I do want to add to that that the tolerance allowed in the Yamaha manual, which specifies the chain be replaced at 2.5% longer than manufactured pitch, is a little generous, and a chain run at 2% or more will misfit the sprockets badly enough to cause accelerated wear. I recommend the chain be replaced at 1.5% over-length if the sprockets are in reusable condition, and you want to keep them that way.
I think the Yamaha
manual has the type of chain in mind when they provide this recommendation. A standard roller chain, that is being well maintained, has about the same growth rate from new pitch to 1% over, as it does from 1% to 2.5% over length. This is due to the chain seeing the very same maintenance and lubrication protection from start to finish. And as engineering standards already state that a quality sprocket of the #50 size can function and remain viable when used with a chain of up to ~2.5% over pitch length. The better the maintenance, the longer the standard chain will last.
The sealing ring chain, on the other hand, has a finite lifespan. The sealed in lubricant can only serve it's purpose for so long before it becomes ineffective and no longer able to protect the wear surfaces. Once the internally sealed in lubrication is no longer viable, the chain will start to see an immediate growth of pitch. Unless you happen to be using a lubricant that can penetrate the more than likely worn rings, there is nothing you can do to stop the wearing of the chains firction points, and the chain will grow at a very, very fast pace.
The graph showing the wear of a standard chain will be a slow steady rising line that is fairly predictible from new to worn. The graph showing the wear of a ring chain will be a flat line for quite some time, however once the lube is not viable any more, the graph line takes a dramatic swing upwards.
So yes, when a ring chain starts to see 1%-1.5% of growth over new pitch, you should change the chain out before waiting, as the wear is happening very fast at this point and only a couple of rides will show you that it can no longer take the abuse. The 2.5% will come and go before you know it when running a worn ring chain. So will the teeth of your sprocket.
But the thing that is overlooked in the above post is the effect of the environment on the drive train. There very simply is no amount of maintenance, and no miracle chain lube that will keep the dirt off the sprockets and chain, especially in muddy or sandy conditions. You can use lubes that will discourage, or at least will not encourage, it from sticking there, but if you're going to ride through the stuff, you can't keep it off. There is also no amount of protection that any chain lube can provide that will prevent the abrasive grit in that dirt and sand from wearing at the contact faces of the sprocket.
If you are using a dry-film lube, you can indeed keep sand and dust from sticking to the chain and sprocket. This I know from not only real world, but from testing as well.
And yes mud will adhere to the chain no matter what you do, but the only pertinent place the mud matters is on the roller faces, as they are all that comes in contact with the working faces of the sprocket teeth. Once the mud gets squished out from the roller area, it will no longer do much damage. Especially if the roller metal has already embedded moly, as the dirt will try to gain the space of the metals asperities, but will lose out to the polar moly.
And yes some face wear can occur, but unless you drive back and forth through a mud puddle for the majority of your riding, the occasional mud bog
won't do too much harm to the drive.
When a person sees lots of wear on the sprocket faces, and KNOWS that his chain is within specifications, he needs to look very closely at the rollers, and how they are functioning. Many times a chain will collect dirt and grit between the ID of the roller and the OD of the bushing, which can cause the roller to not roll. This condition can easily cause tooth face wear, and is more than likely what riders see when the sprocket wears with a chain that is known to still be viable in length.
*Note that all of this is depending on the chain being adjusted properly. I have found that folks running chains with bad adjustments are of epedemic proportions, and if your drive isn't adjusted properly you will start to see damage to both the chain as well as the sprockets...AND other components like wheel
bearings and shaft seals. Often times these maladjusted drives will see blame placed on the equipment, when the fact is that the mechanic is to blame for most of the wear.Improper adjustments of chain drives is the number one cause for failures and fast wearing equipment.
As a result of riding in the desert a lot, I have had more than one rear sprocket wear out while the chain was still under 1% stretched. My CR500 went through 3 on a single chain once. The wear this produces is distinctly different in appearance to that caused by a stretched chain, and you can easily tell one from the other. Stretched chains wear at the tops of the load bearing side of the tooth, and give it a "pulled froward", or "bent over" look. Abrasive wear with a chain the correct length wears the load bearing side of the tooth, too, but it stays the right shape. Under these conditions, the tooth looks upright, and fairly normal, but is thinner than it should be, and the front edges of the roller pockets will look to have moved forward, so that they are half-oval, rather than half-round.
That kind of wear is what can be reduced by the use of either hard anodized aluminum sprockets, like Tag or AFAM, or steel sprockets.
Can you show us any examples of a sprocket worn by an over pitched chain as opposed to one worn by grit? I'd love to see that.
Yes, a steel sprocket of any flavor is harder than an aluminum, and it will resist any sort of abrasion much better. But hardcoat anodizing of an aluminum sprocket is only a very mild surface treatment at best. And the 5 or so thousandths of material that has been slightly hardened up will not fight the degredation of sand or grit very much. Not even close to a point that it makes anodizing a must. If you are concerned about abrasive wear, then steel should be the only material option that will help much.