Comparision of Mamod/MSS/PPS Reverser Valves

[Lner fan Sam]

When I was talking to James about britomart I was impressed by it smooth running even backwards. Perhaps it one of those things that can be delt with just running in. James also mentioned that the loco has been ran backwards more than forwards over its career.

[DolwyddelanLightRail]

. James also mentioned that the loco has been ran backwards more than forwards over its career.

Yeah, that's cause the cab end buffer fell off and I never got round to sticking it back on for a while

In all seriousness though, Brit isn't a role model loco to bounce this theory off, even though it's still the original reverser, I did notice when it used to be the regulator that it ran better in reverse. Since having a cab mounted reg I haven't actually noticed a difference apart from the rather apparent cylinder cock impression it does in reverse.

[Chris Cairns]

That's the point, we are talking about a problem that affected a standard Mamod SL with no real modifications.

I think it may be more to do with the design of the cylinders and their mounting & timing. They have a central pivot bolt and the steam/exhaust ports in the cylinder are symmetrical, yet have they allowed for the effect that the reduced volume the piston rod will cause on one side of the piston. The cylinders are also mounted at an angle and drive the front wheels.

And is the weight distribution of the loco another factor (front heavy)?

But how many people still run an unmodified Mamod SL/MSS these days?

Chris Cairns.

[Narrow Minded]

Running "better in reverse" seems to be a general observation with Mamods - now, this might be completely crackpot but, since a Mamod in basic trim is inherently front heavy could this be a simple law of physics in operation?

"The force applied to move a body depends upon its effective weight."

("Pulling" the effective weight = mg-fsinx and "Pushing" the effective weight = mg+fsinx)

[Chris Cairns]

It will be found that the loco’s weight is concentrated at the front end, where a good deal of brass is used. This can cause the rear end to lift when starting a train cab first. Preference for running backwards is a fault of the Mamod loco, which hopefully can be corrected by following the previous steps.

The only previous step I can see that would affect the reverser valve is chamfering the exhaust hole from 2.5mm to 2.8mm.

The loco can be balanced by adding lead weights under the cab floor. About 160-180g is required. If the lead is cast into two blocks 80-90g each, and a suitable shape, it should be possible to add the blocks outside the frame, and disguise them as cab steps.

Adding a meths or gas burner, a driver figure or a bunker would have a similar effect.

It is an interesting discussion which has never been satisfactorily explained over the years because most owners upgrade their Mamods, and so the reverser valve is never used to the fully open position again.

Chris Cairns.

[Lner fan Sam]

Locos that have been fitted with a meths burner or a gas set up should be more balanced than a straight up un-moded loco. Also the upgrade boilers would have more weight to the rear of the loco with the sight glass adding to maker weight to the rear.

[laurence703]

I have noticed a considerable difference with my Mamod through the many various add ons... most noticeable difference was the first time I used the Meths burner, water top up and uprated SV... I think I added a regulator at the same time... Anyway it became much more powerful and seemed much happier on the rails. This was also the same time that the distinctive wobble was noticed (now rectified). Even when I put one of Gremlin's twin tank burners on, it seemed to weigh more evenly.

The bigger boilers don't really add much to the equation as it is slightly heavier but its still sat in the same place as the original so the centre of gravity doesn't change much at all.

[DolwyddelanLightRail]

Running "better in reverse" seems to be a general observation with Mamods - now, this might be completely crackpot but, since a Mamod in basic trim is inherently front heavy could this be a simple law of physics in operation?

"The force applied to move a body depends upon its effective weight."

("Pulling" the effective weight = mg-fsinx and "Pushing" the effective weight = mg+fsinx)

Interesting...! Another thing that I've thought of (again, another stab in the dark) is if a full boiler wouldn't affect the reverse running compared to a nearly empty one?

[Lner fan Sam]

Could do as the weight would be more with a nearly full boiler compared to a nearly empty one.

[DolwyddelanLightRail]

Also a full boiler I should think would even out the weight distribution a tad more as most of that extra weight would be between the wheels

[Lner fan Sam]

What is the adhesion like on mamod loco?

[dougrail]

Hmm. On Swordbreaker v1, I observed it liked reversing better than forward...!!!! [fresh from building, Jan 2012 onwards]

This was with DS supercyls and a PPS reverser though. O.o

Sadly I cannot see if I can replicate this as I am now on SB v2.2 with the new cylinders, of which I've found near balanced performance i rev and forward - and forward has provided a couple of excellent performances sofar.

The full boiler would act as a weight - but maybe it's the distance the steam has to travel through the system from boiler to being exhautsed-out?? [if that makes sense]

[laurence703]

Adhesion depends on how much added extra's you've put on it also what mods its got and the usual rail conditions, weather and what you've got behind it...
For their size they can shift a fair load though

[dougrail]

Having got bored, and a little frustrated with the current patient [however am due to test new springs....claws crossed...] I took a good look at the actual construction of a Mamod/MSS rotor and block.

For one thing, there's a reason why the slots are one side rounded and one side 'pointed'...and that pointed set of ends is on the left side - where the steampipe from the dome is.

If you try and round those pointed ends out as I had mistakenly, naively in my defence, suggested, it will allow steam to enter the rotor regardless of what position the rotor is in. Cleverly, the PPS one's steam-channel, although rounded, are shorter in length, allowing 'shut off'.

Just a heads up.

[Chris Cairns]

The original Mamod reverser/regulator valve was actually thought out quite well with the gradual opening of the straight sided slots, but the overall width of the slot was too big (particularly when operating above the original design pressure of 1 Bar), and of course it was cheaply (and thus poorly) constructed.

The PPS (now RWM) one would have never been fitted to a standard Mamod SL operating at 1 Bar, so the smaller width of the slots allows for the increase in pressure that the modified Mamod SL would have been operating at. Whilst the slots are smaller in length than the Mamod one the 4 holes in the PPS block have been chamfered.

We still have had no further contributions with experience of using a PPS/RWM reverser/regulator valve in an unmodified Mamod SL/MSS without a cab mounted regulator.

Chris Cairns.

[dougrail]

I could do a bit of temporary surgery...?

[Chris Cairns]

Do you still have the bits for an unmodified Mamod SL (i.e. the old 1 bar safety valve, not the later 25 or 40 PSI ones, and standard pipework - no cab mounted regulators)?

I previously tried it in my Mamod SL3 so I do not think it will be any different in my MSS kit built loco.

Chris Cairns.

[dougrail]

I have a Mamod loco which is boiler > steam dome > rev block for the steam, original cylinders and original brass boiler with sight glass fitting. Alas all I have is 25psi sv's.