Fork Issue

[dane_ra]

Ok, now I know bugger all about suspension, so hopefully someone here might be able to help...

My forks are as hard as hell (1992 CBR900), I've backed off the preload and dampning and replaced the oil with medium/hard (can't remember the viscosity...15 maybe?) And It's still a major bitch to ride, even the slightest bumps shake the shit out of you.

I was wondering if the forks may have been set up for track use or something- heavier springs or spacer maybe. There is a big spacer between the top of the spring and the bottom nut on the top cap and I was wondering if it was original.

Heres a pic, hopefully this makes sense to someone


http://www.streetfighters.com.au/forum/upload/9228271529673.JPG

[Hillsy]

Try lighter oil first - go 5 weight.

[damo1]

Dane thats really wierd because mine are sweet as i just backed the preload right off as well as compression/rebound adjusters and with all the wieght removed and the bars in a different spot it seems to be real sweet.

Ive done track days/road rides/commute every day the only thing im thinking of doing is a couple of extra clicks of compression when at the track next time to try slow down the dive in the forks when on getting up it.

Im with hillsy try a lighter fork oil.

Was it ok before you changed the oil because you never metioned it before has it just started to be a prob.

[damo1]

Oh by the way if you run to much preload in the rear itll weigh up the forks and that willmake them harsh because of the extra wieght being pushed forward on them

[Watto]

mate tend to agree with the fellas a lighter fork oil may help - i recently went from 10w to 5w and the it was noticable - i thought 10w would help with me being a larger type fella but was much nicer with 5w

[Yella]

Most forks have spacers in standard. I would try setting your rider sag to about 30-35mm for road use, if you can't get it to that then the spring is to heavy for your weight if you can get it in the correct range and its still to hard change your fork oil to 5w and back off the compression dampening will help soften the the ride .When you fill your fork oil put a little less in than standard.

[speeddemon]

5wt for cartridge forks.

If you have say 0-25mm rider sag, your springs r too stiff, but if about 30mm rider sag it is your oil wt or oil height too high that maybe causing stiffness

[dane_ra]

I haven't measured the sag, but its hardly anything. Plus I'm only about 72kg so not a lot of weight there to compress them. I mainly commute and when I get a chance I like to do the Old Road/ Putty Rd loop. As you Syd boys would know, the bit through the top is a bit rough, so I definitely need it on the softer side.

Damo- It was shit before the oil change too. I was told when I had the fork seals done that a heavier oil was put in because of the high k's (about 65000) and the springs probably would have sagged a bit. So I changed for a bit lighter oil to try to rectify the problem. Obviously not light enough though.

OK, Looks like a trip to the bike shop for some oil is on the cards for me.

Cheers for all your help guys;)

[Yella]

Make sure you set your rider sag if you have liitle or no sag your forks will top out over bumps/uneven road surface and you run the risk of tyre loosing grip on the road.
I have mine set at 30mm for the road and 23mm for the track at the track on a new bike I set the comp and rebound to about the middle se how it feels and check the tyres after each sesson and make adjustments untill the tyre comes in clean normally it ends up fairly stiff, I dont change anything for the road except the rider sag. If you make the front to soft it will drive under brakes and be unsettled coming into corners.

[dane_ra]

Yella- By setting the rider sag, do you mean with different weight oil? Or spring pre load?

I just checked and it was definitly 15w oil i put in last. I checked the sag with the preload wound right back and it was 12mm

I also firmed the rear dampning a bit (to around the middle) coz it was backed right off too.

OK, hang on, I've just looked back at speeddemons post about the sag. If I've only got 12mm sag, do I need to go and buy new springs??
Any ideas how much they are? And how would I know which to get?

[Hillsy]

Try shortening the tube spacer first - this will reduce your pre-load and should help with your static sag.

[Yella]

You set the sag with the preload adjustment
I wouldn't cut the spacer you can end up with the spring becoming loose at the top of the travel and that can make the front very unpredictable
www.racetech.com has a spring rate calculator on it and a heap of good tips
http://www.ansusa.com/SuspensionGuide.htm some basic set up tips
There are a heap of places on the web to buy springs once you know what rate you want but if you want to get someone to do it for you, I have never used these guys but was talking to a fella at a track day and he said they had good service and knew what they were doing
http://www.teknikmotorsport.com/index.htm
Phone 02 4732 2626
4/5 Harford Street Penrith NSW 2750
sales@teknikmotorsport.com

[damo1]

Ah Dane this may seem like a stupid question but is it when the tank is full or empty?

I do notic that mine gets a little bit funny when its really empty like on res but other than
that it seem sweet dont forget im just over 100kgs how you wieghing up.

[Booster]

12mm difference from front wheel in the air, to you and your gear sitting on the bike in riding position ... WOW ...

[pt]

http://www.zenodamper.com/index.htm

that dude ^^ is very crazy about suspension and i have only heard good words about him. if you want your suspension set up perfectly then he'd be the go. if you just want to fix the stiffness in the front end try yellas sites and do lots of reading:)

[dane_ra]

OK, I thought rider sag would be the difference between sag without the rider and sag with the rider???

Anyway, I've done some more measurements, this is with the tank full, just for the record.

Assuming with the wheel in the air is 0mm:

With the bike both wheels on the ground and standing upright: 32mm

With the rider, upright feet off ground in ride position: +9mm/41mm total

Then if I hold the front brake and try to compress the forks as far as possible: +16mm/57mm total

Does that make sense?

[Booster]

That seems undersprung but preload adjustment may bring it back into line, so your problem is either your 15 wt oil or you are bottoming out.

[dane_ra]

Shit, I'm going from one extreme to the other... lol

It doesn't feel like a harsh 'crack' as you push the forks down, it's still just a dull stop. If it was bottoming out, you would notice a definite stop wouldn't you? Which leaves the oil...

[Yella]

the measurement you want is

With the rider, upright feet off ground in ride position: +9mm/41mm total

you want this around the 30-35mm

If ya want I can have a look at it with ya one weekend just send us a email if ya like

[dane_ra]

OK, after getting the wrong springs sent out, then waiting for teknic to get thier asses into gear, I finally got my new fork springs about 6 weeks later.

They're in, but i haven't had a chance to properly ride the bike or set them up cause I have the tail off, getting the fibreglass treatment... if all goes well, pics soon.

Now maybe I just got them at a bad time, but I wouldn't choose teknic again, I just got screwed around quite a bit. Feel free to make your own judgements though.