crankset stiffness

[Mick F]

The "raw" graph is here. I said that the HR and Cadence was averaged to make the curves simpler.

This shows how complicated the subject is. I can - and do - pedal up to about 30mph with the 53/13 and perhaps 35mph with the 53/12. Some of those hills I went down yesterday were too fast to pedal. The last downhill is the same as the first uphill. 6mph going up but 42mph going down.

I can get this sort of data too. Ignore the temperature, I don't have a thermometer!

[tandemrider]

Mick even if everything were completely rigid, such a crude system as derallier gearing which just works by pushing the chain off one sprocket/chainring onto another while mounted on on roads that are anything but smooth, is bound to fail now and then.!!!
Also I wouldn`t say that 94 revs can be considered spinning out for an experienced cyclist, maybe over 110 rev you may have a point.

[Mick F]

Ok, "spinning out" is a bit strong. Yes, I can pedal at 120rpm but only for very short distances. I'm not happy at pedalling continuously at above 85rpm, so going downhill I'll freewheel if I have to pedal faster than that, so that's what I was referring to.

I don't see why a derailleur should fail now and then. I know it does, and I know why. I referred to a long thread on here about my tests and experiments - and if you go back further, you'll find a thread where I was describing how to increase the jockey cage spring to make the chain tighter in an effort to stop "chain wave".

IMHO, front mechs aren't designed properly. If you have a triple, the cage should closely mirror the profile of the chainset. You can adjust the limit stops of course, and you set the angle and height correct on the big ring, but as the cage goes inwards, it is generally way too high off the middle and inner rings. This allows the chain to go AWOL. If the cage followed the profile, the chain would be contained.

[Mick F]

Can we go back to the original question?

All this is really interesting, but the crankset stiffness issue is there.

[BigG]

I am not convinced by the chainset flexing argument. The deflection that we are considering is a lateral one, i.e. parallel to the bottom bracket axle - towards and away from the seat tube. There are only two forces acting in this direction, the laterally eccentric push down on the pedals and the lateral component of an off-line chain. I believe that the fit of the crank on the bottom bracket axle is (or should be) tight enough to prevent significant movement at this joint. Thus for the pedalling force to deflect the chainring, the axle must bend. I have not done the engineering calculations; but this seems to me to be insignificant. The lateral component of the chain tension is a more likely cause and in extreme off-line situations it certainly deflected my old TA Cyclotouriste rings - and a 38 tooth freewheel sprocket.. These, however, were fairly unsubstantial (laterally) rings fixed to the cranks very close (25 mm) to the axle centreline. More modern cranks and rings have a very stiff casting out to about half the distance from the chain centreline (about 200 mm diameter). Furthermore, the rings themselves are stiffer than the old ones. These individually small deflections will of course add up; but I still suspect that the flexing of the seat and down tubes allowing the bottom bracket to rotate is a bigger problem.

Edited to correct "radius" to "diameter" for chain centreline.

[reohn2]

I am not convinced by the chainset flexing argument. The deflection that we are considering is a lateral one, i.e. parallel to the bottom bracket axle - towards and away from the seat tube. There are only two forces acting in this direction, the laterally eccentric push down on the pedals and the lateral component of an off-line chain. I believe that the fit of the crank on the bottom bracket axle is (or should be) tight enough to prevent significant movement at this joint. Thus for the pedalling force to deflect the chainring, the axle must bend. I have not done the engineering calculations; but this seems to me to be insignificant. The lateral component of the chain tension is a more likely cause and in extreme off-line situations it certainly deflected my old TA Cyclotouriste rings - and a 38 tooth freewheel sprocket.. These, however, were fairly unsubstantial (laterally) rings fixed to the cranks very close (25 mm) to the axle centreline. More modern cranks and rings have a very stiff casting out to about half the distance from the chain centreline (about 200 mm radius). Furthermore, the rings themselves are stiffer than the old ones. These individually small deflections will of course add up; but I still suspect that the flexing of the seat and down tubes allowing the bottom bracket to rotate is a bigger problem.

+1.
If anything is going to flex it's going to be the frame tubes IMO rather than chainset/BB.
That said we have unshipped the chain on a very stiff Cannondale Alu tandem (and we're not the strongest of teams) which I don't think was frame flex.
I'm therefore more convinced of a) "chainwave" b)uneven road surface(has to be the right kind of uneveness ie;bobly ) or c)bad change combination(s)/timing, d) "snap" spring release of the front mech when using STI's/Ergo type levers,or any permutation/combination of a,b,c,d.

[Mick F]

Can we go back to the question please?
What about the stiffness of chainsets?

Judging by manufacturers' claims, making chainsets more rigid is easy, as the % increases are in double figures each year. I've been told that (1) old cranksets (Campag Nuovo Record and it's many copies) are too flexible for "serious" riding [although they worked well enough for Eddy Merckx] and (2) flex felt when standing up is down to the frame and not the chainset. I'm not sure who to believe. Can anyone help me? I've an old chainset I'd like to use on a racing bike. I'm no Bradley Wiggins (I don't swear enough for starters), but I'd hate to have an excuse. I know I'll lose races because my headset is 1" and not MUCH bigger and I'm on 6, 7, maybe 8 speed, but.......

[reohn2]

MickF
I thought we were discussing c/set stiffness,some of us are perhaps not convinced that the newer super duper uber gruber crutchlike chainsets are anything to shout about and are mainly marketing hype to sell more OBB's.
Thinks..............If they were in the sliced bread league,the sale of N-gear Jumpstops and Dogfang gizmos would fall through the floor as there'd be no need for them...........

PS, I like safety nets,they prevent unforseen things becoming messy

[Mick F]

Not that I can see .........
........... we've gone onto chain/derailleur problems - which may or may not have anything to do with chainset stiffness.

What about singe speed bikes, or hubgear bikes?

What is the problem with lack of stiffness?

[reohn2]

Not that I can see .........
........... we've gone onto chain/derailleur problems - which may or may not have anything to do with chainset stiffness.

Which could according to some,have everything to do with c/set stiffness,so is a natural debating point.

What about singe speed bikes, or hubgear bikes?

What about them?

What is the problem with lack of stiffness?

Spaghetti c/sets could less efficient and over a period if time break due to metal fatigue.
Very noodly chainrings would unship the chain especially when changing rings,though I don't think that's the problem with c/sets currently.
It depends how noodly you mean.There's an acre of difference between a chainring moving 1 to 2mm and 5 to 7mm laterally.
It's the same with wheels running out of true when using rim brakes 5 to 7mm can be a problem but if you are using discs the bike can be ridden great distances and the wheel trued up later ie;the difference between multi c/ring deraileurs and hubgeared single c/ring bikes..
IMO crankset stiffness isn't a real problem with modern c/sets.