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YZ fork preload/ Sag/ BMW 650

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hey guys I need a littler help. Here is some back ground on my project. I have a BMW 650 Dakar with YZ forks and a custom rear shock. Stock travel on this bike was 10 front 8 back. Now its 12 front 10 rear. I basically built a touratech customer rally bike for a 1/3 the cost. I had the rear shock custom built for by weight and the bike. I had the forks revalved and re-sprung for the weight of the bike. The forks have .52 spring in them but I think there is a tone of preload on them. I told the guy who did the forks to valve it like a KTM 640 Adv plus 50 pounds. Well it came out way to stiff. since .52 was kinda light for this bike I think he put lots of preload in to help it.

Right now the rear sag is right one.

The front sag is 1.5 inchs on the bike or off. It will not move down any more with me on the bike.

How do I adjust the fork preload?

Here are some pics to give you an idea of what I built.

On center stand, both sides drooped out

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IMAG0221.jpg

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Ok I think I know what to do but I need a picture of what the preload shims would look like and where they go.

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they sit at the top or bottom of the springs , you won't be able to miss them

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if there is 1.5" sag (from bike only) and it does not move more with your weight on it, then this (the lack of extra sag with your weight added)is not a preload issue, as the 1.5" has already taken up the preload force, if this makes sense to you?

The bike weight has already used up the preload force (because you report a sag amount), so it is already active on the spring rate and when add weight from the free-sag position it will stroke according to the force added vs the spring rate (minus stiction).

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if there is 1.5" sag (from bike only) and it does not move more with your weight on it, then this (the lack of extra sag with your weight added)is not a preload issue, as the 1.5" has already taken up the preload force, if this makes sense to you?

The bike weight has already used up the preload force (because you report a sag amount), so it is already active on the spring rate and when add weight from the free-sag position it will stroke according to the force added vs the spring rate (minus stiction).

I thought the bike should use closer to 3.5 inches of travel with me on the bike. Note this is being used for Dual sport, lots of twisties, and the dezert. I checked the sag again this morning and I would say its closer to 1 inch, not 1.5.

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OK so i looks like have 5 plastic rings under the spring. I took two out but now the springs rattle a little when the fork has no weight on it not on the bike. is this normal? 04 yz250 F forks. when the cap is tight there is no prload on at all. little gap between the cap and string guide.

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you should have no gap , also do a search on fork alignment as it sounds like it maybe a issue on yours, also possible stiction due to seals and bushes ?

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Mog thanks for the help, the forks were not aligned and that was making a major sticky spot at the top. Once the forks start moving the valving is very good. Unfortunately i caused another problem. I put the fork back togather as they were from the shop but spilled some of the oil. I noticed the bike dives allot more on the brakes now. Will adding oil help the bike from diving?

Also I'm concerned the springs are too soft. This bike is 400 pounds and I weigh 210 no gear (note the gas tank is under the seat so not all the weight it up top. The bike has .51 fork springs and I was told there are no springs stiffer than that. I know that is not true cause people who have done this yz fork mod on a bmw are running .54 and even some progressive .56 springs. Do you think I should put stiffer springs in or try more oil? How will think effect the valving?

75% of my riding is on the street.

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adding oil will only have a marginal effect on fork dive, if its now diving too much try putting a little of the preload back in?

what static and rider sag do you have front and rear?

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adding oil will only have a marginal effect on fork dive, if its now diving too much try putting a little of the preload back in?

what static and rider sag do you have front and rear?

The dive is pretty bad now with the forks not sticking. Its using alot of travel. I will have to put a zip tie on to tell you how much but around 60-70%.

Once I got one of the forks open I found that it had very little if any preload. It had 5 nylon washers in there to keep it tight. I assume the springs he used are shorter than stock. Just removing one would make the spring not tight. I tired typing in the part number on the spring on line and all i got were Ohlins springs for a KX450. Odd.

I read that 05 yz springs will work on the 04 forks if I use a 10mm spacer. I have more than 10mm worth of spacers to use and I found .54 springs online. If I need springs I can get them.

I have not checked the sag since I fixed the front. I will do it tonight when I get home.

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Is it normal for the lower fork tubes to rock back and forth a little when topped out?

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yes they do have some movement

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Mog,

Here are the sag numbers.

Front of the bike has 11.5 inches total travel. Static sag is 1 3/4 inch, race sag is 2 3/4 inches.

rear of the bike has 10 inch of travel. Static sag is 1.5 inches, race sag is 3 inches

I toped off the right fork leg since I lost some oil looking inside it. They are both at 90mm (I bought the tool to check). I have the compression clickers one out from max. Bike still dives. I took it off road and is plush when riding it fast like a normal dirt bike. I was able to bottom it one time but it took some effort. Bike has 5 weight oil right now.

The front forks are slid up in the clamps 3/4 inche. I think the bike feels high in the back when riding.

Do you think the front springs are not stiff enough. I found a company that says they make .54's for it. it has .51 right now. I also heard 05 yz forks will fit if you use a spacer. Is this true? These are off a 04.

thanks for all the help

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well i am not wel versed on supermoto type stuff which yours is close to, but thats a lot of sag IMO for something that can brake hard on concrete, so you need to either preload the springs more or get those 0.54 , then you can open up the comp a little more.But use a lower oil level when you do so you can bottom sometimes.

If 04 forks fit i am sure you could get 05 ones to but i dont know the specifics, lots of info on here on fork transplants on yz, but i would fit 06 forks, much better.

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well i am not wel versed on supermoto type stuff which yours is close to, but thats a lot of sag IMO for something that can brake hard on concrete, so you need to either preload the springs more or get those 0.54 , then you can open up the comp a little more.But use a lower oil level when you do so you can bottom sometimes.

If 04 forks fit i am sure you could get 05 ones to but i dont know the specifics, lots of info on here on fork transplants on yz, but i would fit 06 forks, much better.

Thanks for the help. The bike has a ya motard rotor on it so it's putting some good force on the forks. I meant to ask will 05 springs fit in the 04 forks?

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no the 04 05 and 06 all have different internals

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Thanks for all the help Mog, I found some .54's from Diverse. Ordered and on the way.

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