Video suspension tuning

[Anssi]

I tried to tune my forks using a video camera. I found out that my forks pretty much bottom the first time I hit a bump on the brakes.

I put a clip on the web, and would be extremely grateful for any comments (it's only 600 kB).

I weigh about 210 lbs. (95 kg) with no gear, have .45 straight-rate springs(from Yamaha) in the front, just replaced fork oil (5wt Showa at 110 mm) and no revalving. Oh yeah, the bike is a 2000 YZ250.

The settings for that clip are C8 R14 (I can see the effects if I stiffen rebound, but I don't think it's packing per se I'm seeing here). I'm thinking the springs are still too soft, seeing how the front settles into that low position? Would adding preload help any?

The clip is at http://personal.inet.fi/luonto/the.man/videos.html at the bottom.

 

[Jeremy Wilkey]

Anssi,

Your springs are ok.. I would think that your midvales have failed on your fork.

Regards,
Jer

 

[Anssi]

Damn. Well, I gotta get them fixed I guess.

Thanks for the reply.

PS. does anybody know of a thread or an article about how the midvalve functions and what happens (mechanically, to the part) when it fails. If you (Jeremy) or anybody would be so kind as to provide a small discourse if one is not already available, I would be forever grateful.

The only local person that I know to have intimate knowledge of suspension is mostly familiar with Öhlins and WP items.

 

[FlyingFinn]

Anssi: Are you referring to Simo Penttilä? I think he can service also Kayabas. We don't have too many suspension guru's here in Finland so there is no choices... I talked to Petteri Gustafson at motorcycle exhibition and he said he is learning secrets of suspension service and tuning. Maybe he can help you.

 

[Anssi]

Originally posted by FlyingFinn
Anssi: Are you referring to Simo Penttilä

Yes :) I'll call him tomorrow. He did good work on my KYB shock so I hope he'll be able to handle the forks, too.

 

[dirt bike dave]

Jeremy or others,
Would raising the oil level help with Anssi's problem? If his bike did not have the midvalves, what would be the recommended solution?

 

[Jeff Howe]

I think it's a CV concern from the way it looks. Still, I would up that oil height to 90mm also. Granted, like Jer said the midvalve will be blown too anyway. When you have them looked at consider new cartridge rod bushings if you have a fair amount of time on the bike. They look really mushy to me.

 

[Anssi]

When the valve was still probably good, I tried an oil height of 100 mm using don't remember whose 5wt oil. That caused the forks to lock at about 10 cm from the bottom on big hits.

By CV you mean cartridge valve ie. midvalve?

 

[JTT]

Originally posted by Anssi
Damn. Well, I gotta get them fixed I guess.

Thanks for the reply.

PS. does anybody know of a thread or an article about how the midvalve functions and what happens (mechanically, to the part) when it fails. If you (Jeremy) or anybody would be so kind as to provide a small discourse if one is not already available, I would be forever grateful.

The only local person that I know to have intimate knowledge of suspension is mostly familiar with Öhlins and WP items.

Anssi, do a search for "midvalve", there have been some good discussion here on this subject. The midvalve is the "active piston" (one moving through fluid, as opposed to the base valve, with fluid moving through a stationary valve). My understanding of a midvalve failure is the shims that control compression (or at least at a particular speed) get bent and do not seal to the piston. This has two effects, one - effects compression, and two - soes not provide proper seal on piston thereby also effecting rebound (by allowing unmetered fluid to pass through valve).

Hope this helps some.

 

[Jeff Howe]

Anssi,

CV is in the top of the cartridge. There is nothing you can do about them except replace the cartridge rod bushing. They leak through the top. What happens is that instead of your fluid working against your base valving, it's going out the top of the cartridge. If the cartridge rod bushings are worn it will leak through there too resulting in a very soft fork.

I have the same bike, and am the same weight as you. I run .44kg springs in mine. I run an oil height of 90mm and I am using the whole travel. Why yours is locking up 4" before the end of the travel at 100mm oil height is beyond me. My 2 125 A class riders run 90 mm in their YZ 125's and they are really happy there so I can't figure why you don't get full travel.

 

[Anssi]

Shocknut,

I went for 90mm today, and the fork bottoms just fine. There must have been something in the way I rode or the conditions (it was pretty cold) last autumn when the locking up happened. I probably was too hasty to remove some fluid before proper analysis.

I will post some more video tomorrow. I think I need a revalve, since my rear end bottoms in pretty much every decent landing or whoop I hit (I ). It was revalved this spring with a GP-racing piston and their suggested shims.

 

[Jeff Howe]

Anssi,

What are you running for a shock spring? 5.4kg? Thats what I'm using. Very happy with that one.

 

[Anssi]

Yeah, 5.4 kg. I'm not quite sure how accurate that number is, it's the Yamaha part rated at 5.4 straight-rate. I will take the whole shebang (shock and forks) to a tuner next month when my vacation is over(too busy riding everyday right now) and let him check the rates.

I feel underdamped and undersprung. I will post sag numbers tomorrow when I get to measure them, but they are something like 10 mm free sag 100 mm race sag(=standing on pegs, support from wall). I can't concentrate on the other damping circuits when normal landings(video tomorrow, this was about 4-5 feet in the air, landed on slight downslope) hurt my wrists and the frame drags on the ground.

BTW, I totally bottomed my dads KTM 380 on the same landing with compression damping set to full hard, so maybe I somehow disturb the laws of physics while riding.

 

[Jeff Howe]

I have played around with my bike quite a bit. You should not be bottoming like that at all. You should talk to GP about it and see what they want to do. If you have a e-mail address let me know and I'll talk with you later.