS. On one of the passes today I thought it hooked but didn't know it didn't until I looked at the data log. It's a far more subtle feel that with the IRS.

XR7 Phillip
09-27-2014, 10:42 PM
I can't help with suspension tech, but that's a sick video. :D Love it. Lol

Phil

JT's03
09-28-2014, 09:44 AM
Looks like it's unloading to quick. Maybe the front is dropping too quickly. Where is your rear rebound set at now?

fastsc92
09-28-2014, 08:17 PM
How'd you run?

SCrazy
09-28-2014, 08:42 PM
How'd you run?

Two 10.5x passes then a 10.48 but none of the three hooked up.

CMac89
09-28-2014, 09:19 PM
This is where 4-links start to help, but above is pretty much right. The rear end is separating too quickly, slamming the tires, then they unload because it wads up the center of the tires; therefore, the contact patch distribution is lower and the tires spin.

Tighten the extension (rebound) adjustment to a high setting (Start at 75% tight and work from there.) This will lower the rate that the tires hit the ground and and will prevent from wadding up the tire. Also, try putting more air in the tire. If it's slamming the tires, then putting more air in the tire will help with the contact patch distribution. Honestly, you should highly consider buying a set of 10.5" slick radials and get rid of the bias plies. Especially with us having a ladder bar setup. Less rolling resistance going down the track, and the tire won't wad up like bologna skin tires.

If you run out of adjustment with the shocks then you'll have to resort to adjusting pinion angle.

SCrazy
09-29-2014, 07:25 AM
This is where 4-links start to help, but above is pretty much right. The rear end is separating too quickly, slamming the tires, then they unload because it wads up the center of the tires; therefore, the contact patch distribution is lower and the tires spin.

Tighten the extension (rebound) adjustment to a high setting (Start at 75% tight and work from there.) This will lower the rate that the tires hit the ground and and will prevent from wadding up the tire. Also, try putting more air in the tire. If it's slamming the tires, then putting more air in the tire will help with the contact patch distribution. Honestly, you should highly consider buying a set of 10.5" slick radials and get rid of the bias plies. Especially with us having a ladder bar setup. Less rolling resistance going down the track, and the tire won't wad up like bologna skin tires.

If you run out of adjustment with the shocks then you'll have to resort to adjusting pinion angle.


I was going the wrong way the shock adjustment, making it looser, thinking that more body separation would make it hit softer. I'll try it the other way. I was also running the tires pretty soft at 11psi.

In real time the 30 second video I posted is probably only about 1 second, stuff happens so fast, heck even at this speed things are happening fast.

Mike8675309
09-29-2014, 02:25 PM
I was also running the tires pretty soft at 11psi.



The main reason for SC folks running bias ply and low drag tire pressure is related to suspension issues that you simply don't have anymore. Not having to deal with an IRS allows you to setup much more like drag racing 101.

CMac89
09-29-2014, 02:53 PM
I was going the wrong way the shock adjustment, making it looser, thinking that more body separation would make it hit softer. I'll try it the other way. I was also running the tires pretty soft at 11psi.

In real time the 30 second video I posted is probably only about 1 second, stuff happens so fast, heck even at this speed things are happening fast.

I'd also like to add that the elasticity and air of the tire will cause the tire to bounce back to its equilibrium point under power, if the compression setting is too loose. The mentality is, "what do I need to set the shocks at relative to the actions of the tire, but not the body?" So you need weight transfer, but the power reaches the tires and starts motion before the car even comes close to transferring weight does. It's a question of what the tires do instantaneously; the car doesn't transfer weight until the tires receive traction and enough acceleration to transfer that weight. So, once again, think about what's happening at the tires first and the weight transfer second.

You can always get a device and a driveshaft speed sensor to log tire slip and another to log the trace of the shock (velocity versus time). Those are both something I use as tools for suspension tuning.

SCrazy
09-29-2014, 03:15 PM
You can always get a device and a driveshaft speed sensor to log tire slip and another to log the trace of the shock (velocity versus time). Those are both something I use as tools for suspension tuning.

I can see tire slip on my data logs via engine rpm but of course there is no way to relate that to shock position.

It all happens so fast and smoothly now that I often do not feel the tire slip at all (or at least I'm not attuned to it yet), so without the data log I would not know things weren't perfect.

I've been doing so much reading on this stuff my head hurts. I need a Jedi Master of Suspension Setup to spend some time with me at the track.

JT's03
09-29-2014, 07:13 PM
I was logging the other day street tuning and with the bigger turbo on the car load actually drops ( .20-.30) when it starts to spin the tires. Crazy because I never knew it would show up like that.

CMac89
09-29-2014, 07:14 PM
I can see tire slip on my data logs via engine rpm but of course there is no way to relate that to shock position.

It all happens so fast and smoothly now that I often do not feel the tire slip at all (or at least I'm not attuned to it yet), so without the data log I would not know things weren't perfect.

I've been doing so much reading on this stuff my head hurts. I need a Jedi Master of Suspension Setup to spend some time with me at the track.

It always helps to quantify it, but you can also see cool things like converter slippage and actual stall speed with those loggers.

Those bologna skins are so flimsy and have so much give that it's hard to tell if they're spinning. Since your 60 foot is down, if you feel it spinning you're definitely sacrificing a good amount. With radial slicks you can feel everything that's happening.

It really is pretty simple. You tighten extension so your tires don't collapse and bounce like basketballs, and once that's under control, then you can use the shock for weight transfer. If the ladder bar makes the separation too violent (long wheel base cars have faster action because of how far behind the CoG the instant center of the ladder bars are at) to the point that it's still causing the tires to unload, then you'll need to tighten the compression so that it helps keep the tires from bouncing back and unloading them (causing them to spin).

Then if that still isn't enough, ya gotta readjust pinion angle and start over with shock settings. Pretty much why it makes datalogging shock information useful and saves a lot of time. Definitely not money, though :)

David Neibert
10-01-2014, 02:28 PM
I don't really understand any of the stuff that you and Casey are talking about, but it's still a cool video. How did you mount the camera under there ?

David

SCrazy
10-01-2014, 02:37 PM
I don't really understand any of the stuff that you and Casey are talking about, but it's still a cool video. How did you mount the camera under there ?

David

The camera is a GoPro with a mount on one of the frame rails right back at the bumper. It's just a sticky mount with a couple of tie wraps as backup. These cameras are really pretty cool.

SCrazy
10-01-2014, 02:40 PM
Those bologna skins are so flimsy and have so much give that it's hard to tell if they're spinning. Since your 60 foot is down, if you feel it spinning you're definitely sacrificing a good amount. With radial slicks you can feel everything that's happening.

The tires are supposedly "stiff" sidewall bias ply. I believe they are similar to what Kevin was running which helped me to lock in my choice on them since he is tickling 1.2x 60' times. Hopefully I can air them up and stiffen up those shock to make a difference.

How much air pressure do you think is a good starting point 14-15 psi?

David Neibert
10-01-2014, 02:59 PM
The camera is a GoPro with a mount on one of the frame rails right back at the bumper. It's just a sticky mount with a couple of tie wraps as backup. These cameras are really pretty cool.

I've got a Hero 3 black edition and might try doing something similar on my 91 SC, just to see whats going on under there.

David

SCrazy
10-01-2014, 03:11 PM
I've got a Hero 3 black edition and might try doing something similar on my 91 SC, just to see whats going on under there.

David

That's the exact same camera I have.

I've had a couple of people taking video of my car with cell phones but stuff happens so fast that I couldn't see what the heck was going on. It's a bit of a pain with the GoPro because it takes a while to process the video....you can't just play it back...but the end results are worth it.

The little clip I posted is really only about a one second piece of a 5 minute long video in real time....the play back was slowed down to 1% of actual speed. The actual video was also a much wider angle that showed the entire width of the rear end from tire to tire....in addition to slowing it down I was able to zoom in and clip the image to only show one shock/tire.....all during editing after the video was shot.

Pretty cool stuff........

SCrazy
10-27-2014, 08:59 AM
I had a new best 60' this weekend 1.397 and I have an undercar slow motion video of the launch


https://www.youtube.com/watch?v=kMy8PoxZVFc&feature=youtu.be

This video clip is about .5 seconds long in real time.

I also have a datalog of the engine rpm:

62253

At the beginning the car is sitting on the trans brake and rev limiter at 4100ish rpm, at launch the rpms climb quickly to about 4800-4900 rpm then stay there for almost a full second before the motor starts to accelerate again. When I first saw this I thought I was seeing wheel spin but looking at the video that is not the case so I'm now assuming this is the torque converter.

I wonder if launching at an rpm that is closer to that 4800rpm plateau will get things moving quicker and eliminate some of that flat spot?

You can see in the video where the car first hits the tires hard then bounces just a little bit before it digs in and the starts to separate again....I wonder if that little bounce corresponds to the beginning of that flat spot in the rpm log?

You can also see the rear end flexing and moving around a little bit at launch.....do you think that is a normal amount of flex/movement?

These videos from the under car camera are really cool!!