Suspension quiz

[Igofar]

This is the shock spring under my police saddle on the Harley Davidson.
As you can see it is threaded and has a lock nut, and an adjustment nut.
Here is the question…
Will loosening the nut, counter clockwise, and removing pressure off the spring, raise the seat height, lower the seat height, or change nothing?
The instructions indicate that reducing the preload, will make the spring taller, and will raise the seat height.
Anyone want to clarify for me please.
The unit is bolted to the frame, and the seat bracket sits on top of that post, so how will any adjustment on the spring change height?

 

[ibike2havefun]

If I remember my high school physics class correctly, coil springs always compress by the same amount (tension) under any given load, whether they start from their fully-relaxed length or from some partially-loaded condition.

My thought is that reducing the preload pressure on the spring might raise the seat when it's not loaded (empty) by allowing a greater degree of extension and overall travel but I'd hazard a guess that when you are sitting on it and applying a static load the height would be the same.

Doesn't pre-load simply reduce the amount by which a seat will travel (recover), and perhaps the recovery rate, after being compressed, and possibly also reduce the speed at which it consumes the remaining travel?

If so, it seems to me that the effect would therefore be to reduce the tendency to launch you, catapult style, after a sudden bump and to somewhat reduce overall "bounciness" but wouldn't change either the maximum amount to which the spring could be compressed nor the degree to which it compresses under some static load that is less than the spring's overall capacity.

Bear in mind that I'm neither a mechanic, nor physicist, nor do I play either one on TV, so this is just me spitballing into the void.

Someone with actual knowledge will undoubtedly come along and expose / correct my ignorance.

 

[RaYzerman]

Reducing the preload on a shock does not change the ride height. It only makes the spring "softer". You first measure sag, then adjust preload to the desired amount of sag. Most shocks do not have a ride height adjustment, but some do (Penske's, e.g.).

Not to be confusing, but increasing preload will increase ride height somewhat as the spring will push back harder on the load it carries.

 

[Larry Fine]

Just as with suspension, it adjusts the loaded ride height.

 

[W0QNX]

The instructions indicate that reducing the preload, will make the spring taller, and will raise the seat height.
Anyone want to clarify for me please.

I'll bite.

Throw those instructions away if they say it will raise the seat height and use these below.

The seat will only be taller if a light rider sits on a heavy preloaded seat. Skinny guy won't compress the preloaded spring as much. Fatboy sure will.

If the shock is fully extended at both minimum prelaod and max preload the length of the shock didn't change, but the preload of the spring sure did.





https://serviceinfo.harley-davidson...69251086968193/288096/J05609_536689_en_US.xml

 

[TerryS]

Let me get this straight...this is a shock absorber solely for the seat, and not related to the swingarm? HD's truly are very different beasts.

 

[W0QNX]

Let me get this straight...this is a shock absorber solely for the seat, and not related to the swingarm? HD's truly are very different beasts.

I’m pretty sure they stole that design from Schwin.

 

[St-Gerard]

Seat height is the length of the Shock, without a load. Spring adjustment will effect how soft or stiff once a Rider is on seat depending on riders weight.

 

[SMSW]

Mr. Fine got it right, unless Larry gains weight after increasing the preload. From the picture, I'd say that is going to be a very uncomfortable seat....but you won't slide off...

@W0QNX - for a given rider, if he is not fully compressing the spring, increasing the preload will raise the seat (unless he eats the whole pizza for lunch). No different than the preload adjuster on a bike's shock.

@TerryS - Not related to the swingarm. I'm sure you've seen old bicycles with springs under the seat. Since mc's go faster than bicycles, it makes sense to dampen the bouncing of the seat - that's what this does. IIRC, over the road trucks - Semi tractor trailers have air seats that absorb jolts from the hard suspension needed to carry 40,000lb loads. These seats have to have some sort of dampening mechanism too or the driver will be pogoing all day long until his eyeballs get a permanent oscillation.

 

[skidgillen]

I'll bite.

Throw those instructions away if they say it will raise the seat height and use these below.

The seat will only be taller if a light rider sits on a heavy preloaded seat. Skinny guy won't compress the preloaded spring as much. Fatboy sure will.

If the shock is fully extended at both minimum prelaod and max preload the length of the shock didn't change, but the preload of the spring sure did.





https://serviceinfo.harley-davidson...69251086968193/288096/J05609_536689_en_US.xml

this diagram gave me flashbacks to when we updated an officers bike to the new coilover shock for the seat compared to the old air ride one that he kept blowing out due to his size

 

[Larry Fine]

It has always amazed me that so many people think that "preload" changes the spring rate or length.