Monday, December 15, 2008

Derailler Spring Tension

So, I had a problem. My cyclocross bike chain was falling off pretty much every race and my homebrew chain keeper just wasn't cutting it. So, I first added an N-Gear Jump Stop (which is a quality product, by the way). That minimized chain drop to the inside of the crank, but couldn't stop chain drop to the outside. I run a 42t single chainring with a 42 tooth equivalent BBG outside chain guard. I wondered if I could go bigger on the outside chain guard, but that would rub against the chain when I was in the 11t sprocket. Then I thought to myself, why doesn't the chain ever come off on my mountain bike? It's a hardtail and I do some much bumpier terrain on that thing, it has a similar rear cogset and I'm frequently descending in the middle ring. The answer- chain tension.

I run a fairly low budget Deore rear derailer because it can handle the 11-32 cogset and has a 'high-normal' spring to match my road shifters. I suspected that the long cage geometry might give lower chain tension, allowing the chain to more easily slip while coasting (over bumpy descents).

How to test my theory? Borrow a digital force gage from work (you can substitute your everyday fishing scale if you want to repeat the experiment) and go to town tugging on derailers over a pre-defined travel range.


CX Bike (Deore) - 2.4 lb
Road Bike (Campy Centaur)- 2.6 lb
Mountain Bike (Deore XT) - 3.0 lb
Touring/Commuter Bike (Deore) - 2.3 lb


Cage length matters not. My Campy Centaur rear derailer is a very short cage road model. My Deore XT rear derailer is a long cage model. It's all about the torsion spring that pulls the cage back. Until I can get my hands on a high normal rear derailler that can handle big cogs and has a high spring tension, I took a couple links out of the chain and called it good. After a few races, things have turned around and there have been no more dropped chains.

On a related note, I used the digital force gage to get a real accurate read on my road bike's weight. (Bike+Pedals+Pump+Computer)+Saddle Bag = 22 + 1 = 23 lbs. Heavy or not, you know what they say... it's the Indian, not the arrow.


-d said...

I know it probably voids any component warranties and scoffs at the design intent, but could you order a spring with a higher spring rate (from some place like McMaster Carr), install it in your current derailler and see if there is any improvement?

-p said...

That's an interesting idea. I think the part cost me $22 bucks, so opening it up doesn't bother me in the least.