What was the hot setup for 1988 KX250 forks?

queen of spodes
Edited Date/Time 9/2/2020 9:14pm
I'm building a 1987 KX250 and got some really nice '88 forks with triple clamps to install. 88 KX forks were cartridge (87s were not).

Anyone remember what the hot setup was for 88 kx forks? Also did anyone make a top triple clamp back then? I am not a fan of how the bar clamp is forged into the top clamp and sits pretty far back.
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queen of spodes 8/21/2020 9:11am
8/21/2020 9:11am
Alternately, where can I get heavy duty springs for 88 KX250 forks? Race tech doesn't list them (they do have shock springs for the 88s).
_X_
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_X_ 8/21/2020 1:10pm
8/21/2020 1:10pm Edited Date/Time 8/21/2020 1:12pm
queen of spodes wrote:
Alternately, where can I get heavy duty springs for 88 KX250 forks? Race tech doesn't list them (they do have shock springs for the 88s).
cross reference kx 500 springs with similar forks, they are usually stiffer. ebay most likely or facebook market place
queen of spodes 8/21/2020 1:27pm
8/21/2020 1:27pm Edited Date/Time 8/21/2020 1:28pm
yes, by .01kg/mm in this case.
Stock KX250 is .38kg/mm.
Stock KX500 is .39kg/mm.
Slightly better.

RT said I need .43kg/mm.
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_X_ 8/21/2020 4:44pm
8/21/2020 4:44pm
queen of spodes wrote:
yes, by .01kg/mm in this case.
Stock KX250 is .38kg/mm.
Stock KX500 is .39kg/mm.
Slightly better.

RT said I need .43kg/mm.
Run what ya brung then, good luck.

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TTperra
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TTperra 8/22/2020 12:51am
8/22/2020 12:51am
Measure the springs, length/diameter, and compare to other 43mm forks, maby there are some other brands that fit?
queen of spodes 8/22/2020 3:06pm
8/22/2020 3:06pm
TTperra wrote:
Measure the springs, length/diameter, and compare to other 43mm forks, maby there are some other brands that fit?
I think RT's phone person checked to see if anything would be a close match and came up empty handed. They have a database of every OEM fork spring OD and length.
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_X_ 8/22/2020 3:52pm
8/22/2020 3:52pm
Just run a spring as close as you can get and valve it stiff. Its vintage racing, not necessary to be exact. The tracks rarely get rough enough to warrant over thinking suspension building/tuning. If you learn where the suspension weaknesses are you can learn to overcome it's deficiency by being a smarter rider.
Matt Fisher
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Matt Fisher 8/22/2020 6:23pm
8/22/2020 6:23pm
You might be able to cut the springs to stiffen them. As long as you don't run into coil bind, the shorter length can be made up with a spacer.

It's been years since I've dealt with shortening springs, but it does work. There are various websites that detail how to measure and count the coil to determine what spring rate you currently have, and it's easy enough to do the math on how many coils to cut to achieve the desired rate.

You may want to reach out to Cannon Racecraft and see if they can make you a set of springs.
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mb60 8/22/2020 10:26pm
8/22/2020 10:26pm
Cannon Springs can make you a set if you have them measured out.
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queen of spodes 8/23/2020 6:37am
8/23/2020 6:37am
Did more research, the XR400R fork is 43mm cartridge. It uses a 470mm x 36mm OD spring,

The 88 KX250 has 549mm x 35.9mm OD.


Could probably make the XR400R springs work with a simple 80mm PVC spacer. RT sells the XR400R springs at 129 + ship.

thanks ya'll
KDXGarage
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KDXGarage 9/2/2020 1:23pm
9/2/2020 1:23pm
Any chance you have the KX125 clamp instead of the KX250 clamp?

queen of spodes 9/2/2020 1:55pm
9/2/2020 1:55pm Edited Date/Time 9/2/2020 2:02pm
KDXGarage wrote:
Any chance you have the KX125 clamp instead of the KX250 clamp?

No diff - KX125, 250 and 500 all came with 43mm forks through 1982-1988.

Same models used 46mm forks in US 89 models (euro models had an early USD fork).
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KDXGarage 9/2/2020 7:03pm
9/2/2020 7:03pm
Also, it's actually cast, not forged.

Race Tech 2040 Gold Valves would be a big step up from the reverse bend shim style. Also, some damping rod bushings to go inside the cartridge "seal head" would be a good thing to look into. 10 mm ID
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KDXGarage
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KDXGarage 9/2/2020 7:22pm
9/2/2020 7:22pm Edited Date/Time 9/2/2020 7:26pm
In addition:

Honda XR400R OEM Fork Spring Information



1996 - 1997 (51401-KCY-671)

510.4 mm, 20.09 inches

service limit 505.3 mm, 19.89 inches

spring rate is .382 according to Race Tech



1998 - 2004 (51401-KCY-761)

536.1 mm, 21.11 inches

service limit 528 mm, 20.8 inches

spring rate is .40 according to Race Tech


EDIT ADD ON:

As best I remember on 1987 - 2008 KX125:

Cast

1987, 1988, 1992 - 2002

Seperate clmaps:

1989 - 1991, 2003 - 2008
queen of spodes 9/2/2020 8:07pm
9/2/2020 8:07pm
Good info. I sit corrected (kind of!). These are def the KX125 clamps. I wonder if the rake/trail differed between the 88 KX125 and KX250 (and 500). Used to have all those old mags to look stuff like this up.
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KDXGarage 9/2/2020 9:14pm
9/2/2020 9:14pm
1988 KX125: Caster (rake angle) 27 degrees, trail 122 mm

1988 KX250: Caster (rake angle) 28 degrees, trail 129 mm

from the service manual

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