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Old 10-30-2011, 10:58 PM   #1
Box'a'bits OP
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An Unholy Union IV - 48mm KYB Forks On An Airhead GS

This is an 'in progress posting' as I work through issues with a fork swap.

WHAT I'M DOING:
So Iíve decided to get cracking on the í05 YZ250F fork conversion. These are 48mm Kayaba (KYB) forks, with adjustable rebound & compression damping. They will replace the standard í91 R100GS Marzocchi 41mm conventional forks, which have no provision for external adjustment. I bought these forks in a rolling chassis 12 mths ago. Iíve recovered my money from the other parts I sold, so this is a Ďfreeí upgrade, at least from a basic component perspective.



Quite apart from the other Unholy Union writeups, for inspiration I've also looked at these other threads.
Yamaha Forks on a BMW F650 Dakar
To Those Who Have Put Modern Forks...
KTM lc4 forks on my gspd

Whilst it would have been nice to go all KTM with the HPMGuy triples, I just did not have the available cash when it mattered. The KYB forks are an economic alternative.

WHY I HAVE DONE THIS:
The BMW forks are adequate, but not adjustable without disassembly. They are not very tuneable, albeit that Gold Valves can be installed to partially improve damping performance. Gold valves aside, they are very much 80's technology. The front end from the BMW weighs a tonne, especially the front wheel. Given I donít use tubeless tyres (I do single & 4WD tracks, with low pressures) I canít see the advantage of retaining these. A flat in the boonies will be a major pain.

My bike isnít concours, has already done 206k kms (129k miles) & is heading towards being a bitza, so Iím not losing any value as an immaculate example of the breed. The conversion will fit what I want from the bike more. Watch any video & you'll see I'm not an aggressive rider. Part of this is concern re suspension compliance & weight. The swap is an easy weight saving & should improve the ride comfort for the bike, and give me some other options later with the bike in terms of ride height.

Originally I was going to do this conversion after HPN style frame bracing. Iíve not gotten' around to this yet, & given I have the forks just sitting there....

STEERING STEM LENGTH & OFFSET:
Standard offset on the BMW R100 GS is 38mm. On the YZ250F triples 25mm. The axle offset I measure as 40mm for the standard forks, 34mm for the YZF forks. So there will be some difference in steering.

I'm not running a fairing, use a Baja Designs 8" light, & also have an HPN tank, so can accommodate bigger fork legs with no clearance or fitting issues.

I will need to do something to modify the steering stops on the YZ250F triples. These hit the BMW frame braces rather than the designated stop on the headstock.

The length of the YZF steering stem is too long for the BMW headstock, & is in an hourglass shape, which limits shortening options. The overall length of the stem needed to be kept consistent with the stock Yamaha stem, given the forks also have a couple of tapers that the stem needs to work with. The brown stain on the stem is where the Yamaha bearings sit in the YZ250F headstock.



The Yamaha bearings are both the same outside diameter as the BMW bearings, but the bottom bearing uses a larger inside diameter. The bearing cup giving the reference nos is pictured. The top bearing has the same I/D is the same as the BMW bearing (ref 320/28 J).



FORK SPECIFICATIONS:
BMW stock springs are said to be circa 0.47-0.48kg/mm. I'm currently running 12.5w & 10w fork oils. Fork travel is 225mm.

Standard, YZ250F KYB fork comes with 43.5kg/mm springs. The forks have come with a 0.41kg/mm spring installed. YZF fork oil is 2.5w standard, & stock free spring length is 470mm. Fork travel is 300mm. The forks are externally adjustable for compression & rebound damping.

WHEEL:
Iím not particularly a fan of the BMW wheel set, which is tubeless Ė great on road, but IMHO a bugger if you get a flat elsewhere, and aren't running a tubeless road oriented tyre. They are also damned heavy (albeit strong as a strong thing). Given this I want to keep the Yamaha DID wheel & rim.

Axle sizes are 25 mm BMW, & 20mm Yamaha.

BRAKES:
The YZ250F brake rotors are 250mm, compared to the BMW's 280mm.

I want to retain the standard GS 13mm mastercylinder, partially because I don't want to swap out the switchgear, & throttle housing. The YZ250F uses an 11mm mastercylinder, & Nissin 2x27 mm calipers.
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It's just better when there's twins involved....

Box'a'bits screwed with this post 11-05-2011 at 11:02 AM
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Old 10-30-2011, 11:45 PM   #2
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STARTING THE CONVERSION:
The stem needed to be matched to the BMW headstock. For this conversion I decided to use the Yamaha YZ250F bearings. From my perspective the more 2005 YZ250F components I can use in the front end, the better for later costs & replacements.

Re the stem, conversion options included:
1. Placing a 20mm spacer at the top of the bearings. However the top bearing would be too low on the stem, i.e. not positioned wholely on machined 'flat'. Looking at the photos, the brown stain is where the bearing would normally sit;
2. Placing 2x 10mm spacers at the bottom & top of the bottom & top bearings. The concern was that the 'flats' onto which the bearings should sit has +6mm of 'slop'. The spacers would push the bearings respectively too high & too low on these surfaces;
3. Get someone with a lathe to spin up a spacer to sit (interference fit) in the bearing cup on the frame, with a new bearing cup formed at the top of that spacer. This would then allow you to use the YZ250F steering stem, unmodified. I've saw an example of this after I've arranged for the steering stem to be modified. The downside is that this should really be welded in – a permanent modification to the frame.
4. Have the stem remade to match the BMW frame. I expected that this would be relatively easy for someone with access to a press (to remove the stem), and a lathe to make a new one;

For my conversion the steering stem has been modified, with the stem spun down 20mm past the top bearing surface, & a steel sleeve made up & inserted to allow the bearing to be dropped to match the bearing cups on the BMW. A 20mm spacer has been fabricated to restore height at the threaded portion, which was necessary to keep the forks / triples in spec.

I have retained the Yamaha bearings. The top one is the same as the standard BMW one. The bottom one is Yamaha spec, with the outside cup diameter is the same as the BMW cup size, but the inner 29mm.



One side effect of this is that either I won't need the Rox Risers, or (more likely) I can go to a lower bar.

No issues with the forks hitting the tank, but I will need to do some modification to the stops to make this work properly - the back of the stops on the triple clamp hit the frame gussets - the front opf the stops don't hit the BMW frame stop.



FORKS:
I will run 225mm fork travel, rather than the 300mm that the YZF conversion could offer. This is primarily to retain stock rake as much as is possible (bearing in mind the different fork & axle offsets), & also because the rear is running standard ride heights. As it’s a Paralever, I don’t want to overstress the driveshaft universal joints.

Special tools - with the 48mm KYB forks you'll need the fork cap spanner, & a compression valve tool, to fit into the top of the fork cap (the tool is basically an octagonal reverse drive, with a hex 19mm top onto which you can fit a socket).



The forks were disassembled & 75mm spacers made to reduce the extension of the cartridge. I have had these cut to 50mm & 25mm lengths so that I can play with ride height in a later iteration. The spacers ID is 13mm (cartridge shaft 12.5mm), and O/D 24mm (cartridge I/D 24.5mm). This picture is from someone else - and may be the Showa twin chamber equivalent? (I need to find the thread this came from so I can do a proper acknowledgement)



Here's a couple of videos on how to disassemble the forks to service seaks & bushes





I have also cut the YZF springs 75mm to match the reduced travel. This has a side benefit of bringing spring rates up to stock BMW nos 0.47-0.48kg/mm. I still likely need to go up from that, but there was no cost in cutting the springs (aside from the potential to sell them before they were cut). The springs were cut with an angle grinder, then the ends heated with a propane / butane torch & bent to create a flat end. The ends were then ground to give a flat surface for the spring retainer to sit against.



The below gives a snapshot of how much I've shortened the forks (assembled with no bushes & seals).



The fork seals & bushes on my forks are completely shot, which explains why they leaked so badly. I'm waiting on the bushes to be able to reassemble & finish installing these on the bike.

BRAKES:
I popped the seals out of the Nissin 2 pot caliper to check the seals were in good shape, & that the caliper bore was still good. I unhooked the caliper from the brake line, tipped out the brake fluid, clamped one piston, & used a mini compressor to pop out the other one. I used an old soft handgrip to 'catch' the exiting piston - otherwise you can end up damaging this when it comes out of the brake DAMHIK. Surprising how many bikes use these brakes / pads



You can get 270mm oversize rotors, however I have bought the EBC 320mm super motard set up. This comes with an adapter mount, & uses the standard Yamaha caliper. I note that the F650GS riders use this set up successfully. Note that you need to swap slider pins between the adapters. I need to get some new ones - these were quite corroded.



The below picture is not of my bike, but does show the EBC disc nicely (yet another picture going unacknowledged - this one from the F650GS conversion thread - I'll replace with one from my bike ASAP).



WHEELS:
I am pretty happy with the DID rims, albeit that there is one flat spot, & I needed to tighten a couple of spokes.

In common with much of what I have taken from this rolling chassis, the wheel bearings were shot. I am unsurprised YZ250F threw a rod through the cases - there was comprehensive neglect on every component I have looked at....

Big difference in bearing size...BMW on the left

__________________
My '91 R100GS thread is here
My Ride photos are here

It's just better when there's twins involved....

Box'a'bits screwed with this post 10-31-2011 at 01:14 AM Reason: Add to content
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Old 10-31-2011, 01:08 AM   #3
macrae85
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I like these sort of mods.Interesting stuff!
I maybe jumping the gun on you,but i'd
recommend running the brake line up the
front of the USD forks-down round the bottom
of the caliper and up.Saves it getting caught
on the wheel on compression!Just a small
observation!
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Old 10-31-2011, 01:19 AM   #4
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so when is the beast going to be ready for a ride?

BTW Waikato Yamaha were selling genuine new fork leg plastics for about $15 a pop
The genuine yamaha plastics are vastly superior to aftermarket ones
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Old 10-31-2011, 01:41 AM   #5
Box'a'bits OP
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Quote:
Originally Posted by Mark_S View Post
so when is the beast going to be ready for a ride?

BTW Waikato Yamaha were selling genuine new fork leg plastics for about $15 a pop
The genuine yamaha plastics are vastly superior to aftermarket ones
I'm fairly grumpy that parts weren't in Saturday - which would have allowed me to finish this last weekend.
Waiting, waiting ....
Quote:
Originally Posted by macrae85 View Post
I like these sort of mods.Interesting stuff!

I maybe jumping the gun on you,but i'd recommend running the brake line up the front of the USD forks-down round the bottom of the caliper and up. Saves it getting caught on the wheel on compression! Just a small observation!
Thanks, I'll keep that in mind. I've seen another photo in the F650GS / YZ fork conversion thread with that brake line routing. I've got a looong stainless brake line from the YZF which I'll be using (possibly as an interim measure - not certain on compliance in NZ) which would allow that routing...
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It's just better when there's twins involved....

Box'a'bits screwed with this post 10-31-2011 at 02:00 AM
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Old 10-31-2011, 10:16 AM   #6
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Love it!
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Quote:
Originally Posted by Stagehand
your bike is suitably dirty. Well done.
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Old 10-31-2011, 01:01 PM   #7
igormortis
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Good stuff! Wanna help me shorten mine?
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Old 10-31-2011, 06:14 PM   #8
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Great write-up. Thanks!
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Old 11-01-2011, 01:28 AM   #9
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Nice work and even better write up!
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Old 11-02-2011, 02:54 AM   #10
Box'a'bits OP
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Quote:
Originally Posted by SOLO LOBO View Post
Love it!
Quote:
Originally Posted by igormortis View Post
Good stuff! Wanna help me shorten mine?
Quote:
Originally Posted by bgoodsoil View Post
Great write-up. Thanks!
Quote:
Originally Posted by advNZer? View Post
Nice work and even better write up!
Thanks Guys. Igor, (& Padmei) I'm happy to assist where I can. Your forks will be a little different though (WP4860, & 46mm KYB). Same principles though. But I am learning this too (definitely no suspension expert)...

Blingtastic, we have a roller...



A simple bounce test seems to show things are firmer than stock with all adjusters set at zero. So we are in the ball park. 5w 160cm3 fork oil in the cartridge (stock 195cm3, less room taken by the spacers). Standard 245cm3 in the fork leg. Haven't looked at sag etc. I'll get everything on the bike & work from there. I'll post more detail once I have things more complete.
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It's just better when there's twins involved....

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Old 11-02-2011, 06:38 AM   #11
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Very nice,
and moving along quickly too!
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Old 11-02-2011, 10:43 PM   #12
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What are the clamping diameters, top and bottom, of the KYB forks?
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Old 11-03-2011, 12:39 AM   #13
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Awesome stuff there Nutso. That disc -
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Old 11-03-2011, 02:50 AM   #14
Box'a'bits OP
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Quote:
Originally Posted by igormortis View Post
What are the clamping diameters, top and bottom, of the KYB forks?
I'll measure more precisely tomorrow, but very roughly 54-55mm top triple, 59-60mm bottom (ish)
Quote:
Originally Posted by Padmei View Post
Awesome stuff there Nutso. That disc -
I'm using the YZ250F 2 pot nissin caliper (2x 27mm). I was concerned that the braking would be marginal using the 'oversized' 270mm. The 320mm should deal to those concerns. I'll see when I hook up the brake lines tomorrow.
Quote:
Originally Posted by ontic View Post
Very nice, and moving along quickly too!
Thanks Ontic. I really want to ride it with these forks on.... The stock forks have some limitations for the sort of riding I want to do...

Re the speed of the fork conversion - appearances can be deceptive. The forks, triple, brakes etc have been sitting on my wall for 12 mths. I got them all together as a rolling YZ250F chassis.

I need to finish this so that I can sell the surplus GS forks, triples, wheel etc, which will give me more money to do other things. I bought Gus in July last year, in standard GS form - he has already evolved considerably since then.
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My '91 R100GS thread is here
My Ride photos are here

It's just better when there's twins involved....

Box'a'bits screwed with this post 11-03-2011 at 03:33 AM
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Old 11-03-2011, 10:28 AM   #15
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Carry on, you'll end up with one similar to this . Spotted in Texas at the Harvest Classic in Lukenbach.
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