The term DERNY is not the right term for the motorbikes, which you see on this pictures.
A DERNY is more like a moped, a bicycle frame with a small motor
Those black monsters on the photo are the REAL Pacemaker-motorbikes from the beginning of the last century.
The picture is taken on the 200 m track in Dortmund/Germany during the "Big Christmas Prize", held each year at Christmas.
The motorbikes typically have a big engine (1000 ccm or more) with a fly wheel and only one or two gears and no mufflers
. Power transmission by a belt. They can achieve speeds up to 130 km/h (70 mph) and were built for the long stayer-tracks of 333 and 400 m, which were so popular in the 1920 and 30ies.
Get some more impressions from this time under: http://home.t-online.de/home/Roland....n/cyclerie.htm
(unfortunately only in German, but good pictures
The motorbikes on the picture are originals from the 1920ies with copper-tanks, which are kept in shape by an enthusiastic pacemaker from Dortmund.
Unfortunately stayer races are not anymore part the UCI-World-Championship programm and therefore have been out of fashion in the last years. But there is still a good community of stayers in Europe, which organises races on a high level.
The race is actually made by the pacemaker and the rider "just has to take care", that he stays as close a possible to the "roll" in order to receive the best wind-shadow and to make riding for him easier.
The long handle bars of the pace-maker allows for fine steering and the speed is regulated by a notched grip, which allows for smooth accelleration. The overall speed can somehow also be regulated by the distance of the roll from the motorbike (shorter=more wind-shield=faster).
Note the position of the pacemakers in their normed black leather suits sitting as much as possible in the back of their bike for maximum wind-shield.
Note also the pacemakers helmet, where the "auricles" are open backwards, trying to hear the riders commands in the noise of the machine.
Basically there are just two commands possible at this kind of speed.
1) Alléz (french) = GO (faster) or
2) "OOOOOOOOH", when it has become too fast for the rider
If the rider comes "off the role" he is virtually thrown back by the wind resistance and the pacemaker must slow down considerably to pick up his rider again behind the motorbike, while the other teams pass above them and gain lap by lap.
There are plenty of clean and unclean tactics in this kind of racing and it can be really fun to watch, when two or three teams next to each other drift through the steep bends of a stayer velodrome at car speed accompanied by the ever-rising blobbing sounds of the motorbikes and a cheering chrowd.
It would be real fun to revive this discispline.