slowandsteady

I assume I need a new Bottom bracket. Correct or no? It is similar to this one. It is ISIS.

It is from 2006.

dabac

The axle is steel, the crank arms are (usually) considerably softer aluminium. It's just about always the crank arms that gets damaged. Sometimes you can get lucky and find a left crank only to use as a replacement, which usually sorts it out. Otherwise what you need is a crankset. The actual BB will usually be just fine - unless there's something else going on there.

And as I understand it GXP means that the crank axle is permanently attached to the right side crank, while ISIS is basically a development of the good ol' square taper. Crank arms and BB(axle) as separate parts. ISIS just means that the square taper is replaced with a splined interface.

slowandsteady

Yes it is a splined interface. The attachment of the crank arm is with a bolt like thingie(see below) that is threaded and attaches to the BB with a simple allen wrench. Because it is splined(similar to Octalink but with 10 splines) and not a square taper(that can get rounded over time) I don't understand why the crank arm itself would be causing it become loose.

I would assume it was the bolt or the threaded part of the BB that is to blame.

HillRider

As dabac noted, the bottom bracket spindle is hardened steel and the crank arm is aluminum so the damage is always to the crank arm if it gets loose.

Those bolts are to blame if you didn't torque them adequately when installing the crank. The recommended torque is very high at 300-400 inch-pounds and MUCH higher than you could possibly apply with a standard 8 mm hex key. If the bolts aren't tight enough, they will back out and the crank arm then loosens and is damaged by the spindle.

slowandsteady

I wasn't the one to install the cranks originally. About an hour ago I put on some locktite as a last ditch effort. I attached a small box wrench to the allen wrench to allow me to really crank it down with additional leverage. I will see how it holds for my 3 hour ride later today.

I guess I still don't understand how the crank arm(splined not square taper) could be damaged to the point that the bolt backs out on its own. I can understand how that can happen with a square taper...since the fact that it is tapered is what accounts for the tight hold. But if the splines(I assume) aren't tapered, then how does it loosen itself?

HillRider

The sequence is the opposite of what you describe. The bolt backs out first due to inadequate tension and THEN the arm loosens and is damaged.

AEO

actually, for GXP, the interface is splined AND tapered.
What's stupid about the GXP design is that the arm is not designed to go in too far and is supposed to perch itself onto the axle with about 0.2mm to spare before being tightened down to pick up the slack. Sure, the design doesn't load the bearings up axially, but if it's not tightened down properly, it can loosen and squish the interface enough to cause problems.

Shimano's design is far superior when compared to GXP, as the fastening and preload adjustment are two different parts. With shimano, if you're not careful, you can strip the threading on the left side arm, but that alone won't cause the arm to loosen and rock.

slowandsteady

Correct, however, then once the bolt is adequately tightened later it still decides to back out on its own.

Sequence: Bolt too loose. Arm rocks around and is damaged. Loose bolt is discovered and retightened. Bolt loosens on its own. I just didn't realize that the splined crank arms were also tapered which explains why the issue is with the crank arm. I had assumed that only square tapered cranks are tapered.

Why don't they just design the darn things with multiple bolts like a car wheel or with some sort of lock washer?

Oh well, I will see if the loctite can bide me some time till I get a new crank arm.

slowandsteady

I know...the octalink is better. The truvativ was stock on this bike. They(truvativ and others) went to the ISIS design to get around the shimano patent.

davidad

There was never anything wrong with the old square taper BB spindle. Everything that followed is just new and improved. If shimano's octalink was that good they wouldn't have replaced it as soon as they did.

AEO

I think the main problem with ISIS and octalink was that the axle diameter caused the bearings to shrink, which caused them to wear out quicker.

The new shimano external BB system is good. I like it, despite some of it's shortcomings.
For one, the expensive drive side doesn't wear out each time you go to remove it, only the cheap left side does. You don't have to worry about stripped extraction threads. On the downside, you can't adjust the chainline too much and it's quite specific about having a properly faced BB shell.

slowandsteady

No, nothing wrong, but they could have the same issues with loose crank arms. Plus you need special tools to service them. They are also much heavier. Not that I care, but weight weenies do.

surreal

couple thoughts:

The early versions of GXP suck b/c the action of pedaling will loosen the crank bolt over time. This is far less likely to happen if it's torqued down properly, but it can happen regardless. Shimano's pinch bolts counter this effect, and FSA has gone similiar pinch bolts. RaceFace sidesteps the issue by having the bolt on the drive-side arm, with the axle attached to the left arm. New GXP have a reverse-threaded lockring to keep the bolt in place after you've tightened it properly.

Square-taper died b/c this interface was the weak link in high-impact situations, as one might encounter in downhill and dirt-jumping disciplines. New technology is new technology, so when they developed stuff to deal with those styles of riding, the market clamored to offer that same stuff to all riders. Octalink and isis sucked b/c, as someone mentioned, the bearings were too small/weak, but they also sucked in almost every way. The octa/isis era was thankfully short.

These outboard bearings are already essentially dead technology. They're moving to bb30, and 2 different oversize bb shells for shimano alone, depending on if it's for a road or mtb. I don't really like this, b/c it kills the super-adaptable nature of bike frames thayt i've been enjoying for most of my lifetime. I can put hollowtech II on a 70s bike-boom sled if i like, but it'll never take a bb30. Thankfully, the shimano systems will accommodate the regular 68mm "euro" BB shell for as long as hollowtech II BB sets are available.

sorry for the rambling post. it's friday.

-rob

operator

Excuse me, which are the new shimano oversized BB standards?

slowandsteady

Loctite plus cranking down the bolt with a box wrench attached to the allen wrench and standing on it with my foot seemed to do the trick...at least for now. 20 miles and still tight. Yesterday just a 30 minute ride had it nearly falling off so this is an improvement. Let's see how long this lasts.

HillRider

I contend the bolt was never adequately tightened. Unless you used a torque wrench and know you torqued it to spec, I'd bet a fair amount the torque you applied was no where near correct.

surreal

slowandsteady, now that yor left crankarm has come loose and been ridden loose, there will be enough play that it will never be truly tight again. I went thru this on an early-gxp version of a truvativ stylo SS crank on my old dianchi mtb. Tried all sorts of things but, ultimately, the crank had to go.

Operator, the oversized BB shell standards being favored by shimano (and others) are bb86 (road) and bb92(mtb). The basic idea is to have a very wide bb shell, both laterally and in terms of circumference, so a unit the size of an entire hollowtech II bb set can be fit inside the shell. The ID is something like 37mm; enough to accomodate the bearings shimano currently uses in its outboard bearings, but *inside* of the shell. The width of the bb shell, left to right, is 86 for road (roughly the same as a 68mm bb shell plus the width of the outboard bearings) and 91.5mm for the mtb stuff (roughly equivalent to a 73mm BB shell plus bearings. Remember, the mtb hollowtech II cranksets have a longer axle, to accomodate 73mm shells, and the bbs come with spacers to effectively make a 68mm shell into a 73mm). Currently, there are both threaded and press-fit designs on the table. I like the idea of the press-fit, but this will almost surely drive production costs up. Regardless, i'm terrified about such a radical change, and i'm already thinkling about hoarding a huge pile of hollowtech II BBs so i can continue to run modern cranks in my 68mm english/euro BB bikes for the foreseeable future.

Then again, i'm weird and paranoid.

-rob

Seb71

They could design those over-sized BB Shells and bearings in such a way that you could still use Hollowtech II cranksets with them.

davidad

3 ounces?

surreal

Seb, that is what they're doing, and that's why i like the shimano standards more than the bb30 nonsense, which uses a different diameter axle. Essentially, if they go forward with bb86/92, new frames will have giant, weird BB shells, which will accommodate BBs with bearings the same size as HTII, which will in turn accommodate cranks with axles identical to the standards of current HTII cranks. So long as regular HTII BBs are available, we'll be able to run whatever BB86/92 cranks come out in the normal 68 and/or 73mm BB shells. If they keep making italian-sized HTII BBs, we can even rock it with 70mm BB shells. The new BBs will be different, but the cranks may stay the same, which i'd like b/c it'll allow me to do the things i like to do....

-rob

DannoXYZ 

I'm sure someone will come up with adapters to fit standard 68mm BBs into BB86/92 bottom-bracket shells. Similar to the ones we currently have for American 1-piece bikes.

surreal

no doubt. My concern runs the other way: fitting the new cranks on bikes with the "old" 68mm english-thread bb. And, they'll surely have ways of doing that for a while, too.

I just enjoy panic
-rob

CACycling

Very doubtfull. The new BBs are larger diameter and the cranksets are narrower than the old 68mm standard. You can overcome diameter with smaller bearings but the width issue would be tough to beat.

AEO

well, the external BB cups give an additional width to the 68, 70 or 73mm BB sizes. Aren't the shells simply larger so that the cups sit inside the BB shell, instead of being external?

The chainline never changed between external and 3 piece splined or square taper systems.

surreal

as it stands now, the bb86 frames accept bb86 BB cups, and these accept the regular hollowtech II and compat cranksets. So, if they keep the current bb86 standard the same, the interface between the bb86 cups and the crank will still be the same as the interface between hollowtech II BB and the crank. So, no worries. Even as it stands now, they have adaptors to run campy ultratorque in bb86 bikes. I guess i'll just have to grab a bunch of hollowtech II BB sets whenever it looks like they'll be gone forever (which shouldn't be anytime soon.)

AEO, your understanding of the design seems to be accurate, which is why i'm not all too worried.

-rob