IF THE MXA WRECKING CREW HADN’T STARTED CHANGING THE FORK OFFSET ON THE 2002 CRF450′S, THE CONSUMER WOULD NOT BE FACED WITH A CONFUSING PLETHORA OF FORK OFFSET CHOICES TODAY.
The MXA wrecking crew feels a tinge of guilt for launching the triple clamp offset revolution. If the MXA wrecking crew hadn’t started changing the fork offset on its late ’90s KTMs and 2002 CRF450s, the consumer would not be faced with a confusing plethora of fork offset choices today. But, we did, and the movement has taken off. We feel guilt because, in many cases, modern motocross bikes don’t need the offset changes the triple clamp manufacturers are foisting on them.
As a rule of thumb, for the 2007 model year the MXA test riders only change the offset on the KX250F, KX450F and CRF250. It is true that over the last five years we have changed the offset dramatically on all of the KTMs and the CRF450. But both Honda and KTM responded to the need for offset change at the factory level. Honda changed their offset at the fork dropout and worked on their frame geometry to lessen the need for 20mm offset clamps, while KTM equipped all of their bikes with adjustable offset triple clamps (KTM owners can choose between the stock 20mm and the optional 18mm).
Most modern motocross bikes don’t need a different offset. They need tender loving care, careful fork adjustment, perfectly matched race sag and the best front tire possible. That said, the MXA wrecking crew has struggled with the cornering prowess of the 2007 aluminum-framed YZ450F.
WE GET MORE MAIL ON HOW TO FIX THE YZ450F’S FRONT END RESPONSE THAN ANY OTHER SUBJECT (AND THERE ARE MORE OFFSETS OFFERED BY TRIPLE CLAMP
MANUFACTURERS FOR THE YZ450F THAN ANY OTHER BIKE MADE).
And we aren’t alone; we get more mail on how to fix the YZ450F’s front end response than any other Yamaha subject (and there are more different fork offsets offered by the triple clamp manufacturers for the YZ450F than any other bike made). It should be noted that we never change the offset on the Yamaha two-strokes, and rarely change the offset on the YZ250F.
As much as we love the lightness of the 2007 YZ450F chassis in roll, we fret over the tendency for the front to understeer from the center-out. Understeer is a handling condition in which the front wheel of the bike turns in slower than the rear wheel. In essence, the rider turns the bars to dissect a given line with the front tire, but the front wheel misses the line. You aim for the inside, but end up outside. In the old days, understeer was called “pushing” or “washing out.” The YZ450F tends to understeer in the center of the corner.
We give Yamaha credit for recognizing the existence of the push and trying to fix it with these three changes: (1) The 2007 YZ450F has a slightly steeper head angle than the 2006 model. (2) The handlebar bend, which was so low in 2006 that it forced the rider to extend his arms downward in the corners, has been raised 8mm, from a 79mm rise to an 87mm rise. (3) The shock length was increased by 1.5mm to load the front end more under braking.
We don’t think Yamaha’s engineers went far enough in 2007. As a quick backyard fix for the understeer, we slid the forks up in the clamps about 5mm (which steepens the head angle). Depending on track conditions, we would change both the fork height and the race sag. Although we prefer 100mm of sag on most tracks, if we raced at a track with exceptionally demanding corners we would jack the sag up to 95mm and correspondingly slide the forks even higher in the clamps to put more weight on the front tire.
These simple-to-do mods helped, but they didn’t fix the YZ-F’s big push in the center of the corner.
BEFORE WE GO ANY FURTHER, WE NEED TO BORE YOU WITH A LOT OF TECHNO-MUMBO-JUMBO ABOUT HOW A BIKE HANDLES. THIS IS THE LANGUAGE OF TEST RIDERS, AT LEAST AT MXA
Before MXA goes any further, we need to bore you with a lot of techno-mumbo-jumbo about how a bike handles and how test riders discuss the subject. This is the language of test riders, at least at MXA, and it is crucial that you understand the basics in order to understand why a YZ450F turns and how to make it better.
First and foremost, the actual act of turning a motorcycle has to be broken down to its most basic elements. The MXA test crew divides a corner into three segments: (1) Turn-in. This is the spot where the rider initiates direction change for the first time. (2) Center-out. Center-out is just shorthand for saying “from the center of the corner to the exit.” It is where actual direction change is achieved. (3) Exit. It might seem obvious that the exit of a corner is where it dumps back out onto a straight, but in the case of a motorcycle, the exit is the spot where the bike is straightened up and steering input is lessened.
Next most important, at least for this discussion, is a frame geometry measurement known as “trail.” Trail is the distance from the where the virtual head angle dissects the ground back to the center of the front tire’s contact path. It is determined by drawing a line through the frame’s head tube angle to the ground and then measuring rearward until it intersects a line drawn vertically through the front axle of the bike.
The distance between these two points is trail. The more trail, the more self centering and more stable the front end geometry. The less trail, the quicker the bike turns and the more likely it is to exhibit headshake.
Trail is measured in inches. The first thing that a frame designer learns is the effects of changing a bike’s trail. If you want to build a chopper, you increase the trail so that the rider can cruise PCH with his feet up and his hands off (of course, the long trail makes the chopper want to flop over in slow speed corners). A chopper typically has seven inches of trail. A trials bike designer decreases the trail to make the bike’s steering light and responsive (with the caveat that if the rider goes fast, he will suffer from uncontrollable head shake). A trials bike can have as little a 2-1/2 inches of trail. A motocross bike designer has to strike a happy medium between stability and agility, because a motocross racer needs both quick turning and high-speed control. Motocross bikes, like the YZ450F, have around 4-1/2 inches of trail.
ON A BMX BIKE, OFFSET IS ACHIEVED BY WELDING THE AXLE DROPOUT ON THE FRONT OF THE BIKE’S STRAIGHT FORK LEGS. ON A ROAD BICYCLE, OFFSET IS ACHIEVED BY CURVING THE FORK LEGS FORWARD.
Trail can be changed in several ways, but for MXA’s purposes, the most economical and understandable method is to change “fork offset.” Fork offset is the distance of the front axle in front of a line originating from the virtual head angle. On a BMX bike, offset is achieved by welding the axle dropout on the front of the bike’s straight fork legs. On a road bicycle, offset is achieved by curving the fork legs forward. On a motocross bike, fork offset is achieved by two methods: (1) Triple clamps. The fork legs are positioned in the triple clamps forward of the steering stem. Every manufacturer offsets the fork legs in their triple clamps by different amounts (Kawasaki 24.5mm, Honda 24mm, Yamaha 25mm, KTM 20mm and Suzuki 23.5mm). The amount varies on different brands because of the other factors that contribute to both trail and fork offset (head angle, dropout offset and rake). (2) Dropout offset. Leading axle front forks were developed to allow the fork’s legs to be pulled back closer to the pivot axis of the steering stem while still providing adequate trail. Prior to leading axle forks, the front axle was mounted on the bottom of the fork legs (in line with the forks leg’s center line). Leading axle forks have the front axle positioned on a dropout in front of the centerline of the fork leg.
Trail can be changed when a frame designer elects to move the axle dropout fore or aft. Honda did this three years ago by moving the front axle back 2mm rather than change their 24mm triple clamps. Moving the fork dropout back 2mm was the same as Honda switching from 24mm offset triple clamps to 22mm offset triple clamps.
All of this brings us back to the 2007 Yamaha YZ450F. There isn’t a single MXA test rider who likes the way it turns. The push at center-out is distracting and requires the rider to readjust his angle of attack to compensate for it. That hitch in the giddy-up doesn’t allow a rider to go through flat corners as fast as possible. Enterprising triple clamp manufacturers offer YZ450F triple clamps with offsets set at 25mm, 24mm, 22mm, 17mm and 15mm. Each manufacturer hypes the benefits of its offset, promising improved turning prowess, quicker lap times and happy faces at the end of the day.
INUNDATED WITH LETTERS ASKING FOR HELP, THE MXA WRECKING CREW GATHERED UP THREE YZ450FS AND THREE TEST RIDERS (INCLUDED IN THE THREE TESTERS WERE FACTORY TEST RIDERS FROM BOTH YAMAHA AND HONDA).
Our goal was to have every test rider ride with every triple clamp (for a minimum of two separate test sessions per offset) and come to a conclusion about which triple clamp offset was the best for the 2007 YZ450F. As a methodology, we started with three different bikes, changing the triple clamps as we went. Then, once we felt that we had a baseline for each offset, we switched to a perfectly stock YZ450F and changed the triple clamps on it five different times (to eliminate differences that might have existed between the three YZ450Fs). To add icing to the cake, we used a Rekluse adjustable offset axle to test every offset in 1mm increments from 25mm down to 15mm. Each test rider filled out test forms (without talking to the other test riders between sessions). The forms graded each offset on a scale of 1-to-10 on turn-in, center-out, exit and overall feel. It was a lot of work.
The results will surprise you. Here is how each triple clamp rated.
To set a baseline, every test rider started the day on the stock triple clamps. All of the test riders have considerable YZ450F saddle time, so we didn’t worry too much about how they felt about the stockers. We knew that they liked the turn-in, hated the center out and were ambivalent about the stock 25mm clamps’ exit. We asked each test rider to rate the stock clamps in the middle of the 1-to-10 scale so that the 24mm, 22mm, 17mm and 15mm clamps could be judged above or below the stockers. It should be noted that each triple clamp manufacturer also makes YZ450F clamps in the stock 25mm offset.
It seems hard to believe that 1mm of offset would make much difference, but the MXA test riders felt that the 24mm Applied clamps were an improvement over the stocker in every area (turn-in, center-out and exit). The 24mm clamps turned in modestly better than the stocker and were more responsive in the center of the corner, but still a little pushy. On the exit, the difference was marginal.
RG3′s four-post triple clamps are an MXA favorite, but test riders weren’t evaluating bump absorption or construction. The 22mm offset RG3 clamps were at their best on the exit of the corner. There was less correction involved in picking the proper exit strategy with the 22mm clamps. We had expected the 22mm clamps to be more planted in the center of the corner, but they felt very similar to the 24mm clamps (better, but not outstanding). We had no expectation that increasing trail would help make the YZ450F more accurate at turn-in, and it didn’t. The handling suffered in precision when compared to the trail of the 24mm and 25mm clamps.
Although we tested both the 17mm and 15mm triple clamp independently, we combined the results because they delivered almost identical handling. First, we were surprised to learn that triple clamps with so little offset existed, but since Ty Davis has been racing the WORCS series for the last few years, we felt that his need for more high-speed stability could offer an explanation. The 17mm and 15mm offset triple clamps reduce offset by 8 and 10mm respectively and produce a massive increase in trail. Corresponding, they offer the most unique feel. The turn-in was very lazy (which a simple course in geometry would reveal), but the 17s and 15s felt the best in the center of the corner. The self-steering and stabilizing nature of increased trail helped slow the center-out down enough that it couldn’t dart away from the rider. Exit was lazy. These wildly offset clamps traded turn-in and exit for a more comfortable feel in the center of the corner.
REKLUSE ADJUSTABLE OFFSET AXLE
The MXA test crew also used the Rekluse adjustable offset front axle to test the offsets that weren’t available with complete triple clamp systems. The results mimicked the triple clamp results, and neither 23mm, 21mm, 19mm or 18mm added any new information to the mix.
IF WE CAME AWAY WITH ANY CONCRETE KNOWLEDGE ABOUT THE 2007 YZ450F, IT IS THAT OFFSET CHANGES CANNOT FIX ITS STEERING RESPONSE. WE WEREN’T THRILLED WITH ANY OF THE OFFSETS THAT WE TESTED.
If we came away with any concrete knowledge about the 2007 YZ450F, it is that offset changes cannot fix its steering response. They can change it. They can move its prowess around. But, we weren’t thrilled with any of the offsets that we tested. Conclusively, we can say that the YZ450F’s cornering is not fixable with an offset change!
The loaded question still remains, though; what offset do we think is the best? And we have an answer. The majority of test riders felt that 24mm was the best all-around fork offset for the 2007 YZ450F. There are caveats: (1) The differences are small enough that poor setup will erase any benefits. This is an improvement on a scale that probably wouldn’t matter to a rider who isn’t pushing to the limit. (2) Triple clamps that change the offset are expensive, because they require both top and bottom clamps. It will cost you close to $600 to feel the small improvement of 1mm. (3) Team Yamaha has tested many offsets. What do they choose? They run the stock 25mm offset.
For riders who absolutely hate the YZ450F’s tendency to push in the center of the corner, the 17mm clamps are a viable option. They aren’t as good at turn-in, but this downside could be offset by the slower response in the center of the turn. The 17mm triple clamps don’t push in the center. They feel more secure because they are more planted to the ground. They also aren’t quick. This is the offset we would recommend to YZ450F haters (although we would also recommend that they switch to a CRF450 if they hate their YZ-F so much).
And there you have it. There is no such thing as a great offset for the YZ450F. Sorry, but if the push bothers you?you will have to compromise. o