square taper BB questions

[KeepItSteel]

Hi guys,

im planning on going back to a square taper b/b for the first time since in years having turned my back on them when ISIS came along.

I never got on with square taper, putting the rounding of numerous chainsets down to the sheer power in my thighs ( ) when in reality I was obviously fitting them badly.
I just got plain sick of loose chainsets falling off miles from home and never looked back when switching to ISIS.
So I guess i'd like to know:

Whats the best way to tighten up a square taper chainset?
How much grease do I apply to the spindle (if any)?
how do I avoid overtightening?
what else should I consider?
is it just an inherent poor design, with no way of making sure I dont work the chainset loose?

I love the way ISIS drive is tightened up to a 'lip' on the spindle, preventing overtightening... ISIS also feels much stiffer to me, am I going to notice much more flex?

Im using an XT M739 chainset... I had'nt considered b/b availability. Can I use a modern b/b with no problems? Which are the best to go for?

Finally, does anyone know the best way to tell how long an axle to use?
Im fitting on my Ballistic frame which has a 68mm shell, but I have no previous b/b to go by for measurements. Im presumming 113mm, but this is a guestimate.
Sorry for all the Q's!

 

[sancho]

Although there seems to be some debate about whether or not you should grease the spindle for a square taper BB, I would go without.

The concern about using grease is that it will allow the crank arm to tighten too far on the spindle, which could ultimately lead to the crank bolt bottoming out against the spindle (leaving you with wobbly crank arms).

Unless it's a titanium spindle, it shouldn't creak if it is torqued to spec even without any grease.

As for BB width, this is always a great starting point:

http://www.sheldonbrown.com/bbsize.html

as is this one:

http://www.bikepro.com/products/bottom_ ... able.shtml

 

[Harryburgundy]

Funny, I don't like anything other than square taper and have never rounded a taper or had a crank arm fall.
Finding the right axle can be the only pain in the butt.
Anyway, as above...no grease. Super clean the axle and crank tapers. Nice and tight, job done

 

[REKIBorter]

I use just the smallest smear of copper grease on each taper, then tighten it till your knuckles are white and the viens are showing on your forehead - works out at about 45NM or bloody tight.

 

[perry]

Can of worms , can of worms

The tapers need to be clean . I use a dab of loctite on each side of the tapers , cranks on , dab of loctite on the bolt threads and under the head , tighten , preferably using a torque wrench . they should never move , if your having to retighten at all they wern't tight enough in the first place .

Not a fan of the completely clean method , fine in workshop conditions , but out on the trail dirt and moisture can cause a creak as well as corrosion . I used to show the axle grease , ie only enough to give it a rust proof coating using an oily rag but stopped and changed purely due to others having problems . Loctite is the way to go , stops the axle corroding , stops any chance of a creak and will hold the arm in place if need be .

I think the biggest problem isn't what you do with the axle/arm interface as clearly people have good and bad results with all methods , it's the eagerness to retighten . Bolt comes loose so it gets tightened more , resulting in pushing the arm further up the taper . Tighten them enough in the first place and loctite the bolt and itl be good whatever your fitting preference is .

 

[FluffyChicken]

The main problem with Square taper is over tightening, the more you tightened, the more is splays apart the crank arm and causing them stress and leads to cracking coming from the edges of the square prematurely. They actually do not need to be on Extremely tight.

They are not suppose to fit flush against the end

I use clean tapers my self.
Prefer the Allen bolt ins and I use the long one that came with my M900's BITD. Do them up pretty tight and they stay there for ages.

M739 will need 107 to 113mm length (though double check this)
My rule of thumb is
Front mech actual clamp size (i.e before any shims)
28.6 then 107mm
31.8 then 110mm
34.9 then 113mm

 

[KeepItSteel]

Can of worms indeed!
Many thanks for your answers so far... now I know that I need to use no grease, but use grease, and tighten really tight, but not too tight, by hand, but using a torque wrench

Seriously though, its good having you offering your experience as im not confident when it comes to square taper... i just shudder at the thought of creaking arms falling off...
the loctite method sounds intreaging, does it not make the crank more difficult to remove resulting in possibly stripping the thread?

Is there somewhere that I can find out how much to tighten up an M739 crank using a torque wrench?

My rule of thumb is
Front mech actual clamp size (i.e before any shims)
28.6 then 107mm
31.8 then 110mm
34.9 then 113mm

this is interesting, my frame is 28.6mm, im currently running a 113mm b/b on my RS8's and I was thinking that the crank arms looked a couple mm too far out on each side. I take it there is no sure fire way of telling the correct length? What am I aiming to achieve? Is there a way to spot if you have the correct length fitted or not?

Cheers!

 

[FluffyChicken]

Those length are specific to the Super Low Profile Shimano cranks, started by the M900.
Shimano run a 47.5 to 50 mm chainline, Shimano modern stuff uses 50mm, but then all their modern front mechs are also 34.9 (just shimmed down).

The reason for the step in size is to allow the front mech to swing properly from it's position over the chainrings.
How this works in practice will probably be minimal, i.e just some annoyance in shifting.


No idea of RS8, Middleburn will (and do ) have their recommendation on their site ...

Having the right BB length is crucial to having gears that work as well as intended.

Middleburn cranks run a 50mm chain line.

For a standard MTB setup a 113mm axle is needed for ISIS or square taper.

However, there are far too many variables to always be able to predict what bottom bracket you need. BB-mount ("E-type") front derailleur and chain devices can throw a stick in the works, as can a lot of oddly designed frames.

If in doubt have your local bike shop fit your cranks.
Copyright ©2009 Middleburn Components, All Rights Reserved.

They obviously use pretty low profile cranks, or more likely designed around the standard ISIS length to keep it simple. (113 for MTB)

 

[KeepItSteel]

thanks, however im still not 100% clear which length to go for...

judging by your advice and the sheldon website I should probably go for a 107mm?

even if I went for trial and error, how do I tell if ive got it right? is there a measurement I can take, i'm thinking probably the chainline?

 

[perry]

taken directly from Barnetts bicycle manual - ( taken from a pdf so sorry for the spacing )

Symptoms indicating loose crank arms -
One of the most persistent problems with crank
arms is that they work loose. This can strand the rider,
and it can easily destroy the arm that works loose -
an expensive concern. Creaking noises from the crank
area are a warning sign that the arms may be loosen-
ing, but loose pedal parts and loose chainring bolts
can cause similar noises, so check all these areas at the
same time. When the arm makes a knocking sound or
feels loose while pedaling, the situation is critical. If it
is not too late, it may be possible to save the arm by
not pedaling on it until it can be secured. Pedaling
lightly, for even one block, may destroy the arm.
With proper installation, most riders do not need
to periodically tighten the crank arms. Other books
and magazine articles often state "re-tighten crank arms
every 100 miles," or something similar. The crank arm
would be the most under-designed part on the bicycle
if this was the case, and bike shops would be selling as
many replacement crank arms as they do inner tubes.
In fact, it is possible to damage crank arms from rou-
tine tightening without a torque wrench, which is why
it is not recommended. With a torque wrench, crank
arms can be regularly checked without risk. If they
are remaining tight (as they should), then the torque
wrench will show this without adding any tightness.
If they have worked loose, the torque wrench will
tighten them back to the original torque. If they work
loose more than once, consider a higher torque. This
"torque checking" (as opposed to periodic re-tightening)
is an excellent form of preventive maintenance.


prepare the arm for installation by
cleaning the mating surfaces of the spindle and arm
with acetone or alcohol. The purpose of this is to re-
move any traces of lubricant. Since these two pieces
are held together by friction, grease or oil may enable
the arm to go on further (not necessarily a good idea).
Further is not more secure if arrived at by using lubri-
cation. Crank manufacturers are unanimous in recom-
mending against lubrication of the spindle when
mounting the arm. Arguments to the contrary have
been voiced, but never lubricate the spindle flats! If there
is a concern about preventing corrosion or about con-
taminants getting in the gaps between the spindle flats
and the hole flats in the arm, then treat the mating
surfaces with Loctite 222 or 242 (bearing-supply or
automotive-supply stores). The Loctite will seal the
surfaces from moisture or dirt, reduce creaking problems
, will not cause the arms to be less secure and will allow
arm removal with normal effort .

He recommends 390in - lbs / 32.5ft - lbs if the manufacturers settings aren't available