My car is chain drive. I have found the chain is tight, then when the car is moved forward it is loose, then tight, then loose.
My first thoughts were that possibly the sprocket is running asymmetric. It doesn't appear so. Maybe running out of true. Visually doesn't
appear so.
Could the chain stretch uneven? I suppose it could, does this happen?
I used to ride a Suzuki GT500 - it was a so-and-so for unevenly stretching its chain. I believe that this was caused by the pivot point of the rear
swing arm not matching the centre of the front sprocket - when you hit a bad bump the bit of the chain currently at the top would get an extra
stretch.
So yes, it can happen.
This happens on motorbikes, you make the play adjustment at the tightest point.
It can, I've had it happen on bikes, I put it down to nailing it from a standstill stretching the chain, thinking about it I do it so often it
should just about have evened itself out. But I just set the tension with the chain at the tightest point, and have never had a problem.
I'd guess the chain will be under more stress on the car, have you checked the gearbox output shaft bearing, if it is worn it could be running
off centre.
I'd say it's the sprocket thats excentic. It'll only need to be a fraction out. I know it only takes a tiny bit from my years of cycle
racing on the velodrome. Any thing but the best (read expensive ) sprockets and chain wheels caused slack/tight symptoms. You couldn't measure it
though.
Technically, chains don't stretch. They wear and all the tiny bits of wear add up to make the chain longer. It's pretty even though.
As adi ^^^^ said almost definately the sprocket which is eccentric, it'll be the rear sprocket, very common.
Davie
Cheers guys will check it out. Not a huge problem as will just get a new set if necessary.
Tight links can also make a chain go tight in some places
I'll second Johneturbo ^^ first thing I'd be looking for is a tight link particularly if you've joined it by peening over the joining
link.
A proper chain tool should allow you to slacken it if it is tight without running the risk of popping the rivet out.
Then it's stretched chain and then eccentric sprockets.
HTH. Cheers, Pewe 
PS be interested to know what it is when sorted.
Your sprockets don't move like the rear on a motorbike going up and down so it has to be an eccentric sprocket or a tight spot on the chain. Watch as the chain goes round the rear and look for it binding. When you say it is moved forward do you mean out of gear as it will loosen the top stretch if you are pushing it but if you drive it it will tighten the top stretch.
Sorry to hijack but my old bike used to have a shimano biopace front sprocket set on it - what was the idea behind this shape of gearset
Incidentally chains rarely stretch in the true sense of the word - that would require plastic deformation to occur, and/or creep under fairly high
load for very long times to be measurable. If it's plastic def you're in trouble. More likely to be excentricity.
prawnabie - biopace was designed to... hang on, sheldon did it better than I...
http://www.sheldonbrown.com/biopace.html
One test is to take the chain off, hold it sideways (so it won't flop) and see how it droops (so to speak). A new chain will be almost
poker-straight, and a knackered one will hang like a banana.
The tired old chains I took off my Suzuki used to have bends that varied along its length! Straightish bit, pronounced bend, straightish bit, bend,
etc.
It may not have been 'stretch' as such, more like worn-out links, but the effect was the same...
Chains DO 'lengthen ( not actually stretch) at first , Initial 'stretch of a chain is when the factory injected lube (in the rollers) gets
squeezed out. Much more likely /early in a non O ringed chain. , which uses the rings to keep the lube 'inside' that's THE tecno
advantage to one.
Once the lube is pushed out, it's direct metal to metal wear.. Very quickly too.
More than possible that Sprockets are off centre as well.. a few minutes with a dial gauge will answer that though.
Best to just bin the chain if there are ANY doubts around it all and buy an Oring version with the highest specs you can afford.
It's NO place to economise.
quote:
Originally posted by prawnabie
Sorry to hijack but my old bike used to have a shimano biopace front sprocket set on it - what was the idea behind this shape of gearset
Normal and due to minor eccentricity in rear sprocket. I built my BDN with brand new sprockets and chain and it still had slightly tight spots. As advised, just adjust it so you have the required play (not much needed on a car unlike the bike) with the chain at it's tightest spot.