Got the car scaled, right track?

Shocks, Springs, Brakes, Frame, Body Work, etc

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Makina
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Got the car scaled, right track?

Post by Makina »

I finally got my car to a set of scales to check the overall weight and to try and get corner weights dialed in.

Brian (MADMAN) has helping get the initial tuneup on the car. He's out in Memphis right now so I'm posting here to see if I'm on the right track.
I have had alot of calls and emails about choosing the correct spring rate for their car so I will give a rough overview of spring rates in hopes this will help.

First this is how spring rate is measured. The springs are measured in lbs per inch. An example of this is the QA1 spring for the front os our cars. It is rated at 315 per inch. This means that each inch you compress the spring changes by the rating. 315lb is 315 for the first inch, the second inch would be 630lbs and so on.

We will start with the front.

Most of the 4th gen f-bodies weigh in at 3500 at the high side, the weight bias(front to rear) is usually 54% front 46% rear. This equals 1890lbs front weight. If you run a QA1 spring at 315lbs usually the spring is compressed 5 inches per side. This comes to 3150lbs of spring on the front. NOW comes the trick. The spring shock setup is actually inboard of the spindle on our cars unlike strut front ends. The spring has to actually hold up more weight because of the leverage of the control arms. This comes to 40% more spring to hold up the car. Here lies the problem. The more you compress a spring the more "stored energy" the spring holds. When you launch the spring tries to throw the car up and relieve the stored energy. This ends up being a wheelstand situation if the cars hook.

The way to remedy this situation is to run a softer spring. We normally try a 275 lbs spring compressed to 5.5 inches. This lowers the total weight to 3024lbs BUT the car runs out of stored energy quicker so it will slow a wheelstand down. You could actually put a 225lb. spring on and compess it to 7 inches and hold the car up but kill the weight transfer.

Tightening the shock up on the front will not control the wheelstands as the shocks work to slow to overcome the spring. Only a softer spring or a limiter will control the front end.


Hopefully this will explain some of my spring theories.
The bold is what I found interesting. Over the phone Brian told me to shoot for a 53% bias, but didn't say front to rear or rear to front. Tom at Coach chassis though I meant rear to front. Brian is referring Front to rear. I'm making this assumption based off of what has been written down on paper from what we measured. The corner weights were as follows.

LF......RF
919....876
LR......RR
754....650

43.89 Rear bias (56.11 FRONT)
Corner weights as I drove the car, no adjustments

Added 83lbs of lead to simulate a full tank of gas
LF......RF
900....882
LR......RR
827....682

45.85 Rear bias (54.15 FRONT)
3291 Raceweight

Adjusted LF shock to move weight towards the RR tire (adjustment maxed out)
LF......RF
928....852
LR......RR
794....717

45.91 Rear bias (54.09 FRONT)
3291 Raceweight

The RR tire should have 10-15lbs more weight than the LR. So somehow I need to get 94 more lbs of weight on there.

I never saw the paper as he was writting the numbers down, so I didn't pick up on the confusion. He had a calculator that just spit out the numbers as the corner weights were entered.

So my thoughts based off of what I saw.

-Loosen up the RF shock to balance out the LR side
-Loosen up both front shocks to shift more weight to the back, at the cost of ride height.

Anybody else agree?

I'm not to crazy about running at a full tank of gas. I may just live with the 56% as long as I can get the rears to balance out.

Billy? Ed?
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BillyShope
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Re: Got the car scaled, right track?

Post by BillyShope »

Makina wrote:Brian told me to shoot for a 53% bias, but didn't say front to rear or rear to front.
Well, it's going to take about a 53% rear bias to get all the weight on the rear wheels during launch, even with the best of slicks. (Check this out with the spreadsheet on Page 9 of my site.) But, as nose heavy as you are, that's not going to be easy!


Makina wrote:Added 83lbs of lead to simulate a full tank of gas
LF......RF
900....882
LR......RR
827....682

45.85 Rear bias (54.15 FRONT)
3291 Raceweight

Adjusted LF shock to move weight towards the RR tire (adjustment maxed out)
LF......RF
928....852
LR......RR
794....717

45.91 Rear bias (54.09 FRONT)
3291 Raceweight
Percentage differences here are due to measurement error. I don't recommend adding ballast to reduce 60 foot times. That extra weight bites back at the top end.
Makina wrote:The RR tire should have 10-15lbs more weight than the LR.
Where did you get this? If you're after equal rear tire loading on launch, you'd need about ten times that much. (See Page 15 of my site.)

Since you have wheel scales available, you should set up a traction dyno (Pages 4, 5, and 6) and directly measure wheel loads during launch.
http://home.earthlink.net/~whshope
Makina
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Re: Got the car scaled, right track?

Post by Makina »

Well, it's going to take about a 53% rear bias to get all the weight on the rear wheels during launch, even with the best of slicks. (Check this out with the spreadsheet on Page 9 of my site.) But, as nose heavy as you are, that's not going to be easy!
I'm thinking it's just going to be an issue I'm going to have to deal with. But I think Brian (MADMAN) was talking about 53% to the front, and not 53% at the back. If I remember correctly his words were "on these cars I typically shoot for 53%". And based off of the quote above he was referring to the front. Now the guy at the chassis shop did say "the weight moves to the tighter spring" which makes sense as it's taking up more load of the car from the preload. So does that mean I can loosen up the fronts to shift weight to the back at the cost of ride height? My car won't yank the wheels so I'm not worried about it yet.
Percentage differences here are due to measurement error. I don't recommend adding ballast to reduce 60 foot times. That extra weight bites back at the top end.
Neither do I, we were just testing out to see what 80lbs of weight would do. Only had 2% of an effect. That was the kicker, I don't car to add another 100lbs of weight to aid traction at launch only to slow me down another .1 or so.
Where did you get this? If you're after equal rear tire loading on launch, you'd need about ten times that much. (See Page 15 of my site.)
Brian (MADMAN) recommended that. He's got a lot of experience with setting up these cars. There was just recently an article published in GM High Tech Performance and the results speak for themselves. I checked the website but they haven't been posted yet and I can't find the rag. To post the article. :x
Since you have wheel scales available, you should set up a traction dyno (Pages 4, 5, and 6) and directly measure wheel loads during launch.
I'm still not sure how exactly to build this rig. :lol: :lol:
Ed-vancedEngines

Post by Ed-vancedEngines »

Brian is pretty sharp. He is also very good with Torque Arm Suspensions.

I am like you, I think he meant 53% bias to the front. Many of these 10.5 cars are running with that much bias to the front and some even more.

Like I said, Brian is pretty sharp and I won't second guess him. He can definetly make them hook. If you do not have one of his Torque Arms, I suggest to talk with him about it. That is if this is still a Torque Arm car.

LIke Billy said, Preloading with springs to get an even right left weigths on wheeels at launch would be very difficult to do. He didn't use those words, but the essence is the same. Billy does have some spring ideas for you to do with the right front spring that are interesting though.

I have one guy that I would literally love to see him not hook. No matter what they are doing or which shop does it, the car hooks too hard. With recent changes from a different shop that I was told about the car may do btter but will still be plagued with too much hook.

Keep us tuned in.

Ed
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Post by Eliminator540 »

What factors are used to determine the ideal weight bias?

I am redoing some stuff on my Camaro over the winter and this might be the ideal time to move some weight around.
It runs 11.0 right now with a 350, TH 400, has a stock front end and 4 link in the rear. I would guess that it weights around 2900 lbs, but will get it on a scale as soon as I get a chance.
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Post by BillyShope »

Eliminator540 wrote:What factors are used to determine the ideal weight bias?
Try some different numbers in the spreadsheet on Page 9 of my site.
http://home.earthlink.net/~whshope
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Post by Bubstr »

One thing is sometimes over looked when you get too much or not enough hook. That is center of gravity hight. We tend to weigh them out and treat it like a flat plane, when some times a little high weight or lowering of weight can change the hook with less weight added even if your front weight bias is heavier than you would like. It's a combination of front to rear and hight that makes weight transfer. It's a juggling act but if you get the balance right you need very little suspension to make it work. Some time try 30 lbs attached to the top of rear hoop instead of the 80 in the trunk. You will see what I mean. When you find balance then move engine or anything that weighs something up or down forward or back and dump the weight.

Think of this weight in terms of an 8 lb sledge hammer. Grab it at half handle extend arm and touch your nose with the heavy end. Now grab the bottom of handle and do the same. You will see what high weight is worth. Watch out for your nose, cause that hammer will hook with high weight.
Older I get the less I know for sure
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