Is this a wives tale?

[Kurt Franz]

My buddy was over last night and he was looking at the KDX when he pulled on the chain about midway on the rear sprocket. When it pulled back about 1/4 inch or so he told me that the chain was too loose.

Any validity to this?

[radonc73]

I just use the 2-3 finger method for my chain. I would think that the other teeth would prevent an accurate measurement. I have seen people do this as a ballpark figure. The more the sprocket teeth are worn the more room the chain will have to strech.

[KarlP]

Naw.

Can't really tell from here, though.

It is really easy to run the chain too tight. I have. Really shortens chain and sprocket life.

Within reason, looser is better than too tight.

[Kurt Franz]

I just use the 2-3 finger method for my chain. I would think that the other teeth would prevent an accurate measurement. I have seen people do this as a ballpark figure. The more the sprocket teeth are worn the more room the chain will have to strech.

The 2-3 finger method being that 2-3 fingers fit in the space between the swing arm and chain mid-point between the sprockets - correct?

[radonc73]

Yes. As long as you don't have piano or sausage fingers you will be fine.

[KarlP]

I like to do a sanity check.

If I lay over the seat and grab the swingarm and compress the suspension as much as I can, that chain better not get near tight.

[Jeb]

I like to do a sanity check.

If I lay over the seat and grab the swingarm and compress the suspension as much as I can, that chain better not get near tight.

That's the key - how tight it is at the point where the chain is tightest. And it should be the basis on which the manual provides for slackness in the chain while it's on the stand, off the ground . . . however the manual states to measure your chain slack.

So follow the manual, forget the "wives tales".

A chain's total slack should be 1%-3% of the distance from axle-to-axle, measured directly in the middle of the length between axles. So if you measured, say, 50" from the output shaft to the rear axle you'll want between about 1/2" to 1&1/2" of total slack or movement at the midpoint of the chain (25" from the output shaft). Total slack meaning up AND down motion, so you'd want about 1/4" - 3/4" slack if you measured it how most do - by pulling it up.

BUT . . . we've got that pesky swingarm that creates sort of a crook in our measurement, so to use the above measurement you'll want to compress the shock enough so that the swingarm pivot point is directly in line with the output shaft and the rear axle (the tightest point that Karl mentioned).

Or just follow the manual - I'll bet they've got it figured out for you!!

[canyncarvr]

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!