After Ferry left the Yamaha team he worked for some time for Ton van Heugten, who was a gifted Motocross rider. When Ton tried his hand at sidecar motocross, Ferry was there to build (of course) a Yamaha XS650 powered outfit. The engine was enlarged several times, to finally (I do think 910 ccm). Here you see Ferry flying through the air (I do hope he knew that this is quite a painful hobby.....) Anyway, Ton van Heugten became European Champion with this bike in 1975.

Ferry Brouwer, XS powered sidecar