Can anyone tell me what we did wrong?


14 replies to this topic
  • Knowledge seeker

Posted December 13, 2011 - 10:01 AM

#1

We changed the fork springs on my buddy's '07 YZ450 to suit his weight. After that the rebound is really fast even after giving it 6 clicks. It was OK before the spring swap so we must of done something wrong which we have no clue. We need some direction to go from. Also, what oil weight do you guys use on these forks. Any help will be greatly appreciated.

Roger

  • grayracer513

Posted December 13, 2011 - 10:09 AM

#2

The rebound at which end? What springs are you using? Are you testing the rebound with the rider on or off the bike?

  • crf450319

Posted December 13, 2011 - 11:29 AM

#3

From what I've found, the most common fork oil is 5 weight, I put 360 ml in the tube but I think the range is somewhere between 340 to 380 ml ?

Has he ridden the bike ? Did the rebound feel too fast then, or like gray said was this a test in the garage ? With heavier springs, the forks will rebound faster and you'll more than likely need to run with more rebound (fewer clicks counter-clockwise from being bottomed out).

Where are you with your 6 clicks ? Do you mean you've turned it 6 clicks counter-clockwise after you've turned the adjuster clockwise as far as it will go ? Or are you starting with the adjuster backed out all the way (maxed out counter-clockwise) and turning it in 6 clicks ? Or do you mean that you put it back to the old setting, turned it clockwise 6 more clicks and it's still too fast ?

We need a little more info...

  • Knowledge seeker

Posted December 13, 2011 - 01:14 PM

#4

Alright, alright... I apologize for expecting a precise answer with not enough information. He has raced it twice & the front wheel bounces off the ground a couple of times when landing from a jump. The 6 clicks we gave it was clockwise but what I don't know is the initial rebound settings he had before the 6 clicks. So, is there any ballpark settings to begin with before making assumptions?. He's a 40 old vet & weights 220 lbs. Thanks guys for your replies.

  • crf450319

Posted December 13, 2011 - 02:16 PM

#5

Haha, no problem...

The standard setting for my '09 is 12 out (12 clicks counter-clockwise), I run my bike at 9 out. What I've done before (although I'm not proud to admit it) is before I've started taking my forks apart I'll record all of my comp/reb. settings, then when I'm done servicing I'll set the clickers back to where I'd initially had them... most of the time.

I have on one occasion, forgot to set the clickers after I've finished working on the forks/shock. It'd be a good idea to go over all of the settings on the forks and on the shock.

Here are MXA's fork/shock settings :

Forks
Spring rate: 0.47 kg/mm
Oil height: 350cc
Compression: 12 clicks out
Rebound: 12 clicks out
Fork leg height: 5mm up
Notes: The stock oil height is 350cc, but if you feel that the forks are too soft in the second half of the stroke, add 5cc of oil to each leg. Kayaba’s SSS forks are very sensitive to oil height changes and 5cc makes a big difference.

Shock
Spring rate: 5.5 kg/mm
Race sag: 100mm
High-compression: 1-1/2 turns out
Low-compression: 11 clicks out
Rebound: Ten clicks out (12 clicks stock)
Notes: Riders who weigh over 200 pounds should consider stepping up to a 5.7 shock spring. On the high-speed compression, make small (about 1/8th turn) adjustments to the dial to adjust the attitude of the chassis at speed. Yamaha’s high-speed compression clicker is very sensitive.

  • grayracer513

Posted December 13, 2011 - 02:34 PM

#6

That the front wheel bounces off on hard landings is not in itself an indicator of overly loose rebound damping, and can be as easily brought on by the rider as anything else. If he leans rearward on landings and pulls up or back on the bars, that will encourage it as will opening the throttle on landings.

  • Knowledge seeker

Posted December 14, 2011 - 02:29 AM

#7

Thanks a lot for the help!!

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  • crf450319

Posted December 14, 2011 - 10:28 AM

#8

Thanks a lot for the help!!


Let us know if any of this made a difference... :bonk:

  • Krannie McKranface

Posted December 14, 2011 - 02:54 PM

#9

If you add stiffer springs, you make the stock Rebound less effective. You have to go more clockwise. Usually, changing springs requires changing shim stacks.

  • crf450319

Posted December 14, 2011 - 03:01 PM

#10

With heavier springs, the forks will rebound faster and you'll more than likely need to run with more rebound (fewer clicks counter-clockwise from being bottomed out).


If you add stiffer springs, you make the stock Rebound less effective. You have to go more clockwise. Usually, changing springs requires changing shim stacks.


I run heavier springs in my '09 but have left the shim stack alone, I'm sure it would help but like you've said running with slower rebound should compensate for the new springs. I'd change the shim stack(s) myself if I wanted to spend the time testing/tuning... don't really have that desire though.

  • Krannie McKranface

Posted December 14, 2011 - 04:37 PM

#11

Every time you turn a clicker, you affect the opposite clicker by a certain percent. The problem is the 'affect' is not linear, so you can't plot the changes ahead of time, and it's not consistent, due to shim stack reaction to speed. When you add stiffer springs, the issue becomes even less linear. When I put stiffer springs in my WR, the clickers has to go to 100% in just to make it rideable....that's 11 more clicks! The dampers were overwhelmed with the new springs. Yeah I know, open bath forks and all that. Still applies though. And the WR left yesterday, that's why I'm lurking here, to see if I want an '08 YZ for my 'light' bike.

  • crf450319

Posted December 14, 2011 - 09:32 PM

#12

I'm 215 Lbs without gear and I went from .47 kg/mm fork springs (stock) to .50 kg/mm springs and on the shock went from a 5.5 kg/mm (stock) spring to a 5.8 kg/mm.

So, a reasonable change but I was no where from maxing out my rebound with the aftermarket springs. I may have gone in 2 clicks max from where I had the rebound set (both ends) with the stock springs in the bike.

I had 7 Honda's before my first '09 yz450f, I just sold that one and bought another new '09 - was starting to get a few hours on the old one and thought I'd freshen up with a new bike. Anyway, I guess it's pretty obvious that I'm happy with the yz's !

:bonk:

  • grayracer513

Posted December 14, 2011 - 10:43 PM

#13

On a YZ450 fork, the rebound adjuster has some effect on compression because the same bleed circuit is used on the mid valve, which is a part of the compression damping in that fork. The compression adjuster, however, has no direct effect at all on rebound, as it bleeds the base valve, which has no rebound functionality.

It's also important to remember two more things:

First, the mid valve is a part of the total compression damping, so tightening the rebound adjuster reduces only a part of the total compression bleed.

The other thing is that the resistance to oil flow of the rebound valve is far greater than that of the mid valve, so changes to the bleeds affect the rebound more than the mid.

The statement regarding heavier springs vs. rebound is only true if the rider weight remains constant. A bike set for the right rebound rate with light springs would need more rebound damping if the springs were changed to stiffer ones, but a bike that ran, say, 10 out on rebound with a 180 pound rider would set up at about that same point when resprung for a 220 pound rider once that rider was aboard. Rebound damping works against the uncoiling spring, but also against the weight of the rider, so as long as the springs match the heavier rider's weight at the same proportion that the old springs matched the lighter one, not much will change.

  • eazrider

Posted December 15, 2011 - 07:45 AM

#14

On a YZ450 fork, the rebound adjuster has some effect on compression because the same bleed circuit is used on the mid valve, which is a part of the compression damping in that fork. The compression adjuster, however, has no direct effect at all on rebound, as it bleeds the base valve, which has no rebound functionality.
.


So...If you increase the compression dampening with the clickers on your forks....does it also INCREASE the rebound, or DECREASE the rebound? This may be obvious to those that understand the physics behind what's going on inside, but to a schmuck, it seems to be an obvious question...Same question on the rear shock: Increasing the rebound is also said to "affect" the compression....how does it affect the compression, and does it affect BOTH high speed and low speed compression..? :bonk:

Schmuck

  • grayracer513

Posted December 15, 2011 - 02:07 PM

#15

The compression adjuster on either the fork or the shock (and either compression adjuster on the shock) has no effect on the rebound directly because it controls the bleed circuit on a valve assembly that doesn't have any influence on rebound. That is not the case with rebound adjusters, since the fork mid valve and the shock compression stack (at least in the case of contemporary KYB and Showa units) bleed through the rebound adjuster. But as I said, the rebound adjuster has a much greater influence on rebound than compression for the reasons I went over.

With any of the adjusters, what they actually do is to control the size of an oil circuit that allows oil to bypass the valve they are connected with. The effect is that as this circuit is opened up by backing out the adjuster needles, the valve assembly can move at a faster speed before there is enough pressure to lift the shim stack and open the ports in the piston. Once that point has been reached, the bypass controlled by the adjuster very quickly becomes almost or completely ineffective. In fact, as the speed of the shaft is increased, the oil velocity through the bypass will eventually reach a maximum, and if the oil pressure is significantly increased beyond that point, the circuit will "lock up" hydraulically, somewhat like a traffic jam, and the flow will be reduced dramatically, handing off the excess to the valving.

In the KYB fork, there is one rebound and one compression adjuster. To the extent that they control any particular valve, these influence both the high and low speed elements of the stack. It's probably true that the clickers on the fork have more effect on the low speed operation than on the high speed because of the locking phenomena just mentioned, and because the major effect of the clickers is to delay the operation of the valve stack. The higher speed elements of the stack will be less affected.

The rebound in the KYB shock will effect both low and high speed elements of the rebound and main compression stack. The shock low speed compression adjuster has a very minor influence on high speed compression, while the high speed adjuster has virtually no influence on low speed.

Also remember that "speed" in reference to damping units refers to the speed at which the unit is being compressed or extended.





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