I'm off to Karlsruhe in a couple of weeks for 6 days and whilst I'm there for a conference, there will be plenty of time to amuse myself in the evenings or mornings. It looks to be a cycling friendly city and there are trails outside the city so I'm pondering getting ahold of a bike. Now they operate one of these public transport bike schemes but the things look heavy and cumbersome. I've tried to find shops which hire but can't find much in English or with half decent bikes. Any ideas/recommendations?

take your own? extra £20 with the plane, isn't it?

"hallo, kann mann hier ein fahrrad mieten?"

or

"können sie englisch?"

Just had a quick look on google and found this:
http://www.rent-a-sportsman.de/ If you have time, it's worth going to Baden Baden, old Spa town where England stayed during the world cup here. You are also in the Black Forest, which cakes aside, is great for cycling.

Alpin, where are you based in Germany?

A quick google of "fahrrad mieten Karlsruhe" brought this up…
http://www.outdoorschule-achim-kleist.de/Fahrradverleih/Preisueberblick.htm

Looks like you're into about 75 euros for the duration.
I'd suggest ringing 'em. Even if your German is not up to much, chances are you'll get someone speaking better English than most of us. 😳