Screaming CRF450 Quad
Custom Bike Buildup
Note: This is a draft report that will be refined and finalized. We have pictures for each step that will be inserted.

Click here for a picture gallary of our project


Background

Sean had a successful first year in the expert class taking a District 34 3^rd place. By the end of the season he was clearly using the full potential of the screaming 425, and going toe to toe with open class power valves, custom YZ426, custom KTM 520, 450s, …….. After going to the Budds Creek Nationals and reading the MX bike reviews, it became clear that the next step was a custom CRF450 based project over the winter for the 2003 season. Our plan is a transition year to a higher level of competitiveness by racing more nationals and maybe some District 6 for variety.


The Plan

How could we do a 450 on a budget. We have seen full out, all new component 450s that cost over $25,000. The least expensive we have seen with less than main stream parts, was $14,000. Our target was $6000 out of pocket plus parts we already had. We already had a new, reinforced, stock 400ex frame that was prepared for contingency if the 425 frame broke beyond repair. It was never used. This would become our platform. Using this platform allows us to transfer the Roll Design Lobo II front end and Custom Axis triple rate, dual adjust shocks. We can also use the same RPM swing-arm and Dominator axle. This saves most of $3500. We also have extra plastic, seat, bars, and stem from our backup parts bin. We need to steal the hubs, spindle, and calipers from the 425.

Now, where to get the motor. We found Service Honda on line selling bare motor for $3500. Dealer cost is $3400. But then where do you get the carb, exhaust, and electronics? RT 15 Honda, our sponsor, came to the rescue with sponsorship assistance on the whole CRF450 bike. After taking what we needed and selling off the rest we netted motor, carb, Titanium exhaust, electronics, full clutch setup, rear brake cylinder, motor mounts, …. for $3300.

Now, how to do the frame and other mods required. We called East Coast ATV and looked carefully at the online pics of their modifications. They want $1000 for labor plus parts. This is WAY out of line and the resulting sharp S bend required in the air tube to the carb severely compromises performance. We went to see a Walsh 450 setup. Walsh (and Loanstar) start with their 250 frame. The problems are the 250 is a 2-stroke which, without the need for top end cam and valves, is a shorter motor. The frame is very narrow. Stuffing a tall and fatter 4-stroke in that frame results in the inability to remove the head/cylinder without totally removing the motor. Since the CRF450 design is a modified 2-stroke piston, piston replacement is recommended frequently. Thus you have a dilemma. Walsh solves the problem by designing in a removable top side frame rail to allow the motor maintenance. This design adds considerable weight, not to mention the huge cost of the product. The stock 400ex frame is taller and wider to accommodate an air cooled, wide and tall, 4-stroke (Click for pic of frame clearance). To the rescue, Rich Buckley, local Pro racer and professional engineer and welder. Ariens is making a modified 400ex frame for $1400, that may be a good choice for those that don’t have a frame to start from and don’t have access to a racer/welder.

The CRF450 is a 4-stroke like the 400ex, but it is water cooled (that’s where the power comes from) not air cooled. Thus it needs a radiator. Good news is it is a wet sump oil system that does not need the exterior oil tank from the ex, therefore there is plenty of room to mount the radiator. The bike has two small radiators which make no sense for the quad, so we needed a large capacity, heavy duty radiator. Mike Walsh sells a super radiator kit, just very expensive. Mike helped us out with the used radiator from the bike he rode to win the 12hour enduro out west. Minor dings but very serviceable. He updated it to the carbon fiber shrouds and included the used, add-on expansion tank. Price was reasonable versus all new.


What did we face?

We placed the 450 motor in the frame to eyeball the task at hand.

Obstacles;

1. Carb inlet is directly centered, and only one inch from the top rear frame cross-member from the stock frame. There is no way to customize a straight shot and efficient air intake (Click for pic of carb clearance).
2. None of the motor mounts come close to matching (Click for pic of motor mount issue).
3. No radiator mounts.
4. Sub-frame front cross-member also in the way of air intake.
5. Motor inserts for the swing-arm bolt oversized.
6. Motor flange for swing-arm bolt too wide and offset to chain side.
7. Kick-start hits frame.
8. Exhaust header to long.
9. Silencer front cone shape and angle wrong.

What did we do?

1. Cut out the rear frame cross-member. Extended the vertical frame tubes by 2 inches. Installed new cross-member. Added flat plate to each side to tie the top, side rails back in. Added a diagonal tube stiffener. Fabricated a new rear shock upper mount (Click for pic of frame cross-member and rear shock mount mods).
2. Cut out the motor mounts and fabricross-member mod points that use the motorcycle brackets.
3. Fabricated radiator mounts and rubber grommets (Click for pic of lower radiator mounts) (Click for pic of radiator mounted with carbon fiber shrouds). Added same for expansion tank up under front cowl (Click for pic of expansion tank mount).
4. Cutout sub-frame cross-member, bridged up and over with new cross-member, relocated the seat mounts (Click for pic of sub-frame mod and shock mount). (We bought a new, stock sub-frame, and added the diagonal braces mentioned in out 425 buildup report).
5. Swingarm bolt motor inserts pressed out, new ones machined, and pressed in.
6. Swing-arm pivot bolt tube machined down by 11 millimeters on the chain side, and the corresponding pivot tube insert also shortened. Swing-arm seam needed re-welding. A pivot tube insert spacer for the right side was fabricated.
7. We bought a Walsh kick-start shaft extender. Nice piece of work, heat treated, but $200 is way out of line. Turned out to be too short (ok for 250 frame but not the wider 400ex frame). Had to cut and weld in a 1 inch extension.
8. Header needs to be bent horizontally to right side of frame approximately 2 inches, then shortened approximately 3 inches. (You loose the pressed on stopper ring but no consequence)
9. Silencer front cone must be cut, a wedge removed, rotated, and welded back on (remember this is Titanium)
10. Extraneous tabs and mounts removed from frame. Battery and other tabs removed from sub-frame (Hooray no battery and associated circuits and wires).
11. New tabs fabricated and welded on for electronics (Click for pic of CDI box mount).