I'm not sure if this is the right place to post this but hopefully it will surface if someone in the future has cooling problems with a Sachs Roadster 800. I found the attached post on a German site called Freebiker.com who have the odd item on Sachs bikes [ us Sachs owners are definitely the waifs and strays of the online biking community as there are so few of us especially in the UK ] . My bike had it's overheating warning light come on in traffic earlier this summer and it turned out to be exactly as per the post, ie the temperature switch for the fan was coming on too late. It's an easy and cheap fix to swap it for a lower operating switch, initially I opted for a replacement switch 95 deg on/85 deg off more or less as per the posting but I found that the switch was cutting in just as the temp light was starting to flicker for a second or two so as a belt and braces I then replaced that switch with a 90/80 degrees switch which has fixed it, I bought my switches from Stoney Racing but they are freely available M14 x 1.5mm thread. I had to remove the radiator to get the switch off but it's not a difficult job and is a good opportunity to flush the radiator and change the coolant .
Owners new to the bike should note that when the handbook says to fill the expansion bottle to just cover the bottom of the bottle they mean just that, I made the mistake of filling mine to the 'Max' line which put a tiny pinhole sized leak in the bottle as the plastic expanded, easily fixed with something like JB Weld or a similar epoxy but lesson learned there, the coolant bottle is off a totally different machine it turns out, Adilmon has identified it as being off a Rieju which I wasn't aware of at the time. Come the winter layup I intend to look into the Cleassou radiator mod which will add a proper radiator cap at the rad rather than at the expansion bottle but I'll get professional advice from a radiator specialist before I commit to that. The pdf drawing for this can be found on the excellent Sachs Biker site but for some reason I cant paste a link to it so anyone interested will need to Google it;
Sachs Roadster 800: Thermal Switch/Temperature Switch - Sachs - freebiker.com
I hope that this helps someone at some point.