From digest.v6.n250 Thu Feb 20 16:08:07 1997
From: Pete Read <read_at_arl.bna.boeing.com>
Date: Thu, 20 Feb 1997 14:02:11 -0800
Subject: Re: <MISC> H&R vs. Dinan vs. Eibach springs

Paul Kunz writes:
>Dinan's theory is that progressive rate springs are a bad idea for
>aggressive driving. The reason is that the soft initial compression of
>the springs allow the car to roll then the angular momentum built up by
>this roll hits the firmer part of the compression. This puts a heavier
>load on the outside than linear springs because the linear springs
>would not have allowed as much roll in the first place.

Filippo Morelli writes:
>>If the car is street-bound, I would almost categorically vote for
>>progressive rate springs -- they handle the bumps and cracks much
>>better and still stiffen tight under load.
>>... When you have a spring that is progressive it is, in effect, a
>>seamless two stage spring.

Filippo, I think you're missing Paul's point. Most progressive springs don't have "seamless" transitions in rate. They change rate in significant steps as active coils bottom on each other. A sudden change in spring rate, while at the traction limit, can result in a nasty slide.

Now if the spring wire is tapered (constantly changes wire diameter), the springs should give a "seamless" transition as you say. However, it's much cheaper to manufacture a progressive rate spring by using constant wire diameter and varying the spring pitch (distance between coils).

Here's an example. I'm looking at a catalog picture of some Eibach Pro-Kit springs. The front springs are constant rate while the rears are progressive. The rear springs have 10 total coils, with the top and bottom coils wound much closer together (less pitch). I'm going to assume the end coils are inactive because they rest on the spring seats. This leaves eight active coils.

Spring turned sideways (because it's easier to draw), 8 active coils

12 3 4 5 6 78 <- coil number
:: : : : : ::
:: : : : : ::

                \ 
                 Spring coils

Spring rate is inversely related to the number of active coils. To use some numbers, let's say that the starting rate is 175 lb/in.

Notice that coils 1,2 and 7,8 are wound more closely together (less pitch than the other coils). As the spring compresses, those coils bottom on each other, reducing the number of active coils and increasing the spring rate.

Initial rate:

175 lb/in, 8 active coils

Rate when coils 1,2 bottom

200 lb/in 7 active coils (175 x 8/7)

Rate when coils 1,2 and 7,8 bottom

233 lb/in 6 active coils (175 x 8/6)

In this example, the spring rate changes in 25-33 lb/in steps. Not exactly "seamless". It may only be a problem for cars driven at the limit, but I think Paul's point is valid.

Hope this answer is "rated" well <g>,

Pete Read
'88 M5