nlerner

Yesterday, I was out for a ride on my '71 Raleigh Int'l. I have this set up with a six-speed rear cluster, a Stronglight 99 crankset, and a Campy Super Record FD. I soon realized a problem: When I was shifted on to the 2 or 3 smallest rear cogs and large chainring, I encountered chain rub on the FD cage. If I shifted the FD a bit more outboard, it made contact with the crank arm once each revolution (Stronglight cranks are well known for having very little clearance between arm and big ring). So my physics problem was whether I should opt for constant chain rub or for once every 360-degree crank arm contact with the FD. Which would create more resistance in the system as a whole? I would guess the chain rub, but then I figured the force of chain rub and the force of crank arm contact with the FD weren't equal. Help--I need a physicist (but I'll settle for an engineer)!

trailangel

re-adjust the FD

rhm

I assume your derailleur has the safety lip on the front of the cage? I'd switch it out to one with a flat outer plate.

corrado33

Either is bad. I'd suggest replacing something until nothing rubs. Alternatively, adjust the chainline so that it lines up better. You're either going to prematurely wear your chain, or eventually knock your FD out of position with your crank/put a nice big scratch in your crank over time.

Alternatively you could file a bit of material out from behind the crank, but that's probably not a good idea either.

USAZorro

Re-do the spacers on the rear axle and re-dish the rear wheel. It sounds like 1 mm towards non-drive side ought to do it.

southpawboston

For Neal to pose a mechanical inquiry indicates a serious problem given his Mad Mechanix Skilz.

I don't think the resistance caused by either rub scenario is significant enough to worry about. I would be more concerned about the annoying rubbing noise and eventual wear on the cage (or crank arm).

As Rudi suggested, an FD with flatter cage will solve the problem (and there are plenty of flat-cage FDs out there to choose from), but so might using a slightly longer BB spindle.

You could also file away that lip on the front of the cage, but then it might start rusting.

repechage

Yes, what brand and age/model of front derailleur?
I would be looking for a Campagnolo Record front changer pre CPSC modifications. Set it up where the outer cage plate is parallel to the chainrings and no more than 3 mm above the large ring.
Also, what size of big ring?
Are you using period Stronglight chainrings?
Of course, you could be so strong that you are just distorting the system.

rhm

The crank is designed for a thinner derailleur.

Velognome 

Before messing with FD Plates or dishing the wheel, may I suggest a quick check of the Chain Line. Once you settle on a good workable line the issue whether a flat FD plate or a spacer or both will become clear or maybe the FD is tweaked a bit? But I'd look at the Chain Line first.

non-fixie 

Had the same problem with a Stronglight 49D. Changed Campagnolo FD to older, flat model. Problem gone.

repechage

Glossed over that Super Record description, yep. Wrong changer.

nlerner

Thanks to Anton for being the only one to address directly my question about one force versus another (though I'm still waiting for the calculation as to which force is greater). And thanks for those suggestions on how to make the rubbing go away though that's not quite what I was asking.

rhm

You're welcome! I figured if you really wanted the question answered, you would have posted it on www.physicsforum.net.

3alarmer 

.
...you should cross post this in the fixed/SS forum. The answers would be funnier.

SJX426 

The total force (friction) would likely be with the constant chain rub vs a cyclical rub on the crank arm. If your question is not related to the impact, this is the answer. Of course how much interference can impact the equation. With out knowing the amount of surface area contact and the interference dimensions, you can't accurately answer the question.

USAZorro

For a thorough calculation of the forces, we'd need to know the number of teeth on your outer chainring, the surface area of the chain that is in contact with the FD, the length of contacting surface on the front derailleur with regard to both the chain and the crank arm, the flexibility of both the chain and the FD mounting, rate of movement of the crankarm and chain at the point of contact, etc...

pastorbobnlnh 

A question such as this coming from a former professor at MIT (albeit a writing and English one), is just mind boggling.
@jimmuller might be able to answer the question about the force differential. In the mean time, I suggest you examine which you find more annoying: "click--- click--- click----" or "ratatatratatatratatatratatatratatatratatatratatatratatatratatatratatatratatatratatatratatatratatatr atatatratatatratatatratatatratatatratatatratatatratatatratatat...."

Jus' sayin'

P.S. Is this at sea level or at some other altitude? Air pressure will make a difference.

Fahrenheit531 

re: several entertaining responses:

Down with physics! Viva la Revolucion!

3alarmer 

.
...I have an answer, but it is expressed in furlongs per fortnight as a function of angular momentum, so I need to check my work.

due ruote 

Given how many great bikes the OP has to chose from (presumably without annoying rubbing noises) I assume this to be a strictly academic inquiry.

This class is too hard, and the prof only wants answers to questions asked. I'm going to drop it.

gugie 

If you're settling for an engineer, I is one.

The one thing you'd need is the coefficient of friction from both sources, neither of which can be calculated from first principles, nor from any known experimental sources. One way to calculate this would be to use a power meter, which are readily available specifically for bikes, and a cyclometer to measure speed. Create an experiment where you ride the same route, repeat several times for each type of friction, run the results in your favorite stats package, and report the results to us. Easy-peasy.

But what you really need is a mechanic, not a physicist or engineer.

nlerner

Oh, come on you engineers, you just look all of that stuff up in a reference book! They might even be online now.

nlerner

Unfortunately, my mechanic seems to be spending all of his time these days riding rather than wrenching.

Sir_Name 

Assuming the friction from each is comparable, the chain rub would rob more power as it's a constant drain on the system vs the periodic annoyance of the crank arm interference.
Ref: post #17
Installing a hamster on top of the chainring may offset lost power.

jimmuller 

FIFY.

Consider the time duration of each of the rubbing events. The crank arm thickness is maybe equivalent to two chain links. The ring was, what, a 52T? The constant rubbing would be in effect for 26 times longer. Both would be at approximately the same radius to the spindle. So unless the derailleur-strike event applied 26 times more ******ing force, the rubbing would definitely produce more drag.

I know someone who prefers not to shift his Campy FD at all lest he wear away the inside of the plates. (Seems kinda' silly to me.) But if I wuz you though, which I'm not for obvious reasons, I'd be more concerned about the stress on the FD from the potentially much-higher-stress impact of the crank arm.

And I'd check its alignment on the ST.

You can correkt my speling if you wish.