Datsun Roadster Forum-311s.org

I ordered the nissan comp springs last fall and I have to agree with SloRoadster the front end sits to high. Short of chucking these springs and going with the datsun sports springs can I cut mine? I don't mind the spring rate being a little stiffer than the datsun sport springs.

Thanks for the advice.

I would not recommend cutting them. The cut will never fit well and could cause problems. It would take a lot of grinding to even come close. Do not do it. Heating them and compressing them is a better solution. Not perfect but better than cutting.

Steve

If you cut your springs, you loose spring rate (you make them too soft). The comp front springs make the front end sit higher with the comp rear springs in place. With the stock rear springs the front will sit lower. If you look at the May 03 car of the month you will see that my front end sits higher than the rear. Heating and compressing the springs is also a bad idea, it will do strange things to the steel, however that is the way the Datsun compitition manual says to do it.

Will

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Correct me if I am wrong but, when you remove a coil you loose the spring rate that was found in that coil. (Thats what I have found with working on mountain bikes and front suspension. I have cut springs to fit into different types of forks and found that when I swapped over the spring rate was way less. I could however be smoking dope)

Will

Time for an engineer or physicist to chime in here, but my understanding is that the removal of a coil (assuming that the temper of the spring is unchanged) effectively increases the spring rate. Think of it this way. You have an O ring that will compress 1/32 of an inch with 30 lbs on it. You stack ten of them and put the same 30 lbs on it and the whole shebang drops 5/16 of an inch. Remove 5 and it only compresses 5/32 (half as much). The spring rate (compression) has been increased by the decrease in the number of coils. (OK, it is an overly simplistic example, but I think it illustrates the point.) If I am wrong, don't sue me - I was a Political Science major in College and spent most of my time doing ROTC stuff....

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OK, I'll bite and get real geeky!

All linear springs obey this law:

F=Kx

Where F is the force, K is a constant based on material and geometry, and x is a displacement. This K is the often referred to value called the spring constant. Basically, this equation states: Apply a force to some element with a certain K and you get x deflection. Another fun equation relates a general force and displacement to the geometry. It is basically the same equation, but it is an easy way to show what the spring rate relates to...

x=FL / EA

I like to call this "Deflection equals flea"...Here x is again the displacement, F is the Force, L is the Length, E is the Modulus of the material and A is the Area of the member. For a coil spring, the L variable can be replaced with combinations of Coil Diameter, Pitch, Number of Coils, etc. that relate the total length of the wire used to make the spring, but to simplify the discussion...

If you combine the two equations and solve for K, you get:

K=EA / L

Soo...Drumroll...As L is increased (Length is magically added to the spring, number of coils is increased), the spring rate decreases...And as L is decreased (cut off a coil), the spring rate increases. Note that this is only determining the spring rate when the geometry is altered. If a spring is made shorter by applying a force to it (without yielding), the spring rate does not change, the spring just returns more force.

As for mountain bikes and dope...

What you might be seeing is a decrease in the preload of the spring. The spring must first provide enough force to balance the weight of the vehicle. This initial deflection and force response is the preload. Then as inputs go into the member (bumps and such) the spring is compressed further and the force rises with the rate law. If the preload is reduced (via shortening the spring, some adjustment, etc.), it is possible that the spring may not be deflected enough to provide enough force response and therefore feels soft. Spacers may be added to get to the same preload as before the shortening, then the suspension will feel stiffer...

Enough Geeking for tonight, TR

Hi all,

I am not a frequent poster, but because I had a large part in the design of the Datsun Sports springs I will chime in. TR, Alvin and Sid are correct. A springs' freelength does not have a part in its rate but coil count as well as wire diam, coil diam, modulus of the steel all do. When shortening a spring rate will increase because coil count has been reduced, even if only fractionally.
Think of coil count in an angular way, with the coils acting with leverage agianst one another. The greater the coil count the flatter the coils lay as coil count decrease the coil angle moves towards perpendicular to travel the given freelength.

Complicating issues,


Datzoom

Datzoom,

You are absolutely correct in your statement that, "When shortening a spring [, the spring] rate will increase because coil count has been reduced." If I have read everything correctly, this reiterates what everyone has concluded.

Think again about the free length statement, though...Imagine a pair of springs made from the same wire (same diameter and Modulus), made to the same spring diameter, both with the same number of coils...What if the free lengths were different, let's say one is double the other. I think you see what I am getting at, as in your example, the angles have changed greatly and the only thing different is the free length...

As I wrote in the second paragraph...

"the L variable can be replaced with combinations of Coil Diameter, Pitch, Number of Coils, etc. that relate the total length of the wire used to make the spring"

But I decided to leave out the many different combinations of these variables for simplicity. If you look at any of the spring formulas, they all calculate the total length of the wire used for the spring (not including the small corrections for types of ends).

That is why I like to think of a coil spring as a very long straight slender beam that has been rolled. As a matter of fact, all of the engineering equations for a coil spring are derived from the same straight beam theory. TR

I am smoking dope. I did some research and I am indeed wrong. I have no clue what I was thinking.

Will