Garden Railway Forum

Chris Cairns wrote: ↑Mon Mar 18, 2019 5:35 pm One thing I was not aware of until a discussion over on Facebook last year is that the Bertie has a high centre of gravity with more weight on the rear axle thus can be prone to centrifugal force rollovers on tight curves - mine has done it twice now.

I've had a couple of derailments but thankfully no rollovers!

Keith S wrote: ↑Mon Mar 18, 2019 3:30 pm Since Roundhouse engines don't have a steam dome, the only place for steam to accumulate is along the top of the boiler where you've left some space.

So does that make my Mamod SL3 more sophisticated than the Bertie in that respect?

cncmodeller wrote: ↑Sat Mar 23, 2019 12:45 pm I'm not a great fan of Gas fired locos

I've definitely got a soft spot for my ancient meths-fired Mamod SL3. No "roar" and that smell...

EDIT: corrected the type of Mamod to what I actually have...

CSL wrote: ↑Wed Mar 27, 2019 10:18 pm

Chris Cairns wrote: ↑Mon Mar 18, 2019 5:35 pm One thing I was not aware of until a discussion over on Facebook last year is that the Bertie has a high centre of gravity with more weight on the rear axle thus can be prone to centrifugal force rollovers on tight curves - mine has done it twice now.

I've had a couple of derailments but thankfully no rollovers!

The weight distribution issue (i.e. too much weight on the rear axle) is why the DHR B class locos have their little saddle tanks - or to put it more poetically - "why the Dachshund has its rucksack!" Apparently, even with the forward wing tank they were a bit bottom end heavy and would occasionally waddle off the track... and there are plenty of places on the Darj where you don't want to do that!

cncmodeller wrote: ↑Sat Mar 23, 2019 12:45 pm I'm not a great fan of Gas fired locos

I've definitely got a soft spot for my ancient meths-fired Mamod SL1. No "roar" and that smell...

I do not mind gas firing, but I do prefer pot boilers to internally fired locos. The behaviour of the burner seems a little more stable to me, and they usually have a longer run time if the fuel supply is adequate.

Just my 2 rupees worth!

Peter in Va

IrishPeter wrote: ↑Thu Mar 28, 2019 3:20 am The weight distribution issue (i.e. too much weight on the rear axle) is why the DHR B class locos have their little saddle tanks - or to put it more poetically - "why the Dachshund has its rucksack!"

Does that mean that the saddle tanks must always be full?

To bring this diversion back on topic, that would surely reduce the B class's run time compared to what you might think!

The saddle tank in front of the dome is the first to empty, but it still helps even out the weight distribution on the full sized the loco given that the tank itself weighs something. It also has to be kept in mind the coal bunker is being emptied at the same time, so weigh is disappearing both sides of the dome. However, the really big help with adhesion on the Darj is the swinging link coupling, as opposed to a conventional chain or hook, which eliminates a good deal of coupling snatch, and also allows the loco to swing in to the bends more easily. Even without the two guys on the front, the B class are unusually sure footed, which with gradients as steep as 1 in 18.6, and sustained banks of 1 in 23 they need to be!

I don't know how the weight distribution is on the model, but one usually has the option of finding somewhere to stuff some extra weight if needed.

Peter in VA

CSL wrote: ↑Wed Mar 27, 2019 10:18 pm

Keith S wrote: ↑Mon Mar 18, 2019 3:30 pm Since Roundhouse engines don't have a steam dome, the only place for steam to accumulate is along the top of the boiler where you've left some space.

So does that make my Mamod SL3 more sophisticated than the Bertie in that respect?

Yes, it does, if by "sophisticated" you mean "like the big engines". The Mamod design, having a steam pipe protruding into a discrete space for the accumulation of steam is quite elegant and I've always admired it. However, it leaves little provision for a proper regulator without a great deal of added sophistication, which is why other engine designers in this size range do away with it.

"Sophistication" is in the eye of the beholder. A fully "sophisticated" model steam engine would include an internally fired boiler with a draught provided by blower and exhaust, steam accumulation in a proper dome with a regulator valve incorporated into the dome, either externally or through the backhead, and superheater pipes in the smokebox. Many models incorporate one or two of these features, but most of them are either unnecessary or expensive to fit, or they are fiddly and irritating.

Keith S wrote: ↑Mon Mar 18, 2019 3:30 pmSince Roundhouse engines don't have a steam dome, the only place for steam to accumulate is along the top of the boiler where you've left some space.

Without exception, I believe, all Roundhouse locos have a turret on to which the regulator is mounted. The turret performs the same task as a dome, i.e. keeping the steam take-off above the boiler water level.

Some smaller locos (Wrightscale Wren, ELR Quarry Hunslet, for example) have the regulator plugged straight in to the boiler backhead, which can create some lively performance characteristics if the boiler is over-full.

TonyW wrote: ↑Fri Mar 29, 2019 9:04 am

Keith S wrote: ↑Mon Mar 18, 2019 3:30 pmSince Roundhouse engines don't have a steam dome, the only place for steam to accumulate is along the top of the boiler where you've left some space.

Without exception, I believe, all Roundhouse locos have a turret on to which the regulator is mounted. The turret performs the same task as a dome, i.e. keeping the steam take-off above the boiler water level.

Some smaller locos (Wrightscale Wren, ELR Quarry Hunslet, for example) have the regulator plugged straight in to the boiler backhead, which can create some lively performance characteristics if the boiler is over-full.

You're right, I guess the turret is a steam dome isn't it? Well, it's not a very big one anyway.

A couple of runs with the Bertie over the weekend and it's nothing if not consistent! The stopwatch showed 9 minutes 45 seconds when it cleared that difficult curve for the first time neither too fast nor too slow, and carried on smoothly and without stopping for 17 minutes before the gas ran out and - with encouragement - made a further short run before coming to a halt.

An interesting inconsistency (with the previous consistency) today.
The Bertie ran smoothly round and round my little circuit without stopping - if at varying speed - virtually from the first turn of its wheels, instead of the usual first ten minutes of start-stop before steady running is established.
Run time a few minutes (at least) shorter than usual: 23 minutes. Which is entirely consistent with the excessive (to my eyes) blowing off from the safety valve for about the first half of the run. And when I adjusted the gas regulator down, the stop-start set in intermittently on the principal bends.
So it seems to be a trade-off between steady running and long running, with the point on the scale determined by the size of the plume from the safety valve.

I was able to complete the full tour of our large club layout, 7 loops, with a few extra & a couple derailments on the less used loop, & back to the steaming bay in 24 minutes with my Lady Anne yesterday. The gas ran out at 27 minutes. There was a bit of blowing off at times so likely still room to extend the run a bit.