Spring preload

[Paul Szilard - Australia]

I've had an interesting discussion with an ex motocycle journo about spring pre loads and the question of using pre-load to make the ride harder.

 

His argument, which I can see no flaw with, is that ignoring coil binding (that is when adjacent coils are so compressed that they touch each other) changing the pre-load does NOT make the ride any stiffer:

 

Imagine a theoretical spring, say 1 foot with no load, that compresses 1" for every 100lb pressure.

 

Assume that the unladen bike weighs 100lb at the rear suspension, so the spring is compressed by 1" to 11". Now I sit on the bike and I weigh 200lb so the spring has 300lb compression and is pushed down to 9".

 

If I use the pre-load to pre-compress the spring so that the unladen bike sits on 10" spring and then sit on it, it will only compress to the same 9" due to the same combined pressure of 300lb.

 

The only noticeable difference is if when riding, the bike rebounds after a bump, and the spring can't extend as far, but in riding normally, in both cases I am riding on a 9" compressed height spring, therefore no difference in hardness.

 

Any comments???

[T__]

Paul, within reason,, & assuming non progressive springs,, & assuming very linear strut compression the spring pre-load won’t change anything except the ride height.. As you say if you get 1” per 100# spring deflection you get that no matter what the preload as long as the number of WORKING coils remains the same..

 

BUT, in real life maybe not so.. Progressive type springs will coil bind the soft coils first (usually by design),, Springs with the end coils closer together will coil bind those first then both the spring load & spring rate will increase..

Probably the most difficult thing to predict is the strut angle vs the compression/extension travel..

Unless the working angle of the strut stays the same throughout it’s travel the EFFECTIVE spring rate & load can change as the working angle changes in relation to the rear wheel travel..

Then there is the control arm working angle in relation to the bushing pivot center vs the wheel center.. The more angled down the rear control arm is the more vertical input that is transferred into the pivot mounts at impact time..

 

There might also be some slight weight transfer difference due to front/rear height differences..

 

Twisty

 

[swfraley]

 

Imagine a theoretical spring, say 1 foot with no load, that compresses 1" for every 100lb pressure.

 

 

If his argument fails, it fails here. If your theoretical spring takes the same 100# to compress each inch, then the conclusion is valid. Unencumbered by data, I don't think there is such a spring anywhere, but especially not on a motorcycle.

[T__]

Stu, not exactly.. Most straight rated springs are rated in pounds per inch or newtons per mm..

 

In his 100 lbs per inch deflection each additional 100 lbs deflects the spring another inch.. So 3” deflection (compression) would be 300 lbs..

 

So if you have a motorcycle rear that weighs 300 lbs that would compress the rear spring 3” (assuming direct acting strut & no compound working angle).. Then if you turn the rear lower spring collar adjuster up 1” that will just raise the rear of the bike 1”.. You haven’t changed the rear load on the spring (still 300 lbs) you have just moved it’s seat up 1”..

 

Now that works until the rear shock runs out of up travel.. Once the rear shock runs out of extension travel the spring load would increase it’s load by 100 lbs for each inch you shorten the spring.. You would then be more than likely close to coil bind on compression.. The suspension would also be topped out..

 

Springs are kind of odd ducks as long as they don’t coil bind or ground out coils.. The material use to make the spring has a fixed linear deflection rate as long as the coils are all the same diameter material..

 

 

Twisty

 

[Haynes]

The logic is good. In reality, a torsional twist spring does not compress but twists along its length.

In the main, adjusting preload is more about ride height.

[IanW]

Information here:

 

http://www.promecha.com.au/

 

Ian