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Old February 16th, 2018, 09:49 AM   #1
ck.mecha
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Some Observations on 250R vs 300 Forks

Im not making any statements about how these differences affect ride, but more posting this for posterity and in case anyone ever is wondering if they can use 300 parts on the 250.

I recently had a couple sets of both 250 and 300 forks apart to pick the best condition parts & put together a set for the coming race season.

The first main difference you will note is on the inner fork tubes. The ones on the left are 250R, the center is 300, and the right is an aftermarket unit for the 300.

Other than this pressed in piece looking like a forged/cast piece in the 300, the inner fork tubes are essentially dimensionally identical and can be interchanged. I noticed on all the fork tubes that the holes in this pressed in piece was never indexed with the holes on the inner fork tube. Obviously either Kawasaki cares nothing about this alignment or it is unimportant.

The major differences were in the damper rod and guide. The guide is taller and the compression damping holes are placed higher on the 300. I didnt take a picture of it, but the rebound damping hole on the 300 is also slightly larger than the 250R. The ID of the 300 damper rod is also about 0.25mm larger. Despite these differences, they are interchangeable provided you replace the damper and guide as a set.

I hope this information is of help to someone.

PS. All these inner tubes bend extremely easily, I took apart 4 sets of forks and all 8 OEM tubes were at least 20 thou out. I would not recommend buying used 250 or 300 forks with the idea to slap them on and run them as is
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Old February 16th, 2018, 03:45 PM   #2
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great info, many thanks. i will bookmark this 4 sure.
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Old February 16th, 2018, 08:20 PM   #3
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I kinda wonder what bent those tubes 20 thou, and if 20 thou really matters?
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Old February 17th, 2018, 07:52 AM   #4
Zaph42
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Quote:
Originally Posted by Z1R rider View Post
I kinda wonder what bent those tubes 20 thou, and if 20 thou really matters?
What I found over the years on various crashed bikes is that fork tube runout numbers matter less than the old "stiction" test.

Stiction results range from pushing the forks down and they stay all the way down, (oops, oh oh hahah) to coming all the way back up to where they started. Generally, if within 1/4 to 3/8 of where you started, I'm not even going to bother taking the forks apart for a runout test. Stiction like that is fine and acceptable. Anything more and it's time for new fork tubes. Just remember that new fork seals have higher friction and they do break in after a few rides.

I don't think 20 thousandths is all that bad on skinny fork tubes. I've seen worse numbers still have good compliance and low friction. 20 thou on a supersport fork tube might be a problem though
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Old February 20th, 2018, 07:59 AM   #5
ck.mecha
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The inflection point on each was where the inner tube meets the lower triple. This would mean even a relatively large bend would have minimal impact on the movement of the lower - compressing the forks would not be a good test.

I must clarify this is for a race bike so yeah these MIGHT be OK for a street bike depending on your standards. Twenty thou was the smallest variance, one was nearly 1/4"!! Ill write out all the values if there is interest. The 20/1000 off forks I had actually straightened to around 8/1000 over a year ago and they worked themselves back out (no even a lowside since)

Consider that if its bent 20 thou. at the triple, the distance its off at the wheel axle is off nearly 3x that when you do the geometry calculations.
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