I said in my previous post that it didn't matter what kind of bike you get, any bike is better than no bike and nobody should let lack of a bike prevent them from starting their fitness journey.
With that said, I have pretty strong opinions of what sort of bike a noob should be looking at. What I'll do is layout a few rules that will be a pseudo buyers' guide so you can arrive at your own decision, but give you a framework to avoid big mistakes.
I said it didn't matter what type of bike you had. I meant it. If you have a perfectly serviceable mountain bike in your garage, use it! If you have no bike, I would strongly suggest getting a road bike. Why? You can ride farther and faster on a road bike than any other kind of bike. The farther and faster you ride, the more calories you burn and the fitter you become. There are plenty of fit people who exclusively mountain bike, but its a harder journey to get there.
The exception to the rule is based around where you live. If you live in some area that has no good paved roads with decent bike lanes, no paved cycling trail networks a road bike might not be great for you. If you live in an area with amazing single track mountain bike trails of varied terrain and difficulty, you might be better off mountain biking. If you watch mountain biking videos on youtube and simply identify more with that side of the sport, by all means get a mountain bike.
But for everyone else, 99% of first worlders live near good road cycling roads or trails (even if you don't know you do yet) and a road bike will be perfectly awesome. So get a road bike. Faster, farther, fitter.
Rule #2: Get an aluminum frame
Bicycles are made in every material from carbon fiber to aluminum to magnesium to bamboo. If you read a bicycle magazine or talk to a salesman at a less reputable bike shop, they will tell you that carbon fiber is the best material ever for bikes and that you NEED it.
You don't need it. Not for your first "real" bike. For starters, carbon fiber bikes are expensive. I am not going to recommend a bike to a newbie that costs several thousands of dollars. That isn't appropriate. Carbon fiber bikes are for dedicated racers, or cyclists who are committed to cycling enough that spending $3,000 on their hobby isn't a big deal.
People that don't know shit about cycling will regurgitate industry sales mantras that aluminum frames are harsh rides. They aren't. They will say that aluminum is easier to break than steel. It isn't, and unlike steel, it won't ever rust.
The simple fact is, aluminum rides are 99.9% as smooth as any other material, and it's lighter than a steel bike too. Oh yeah, and aluminum bikes are much cheaper than steel or carbon. That's a big plus. Lastly, if this guide is for you, you're a newbie. You will dropped your bike, bump into things, and you will almost certainly crash at some point. Aluminum will simply hold up better to the abuse that you will put it through as an inexperienced cyclist. After a year on the aluminum bike, if you decide that you "are a cyclist" as opposed to "someone who rides a bike" and you want to buy a bling bling carbon bike... go for it. But take my advice and buy an aluminum bike first.
Rule #3: Buy a compact crankset with a wide gear range
If you don't know anything about bikes that probably sounded like meaningless jargon to you. Sorry about that, but this is an important rule. In the future I will go in depth about why compact cranks and wide gear ranges on your rear cassette is superior, but you'll have to trust me for now that it is.
The short version of the story is that pedaling at a high cadence of 85-105 RPM in an easier gear is better for your knees, your cardio system, and your lactic acid management than grinding at a slow RPM in a big gear. 'Spinning' at a high RPM is just as fast as pushing a big heavy gear, and considerably easier on your body so you can ride faster and farther. Also, compact crank and wide gear range is going to make it so you can 'spin' even going uphill. Going uphill is where your bread is buttered as a cyclist. You work the hardest, burn the most calories, and you have the most fun when the road turns upward. A hill is not an obstacle to be avoided: it's a challenge to be overcome; a foe to be beaten. You will go uphill faster, easier, and more often if you are spinning in an easy gear than if you are grinding down your knee cartilage in a heavy gear.
Some people will dispute this. That's nice, but they are wrong. You know who need big gears? Racers who need to be able to ride at 30+ mph without running out of gears to shift to. Racers who need to sprint at 40+ mph. People who are going to be riding on country roads or on paved multi-use trails at 17mph do not need a 53 tooth chainring. Do not buy a bike that comes equipped with one.
Rule #4: Buy a bike with wide tire clearance and wide tires
For years the industry standard was 23 millimeter wide tires on road bikes. These can be run at high pressure and ostensibly were the fastest tires possible. Until people actually did the science and found that in many cases a wider tire (25 - 27mm) was actually faster because what it lost in aerodynamics it more than made up for in better rolling resistance.
Now, for our purposes today speed isn't relevant because you readers are not racers you are first time bike buyers. But the reason I mentioned speed is because I do not want you to think you are making a trade off or compromise when I recommend wider tires. You get speed AND comfort. It's a win win and there is no reason not to ride wider tires in 2016 and beyond.
So why are wider tires important if not for speed? Number one is comfort. Your bike tires are air cushions. The bigger the cushion, the smoother and more comfortable the ride. Simple to understand. A 25mm tire is simply a better feeling ride than a 23mm tire. And a 27mm tire is even more comfortable than 25. Beyond 27 you do start slowing down a bit, and not many road bikes are equipped to go much bigger, so 25-27mm is the sweet spot in my opinion.
Lastly, riding on gravel roads or smooth dirt is becoming quite popular. This is known as "adventure cycling" which is a dumb name for what is essentially "riding on a non-pavement surface on your road bike." This is a small consideration to you as a newbie, but why buy a bike that CAN'T do a gravel road when you can buy one that CAN and is also faster and smoother to ride? Kind of a no-brainer.
Rule #5: Fancy wheels don't matter
The stock wheels that come with whatever bike you buy are exactly all you need for now. When will you know when you need better wheels? The truth is you never do really, but I would say if and when you start winning USAC bicycle races and become ultra competitive you get a pass to buy some bling wheels. For now, don't pay extra for fancy wheels.
Rule #6: Be Prepared to Spend at least $1,000 US
I just heard a collective gasp. "Spend a grand on a bicycle to help me lose weight and get fit? I don't even know if I like biking yet!"
I know. I am well aware that this is a shocking price for people whose previous experience buying bikes comes from Wal-mart bikes that cost $150. But you should know that everything is relative. Walmart bikes are pieces of crap that will break into pieces after a week of hard riding. A $1000 road bike could last you the rest of your life with proper care and maintenance. There is definitely diminishing returns with cycling but those returns kick in around $2,500+. The gulf of quality that exists between a $150 bike and a $1000 bike is vast. Enormously vast. I barely consider a Walmart bike to be a real bike.
If you're reading this, you're a first worlder. You're the top 1% of the world's population. If you want a $1000 bicycle, you can get one. Are you interested in fitness and changing your life? Or are you committed to it? If you're committed, you'll get shit done. If you're merely interested, you'll find some excuse not to. I can't make you buy a quality bike, and frankly, if you have a perfectly servicable bike sitting in your garage already you can and should use that for as long as you can. But if you don't have a bike at all, you should buy a quality bike not a piece of crap. Quality will be remembered long after cost is forgotten. Work a few more overtime hours, collect cans. Do whatever the fuck you have to do. This is your health and your life. It's a small price to pay.
If you come to hate cycling and decide it isn't for you, you can sell the $1000 bike on craigslist for $750 as "almost new" and the experiment will have cost you $250. If you buy a $250 walmart bike and hate cycling, nobody is buying your shitty walmart bike so it costs you $250 anyway.
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What bikes would I recommend that fit my rules? There are dozens. But I'll give you what I consider the best three options.
Trek Emonda 5 ALR - $1,350
Pretty sweet looking bike if you get it in the red and white. Excellent aluminum frame, nice shimano 105 components. Only con is it comes stock with 23 mm tires. It can easily accomodate wider, and I have no doubts that your local Trek dealer would throw on some 27mm tires free of charge if you purchased one of these.
Giant Defy 2 Disc - $1,100
I have to say, this bike is better than the Trek. It has disc brakes, better wheels, wide tires we like and has the exact gearing I recommend. It's also a couple hundred dollars cheaper than the Trek. Those extra dollars will pay for your helmet and cycling shorts. This is simply a better bike at a cheaper price point. The gearing is spot on and perfect for our needs. I really rate the bikes Giant is making very highly and this bike is really built to suit our needs.
Specialized Diverge Sport A1 - $1,150
Full disclosure: I own a Specialized and I rate their bikes quite highly. So perhaps take it with a grain of salt when I say that this is the best of this bunch. This bike fits all the requirements we listed and is quite stylish. Some people prefer the curved tubes of that is Specialized's style, others like the straight tubes like the Giant. Both these bikes are great and you won't go wrong with either.
The Diverge, however, isn't a stripped down version of a race bike like the Defy. It's a purpose built bike for people who aren't racers, but who still want a fast, nice bike. It is built to be ridden both on and off road, and is built to take that kind of punishment. The gearing is perfect for spinning and this bike, like the Defy, is built for climbing uphill. This is an Adventure Bike. Sorry for being a Specialized fanboy but it is what it is.
Questions? Comments?