February 26, 2008
Comments off

The AMA’s Steve Whitelock has ordained that 450cc motocross bikes are too powerful. If he gets his way, they will be banned. Once rid of them, he can move on to detuning those pesky Formula 1 cars, F22 Raptors, and that thing that John Force drives. According to the AMA, all will be right with the world when political correctness controls the racing world.
But Steve does have one point that Yamaha, Honda, Suzuki, Kawasaki and KTM agree with: racing motorcycles isn’t all about horsepower. The perfect bike is a delicate balance of power, manageability, handling, suspension, weight and usability. But where Whitelock wants to swing the axe, the factory engineers are using their slide rules (computerized, of course) to try to choreograph 450cc motocross bikes that aren’t just fast, but can be ridden fast.
For 2008, Yamaha’s white-lab-coat gurus engaged in a tango of give-and-take to try to build a better YZ450F. The MXA dance masters tripped the light fantastic with the all-new 2008 YZ450F. Here is our report.


A: No surprise here; Yamaha’s dance card had the same four partners on it that it had in 2007:
(1) Power. Yamaha didn’t try for more peak power, they wanted more usable power.
(2) Weight. The goal of losing weight wasn’t as important as losing weight in specific areas.
(3) Sound. Don’t get too misty about Yamaha’s new mechanical muffler. Although it meets AMA sound limits, its real purpose is to one-up the competition and save weight
(4) Handling. The MXA test riders have complained vigorously, bordering on whining, about the handling of the YZ450F (a complaint that we don’t have about other YZ’s). Yamaha has responded in ’08 with a more resilient chassis.


A: Yamaha changed lots of hardware on the 2008 YZ450F engine, but what’s important isn’t the hard parts as much as the intent of the changes. The new parts include a new piston, cylinder head, ignition, camshafts, head pipe and muffler.
The reasons for the changes are an intriguing interplay of give-and-take.
Muffler: No matter what anyone tells you, the 2008 Yamaha muffler is a taker. It costs the YZ450F power. It delivers a corked-up feel that had every test rider complaining about a dead sensation from low-to-mid.
Head pipe: The 2008 head pipe has been upped in diameter from 41.3mm to 45.0mm. That’s a big increase in volume, which would normally result in more top-end power. But, since the new muffler is six inches shorter than the 2007 muffler, the increase in head pipe diameter is neither a give nor a take-?it is a push that compensates for the shorter overall length of the 2008 exhaust system.
Camshaft: By altering the event angle of both the intake and exhaust cams, the YZ450F gets perkier response and a cleaner pickup. This is a give, but was done to increase horsepower to compensate for the loss of throttle response caused by the restrictive muffler.
Cylinder head: The new head has straighter intake tracts to improve fuel flow. This is a give (and while it may help offset muffler power loss, it is good all-around engineering).
Piston: The 2008 piston is new, but it isn’t different. Side by side we don’t think anyone, even equipped with micrometers, could tell the difference between the ’08 and ’07 piston. So what is the difference? The manufacturing tolerance on the 95mm piston has been cut in half. What does that mean? In the past, quality control allowed a 95mm YZ450F piston to be undersized from 0.040mm to 0.055mm. In 2008, YZ450F pistons will be within 0.020mm to 0.030mm of 95mm. Why the hubbub? Many riders complained that they could hear piston slap on deceleration. The tighter tolerance will keep piston rock to a minimum.
 Q: IS THE 2008 YZ450F FASTER THAN THE 2007 YZ450F?
A: No! Not even close. The 2008 YZ450F feels more like a hopped-up WR450F than a 2007 YZ450F. It’s that slow! Suffice it to say that Yamaha’s goal was to make the bike easier to ride, which entailed taming the low-to-mid transition. At the same time, they wanted to gain both handling benefits and Brownie points with their new muffler design. Paradoxically, the new muffler may have helped them on the low-to-mid manageability scale, while hurting them in the overall power department. It looks like Yamaha’s engineers got themselves in a pickle. On one hand they were detuning the engine, while on the other hand they were hopping it up. Although not fraught with as much peril as juggling hand grenades, Yamaha’s approach still smacks of “edge of disaster” engineering.   


A: No. And the reason is simple: we don’t like the low-end throttle response. It feels dead, listless and flat. Yes, Virginia, it is easy to use?-but we could have stuffed a rag in the muffler of the 2007 model to get the same feel. Once past the bottom end, every MXA test rider liked the fluidity of the YZ450F powerband. But in juggling, you can’t catch half the balls and consider that a good performance.


A: You bet. The 2008 YZ450F powerband can easily be turned into an adequate performer. Here are two simple choices:
Tooth fairy. The cheapest way to perk up the corked-up low-end transition is to gear the 2008 YZ450F down. By adding a 50-tooth rear sprocket, the power is strengthened in the lower range. This is a decent fix. It is a band-aid for the low-to-mid transition and we think every 2008 YZ450F owner should do it immediately. The downside is that it condenses the gear ratios a little too much for fast tracks (although it is great on tight tracks).
Pump up the volume. The best fix is to drop-kick Yamaha’s stubby little mechanical muffler in favor of an aftermarket pipe. This change, which many riders will make as a matter of course, is a take-take solution. With a straight-through aftermarket pipe, the YZ450F gets to take full advantage of its beefed up camshafts and new cylinder head.
We would be remiss if we didn’t tell you that the 2008 YZ450F with an aftermarket pipe installed will still not be as powerful as a stock 2007. You read that right. The 2008 YZ450F gives up six horsepower at 6000 rpm. In fact, it gives up anywhere from one to six horsepower from idle to 9400 rpm. And, it never makes more power than the 2007 model at any point on the curve (and the 2007 YZ450F was a detuned version of the more powerful 2006 model).


A: We had to look at the 2008 YZ450F engine with an open mind. There is no denying that it is slow. And, deep in their heart of hearts, every rider, regardless of their skill level, will want more punch. Faster riders will demand it, but Vet, Novice and Beginner riders should hold judgment until they race the 2008 YZ450F. The slowness isn’t necessarily a bad thing if it mates well to the rider’s talent. Every MXA test rider admired the usability of the 2008 YZ450F powerband. They were amazed that they could hold it wide open out of corners and ruts without fear. Perhaps, the mellowness of the 2008 powerband was a plus for the less skilled among us. It was easier to ride in the technical sections, which was an improvement. It was, however, slower in the pure power sections, which was a liability.

A: No problems. Apart from spending a little time adjusting the fuel screw whenever the temperature or humidity changes, the YZ450F runs like a top.
The stock jetting is as follows:
Main: 160
Pilot: 45
Needle: NFLR
Clip position: Third from top
Fuel screw: 2-3/8 turns
Leak jet: 55
Notes: Yamaha also made MXA’s 2007 fuel screw settings standard on the 2008 carb. Last year it was 1-1/4 turns out. This year the stock setting is 2-3/8 turns out.

A: Kayaba is sitting pretty with Yamaha’s SSS forks. We tested with a wide range of skill levels and there was always a clicker setting that worked for everyone from Pro to Vet. As a rule, we started with the clickers out during break-in and then turned them back in as the forks became more fluid.
For hard-core racing these are MXA’s recommended 2008 Yamaha YZ450F fork settings:   
Spring rate: 0.47 kg/mm
Oil height: 350cc
Compression: 12 clicks out
Rebound: 12 clicks out
Fork leg height: 5mm up
Notes: The fork stanchion tube is 4mm shorter for 2008 than in 2007.


A: Before we talk about the shock, it is important to note that Yamaha changed the shock’s rising rate for 2008. The leverage ratio has been revised so that the rising rate has three percent more leverage at the beginning of the stroke (which makes the shock feel softer) and six percent less leverage in the last 100mm of travel (which makes the shock stiffer at the end of the stroke). The shock’s damping has also been softened up a little in the high-speed compression circuit.
Although the YZ450F still has a tendency to kick at the end of consecutive bumps, the shock tracks better under acceleration and is more resistant to bottoming over big jumps.
For hard-core racing these are MXA’s recommended 2008 YZ450F shock settings.   
Spring rate: 5.5 kg/mm
Race sag: 100mm
High-compression: 1-1/2 turns out
Low-compression: 11 clicks out
Rebound: 10 clicks out (12 clicks stock)
Notes: Make small (about 1/8th turn) adjustments to the high-speed compression clicker to adjust the attitude of the chassis at speed. Yamaha’s high-speed compression clicker (the large dial) is very sensitive to adjustment.


A: This is the $64,000 question. Every MXA test rider insists that the 2007 YZ450F pushed in the center of corners. It was an irritating trait, and the faster the test rider, the more the understeer bothered him. Here are the major questions about the 2008 YZ450F’s handling.
(1) Did Yamaha change the geometry? No. The frame’s dimensions are unchanged.
(2) Did Yamaha do anything to fix the handling? Yes. Yamaha’s approach to the YZ450F’s handling woes were very creative (and also very similar to the steps that Honda took when the CRF450 had handling problems). Yamaha, like Honda before them, redesigned the frame to feed more flex into the head tube area. The steerer tube was shortened by 5mm to lessen the span between the backbone and down tubes. The ribbed gusseting (inside the frame’s down tubes) was reduced in height to take some stiffness out of the chassis. The bottom triple clamp, which previously was a solid casting on its upper surface, was changed to a ribbed design-?again with the intent of feeding in more flex. Yamaha knocked one pound off the front of the bike with smaller dropout castings on the fork legs, a smaller front brake caliper and lighter fork guards.
(3) Does the 2008 YZ450F handle better? Yes and no. Every test rider, even the Pro-level test riders who were the most vocal critics, believed that the 2008 YZ450F turned better than the 2007 model. Best of all, hard-core Yamaha riders, who lived with the push of the 2007 YZ450F, loved the cornering of the ’08 model (but they are true believers). But, and this is a big but, no one believed that the handling was night-and-day better than last year.
(4) Does the YZ450F handle better than a CRF450? No. No. No. We are willing to give Yamaha props for their more resilient frame and lighter feel, but this is still a troubled chassis that isn’t as settled in flat turns as it could be. It’s not bad in berms, ruts and loam, but the front just isn’t confidence-inspiring in front-wheel generated direction changes. It still pushes from the center-out.
(5) Can you make it better with an offset change? No, but you can make it different. In the crazy mixed-up world of give-and-take, changing the offset just moves the problem around. You can eliminate the push at the cost of slower turn-in, but there is no magic bullet.


A: It feels light (not when picking it up off the stand, but when riding it). Thanks to the wonders of gravity, rotational moments of inertia and mass centralization, Yamaha found a way to make a two-pound weight savings feel like a ten-pound savings.


A: The hate list:
(1) Cornering: Yamaha made baby steps towards fixing the problem, but they need to take larger strides.
(2) Shifting: Yamaha says that we are the only ones to complain about this, but the tranny is stiff. Shifting requires more effort than it should. The second to third shift often ended up being second to second.
(3) Muffler: We don’t think that weight is at the top of the stock muffler priority list. Power tops MXA’s hit list. Plus, this muffler isn’t all that much quieter than last year’s muffler. And regardless of how light it is compared to last year’s stock muffler, aftermarket exhausts are still two pounds lighter (without the power loss).
(4) Rear tire: Yamaha went to a 120/80-19 rear tire. Most MXA test riders prefer the 110/90-19 because it is quicker in roll and has less tendency to push on the entrance of turns (something Yamaha should be concerned about). Why did Yamaha make the change? The wider 120/80 is 300 grams lighter.
(5) Power: Steve Whitelock would love the 2008 YZ450F because Yamaha made the 450cc engine feel like a 350cc engine.


A: The like list:
(1) Brakes: Consistent, predictable and trouble-free.
(2) Weight: Yamaha saved 1-1/2 pounds last year and two pounds this year. That’s a program that pays noticeable dividends. Unfortunately, we will be giving back the 0.6 pounds of muffler weight and the ten ounces of tire weight when we switch the bike to race trim.
(3) Black cases. We loved the black engine cases for the 15 minutes that they were black.
(4) Aluminum plugs: The YZ450F gets aluminum engine plugs instead of the plastic plugs on the YZ250F.
(5) Footpegs. Yamaha increased the width of their footpegs from 45mm to 55mm. Great stuff.
(6) Shock spring. The only titanium shock spring on the showroom floors.


A: We think Steve Whitelock is wrong about 450cc motocross bikes being too powerful. Why? Because the first thing we wanted out of the 2008 Yamaha YZ450F was more power. The first place to start is by round filing Yamaha’s dual-cone muffler.o


Comments are closed.