What is a better gearing combination for downhill?

Question

One of the tracks I've been riding recently is testing both my skill level and my bike. The track is a combination steepness and roughness that makes it significantly different to what I've previously riden. The track is probably more suited to an AM FSR but I'm riding an XC hardtail.

The frequent chain slapping and rear derailleur bouncing around has led me to question what the best gear ratio to ride is and if this will damage my drive train.

Is it better to ride downhill in the larger chainring (42T) on a lower gear with a tighter chain or in the middle (32T) in a higher gear assuming the ration at the end is roughly similar?

Answer 1

I ride a hard tail over all sorts of terrain. Generally I ride on the middle chain ring in these situations. Its less to do with the mechanics, and more to do with whats possibly coming up, and fast + reliable gear selection. When on the large chain ring, you can get caught out by a short (or long), steep uphill, and not be able to get a low enough gear to climb it just changing the rear cluster. I reserve the large one for easy predicable ground, or tracks I know really well - for instance sealed/gravel roads, manufactured fast tracks with sweeping / bermed corners where speed is all important, and you are unlikely / have no need to slow down below about 20km/h.

If the track is steep and rough, I am unlikely to run out of cadence before 'balls' - if you find you (like many others) have a "lower center of gravity" than me (i.e. smaller brain and/or bigger balls), and you run out of cadence, then go for the top chain ring. Also the chain can be dropped far more easily off the top chain ring - although on a well tuned bike this should not happen.

As far as chain slap and derailleur bounce - I ignore it. Its a mountain bike designed to be used and abused. If you want, you can get neoprene coves for the chain stay which will quieten it down, or wrap it in duck tape... I have never heard of derailleur damage caused by slap, but that is not to say it does not happen.

Answer 2

I run a single front with a "Single Ring Chain Guide" very similar to the pic on the left. There are also similar options for multiple rings like you see on the right. They keep the chain from slapping, but more importantly (for me anyway) they keep the chain on the rings. The more rings you have, the more options you can choose from, the longer the chain has to be, and this will cause more slap and more chance for the chain to come off. You can see the bike on the left also uses a rubber slap pad.

If you feel like you need all 3 chainrings in the front, you might want to chop the chain down to the minimum length you will need for both big front and rear, but this "cross-gearing" shouldn't be used anyway. (I have known some to chop it even more and never use the outer chain until you are in the lower 3 rear sprockets, this will also help server as a chain guide to reduce the chain from bouncing off the front rings).

The downside to using a single front is that you are basically stuck in your middle gear in the front, but for the riding I do, almost exclusively downhill, it is worth it to me to jump off my bike and walk it the few times I need to go uphill (its almost as fast as pedaling it in hamster-wheel mode anyway), I don't miss my other gears. You can also buy just the pulley and attach it to your chainstay, but I didn't like the way it worked on my 3 chain bike.