small problem with crank

[cyclightning]

I have a Shimano 105 groupset on a TT bike and recently noticed a small problem. Pedaling up a steep hill I lost power transmission through the crankshaft.. the chain was just sitting still while the crank turned normally. It is a strange problem since it only happens very rarely, but when
it happens it was only on a steep hill > 10% gradient. What is wrong here ?

 

[dabac]

Well...
The chainring can't slip WRT the crank arms, so probably you missed a shift a little, which made the chain ride on the top of the teeth for a moment before settling.

 

[cyclightning]

Hi.. yes, except that I didn't shift upon entering the hill. I definitely had power transfer and then lost it on the hill without shifting. That's what I thought too that the crank and chain-ring are physically connected. It seems the chain was lifted and slipped on the teeth under high load - maybe my chain-ring teeth are worn ? This bike has seen about 3 years of use at about 4000 km a year. I have another bike that has many more kilometers on it (30,000+ on old campy groupset, but with a new chain) but with no slipping on steep hills. It's a bit noisy but it still works.

 

[oldbobcat]

Take the bike to a shop and have them adjust the front derailleur and shift cable.

Regarding indexed shifting systems, practically any monkey that can follow directions can get the rear working right, but getting the front to work really well requires finesse.

 

[alfeng]

Originally Posted by cyclightning .

I have a Shimano 105 groupset on a TT bike and recently noticed a small problem. Pedaling up a steep hill I lost power transmission through the crankshaft.. the chain was just sitting still while the crank turned normally. It is a strange problem since it only happens very rarely, but when
it happens it was only on a steep hill > 10% gradient. What is wrong here ?

Where you actually looking DOWN at the chain & not seeing it move OR are you presuming that it wasn't moving because you "lost power transmission through the crankshaft" on that steep hill?

 

[alfeng]

Originally Posted by oldbobcat .

Take the bike to a shop and have them adjust the front derailleur and shift cable.

Regarding indexed shifting systems, practically any monkey that can follow directions can get the rear working right, but getting the front to work really well requires finesse.

Bar-end shifters & Campagnolo shifters should not require "finesse" to adjust the front derailleur ...

• heck, I guess it is partly BECAUSE Campagnolo shifters do NOT require "finesse" to adjust that 'I' prefer them!!!

 

[cyclightning]

Yes, I looked right down at it.. this seems quite natural, no ? I was pedaling and the chain just stopped moving completely, in addition I could have sworn the chain-ring wasn't moving either - just going up a short steep hill coming up from a bicycle path underpass under a highway. The bike somewhat slowly came to a stop with no power transfer. If I recall correctly, the issue resolved itself. I just walked to the top of the hill, got back on the bike, and it didn't slip over the chain-ring after that. That's what confuses me. This happened just twice, and very briefly ( a few seconds), but otherwise it seems normal. Hammering up hills, going fast on the flat, or pedaling very quickly down hills after this there doesn't seem to be anything wrong. It hasn't done it for a while.

 

[cyclightning]

Well, I've been riding bikes for over 10 years and have other 40,000 km on this bike and another road bike. I certainly can switch gears quite well, i.e, with finesse.

 

[alfeng]

Originally Posted by cyclightning .

Yes, I looked right down at it.. this seems quite natural, no ? I was pedaling and the chain just stopped moving completely, in addition I could have sworn the chain-ring wasn't moving either - just going up a short steep hill coming up from a bicycle path underpass under a highway. The bike somewhat slowly came to a stop with no power transfer. If I recall correctly, the issue resolved itself. I just walked to the top of the hill, got back on the bike, and it didn't slip over the chain-ring after that. That's what confuses me. This happened just twice, and very briefly ( a few seconds), but otherwise it seems normal. Hammering up hills, going fast on the flat, or pedaling very quickly down hills after this there doesn't seem to be anything wrong. It hasn't done it for a while.

For clarity ...

So, are you saying that the chain was nested on the teeth of the (inner?) chainring but that you did not see it-or-the-chainring moving rather than that the chain was skating atop the teeth as suggested by dabac & oldbobcat?

Presuming that you weren't hallucinating, could you indicate what type of crankset & BB your 105 equipped TT bike has?

 

[cyclightning]

Outer chain-ring. The chain wasn't moving at all, it was still (relative to the bike frame, so to speak). Definitely remember that but it was a while ago. I don't have the other technical details. I don't use the inner chain-ring at all on the TT bike, no long or really steep hills where I am. In any case, the issue occurs so rarely I suppose it's not worth mentioning. It just really surprised me. I've been riding all kinds of bikes and this was a unique experience

 

[sitzmark]

Unless you have a splined ring spider that pops free from the crank arms under heavy load - something like a Quarq spider - there is really no physical explanation for what has been described. Virtually impossible that a decoupled Quark spider would "self-repair".

The only other explanation is that the chain was skipping on top of the ring teeth. The amount of torque and RPM (i.e. cadence) needed to keep the chain elevated and sliding over all the teeth tips without any forward motion of the chain would be exceptional.

More of a possible explanation (though still improbable) would be if the hub pawls failed to seat under extreme torque and allowed the driveline to freewheel under load. However, the chain would definitely be spinning (for as long and fast as you continued to pedal). This kind of failure mode is not a hit or miss, self-repairing thing - it's usually a permanent failure.

 

[cyclightning]

Sitzmark - sounds like you know what you are talking about. I don't know what a Quarq spider is - my groupset/crank is just an entry level
Shimano 105, don't have more technical details. Now I remember what happened, I was pedaling with the chain moving but no power transfer - my mistake. Definitely had no power transfer to the rear cog. The cranks just seemed to be spinning with zero power transfer. The issue did resolve itself, and only occurred on a short, steep hill. However, I completed a time trial with a couple short, steep hills and didn't have this issue. I did notice my rear wheel
quick release was open and I think I may have ridden the bike in training with the quick release open on the rear wheel when this happened.

 

[dabac]

Sometimes excessive amounts of grease can cause a freehub/freewheel not to engage intermittently if it's a cold ride. Not that it sounds lika good explanation for the OP though.

 

[alfeng]

Quote: Originally Posted by dabac .

Sometimes excessive amounts of grease can cause a freehub/freewheel not to engage intermittently if it's a cold ride. Not that it sounds lika good explanation for the OP though.

Similarly, too heavy a grease will have a negative impact on the ability of the pawls to move freely within a Freehub/Freewheel mechanism.