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daywilmOffline
Post subject: Front Shocks on Replica Chassis  PostPosted: Apr 08, 2012 - 03:17 AM
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I have a replica chassis (Tubular Dynamics) and have adapted the NAERC suspension to fit. I have Heidt shocks and have taken Jim Dinner's advice on spring rate (450 lb. up front). The springs are 10.75" long when mounted on the shock and the shock is fully extended. My problem is that this just seems too long. Due to the binding issue with the upper ball joint (Honda) the suspension will not even drop low enough to get the shocks mounted so I have to use spring compressors to mount the shocks. The 450# springs are really stiff and it would seem they would put a lot of pressure on the upper ball joint. There isn't enough weight to compress the springs any at all when the car is sitting on all four wheels. I realize I have a lot of weight to add (engine, tranny, etc.) but most of that is from the center back and shouldn't add a lot to the front.

Any suggestions on spring length? What are you other replica frame guys using?
 
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EAMartinOffline
Post subject:   PostPosted: Apr 08, 2012 - 03:31 AM
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450 lbs. sounds rather high to me. I use 240 lb on most frames and that seems about right. The shock should be compressed 1/3 of its travel when the car is at ride height.
 
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dleachOffline
Post subject:   PostPosted: Apr 08, 2012 - 04:13 AM
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Isn't 400# to 450# the recommended spring rate for the rear suspension? As EAMartin says, somewhat less for the front springs.

Don
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daywilmOffline
Post subject:   PostPosted: Apr 08, 2012 - 05:01 AM
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I may be misunderstanding but from what I can tell Jim Dinner insists that on a replica frame you should use 450# springs up front and 250# springs (times 2 per wheel) for the back. MMBRAZIL even went as high as 500# up front if I recall correctly. However, I may just have to play with it a little until I find the right combination. I just would like to avoid shelling out a lot of money for different spring rates until I find the right one.

I would be interested in what length others are using so I could rule out one thing at at time.
 
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guywithlamboOffline
Post subject:   PostPosted: Apr 08, 2012 - 10:58 AM
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I have a 450lbs on front 11".
 
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jdinnerOffline
Post subject:   PostPosted: Apr 08, 2012 - 01:33 PM
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I am pretty sure mine are 10" springs. I could not fit 11" spring into the suspension .

450# is about right. I might go to 500# still to get a smoother ride.

Our problem is the geometry, normal spring rates for vehicle weight do not apply here.

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RTOffline
Post subject:   PostPosted: Apr 08, 2012 - 03:21 PM
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Jim is right about the geometry coming into play with spring rates. The load rating applied to the springs is how much force is needed to compress the spring one inch. A 500# spring requires 500# to compress one inch. If the wheel on your suspension moves one inch and the spring compresses one inch, your ratio is one to one. If your your wheel moves two inches and the spring compresses one inch, your ration is two to one which would mean your spring would have to be twice the rating to carry the same load. Of course anything between or above would be a proportion of that.
The angle of the spring is also a factor but if you just determine the amount of travel for the amount of compression you won't have to be concerned. After all, you are not mounting your springs completely sideways here and there will be a realistic amount of compression for wheel travel.
The length of the "A" arm comes into play too because the spring cannot be at the end of the arm where the ball joint is. It will be located about halfway between the ball joint and the inner suspension pivot. That alone will mean the spring will compress about half of the wheel travel.
Measure your distances and draw a little sketch. The ratio you need will be a little easier to understand and get right.
So estimate the weight loading at the corner and apply the ratio you have to estimate the spring rate you need.
The length of the spring is based on the space you have for the spring and how much it will compress when the weight of the car is applied compressing the spring, per the ratio you determine, to give you the ride height you want.
Stiffer springs produce a harder ride. The shocks are dampers that keep wheel bounce down and don't affect the spring rates.
Easy huh?

RT

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jdinnerOffline
Post subject:   PostPosted: Apr 08, 2012 - 10:05 PM
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OOOORRRRRR, get to know your local spring supplier and explain the problem you have. My guy has springs from 100# to 550# for the same price. He said as long as I don't scratch the powder coat off I can try different springs until I get one that feels right.
What I meant about a smoother ride sometimes a pothole will make the suspension smash against the bump stop, a stronger spring will prevent that.

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RTOffline
Post subject:   PostPosted: Apr 08, 2012 - 10:17 PM
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Jim is right about a smoother ride with harder springs that prevent you from bottoming on the bump stop.
Sometimes trying to explain things in writing, isn't as simple as you think.
Up travel bump stops are a very good idea but you really don't want to use them if you can help it. If your driving makes them bottom out a lot, you need a stiffer spring rate.

RT

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daywilmOffline
Post subject:   PostPosted: Apr 09, 2012 - 01:06 AM
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Thanks RT and Jim. It all makes sense and I may not be able to figure out the exact spring I need until I get more stuff connected to the chassis so the finished weight (or something close) is there. Sometimes things don't work out like I envision they should and I come to this forum for advice. I am never disappointed with the help I get and hope to be able to return the favor sometime.
 
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