Sunday, July 28, 2019

BMW plunger-suspension cutaway

Thanks, Rolf!


pzak said...

Great blog! However this is not really a plunger which by definition uses no swingarm. In a plunger frame the rear axle is located by the plunger units alone. I think the last plunger frame BMW was built in the mid 50's. I believe this design was BMW's attempt to create a swing arm rear end without changing the existing plunger frame too much.

Mister G said...

Thanks for your comment! So on a "real" plunger frame, there would be no provision for keeping the wheel vertical through its travel? This BMW arrangement would also eliminate the need for a second universal joint in the drive shaft. I guess I need more plunger experience. Thanks!

pzak said...

Yes, That is right. Picture a chain drive bike, easier to imagine than shaft drive. Remove the swingarm and position the axle in two sliding pillars that are tightly constructed enough to eliminate flex but permit an up and down movement. That is a plunger frame bike. Plungers are not just shocks, they control both up and down movement with springs, holding the axle at the midpoint.They wore quickly and became sloppy which is why they rapidly fell from grace. The rear fork type has the axle describing an arc in motion and is very resistant to wear. Although the R60/2 type frame does resembles a plunger with the shock sticking up like they do, if you think about it is is just a normal swingarm with oddly placed shocks. The driveshaft in a plunger framed BMW does nothing to locate the axle in the frame, just drives the bevel. On those BMW's the shaft is not enclosed in the swingarm as in your pictire. If you google a picture aof a plunger chain driven bike it should all become clear. I read your blog everyday and always enjoy Sidecar Sunday as I have a few! Thanks for all you do.

pzak said... If you look at this pic of a plunger framed BMW you will see there is no swingarm at all. Pretty neat!