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General Category >> Rubber Side Down! >> Stock shock spring rate and travel?
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Message started by arteacher on 07/17/12 at 11:04:00

Title: Stock shock spring rate and travel?
Post by arteacher on 07/17/12 at 11:04:00

Anyone have this info?

Title: Re: Stock shock spring rate and travel?
Post by thumperclone on 07/17/12 at 11:32:49

rear wheel travel 3.1"

Title: Re: Stock shock spring rate and travel?
Post by arteacher on 07/17/12 at 11:43:30

475B465E435641505F5C5D56330 wrote:

rear wheel travel 3.1"

Thanks Thumperclone. I saw that in the specs as well. It doesn't really answer my question though, as I am trying to compare shocks.

Title: Re: Stock shock spring rate and travel?
Post by engineer on 07/17/12 at 18:28:05

I worked out the spring rates about a year ago for the Savage and a Harley Sportser because I wanted a softer ride and had some Sportster shocks. I made the assumption that the spring material was typical of the stuff used in automotive OEM springs and made the computations based on that data and measurements of the actual springs. My numbers may be off a little because they might use a different alloy and heat treatment than is typical in the automotive industry but I didn't care because all I wanted to do was to compare two different shocks.

The spring rate I calculated for the Savage is 140 lbs per inch. It has about 10 active coils and the total travel is just the sum of the spaces between the coils. The spacing is equal and they all bottom up at the same time.

The Sportster shocks are progressive with the space between the coils being progressively smaller at one end so that as the spring compresses under laod some of the coils "bottom out" leaving fewer coils in operation. The Sportster shock starts out softer at 85 lbs per inch. But the HD spring rate goes up to about 140 lbs per inch after the first inch of travel.

Remember that these shocks are mounted at an angle and ahead of the axle so the wheel travel is longer than the shock travel.

So by my calculations the Sportster shocks do start out considerably softer but after they are compressed an inch or so they are about the equal of the stock springs with one major advatage, spring travel is longer. So that spine jarring impact when you hit a pot hole and the coils bottom out may not even happen.

Also keep in mind that the force required to compress the spring is cumulative, every inch you add on that much more force. The Harley shocks are mounted at a different angle and distance which is why it has a lighter spring rate even though it is a heavier bike.

I haven't mounted mine yet and don't intend to in this hot weather. An old man like me isn't happy sitting over a hot engine on hot asphalt in 90 degree air. I'm waiting for September.

Title: Re: Stock shock spring rate and travel?
Post by arteacher on 07/18/12 at 06:57:55

Thanks very much, engineer.
Travel is limited by the length of the exposed shaft in the shock itself, not the distance between coils. I just measured it at 1 3/4 ".