An Unholy Union III: WP 4860 USD Forks on an Airhead

Just came across the below blurb about setting up the WP forks. I'm hoping to make the same change in the future and thought it may interesting to someone...I've put the interesting bit in Bold and italic typeface.



WP FORK SETTINGS
For hardcore racing, we ran this setup on the 2012 KTM 250SX (stock specs are in parentheses).
Spring rate: 0.46 kg/mm
Oil height: 370cc (380cc)
Compression: 7 clicks out (12 out)
Rebound: 12 clicks out
Fork-leg height: 5mm up (flush on a sandy track)
Notes: The cheapest way to fine-tune the KTM’s WP forks is to raise and lower the fork-oil height to get the smoothest midstroke action without bottoming, which is why we lowered the oil height by 10cc. KTM’s forks come with different oil heights for each model. The 125SX and 150SX have their oil set at 360cc, the 250SX has 380cc, and the four-strokes roll out of the factory with 390cc.

WP SHOCK SETTINGS
For hardcore racing, we ran this setup on the 2013 KTM 250SX (stock specs are in parentheses).
Spring rate: 5.4 kg/mm
Race sag: 105mm
Hi-compression: 2 turns out
Lo-compression: 15 clicks out
Rebound: 12 clicks out
Notes: We ran a 1.25mm-longer Pro Circuit shock linkage. Not only did it lower the rear of the bike by 10mm, but it stiffened up the initial part of the shock’s stroke to help widen the range of the stock 5.4 shock spring. Faster or heavier riders might be required to go to the stiffer 5.7 kg/mm shock spring. If you are savvy, you will use free sag to make the decision for you.

Not sure what exactly is interesting about that, unless there is anybody that didn't know that changing fork oil level will also affect the fork action? :huh

The specific numbers you are listing are completely irrelevant to anybody but that exact rider/bike/forks. For example, he is measuring ccs because they use closed cartridge forks on the modern KTM motocross bikes. On an open cartridge fork which pretty much everybody uses on their airhead conversion you tune by measuring air gap with the springs removed and not by measuring oil volume inside the cartridge.

Prutser could you explain a bit more about the modification you mentioned on the last page about somehow shimming one of the bushings in order to reduce stiction? I haven't seen that modification mentioned anywhere else. Also, I've been looking around for a DIY 4860 rebuild thread, but I've come up surprisingly empty considering how common these forks are on new KTMs, Husabergs, etc. Anybody got a lead?

I was going to write at the end of the paste... I know these number are completely different but it may help someone when setting up from nothing/no baseline... but I figured everyone who read it would have figured that out for themselves and ignored. I always assume wrongly it seems. With that said... I didn't pick up on the closed cartridge bit... why aren't more people using these on airhead conversions, as they seem to be readily available on german ebay? Further, I liked that fact that someone posted additional details about fine tuning the forks... yes the weights are different, the bikes are different etc etc etc but... this would be useful info for those in the know to chime in with to help us dotterels!

Anyhow moving along... the reason for the post was that I too hear about stiction issues and, like the airhead wrangler have not managed to find a thread about how to set these up on a bmw. I think many people have done the swap over but no one has written about it (in terms of tuning).

I too would like to do a similar thing but have a basic knowledge of suspension (no probs with mathematics but tuning them I have no idea... I twist the preload and compression until good and voila!) and was hoping that someone would chime in with how they set up their forks after swap over to a airhead. I also hear some people use Kayaba forks (honda/yamaha) as they have lesser/fewer stiction issues and parts are readily available though most here seem to be going down the WP route.... curious why to be honest.

Thanks for inspiring further reading folks, I had heard about the mechanics behind closed and open cartridges, but didn't know the practical differences. Found some good reading here on the racetech site and here on another site. I think most people go for open cartridges on these bikes because they are not meant for or planning on doing pro circuit motocross on a beemer , and open cartridges are more durable (but that's from my reading just now, I really have no experience in the matter).

If you're worried about air gap, you could always install a set of air chambers for tunability and balance between forks. I saw at least one guy do this with the WP50s. Mo money tho. And as long as you're throwing money around, you might as well take it to a professional suspension guy for a few bucks. I do however plan on doing as much research into it as possible to try and learn it myself.

The WP 48mm forks do have a lot of friction. One big problem are the seals. SKF did make special new seals to help solve this problem. They are standard in the recent KTM models. There is a You tube film about these seals. What they show is true.
A lot of people complain about a firm feeling with the KTM's. They don't filter away the hard firm small bumps but still dive down when braking. IMO the discomfort comes from the friction.


The 48mm WP that I used for my ST did the same thing. I think it was the worst fork I ever felt.

There was a lot of movement between the inner and outer tubes. Even with new bushing.
This pushes against the seals. So more friction and the seals will wear out faster.

I used feeler gauge to shim out the bushings. Its a terrible job and will take a lot of time.
But for my fork it helped a lot.
The pics just show "where" to put it. Make sure its al the way round the bushing.

Another thing that I felt, was that the sharp edges from some bushings tend to scrape away the oil film.

So I made a less sharp edge on the bushing. Hoping it will not scrape away the oil film.
And it made some improvement.

Ok Sorry, I know some things about forks but not about my mac.

But they do show on my screen.... Even when you reply.

They show up for me.

They come up fine for me. Thanks for the explanation. I've never seen that shim material before. Cool. After some more reading around the interwebs apparently the '09 and later forks have lower sliders that are made by showa and are much better quality (slipperier) than the previous WP tubes. Everyone seems to agree that the later lowers as well as the SKF seals are both big improvements.

Its really a lot of work. The feeler gauge cost's about 8€ a roll of 4 meter. Not that bad.
My tubes have the gold colored coating. That should be slippery...

I have these drawings for the people who like them.... Sorry for the bad quality.

I don't think so, where did you read that? It rather has to do with WP moving it's entire production from Holland into a completely new factory next to KTM in Mattighofen around that time. New machines, tighter QC, revised designs, etc all make for better quality parts, not just the quality of the chrome but all over.

Hi Prutser,

Do you happen to have a drawing that shows the dimensions of the left axle clamp (where the caliper bolts to) more clearly? That would be very helpful for me...

Thanks,
Lukas

I found an interesting article on the new twinchamber/closed chamber versus the old open chamber KTM Forks.

http://www.dirt-bike-secrets.com/motorcycle-forks.html

Well, I got the last of a run of HPMGuy 4860 triples last fall after selling my touratech paniers in order to afford them. And this spring, it took all of 5 minutes of a wanted post in classifieds to find a rather nice fella selling these open cartridge forks from a 2006 KTM 250SX-F. From what I've read, this was a good year for these forks.


He's also sending me an axle and brake rotor. I was thinking of buying the hub he had off of him, since my ulimate goal is to send both wheels into Woody's for "supersize superlacing," but in the interest of getting it on the road sooner, I found a full wheel on ebay with new bearings and seals for cheap. Now all I need is a tube+tire, a brake caliper, and a custom brake line. Oh yeah, and some new springs and seals from Jeff Slavens. I hope to get this done in time for spring along with the loss of some weight from my midsection . Gotta make sacrifices, so much for beer for a while. The bike's about to go on a diet soon too .

Btw, while I'm on the subject, what KTM models have the handlebar hardware I need for these HPMguy triples? From what I can tell, a 990adv will work, but I'm not sure.

Thought I'd share some recent research into these forks and post some links to helpful info.

First of all, there were some changes made in 2005 to reduce stiction. Prior to this, there were 3 bushings and no oil relief, so they liked to blow seals. Many were drilling holes in the tubes to relieve this pressure, and they were removing the third bushing and spacer, replacing them also with KYB parts. See this thread for more info, although you'll have to dig a bit. Pay attention to the Terry Hay member. Oh yeah, you'll need to register to see these KTMtalk threads.

Although, after 2004, WP installed an oil channel for pressure relief, they still recommend drilling the hole as it may not be enough.

Also, this thread is quite useful for parts ID and breakdown. It also details some of the mods.

I was originally looking for recommendations on shim stacks. I first found this thread with several examples for KTM 990 adventures which are close in size to our bikes. Slackmeyer seems to have the most input with a very nice chart of changes he has made. The other charts only include the compression stack, but slackmeyer has covered the midvalve and rebound in 2 revisions. Here's another thread with more options and links. Head 2 Wind may be a setup I'll try since he's about my size.

If you just want the easy way out and have someone send you the proper parts and valves, I found several companies that seem helpful in the matter. There's SuperPlush who not only cater to the bigger bikes, but also have perfect, short, heavier springs and custom spacers for shortening . They were willing to take my specs and send me everything I need to set my valves and rebuild with new springs and spacers, I've read people gushing over their setups. There's also MX-Tech who have dealers in several places and are based out of Illinois.

If you REALLY want to get all technical, you can struggle on the uphill battle of figuring out how to use Shim Restackor and apply it to real life. One of the previously linked ADV threads goes into more detail about it and gives good examples, but not much in how to actually USE it. I think I'd rather simply have someone tell me what to do and sell me the proper parts.

Custom springs can be had from Sonic or Slavens. Others previously mentioned may be able to help with that as well. But, if you are shortening these forks to match OEM height, be sure to order shorter springs or you'll be adding preload. Slavens also has some great how to vids. Recommended spring rates seem to range from .52 to .65 for bikes our size. Head2Wind has .6 in his and are slightly harsh, that's 250lb rider + 460lb dry weight bike, almost exactly our bike size. Slavens recommended .64 for me. I may order two different sizes.

And make sure to use the SKF fork seals for reduced stiction.

Prutser mentioned here somewhere that you may wish to shim out and file the edges of your bushings. There are a bunch of other mods that have been discussed in these threads I've posted. Some of them sound downright sensible, Head2Wind gives quite the list of them.

More good info here, here, here, here, comprehensive video for sale here, and dis/assembly manual here.

Thanks a lot for that. To keep some of this info in one spot, the front axle on the WP4860 switched from a 20mm to a 26mm in '03. There are axle adapter kits available to fit old axles in new forks. 2007 was the first year that the fork went from an open chamber fork to a closed chamber(edit: only the SX. some switched later, thanks AW). The article posted earlier makes a strong argument for sticking with the open chamber fork for a travelling bike. Sounds like an '05 or an '06+ would be the best years to get.

Not ALL of them went to a closed cartridge or "dual chamber" as they call them. Only the SX models at first in '06. I think some are still open cartridge to this day. Some say that quality control got a lot better in 2010 when KTM moved production to austria and the quality of the sliders in particular is much better which helped with the stiction issues. Also SKF seals were used from 2010 on so that helped with the stiction a lot too.

EDIT: current model year EXC and XC-W models still use open cartridge forks. KTM's website lists open cartridge forks as 4860 MXMA PA forks where as closed cartridge ones are listed as 4860 MXMA CC.

Ok, I have unsuccessfully contacted several suspension shops who might cater to a DIY rebuild and revalve. None of them seem to want to work with me . Well I found out why today.

The one, most friendly, helpful contact I have made is with Kelvin at CSR, who says he was taught by WP, taught many others himself, and was an initial distributor for WP, is very experienced in them. He also said that many of even the big name suspension tuners out there are doing it wrong. For instance, the WP manual details oil level setting wrong because they copied the old method by accident. This guy seemed very knowledgeable about all the subtle changes made in WP forks (and there have been A LOT) that drastically change behavior and has all the fixes for them like bigger valves to reduce deflection. They may look the same to the untrained eye year to year, but said that in order to work on these older forks, he'd have to dig up some notes. He mostly works on newer ones which are much different despite the similarity.

One major issue is shim deformation, especially in the older models. Apparently WP has implemented only in the past couple of years a fix for this, that no one, not even he or WP understands clearly. I'll give more details when I get the parts and dig in.

Kelvin also said that this is NOT, I repeat NOT a job for the unskilled or faint hearted, which is probably why I've been having a lack of response. Suspension guys are laughing at my emails. With an increase in complexity of valving, it becomes very difficult to simply say, "this is what you need." Change one valve wrong and it isn't balanced with the other two AND the spring rate properly, and the whole system is mucked up. There are also many things to know about before wrenching on the insides. One small tip for instance, the threads that hold the shim stacks must be cut in order to get the nut off, it is deformed in order to keep the nut from falling off should it loosen. I mentioned the rebuild videos that Slavens has posted, and he said, "yup, I'm the one who taught him." So Slaven's videos are the ones to watch. But despite the reservations, he said he's willing to work with me and answer any questions. Really a great guy!

So anyway, I'm gonna try to do this myself, and he'll make me up a set of shims and springs and all the parts upon request and build me a kit. He said that having him do the rebuild, lowering, and revalve would cost $7-800 and I don't have that kind of cash. Once I get the parts, I'll let you all in on the details.

Off....stings...don't it? I was at 950 with respoking and truing the front wheel. Was worth the cash, but I couldn't get any details on the dark art from my guys wife....who is the office manager.