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Discussion Starter #1
Converting a former track car into a drag car. looking for some info on what a good spring rate would be. The car currently has a MM torque arm & rear coilovers with a super stiff spring. Race weight is around 3000lbs.
 

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For drag racing use, you won't be able to install a soft enough coilover spring. I would convert the car back to RLCAs that have the springs mounted on them. Something in the low 200lbs/in range will work fine. Its really not that important. For drag racing using a stiff enough rear swaybar will have much more affect.
 

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Discussion Starter #3
For drag racing use, you won't be able to install a soft enough coilover spring. I would convert the car back to RLCAs that have the springs mounted on them. Something in the low 200lbs/in range will work fine. Its really not that important. For drag racing using a stiff enough rear swaybar will have much more affect.
I don't plan on ditching the rear coilover setup. Just looking for what spring rate would work best. The torque arm helps the car hook from a roll but the initial hit is making the car 60ft suck. I see a lot of F body guys running in the 130lb range, Maybe ill try those.
 

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With a 130lbs/in coilover spring rate in the stock shock location on a Mustang, that gives a wheel rate with the TA of about 170lbs/in. If the car had a 200lbs/in spring located on the RLCA, the wheel rate would be 99lbs/in. That is more inline for what you need in a drag car.

To make matters worse, if the car has the rear shocks cantilevered off of the rear axle axle farther than stock, that will increase the wheel rate even more.

To get down to a 99lbs/in wheel rate, you need a coilover spring in the stock location of about 75lbs/in. The problem with this is that even if the spring is a 2.5" model, it will need to be about 20" long to have enough travel to not coilbind. Even if you could find someone who manufacturer a spring like this, you are going to have to use a spring compressor to install it because even at full droop travel, the spring is going to be preloaded about 13" or so.

If the car doesn't hook up well on initial launch, take high frame rate video from both sides of the car and from directly behind it, under the same conditions. You can look at the video and determine what the average (left+right/2) ride height is doing at launch. You want the average height to not change. When that occurs, the rear suspension is setup for 100% antisquat. This transfers weight to the back tires as quickly as possible, because no acceleration forces go through the rear springs or dampers. Another advantage of this is since no acceleration forces go through the shocks, it minimizes their affects on traction.
 

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Discussion Starter #5
With a 130lbs/in coilover spring rate in the stock shock location on a Mustang, that gives a wheel rate with the TA of about 170lbs/in. If the car had a 200lbs/in spring located on the RLCA, the wheel rate would be 99lbs/in. That is more inline for what you need in a drag car.

To make matters worse, if the car has the rear shocks cantilevered off of the rear axle axle farther than stock, that will increase the wheel rate even more.

To get down to a 99lbs/in wheel rate, you need a coilover spring in the stock location of about 75lbs/in. The problem with this is that even if the spring is a 2.5" model, it will need to be about 20" long to have enough travel to not coilbind. Even if you could find someone who manufacturer a spring like this, you are going to have to use a spring compressor to install it because even at full droop travel, the spring is going to be preloaded about 13" or so.

If the car doesn't hook up well on initial launch, take high frame rate video from both sides of the car and from directly behind it, under the same conditions. You can look at the video and determine what the average (left+right/2) ride height is doing at launch. You want the average height to not change. When that occurs, the rear suspension is setup for 100% antisquat. This transfers weight to the back tires as quickly as possible, because no acceleration forces go through the rear springs or dampers. Another advantage of this is since no acceleration forces go through the shocks, it minimizes their affects on traction.
Thanks for all the info. I think I will end up trying a spring in the stock location. The car seems to squat pretty level on the launch. I have some videos of it leaving Ive studied. Do you know what the spring rate for fox v8 rear springs are?
 

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How long are your shocks? I would not go back to springs on control arms if i had coil over.
Think I got 6 inch of travel on mine 12 inch shock running 14" 120 lb spring. Still testing the car. Do not have tq arm. Car is 3100 lbs. 1.19 60s
What 60 are u doing? What type of tiire? Track prep?
 

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All Fox rear springs are progressive rate. This makes it impossible to define them as having one rate. 1987-93 V8 Fox springs vary from 200-300lbs/in. 1979-84 GT springs and 1987-93 LX springs are rated by Ford as being 160lbs/in. They are the most linear springs that anyone makes for the car. These would be best for drag racing.

Do you have links to the videos?

If the car squats on launch, that means that the rear suspension has less than 100% antisquat. You can adjust the rear ride height to get the AS to 100% and the car will have more forward grip when the clutch is dropped.
 

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Discussion Starter #8
How long are your shocks? I would not go back to springs on control arms if i had coil over.
Think I got 6 inch of travel on mine 12 inch shock running 14" 120 lb spring. Still testing the car. Do not have tq arm. Car is 3100 lbs. 1.19 60s
What 60 are u doing? What type of tiire? Track prep?
The shocks are whatever stock length bilsteins for a fox are. The coilover spring is very short 9" its a MM setup, The spring is also only 2-1/4 wide so Finding replacements are hard. Im sure the bilsteins aren't helping much either. Tire is a mickey Thompson stiff wall 26" slick. Best 60ft was 1.47 Track prep is very good. the stiff walls def made a difference vs the normal M/T
 

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Discussion Starter #9
All Fox rear springs are progressive rate. This makes it impossible to define them as having one rate. 1987-93 V8 Fox springs vary from 200-300lbs/in. 1979-84 GT springs and 1987-93 LX springs are rated by Ford as being 160lbs/in. They are the most linear springs that anyone makes for the car. These would be best for drag racing.

Do you have links to the videos?

If the car squats on launch, that means that the rear suspension has less than 100% antisquat. You can adjust the rear ride height to get the AS to 100% and the car will have more forward grip when the clutch is dropped.
The car does squat on the launch. I don't have any videos. Im going to raise the rear up about a 1" for now and see if this helps for the time being.
 

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You sound like u are doing really good. Video helps out a lot. You can also try a taller tire. 28 see if that helps if it does not kill your rear ratio.

Hope u got good parts in the rear. This is the time u start breaking stuff.
 

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You can't look at one side of the car on launch to tell if it squats or not. On launch, with a solid axle rear suspension, the driveshaft torque reaction is going to cause the chassis to roll. From one side of the car, the ride height will go up from this and on the other side of the car, the ride height will go down. In addition to this, the amount of antisquat in the rear suspension will determine if the squats or rises. On each side of the car, you need to add both of these affects to see what is happening.

For example a car with 100% antisquat in the rear suspension, but very little roll stiffness is going to look like it is squatting a lot on the right side of the car and rising up on the left side of the car. The only easy way to know if the car is squatting or rising is to watch the launch from behind the car so that you can see what the middle of the car is doing.

With the TA rear suspension, if you increase the ride height in the rear suspension, the amount of antisquat will increase. If you can tell me the distance from the center of the rear RLCA bolt to the ground and the distance from the center of the front RLCA bolt to the ground, along with the front ride height, I can tell you what the antisquat percentage is for the car. This is fairly easy to calculate. There is no reason to experiment.
 

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Gotta experiment to figure out what the car likes. Test test test. Data helps u figure out what going on.

Look at you tube 03 cobra test at atco. That is 1.19 60 gotta go 1.10 Think I gotta change my IC? Not sure friend thinks softer shocks in the back. More rebound. Test test test.


Multiple video angles help to. See if the front is lifting how hi up. How far the car is squaring. Tire getting smashed. How much the body is leaning. Video video video helps
 
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