I must admit this has been driving me nuts! I have a Gary Fisher Rig with an eccentric bottom bracket chain tension system. I've just fit a brand new set of Shimano SLX cranks to replace a slightly older set of SLX's I managed to snap (crash damage, not a failure).

Problem is when the drive side crank points towards the back of the bike, chain tension is fine. When you rotate the crank to point forward it seriously slackens off the chain tension. If I adjust the chain tension so it's fine when the crank points forward, the chain is far too tight when the crank points back. The result of which is that it's impossible to get correct chain tension.

I'm pretty certain the problem is due to the spider not being completely central on the SLX cranks because, A) I’ve checked to make sure the rear sprocket it central, B) The chain is brand new, and C) the chainring bolts hold the brand new goldtec chainring tightly (so the chainring is central on the spider).

So the questions are has anyone else had this problem with Shimano cranks (SLX or other) and is the spider designed to be slightly off centre (to aid shifting) or have I got a set made on a Friday afternoon after one too any saki’s (could it be a warranty job even)?

I can’t really justify buying another set of cranks so it looks like I’m going to have to buy a tensioner to take up the slack (fortunately the rig will take a mech hanger) . How annoying is that!

Cheers

Chris

Why could you not return them? Might be worth a try? Not sure what you would replace them with though.
Are you sure it is the spider and not the chainring? Rotate the chainring 180 degrees and see if the problem is identical?

Slacken the chainring bolts & tap the chainring to centralise it. Might take a bit of fiddling but normally works.

my ht2 deore is the same on my sanderson slack in some and tight in others ovalised chain ring on mine me thinks.

My SLX cranks on my Genesis IO run perfectly round with a cheap taiwanese ring, but i think it's just your luck…

Haven't come across a singlespeed setup yet that doesn't have a "tightspot" on the chain somewhere…

As takisawa says, loosen the chainring bolts a bit and tap it where it's tight, see if that makes a bit of difference. Due to the nature of not having a derrailleur though, an SS drive train will be much more unforgiving of any imperfections in its components, such as a slightly oval, or at least not perfectly round, chainring.

My Goldtec SS ring seems acceptably concentric. Maybe try the slackeing of the bolts but maybe just slightly and spin the crank and see if it pulls the ring into a more central position. I'd be surprised if it was the crank but maybe one of the forging dies was poorly made? As said previously on a geared (dérailleur) bike it would hardly be an issue but more important on a SS. The spindle of the crank isn't bent by any chance? That would make the whole crank run eccentric around the centreline of the BB. Poorly installed BB? Does it run smooth?

I've just had another thought. You said that your previous SLX was snapped in an accident. Now to my way of thinking a crank arm is a pretty stiff lump of metal. I'm wondering if the force that could snap a crank arm didn't also twist your frame? Just by instinct I feel frame tubes would give more easily than a crank arm. So maybe the whole shell is twisted and is not parallel to the centreline of the back wheel centreline. I'm sure it's not something you'd want to consider but I think it may well be worth getting checked at the LBS.

what 1961Bikie said -it could just be that the BB shell is no longer parrelel to the dropouts – might be worth getting it checked.

maybe put the old crank/another 'known good' crank back in and see if that has the same problem now (assuming you broke the crank arm not the spider?

Not saying the frame isn't bent, but the frame doesn't rotate so won't affect the chain tension at certain points only.Try rotating the ring through 180 degrees to see if the tight spot is on the ring or the crank.

My Tinbred/EBB with XT is the same, but it seems to only be an 'issue' when retensioning when I have to find the compromise between the tight spot being too tight and the slack spot being too slack. From experience, the slack bit has to be very slack before it creates riding issues so I tend to err towards making sure the tight bit isn't too tight and live with a slightly slack chain at the other end as in practice that doesn't matter.

Slacken off and then re-nip the chainring bolts enough to hold it firmly but not completely tightened. Find the tightest spot and give the front facing side of the chinring a tap backwards with a block of hardwood or a soft face mallet. Keep doing this until you get the best result possible and then tighten the chainring bolts.
If you don't get anywhere with this, then as above – try it with the ring rotated 180 degrees and go through this again.

I agree with theotherjonv – you don't need to be too anal about chain tension if you've got your chainline nice and straight. Just ride 🙂

Yes had a think after my last post and realised that a twisted frame would not vary the chain tension. However GF Rigs have an eccentric BB n'est pas? Could it be a problem with the eccentric? Or as I said could the chainset spindle be slightly bent? Put it against a straight edge. If bent take it back for a replacement.

I'm still concerned about the frame following the accident though.

If the tension is varying it is due to something not being constant. Bent frame wouldnt change, neither would the BB cups. So it has to be either the BB axle, crankarm or chainring.

I'd think that it was more likely that the spindle isn't installed perfectly perpendicuar to the spider than off centre or bent.

This could be checked by putting a dial test indicator on the frame, (any mate who works in industy would likely be able to borow one of these) with the tip against inside of the ring and spin the cranks.

I had the exact same problem until a week ago, with some Shimano Alivio cheap-as-chips cranks.

To confirm it, I attached a cable tie to the frame so I could measure by eye the variation in the length of the spider's tabs as it rotated. They were all spot on.

I then measured the distance of the holes in the tabs to the ends, and that varied by about 0.8mm. That was enough to cause the same huge variation in chain tension you are seeing – and have the chain fall off regularly.

Replaced with Deore LX and it's now completely fine.

As above, with a decent chainline, and proper SS rings and sprockets, you can run the chain fairly slack on a ss*. If tweaking the ring 😯 doesn't work, just loosen the whole thing enough to get rid of the tightness.

*on a fixie, you need the tension more even, or there's sometimes a 'delay' when resisting pedalling.

the only single speed i've ever owned that doesn't have a slight variation in chain tension has Dura-Ace track bits, most cranks / rings aren't 100% centred and it only takes a tiny inaccuracy to vary chain tension. square tapers also sometimes don't sit totally in line once they've been ridden a bit and tightened back on a couple of times. an EBB may be off-axis fractionally but i duobt i'd be by much.

Andy r's tip is best fix – loosening the chainring bolts and seeing if it settles – if not don't stress it's never been an issue in my exp.

Another thought: might be worth rotating the chainring 90 degrees. if the cranks are slightly out and the chinring is also slightly out they could add together to cause a problem.

Hi,

Thanks for all your replies. I'm pretty certain the frame isn't the cause, as it's been said, a bent frame would hold the chainring at a single angle and not cause tight spots. I checked the frame welds for cracks after the crash but I hadn't considered I might have bent the frame. I might get the LBS to check it’s in line next time it's in the shop.

I can't put the old crank on as it completely snapped off one of the tabs and bent another one. I've had a cursory look to see if the chainring is parallel to the frame but I will check it out properly.

I've also tried flipping the chainring over and even another chainring, but the tight spot remains in the same place, so I’m pretty sure the rings are straight.

Top of my things to try is the loosen-off-bolts-and-tap-with-soft-hammer plan. I don't think there's that much room to move it on the spider, but I'm willing to give it a go and be proven wrong. It won’t be the first time. :o)

I had considered just running the system a bit slack but I'm covinced the chain will come off. I don't really want to run it with a tight spot as I’m convinced it will wear all the components out quicker, particulaly the freehub and bottom bracket bearings. I’d rather run it with a tensioner.

They're bl00dy silly these singlespeeds aren't they! Still, I can't stop riding it though. :o)

VC