In my last post I mentioned I'd been working on a lot of minor things, and now they've added up to be something to report. Whenever I went down to Baja I always had some envy of those dirt bikes. I tried, but just couldn't keep up. They had such nice suspensions compared to the GS.
Some time ago I got these CRF250R forks off ebay.
All the CRFs, 125, 250 and 450, use the same fork with minor changes in spring rate and valving.
Here is what I found when I compared the GS to the CRF:
R100GS CRF250R difference
wheel base 1513 mm 1477 mm -36 mm
steering stops 90 deg 90 deg 0
bearing 28x52x16 30x51x15
tube length 168 mm 192 mm +24 mm
rake 28.0 deg 27.5 deg -0.5 deg
trail 100 mm 125 mm +25 mm
triple offset 37.5 mm 24.0 mm -13.5 mm
fork lead 38.0 mm 32.0 mm -6.0 mm
total offset 75.5 mm 56.0 mm -19.5 mm
In the table, total offset = triple offset + fork lead, which is the distance the wheel's center is from the steering axis.
Here is my first attempt at adapting the CRF forks on another bike.
I just added on another 25mm to the top of the steering tube and fitted some 30x52x16 bearings. It was a relatively simple mod, and it worked out OK, but there were several problems with it.
Because of the shorter offset and the higher front end the trail was jacked way out. It carved around turns and was really stable on the highway, and I found I really didn't need a steering stabilizer. It seemed tiring to ride through tight twisty stuff though, and was also hard to turn when stopped with a lot of weight on the bike.
Another big problem was the loss of steering angle. The lower triple would hit the frame at the down tube gussets. I really missed those extra few degrees. It was very hard to do slow technical riding. When you need to turn into the falling bike to keep it up. There was just no way... I've seen some similar adaptations that put a spacer between the lower bearing and the lower triple clamp. That would allow more steering angle, but would raise up the front end.
Anyway, my list for ORGS was:
- 90 degree stop to stop steering angle
- About 110 mm trail
- Minimize ride hight
The solution I came up with this time was to fit another steering tube that would mount the CRF triple just ahead of the original steering tube, a pretty radical mod.
I found a hydraulic cylinder tube and a chunk of 2.5" round stock at the scrap yard that I though wold work. The OD of the hydraulic cylinder tube measured 65.0 mm. Here is the plan for the tube ends that would take the CRF's 30x51x15 bearings.
And here is the hydraulic cylinder tube, and the finished tub ends.
To fit the tube length I just assembled the bearings and tube ends in the triple clamp and marked off how long I needed it.
Here is the finished head assembly. You can see here where I had filed grooves in the lower triple clamp to get more steering angle clearance on the old bike.
Based on measurements and trial fittings I figured I needed to set the bottom of the new tube about 40 mm in front of the original tube to give me enough clearance between the lower triple and the frame down tubes to get the 90 degrees of turning I wanted. The original steering tube diameter is 60 mm, and the new tube 65 mm, so if the new tube goes 40 mm in front of it I would need to cut the old tube where the new and old tubes intersect, then weld on the new tube.
But wait, I also needed to set the new steering tube at a steeper angle than the original to get the reduced trail I wanted. I did a some calculations based on the geometry of the two bikes and found I needed to cut about 6 mm less off the top of the old tube than at the bottom of it.
Now the new tube is a perfect cylinder, but the old tube has a reduced center section, a complicated intersection to figure out... I only had one chance to do the cut, so I wanted to be pretty sure it would be right. I figured I'd better have a pretty good handle on that intersection before cutting. I used a graphical geometric calculation to get the four intersection points of the very top of the tube, the top and bottom of the reduced section, and the very bottom of the tube.
Being a bit nervous, I made a trial cut and checked the fitting.
Then I transfered the intersection points to the tube then sketched in the rest of the cut.
Then did the final cut.
I put the swing arm and rear wheel on the bike to use as a baseline to align the new steering tube, then ground the sides of the cutout with an air grinder until the two pieces mated up and the new tube was aligned with the rear wheel.
Then, with the two straight edges aligned I tacked the new tube in place.
After a lot of checking and a break for coffee I welded the new tube on.
To add strength and cover the hole of the old tube I made some gussets from 1/8 stock.
I wanted to mount a Scotts steering damper (http://www.scottsperformance.com
), and the stock CRF triple just doesn't look very cool, so I bought an Applied Racing Stabilizer-Ready triple clamp (http://www.appliedrace.com
) that had the same offset of 24 mm as the stock CRF.
To fabricate a tower for the damper I got a 3/8-16 brass screw, coupler and jam nut. I cut the head off the screw and filed the sides down until the screw just fit into the slot of the damper arm.
I welded the coupler to a bracket made from 1/8 flat stock, then welded that to the top cover. I needed to chase the coupler threads with a tap after welding it.
This shows the steering angle is close to 45 degrees. The limit is in the damper, not the steering stop.
I'll need to weld some small shims to the frame so the steering stop will hit it before the damper reaches its limit.
It was a big mod, but I am very satisfied with the result. This photo just doesn't present what it looks like, it looks really cool in person.
I'm really wondering how it will ride, and where it will crack if it does.