Softer shock spring rebounds faster ?

[Swappa]

Recently purchased 2006 YZ250 with Graeme Brough susp setup for playriding/MX for 185lb.

Fork and shock serviced approximately 10hrs ago with fork and shock rebuild.

Shock build sheet reads: 5.4 kg/mm, sag 104 mm.

Fork spring rate: 0.46 kg/mm, 345cc

 

I am an expert vet, but only weigh 150 lb without riding gear.

 

The spring rates are both much higher (3 rates on the front) than stock. The ride is very firm with this setup and it's only when casing a landing that I'm able to push the fork to the bottom of the travel. Typical moto laps leave an indicator o'ring about half an inch off bottom. I've also noticed some knifing and twitchyness on the front with the graeme brough setup.

 

Long story short, I switched back to the stock rear spring and to my surpise, the shock rebound rate increased. This seemed totally counterintuitive to me: I would have thought that a stiffer spring would take more force to compress and rebound with greater force. I rode today and the rebound damping was too light and I had lots of wiggle on the entrance to turns due to 'pogo' and a similar level of knifing and twitchiness on the front end. I'm at 7 clicks out on the rebound adjuster 9 clicks on lsc and 2 turns on hsc.

Plan B will probably be to put the shim stacks back to near-stock if I can't get more rebound damping out of this shock.

I'll also reinstall the stock fork springs and see if I can get a comprimise click setting and closer to a more balanced chassis setup.

I ride 50/50 moto and west coast offroad.

Edited August 25, 2014 by Swappa

[GHILL28]

I had his setup on a YZ250 desert/occasional MX setup and it was pretty mint.  I'm the same weight too.  Nothing exotic, just good and sensible.  Give him a call and I'm sure he'd be happy to help.

 

You're probably just using a lot more travel and getting more force transmitted.

 

Those springrates don't sound too high.  I rode with 0.46's on that bike and a 5.5kg rear spring on the YZ with a 3G tank.  On my RM250, I run a 5.7kg rear spring (longer swingarm and higher leverage), and 0.48's because the longer arm puts more weight into the front.  I might be able to get away with 0.46's, but haven't tried it yet.

 

Since riding setups like that, bikes with much softer rates just feel flat out scary to ride.

[EnglertRacing]

Recently purchased 2006 YZ250 with Graeme Brough susp setup for playriding/MX for 185lb.

Fork and shock serviced approximately 10hrs ago with fork and shock rebuild.

Shock build sheet reads: 5.4 kg/mm, sag 104 mm.

Fork spring rate: 0.46 kg/mm, 345cc

 

I am an expert vet, but only weigh 150 lb without riding gear.

 

The spring rates are both much higher (3 rates on the front) than stock. The ride is very firm with this setup and it's only when casing a landing that I'm able to push the fork to the bottom of the travel. Typical moto laps leave an indicator o'ring about half an inch off bottom. I've also noticed some knifing and twitchyness on the front with the graeme brough setup.

 

Long story short, I switched back to the stock rear spring and to my surpise, the shock rebound rate increased. This seemed totally counterintuitive to me: I would have thought that a stiffer spring would take more force to compress and rebound with greater force. I rode today and the rebound damping was too light and I had lots of wiggle on the entrance to turns due to 'pogo' and a similar level of knifing and twitchiness on the front end. I'm at 7 clicks out on the rebound adjuster 9 clicks on lsc and 2 turns on hsc.

Plan B will probably be to put the shim stacks back to near-stock if I can't get more rebound damping out of this shock.

I'll also reinstall the fork springs and see if I can get a comprimise click setting and closer to a more balanced chassis setup.

I ride 50/50 moto and west coast offroad.

 

your using more travel.

and its possible more preload may make the last little 1-2 inches before top out quicker.

[mog]

Stick was a light on rebound , you need at least nine face shims if not a few more

[Swappa]

Does 0.46/5.4 sound like a match? RT spring calculator indicates 0.457/5.121 with 3 gal tank.

[mog]

Yes it does

[Swappa]

 Give him a call and I'm sure he'd be happy to help.

I'm going back to 0.46/5.4 and removing 20 cc from the outer chamber of each fork. Once there, I'll work from the middle clicker settings/104 mm to try and find some comfort on the clickers and add fork oil in increments. I just hope that increasing the fork air spring/reducing oil volume gives  more compliance in the first 1/3rd of the travel.

[Swappa]

The ride was better, but still not where i need it to be.

How much are my forks supposed to be sagging under rider/fuelled weight?

Overall, the fork is firm with lots of preload/high in the stroke even with greatly reduced fork oil, and the rear is stiff on compression and very light on rebound damping (pogo).
The good news is I managed to finish my races (approx 4hrs of bike time), but very tired by the end. Somehow, my hands weren't shredded, I think because of my combo of padded gloves and cushy Tag grips more than anything else.

[mog]

Sounds like it needs valving softer

[AEES]

Just posted this in another thread reg fork sag:

 

Harder to measure, easier to check on track.

 

It is a mix of:

1. how high you want the fork to run on the track during normal riding (typically pretty high in the stroke)

2. Using the full amount of front fork travel (on jumps)

 

If you have around 10mm left of the travel on the fork on a track that has decent flat landing jumps you are ok. More than this, it will affect your turning capability on hard packed non-rutted tracks.

 

Best is if you can get the fork to bottom slightly when you flat land something light to medium hard with compression clickers set around middle of the adjustability on a hard packed track. Then you can still go stiffer (5-6 clicks) when there is reason to land HARD on certain tracks (hard packed), and you can go softer on tracks with not so much jumps. You can also go harder on softer/sand tracks.

 

To get to this point, you will have to test around a bit with spring preload, oil level, spring weight and compression clickers. I will give you some examples:

 

I currently run, on my WP (re-valved) 5.1 springs with 488mm spring+spacer length (like 2mm preload) and 375ml oil. I then have 10mm left with 16-18 compression out on hard packed medium hard landings. Ride hight is "ok" for standing up and riding. It is not perfect for turning on hard packed but if i go to 5.0 springs, or 370 oil it rides to low in the stroke on sand/softer tracks where you have to stand up a lot. Of course fork position in triple clamps can be an factor aswell.

 

To much preload, (above 10-12mm) on shock spring will be felt as the bike starts kicking sideways and you have excessive rebound.

[Swappa]

You're probably just using a lot more travel and getting more force transmitted.

I think I had some of this going on, creating the knifing and unpredictability in the front wheel.

 

Just to recap:

Fork: C 9 out, R 13, 325 cc, 0.46 kg/mm, fork caps flush triple clamp

Shock, C 10 out R 7 out, sag 105 mm, 5.4 kg/mm

Rider, 150 lb naked, racing motocross and offroad in the south western USA.

 

Overall, I feel like I've achieved a good fore-aft balance in the chassis, but both ends seem overdamped and not very comfortable for offroad racing due to a damping wall at 75-80% travel. Reducing the oil volume in the fork has been my best improvement.

 

I can easily charge for half an hour with good track prep, but as I try and do this pace for 3 hrs on a rough track, I am definitely suffering by the end. I've applied a lot of clicks to the front to stop it from diving.

[numroe]

I think I had some of this going on, creating the knifing and unpredictability in the front wheel.

 

Just to recap:

Fork: C 9 out, R 13, 325 cc, 0.46 kg/mm, fork caps flush triple clamp

Shock, C 10 out R 7 out, sag 105 mm, 5.4 kg/mm

Rider, 150 lb naked, racing motocross and offroad in the south western USA.

 

Overall, I feel like I've achieved a good fore-aft balance in the chassis, but both ends seem overdamped and not very comfortable for offroad racing due to a damping wall at 75-80% travel. Reducing the oil volume in the fork has been my best improvement.

 

I can easily charge for half an hour with good track prep, but as I try and do this pace for 3 hrs on a rough track, I am definitely suffering by the end. I've applied a lot of clicks to the front to stop it from diving.

Weird stuff. Lots of damping, stiff springs, but still lots of knifing?  Are you sure it's knifing (tucking under)? Perhaps a simple rebound tuning issue.  Perhaps too stiff so it simply breaks traction and pushes wide and then bites on something and that's the knife sensation?

 

I've used .44 to .46 front springs and the difference was huge. Settled on 0.45 for a nice combination of holding up under hard braking while getting enough travel for turn-in on lighter braking corners, and some fork travel over bumps from mid corner out.  160 lbs at intermediate pace.

 

What are your valve stacks on each end?   Perhaps you have some odd fork reb stack or an imbalance between the damping workload asked of the mid vs base or some strange high/low speed combination. Maybe the shock setup is throwing out the front end behaviour.  I believe the SSS fork mid comp has most effect on initial stroke movement (think time not position) and potentially on high speed damping.

 

I've played around with my valving a fair bit. 8 times in the SSS fork and it surprises me how quickly things get messed up if I deviate much from stock 2012 YZ settings.  For example yesterday I tried a 1mm smaller diam cross-over shim in the fork reb stack and I didn't like it. I was seeking better front end connection over smaller stuff from mid corner out, but I couldn't make sense of it. Thought I must just be having an off day, which happens a lot to all of us, however I tried various tracks (natural terrain grass/loam MX) had some rest breaks then kept persisting and concluded that tiny change was very real, and worse. Today I'm putting it back. The combination of reb bleed vs damping seems critical.  I think the reason is a combination of me being fussy (although I try not to be) and the stock settings being just very refined.

Edited September 27, 2014 by numroe

[numroe]

Regarding your original question/problem, it could be a case of:   Old: firm spring + soft comp damping.  New: softer spring so pushing deeper into the stroke (stored energy) and the reb valving does not cope with the reb speed and duration.   Or maybe cavitation on comp so no reb damping.  Just guesses, but in any case I don't think of it as being very surprising.