Sugino RD Messengers, not 100% round?

I've been having a bit of grief with chain tension, I have come to the conclusion my Sugino cranks do not seem to be completely round. I'm running RD messengers with a TA 50t chainring, I have used both a shimano and the Sugino 103mm (actually RPM 7420 if you peel the sticker off) bottom brackets. I did have a loose crank bolt which lead to creaking at one point but was tightened up again pretty soon. Could I have warped the square taper in the crank? I am a heavy fecker (down to about 19stone now ;) ) I've also slid sideways down the road more than once so could have possibly bent the BB (although unlikely as both BBs have wandering chain tension):

could I have warped the square taper

Am I just expecting too much from a pair of cheap cranks?

Surely the chainring aint perfectly round either... not truly. There was a thread on here a while back about this, a lot of people swore by trying rotating the chainring on the cranks by one one bolt, trying the tension, and then try them all if necessary. One of the positions will be the sweetest for tension.

Definitely try what Skullhead says.

It worked for me. Perfect smoothness and a silent drivetrain is worth putting some effort into.

I did try moving the chainring round but it didn't make much difference tbh.

how bad is it?

is the ring new?

sometimes they take a little while to bed in, I've not seen many new set ups that have perfect tension all the way round anyway to be honest. the BB, the BB threads, the cranks, the ring, even the frame, none of these things are always perfect..

the usual advice is to set it so it isn't binding at the tightest point and RIDE!

Its not THAT bad, it varies from good tension to beginning to bind, so setting it up to be reasonable tension leaves it a little slack in the worst position. the ring is new but all rings have done this, I did have the ring on the inside which improved chainline but going from 44t to 50t mean it then caught on the chainstay in one position. moving it round on the BB helped a little, moving the ring on the crank didn't do much.

It is OK, I guess I'm aiming for perfection :)

TA chainrings aren't the most round to begin with. Sugino RD are a 'budget' crank so don't expect those tolerances to be very exact either.

Meh, not what I was hoping to hear, but probably what I expected.

So a 'good' set of cranks that will run nice and true would be???

sugino 75s
Dura-ace

what else should I be looking at?

I recently bought some Condor-branded Miche ones for £100. They come in black and silver. I can't find any tight spots. They also take 144mm BCD chainrings which is the track standard, unlike the other Miche cranks which I believe are 130mm. I think 130mm BCD is Campagnolo and would limit the choice of replacement rings.

I really like the Sugino 75's but couldn't justify it for a bike that's going to be left locked up.

I did try moving the chainring round but it didn't make much difference tbh.

;142286']TA chainrings aren't the most round to begin with. Sugino RD are a 'budget' crank so don't expect those tolerances to be very exact either.

Here we go, this is as I understand it. This may help some folks in future so I'll write a pretty full description...

Changing the rotational position of the chainring to correct the tension problem relies on both the chainring and cranks having some degree of machining inaccuracy. If one of the two is pretty near perfect moving the chainring round will have little effect.

If the two have some innaccuracy, by moving them round you are trying to minimise the compound machining error. Say the chainring BCD (bolt circle diameter) was machined 0.5mm off centre from the teeth and the crank spider BCD was machined 0.3mm off centre from the square taper, at the worst position the teeth could be running 1.6mm out of round:

INDENT = 0.8mm off centre, but the problem is doubled because the error moves from side to side as the crank rotates; 0.8mm is the radius of a circle about the centre of the BB spindle described by the centre of the chainring teeth.

Total movement side to side = 1.6mm
At its minimum, in this case the error could be reduced to (0.5 - 0.3) x 2 = 0.4mm
[/INDENT]

Because of machining tolerances, there may be some play between the chainring and spider where the diameters of the seating surfaces are different. If this difference in dia. is 0.2 mm the error could be reduced to 0.2mm overall using the method described on Sheldon Browns website (loosening ring and tapping it over to counteract the error).

However, there is another potential machining innacuracy - the chainring teeth are supposed to be mechined in a circle, this may not be the case. If it is perfectly oval ( like a perfect biopace chainring) it should still be theoretically possible to almost get a tight chain, but this depend how much of the diameter of the chainring is wrapped by the chain; at exactly 180 degrees wrap it is possible, as a greater angle is wrapped around (larger rings and smaller sprockets) the ovality will cause a more pronounced change in tension as the cranks are rotated.

If the chainring is not perfectly oval, but is only ovalised in one area (because it has been hit at one point against a log, low wall, or by a hammer), tensioning will be a problem again.

If this is not so, someone please correct me.

Dura-Ace FTW.. :) (ok, i need to justify using ~190£ on 7600-cranks with Dura-Ace chainring for my Vivalo;)

I recently bought some Condor-branded Miche ones for £100. They come in black and silver. I can't find any tight spots. They also take 144mm BCD chainrings which is the track standard, unlike the other Miche cranks which I believe are 130mm. I think 130mm BCD is Campagnolo and would limit the choice of replacement rings.

I really like the Sugino 75's but couldn't justify it for a bike that's going to be left locked up.

miche old 135 bcd
miche new 144
sugino messenger 130 bcd

shimano road 130bcd
campag road 135bcd

just for the record

@ ChrisNW - good stuff.

DA chainrings are built like nothing else (apart from the other good ones). seriously solid pieces of kit.

Basically the better your cranks, chainring and sprocket the more even your chain tension will be. I have Campag Record (new ones) running to an EAI cog on one of my bikes and it's pretty perfect. But the drivetrain also costs an absolute shitload. To be honest I'd try and minimise the uneveness and then stop worrying about it. As long as it isn't varying wildly then you'll be fine.

It's true. I have an early 80's (7400? 7420?) dura ace groupset including rear cog which runs as tight as can be.

I've also run low-end campy xenon road cranks, TA chainring with formula hub and no name cog that was absolutely pearshaped! I noticed when spinning the wheel, the forumula hub wasn't even round. Given all those variables, I'm sure I could have sat there for days and come up with the winning combination for an even chain tension but really couldn't be bothered.
Rode it with no ill-effects...