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Old 05-13-2013, 03:49 PM   #1
kellymac530 OP
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Spring change ??

I need to change the springs on a pair of 46mm KYB forks from an '03 Yami YZ. I have watched tons of how to videos but I still have a quick question. All of the videos show the entire rebuild process with seals.....
I just want to swap the springs and oil for now.

Do I HAVE to remove the bottom rebound dampner valve to take the upper cartridge out to access the spring?

I guess I can also ask, if I were to put more than the reccommended preload shims, to the point of spring bind, what does that hurt?
I do not care about loosing 10/20/30mm of travel or even if it is a bit harsh at the end of travel/bottom out, but more about lessening the fork sag since this is at least temporarily going on my R1100RT to DS conversion.

PLease do not get sidetracked telling me the forks will not work and not to bother and just buy some Ohlins or WP forks...in time.
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Old 05-13-2013, 07:06 PM   #2
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No you don't need to remove compression stack.

Increasing preload does just that. I'm not qualified to say what will happen but I imagine the stroke will be reduced.
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Old 05-14-2013, 12:36 PM   #3
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I'm pretty sure you'll need a lot of preload on the springs before you risk coil binding. Just measure the wire diameter and coil spacing and do a little math and that'll tell you about how much compression you can get. Even then, bottoming out by binding is probably no big deal. Much better than bottoming out by hitting the tire on the triple clamp or something.


Too much preload will ride badly. On uphills or anywhere you get on the power the front end will top out and you'll lose traction over every tiny bump. I saw that with the first set of fork springs. It was annoying but not really dangerous. The mfg recommendation for how much preload is going to be pretty worthless, as they're meant for a bike with less than half the weight. IMO, best to install what you've got, measure it and change things to get it to ride right.


Finally, I'd say the YZ forks should be great. They're valved to be really harsh so they're good for MX, but with an extra 300# on top of them they should smooth right out.
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Old 05-14-2013, 04:31 PM   #4
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The stock springs are 42kg and I found a set of new 50kg I am going to throw in there, couldn't say no for $45.
I found a place that can make about any spring rate I need for under $200 so I can change later once I have it on the road and have a better feel for what I need.

Just to make sure I have it right, I can just unscrew the top fork cap and pull out the upper cartridge with the spring on it right from the top, correct?

Thanx.

PS, I really need to make a build thread, but it is REALLY coming together. All of the chassis work is done, the seat and rear fender is all build {I just need to upholster the seat} and the tank is fit {I still need to finish shaping it and enlarging it and moving the fuel pump mount ring lower} but it is all a roller now and getting put together. Half done with the custom exhaust too.
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Old 05-14-2013, 05:13 PM   #5
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Quote:
Originally Posted by kellymac530 View Post
The stock springs are 42kg and I found a set of new 50kg I am going to throw in there, couldn't say no for $45.
I found a place that can make about any spring rate I need for under $200 so I can change later once I have it on the road and have a better feel for what I need.

Just to make sure I have it right, I can just unscrew the top fork cap and pull out the upper cartridge with the spring on it right from the top, correct?

Thanx.

PS, I really need to make a build thread, but it is REALLY coming together. All of the chassis work is done, the seat and rear fender is all build {I just need to upholster the seat} and the tank is fit {I still need to finish shaping it and enlarging it and moving the fuel pump mount ring lower} but it is all a roller now and getting put together. Half done with the custom exhaust too.
I started with .44 or .46, was way to soft. .50 was about right. I'd guess .42 wouldn't work too well for you.

On the cap; almost.
Back the rebound adjusters all the way out. This is important for re-assembly.
Unscrew the cap from the top tube.
Unscrew the damper rod from the cap.

You'll need to pull the spring down to get a wrench in on the damper rod. A helper is really nice here. Especially when you have a bunch of preload spacers in there. The wrench doesn't actually engage on the rod, but on the spring guide/bottoming stop.
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Old 05-14-2013, 07:54 PM   #6
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That is what I needed to hear, thanks Luke.
Yeah, I know .50kg are still going to be too soft, but still better than the .42kg that were in there.

So, I understand that preload is only going to raise the forks back up into the stroke of the valve, and not change the spring rate. I also know that heavier oil would hurt me on the rebound side, having heavy oil and soft springs will ruin the rebound making it slow and heavy feeling. Is there anything else I can do as a temporary bandaid to firm up the forks a bit and make it more rideable until I can afford some better springs or legs all together?

I have not ridden it yet so it may actually be rideable as it is {once I put in the .50kg} but just thought I would ask if there are any "cheats" I can do for the moment?
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Old 05-14-2013, 11:12 PM   #7
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Quote:
Originally Posted by kellymac530 View Post
...
I have not ridden it yet so it may actually be rideable as it is {once I put in the .50kg} but just thought I would ask if there are any "cheats" I can do for the moment?
I'd leave the valving and oil alone before riding it. (Aside from putting in fresh oil) Any arbitrary change is as likely to make things worse as better. You could do the full computer simulation an re-valve the forks based on that, but that's a major project (aka: time sink and potential stumbling block) best done after the bike's running.

Set the preload to get the right loaded sag with whatever springs you have before riding it. Then if you need to, change springs to get the right unloaded sag. I used standard dirt bike sag: 25% loaded, 10% unloaded for the front. That rule wasn't meant for big bikes, but it worked out for me.

Luke screwed with this post 05-14-2013 at 11:19 PM
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Old 05-15-2013, 09:04 AM   #8
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Thanks Luke.
That is my plan. Get it on the road and then see what it need from there. I have a suspension buddy who will help me if and when it needs valving, springs are the hardest issue with MX forks. That is why most people use WP forks on bike bike projects, as you know.

I just could NOT justify that kind of outlay of cash on a fun project. I looked and looked and to get a complete front end from triple clamps to wheel and brakes was about a grand {USD} and then it would still need rebuilding and spring/valving.
I got the complete set up of the KYB for about $200.00 and it is in perfect condition and has VERY low hours on it. Not even a seal weep at all and hardly a scratch on it....couldn't say no. It will get me up and running and see if I even want to continue improving it later.

Thanks for your help and I look forward to any future input. You, Sailah, DRJoe, JTH and a few others are always a great resource for us garage flunkees.
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Old 05-17-2013, 06:11 AM   #9
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I have to say I usually use KTM forks for the reasons you mentioned. I have KYB closed chamber MX forks on my Husky TXC and they are the best front forks I have ever ridden. Probably a little more modern than yours but not too far off.

You could also cut the spring and use PVC spacers effectively increasing the spring rate, but decreasing the travel. You probably don't want or need 300mm of travel anyways on the bike.

Always best to just run it and see how it feels. Take a zip tie around the fork leg to mark how far the fork is going in its stroke, should give you a good indication of how your springs are doing on a spirited ride.
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Old 05-18-2013, 08:46 AM   #10
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Got the springs all in and fresh oil, seems to be ALOT better.
It is still not done enough to ride, but getting there. But before when I dropped it down on the wheels the forks dropped about 3.5-4" down in the stroke just ubder bike static sag. Now with the .50kg and I made 2 spacers out of steering bearing inner races I had laying around it only drops about 1.25" under static sag and when I hold the front brake and push down on the forks it actually feels decent. Not perfect of course, but seems like it will be rideable. The races fit perfectly. They were .625" thick and the ID slid exactly over the plastic spring guide in the center of the spring and the OD was the same as the fork springs themselves. It was not so much preload that it was super hard to compress and get a wrench on the top of the rod and start the cap by myself. I think it will be plenty rideable to initial testing and bike set up.

Sailah, I agree. I am a KTM and a Yamaha guy personally. I have owned many of each. Most of my race bikes were Yamahas and I always LOVED the KYB twin cartridge forks in both 46 and 48mm and they are great. Very good stock but once valved and sprung for my weight and riding style they are perfect for me. I have also owned 4 KTMs all with the WP inverted forks and they are good. Verstaile. Nice that you can get heavy springs for the bigger bikes, but no matter who re-valves them or how I have them set up I have never gotten them exactly how I want them. If I get then stiff enough at the end of stroke to prevent harsh bottoming out then they are chattery and a bit harsh in the beginning of the stroke on small stuff. When I get the intial stroke plush and smooth they usually buck bit in the mid stroke and/or bottom out harshly. Nothing that I can not live with, but not as overall smooth and compliant yet firm and bottom resistant as my KYBs. But maybe that is just me...not sure.

For what a good useable set of WPs and the TC and front wheel and brake set up goes for compared to what I have paid for these KYBs complete, from Protaper bars to tube and tire, TC, brakes, and even the second set of springs I have 1/4th of the cost so far....I had to start somewhere and being on a tight budget for now I could not say no.

If I love the bike overall and decide to make it my permanent rider but feel the forks are lacking maybe I will save up and buy some custom Ohlins or something later....I know I will want a new shock eventually anyway, but a really good shape stock GS shock for $50 was a great starting point as well.

You guys have been great already and I am sure I will need some more help in the wiring area and the tank set up.

I know I will need some advice on the fuel pump and fuel level gauge since the RT and the Rockster tank I am using use different set ups.

Thanks a ton.
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