TJHOO

Have been enjoying compact chainrings (50/34). Have been able to stay in large chainring longer than with 53/39 combo. However, in doing so, this causes more bend in the chain than if had used 53/39 and shifted down to the 39 earlier (rather than staying on the 50).

Has anyone figured if some of the efficiencies of having compact chainrings (being able to stay in the large ring for a lower gear than a 53, and lower rotating weight) are offset by more cross chaining?

Had been considering switching from 12-25 to 13-29 (w/ 50/34 up front) for very hilly 127 mile ride coming up. Just got the guts to stay with the 12-25...unless the 13-29 will be more efficicient b/c of less chain bend/cross chaining (I never use the big/big or small/small combos)

worker4youth

50/34 with 12/25 should suffice for any hills you might have, unless you crawl up at 2mph. Forget about chainline...just ride

reef58

Poppycock! Use whatever gearing you need to make the ride comfortable. There are a lot of people that walk their bikes on rides with very steep climbs due to the above logic. Not everyone can or should mash up a 3 mile 15% hill.

Richard

johnny99

Where is this 3 mile 15% hill?

reef58

Check out many rides in the mountains of NC. I was in a group of 50 people in the HHH last year, and me and one other guy were the only ones not walking our bikes up Bearwallow Mountain. It was not 3 miles or 15% either. Beech Mountain will get you attention as will Brasstown. The Assualt on Mitchell includes something like 7000 feet of climbing in the last 27 miles. Not everyone wants to do that with a 34-25. Some think they do, and end up walking or bowing out. Unfortunately many are told a 39-25 is fine.

Richard

worker4youth

Oh please, 7000 ft over 27 miles is 5% average...plenty doable in 39-25, let alone 34-25.

I do 5% climbs (albeit, never for 27 miles), regularly on my fixie 48-18.

reef58

Yes it is plenty doable for you. Keep in mind there are people on the forums and on the road with various fitness levels. Since the poster is asking about a large cassette I suspect he may not be the strongest climber out there, but I could be wrong.

Later,
Richard

TJHOO

I was primarily asking about the "chainline". Now I confirmed the correct term on Sheldon Brown's site:

This refers to how straight the chain runs between the front and rear sprockets. Ideally, both sprockets should be in the same plane, so that there is no sideward motion or stress to the chain. This constitutes "perfect chainline".

"Chainline" is more crooked with some of the larger sprockets on the cassette when using a 50 chainring (compared to equivalent gear inches when using smaller sprockets on the cassette with the 34 ring.

This difference is accentuated compared to using standard 53/39 b/c you have lower gears on the 50 than the 53. Eg when in an increasingly steep hill you can stay in the 50 longer than the 53 while you move to sequentially larger sprockets on your cassette.

I am doing a long, arduous ride ogf 127 miles w/ >13k ft climbing including 12% average 3 mile climb at the end. So I was wondering if by switching from a 12-25 to a 13-29, I might have a straighter chainline for this ride. If so, does it matter?

Now that I learned the correct term "chainline", I will repost with that name under new thread.

rruff

Neither the rotating weight nor the cross-chaining is going to slow you down. Spicer et al tested efficiency vs chainline and found no measurable effect. It may wear out your cogs faster, though... so I wouldn't advise doing it all the time.

bluecd

TJ i think you're needlessly worrying yourself. you're an excellent climber and your 12-25 should do fine. i wouldnt worry about how straight your chain is on the MOM, just ride it.

whitemax

I was the other guy, I didn't have to walk

whitemax

Absolutely, I have done the NC 80 up to the summit on Mt. Mitchell part in 53/23 just to see if I could it. 39/25...are you kidding me?

Mo'Phat

I just did the Alpine Challenge in San Diego (72 miles...7000' climbing...pretty much an out and back) with some pretty good sustained climbs. I rode an FSA Compact (50/34) with a Harris Cyclery 'High & Wide' 11/28 cassette. I never made it into the 28 gear, but it was nice to know it was there.

A 50/11 for the downhills was excellent, and I was routinely spinning out of my 50/13. So, it's something to think about if you like going quickly down the hills you just climbed.

I also have an 11/23 as my main training cassette...which is nice.

roadfix

But yes, as the OP pointed out, I find myself crosschaining quite frequently with the 50 ring.......and I love it...