Loose crank - Sugino 75

Since I finished the bike about 6/8 weeks ago I seem to be having to tighten up my crank(both sides) atleast once a week. I'm slightly concerned as I've never had to do this before on my previous bikes. I have to tighten over 180dg sometimes.

I have a Sugino 75 on a Sugino 75 bb

Any help much appreciated

erm... check to make sure there is absolutely no grease on the tapers?

Thanks I will. I never put any on so I'll find it difficult to see how any got on there.

The little people apply it when you're asleep.

Sounds a little worrying. I don't think grease or lack of it would cause the problem.

Are the cranks themselves loose or just the bolts? If it's just the bolts then it may not be anything to worry about as sometimes the bolts loosen as the cranks work onto the tapers a bit further. Have you pulled the cranks off to see if there's any damage to them? Worth doing even if it gives you an unpleasant surprise. How much torque did you put in whn you installed the cranks? Cranks need a hell of a lot so they might be undertorqued. Maybe get Brixton Cycles to have a go with a torque wrench or buy one yourself?

Thanks John but it seems to be just the bolts. I'll take them off when I get a chance, have a proper look - if not I'll get someone more clued up than me to have a look :-)

erm... check to make sure there is absolutely no grease on the tapers?

Oh !! The old 'should I grease my tapers or not controversy' !!! :)

I always grease my tapers and always use a torque wrench and have never had a cranks arm loosen even slightly and that is with around 12 bikes over many many years.

Pistanator, Jobst Brandt (Sheldon Brown) pretty much gives the complete answer.

My cranks get loose, quite quickly too; over about 10 miles or so from being solid to flopping about in the breeze. Any suggestions?

One or both of the cranks are ruined! Once ridden in the "floppy" mode, the tapered square bore of the crank has been deformed and can no longer be secured on a spindle. Install and properly tighten new cranks on the spindle after greasing the tapered square ends of the spindle. Proper tightness should be achieved with a torque wrench or by a skilled hand.

The admonition not to grease the spindle finds life mainly in the bicycle trade. When I discussed the "dry assembly" rule with crank manufacturers, I discovered that they had warranty claims from customers who split cranks. However, cranks cannot be split by overtightening them. This can be proven by attemting to do so. An M8x1 bolt is not strong enough to split a major brand crank.

Failure from "over-tightening" is caused by repeated re-tightening of properly installed cranks. In use, an aluminum crank squirms on its taper and, because the retaining bolt prevents it from moving off the taper, it elbows itself away from the bolt and up the taper ever so slightly. The resulting loss of preload, after hard riding, can be detected by how easily the bolt can be turned.

Loss of crank bolt preload is greater on left than the right cranks, because left cranks transmit torque and bending simultaneously while right cranks transmit these forces separately. The left crank transmits driving torque through the spindle to the right crank and chainwheel while the right crank drives the chainwheel directly. Besides that, the right crank transmits torque to the spindle only when standing on both pedals. Doing this with the right foot forward (goofy footed) is the only time the spindle transmits reverse torque.

Mechanics, unaware of why crank bolts lose preload (and commensurate crank tightening), have re-tightened bolts until cranks split. No warnings against re-tightening properly installed cranks are evident although it is here where the warning should be directed rather than at lubrication.

Because friction plays no role in torque transmission, preload in the press fit must be great enough to prevent elastic separation between the crank and spindle under torque and bending. This means that no gap should open between crank and spindle facets under forceful pedaling. Crank bore failure occurs when the press fit is loose enough that a gap opens between spindle and crank. Torque is transmitted by both leading and trailing half of each facet, contact pressure increasing and decreasing respectively. In the event of lift-off, the entire force bears only on the leading edge of facets and causes plastic deformation, causing the bore takes on a "pin cushion" shape (loose crank syndrome). Subsequent tightening of the retaining screw cannot correct this because neither the retaining bolt nor crank are strong enough to re-establish the square bore.

The claim that a greased spindle will enlarge the bore of a crank and ultimately reduce chainwheel clearance is also specious, because the crank cannot operate in a plastic stress level that would soon split the crank in use. However, increased engagement depth (hole enlargement) may occur without lubricant, because installation friction could ream the hole.

With or without lubricant, in use, cranks will make metal-to-metal contact with the spindle, causing fretting erosion of the steel spindle for all but the lightest riders. Lubricating the spindle for assembly assures a predictable press fit for a given torque. Without lubrication the press is unknown and galling (aluminum transfer to the steel spindle) may occur during assembly. After substantial use, spindle facets may show rouge and erosion from aluminum oxide from the crank, showing that lubricant was displaced.

Crank "dust caps" have the additional duty to retain loose crank bolts. Because crank bolts lose preload in use, they can become loose enough to subsequently unscrew and fall out if there is no cap. If this o

What Momentum said.

One important public service announcement to all:

If you ever even feel the slightest wobble in your crank arms, stop riding immediately. I have seen people total expensive cranks after twenty-odd minutes of riding.
Once you flair out that taper your lovely crank arm becomes an elegant njs paper-weight.

Too slow.
What tynan said Sheldon said.

Thanks again all. I've read that Sheldon info now so I'll have to have a proper look

Since I finished the bike about 6/8 weeks ago I seem to be having to tighten up my crank(both sides) atleast once a week. I'm slightly concerned as I've never had to do this before on my previous bikes. I have to tighten over 180dg sometimes.

I have a Sugino 75 on a Sugino 75 bb

Any help much appreciated

I had this problem and eventually resorted to lock-tighting the chain ring bolts. This solved the problem though I presume it would have been a bastard to change your chain ring if needed!

One trick I use - when fitting the cranks, use a hex bolt and a long-handled wrench to apply the appropriate amount of torque. Then remove the bolts and put in little allen-key ones with dust caps.

Managed to screw up a cheap crankset by just using the allen key bolts to fit the cranks, there was no way in hell I could get enough torque with a little key. Learnt my lesson when I read that Jobst Brandt thing Tynan posted.

I use an allen key slotted through the end of an adjustable wrench to get some torque

Thanks again but isn't there a possibility of overtightening them?

You can overtighten them if you're re-tightening them. If you're fitting them fresh (if necessary, pull them off and start over), and you're using a normal long-ish wrench (like the length in the photo), then you're unlikely to over-tighten them if you stop when it gets a bit too hard to turn. If you start using longer stuff or using your whole body weight you might.

Right, thanks. I'll take them off and start from fresh :-)

Try and get hold of a torque wrench; takes the guess work out.

on the subject of cranks.
I've just bought a new bike. the FSA carbon cranks it comes with turn out to be shit, there's a wealth of evidence on other forums to show them to be totally defective.
I can retrieve the warranty from the person I bought the bike from. What I want to know is if a warranty entitles you to a refund, or just a replacement. I don't want no more shit cranks.
grateful for any advice

most likely just a replacement, which you could ebay, for some funds.

yep, replacement. if an item is faulty the retailer has to give a refund if you want one, but if you bought the bike whole it will be the whole bike refunded not just the cranks.