Ohlins' FGR series motorcycle forks are only available as a universal set that requires substantial modification to the front end of the bike. I could not find any information on how to custom fit a set of these forks, so I decided to document the entire process for other riders to reference. This guide is applicable to all Ohlins FGR forks (800, 900, 100, 300, 250, etc) and most modern super-sport bikes. I will use my 2009 CBR1000RR as an example, but the process mainly differs in triple clamp selection/availability.
The most critical 1st step is choosing a rotor/wheel combination for which there is a custom triple clamp (Attack, IMA, etc) available for the rotor-rotor spacing. FGR forks have a caliper mount offset of 16.25mm. 108mm brake calipers, such as the Brembo GP4-RR, have a brake pad offset of 22.5mm. Note that FGR forks only have 108mm caliper mount spacing, so if your bike (Ducati, BMW, etc) has 100mm calipers, they cannot be used. Since the calipers are bolted to the fork and the forks are bolted to the triple clamp, it's the triple clamp spacing that centers the caliper over the rotor.
For example, on my 09 CBR1000RR the rotor-rotor spacing is 133mm with stock rims and Galfer wave rotors.
133 + 2(16.25) + 2(22.5) = 210.5
The closest triple clamp available is therefore a 210mm. I chose the IMA Evo 4 billet triple clamps in 52mm upper and 58mm lower diameter, which is the size of FGR fork upper tubes. Reference the service manual to mount the triple clamps and set the travel stops. Most aftermarket clamps do not accommodate an ignition key, so the ignition switch will have to be relocated or bypassed. 52mm clip-on handlebars are needed as well, and I went with Woodcraft.
FGR forks bottoms have a 30mm hole (same size left and right) to clamp the axle, and the 09 CBR1000RR rim has a 25mm hole (common to many sport bikes) for the stock axle. The axle can be fabricated from scratch, or can be machined from a donor axle. For a 210mm triple clamp, a Suzuki GSXR nut paired with a Kawasaki ZX axle results in exactly the correct dimensions. For triple clamps less than 210mm, 1-1.5 internal threads in the nut must be removed to allow the axle to thread in slightly more. The GSXR nut is selected because it has a large flange and more than sufficient number of threads for pairing with the ZX axle. The ZX axle is selected because the wide portion is 44mm long to match the Ohlins clamp width and is reinforced in a manner that allows it to be safely turned down (GSXR axle is too thin and wide portion is too long).
FGR fork bottoms are 44mm wide at the axle clamp, and can be divided into 22mm halves by the fork tube center line. The axle nut must not protrude past the axle clamp to ensure that the spacer adjacent to the axle nut (red) is firmly clamped against the fork leg when the axle is torqued (~50 ft-lb). In other words, the axle and nut clamp the left fork leg, red spacer, wheel bearings, and blue spacer but not the right fork leg, which will clamp onto the axle directly. This setup is needed to stabilize the left fork leg with the axle nut, which cannot directly clamp the axle.
The wheel bearing spacing required for calculating the size of this red spacer can be determined by inserting the wheel axle into the axle hole, marking a line, and measuring the marked axle with a caliper. Be sure to exclude the width of the old spacers if left in.
Prior to machining the spacers, use a plank of wood (with the same wheel spacer for both sides) to check if the bearing to rim offset is the same on both left/right sides of the wheel. The 09 CBR1000RR has a symmetrical bearing to rim offset, but if for some reason your bike has a different rim to bearing offset on each side, the wheel spacers must be offset to match (uncommon).
For a 09 CBR1000RR, the wheel bearing spacing is 135mm. The size of the red spacer is calculated below.
1/2 (210 - 135 - 44) = 15.5
The blue spacer should be slightly smaller to allow for some clearance to install the front wheel. I made this spacer 15mm (0.5mm smaller). This 0.5mm clearance does not affect fork alignment, since the 30mm OD section of the axle will thread in 0.5mm more and take up this space. 6061 aluminum round stock can be used, but I started with a pair of Kawasaki ZX spacers to reduce the amount of machining required. The OD of the spacer needs to match the OD of the OEM spacer to fit the bearing seal. For the 09 CBR1000RR, the OD needs to be 31mm. A flange is needed on the red spacer but not the blue spacer.
At this point, the calipers should center perfectly on the rotors if all the measurements, calculations, and machining was done properly. Four spacers (24mm OD, 10mm ID) are needed to position the calipers so that (specs. from Brembo):
(a) entire brake pad must contact rotor
(b) brake pad must not leave >0.5mm of rotor outer edge exposed
On a 09 CBR1000RR, the caliper spacer height is 15.5mm for GP4-RR's. If you are personally machining the spacers but do not know the size needed, it's best to make a set of oversize spacers. Do a test fit and reduce the size until the outer edge of the brake pad is barely over the outer edge of the rotor. If using a machine shop, stack some washers to estimate the spacer height. Select the appropriate length caliper bolts to have sufficient threads of engagement. I used 75mm titanium bolts. Verify that the rotors are perfectly centered in each caliper.
A Ducati 848, 1098, 1198 or similar fender and a pair of adjustable fender mounts with 70mm spacing are needed. I ordered a set of mounts from Opp Racing, but it's not a difficult to make if you have access to a milling machine. The hole on the fender is almost as large as the post on the mount, so a shoulder washer (6mm ID, 8.5mm OD, 14mm flange) needs to be made to secure the fender and provide a metal-metal interface so the mounting bolt can be tightened without crushing the plastic. Adjust the height and angle of the mount and use Loctite to secure the bolts.
Other Points to Consider
A "spacer-less" axle design may be possible with aftermarket rims, but OEM rims use a 31mm dust seal and since the fork clamp is 30mm this won't work on the stock rims.
You can make the spacers captive by leaving a small raised ring (around the spacer) that fits between grooves in the dust seal. I did not bother since aluminum spacers are light compared to the OEM steel spacers and don't fall out.
Ducati fender lacks brake line mounts, so brake line needs to be routed in front of the forks. I chose Galfer "direct thread-in" superbike lines because the curved end adapter allows you to easily adjust the angle of the brake line leaving each caliper.
The right fork leg bottom has 2 tapped holes (20mm apart) to mount a wheel speed sensor. To my knowledge, only brackets for Yamaha R1, BMW S1000RR, and a couple others are available (but not for CBR1000RR), so if your bike has a front wheel speed sensor (ABS, traction control, etc.) a custom mounting bracket needs to be custom made. I am in the process of milling a custom mount for the GripOne traction control sensor. Will update later how this works out.
Good surface finish (no ridges) is critical for all spacers. The wheel axle 30mm OD surface needs to be smooth, polished, and lightly greased to slide in and turn smoothly. Make special note of this to the machine shop if not turning the parts yourself.
The ignition key switch on a CBR1000RR is a high current switch (>15A) so you cannot replace it with a small handlebar switch (most can only handle 3A) unless you wire in a relay.
If using an aftermarket linear steering damper on 2008-2016 bikes, the only option I know of is the HyperPro mount that mounts under the tank fairing but still uses the center-hole in the triple clamp shaft for added stability. The problem is that this hole will be a smaller size for aftermarket clamps, so the OD of that part needs to be reduced, which is not easy because it's essentially a cylinder cut at a slant. I will try super-gluing the two halves for machining then soak in acetone to separate them. Also a 52mm damper mount is required to clamp the fork upper tube directly, since aftermarket clamp will only have 1 of the 2 holes needed to secure the other side of the HyperPro mount.
OEM torque values (20 ft-lb) for the lower clamp must not be used for Ohlins. 12-15 Nm or 9-11 ft-lb is specified in the manual to avoid deforming the upper tube.
***Will update with more pictures and how to mount the damper once I have the rest of the bike put together.