Chain tension varies

[chancer]

Hey guys
I just almost finished building my first kart many thanks to info on this forum. I am having troubles with the rear axle sprocket and clutch alignment. I have a 36"live axle 72t sprocket and 12 tooth clutch. everything is squared up the axle with the motor and the sprockets etc. BUT when I spin the axle the tension goes from no play whatsoever to 3/4" every time the axle spins 180*. I have moved the engine back and forth, lined up the sprockets. I can't figure this out. I took everything apart tonight and it seems the axle has a slight bow in it. I put it on a 48" level and with both ends touching there is a slight gap in the center, if I roll it over it only touches in the middle. I am talking less than 1/16 from end to center. Is this possibly my problem. I checked the sprocket and hub they are not visibly or measurably bent. If the sprocket was bent or "taco ed" I think the tension would come in and out every 90*. Thanks for any help.

 

[landuse]

I am not too sure what you are asking, but if your axle is bent, then that would be the cause of your chain slacking off each 180 degrees. You can either bend it straight again (or have someone do it for you, or get a new one

The most concerning thing is HOW the axle got bent. Was it always like that? Did it happen while riding? What thickness is the axle? You might find that you do not have enough axle support, which allowed it to bend

Pics would be good

(and why are we only hearing about this kart now? Don't you know that it is pretty much sacrilegious to not have a build thread going

 

[Doc Sprocket]

Yup- the symptoms point to a bent axle, and/or a sprocket out of concentricity. This means that the sprocket is NOT centered perfectly on the shaft. Since you have areaddy detected a bend in the axle, I'd say that's it.

Landuse is spot on about finding and correcting the underlying issue.

 

[Poboy kartman]

The bend in the axle may be a contributing factor but with the amount of bend doesn't jive with the amount of tension change. I would look to concentricity for the cause. I'd straighten that axle though. A little persuasion with a sledgehammer should do it.

 

[Scout]

Try a new chain. They do stretch.

 

[mckutzy]

What is the dia. of the axle in question? Is it a solid round bar w/keyway or is it threaded?
Pics?

Edit: sorry landuse, I didnt read carefully your comments.

 

[chancer]

Thanks guys for your answers and questions
It is a 1" solid axle with a 1/4" key way and threaded ends.
How it got bent? I did not drop it I just installed it I assume in shipping fro B M I karts.
The chain is new un stretched unused.
Like Doc said It sounds like that is the problem but like Poboy said The slight bend does not seem to "JIVE" with the amount of chain slack.

Concentricity? If the sprocket is not centered on the shaft. But it is keyed and set screwed tight on the axle. How can I adjust concentricity. (cool word).

Ok as far as build pics and a build thread, Well I may be a little shy but for sure I am a better fabricator/Mechanic than Techy. Would rather build than blog. Heres the teaser though.... The cart is a two seater with the passenger behind the driver. Pics soon enough.

 

[chancer]

My go cart

 

[qtband]

Does your sprocke bolt onto the hub, or is it a one piece unit?

 

[Poboy kartman]

Here's what I'm suggesting: measure from the axle to the outside of the chain on the sprocket as you rotate the axle. Note where the master link is and make a complete rotation, measuring every 90° on the sprocket. Not only will this detect an out of center sprocket but mis-spaced teeth that don't allow the chain to fully seat.

 

[chancer]

I have a Azusa "unihub" with a sprocket bolted to it both new bought from BMI. They did say that they have seen this before with the uni hub and suggested buying a racing hub. (after the sale)
Not on this site, but I have seen some ratty looking old karts. Free sometimes and then rebuilt. (Kart Rods) and they do not seem to have this problem. My parts are new but clearly are not purrrfect. The imperfections in these parts are less than or about 1/16 or 1mm so it is hard to actually measure. I can just see a wobble when I spin the parts by hand.
My real question I guess is Should these minor imperfections cause such a huge difference in chain tension. (3/4" play too 0") Or. Did I build something waaay out of shape and that is what is causing the problem.
When building a kart what are the main super important things that must be perfect on the rear end and engine mounts to prevent chain alignment issues and destroy clutches?

 

[qtband]

Can you slide the sprocket right next to the axle bearing? If you can, check the wobble there. If it disappears, the issue is with the axle. If it's there, remove the sprocket and check the hub and see if that wobbles. If it wobbles without the sprocket, send it back and get a different one.

 

[chancer]

Well I took everything apart and screwed the sprocket to the hub on the other side and lined up the holes 180* opposite from before. I put everything back together and now it works fine. WTH? I dunno, but it works now.

Just wired up the kill switch and will be testing it out today
Thank you thank you

 

[Oxymoron]

Well I took everything apart and screwed the sprocket to the hub on the other side and lined up the holes 180* opposite from before. I put everything back together and now it works fine. WTH? I dunno, but it works now.

Just wired up the kill switch and will be testing it out today
Thank you thank you

That makes it sound like the hub and sprocket are both running out the same amount, and rotating the sprocket 180° cancelled it out. Weird.

 

[Poboy kartman]

That makes it sound like the hub and sprocket are both running out the same amount, and rotating the sprocket 180° cancelled it out. Weird.

No- not weird at all. What I said from the get- go. A concentrictry problem coupled with a bent axle as a contributing factor. Measuring the way I suggested in my last post would have made it obvious.

If something is off by a little bit and the position is 180° relative to each other the variance doubles.

Anyway, glad you got it going.

 

[chancer]

ARRG CLUTCH will not engage. Works with wheels off the ground but will barely move the kart. I have a 6:1 gear ratio 72t sprkt and 12t clutch with 13-6-5 tires. Motor revs and it just does not move

 

[landuse]

ARRG CLUTCH will not engage. Works with wheels off the ground but will barely move the kart. I have a 6:1 gear ratio 72t sprkt and 12t clutch with 13-6-5 tires. Motor revs and it just does not move

Have you got the key properly installed? Are the setscrews tightened down? Post a vid of what is happening so that we can see.

 

[OzFab]

Like Poboy said, the runout issue makes perfect sense; think about it:
If the axle is bent by 1/16", that's a total runout of 1/8"
If the sprocket is off centre by 1/16", again, that's a total runout of 1/8"
Add the two together & you have a total runout of 1/4"...

Now, rotate the sprocket 180° & it still has 1/8" of runout but, in the opposite direction which is then counteracted by the bent axle...

 

[Oxymoron]

No- not weird at all. What I said from the get- go. A concentrictry problem coupled with a bent axle as a contributing factor. Measuring the way I suggested in my last post would have made it obvious.

If something is off by a little bit and the position is 180° relative to each other the variance doubles.

Anyway, glad you got it going.

Like Poboy said, the runout issue makes perfect sense; think about it:
If the axle is bent by 1/16", that's a total runout of 1/8"
If the sprocket is off centre by 1/16", again, that's a total runout of 1/8"
Add the two together & you have a total runout of 1/4"...

Now, rotate the sprocket 180° & it still has 1/8" of runout but, in the opposite direction which is then counteracted by the bent axle...

I understand that part. I just mean weird that the two parts that are out, are out equal amounts. I mean what are the chances of them both being out by exactly 1/16”?

Good thing they were though... easy fix

Though, It'd probably be better to straighten out the axle and get a different sprocket.

 

[OzFab]

I understand that part. I just mean weird that the two parts that are out, are out equal amounts.

Happy coincidence

I mean what are the chances of them both being out by exactly 1/16”?

Knowing BMI, I'd say pretty good

Though, It'd probably be better to straighten out the axle and get a different sprocket.

True but, if it works...