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Entry level bikes are becoming more and more popular, the range of bikes to choose from is almost infinite. Entry level bikes are made to be affordable and for that reason there will always be some kind of compromise, whether it be on the frame material, gearset, forks or wheels there will be something that makes it possible to sell it at a low (relatively) price. I cannot possibly cover all the different bikes available to you but I can give you some advice on what to look for.
Beware of second hand bikes from a certain auction site - you can get some really great deals but think about why the deal is so good. To be honest, my first road bike came from an auction site, it was a good deal and a great bike (it is still my main bike) but it didn't come without it's own fair share of problems, the back wheel was knackered so I bought some new wheels, the cassette, bottom bracket, chain and inner chain ring all needed replacing. The cables were worn and there is some evidence that the previous owner had at least one crash. Getting up to a good standard has cost almost the same as what I paid for it in the first place. Don't get me wrong - I love it, and at the time I was a still a student so it was the only way for me to get the bike that I wanted.
If the bike is too good to be true - then there is a good chance it is stolen. These auction sites are probably the fuel behind bike theft.
If I can offer you one piece of advice here it's that you should get yourself sized properly - go to a local shop and get them to size you properly, frames come in many different geometry layouts these days so it is vital to know that your new bike will fit your body. Otherwise you will feel some discomfort on longer rides and probably will not perform at your optimum. A cheap but well fitted bike will be a far better investment than an expensive but improperly fitted bike.
Look for a bike with carbon fibre forks, not only are these lighter but they will massively reduce the amount of road vibration coming through the handlebars which is a godsend on longer journeys. Don't worry about bikes with a poor wheelset, these can easily be upgraded when you have the money and the inclination.
You will have to make a choice between gearsets, you will either choose Shimano, Campagnolo or the new gearset company SRAM. Most entry level bikes seem to be supplied with Shimano gearsets. They are a good bet, easy to use, dependable and the brakes are strong. As with any gearst there is a range, for Shimano it starts with "Sora" at the bottom, then "Tiagra" followed by "105" then "Ultegra" and finally "Dura-Ace" at the top of the range. Each jump up the ladder can add a significant amount of money to the cost of the bike. If your budget allows it try and get at least Tiagra as there is a big difference between Sora and Tiagra. The higher up the range you go, the smoother the gear changes, the lighter the gearset and the shinier the components become.
There is always an argument about what is better to compromise on - gearset or frame material. The best bet is to read plenty of reviews (preferably from users rather than bike shops) and look around. Entry level frames are pretty much all Aluminium, of varying quality. You want something that is reasonably light but preferably it will also be good and stiff allowing for better power transfer and handling. Without a doubt you best bet is to try before you buy. If you desperately want a lighter frame with juicy components then consider the "cycle to work scheme" or some sort of interest free finance deal - but remember, it won't make you a better cyclist. Consider the weight difference between an expensive bike an a cheap bike, you could probably lose that same amount of weight off your body in a week - and it'd be far cheaper!!!
If you want some typical entry level brand suggestions then you can't go wrong with Bianchi, Focus, Trek (bottom of the range).
Top Tip
If you're buying a new bike, wait until the end of summer, this tends to be when the next season bikes come out. The latest and greatest is always the most expensive, a 2009 bike can drop in price by as much as 40 percent when it's 2010 equivalent model comes out!!! Look in the clearance sections on-line and in-store - you can get some awesome deals, just make sure you get the right size.