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Only $10 for all 2026 SX, MX, and SMX series.
I was a 2/7 guy (29 palms, CA).
Sweet build.
Very cool. I know you're into KX500 development, but do you have access to a KDX to try some of this stuff on? A KX front end is basically bolt-on for an E-Series KDX. The shock I believe is similar.
Obviously there's probably more money to be had with the KX guys, but the KDX still has a pretty large following and guys are always looking for upgrades along these lines. If some of the stuff you offer applies to both, it's a win-win.
I don't have access to a KDX, but the shock body looks identical to the KX500 and the 1989 KX250 or KX125. I almost bought a few KDX shocks for parts a while back.
I think the KDX has a 44mm piston like my original 1990 KX500 shock. KX shocks after about 1994 have the 46mm piston. I believe the KDX has a 14mm shock shaft while the KX and most modern MX bikes have a 16mm shaft. All it takes to convert the shaft diameter is the 16mm shaft and appropriate seal-head assembly and bottom cap. The KX shock is around 450mm long, so that would be the first thing you'd want to check before swapping things around. Shock length can be changed pretty easily by a good tuner or a savvy guy with a lathe. The larger shock shaft gives more flow through the compression adjuster.
At present, I'm setting up a machine to make another type of KTM WP to KYB conversion kit.
I'll be revisiting KX500 stuff in a month or so after I get some test riding on this shock.
The Shop
DeCal Works Huge Plastic Inventory of UFO and Polisport kits.
Sunday I took the big KX out to a really fun local track called Burnt Ridge to finalize my jetting for the 2024 Two Stroke Dream Race at Washougal MX park. This bike feels a touch heavier to lean into a corner than my GasGas MC350F or my YZ250, but it actually handles as good as anything modern at this point. I went down one size on the pilot jet to #50 (was #52) and it cleaned up the initial crack of the throttle.
Looks like fun.
I'm in the process of building a 94 for moto and preparing to do a fork swap. I read in your other thread that you built some triples with a 22mm offset to match up with the 32mm axle lug offset on the KYB's. If you were doing it again would that 54mm offset still be the target or would you try something different?
By the way, I've done quite a bit of machining and those triples you did looked great.
At this point I wouldn't change anything from the current setup.
I'm not usually a guy who likes reduced offset in the triple clamps, but I feel like the small change has helped at turn in without giving up any stability in corners or in rough stuff. The bike blitzes through the Washougal whoops like a Cadillac on a country road if I don't make any mistakes.
The goal was an old bike that turns like a modern bike, can be ridden fast with confidence and has a modern rider triangle. I feel like I have that and feel as confident on this bike as I do on my 2023 GasGas MC350F or my 2019 YZ250. The rider triangle is basically equivalent to the YZ250 thanks to the top clamp, the pegs and the tall seat. I'm 5'9" and currently 164lbs.
If I compare this KX500 to my '23 MC350F, it feels a bit heavier leaning into corners the same way a 450F does. The positive is that it has all that 500cc torque with a mellow delivery and four-stroke like traction.
The most significant changes on my KX that contribute to the way it handles are:
1) Fork and triple clamp offset changes.
2) Lowered rear by reducing shock length 1.5mm (in conjunction with the OEM linkage).
3) 116 link chain with 14:49 sprockets and axle moved back between the last two marks on the swing-arm.
4) Modern wide foot pegs with camber.
5) GUTS 30mm taller seat foam.
6) YZ location of handlebars relative to the steering stem with Pro Taper SX-Race bend or similar.
A lot of making this bike work well is in having great valving and spring rate choices. I didn't list it above, but the adjustable high speed compression on the shock has a ton of value in tuning the rear end feel and balancing the bike. I don't believe that changing the linkage is necessary on this bike even though it seems a very popular mod. I'm running my shorter shock with stock links and about 114mm of sag. I measure sag standing on the pegs for consistency.
Something to keep in mind on the fork swaps is that a two stroke YZ fork or a fork from 2008 - 2009 YZ450F and 2008 - 2013 YZ250F will be approximately the same length as a standard KX500 fork. The later model four stroke Yamahas have forks up to 10mm longer. I don't think the longer forks are a net benefit for this chassis.
Hopefully that information helps.
Great info, thanks for sharing. But now I have more questions.
You mentioned that your triples move the bar position relative to the stem to mimic the YZ, is that further forward than the stock KX5 position?
Interesting that you went lower with the rear and lengthened the wheelbase. I’ve seen several references to the 132.5 linkage arms and raising the rear end up. It’s been twenty years since I spent a lot of time on a KX500 but I thought I remembered it always feeling low in the back even sprung correctly and sag at 100mm. Part of that may have been the seat, which I will follow your lead with the tall foam. I was only doing high speed desert racing back then so I wasn’t real sensitive to how it turned . I can see how adding the HSC adjustment to the shock would be a big help with setup. I’ve really gotten into the habit of using it on my modern Austrian bikes. How did you shorten the shock and what made you decide to go that way?
Thanks for the info on fork lengths. I figured I could deal with a longer fork by just running it up further in the triple clamp but it would definitely be better if I didn’t have to.
No problem. I sure hope it helps you or some other fellas to enjoy these awesome old bikes.
Riding mine, or even thinking about riding mine puts a big smile on my face.
I considered making a few sets of links in various lengths, or making one set with some adjustment similar to the Devol system for testing. I saw that everyone was raising the rear of these bikes and for me that felt like the wrong direction, so I did the right thing and ignored the crowd.
With my 114 link chain and 14:49 sprockets I had black rubber marks on the airbox and the fender and more importantly the back of the bike was hopping side to side on braking bumps entering a turn. That was way worse with the 5.5kg/mm spring on the stock shock. After the shock upgrade, the rear end was calmed down a ton, but adding wheelbase helped reduce wheelies on throttle and hopping off throttle.
I saw guys suggesting the shorter 132.5mm aftermarket links and running 120mm of sag. Maybe that works with the stock forks and clamp offsets, maybe I'd like it if I tried it too. From my testing it seemed like raising the rear was going in the wrong direction so I never tried the links.
I was running close to 120mm sag before and I estimate the shock change drops the bike 5mm to 6mm in the rear, so the 114mm sag I have now is pretty close to that same rear ride height at the sag point.
To shorten the shock, all you have to do is machine a new base washer with a counterbore larger than the shock shaft on the bottom surface. I added 1.5mm to the total thickness and counterbored it 1.5mm at about 20mm diameter. The base washer goes under the compression shim stack and replaces the standard washer. It effectively causes the shock to top out 1.5mm sooner. I used some 4340 alloy steel leftover from a customer job to make that part.
My fork height is about 2.8mm, but my custom clamps make that a number that doesn't mean anything without taking the clamp thickness into account. My top clamp is 17.25mm thick from the OEM 1990 KX500 steering stem nut to the top surface where you measure fork height.
Handlebar offset on the stock KX500 triple clamps wasn't something I worried too much about. It has been three years since I did the top clamp machine work and longer since I did the design... I'll pull up the CAD drawings when I get a few minutes and refresh my brain on the differences.
The GUTS 30mm foam doesn't add anything at the rear of the seat, it just flattens out the pocket at the front. This makes the bike easier to transition from sitting to standing and makes the seated position more relevant to modern riding technique and bike setup.
I'm not afraid to experiment, make mistakes and admit to them so I made some bold choices based on riding the bike back to back with modern motorcycles. I combined that testing with plenty of measurements and conversations with two savvy friends who rode the bike. I like a pretty neutral bike and I won't tolerate headshake or twitchy behavior. The KX was way too tall in the rear for me. Now it feels relatively flat and balanced. There's also a lot of comfort, plenty of bottoming resistance and confidence to really hammer the throttle without losing the rear end.
Thanks again for all the info. I realize there are a lot of variables in suspension setup and it can be very subjective. It’s great that you went in with an open mind and were able to find a setup that is perfect for you.
I finished all the welding on my frame last week and get it back from powder next week so I’m excited to get started. Obviously I’m hoping to get in the ball park with the first suspension setup and your tips will definitely help that. After that the tweaking will begin.
Let me know when you put together a kit, or plans, to add the HSC adjustment. I’m in. Maybe we could also work out something for the solid models of your clamps if that’s something you’re interested in.
Have fun at the Two Stroke Dream Race.
Jim
I'll update this thread when I have some time to do the HSC shock project. I have production of my KYB conversions on the machines now (running parts while also watching High Point today). After that I have a bunch of customer projects and then I can squeeze in a few more of these personal projects along the way.
The solid models for my clamps were AutoCAD, just for visualization. I programmed them from 2D profiles using a post-processor for Fanuc that I wrote years ago.
We need pics 😎
These are pretty recent.
That is a sweet bike. I like that’89 tank. You were riding it pretty freakin well in the vid 👍
Thanks, I really enjoy riding it.
The combination of that tank with the other mods to the rider triangle makes the bike feel a lot more modern.
Based on great info from this build I decided to go with a YZ fork on my KX500 build. I found a complete front end from a 2011 YZ450 and mounted it to my KX500 using the YZ triples (22.5mm offset). I rebuilt the forks and noted that the shim stack was not stock, but decided to give it a go. I have had the bike at the track three times now and it works really well when the track is fresh. It turns well, holds its line and jumps neutral. It’s all great until the track develops braking bumps and potholes, then the bike becomes a handful. Small-bump compliance is horrible through to mid-stroke. It makes it very difficult to hold a line through a rough corner or brake into a corner. I’ve chased clickers and rear suspension setting without any improvement. Based on some measurements I believe the fork springs are 4.7’s and it feels like it holds up well. So my next step is to revalve but I’m not sure which direction to go and clearly not a lot of KX/YZ experience to draw from.
@cwtoyota Would you be willing to share the shim stack you’re using on your KX500 with YZ forks? I weigh more than you (185) but from your comments I may be a skill level lower. Your stack may not be perfect for me but I feel like it would be much closer than me just shooting in the dark. Thanks, Jim
Jim, I'll post my shim stack here, but it will take you a functional change to duplicate it. I'll describe all that below, but first I had a few notes you might want to consider in terms of my chassis setup versus what you have.
I believe that the 2010+ YZ450F forks are longer than the 2013 YZ250 two stroke forks I am running as well as the stock KX500 forks. My fork height is around 3mm. Also, I am running with a shock that is about 1.5mm shorter than stock with a 5.5kg/mm spring set to 113mm rider sag. I also have the rear axle slid back as far as I can with a 116 link chain.
Fork settings:
My fork springs are 0.49kg/mm.
You won't be able to just drop in a couple shims to make my setting work. I've converted the forks to use a "leaf spring mid valve" like the 2018+ YZ450F and 2019+ YZ250F... I've written a forum post about that conversion on older Yamaha forks, so do a search for that under my user name. The issue with that conversion is that you can't duplicated it 100% because I manufacture my own cups for the MV leaf spring. Suspension Direct sells a similar part for about $13, however their inside diameter is approximately 19mm and mine is approximately 18mm. The SDI cups will result in a softer leaf spring rate due to the difference in diameter. I can offer you a correction factor (different shim arrangement) for that if you are interested. You could also skip the leaf spring conversion and just start out with something like 0.25mm float and adjust to your liking after some test riding.
Another issue with running the SDI cup vs my parts is that the total height of the shim stack will need to be adjusted. I designed my parts and started running them before someone told me that SDI already makes something just like it. The SDI parts have a different cup depth and assembly height, so the small shims between the leaf spring stack and the MV stack will have to be adjusted to result in zero preload.
The plot shows a few different forks plotted for comparison... The blue line "FT.clk" and corresponding dashed lines show the damping force in my setting at 7 clicks and through the clicker range. The pink line is a YZ250 setting. The very dark line labeled MC350F is an early setting from my GasGas. The green diamonds are actual measured dyno data from a 2014 YZ450F. I believe a Yamaha 450F setting is too firm for a two stroke, even a big bore.
Thanks again for all the great info.
I was also under the impression that the 2011 YZ450 forks would be longer but whatever these forks are that I ended up with are very close to my 94, KX500 forks in length (37.1” axle cl to bottom of cap). I’m running the forks at the bottom of the caps. I took your advice from our previous chat and I’m running a 116 link chain with the rear wheel back. I’m using 132.5 dogbones and 110mm sag.
You’re running stiffer fork springs than I am which makes me wonder if maybe I’m getting enough hold-up. Have you run your setup with .46 kg/mm springs?
I just read the thread on the leaf spring valve and it was very informative and convinced me I should give it a shot. It seems to offer much more tuning ability without the related issues with traditional float. Are you selling your cups? I’d be interested in buying a pair and mimic your setup.
Love the graph and how close the KX and YZ data is (pitch and magnitude). The empirical data from the YZ450 shows a significant difference from where you are even at low shaft velocity. I’ve had several people tell me the 450 forks from that era were stiff. Do I understand correctly that the blue line is your KX at your preferred clicker setting and the broken blue lines are open and closed clickers? If I understand correctly, it doesn’t appear to make much difference.
Pit Row
No problem. You've got it 100% on the plots...
The clicker is low speed only so it has less effect on the flatter higher speed portion of the curve. Two clicks is a noticeable change on the track.
Shim changes are required to change the shape of the curve and to make significant mid or high speed damping changes.
Bottoming velocity on a set of forks can be three times higher than what is shown on that plot. I only generated the plot up to where the sample dyno data ended for the 2014 YZ450F. You can consider that a low/mid speed plot.
I have not tested with softer springs in the KX, but 0.46kg/mm might be just fine if you like a softer front end.
I run a 0.47kg/mm spring in my YZ250 forks with similar valving (I believe the YZ250 setting has three fewer face shims on the base valve).
I don't have any conversion cups left in the drawer and my CNC machines are currently running other parts. Functionally, you can get a similar feel with the SDI parts with a few corrections to the leaf spring stack and you wouldn't have to wait a month or two for me to make those parts.
Good to have some context with the shaft velocity numbers. I imagine the gap would look even more significant if the graph was extrapolated out to 3x velocity. I'll stick with my fork springs until I get the valving close to where I want it. Sounds good on the cups, I'll order up a couple of from SDI. Really want to have the bike close for 2 Stroke Nats. Thanks again, Jim
I think trying the 0.46kg/mm springs is a good plan.
I purchase my shims from SDI as well and highly recommend them if you don't already have a supplier or a stash of shims.
It would be wise to grab some extra shims for the mid valve spacing in 8id-11od with a selection of thicknesses.
Also consider getting a handful of extra 8id-20x0.10mm shims for the mv leaf spring.
Beyond that a few extra BV face shims in 6id-32x0.10 or 6id-32x0.15 and perhaps some 6id-16x0.25 and 6id-17x0.25 shims so you can change the clamp diameter if necessary.
I don't know if many people know about this but there is a company that makes reproduction 1988-89 (until 2004 for 500s) KX 250/500 fuel tanks and 250 specific tanks. I'm told they are pretty nice. They also have other brand tanks for sale.
https://www.vmxracing.com/vmx-tanks/
Wow, that’s a good resource 👍
Good post. I'm using the VMX '89, 250 tank on my 500 and have been very happy with it.
It's cool that there are companies making repro parts like that gas tank for these old bikes.
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