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Old 10-21-2011, 08:46 PM   #1
Airhead Wrangler OP
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An Unholy Union III: WP 4860 USD Forks on an Airhead



Since I happen to know there will be (at least) 6 people fitting WP 4860 forks to various airheads in the VERY near future and since I know Solo doesn't want us cluttering up his thread with our BS, I thought it'd be a good idea to have all the USD folks solve our problems in the same spot.

I'm using "open chamber" 4860s off of a 2006 KTM 250SXF. They'll be going on a reinforced ST frame by way of HPMGuy's triple clamps. I plan to build my own front wheel using a 1.85"x21" rim on an aftermarket KTM hub and use the stock KTM master cylinder and caliper.

Here's a good writeup of the differences between various years and models of 4860 (and 4357) forks as well as servicing data and some relevant info about fitting them to other bikes.

http://crossroadz.com.au/F650GS/Modi...orkNotesWP.pdf

A good service manual available free from KTM:
http://slorider.com/KTM/manuals/WP4860MXMA2003.pdf

Two videos detailing all the considerations of fitting 4860s to the F650 (but plenty of relevant info):
http://www.garagenight.tv/ep-11-ktm-forks-on-a-bmw/
http://blip.tv/garage-night/garage-n...part-2-2375953

Anyone else?
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R80ST Gets The HPN Treatment
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Airhead Wrangler screwed with this post 10-27-2011 at 02:27 PM
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Old 10-21-2011, 09:12 PM   #2
bgoodsoil
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Thanks for doing all the research on this, man. I'm hoping to do this next year.

For anyone that didn't get in on the group buy you can get triples for these forks from EMIG racing. They make triples that have 30mm of offset. I'm sure they can machine the stem down to BMW specs.

http://emigracing.com/
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Old 10-22-2011, 07:41 AM   #3
SOLO LOBO
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Quote:
Originally Posted by Airhead Wrangler View Post
I know Solo doesn't want us cluttering up his thread with our BS
Ouch

Lookin' forward to seeing this thread move ahead!
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your bike is suitably dirty. Well done.
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Old 10-22-2011, 09:07 AM   #4
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thanks for initiating this AW.
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Old 10-22-2011, 01:41 PM   #5
igormortis
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Yeah, I'm in.
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Old 10-22-2011, 02:39 PM   #6
Houseoffubar
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BMW engineered this bike to be PERFECT in every way, if there seems to be a flaw in your machine, it is because YOU are not worthy!

This thread can now be closed.............











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Old 10-22-2011, 02:51 PM   #7
Airhead Wrangler OP
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Quote:
Originally Posted by Houseoffubar View Post
BMW engineered this bike to be PERFECT in every way, if there seems to be a flaw in your machine, it is because YOU are not worthy!
...yeah, exactly how the fine bavarian craftsmen built it 35 years ago:



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Current rides: HPN #834, '93 R100GSPD "red rocket", '73 R75/5 Toaster mongrel, '80 Ducati Pantah 500SL, '92 DR350, '67 Honda SS50, '80 Honda Chaly.
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Old 10-22-2011, 02:58 PM   #8
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Nobody likes a smartass!


I like to think, how they intended, you know, budgetary, and marketing concerns aside.........Oh, alright, I'm full of shit as usual!
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Old 10-23-2011, 06:21 AM   #9
AntonLargiader
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I looked at this a year or two ago. I think the forks I have are from a 450EXC.

The biggest issue was that the clamping distance on the forks was barely short enough to cover the BMW head tube length. The KTM has a longer head tube, and in order to mount these they would have been too far down in the clamps. I'm not looking for a tall bike, nor 300mm travel.

The stem was also too long; it had a taper in the middle and the 29mm diameter parts (for the KTM bearings) weren't close enough together. Even machining them down to 28mm (for the BMW bearings) barely brought them close enough together. But a new stem is not so hard.

I think different, shorter forks (like on the 950SE) would have a much easier fitment for my purposes.
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Old 10-24-2011, 07:39 AM   #10
Airhead Wrangler OP
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Quote:
Originally Posted by AntonLargiader View Post
The stem was also too long; it had a taper in the middle and the 29mm diameter parts (for the KTM bearings) weren't close enough together. Even machining them down to 28mm (for the BMW bearings) barely brought them close enough together. But a new stem is not so hard.
Here's what we did about the stem:



Quote:
Originally Posted by AntonLargiader View Post
I think different, shorter forks (like on the 950SE) would have a much easier fitment for my purposes.
The 950 uses the same forks, but with shorter springs and a spacer under the topout spring to make it top out earlier. Other than that and the clamps for the larger axle and dual discs, they're the same.
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R80ST Gets The HPN Treatment
Seattle to TDF on an airhead

Current rides: HPN #834, '93 R100GSPD "red rocket", '73 R75/5 Toaster mongrel, '80 Ducati Pantah 500SL, '92 DR350, '67 Honda SS50, '80 Honda Chaly.
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Old 10-24-2011, 10:15 AM   #11
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can anyone with a pair of 4860 forks measure both the length of and distance between the two clamping areas on their fork legs? I would measure mine, but my forks are in Seattle and I'm in Mexico.

Also, a bottom of bearing seat to bottom of bearing seat in the steering head would be a useful measurement as well. I'd like to do up a quicky CAD drawing to see how much height adjustment we can get sliding the fork legs up and down.
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R80ST Gets The HPN Treatment
Seattle to TDF on an airhead

Current rides: HPN #834, '93 R100GSPD "red rocket", '73 R75/5 Toaster mongrel, '80 Ducati Pantah 500SL, '92 DR350, '67 Honda SS50, '80 Honda Chaly.
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Old 10-24-2011, 10:19 PM   #12
igormortis
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Quote:
Originally Posted by Airhead Wrangler View Post
The 950 uses the same forks, but with shorter springs and a spacer under the topout spring to make it top out earlier. Other than that and the clamps for the larger axle and dual discs, they're the same.
I was led to believe that the 950 outers (legs? sliders?) were longer than ‘standard’ 4860s. See here.
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Old 10-25-2011, 06:19 AM   #13
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Quote:
Originally Posted by igormortis View Post
I was led to believe that the 950 outers (legs? sliders?) were longer than ‘standard’ 4860s. See here.
That's a new one on me. Everyone so far has told me they're the same. I guess they definitely aren't. Thanks for pointing that one out. One more consideration to add to the list when choosing your forks.




Being a bit caught off guard by that, and the ground clearance concerns in the thread you linked to I ran a few numbers:

R100GS forks:
Top of leg to axle center (fully extended): 831mm
travel: 225mm
Top of leg to axle center fully compressed: 606mm

WP 4860 (SX model):
Top of leg to axle center (fully extended): 935mm
travel: 300mm
Top of leg to axle center fully compressed: 635mm

To sum it up, using dirt model KTM forks as opposed to 950/990 (S model spec, long travel, not short) forks will not hurt your fully compressed ground clearance as compared to stock R100GS forks. You'll still gain 29mm of fully compressed fork length.

I'd still like to get measurements on the machined clamping sections of the fork legs though as they appear to be dramatically different between the 950/990 and dirt model forks pictured above.
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R80ST Gets The HPN Treatment
Seattle to TDF on an airhead

Current rides: HPN #834, '93 R100GSPD "red rocket", '73 R75/5 Toaster mongrel, '80 Ducati Pantah 500SL, '92 DR350, '67 Honda SS50, '80 Honda Chaly.

Airhead Wrangler screwed with this post 10-27-2011 at 07:40 AM
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Old 10-25-2011, 07:42 AM   #14
espressodrinker
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Take cartridges out of modern forks - put into R100

Small hijack!
I upgraded my R80G/S forks to the R100GS, and put in a Gold Valve. Easy painless conversion. Would it be possible to take the cartridge out of one of these modern forks and then fit them inside the standard R100 forks? Similar to the hugely expensive HPN cartridge that isn't available any more?
Seems to me to be an easy upgrade without the pain of all the extra machining mods - although you don't gain any extra suspension stroke.
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Old 10-25-2011, 07:49 AM   #15
Airhead Wrangler OP
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Originally Posted by espressodrinker View Post
Small hijack!
Would it be possible to take the cartridge out of one of these modern forks and then fit them inside the standard R100 forks? Similar to the hugely expensive HPN cartridge that isn't available any more?
Seems to me to be an easy upgrade without the pain of all the extra machining mods - although you don't gain any extra suspension stroke.
Anything's possible, but there would be a lot of machining involved probably. Also, the stock r100gs forks are 40 or 41mm or thereabouts if I recall correctly. Most modern USD forks are significantly fatter, so fitting one of their cartridges inside might be tough. It'd probably be easier to fit a new front end rather than going to so much trouble machining the cartridge/forks/damper rod
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R80ST Gets The HPN Treatment
Seattle to TDF on an airhead

Current rides: HPN #834, '93 R100GSPD "red rocket", '73 R75/5 Toaster mongrel, '80 Ducati Pantah 500SL, '92 DR350, '67 Honda SS50, '80 Honda Chaly.
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