Can someone explain shock spring pre-load
Road Captain
Joined: Sep 2013
Posts: 687
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From: Maryland
That's not really an accurate analogy. Putting more air in your tires (or shocks if you have them) actually makes them stiffer. Putting more preload on a spring does not make it stiffer. Spring rate is a function of coil diameter, wire diameter, and number of coils.... nothing else. If you preload a spring, you reduce it's travel, that's all. For example...
Say a spring has a 100 pound per inch spring rate and is 10 inches long. When installed it's preloaded 3 inches. It now has to see 300 pounds before it will start to compress. Take an identical spring and preload it 5 inches. It now needs to see 500 pounds before it will start to compress. Assuming both springs will go coilbound at 8 inches compression, they will BOTH be exerting 800 pounds of force. The only difference is that the 5" preloaded spring will not move under 300-400 pounds load and the 3" preloaded one will. The 3" spring will give a softer ride over small bumps because the 5" spring is like a rigid frame at that point.
The 5" preloaded spring will not bottom less under equal load.
Say a spring has a 100 pound per inch spring rate and is 10 inches long. When installed it's preloaded 3 inches. It now has to see 300 pounds before it will start to compress. Take an identical spring and preload it 5 inches. It now needs to see 500 pounds before it will start to compress. Assuming both springs will go coilbound at 8 inches compression, they will BOTH be exerting 800 pounds of force. The only difference is that the 5" preloaded spring will not move under 300-400 pounds load and the 3" preloaded one will. The 3" spring will give a softer ride over small bumps because the 5" spring is like a rigid frame at that point.
The 5" preloaded spring will not bottom less under equal load.
You described a linear spring rate...I was talking about a progressive rate spring...which I believe Progressive shocks (and most MC manufacturers) still use.
Like I said...I was trying to make my reply simplistic. I wasn't going to get into different spring types...e.g. linear vs. progressive, etc....let alone all the other factors associated with suspension set-up. Simplistically, heavier load means dial-up the preload...just like putting more air in the tires... I know...a less than perfect analogy...but simple...
http://automotivethinker.com/suspens...-rate-springs/
That link isn't the best available, but gives a basic explanation of what I was trying (poorly) to explain. I believe the Progressives are truly 'progressive', not linear...at least mine were.
Last edited by Wheels24; Mar 17, 2015 at 08:56 PM.
Road Captain
Joined: Sep 2013
Posts: 687
Likes: 5
From: Maryland
Not sure what you mean by reducing travel...unless you mean that by reducing preload, the shock will initially be 'shorter' under the load of rider (and passenger) because it will sag more initially...and therefore have less useable travel. Increasing preload decreases sag, which - in a way - makes the useable shock travel 'longer'.
Suspension tuning damn near requires a college degree in physics and geometry these days, including fluid dynamics. Spring preload, rebound damping, compression damping, rake, trail, spring rates, fork oil weight, valving, tire choice/pressure...my least favorite part of race bike set-up...and by far the most frustrating...
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