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Old 04-28-2009, 03:22 PM
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adjustable coil-over shocks for jag rear

My 31 ford tudor has a 76 jag rear. I am trying to determine what size adjustable coil-over shocks I should purchase. The original builder put two different size coil-over shocks on the rear. The front set are 7 in. springs and the rear set are 9 in. springs, shock length hole to hole is 12 3/4 in. The axle bell is an inch and a half below the rubber bumpers. With the shocks removed, the distance between the shock pins is 10 in. With the current set-up, it rides like an empty dump truck. If I try to use the 9 in. only, it bottoms out. I want to get one set of adjustable coil-over shocks to replace the two sets. Based on a chart I retrieved from Speedway Motors, I figure the rear weight of the car to be 1525 lbs. Can anyone suggest the proper length and spring weight I should use.

earlf31

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Old 04-28-2009, 10:27 PM
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You need to determine what the rate is for both sets of springs you have on the car now and go from there. I would be uncomfortable running just one set of coil-overs on a Jag rear end due to unequal loading of the lower suspension arm. If the 9" spring alone is too soft, and the combo is too stiff, the right rate is somewhere in between. If the shocks are adjustable for dampening, set them full soft for initial testing. When deciding on springs, going on the soft side and pre-loading them is better than going too stiff.

If one spring is 200# and one is 150# for example, the overall rate is 350# per inch of travel. A lighter set might be 2 150# springs per half shaft to drop the overall rate to 300#.

As for the shocks themselves, I usually try to have about 60-70% of the available travel for compression or bump travel, and the remainder for droop travel when at ride height.

Andy
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Old 05-01-2009, 11:43 AM
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Adjustable coil-over shocks for jag rear

Andy:

Thanks so much for the response. If you will, I would like to pick your brain a little more. Before your response, I spoke to a tech guy, (didn't get his name) at Aldan Eagle and he asked me to get the car weighed and to jack up the back end to the height I wanted the car to ride. I jacked the car up to get a 3 in. gap between the rubber bumper and the bell on the lower suspension bar as he said the shock should drop 1 in. or a 3rd of the shock travel to give me a 2 in. gap. At 3 in., the distance between the shock holes measured 12 in. The car weighs 2680, back end 1440, front 1240. I then called Aldan Eagle but, I didn't get the same guy and the guy who answered started out by telling me that weight doesn't matter and that the 76 jag rear takes their jag model no. 654, with the following shock application, Extended: 13.4 in., compressed: 9.7 in., travel: 2.8 in. adjustment: rebound. Coil spring: 10 in., 250#. He said I would need 4 shocks to carry the weight and ride for my car. He said if I only ran one set of shocks, I should probably go to a 300# spring. I don't know how to determine the weight on the existing springs as you suggested. Whatever they are, combined, it is not enough. With the old shocks in place with the 1 1/2 in. gap, I stepped on the back running board, (no bumpers) and it bottoms out. Any added advice or suggestions you can give me will be greatly appreciated.

Earl Fauerbach (earlf31)
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Old 05-01-2009, 01:24 PM
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Earl,
To work through this I will need a bit more info. The way I understand the situation is that at your preferred ride height, your shock mounts c/c are at 12", correct? If so, that puts you exactly in the middle of your shock travel. That will work, but I would like to see a bit more travel on the bump side (compression). If you can extend your shock mounts apart about a 1/2" or so that would be great, (or raise the car...not so great) to get more bump travel it would be to your benefit. If not, we will deal with it the way it is.

As to the springs, since even with both installed the ride is too soft, we need to figure out the spring rates and the load the springs are seeing. Since the coil-overs are mounted so far inboard on a Jag, the spring rates need to be higher than a more conventional suspension set up. That throws a lot of folks off on guessing spring rates for this suspension. If you can measure the wire diameter of each type of spring, the inside diameter of the springs, and the number of free coils I can closely calculate the spring rates for you. The wire diameter is a critical dimension and should be measured with calipers. Once we have that, I will need to know the spring height for both springs when they are in the car, and the car is at the proper ride height (installed height). I will also need to know the uninstalled spring height.

In regards to your talk with the tech at Aldan, I don't understand how if it theoretically takes two 250lb springs on each side to hold up the car, if you went to one it would take only a single 300lb spring. At your ride height at 12" shock length, with the two 250lb springs, they would be supporting about 750lbs per side. With one 300lb spring it would be only supporting 450lbs. You would have to preload that 300lb spring another full inch to get to 750lbs. Depending on the designed spring length when fully collapsed (coil bind), that might not be a good idea.

Andy
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Old 05-01-2009, 05:05 PM
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adjustable coil-over shocks for jag rear

Hello Andy: Thanks for your reply and support. Here's everything I know and have to offer.

With a 3" gap between the rubber bumper & the suspension arm bell, the c/c is 12". I cant move the shock mounts.

Front Shock: c/c open is 13". c/c closed 10 1/2", c/c compressed on car 11".
Front Coil: off car 7", compressed on car 6 3/4" in. Wire diameter is 3/8", inside diameter 2 3/4", free coils 5.

Back Shock does not travel: c/c closed 13", c/c compressed on car 11".
Rear Coil: off car 9", compressed on car 7 1/4 in. Wire diameter is 1/2", inside diameter 3", free coils 5.

With both shocks on the car sitting on the ground, there is only a 1" gap between the rubber bumper & the suspension arm bell. As stated in my previous reply, I stood on the back of the running board and it bottomed out.

What I don't know: is the shock system too hard and creating the hard ride over bumps or is the shock system too soft and the car is actually bottoming out on the slightest bump in the road?

I'm planning on using adjustable coil-over shocks so this may help you with your calculations for what I need. I have a limited expense account so, I would appreciate your suggestion on the shock manufacterer you think would do the best job.

I hope I have given you enough info to give me a final opinion and recommendation on what to do.

Thanks again,

Earl Fauerbach
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Old 05-02-2009, 11:36 AM
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adjustable coil-over shocks for jag rear

Andy,

Hope you haven't started on the calculations. I made a mistake in the measurements. Corrected below:

Front Shock: c/c open is 13", c/c closed is 11 1/4", compressed on car 11"
Front Spring: off car 7", compressed on car 6 3/4". Wire diameter is 3/8", inside diameter 2 3/4", free coils 5.

Back Shock: does not travel, c/c closed 13", c/c compressed on car 11".
Back Spring: off car 9", compressed on car 7". Wire diameter is 1/2", inside
diametere 3", free coils 5.

When I started trying to figure out the calculations, I realized the c/c started out at 13" for both shocks but, didn't didn't calculate correctly when compressed on the car at 11". I remeasured everything more precisely as shown above. The conclusion I reached was that the front shock c/c dropped 2" and the spring compressed 1/4". the back shock c/c dropped 2" and the spring compressed 2".

Sorry for the screw-up. Hope this helps.

Earl Fauerbach
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Old 05-03-2009, 07:30 AM
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To determine the spring rate on a IRS is more complicated than on a leave axle.



The formula is (a / b) x (a / b)
On an un shortened xj type Jaguar axle "a" is app 24,8" and "b" is app 14,17".

Unfortunately I don't remember where I found it, but I have certainly not invented it myself!

Then take the weight over 1 rear wheel and divide that number with the answer from the formula, and divide this on 2, since you want to use 2 springs on each side.
This is with coil-overs mounted straight up. If you for example mount them 35 degrees away from vertical, you must divide your answer with 0,66.

Speedway motors also have this table with recommended spring rates for their coil overs kit w/item number 255-2254:

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Old 05-03-2009, 01:29 PM
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adjustable coil-over shocks for jag rear

Staleg:

Thanks for the info.

earlf31
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Old 05-06-2009, 11:49 AM
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Andy Osborn - HELP - Coil-overs for jag rear

Andy:

Hope you haven't abandoned me. If so, let me know.

Thanks again,

Earl
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Old 05-06-2009, 12:50 PM
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Earl, I had a week like you wouldn't believe. I will run the numbers and get back with you. Are you sure the one spring you noted is 1/2" wire dia?

Andy
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Old 05-06-2009, 03:22 PM
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coil-over shocks for jag rear

Andy:

Could possibly be 7/16th at the least.

Earl
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Old 05-08-2009, 09:07 AM
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Earl, sorry for the delay.

These are the spring rates I came up with using the figures you gave me.

Front shock:
.375 wire dia
2.75" ID
5 coils = 186lbs/in

Rear shock:
.500 wire dia
3.0" ID
5 coils = 419lbs/in

or if it is 7/16" wire diameter, it is a 231lb/in spring.

You can see, as I stated earlier, the wire diameter dimension is critical as a small change has a large affect on the rate. You might want to measure it with some calipers. I would also think that the ID would be the same on both springs, but maybe not. My guess is that it is closer to the 7/16" wire dia, as a coil-over spring with 1/2" wire is pretty beefy. Let me know what figures you are comfortable with and we can go from there.

Andy
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Old 05-08-2009, 11:12 AM
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adjustable coil-over shocks for jag rear

Andy,

You're a life-saver. I will get a set of calipers and get more specific calculations. Your advice on the difference between 1/2" and 7/16" relative to the diameter of the spring is convincing enough with the variable in the spring rate (lbs.).

Sincere thanks,

Earl
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Old 05-08-2009, 01:44 PM
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adjustable coil-over shocks for jag rear

Andy:

First, I want to apologize for not going the caliper route when you suggested it on 5/1. You were right, my efforts to give you the correct dimensions was demented.

I went to my mechanic who has a veneer caliper and here is what "he" came up with for measurements in inches. This way, I can now blame him.

Front Shock:
Spring diameter: .45 in.
Inside diameter: 2.5 in.
4 coils

Rear Shock:
Spring diameter: .45 in.
Inside diameter: 3.0 in.
5 coils

The only thing I had right was the inside diameter and coil count on the rear shock.

I'm sorry and hope that you will continue to help an old oaf like me.

Eternally greatful,

Earl
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Old 05-08-2009, 08:30 PM
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We are getting closer, but I think we still have some issues.

Front spring: 574lbs/in

Rear spring:287lbs/in

To only have 4 active coils, that is unusual and a very stiff spring. When you are counting the free or active coils, begin your count at the point in the spring windings where the coils no longer are touching. Count straight down from there until you get to the bottom of the spring and then add any partial turns until the coils touch again. I hope this makes sense.

If the spring with 4 coils indeed has 5 coils active, the rate goes to a more believeable 459lbs/in.
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