F2 Wheel Conversion - Rear
by Steve Fischer

Converting the rear to that of a CBRF2 or F3 is not as hard as it may seem. Though the F3 rim is only a half inch wider and should work just fine (spacers will be different), this article will focus on the conversion I did--the F2.

Here are the parts you’ll need:

F2 wheel
F2 cush drive/dampers
F2 wheel bearings/dust seals
F2 distance collar
F2 cush drive bearing
F2 rotor
F2 rear sprocket*
VFR axle/nut
VFR alignment markers
VFR caliper
VFR 15t or 16t (stock) front sprocket*
110 link 530 o-ring chain
Ride-height adjustable shock**

* See gearing section below to determine how many teeth to run on the sprockets.
** The Fox Twin-Clicker makes this simple. Otherwise, you'll need to have an extension custom made for your shock after you've determined the new ride height. This is a critical measurement and could be quite difficult to get right without an adjustable shock!

Before you start, take measurements of all stock clearances. You will want to refer to these throughout the project.

First you will make room for the wider wheel. The "I-beam" caliper stay will need to move in order to make enough clearance for the F2 wheel. This can be done the barbaric way—by grinding the "][" down to a "[". Or you can choose, as I did, the aesthetically superior way (yes, art snobs even exist in the motorcycle community). This can be done by moving the stay to the outboard side and bending it in a slight "S" shape. I did this after the wheel was properly positioned so that the spacing would be right. It was bent easily with nothing more than a vice and some elbow grease. Note: Make sure you use a bolt that clears the rear brake master cylinder on the swingarm stroke!

Next you will make the spacers. In all cases, you will want the outer diameter to match the bearing or swingarm in such a way that it does not bind. For the bearing sides, this is especially crucial. Use the surplus VFR and F2 spacers as your raw material, or dig through your local machine shop’s bone pile.

There are three issues to consider for spacing out the wheel.

1) The centerline. The wheel should be in the center of the common plane that the properly aligned front and rear share.
2) Sprocket alignment. I used the distance of the stock sprocket from the swingarm as my target.
3) Caliper arm distance. I measured the distance from the stock rotor to the wheel bearing and compared that with the same measurement on the F2 wheel. The difference became the size of a spacer.

Here are the spacers I made:

Spacer#1 (between the swingarm and the cush drive) I used the stock spacer, but had to grind it down to 14mm. This brought the sprocket bolts to within 3mm of the swingarm.

Note: The old Cobalt Racing spacers made it necessary to shave down the sprocket bolts—suggesting a different centerline. I have checked, re-checked and laser-aligned the wheels and still haven’t found this necessary. After extensive riding, I haven’t experienced any of the tell-tale signs of mis-alignment, so I’m confident that my spacing is within spec.

Spacer#2 (between the wheel and the caliper arm) I used a front wheel spacer because it had the proper thickness to match the wheel bearing’s inner collar. I made it 16mm to ensure caliper clearance.

Spacer#3 (between the caliper arm and the swingarm) I used another thick front spacer to ensure a good bite on the swingarm. I had to keep grinding this one down until it fit snugly—it ended up at 8mm.

OK. The rest is easy. Take the caliper arm down to a good welder and have him cut 21mm off and weld it as straight as he is able. A local guy charged me $20 to do this, and did an excellent job.

Now for the gearing. The new sprockets will be determined by your riding preferences. Download the GearCalc program from this website, or from www.ironjungle.com Input the transmission reduction values found in your shop manual. Determine the circumference of the tire you’ll use (Dunlop shows the diameter—multiply by 3.14159 and you have the circumference). Enter the data into GearCalc and, voila!, you get a chart showing the rpm vs mph for each gear with any given sprocket! Bear in mind that your final reduction value will need to change to compensate for the new wheel circumference. To match gearing, compare the rpm-to-speed for each gear (with the different wheel circumference) to the stock gearing (with the stock wheel circumference).

When Cobalt Racing was doing this mod, they recommended a 15T countershaft sprocket and a 43T rear sprocket. The net result was a much lower gearing (producing more pull at lower rpms, but increasing vibration and reducing top speed substantially). According to my calculations, keeping a 16T countershaft sprocket (same as stock) and combining that with a 43T rear (stock is 46T) makes the closest match to the original gearing. Perhaps Chuck Crites recommended lower than stock gearing for overall track performance, but the 16/43 combo will give you less vibration and a higher top speed in each gear. It is a good starting point, since it will be the closest match to what you started with.

Now for the ride height. Refer to your notes on the stock ride height measuremnts you took before you started. Use that as a starting point to dial in your ride height. If you don't have a shock with an adjustable height, bear in mind that extending the shock length has a multiplied effect on the actual ride height. I increased my shock length by 13mm, though I did this for many reasons--mainly preferencial. This setting will affect the front rake and trail, ground clearance, seat height and chain clearance. If you are using a 45T rear sprocket, make sure your chain clears the exhaust crossover before you commit your adjustments. If you have converted the front end, here's a chance to quicken the steering without sacrificing ground clearance. Make sure you have this right before welding an extension onto an existing shock!

Use a 110 link 530 o-ring chain and you’re good to go! Enjoy those radials!!

-Steve Fischer

Disclaimer: This information has been been reviewed and, where possible, verified. We are not, however, responsible for any mistakes or omissions that have slipped past us. When in doubt, seek official verification.