Spring rate calculated for the bike or the fork?

Questions and comments about converting to beefier forks..
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KDW
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Spring rate calculated for the bike or the fork?

Post by KDW »

When we change the front end on a motorcycle with that of a different motorcycle which bike should we use when calculating spring rate? Without thinking I would use the bike that the fork came from. What if that bike is significantly heavier than my bike? Would you subtract that weight from your body weight? I ask because I am not sure how Race Tech or any other company figures these things out. I am putting 1999 KLX300 forks on my 1992 KDX200. The weight difference is about 9lbs but the fork is reportedly 5lbs lighter so this would mean the bike is 15lbs lighter with my new setup. But wait a second the KDX fork weight is not sprung so how do you factor that? What about geometry differences between the two? Anyway A few lbs here and there are not going to make a big difference because there are only so many spring rates.I am 185lbs and even if I take 15lbs off my weight to account for the difference the calculator wants to sell me 0.46kg/mm springs but recommends a 0.419kg/mm. For the KDX it recommends I buy 0.40 springs. Am I over thinking a 25 year old dirtbike with marginal performance? I think so. But it made me think a bit.

0.46 springs is probably the right answer but I am interested in hearing what others think. I will however try the springs in the forks for a bit before I make the switch and I guess I should also look to make sure they are stock springs before I go making any assumptions. Reading on the KLX it is the first mod that people do so who knows, the PO of these forks may have already put the .46 springs in.

k
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Re: Spring rate calculated for the bike or the fork?

Post by bufftester »

Use the fork, not the bike. For the most part, there isn't that huge a difference in weight between bikes, and by far the largest variable is you. See what the calculator says and pick the closest higher spring value.
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Re: Spring rate calculated for the bike or the fork?

Post by KDXGarage »

Actually, one goes with the bike. The weight of the rider, water backpack, tools carried, etc. has not changed. The weight of the bike has only slightly changed after a front end swap.

What is your weight with all riding gear on and all accessories such as water backpack, extra parts, tools, etc.?

What kind of terrain do you ride on?

What skill level are you? (BE HONEST) :grin:

I would assume .44 or .46 for right now.

On the geometry, it is not like you had it perfected before with the stock forks. KLX forks should put you in the ball park. Take it slow and try a few different fork height settings. Start with maybe 5mm below flush, then you can try fully flush and then 10mm from flush.

Also, be sure the rear spring is correct rate for you and the sag is correct AFTER the front end swap.
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Spring rate calculated for the bike or the fork?

Post by KDW »

Here are the parameters I am putting in the calculator on the Racetech site:

Trail/Enduro

Age 38

Dirt Novice/Intermediate/C Class

Height 5-11

Standard Gas Tank

Rider Weight (without gear): 185

The results are as follow:

KDX200 1992

Recommended Fork Spring Rate: 0.378 kg/mm (use closest available)
Stock Fork Spring Rate: 0.30 kg/mm (stock)

KLX300 1999

Recommended Fork Spring Rate: 0.416 kg/mm (use closest available)
Stock Fork Spring Rate: 0.37 kg/mm (stock)
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Re: Spring rate calculated for the bike or the fork?

Post by KDW »

Racetech spring upgrade starts at .46 kg/mm

As said I will just have to try and decide if I need the heavier spring.

To me I would think motorcycle weight and where it is placed on the bike would be more important than rider weight. A 4stroke is going to carry its weight differently than a 2 stroke. Rider weight is dynamic and can be moved front to rear to achieve different things. I am not saying that the KLX fork isn't going to be soft for me but I think a jump of 9 kg/mm is allot and either way I may need to ride around it being too soft or too hard.
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Re: Spring rate calculated for the bike or the fork?

Post by KDXGarage »

No way in hell .38's is correct.

.44's. LIGHT preload
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Re: Spring rate calculated for the bike or the fork?

Post by KDXGarage »

What a bunch of *&*&. They only offer one rate now?!

If you want to jump through hoops, get longer springs and have them cut down, or cut them down yourself. Look on eBay to see if someone has some older springs from when they used to make more options. Early 90's KX and KLX had several options ten years ago.

They used to make them. Here is some info on the great KLX site:

http://www.planetklx.org/techtips/spring-specs.html

MAN! That is irritating.
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Re: Spring rate calculated for the bike or the fork?

Post by KDXGarage »

http://www.planetklx.org/techtips/spring-rates.html

This happens to show the old results from Race Tech. My experience more closely mimics the older charts.

They changed about a year and a half ago for some reason.
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Re: Spring rate calculated for the bike or the fork?

Post by KDW »

Yeah interesting I tried the same parameters for a 1994 KDX and got the same answer as the 1992. .38 kg/mm. My thinking was that the upside down fork might want a different spring rate but clearly not. These findings all fall in line with the FRP suggestions http://www.frpoffroad.com/kdxposed.htm. The USD e series fork should be a close relative to the KLX fork. Again this is all just me looking up information on the internet and not having made the switch yet. I am going to take my bike out for a nice hard ride on the current .40 sprung 92 fork setup before I do the swap so I have something to compare with.
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Re: Spring rate calculated for the bike or the fork?

Post by KDXGarage »

No offense to the best KDX guy ever, but his suspension spring suggestions are way off, to me. Soft springs and heavy preload are early '90's thinking.

Have you ridden the bike with the stock .30 springs?

Without breaking out a ruler to measure sag and trying a few different rate springs, it is hard to know you got it just right. Riding the stock .38's in KLX300 forks compared to the stock 1989 - 1994 rate of .30 is like night and day.

If you have ridden the stock .30's and go to .40's, you will have difficulty riding as you will be slightly blinded by THE LIGHT of riding on something much closer to what is an ideal setup for you. :prayer:
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Spring rate calculated for the bike or the fork?

Post by KDW »

Yeah to be honest anything will be an improvement. I've had the bike for 2 seasons now and haven't even bothered to measure sag. It had stock suspension when I got it and I upgraded the front springs to .40. Big improvement for sure. I plan on doing better sag adjustments... this season.
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Re: Spring rate calculated for the bike or the fork?

Post by KDXGarage »

Good luck with it. Getting it set up well really makes a big difference.
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Re: Spring rate calculated for the bike or the fork?

Post by 6 Riders »

Yeah, I'm 165+/- and ride on .41 springs in both my 93 forks and my 95 RM forks. I say go by the bike, not the forks and then upgrade your riding style to "B" at the least. I think I got my spring rate by putting in MX riding, but I can't remember for sure. Anyways, I ultimately took my forks to a suspension guy and had him give them some love....That was some of the best $$ spent on my bikes.
newbbewb wrote:^what he said.
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*side note...I'm drunk, so try to read what I'm trying to say, instead of what I actually type
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Spring rate calculated for the bike or the fork?

Post by KDW »

I just wanted to update where I ended up with on the spring rate and add a neat piece of information for future reference.

I purchased the .46 springs worried that they would be too stiff but options are slim as noted above. I got thinking that I could just change one spring which sounds almost stupid until you think about it a little harder. The forks are a single circuit once everything is bolted together. I know for my Ninja 300 they sell fork cartridge upgrades that have compression dampening in one fork and rebound in the other which again sound weird until you think about the forks as one complete unit. A quick search I found that people do this to fine tune spring rates but I couldn't figure out if a spread between .38 and .46 was a good idea. I contacted Racetech's tech department and this is the answer I got.

"Yes, we do this all of the time. It works fine."

The difference should land me in the middle at .42 so I have three options now .38 .42 and .46.

Neat eh?
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