I have a road bike with a compact crank and 11-25 rear cassette(9 rings) at the moment. And my local climb is 35k long with an incline between 10- 30%. I really struggle with it. I have to stop like 5- 10 times to complete it. I am thinking of upgrading the cassette. I have an option between 11-30,32,34. I was wondering if it is really worth it to get the biggest cassette? Any opinions would be appreciated? Thanks.
For grades beyond 10% having a gear that you can spin at the rate you can climb makes a big difference.
Only you can know exactly what gear that should be. If you can find a gear ratio tool that displays speeds for a given gear, wheel size and rpm. This one seems pretty good.
Then think about your typical speeds while climbing on the steep sections. You need a gear that will allow you to spin at a least 60 rpm and still keep those speeds.
A gear that works at 30% and 60 rpm probably isn't possible with a compact crankset.
FWIW, Where I ride has lots of climbs above 10% in the 5-10k range and I see lot's of riders with compact cranksets and 11-34 clusters.
Personally, I think if you are really considering this, you'd be much better off getting a compact 110 Bcd triple and something like 24/36/48 gearing. It's not that you get that much lower gears, but you get many more useable gears. Having a choice of a few gears in the low range makes a lot of difference compared to just the one or two extra low gears you get by going to a wide range cluster.
Get the biggest. The down side is some bigger spacings. If money is an issue consider used.
You have the option to not use a gear you do have. But you can't use a gear you don't have.
Cassettes wear out. If you find yourself rarely using the 34 and/or 32 then get tighter for the next. Start with the biggest for a data point. If that is not enough then go triple next.
I assume you already have the smallest possible chairing on the compact?
Yes, if you like to climb like me and live in an area where there are long steep climbs over 10% grade, I highly recommend a compact 50/34 crank and a wide ratio cassette.
I have asthma and have provisioned all of my road bikes with 11-34 cassettes. Shimano makes 10 speed mountain cassettes that will work with an Ultegra long (mid) cage derailleur. In addition, if you are running an 11 speed Shimano drivetrain, you can buy an 11 speed 11-34 wide ratio cassette from IRD: http://www.interlocracing.com/cassettes-freewheels/11-speed-elite-cassette-shimano-compatible
I do not fear and can climb just about anything now because I know I will always have that 1:1 gear ratio bail-out gear.
On wiggle they sell chainrings from a French company TA Specialties and you can get a 33 tooth for both ten speed and eleven speed. Another option is rotor Q rings in 50/34 size. Get those, if you are running a ten speed you can get a Tiagra mid cage derailleur that can run an 11-34, the maximum range is 11-32. A Japanese company Sugino make an ultra compact twin that has two separate bolt circles and can fit chainrings down to 24 teeth.
Everything in bicycle design is a tradeoff. The right cassette for flatter terrain won't be the same as for climbing big walls. I find most cyclists cheat themselves out of potential wattage on steep hills by not having a short enough climbing gear. Sure, that straight cob looks pretty macho on the rail trail, but it doesn't look so impressive with a weekend warrior on it having to come out of the saddle just to grind up some wimpy 8% grade. And even strong riders can benefit from a climbing gear they can spin at 95 RPM up a tough climb. That's what Chris Froome does to go up hills almost as fast as the dedicated climbers. The reason this works is that the lower pedal force required to turn over the cranks avoids activating too much fast-twitch muscle fiber which, though very powerful, consumes glycogen in a horribly inefficient way, leaving you depleted very quickly.
It's all a compromise. I will tell you a little story for illustration.
At the top of a mountain not so far from home, there is a beautiful willow tree. I like to take a break under its shadow when I cycle, after going all the way up. It creaks and sways along with the wind. It's a relaxing, quite zen experience.
So, there I was, under that tree, just enjoying life while eating a watermelon with a big spoon. Yes, I rode up the mountain with it in the rear bags. I fancied eating it.
And then comes a bunch of spandex-clad guys on racebikes.
I say hi, then resume eating my watermelon, because it tastes good.
I notice some of the lycra fans ogling my bike. It's an old piece of shit, heavy, and it looks tired because it ran more than 15000 km... One guy is looking at the bags on the rear rack like they're aliens from space about to lay eggs inside his head or something.
So I open the bags, pull out a brick-thick tome and simply say "yeah, I was going to the library. You guys ever read some Solyenitsin? I recommend it."
Then I turn around, point to a spot in the foggy distance down below, and say "I live there."
I turn around again, point to another spot in the foggy distance, and say "I signed up on the public library over there. For exercise. Also I like reading."
Everyone looks at me like I'm some kind of space alien. Jaws pop.
Then someone points their finger in the distance and mentions "you could get from your home to the library via this route, it's flat and fast and..."
"Lots of cars, smells like exhaust," I say, "besides, I like the climb here, the view is fantastic, carrying the weight up only means I get to watch the view a bit longer. I like this place. In fact," I pointed down to the other side of the mountain, "I'm early, so I'm gonna go down there in this other valley, then up here."
-- End of story --
I put a 34t cassette on the bike. I also changed the lower chainring to 24t using MTB parts. Unfortunately I can't go lower without changing expensive parts, else I would have.
I like options. Near where I live, I can ride the highway, or a bit of unforgiving 20% grade, followed by very nice secluded roads with very little traffic which are a joy to ride on a bike.
Very low gears widen your options. They make riding lots more fun! Why not use them?
I had an 11-25 10 speed cassette on my road bike with a compact crank (50-39). I did some long, hilly rides with it and managed, but eventually switched out the cassette for a 12-32--the biggest my derailleurs could accommodate.
I kept the original cassette with its chain in a bag in my shop in case I wanted to go back.
I haven't opened the bag in three years.
Everybody's different, but I enjoy having the easier granny gear, and don't really notice the wider range between cogs.
It depends on your power to weight ratio, the grades you are climbing, the cadence at which you are efficient, your riding style and if you ride alone or in a competitive group. For example, I am at about 3.0 W/kg for 1 hour or more, and where I live there are 8% climbs with long stretches at 10%. That means that I need at least a 1:1 gear ratio to maintain 70 rpm's. I therefore abandoned my compact gearing for 42-28 mountain bike cranks (used to be 39-26) with an 11-32 in the back. The 28 x 32 is essential on rides where I'm not feeling good. My high gear is 50-11 which gives 120 rpm's at 55kph.
My choice is also based on my riding style: I am almost never out of the saddle on climbs, I maintain a high cadence on flats and am heavy enough that I don't need to pedal on descents to keep up. I also don't do rides with people who are pushing pace at over 55kph. So for me, I don't need tall gearing.
Hope that helps.