Building The World’s Fastest Dirt Bike

 This whole thing really jelled because of Malcolm Smith. I started working with him about two years ago when he started using my Power Now carburetor device. I had built some bikes for him to auction for his charity in Mexico. Then the movie ‘Dust to Glory’ came out and this is when I learned about desert racers like Steve Hengeveld, places like Malcolm’s Orphanage and the whole •speed’ light bulb started to glow in my noggin. About that same time I had gone off to England and was hanging out with my friend Alan Pickard. ‘American boy have you seen that movie •The Worlds fastest Indian’? It’s a proper movie I tell you, It’s gonna be a cult classic.’ The combination of Dust and Indian got me motivated so I hooked up with Steve and the entire Hengeveld clan who owns Northland Motorsports up in Flagstaff, Arizona and started to plan out building the world’s fastest production dirt bike.

 

PICK(ING) THE BIKE

The next step was to choose which machine we’d use for setting a speed record. Because of Steve’s relationship with Honda that choice was easy, but the big question remained as to whether we’d go with a CRF450X, or the motocross R model. I felt that the R had advantages especially since we were entering into a stock (motor) class. Gearing would be an issue but the R’s higher top-end horsepower overcame the close ratio gearbox.

INITIAL TESTING SETUP

The first thing to do was to set up the 450R with proper instrumentation so we could monitor the engine and know how fast we were going. The obvious things to take note of would be rpm, water temperature and speed. Speed was monitored with a GPS because it was the only really accurate way to measure it. Plus it told us elevation. We also needed to know outside air temperature for proper jetting setup. A critical piece of information came in the scrutinizing the air/fuel ratio with a wideband air/fuel ratio sensor. This gives you a digital readout of how the engine is burning, which really means whether you are running lean or rich, or just •fat’ in a certain area. We use them with F2 Racing Powerback kits and they work well. For this we needed battery juice, so we fit a lightweight Ebatt unit and mounted it under the skid plate. This solved our instrumentation problems and allowed us to take some initial notes.

 

INITIAL NOTES

Our initial test run on a bone stock Honda CRF450R put the top speed at 82 mph at 10,950 rpm. The water temperature was on the low side at 150 degrees and we used standard pump gas, 91 octane. The test was performed at 7000 feet with an ambient temperature of 65 degrees. The O2 sensor showed that the bike was running pretty rich with stock jetting. Wide open it was 10.1 to 1. It should have been somewhere between 12.8 and 13.1 to 1 to make maximum power, but the richness in the jetting was not affecting the top speed because that is determined by gearing, rpm and tire size. So with our baseline in place, development could begin.

DEVELOPMENT PROCESS WAS TO BE AS FOLLOWS

The bottom line was to gear it up until the rpm start to fall and the power won’t pull the gearing. At the same time we wanted to work on the aerodynamics of the bike so it wouldn’t suck power just to cut through the wind. The critical part of my job was to locate some additional power hp (staying within the rules) so we could gear it taller AND pull additional rpm with the goal of 10,800 as our guideline. Before we started all of this, we would need to find tires that would handle the speed and be able to run on the road. We didn’t want to lose wheel diameter, because the larger wheel diameter allows a higher top speed. Finally we found them in the form of Avon Road Rider tires. They are V-rated tires that would fit our 19 and 21-inch stock wheels. Even though they were tubeless, we fitted them up with stock tubes and went out for another run.

 

ENTER THE MAIN PRO-BLEM: FINDING A PLACE TO RUN THIS THING

Do you know how hard it is to find a long flat section of asphalt that no one drives on in Arizona? We finally settled on a piece of frontage road along I-40. It was hardly traveled, and the asphalt wasn’t too cracked up, but it was only 0.9 miles long. That was bad. It was also along the Interstate. Keep in mind at Bonneville you can have up to five miles to get up to speed. We were trying to do it in maybe 0.7 miles and still needed room to stop. This is the road that we actually hit 121 mph, but the bottom line was that we needed more room.

Getting back to our test sequence, the next thing we did was try an array of gearing to see where we would start to lose rpm. Stock gearing is 13/48. We went 82 mph with that gearing. We increased it to 15/42 and went for a run. We went 104mph at 9220 rpm. Then we took the radiator shrouds off real quick to see what getting those big scoops out of the wind would do. This got us 106 mph at 9300 rpm, and  we noticed something while doing this. While running at speed for a while, the air fuel ratio was climbing and the rpm were dropping. This meant that the bike was running out of fuel. We tried the easy fix of changing the main jet. This only made it richer initially, but it didn’t cure the starvation issue. There was a problem keeping fuel fed to the main jet. We went back to the shop and looked at all the parts of the fuel system. We measured flow rates and how the fuel was getting to the carburetor, then how it was getting to the float bowl. That brought us to the float bowl cap and we solved the dilemma. The float bowl cap was slowing fuel flow and the F2Racing Fast Cap was born.

 

We went to our test area with our prototype Fast Cap in hand and did two more runs-one with and one without. The one without showed the same results as before, 106 mph at 9300 rpm. The run with the Fast Cap showed a big improvement of 111 mph at 10,040 rpm. Ironically, it also gave the bike enhanced throttle response. This makes sense, since all the fuel circuits minus the accelerator pump are fed from the float bowl cap. We put on a 16-tooth countershaft sprocket, and we cheated a bit by replacing the stock number plate with a small hand-made fairing. This combination gave us 115 mph at 9220 rpm. This run opened up a whole new set of problems that put a kink in our testing schedule. At this point we spent more time fixing problems than making any headway.

Both brake rotors turned blue from just the friction of them touching the brake pad. The chain guide on the top of the swingarm got completely chewed to pieces, and the chains were stretching faster than you could put them on. These are problems you just don’t have when you are running 20 to 50 miles per hour on and off the throttle in a normal dirt bike world. So we had to build our own parts. We found the right material to make the chain slider and rollers out of. Then we had to find the right shapes and angles to keep everything rolling with the least amount of friction possible. The brake problem took a whole day to figure out. We had to figure out how to eliminate brake drag and still have brakes. We didn’t need to have a front brake in this class, so that solves half the problem right there.

 

The wheels didn’t turn that freely all by themselves, even with the brake and the chain removed. One of the problems with the rear wheel was that when the axle was loose the wheel spun more freely. On further inspection, we realized that the bearings were too tight and caused a binding on one side. It still rotated fine, just not as freely as it could have. While we were trying to fix the problem, we decided to use ceramic bearings. Since we had the wheels apart we figured this was the perfect time to do it. After rebuilding the wheels and fixing our binding problems, we had some real spinners. Instead of the rear wheel spinning 1.5 times around, it would spin three times with the same push.

 

With our attention still in the wheel department, we decided to try to make them tubeless. The rear tube weighs a pound and a half and the front tube weighs a pound. If you ever have done any bicycle riding, you know how critical rotational weight is to speed and acceleration. Now I won’t tell you how we did it, but man did it work.

 

BACK TO THE TEST ROAD

With the motor, exhaust and airbox still untouched, we ran back to back 121 mph runs at 9800 rpm. And that was into a five mile per hour diagonal head wind. The record in the class we had entered was 120.553 mph. Not only had we just broken it, but we were 1000 rpm short of where we should be running, 2000 feet higher in elevation than Bonneville and still had aerodynamic mods to make and some legal motor modifications. This was a good day.

For the motor, everything was done within the rules that really could be done. This bike is in the Production Motor class, which means it has to have the stock bore and stroke, OE head, cylinder and cases, along with the OE carb or throttle body. The motor was taken apart and rebuilt very correctly. I won’t tell you exactly what I did, other than use ceramic bearings everywhere, but the exhaust is still the stock length and diameter and the muffler is stock, and it’s running on legal gasoline for the event and puts out 63 horsepower.

 

As far as aero mods go, some of the original goals were thrown out the window. You see, initially there was a goal of 150 mph on a bike that the tires and gearing could just be switched and then the same bike run in a desert race. The idea was admirable, but the cold reality is you can’t go that fast with that little power with an incredibly large frontal area. And by that I mean that the wide handlebars used in off-road make for too large of a surface area that has to cut through the wind. So we went narrow and aero, because I decided that if Steve was going to set a record, then he was going to set a record, and not just break another.

 

LOOKING AT THE BIKE

The bars are homemade and are as narrow as possible while still being able to turn and use the controls. The clutch lever was cut and re-welded to fit the bars and fairing. There is no front brake. There also had to be room for instruments and the steering stabilizer behind the fairing, and it had to be covered from the on coming wind. A Scott’s stabilizer, borrowed from one of Steve’s practice bikes, worked nicely. You have to use one if you’re planning on going over 125 mph for the event The skid plate/lower fairing is an XR650 unit, which is probably the best plate you could get to protect this bike and what Steve uses for racing. I hand made a little aero device that was added on the bottom to promote the aerodynamics along the bottom of the bike. Yes, it helps, but you have to use a road race stand from now on to hold the bike.

 

The final aero aid is the radiators, or as I like to call them, Raciators. These are hand-made units that not only aid in aerodynamics by letting unused air flow around the radiator, but they also cool more that a standard radiator. Cooling area hasn’t been lost, but gained. Anyhow, that’s the gist of the deal until now. Hopefully, she doesn’t blow or the tires fly off. I’d much rather lose the record than something go wrong that causes Steve to get hurt. That’s actually my single biggest fear in this whole thing, because if he hits a cow in Baja at 80 mph in the middle of the night, then that’s the cow’s fault and not mine. Let the cow feel bad-and I’m guessing it will. Until Bonneville, sayonara.

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