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  #16 (permalink)  
Old 02-08-2013, 10:17 AM
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[quote=jaw22w;1643978]
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Originally Posted by aosborn View Post

No, not referring to the preload. When I input the weights and angles for my car using a 6.3" travel shock and a 12" spring, performance mode recommends a 150# spring with .47 preload. Then when I input the same numbers but change to a 4.1" travel shock with a 10" spring, performance mode recommends a 200# spring with .52 preload. I don't understand why when changing spring lengths, the spring rate needs to change.
I think what they are doing is giving you the fully extended preload setting for each spring rate. To end up at the proper "at ride height" dimension with the different rate springs with the same given load, there might be packaging constraints (length of coil-over threads for example) figured into their formula that would require you to have a different rate to give you enough bump and rebound travel and still be within the adjusment range of the shock.

It is somewhat of an akward calculator, but I think it will get you very close to the right rate, though their ride catagories seem to be a bit on the stiff side. I would think going one catagory softer than what they recommend would be a good idea. If that makes your car a little on the softer side in springing, make up for it with stiffer anti-roll bars.

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Old 02-08-2013, 01:04 PM
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On a street car, you would normally want the shock in the center of travel at ride height. With a 12'' spring, you have 6'' of shock stroke (3'' to center of shock stroke). A 10'' spring will have 4'' of shock stroke(2'' to center of shock stroke).
Using the same front end weight with both springs, the 12'' spring will need less spring rate so it can compress the additional 1''.
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Old 02-08-2013, 01:26 PM
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Originally Posted by cal1320 View Post
On a street car, you would normally want the shock in the center of travel at ride height. With a 12'' spring, you have 6'' of shock stroke (3'' to center of shock stroke). A 10'' spring will have 4'' of shock stroke(2'' to center of shock stroke).
Using the same front end weight with both springs, the 12'' spring will need less spring rate so it can compress the additional 1''.
I have always used 2/3 travel in bump (compression) and 1/3 travel in rebound (extension) as a basic rule for designing mounts and specing out shocks. The length of the spring won't affect the rate necessary. The same load is applied to a 10" or a 12" coil-over with everything being equal other than shock length. The only real difference we are talking about is more travel with a longer shock which is what I would recommend.

With that being said, If the rate I am after falls between two available choices, I go with the softer spring and just add pre-load to make up the ride height. By going this route, having the longer coil-over and spring is good so you will have plenty of travel without the spring binding up.
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Old 02-08-2013, 05:34 PM
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I always use 60% bump and 40% rebound. Yeah, that calculator is a little awkard and is for an a-arm suspension front end . I have a straight axle. For the motion ratio, I jacked up one front wheel 4". When I did that the shock moves only 3". So I used a .75 motion ratio. Not sure if this is completely accurate. I based my original spring choice off of the sprint car. We used 250 to 300# springs on the front of the sprint. Of course the sprint only has about a 300# corner weight in front. The hot rod has a 525# front corner weight. So I decided on a 225# spring. It is not uncomfortable to drive and handles well, but could be a little softer. I was just looking for some confirmation before I spend another $200 or so on new springs.

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Old 02-08-2013, 06:04 PM
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I don't usually factor a motion ratio on a live axle rear or beam axle front. Just use 1-1. Typically they have more unsprung weight which can throw you off if you don't factor that in. Sounds like you are on the right track though.

Sprint car huh, I would love to take one of those for a few laps sometime. I had an opportunity to drive a midget for a friend of mine, but he got out of racing before we could make it happen.

Regards,

Andy
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Old 02-09-2013, 07:35 AM
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I raced dirt late model stocks. 800 hp @2350#. A lot of fun. But my son races the sprints, 850 hp @1200#. I have only driven it at home on the country road while test running before a race, Man oh Man! will they run.
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Old 02-09-2013, 11:33 AM
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Sideways in a car with 850hp and weighs in at 1200lbs! And these drift guys think they invented going sideways with style. They are not even close.
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Old 02-09-2013, 12:23 PM
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No, not even close. Sideways all the way around the track. 12" of stagger will do that for you. Takes *****s big enough to carry around in a little red wagon!!!
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