Uniform Spring Force: NASCAR Engineers' Innovation?

[Matt_B]

Not really something I am working on at all, but a while ago I had heard that engineers for some of the race teams, mainly NASCAR have developed and put into use and compression spring that has a uniform force of the entire distance of deformation. To put it better, you have a suspension spring in a car, at the exact start of travel it is at 600lbs, and remains at 600lbs until coilbind. Has anyone ever heard of this? Is this even possible? I have been rolling this around in a my head for a long while, and no matter how i come up with it, I do not see how this could be possilbe.

Any insight or opinions?

 

[berkeman]

I googled "constant force spring" and got a lot of hits, including some patent links. Here's the hit list.

http://www.google.com/search?source...=1T4SUNA_en___US232&q="constant+force+spring"

Looks like the most common CF Spring is made of a coiled ribbon...

 

[Crosson]

Thank you for bringing up an interesting topic. Attached is a plot of amplitude vs time for the two different kinds of springs, assuming that they start at the same initial condition and both of them have damping proportional and opposite to their velocity. The solid blue line is for the constant spring, while the dashed line is for the normal 'Hooke's law' spring which has the spring force proportional to displacement.

As you can see, the constant force leads to much faster oscillations that are more quickly damped. I suspect that the increased damping, which results from the higher velocity and hence the greater friction, is the reason why NASCAR engineers are using shock absorbers with these characteristics.

I do not see how this could be possilbe.

I can imagine a few practical ways to implement a constant spring force like this. The Bose coporation, which is mostly known for its excellent audio products, also holds patents for an electronic suspension that uses motorized actuarators to dynamically respond to bumps in realtime. This is already a significant step of complexity beyond the constant spring force, since there is an on board computer that optimizes the spring force individually for each imperfection in the road. Here is a quick article:

http://www.edmunds.com/insideline/do/Features/articleId=103183

It has been a few years since I looked at this Bose technology, so I might be fuzzy on the details, but this system is certainly capable of the constant spring force you described.

 

[Matt_B]

From what i am reading on that hit list you provided what they are calling "constant-force springs" is a roll spring

which is not what I am talking about, i am talking about a compression spring

 

[Matt_B]

What i am saying though, is that it is not hydraulic or pneumatic, nor electronic. It actually looks like a normal spring you would see in a car when you look at the suspension. The only difference is that it does not raise in resistance when deformation increases

 

[jamesrc]

If you're asking how it can be done in general (I don't know how this specific spring was made since I'm not looking at the design), some of the factors influencing the stiffness of a coil spring are the diameter of the spring, the diameter of the coil, the number of coils and the material being used. If these are all constant over the length of the spring, you'll end up with a linear force versus displacement characteristic. But if you vary the parameters by, say specifying the shape of the spring as conical rather than cylindrical or if you vary the pitch of the coil (number of coils per unit length) you can design the spring so that it approximates a constant force over a given displacement range.

 

[Matt_B]

If you're asking how it can be done in general (I don't know how this specific spring was made since I'm not looking at the design), some of the factors influencing the stiffness of a coil spring are the diameter of the spring, the diameter of the coil, the number of coils and the material being used. If these are all constant over the length of the spring, you'll end up with a linear force versus displacement characteristic. But if you vary the parameters by, say specifying the shape of the spring as conical rather than cylindrical or if you vary the pitch of the coil (number of coils per unit length) you can design the spring so that it approximates a constant force over a given displacement range.

here is the main problem with that theory... if you make it conical, or vary the pitch or even vary wire thickness, you are still going to reach a point where somewhere in the travel the force applied to continue the compression will have to increase.

 

[berkeman]

here is the main problem with that theory... if you make it conical, or vary the pitch or even vary wire thickness, you are still going to reach a point where somewhere in the travel the force applied to continue the compression will have to increase.

Of course. Even with an F=kx spring, it will bottom out at some point. Every spring assembly has a limited range of useful travel. It's likely that a constant force spring will have additional limitations on the range of travel where the force is approximately linear.