As you may know, I am a geography student, so why not utilize my education? Forgive me if I'm dry, I'll try to be at least informative :)
There are 2 types of suburbanization: British and American. Both mean that people desire to live outside the city center. The British suburbanized in the 19th century by developing public transit system. The American way was to get to the suburbs by private automobile, which also enables the low density population structure and urban sprawl. Nowadays there the both suburbanization types occur in Europe too. The American type has proved to be particularly detrimental for cycling, because the distances are too long and often the necessary and safe light traffic infrastructure does not exist.
Low density housing in America pretty much destroyed community walkability and light traffic infrastructure, but in those European towns that have managed to invest in cyclist friendly infrastructure the cyclist rates are still very high (Follman 2007). The examples of 20th century Amsterdam and Copenhagen prove this point but they also show that it is possible to reverse this development and break the connection between suburbanization and lower cycling rates.
In the 1st half of the 20th century up to 75% of trips in Amsterdam were done by bike. 75 percent! After the second world war cars took over Amsterdam and the city expanded. This obviously had a massive effect on cycling, and the cycling rate dropped to the all time low of 25% in Amsterdam and 10% in Copenhagen by the 70s. How ever, the city road capacity couldn't handle many cars and the oil crises forced the government to think of measures to decrease oil dependency, so they began aggressively promoting cycling among other things by investing in bicycling infrastructure and enhancing cyclist priority in traffic. They succeeded and nowadays the cities have 35-40% bike trip share. That is impressive! Nowadays the cities are known for their cyclist friendliness and are among the safest cycling cities of the world (Jacobson 2009).
So, usually suburbanization and cycling decline go hand-in-hand, but it does not have to be that way. Suburbanization can also promote cycling if the suburb is not too far, but that would be on the expense of pedestrians or public transit, not cars. It is better to reign in the (mostly the American type) suburbanization a bit by supporting strong city centers and restricting suburban mall-building. E.g Muenster has some positive experiences of that strategy (Pucher and Buehler 2007).
Turku has both American and European type suburbs. Less than half of the households on the closer and denser (British type) suburbs have cars, but 70% of the more distant ones have at least one. The highest cyclist rates coming to the center of the city are from the closer suburb area. Surprisingly the cyclists are not coming to the grid plan center as often as the average cycling rate (11,5% of all trips) suggests. From the more distant “American type” suburbs almost no-one bothers to ride a bike to the center (numbers are from Turku Bustrip self-assessment report 2006, in Finnish).
So, it looks like these suburb theories do fit pretty well on my town. How about you, do you have any theories about the connection of suburbanization and cycling or would you like to share your experience? Is this kinda stuff too boring for you or do you wanna hear more of my academic brain farts?
P.S. Here's a video about the solution to the problems presented by urban sprawl (which is not the same as suburbanization, but closely related to the American type)