Re: 'Is there any validity to this?'
Short answer: No. None. Zip.
Longer answer:
The devil's in the details..and there are some missin' in this conversation.
The original post had nothing to do with chain tension. Yes, that's what was SAID..but that is not the point of pulling on the chain at the back of the rear sprocket. The guy that said you checked chain tension by pulling on it at the rear sprocket was missing the beer from his lunchbox. Maybe 'cuz he'd already done drunk it!
Really, now....what difference is it going to make if you have two, three, or six fingers between the swingarm and chain if what you're checking is the 'gap' at the back of the sprocket?
None. Zero. Nada, even.
The accepted way to determine
slack in a chain is to follow the specification. On the KDX, that is: Bike on the side stand, measure gap between the swingarm at the end of the slipper and the bottom of a lifted chain. 2 1/4" is what you're after. Some liddler is OK, some bigger is OK, not a whole lot of either. Any given spec will vary from bike to bike due to centerline of the final drive compared to the pivot of the swingarm, swingarm length, suspension travel and such.
But...
wear? THAT is where the comment came from, not slack.
The accepted way to determine chain
wear is to measure its tensed
length. That doesn't generally happen 'cuz it involves laying the chain out straight and measuring a given number of links. You're looking for a % number. The more links you measure, the greater the accuracy. 2% is generally considered the outer limit of acceptable wear. 1% is more reasonable.
Sooo...a 520 (what's on a KDX) chain has a .625 pitch (distance between pins). 100 links will get you 62.5" of chain..2% of that will get you an additional 1.25 (1 1/4) inches. It's additional 'cuz were looking at added length due to wear.
1% gets you...what?
ONE PITCH length!
That's why measuring 100 links works!
Now this is where the subject of the thread comes in. By pulling on the chain at the back of the sprocket, you are roughly checking chain wear.
From:
Here:
Pull the chain of the back sprocket, if you can pull it one half of one of the teeth - you’re up for a replacement. Also check for nice sharp sprocket teeth.
Not all that's on the above site is right....but it's pretty good.
Oh...the reason the method works (in a kind'a sort'a way) is because there is a relationship between the distance between teeth on the sprocket and the pitch of the chain (DOH! Really Mr. Weezard?
).
The distance moved away from the sprocket does relate to the distance the pin centers 'move'. But...the angles, squares, and hypotenuses of the matter kind'a escape me right now.
I'd be glad to know how a +2% chain measures out that way.
Notice: I didn't say 'stretch' anywhere. Chains don't stretch. They wear. Mfgs 'pre-stretch' chains..but that has more to do with mechanically seating parts than it does actually stretching anything.
Signed,
Jaybird
BTW: In the case of not being able to even SEE the teeth on the sprocket 'cuz they're all broken off (like a particular machine in my garage right now), don't worry about measuring anything. Just buy all new stuff!