Topic: Adding stronger fork springs = "klunk!"

[Kawahonda]

Working on a little mini bike.

I've added stronger springs to the front end of it since I'm well, an adult.

I also went to 20w fork oil, and put 4Oz in each leg. Factory spec was 3.2-3.6oz per leg.

Now the rebound is too much. Going down my curb, or going over bumps of any sort, will cause the forks to top out on rebound which creates a klunk sound.

I don't have the best understanding of fork dynamics, but I'm thinking I should start to lower the weight in the forks, and possibly put less oil in each one?

Thinking that I may want to try 10w next at 3.5oz instead. Or, should I do something more drastic? Or, does fork oil and fork weight actually play a big part in what I'm trying to solve?

[scottly]

You could try heavier oil, but that will also increase compression damping. Your springs may be too stiff.

[Kawahonda]

I think the goal here is to decrease dampening (i.e. slow it down) and not to increase dampening (speed it up), right? So isn't moving towards heavier oil moving in the wrong direction?

[scottly]

I think the goal here is to decrease dampening (i.e. slow it down) and not to increase dampening (speed it up), right?

You've got it backwards.

[Kawahonda]

Yep, my suspension noob-ness is showing.

So putting lighter weight fluid is going to make the rebound force higher.

And going to a heavier fluid may help, but it's going to most likely cause the bike to ride like a damned pogo stick...

So more damping means to slow it down.

Less damping means to speed it up.

So I would want more damping. More damping = heavier weight oil. Could it also mean more oil?

[scottly]

The new springs are over-powering the available damping, which causes the klunk. How far from being topped out are the forks compressed with you sitting on the bike?

[Kawahonda]

Essentially, that's a measurement of the front "race sag", correct? I do think that would be helpful to know in this case to see if optimizing for these springs is even worth it, or a waste of time. I'll measure the movement today for that.

Basically, we want to see a healthy amount of sag. To little sag means these springs should be scrapped.

[scottly]

How much travel do your forks have? Sag should be about 1/4 to 1/3 of travel.

[Kawahonda]

Thanks scottly.

Also, one last (easy) adjustment is draining an OZ out of each of these forks to land me at 3OZ, which I believe is still acceptable for my forks. That should in theory reduce the knocking, and possibly allow me to dial in the race sag?

[Kawahonda]

Hi Scotty,

My race sag was like, 5%. I drained some oil out (but still left it in spec per the manual). It's now at 20% race sag, and the problem is 98% gone (which to me is good enough because it's actually REALLY hard to get it to knock again!).

I'd like to further get my sag to 25%. Would going to a lighter weight oil do that?

I like where you're going with the sag, because dialing in the sag is in essence, dialing it in from NOT toping out!

[scottly]

Normally the sag is only controlled by the spring rate, but over-filling the forks may have been the problem, since lowering the level helped. The weight of the oil has no effect on the static sag.
Since the problem is 98% gone, I wouldn't worry about 5% sag.

[Kawahonda]

Thanks.

Question, let's say I drain out the 20wt oil and put in 15wt oil, and refill to the same level...

Which would would help it further from topping out?

Or, would you expect zero difference.

[scottly]

I would expect the topping out to be worse with 15wt filled to the same level.

[Kawahonda]

Super awesome education, thanks Scottly!