Progressive Fork Springs Issue

[JimVonBaden]

I installed progressive fork springs in my 07 Scrambler a few weeks ago, and left the stock oil in it, I was in a hurry. I have 3000 miles on the bike. I also shortened the spacers from the recommended length by about 1 inch so that the spacer was flush with the top of the forks. I have done this on several BMW K-bikes and it worked well.

However, I at first thought I had an issue with the tires (and I did, but not this issue), but I changed tires and this issue remained. At about 20-35mph I have a distinct bouncing on the front end, not out of round wheel or out of balance, but acting like it. I was wondering if this would most likely be a preload issue or a fluid viscosity issue?

I am very good with the BMW suspension, but this one is new to me.

BTW I weigh 230 pounds and tend to ride pretty hard.

Any thoughts would be appreciated!:wave:

Jim

 

[Carnation]

Sounds like insufficient preload - did the instructions say to cut the spacers? I have never done that with progressive springs, any I have used have required the stock spacers be left alone.

 

[JimVonBaden]

Sounds like insufficient preload - did the instructions say to cut the spacers? I have never done that with progressive springs, any I have used have required the stock spacers be left alone.

On the Scrambler the stock spacers are about 8" long. The stock springs are, I believe, the same as the Bonneville, but the travel is more so they need longer spacers.

The Progressive springs are much longer, making the spacers about 3.5" by the book, but that made them stick out of the fork tubes a lot so I cut off 1/2" to shorten them.

It is possible I should have left them longer.

Jim

 

[schnabba]

could be preload- what is your static sag now? Usually that just means you bottom out faster and the front end is generally lower.

I would guess you need more rebound damping! Since you can't adjust it externally, you'll have to use heavier fluid. I have that a little bit in my triumph tiger - went lighter on the fluid, and need it thicker for those springs (even with cartridge emulators). It's minor enough that it is barely noticeable, so I haven't bothered yet. Next tire change I'll do that.

explanation of stuff here

thicker oil would be my first try (relatively cheap to do).

 

[JimVonBaden]

I'm definitely not near bottoming out. There is little movement when I feel the "bouncing" feeling moving sown what would seem to be a smooth road.

That said, I "could" need more preload, or heavier fluid from not enough damping, but it really feels like overdamping to me.

Thanks, and keep it coming, I am developing some ideas to try out.

Jim

 

[schnabba]

I figured overdamping would be really harsh.... not pogo bouncing, but transfer hard bumps to you.

one other thought - with a change in the front, the combination of the front / rear ... you might be feeling the not so good stock rear of the bike! Try setting the preload on the rear to a higher setting, see if it changes. that's a free thing to try.

This is a good site - there is a nice explanation of suspension stuff, and a spreadsheet for setting up a bike (mostly track related to aprilia bikes, but gives common symptoms of over / under damped, and over / under sprung on one of the tabs - a decent reference at the very least).


website linky

 

[DIRK]

if there is to much you can have that bouncy feeling, it is like you've over loaded your rear shock

 

[JimVonBaden]

if there is to much you can have that bouncy feeling, it is like you've over loaded your rear shock

It was there with the stockers, and there with the Hagons. I doubt it is the rear shocks.

It really feels like it is over damped.

Jim

 

[schnabba]

ah. It could be overdamped. What I interpreted "pogo" as was a lack of damping type thing - where, on a relatively flat road surface, the front end will bounce.... I guess an overdamped front end might cause the front to rise with each imperfection, causing a bouncy feeling.

that's usually noticeable though - it's like the front end is overly harsh / hard. Transfers a lot of vibration and shock to your handlebar...

well - start trying some things man! Set your preload / ride height (static sag), since that isn't going to change with the fork oil.... then, start with a known fork oil (no telling what's in there now). Once you have that baseline, you can move up or down on the viscosity.