Having a bike is actually really practical in Frankfurt and can save you a lot of money on your commute if you live inside the city. Beyond this there are the benefits of the area being relatively flat and having a lot of bike paths alongside rivers. If you are after something a little more exciting, then you can head up in to the Taunus for some relatively interesting forest trails. Nothing that will excite an experienced mountain biker, but certainly enough for some good exercise.
Depending on where you will store it you will have the option of second hand or new 🙂 (obvious, right?). The reality is that while crime is relatively low in Frankfurt, bike theft is relatively high. If you store your bike outside overnight or regularly park it around the city, then head down to the museum flea market on a Saturday morning and find yourself a not-too-attractive second hand bike. There are lots available, some of which are allegedly stolen bikes being resold, but be careful of the quality. Some of them are in really poor condition. But if you look around, you should be able to find something suitable for a fair price.
If your budget stretches further and you are looking for something better, then you may be frustrated trying to find a really good selection of bikes within the city limits. An excellent choice is to head out of town to Denfeld in Bad Homburg. Main drawback is obviously that it is in Bad Homburg though, about a 15 minute drive north of Frankfurt. The SBahn will take 30 minutes but then there is 20 minute walk to the store. UBahn (Gonzenheim) will leave you a 40 minute walk. There are buses in Bad Homburg, but nothing direct. Only real solution is to drive.
This is a seriously big bike store though, if you are able to get there ;-), with a huge range of bikes. Something for everyone’s needs, from the €500 city bike to the €10,000 specialist mountain bike. Just watch out for the kids testing the bikes on the in-store track – they can be lethal! 🙂
There is one large store in the east of Frankfurt on the U6 Ubahn line called Zweirad. It has a big selection and is part of a chain of stores but I wasn’t impressed with the quality of service. When I had a look online I saw plenty of negative reviews on Google, so will probably be staying away from here next time I need a bike.