Lowering the front of your Harley

[Dave63]

Found this on another site, but though it might help some people here with a VISUAL of how front forks are lowered.

http://www.progressivesuspension.com/video02.html

Keep in mind, this is not exactly an original idea. Guys have been cutting springs and adding heavier fluid and a longer preload spacesr for decades. The fact that Progressives springs are just that: progressive compression rated springs, makes it better then just cutting springs, as it prevents bottoming out due to becoming stiffer (higher spring rate) the more you compress them..

You'll note the top red spring they used is fully compressed just on static weight, but will aid in keeping preload when the bike's front lifts, as in over a swell in the road, preventing banging and clunking in the front end. Much better than a piece of PVC pipe, or steel tubing.

The nice part about Progressive's stuff is that it comes with easy to understand instructions and for the 15+ sets I've installed, I never got wrong parts, bad parts, missing parts or anything negative from them, ever.

[Kainam]

Interesting. I wonder how the new "drop in" fork springs compare to the "lowered" fork springs? I have the lowered set on my bike with the PVC tubing, curious if the drop in set offers enough of an improvement to be worth the hassle of dismantling my forks to remove the old set (top out springs) to install the new set?

[Dave63]

Brian, my guess is the top "preload" spring that eliminates the solid, preload spacer, all in itself. Outside of that, why didn't they just make the spring all one length, and a small preload spacer, like the set I have? The only difference seems to be that red spring....

I can reproduce that on my own.....

[EZE RIDER]

I dont understand what is so radically different. Isnt the old kit installed much the same way?

[Kainam]

The new "drop in" springs don't require the top out springs to be installed on the damper rods. In my case, I have a total of 3 top out springs on each of my damper rods (1 stock + 2 Progressive Suspension). Much simpler installation when starting from stock.


See step 4 in these instructions:
http://www.progressivesuspension.com...rkLowerKit.pdf

[Dave63]

Originally Posted by Kainam

The new "drop in" springs don't require the top out springs to be installed on the damper rods. In my case, I have a total of 3 top out springs on each of my damper rods (1 stock + 2 Progressive Suspension). Much simpler installation when starting from stock.

Yep....yer right. I forgot about the top-out springs that I added under my dampers.

So now, it's facotry top-out springs, shorter progressive springs, preload maintaining spring and small preload spacer at the top....

Heavier fluid would be good too......15wt for larger guys, at least.

[EZE RIDER]

Originally Posted by Dave63

the top-out springs that I added under my dampers.

I forgot those are changed out when lowering the bike. I used a stock length replacement spring which didnt require changing the top out spring.

Thanks for the refresher Brian.

[Nathang]

makes it better then just cutting springs, as it prevents bottoming out due to becoming stiffer

'Course you know, cutting any coil spring shorter increases it's stiffness dramatically.

[Kainam]

Cutting coils in the amount that we are talking about does increase the spring rate (k), but not dramatically.

Stock springs for my FXST have a spring rate of ~32.5 Lb/in. Cutting 3 coils (total of 1") off would increase the spring rate to 34.66 Lb/in.


Wire diameter (d) and coil diameter (D) play a much larger role in determining spring rate than the number of coils (n).

Spring rate calculator

Stock FXST spring values:
G = 11,250,000 psi
d = .192 in
n = 48 coils
D = 1.07 in



From this formula, you can see that the number of coils (n) is a first order variable, and that the diameter of the spring wire (d) is a fourth order variable and the diameter of the coils (D) is a third order variable.

[Nathang]

Accurate, I'm sure, but doesn't do much to make the concept clear.

Want to try it w/o a formula or any math?

A coil spring is a torsion bar in a compact package.

I laid under the front end of an old Chrysler Imperial once, changing a starter for a friend. It was so big I didn't need to jack it up, just running one front wheel up on the curb gave me room to work under it, and meant I didn't have to have any ramps or jackstands.

I wasn't real familiar with Chryslers, something didn't look right about the suspension, then I realized, I could see the front shock clearly, there was an upper and lower A-arm assembly, but there was no spring. I pondered what the hell was holding it up, it was a monstrous boat, had to weigh over 7000 lbs. It was a creepy viewpoint from which to study the physics of a torsion bar suspension.

The front suspension on was a Torsion Bar about a foot long and an inch in diameter. I was shocked to see it had no coil spring at all. Scared me at first, it seemed to have no support, as the torsion bar looked like it was only part of the pivot for the lower A-arm. The entire support of the front end was done by that bar allowing and resisting torsional twist over it's short length. When the front end bounced up and down that bar was twisting significantly. If it had been a little shorter, the severity of that twist would have been magnified, if it had been significantly longer, it would have much easier to twist it the same number of degrees.

It was clear to me that if that bar had suffered metal fatigue at that moment, the last sound I ever heard would be " tink!BOOM! "