96 KDX200 - Suspension Setup

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MoonStomper
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Re: 96 KDX200 - Suspension Setup

Post by MoonStomper »

The Gold valves are a worthy investment for the riding you are doing. In the rear tire… Your arms will thank you. Sounds like
you should save for a IRC VE33s or a Pirelli Garacross for the type of terrain you are riding. Install a Ultra HD tube and 6-7 psi with slime in the tube and tire or if you can, do Tubliss in the rear using the Slime Method (see YouTube) and run at 3-4 psi. You will roll up that root staircase like an escalator. That’s our local Appalachian mountains hard enduro setup. You’ll giggle like a school girl.
~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~
Just trying not to get lapped again by my son!
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Charles
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Re: 96 KDX200 - Suspension Setup

Post by Charles »

Hello guys, I just came back from a nice bootcamp / tour ride in the south east of France, on the outskirts of Cannes. (EnduroEvasion on social medias)

As always the old KDX behaved perfectly and followed along the KTMs. Instructors had 300 and 250 2st, and offered 250f/350f for rental. My wife's little Husky 125 did well also. We did a mix of fast fire roads, slow single tracks, rock gardens, steep descents, lots of river crossings, and a few harder sections. The instructors chose the path difficulty to keep us just outside our comfort zone while avoiding being in too much trouble.

So, I kept the same settings as before and worked much on body position, clutch control, static balancing... with the instructors. I would say now that:

The good stuff:
-Riding with instructors is invaluable. Will do again and again. I'm getting comfortable with the KDX and I'm getting much less deflection of the front fork because I'm riding it better.
-4.8 rear shock spring is definitely good, going slow and fast.
-Lower air pressure is nice. The terrain in the south being harder, the Michelin Tracker did fine but here is its limits. I will replace them in the future with softer compounds. (@Moonstomper: Thanks for the recommandations, IRC tires are not available here but you let me discover their awesome youtube channel!) I am also considering Mousse and Tubliss. Almost 100% of local riders use BibMousse, they come in 0.9 or 0.5 bar equivalent.
-Having no e-start is becoming less of an issue as I get more clutch dexterity and crashing dexterity.

The less good stuff:
-I experienced my first fork bottomings! I took natural little jumps in 4th and 5th gear full speed, and just getting the bike barely flying made the fork bottom really hard on the landings. Feels nasty and dangerous. I felt the tire squish, and the fork itself flex. The fork also dives under braking. It also dives a lot in slow passing of big rocks or steps, when the whole weight of me and the bike smashes on the front wheel.
-Compared to the modern KTMs, the KDX seat is tall and wide! Can't have my feet flat on the ground. I'll just have to do with it.


So now I'm having the dillema:

Should I keep the stock forks and work on it? In the crawling stuff it was doing ok... but will it stand as the riding rythm increases?
Stiffer springs seems not easily available here, same with gold valves (racetech shipping is a quite expensive). I am also definitely unsure of which spring rate to get.
But the maintenance is so easy on those forks!

Or should I go directly for a full front-end conversion? That would require a pro tuner (250 euros) and some machining work.
It would kill the old-school looks with the purple boots and also could make maintenance more complex... or not.
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Re: 96 KDX200 - Suspension Setup

Post by SS109 »

Hmm, only you can decide what is better for you and I think that depends on budget and what you plan to ride primarily.

Personally, I'm a champion of the "swap for better forks" crowd. I hated the stock forks for anything other than slow and techy stuff. Once the speeds picked up the fork flex, and lack of external rebound control, had me always fighting the bike to stay on the trail.
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Re: 96 KDX200 - Suspension Setup

Post by Charles »

Well, decision is made. I can't find a decent complete front end, only various overpriced and suspiciously worn bits are available.

That lefts me with the choice of fork springs.
Racetech's are available on Ebay for 150-200€, rates .42 and up
I emailed LRD, a french spring making company, which is renowned for making custom linear springs for motorbikes (mostly road and track), 110€ a pair. I don't think that they will be able to recommend spring rates but they should be able to coil whatever is asked.

Now, the tricky part, shooting for the correct spring rate!

I'm leaning towards .41 / .42 with a very small preload but am I just too shy?
Like KDXgarage said, I checked spring rates for modern bikes (enduro, not XC or MX): Yamaha WR250F 2021: .46kg!!!! KTM EXC250: .42kg Beta 250RR: 4.2 ... 250 Sherco is also .42 to .46 depending on sources.
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Re: 96 KDX200 - Suspension Setup

Post by Slick_Nick »

Always go stiffer on the springs and valve softer, rather than ride a bike thats undersprung.
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Re: 96 KDX200 - Suspension Setup

Post by Charles »

Thanks Nick, I'll take note of that. I need not to be afraid of spring, seems I was traumatized by that 5Kg stock rear spring :roll:

Got an answer from the spring makers, they'll do it in about a month. They will wind it up to spec so they need to know length, diameter, rate, and travel. It will be the occasion for me to measure everything and try 5wt oil, before changing the springs.

While waiting, I had fun studying Racetech's bible spring chapter, and drew a little graph showing spring rates. Please feel free to correct if my calculations are off.

The horizontal axis shown spring compression, in mm. The vertical axis show weight applied on spring, in KG. I drawed the lines for a pair of springs each time (double the weight, for the same length).
I added preload force ( F = Rate x Preload length, ex: 0.42Kg/mm with 10mm preload: 4.2Kg, times 2 for your pair of spring, 8.4Kg of preload), which does not affect the rate (the slope of the line), but moves the line up or down.

The first line I drawed, in black, is the stock spring. 0.345Kg/mm with 28mm of preload (Dear KDXgarage, I saw posts from you talking about 28 or 38mm, and from other post with measurements I guessed 28mm).

Now, I got on my bike and measured free sag (26mm) and race sag (57mm). I crossed this with the Stock Spring rate, and went horizontally to deduct the applied weight of free sag and race sag. This weight being independent of the spring rate, I should now be able to predict free and race sag with other spring rates and preload values.
I highlighted in yellow the recommended race sag range from Racetech. I added recommandations from other tuners, but they feel a bit odd to me.

I should add now that this graph only shows the spring rate, omiting the "air spring" effect that is tuned by the oil level. This curve, according to RT's book, is quite like an exponential, with almost no effect at the start of the stroke, and huge effect at the full compression. However, this air spring rate is the same whatever spring rate we're using so we can forget it and compare springs.

Now the fun begins: I drawed the common used spring rates, with 4mm preload. 4mm preload with various spring rate gives about 3.2Kg (.40 springs) to 3.7Kg (.46 spring). In comparison, the stock weak ass .345 spring, once preloaded to 28mm, produces almost 20kg of preload force!
I drew small circles where the lines cross the stock line.

The graph shows clearly that there is lots of ways to get the same Race Sag, heavy spring with small preload or light springs with stock preload. However, albeit having the same sag measurement the soft spring will be stiffer in the first half of the stroke and weaker after that.

We can see here that a .40 spring (blue line), will be weaker than stock until mid-stroke (145mm), and stronger to the end of the stroke.
Even the .42 spring is lighter until being 105mm in the stroke.

I added a line for .41 springs with 10mm preload to show the effect (dotted black line). While having the same full-stroke force a the .42/4mm, it will be harder throughout most of the stroke, albeit quite close.

Lastly I compared the force needed compress the springs 290mm, based on the stock spring. (Mistake on the .46, it's 270kg, +23%). The difference seems a bit small to me (but what do I know?) compared to the huge effect of oil level at the end of the travel. So maybe the bottoming problem should be dialed in with oil level. We'll get to that once the spring is sorted. :rolleyes:


Now, I could just get whichever spring rate has been advised for 30 years but, isn't it enjoyable to get some stuff down on paper and verifying it afterwards? I'm going for .42 springs.

I should add now that while this graph compare spring rates(in the event that it is not completely wrong), it does not compare riding performance! I do not have the experience to tell if weaker or stronger springs in some parts of the strokes will achieve better or worse behavior, but my goal is to measure and test, trying to understand the relatioship between rates, preload, sag and bike handling. This will require lots of riding.

:drinkers:
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Re: 96 KDX200 - Suspension Setup

Post by KDXGarage »

:partyman:
:drinkers:
:rock:







Thank you for doing the math and posting it. :bravo: I think you will find the 0.42 with little preload much better than what you have now.
Thank you for participating on kdxrider.net. :bravo:
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Re: 96 KDX200 - Suspension Setup

Post by MoonStomper »

Wow! You really dove from the 10 meter platform straight down into the rabbit hole! I applaud your commitment! Nice graph too.

For the riding you are doing, you are on the right forks. Those springs will really help. Save your money and get the gold valves done and you will be all set. I’m riding similar terrain here in Virginia/ West Virginia and both the slow knar and fast flowing jumpy stuff. My forks leave nothing to complain about. Improving my riding skill and having my weight in the right place at the right moment has also improved my bike’s handling.

Something else that has really helped when racing or trying to keep up with fast riders is using a taller gear. I’m on a 13/46 (Jeff Fredette recommended) and try to use third gear whenever possible. This seems to effect the suspension in a very positive way. It feels smoother and has improved my control in choppy terrain. It has definitely made me faster o the clock.
~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~
Just trying not to get lapped again by my son!
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Re: 96 KDX200 - Suspension Setup

Post by Charles »

Little update,
The local shop is moving and unavailable for a while so .42 Racetech's are on their way from the US :supz: I found a shop on ebay "OEMCycle" with reasonable international shipping.

Got the KDX out for a quick solo ride a few days ago. It has been a bit of a drought this year so the terrain was really hard, with random holes where the mud puddles used to be. I'm bottoming the fork quite a lot now in this, as the riding speed goes up.
4th gear and up through holes feels clearly dangerous. I hope the stiffer springs will get that right.


Moonstomper, this is a very interesting remark about your gearing. I heard from a french MX coach that riding a higher gear, lower rpm and on the gas will ease the suspension's work, that seems to be the same thing you're describing.
However, how do you manage the slow difficult stuff? Do you use first gear, or second? Do you slip the clutch a lot?

I find myself quite often in 1st gear slipping the clutch through rock gardens or rooty uphills sections.
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Re: 96 KDX200 - Suspension Setup

Post by KDXGarage »

I am glad to hear you got some springs. I have ordered from them a couple of times a few years ago, no issues.

That is an awesome chart. Thank you for posting it!
Thank you for participating on kdxrider.net. :bravo:
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Re: 96 KDX200 - Suspension Setup

Post by Charles »

Springs installed!!!

I took some measurements and I confirm that stock preload is 28mm. I used Racetech's measurement: https://racetech.com/page/title/IP%20Fk ... pen%20Cart

Nice thing, the springs were provided with an aluminium tube for cutting spacers and a few washers. They came in handy cause I cut the spacers a bit short :doh:
I also re-used the springy washer that clips inside the spring and the cupped washers from the stock spacer.

I stayed with the exact same oil (7.5 Motorex), 120mm level, 4mm preload.

Low level of preload are tricky to measure, 4mm is almost nothing. After doing the first leg, I compared it with the stock one by compressing it. It feels really softer in the first part of the stroke and significanlty harder after that.

Can't wait to try the bike!!
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Re: 96 KDX200 - Suspension Setup

Post by MoonStomper »

To answer your question about riding the slow knar…

That’s where these forks can really excel especially once you get your gold valves done. In that 13/46 gearing I try to stay in 2nd/3rd and 3rd as much as possible using full benefit of the clutch with a good steady dose of throttle. If you set up a practice loop, you’ll see your lap times drop with this method. Stay on your toes, elbows up, nose over the bar, squeeze with your knees but let the bike work keeping the front end light and let her eat!
~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~
Just trying not to get lapped again by my son!
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Re: 96 KDX200 - Suspension Setup

Post by Charles »

Hey guys!

I finally got the chance to test the bike today, a quick 45 minutes ride in the easy trails around the house. Didn't dare to push too much though, having not ridden for a month.

First impressions:
Definitely less deflection, the front wheel tracks the terrain variations and the fork actually function. I was able to speed things up on little rocks that used to send me off the bike (5th gear wot, 2-3 inches rocks, didn't dare more)
Less dive under braking, which is nice because it allows to brake much harder, and on steep descents it's less scary.
Less preload on the front, eases up the load on the rear spring => More grip exiting corners while pushing the pegs. More grip on uphills.
More lively and faster response when wheighting the pegs to pop the wheel up

On the donwside, on the top part of the stroke (which is used when accelerating), the fork feels very soft, like being on a flying carpet. Like there's no feeling at all. But maybe I'm just too much used to feel each little rock on the trail impacting my arm bones!

Overall the bike just feels "proper" now, and I believe it was weirdly unbalanced with stock springs.
Next time I'll push harder, try some jumps and some rough stuff to test the deepest part of the stroke.


I definitely recommend changing both springs to anyone hesitating. Don't lose time like me, there's no point bothering with the clickers or shim stack mod or whatever before getting the right springs. Especially the rear.
I would love to try making longer spacers to get a little bit more preload. Or maybe fabricate an adjustable preload fork plug, to get the things just perfect before digging more into suspension black magic.


Next step: Gold Valves or... Shim Restackor and more hand drawn graphics?
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Re: 96 KDX200 - Suspension Setup

Post by KDXGarage »

Congratulations on your success! Thank you for writing it all up for the benefit of others to read and learn.
Thank you for participating on kdxrider.net. :bravo:
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