Le Corbusier wrote:Enigma wrote:Regarding weight = pull, didn't a Class 37 diesel win the Deputy Chairman's Cup once (including the 'pull test') and weighed practically nothing? It was quite a few years ago so can't remember all the details. Owned by Robin (?! - High Peak layout?).
Thinking about this might this not be (like I think on the prototype) something to do with wheel size and number. I always understood that the more wheels and the smaller these wheels were the greater the traction? Hence the 9fs. I always understood that the greater performance of diesel and electric had as much to do with the gearing allowing much smaller wheels to be used and the drive allowing more of them to be powered than on a steam engine? If so, the class 37 should have more pulling power without the need for excessive weight? Could be wrong of course both in my understanding and because such things don't scale down.
Sorry guys, but weight on the drivers is the prime determinant of the pulling power of any loco. The way it is distributed across multiple drivers will have an effect on how efficiently a loco will use its weight (I.e. locos with the same weight can behave differently) but only within a limits which is set by the only other relevant factor, the coefficient of friction between wheel and rail, (hence traction tyres of some early very light plastic RTR offerings).
On the prototype the weight on each wheel was strictly limited by the civil engineers who build the track. So as trains and loco's got heavier they developed more driving axles so they could apply more adhesive weight to the job. As weight and material strength really don't scale, we don't have the same problem and can load axles out of all proportion. So a single driver, with a well balanced depleted uranium boiler would be capable of pulling really remarkable loads.
To get max aheasion you want an even weight distribution across all the driver axles, not always easy to achieve on a steam loco's rigid chassis, although in a model CSBs make it a lot easier, (Ok Andy, as will equalization and well designed compensation). On modern bogie mounted locos it is very very much easier to get equal weight distribution.
As for the the deputy chairman cup, as I remember this, they were interested in how efficiently a loco used its weight, so the load pulling award went to the loco that used its weight most efficiently (pulled more grams of train per gram of loco weight), not the loco that pulled the most. So a light loco could win over a heavy one even though it couldn't shift as much, and as explained above a modern image bogied loco had distinct advantages.