Tractive Effort......in practical terms.

 

I have searched the forums here for an existing thread on Tractive Effort, and other than some mentions in other threads have not found one on this specific subject.

 

The 4mm scale, OO gauge diorama that I'm building necessitates gradients down to and UP from a lower storage level - or else I wouldn't be writing this. As a corollary of deciding what maximum gradient to allow, it follows that some sort of assessment of what it takes to pull a train - and what the locos are capable of - naturally follows, to ensure that I don't create a massive problem for myself.....

 

I should say from the outset, that this is purely a practical, empirical exercise - so no formulae, calculators or slide rules allowed....! This is not an engineering dissertation - it's simply a practical exercise. I am starting - no pun intended, from the premise that no run-up is allowed, and that a start from stationary must be made - either on the level - or at two pre-set gradients. Having picked-over whatever I can find on the internet, two approximate benchmark gradients seemed to stand-out. Firstly, that serious problems started to occur at steeper than 1:50/2%, and that few problems seemed to occur at 1:100/1% or shallower. Whether this assumption is any good will be put to the test in a later series of tests. Thus the Loco Tests will be at;-

 

1) LEVEL.

2) 1:50.

3) 1:100.

 

For simplicity the track will be standard Peco Code 100 (As that's what will be laid on the hidden gradients anyway.). The track will be cleaned after every test. Tests indoors at room temperature.

 

Bearing in mind that the aim is to test what the locos can START - only the maximum drawbar force is being measured. In practice, all the locos ended-up slipping - so this is ignored. Only the MAXIMUM force is relevant. Interestingly, the maximimum TE recorded is always AFTER the loco begins to slip, though the difference between just before and after it starts to slip is small. This may be at variance with 1:1 loco performance - but I'm merely reporting what happens in this model test scenario.

As a matter of interest - all tests were carried-out with the locos running FORWARDS. A comparison between the loco's PUSHING & PULLING was made - and the results were identical. (The Test Gauge will measure both tension and compression.).

 

A Digital Force Gauge was used, with a range of 0 - 20N. (2.039432426Kg). In practice, thus far, the sample batch of locos have struggled to get to 1.00N, so for the sake of simplicity, all results will be given in Newtons. A simple free online converter can be found here ;- 

https://www.unitconverters.net/force/newtons-to-kg.htm

 

Bearing in mind the simple, empirical nature of these tests, it's worth underlining it's simply not relevant to start discussing the power of the locos - it matters not. In fact, there are a host of variables aside from the power of the motor, such as number of wheels, type of metal the wheels are made from, weight etc etc. All these peripheral factors are ignored - only what is delivered to the drawbar matters.

 

The plan is to carry out three series of tests ;-

 

1) Test some locos.

2) Test some sample scale-length trains - to see what it takes to start them, as per the locos on the Level, 1:50 & 1:100.

3) Test to confirm whether the T.E. of locos in (1) is actually enough to start trains of that rolling resistance (2) or less. (I'm                deliberately ignoring the fact that weigh, is technically separate, as it adds to the rolling resistance - it's just simpler.).

4) Test the efficacy of adding weight to locos.

 

This won't all happen at once, but from time to time, as I get the time.

 

I have a lot of locos and they are a mixture of kitbuilt, scratchbuilt and RTR. In the case of RTR - assume they are standard unless any mod's are specifically mentioned.

Below is an initial test of a totally random box of locos. The results give a taste of what can be expected as more locos are tested. Of course, individual examples of a model will vary, but even in this small sample, a pattern is starting to emerge, and, not surprisingly, weight is a noticeable factor. The range here is from 0.35Nm up to a nice round 1.00 Nm. My guess is that, the likes of Garratts aside, few UK steam-profile locos are going to have a drawbar force above much above 1.00Nm.

(Older diesel-profile models may not be much better, but obviously, more modern examples have both bogies powered, so would give much better results - but as I have none - it's a moot point.)

 

Kernow GWR SRM   *                               4-4         RTR.      0.22Nm.

MR 4P Compound.                                   4-4-0.     SB**.    0.35Nm.

Jin brass GWR4500. (Compensated.)    2-6-2T    RTR.      0.37Nm.

Bachmann GWR 4575.                            2-6-2T    RTR.      0.44Nm.

SECR Wainright C-Class.                          0-6-0.     SB**.    0.51Nm.

Bachmann N-Class.                                  2-6-0.     RTR.      0.58Nm.

Brass LNWR G2                                         0-8-0.     Kit.        0.70Nm.

Hornby BR 8P                                            4-6-2.     RTR.      0.75Nm.

Hornby West Country                              4-6-2.     RTR.      0.77Nm.

Bachmann LNWR G2A                             0-8-0      RTR.      0.86Nm.

H'Dublo 8F.                                               2-8-0.      RTR.      0.88Nm.

H'Dublo Coronation 7P/8P                     4-6-2.      RTR.     1.00Nm.

 

(**SB=Scratchbuilt.)

 

    Interestingly, The HD 7P/8P Coronation was a bit of a reluctant runner, but it came-out well ahead. Both this and the HD 8F are notably heavy. The centre wheelsets are flangeless, and I'm not even sure they actually touch the rails......so these two might even be operating as 4-4-2's.....

    I will also be adding a column for loco weight - excluding tender. I was tempted to break this down further to ascribe the weight per driven wheel. This is not really practical, however, because many locos have flangeless wheels and hardly any are compensated/sprung, so this would very much muddy the waters and complicate what is intended to be a fairly straightforward survey of what locos can pull - and what is needed to pull realistic length trains.

 

    Finally - remember this is really all just empirical observation - not science. Theoretical formulas have no place here - only actual results.... I shall return to this when I have got some more results

 

* PS;- I have added the newly-arrived Kernow GWR Steam Railmotor just out of interest. However, mine is a DCC version running on DC for the test. In this mode, it is pre-programmed with s soft-start/weigh & inertia simulation. When it started to slip, it kept 're-booting', so the small 0.22Nm may not  quite be it's best performance. I got the impession that it needed more weight over the power-bogie, but since these models are not really likely to have to pull much - it won't be a problem for most users. In my case though, again, the gradients will be a factor.

 

PPS ;- I still havent found my digital scales, but will add all the weights as soon as I do.