Interesting Insight into '10 to '13 YZ Characteristics

[FZ1426]

http://cyclenews.com/articles/off-road/2012/09/26/first-ride-2013-yamaha-yz450f/

I think this article touches on the unique handling characteristics of the YZ450 without getting into the typical moto journalism cliches or misunderstandings.

The key is where they discuss the RPM vs. stability. My take is the position (closer to CG) of the cams allows for really good feel and manueverability at lower RPM. At higher RPM they say the bike gets "twitchy".

Given my experience with the bike I think that's probably a fair statement. But I think a better way to say it would be that other bikes with conventional cam placement actually gain stability due to the gyroscopic force of the cams as RPM increase, whereas the YZ does not. It stays loose and flickable.

This may be one reason why some pros may not favor the bike...Compared to bikes with traditional (forward) cam placement the Yamaha is not as stable at the high revs where pro's live.

The good news is, in my world it works REALLY good.

[grayracer513]

The gyroscopic influence of the camshafts is nearly non-existent. They don't weigh enough or spin fast enough to affect handling in that way, particularly in comparison to the crankshaft, which spins twice as fast and weighs well over 10 times what both cams weigh.

In any case, gyroscopics would tend to stabilize the chassis, not make it act nervous. That it becomes less "flickable", which is arguably reasonable to blame on the gyro effect, is not the issue they are raising. My experience with thumpers in general is that they mostly have all always been the most stable and manageable while pounding away at lower RPM's. At extremely high speeds, they become more like a revved up two-stroke, with the power becoming harder to control at the same degree of finesse.

[FZ1426]

You can say the gyroscopic influence of the cams is nearly non existent if you like, but it does exist. To be fair you're not backing up your assertion with any data or substantiation.

This effect is one of the main differences between two stroke and four stroke handling characteristics. Reducing the cams effect on roll and yaw is one of the reasons Yamaha designed the rear canted motor. It is also a factor in the decision of a manufacturer to design a single cam motor vs dual cam.

I agree the cams gyroscopic influence will cause a stabilizing effect regardles of position, but by placing that force closer to CG it will have less stabilizing effect as compared to conventional layout.

[KJ790]

Gray is right, the cams contribute very little when compared to the crank shaft, which is still very minor when compared to the wheels. I wrote this a long time ago, but I will repost it here:

Remember that precession is defined by angular momentum, which is based on the object's shape, mass, and radius squared. This means an object of the same mass but twice the diameter will have four times the effect. A cam has an average radius of about 0.5 inches and is relatively uniform in mass distribution (from a calculation standpoint). For simplicity sake, we will model both the cam and the wheel as solid uniform cylinders. This is a fairly good model for a cam, but since a wheel has most of its mass near the outer edge (rim, tube, and tire), it actually has a much greater angular momentum than a solid cylinder model represents.

A wheel has an average radius of about 21 inches. This means if the wheel and cam were the same mass, the wheel would already have 1764 times larger gyroscope effect. Then consider that the wheel has at least 10 times the mass of the cam, so now we are up to 17640 times the effect. The cam spins at half the rate of the engine, so in third gear your cam will be spinning approximately 6 times faster than the wheel. Taking this into effect, the wheel still has 2940 times the angular momentum of the cam. In reality the effect of the wheel would be much greater because most of the mass is near the edge rather than being evenly distributed.

Using these numbers, let's say on a two stroke it takes 20 lbs of force applied perpendicular to the seat to lean a bike to a specific angle (I'm just making up a number here for something to work with). On the modeled four stroke it would take 20.006 lbs to lean it equally, which seems like unnoticeable difference to me.

Look at it this way, try to balance on you bike by sitting still and revving it. It will still be hard to balance. A bike rolling with the engine turned off will be easy to balance. The spinning wheels are what makes a bike so easy to balance when riding.

However, we can also look at the effect of the placement of weight. Let's say the seat height of a bike is 3 feet above the ground. Let's say the center of gravity is 1.5 feet above the ground. Now let's say to take a certain corner at a certain speed and maintain a certain angle to the ground, 20 pounds of force must be applied to the top of the seat perpendicular to the ground. Now add 2 pounds to the bike 1.5 feet above the ground (approximate mass and location of cams). To take the same corner at the same speed and at the same angle you would have to apply 20.2 lbs of force to the seat or the bike would stand back up, this seams more noticeable.

Now, all of these numbers are just made up for demonstration purposes, and there are many more factors coming into effect than these, but it does become clear that weigh and placement of the weight affect handling much more than the gyroscope effect of the cams.

Edited September 28, 2012 by KJ790

[grayracer513]

You can say the gyroscopic influence of the cams is nearly non existent if you like, but it does exist. To be fair you're not backing up your assertion with any data or substantiation.

This effect is one of the main differences between two stroke and four stroke handling characteristics. Reducing the cams effect on roll and yaw is one of the reasons Yamaha designed the rear canted motor. It is also a factor in the decision of a manufacturer to design a single cam motor vs dual cam.

I agree the cams gyroscopic influence will cause a stabilizing effect regardles of position, but by placing that force closer to CG it will have less stabilizing effect as compared to conventional layout.

To be fair, neither are you.

The cams simply don't weigh enough to be a factor. What miniscule gyroscopic effect the cams have on roll is unchanged by the new location of the cam box for the simple reason that it is virtually the same relative distance from the x axis roll center as it was in the '06, for one thing, and for another, a gyroscope used for the purpose of stabilizing a vehicle or other object is most effective when located at the overall center of gravity, the intersection of all 3 major axes. I could hang flywheels weighing 3 times the weight of the existing cams on the cam sprockets of your bike and the only thing you'd notice is that the chain would make less noise.

Also note that a gyro rotating on the Z axis as the crank does has no influence over pitch, only roll and yaw. You didn't say otherwise, just pointing that out.

The main difference in handling characteristics between most 2 and 4-strokes is the combination of a higher center of gravity and a larger amount mass located away from the CG. The CG simply sits higher in the chassis in a 4T than it does in a 2T, and that's most of the problem. It's worse at low speeds than very high speeds, but with an elevated CG, the lean angle at which balance is achieved is often higher than that at which the wheel naturally rolls around the turn without turning the steering inward significantly, which leads to a tendency for understeer (pushing the front).

The "problem" with an extremely centralized overall mass compared with a more evenly distributed one is that the bike still weighs the same but is much more easily rotated around its CG by the influence of forces acting on either end. For example, when the front strikes a round rock 45 degrees down from the top, the wheel tends to pitch the front end up and yaw to one side more readily than if there was more weight directly over the wheel. That tendency to roll around the center also transfers some of the force of the strike to the rear end, as well. What that means is that suspension tuning is more important on a very centralized chassis because the suspension must be both robust enough to support the entire weight of the bike when it lands flat, but still be supple enough not to pitch the bike around excessively when the wheels hit things independently.

[Ben Mens]

Awesome way to put it and an excellent defense of the bike. I doubt it would of left Japan if it really was such a terrible design.

[FZ1426]

To be fair, neither are you.

The cams simply don't weigh enough to be a factor. What miniscule gyroscopic effect the cams have on roll is unchanged by the new location of the cam box for the simple reason that it is virtually the same relative distance from the x axis roll center as it was in the '06, for one thing, and for another, a gyroscope used for the purpose of stabilizing a vehicle or other object is most effective when located at the overall center of gravity, the intersection of all 3 major axes. I could hang flywheels weighing 3 times the weight of the existing cams on the cam sprockets of your bike and the only thing you'd notice is that the chain would make less noise.

Also note that a gyro rotating on the Z axis as the crank does has no influence over pitch, only roll and yaw. You didn't say otherwise, just pointing that out.

The main difference in handling characteristics between most 2 and 4-strokes is the combination of a higher center of gravity and a larger amount mass located away from the CG. The CG simply sits higher in the chassis in a 4T than it does in a 2T, and that's most of the problem. It's worse at low speeds than very high speeds, but with an elevated CG, the lean angle at which balance is achieved is often higher than that at which the wheel naturally rolls around the turn without turning the steering inward significantly, which leads to a tendency for understeer (pushing the front).

The "problem" with an extremely centralized overall mass compared with a more evenly distributed one is that the bike still weighs the same but is much more easily rotated around its CG by the influence of forces acting on either end. For example, when the front strikes a round rock 45 degrees down from the top, the wheel tends to pitch the front end up and yaw to one side more readily than if there was more weight directly over the wheel. That tendency to roll around the center also transfers some of the force of the strike to the rear end, as well. What that means is that suspension tuning is more important on a very centralized chassis because the suspension must be both robust enough to support the entire weight of the bike when it lands flat, but still be supple enough not to pitch the bike around excessively when the wheels hit things independently.

Really?... I don't think so.

As far as I'm concerned my point stands until someone can prove otherwise by explaining the following:

1. What is the combined force potential (inertia) of the two cams spinning at 4000 RPM ? (example)

2. What is the arm of that force in relation to CG?

3. Now move that force away from CG and closer to the front wheel. What effect will that have?

[KJ790]

Really?... I don't think so.

As far as I'm concerned my point stands until someone can prove otherwise by explaining the following:

1. What is the combined force potential (inertia) of the two cams spinning at 4000 RPM ? (example)

2. What is the arm of that force in relation to CG?

3. Now move that force away from CG and closer to the front wheel. What effect will that have?

Read my post, I give examples. Poor handling on bikes like this is a chassis issue, not a gyroscope issue.

[WouldsAssassin]

I was a gyrocompass technician in the Navy and am well studied on gyroscopic forces in my current career as a pilot. Too pop my collar a little more, I even took a few physics classes in college. While I won't get into mathematical computations I can tell you that the cams themselves would not create much more gyroscopic forces at redline as they would at idle. The diameter is too small in relation to their length to produce any appreciable force. The camwheels would likely create more rigidity in space and precessive force, but again, not much as their weight is pretty low.

The crankshaft is the big gyroscope in the bike. Counterbalancers, if installed would be second. Clutch would be next.

Please don't talk about the bikes exibiting poor handling, I just brought home a 2011.

Edited October 4, 2012 by PilotHonda

[rr558]

Awesome way to put it and an excellent defense of the bike. I doubt it would of left Japan if it really was such a terrible design.

Ever ride the 1997 cr250 ? It is one of the worst bikes ever made.

[Ben Mens]

Ever ride the 1997 cr250 ? It is one of the worst bikes ever made.

No I have not...but it was quite a revolutionary bike don't you think? And I love my '10 YZ

[grayracer513]

Ever ride the 1997 cr250 ? It is one of the worst bikes ever made.

Wasn't very popular, was it? But it felt like a world beater to someone.