I've had a few comments about a posting on this subject by me that appeared on the L1 thread. I thought that it was worth expanding on this but in a more appropriate place. There is a belief that 3 cylinder locos are better at starting than 2 cylinder ones as they cannot 'stick on centres'. The reality is different and I've produced some tractive effort curves for 2 and 3 cylinder locos to try and explain it. I have been unable to find examples of locos that were otherwise identical; except in the number of cylinders so, to overcome this I have used a Black 5 for the 2 cylinder example and created a virtual 3 cylinder Black 5. This 3 cylinder version has exactly the same nominal tractive effort (25,455 lb) and cylinder layout. The cylinder diameter has been reduced to 15.1" and the piston rod diameter to 2.25" to compensate for having 3 cylinders and the cranks are at 120° but everything else is the same, including con rod length. Diagrams 1 & 2 show the two tractive effort curves for one revolution of the driving wheels for both types. The effort for each cylinder is shown, together with the combined total tractive effort. The curves are not symmetrical because of angularity and the effect of the piston rod on the return stroke. An allowance for expansion of the steam beyond 75% has also been made. It will be seen that the peak TE of the 2 cyl loco is just less than 33000 lb (compared with the nominal TE of 25455 lb) and that of the 3 cyl loco just underr 30,000 lb. Both locos exhibit a tractive effort curve that is above the nominal for the majority of a wheel revolution. The 3 cyl loco has a much smoother tractive effort curve. So far, so good, there is nothing revoluntionary in this. However, if we now look at the tractive effort available at the moment of starting, a different picture emerges. A cylinder will only receive steam if the appropriate port is uncovered and I have based this on 75% cut off so, for one quarter of its stroke, any one cylinder will contribute nothing to providing starting force. The variation in tractive effort of the 2 cylinder loco is quite large, depending on theactual wheel position but the loco will still produce a peak tractive effort of about 33000 lb. However, the maximum starting tractive effort of the 3 cyl loco is only just about 28000 lb for three short peaks and, for much of the wheel revolution, it is below even the nominal tractive effort. The 3 cyl loco only starts to produce a reasonable tractive effort once it gets its train on the move and is admitting steam to all three cylinders. What I haven't done in these diagrams is include for lead steam. This makes the situation even worse. The lead steam on a Black 5 starts to be admitted at about 93% of stroke and this will oppose any tractive force applied. Obviously, with 3 cylinders, there are six places in each wheel revolution where this has an effect. (I haven't included for this as I haven't worked out how to factor it in to the spreadsheet!) Harry Holcroft certainly appreciated this problem with 3 cylinder locos, realised when investigating why Schools class locos struggled to start trains that their lesser and less powerful 2 cylinder cousins could start with ease. Gresley probably didn't because his 3 cylinder locos were originally limited to 65% cut off as he thought that long cut offs was unnecessary with a 3 cylinder loco and this makes starting even worse. It's only post war that they were modified to give a longer available cut off.