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

[Clay Spinner]

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.

[LukasM]

Originally Posted by Clay Spinner

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?

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]

Originally Posted by LukasM

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?

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.

[Airhead Wrangler]

Originally Posted by Prutser

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?

[Clay Spinner]

Originally Posted by LukasM

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?

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.

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.

[naginalf]

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.

[Prutser]

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.

[Prutser]

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.

[Box'a'bits]

Originally Posted by Prutser

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.

Prutser, none of your pictures are coming up?

[Prutser]

Originally Posted by Box'a'bits

Prutser, none of your pictures are coming up?

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

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

[igormortis]

They show up for me.

[Airhead Wrangler]

Originally Posted by Prutser

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 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.

[Prutser]

Originally Posted by Airhead Wrangler

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.

[LukasM]

Originally Posted by Airhead Wrangler

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.

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.

Originally Posted by Prutser

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

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

[bgoodsoil]

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

What Motorcycle Forks Work Best For You? link here
What motorcycle forks do YOU need? Should you buy an enduro bike with open cartridge forks or a MX bike with closed cartridge forks and convert it? How big or little is the difference between the two systems? And what is a cartridge anyway?

The suspension specialist in our family is my husband who worked several years in the R&D Department of WP at the KTM Factory in Austria. Even though I never personally considered putting different forks in my bike I know that a lot of guys wonder about upgrading their suspension.

The bottom line is that in extreme conditions and for pro riders the closed cartridge fork is the fork of choice. But then pro riders have a specialist technician to look after their forks. Hobby riders are better off with open cartridge as it is easier, cheaper and more reliable in the long run. Lets see why...

The cartridge is the part that contains spindles, needles, pistons and shims to make your fork act like a serious damping system and not just bounce around like a single spring would do. Just as it used to be on vintage bikes. When the fork compresses in, the oil is forced out the cartridge. When it rebounds, oil enters the cartridge again.

The closed cartridge system
In the closed cartridge fork the oil is displaced into a pressurised area at the end of the cartridge. The entire cartridge containing the damping system is therefore sealed off. The oil which lubricates the bushes, contains the fork spring and seals, is not mixing with the cartridge oil.
There are 2 oil systems:

•one in the cartridge
•one around the outside of the cartridge

That’s why it is called closed cartridge or twin chamber system.

Advantage of closed cartridge forks
This system offers more consistent damping due to the pressurised cartridge. Problems like cavitation (turbulence related air build up behind the piston) are far less. Cavitation can produce a split second loss of damping. Personally I have never noticed that in any suspension. If it occurs it might be affecting the performance of a pro rider though.
Closed cartridge motorcycle forks also work better in extreme conditions. Due to the two separate systems the oil inside the cartridge gets less contaminated by rub off from bushes, spring, inner tubes etc.

The pressurized system also helps to maintain damping at extreme damper speeds. Damper speeds of 8m/s are reached at pro level MX in comparison to road bikes where only 2m/s are reached. That is why all MX and all factory riders use closed cartridge forks.

Another consideration is cost. Closed cartridge is expensive to produce and money saved on MX bikes by not having lights, lighting coils, big silencers, side stands etc. helps to make up for the extra cost. Enduro also has a lower damper speed due to less big jumps.

Disadvantage of closed cartridge forks
The drawback of the closed cartridge is that it is more complex and requires more time and knowledge to maintain. Mechanics working on a closed cartridge fork have to be much more careful as it is easy to make small mistakes with bad consequences. Service intervals are shorter. Even though the oil inside the cartridge gets less contaminated. But the complexity of the system gives way for little hick ups in the cartridge. Regular checks are necessary.
The mechanic also needs more knowledge for personal settings in these motorcycle forks. It is more complicated to open the sealed cartridge to change settings and better skills are needed to find the optimum performance of the fork.

The open cartridge system
In this system the oil within the cartridge and outside is the same. The cartridge is at the bottom of the fork and remains under the oil at all times. The oil circulates in and out of the cartridge. There is one common oil chamber. That's why it is called open cartridge fork.
As we have seen the reason why enduro bikes come with this system is production costs. Enduro is also not as extreme as MX in terms of damper speed and therefore open cartridge units cope easily.

The open cartridge fork tends to offer slightly better comfort than the closed cartridge units due to little air mixing in the oil. This also means that damping may not always be as precise and consistent as with the cc units.

What is the right choice?
The average enduro rider does not need such a specialized motorcycle fork as the closed cartridge unit. An enduro bike comes well set up and is suitable for enduro right out the box. Obviously fitting springs to suit your weight is all that is needed in order to have an excellent set up enduro bike with standard open cartridge forks.
In order to use closed cartridge forks from a MX bike for Enduro it would need to be internally valved. The shock as well needs to be changed accordingly. This job is fairly complicated and if done wrong your closed cartridge fork would feel much worse than a standard enduro open cartridge fork straight out the crate.

Also don't forget that you need to service the closed cartridge forks more often. Makes no sense to use the better material and then have the performance suffer due to lack of maintenance.

Pro riders do not need too much comfort. What they expect from their motorcycle fork is consistent damping and big safety. If you go the closed cartridge route then make sure you have the time and money to experiment with different settings in order to get your cc units set up for enduro correctly otherwise they will be wasted. If you get the cc unit right, then you can be sure your motorcycle fork is as good as it can get and on par with the pros bikes.

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