corwin1968

I was out riding in the subdivision today and my left crankarm (Hollowtech II) fell off mid-ride. Luckily, I was just cruising and came nowhere near crashing.

The pre-load nut was less than finger tight.

The bolts still seemed to be in place.

The plastic safety tab was still in place.


There were a couple of construction guys and they offered me an allen wrench so I undid the bolts, put everything back on and headed toward home. After about 20 pedal rotations, the crankarm was working it's way off again. I simply pedaled about 10 times, used my left foot do drive the crankarm back on, pedaled about 10 times, etc.

When I got home, the pre-load nut was gone (I had a good idea where it probably was and headed back in my car and found it right away), the two bolts were still pretty tight and the safety tab was broken.

Any idea what happened here? It's possible that last time I installed it, I tightened the bolts and then tightened the pre-load nut. Would this cause what I experienced? I'm sure this is what I did during my field repair.

I just watched a video as a refresher and re-installed it correctly and it seems to fit fine, nothing seems stripped or broken (except the plastic tab).

DrIsotope

If you tightened the two bolts on the arm before the preload bolt, or didn't make sure to evenly torque the arm bolts, the non-drive arm can work itself off. I neglected to check the torque on the arm bolts, allowing the spindle to slide out to the drive side about 1/4", and completely eliminating my ability to operate the front derailleur. So on the non-drive arm, tighten the pre-load cap down to spec, then alternate tightening the arm bolts until they're to spec. Be careful not to go too tight on the preload cap-- I did, and introduced a nice (permanent) click into my cheap, factory-supplied BB. I consider both events to be valuable lessons learned.

corwin1968

I suspect that this is what I did but I couldn't convince myself that it would result in the crankarm coming off.

FastJake

The preload bolt needs to be tightened first, then the two bolts need to be tightened evenly. I like to do a quarter turn on one and then back to the other. Sometimes less than that as they get tight.

Hollowtech II is an elegantly simple design so I think you must've done something really bad to have the left arm fall off. Like having the bolts WAY too loose or tightening one completely before the other. Keep in mind, if this is anything like a square taper crank, the left arm might be ruined now and you might need a new one.

hueyhoolihan

if the driveside crank is not pushed in far enough, the outside non-drive side crankarm pinch bolt may not engage the spindle when tightened. that could lead to the non-driveside crank arm falling off. the preload bolt wouldn't, IME, be a factor. i have on occasion dispensed with them altogether.

corwin1968

Well, my Bottom Bracket is a gooey, crusty mess so this will be a good opportunity to buy a new one and brush up on how to install everything correctly. I know for certain I alternated bolts when attaching the crankarm but I'm not sure where in the sequence I installed the preload bolt and it's possible the driveside crank wasn't in far enough. This will be a good learning experience and I now have a small set of tools on my bike!

hueyhoolihan

sometimes i think everything i think i know is a result of some FUBAR or another.

FastJake

I'll admit, I didn't learn the proper torque for a square taper crank until I ruined one... 30 ft lbs is a lot more than it feels like until you actually do it.

Andrew R Stewart 

Just like a tapered square classic retainer nut being loose and causing the arm's hole to get molded so too can a pinch bolted arm get ruined by riding it loose. As long as the splines were somewhat engaged and the pinch bolts were near equally tight enough (two big possibilities of mistake) the preload bolt can be removed and never used when riding (just like a threadless headset cap). Once the arm starts to move around (and it doesn't take much movement. One's legs don't have the tactile senses to pick up on this slight movement) the AL arm gets augered by the steel spindle. The damage is done.

I gently tap the crank arms with the wood handle of a hammer to seat them then install the preload bolt. The two pinch bolts get tightened next and with the last bit of torque I'll go back and forth. This is one of the few times I use a torque wrench. Andy.

Camilo

Shimano publishes installation instructions for all their stuff online. When in doubt......

corwin1968

Would this be apparent on visual inspection?

I've learned a lot from screwing up! After breaking two bolts in the same Ergo grip and then breaking one in a stem (my first forays into bicycle maintenance), I bought a 5 NM torque wrench for everything on and around my handlebars.

HillRider

It would be apparent after the damage was done. The HTII non-drive side arm has multiple interior grooves inside that mate with splines on the steel spindle and are keyed so the arm can only be (properly) installed in one orientation. If the arm was installed improperly and moved around, the interior grooves will be noticeably distorted.

Andrew R Stewart 

If the damage was enough, yes it could be seen by eye. Holding a good arm next to a damaged one might make it easier to spot damage.

Having a sensitivity for torque is but one aspect of a mechanic that's needed. I have worked with a few budding wrenches that didn't have this skill/touch (and was LONG before the wide spread awareness of using a torque wrench). One had one torque level. Little bolts like cable anchor bolts (the classic type with the cable passing through a small hole in them) were stripped/broken regularly. Large fittings like axle nuts or headset top nuts never were tight enough. Trying to explain to him the correct ways was hampered by his lack of English language (his Laotian younger brother had to translate, when he could be around). Another short term coworker was a fruititarian (as he called his self). He never got the large fittings tight enough. We all thought it wasn't from understanding or trying but from a lack of strength. He ended up leaving after a week or so because he couldn't tolerate the lube and solvents we used. (Which is another aspect of a long term wrench, the acceptance og the conditions they work with). Andy.

corwin1968

The splines look normal but I don't know if they should extend the full depth of the crankarm or if there should be several mm's of smooth metal. Mine has the smooth area (I believe on the pedal side) and it's shiny but I can't tell if there were previously splines there that were sheared off.

I'm planning to just get the parts I need, install it and then watch it very carefully to see if becomes loose.

Andrew, I feel like I'm pretty sensitive to torque levels but the problem is that I don't have a reference point. I assume that as a fastener reaches the correct level of tension, the "feel" becomes very specific but I don't know what that "feel" is. I've been tightening firmly and stopping at the point where I feel I would be working hard to tighten it more. I have big and strong hands so I can apply some torque but I'm careful not to over torque but I don't know if I generally succeed.

well biked

Never install a Hollowtech II crankset without using a torque wrench on the two pinch bolts, and as has been noted earlier, tighten one and then the other as you increase the torque so that you bring both sides up evenly. I'm not one to use a torque wrench in too many places on most bikes, but I would never install a Hollowtech II crankset without a torque wrench. 12-14 Nm for those two bolts. It's critical, and it's important, as you have found.

andr0id

When it comes to torque, your feelings and beliefs don't matter, only what the torque wrench says.
The Navy taught me that in aviation mechanic school.

Pro mechanics tend to over torque things, amateurs tend to under torque. That's why we always used torque wrenches on airplanes.

Some stuff on bicycles can be done by feel, for example, a brake bolt or pedals. But on new bikes, stems, cassette lock rings and HTII really need to be torqued to spec.

I do HTII in a very specific way.

Tighten preload cap as tight as I can by hand then back it off, tighten again about 1/8 turn.
Set torque wrench to about 8Nm and hit both bolts until the click.
Move up by 1Nm and alternate bolts back and forth until I get to 13Nm.

I've never lost a cap, had a crank come loose or had a creaking BB doing it this way.

fietsbob

Do a safety check before every Ride?

corwin1968

I will now!

The funny (or maybe not) part of this is that as I was getting on the bike I noticed a gap between the back of the left crankarm and the bottom bracket, where the spindle was showing. I had about a second of concern and then the 17 years conditioning from when I rode square taper cranks kicked in and I decided that the gap is supposed to be there. I'm really surprised the crankarm stayed on as long as it did.

I've thought about buying a torque wrench and I would prefer the kind you set an amount of force on but I'm intimidated by what I've read about having to have them periodically recalibrated. The beam type just looks like a PITA to use, so I've refrained from buying one of those.

melloveloyellow

On my HTII leftside crankarm, there is a "plate" which must be pressed inward against the spindle to engage a dimple in the spindle. This secures the left arm from working off (along w/the pinch bolts, of course). I once neglected to do this, and the arm began working it's way off. It's part #5 in this diagram:

Camilo

Torque wrench + Instructions = simple ways to avoid problems.

Any item with more than one fastener should be tightened in small steps and in a pattern, whatever pattern makes sense for the number and configuration of the fasteners. Back and forth for two inline, inside/outside for three, X for four, star for five, etc. Basic principle of tightening.

The $25 beam is not a PITA, just requires that you actually look at it. But it's dead, fool-proof simple to use (nearly so anyway) and time proven. If you're not willing to pay a LOT more than that for a decent quality click type and keep it calibrated, go with the easy to use, reliable beam type. It seems that if I had a crank arm fall off, the PITA would not be a consideration. Seems a silly choice to me. Get it at Sears or some discount tool supply. No need for a bicycle specific one if they're over priced.

I would recommend supplementing your set of two $25 beam wrenches (one small, one medium) with one of the "torque key" type of devices. They are, cheap, very handy and useful for a whole bunch of little fasteners on a bike. I hardly ever use my small beam wrench any more.

And again, instructions are so easy to find online, I can't imagine why anyone would just guess at something that has multiple fasteners and/or steps if they'd never done it before. Yea, this stuff isn't rocket science, but it's so easy to check how to do it, why wouldn't you (again, if it's something you've not done before).

davidad

The last resort when everything else doesn't work.

corwin1968

I want to say thanks to all who pushed me to do what I knew I needed to do quite some time ago.

I went to Sears and picked up a 3/8" beam torque wrench and a set of allen key adapters. Total cost for all of it was about $30. I just installed my old Hollowtech II BB and crankset, using the torque wrench for the BB cups and the pinch bolts. It was a very satisfying experience. Both knowing the torque amount was correct and the incredible leverage the wrench gave me actually made it fun. I had been intimidated by the beam torque wrench but it's actually very easy to use. I'm a happy camper.

I'm still not sure on the pre-load cap. I can't get it tight enough with my fingers to make any difference in how the cranks spin but I'm erring on the side of making it a little too loose. I figure if the cranks aren't rattling, the pinch bolts will keep everything in place.

Camilo

I wonder what the range on your 3/8" torque wrench is? I needed to get two - a 3/8" for the heavier stuff, but the one that's "light" enough for the small stuff is 1/4" drive, much smaller. A general rule of thumb is don't use a torque wrench at the very low end of it's range, tougher to be accurate.

As for the pre-load cap: if you're using the Shimano tool

I believe it's designed so that hand tight with it is in the range of correct torque. My experience with cartridge bearings like in the Hollowtech II external cups (and other similar bearings I've worked with) is that I've never felt much difference in the resistance of the bearing when I tighten the pre-load. It's not like the old cup and cone bearings where you feel a huge difference with just little adjustments. First time I installed a Hollowtech II crank, I didn't have the little tool and just used the jaws of a tiny needle-nose pliers I had to spread against the notches, and the crank worked fine forever.

Just use that Shimano tool to tighten the cap and you'll be fine.

corwin1968

I downloaded Park Tools torque chart wrote down the ft/lb ranges of all of the parts I plan to ever mess with. My wrench goes from 0 to 75 Ft/lbs (or 0 to 95 NM..I'm using Ft/lbs simply because it's easier to get an accurate reading on my wrench on that scale). I felt comfortable doing an 8-10 Ft/lb range for the pinch bolts on my crankarm and I tested it by undoing it with a typical "L" shaped allen wrench and it was pretty hard to loosen with that small tool (what I was using to tighten them). I've got a 5 NM torque key for really small stuff.

grall1126

I had this happen to me today. I was out riding and luckily I didn't crash and the crank was on the roadside. I lost the pre-load cap, luckily I have an extra one. I have lost three of these caps. I do have a torque wrench and I need to figure this out before I go bonkers.

Is there anything you can use to have a secondary line of defense on this falling off again?

1. I cleaned the crank grooves and bottom bracket of all built up grease.
2. Next, Place the crank in the bottom bracket and tap lightly with a piece of wood to make sure the crank is in the right placement.
3. Place the pre-load fixing arm cap on the crank and tighten.
4. Tighten the crank bolts with a torque wrench in small increments. 12-14 N/MS
5. By the way, best spot to buy the plastic caps and do I have to buy only the fc6700 caps? Any others work with it?

Am I missing anything?
Thanks