Tuesday, 25 February 2014

Meridian MPD18 Chassis Mk2

I mentioned back in November that I'd acquired two of the Meridian MPD18 chassis kits, but despite only having bought them a month apart, one was for the Mk1 design and the other the Mk2 design. I built the Mk1 already so I thought I'd build the Mk2 and see how it compared.

There are some important differences in the design:
1) Top-hat type bushes are supplied for the axles
2) The axles are fitted into the chassis from below after assembly with bushes and gears
3) The layshaft is assembled in it's own carrier and fitted to the chassis from above
4) The pick-ups are suggested to be fitted to the upper side of the chassis, as I did for the Mk1 but not as the Mk1 instructions suggested.
5) The brackets for fitting to the Meridian body kits that use this chassis are changed, and look much improved.

So here is the Mk2 assembled. No soldering is required except for the pick-up wires, and to attach the nut to the body fitting bracket (not shown), and can be put together in a couple of evenings' work. I did deviate from the instructions slightly:
- I used phosphor-bronze strip rather than the wire provided for pick-ups as I think it is more reliable with less drag
- I was concerned that the motor was not secure enough relying on the strap alone (there is no locating end-plate as on the Mk1) so used a little Bostik glue (the rubbery stuff) under the rear end.

Side by side (Mk2 on right) the obvious difference is the size of the motor, the newer one is small making the chassis narrower, though it is mounted higher so they are about the same height overall. The wheels are the same, I think they would look better and probably pick-up power better if they were of a larger diameter. A shorter wheelbase (or variable options) might give the chassis a wider appeal for other prototypes.

From the drive end (Mk2 on the left) it can be seen there is no motor locating plate, the newer design relies on the motor fitting into the chassis frames and the strap holding it secure. I think a locating plate as for the Mk1 design would be a good idea and the strap alone didn't seem to hold the motor securely enough.

From below the removable axles with bushes of the Mk2 (right) can be seen, the keeper plate is held in place by the bolt that secures the chassis to the body. As well as providing a better wear surface for the moving parts this design is a vast improvement in terms of assembly, as the worm-gears and the adaptor tubes (to go from 1.5mm dia axle to 2mm dia gear) can be super-glued in place before the axle is fitted to the chassis. The Mk1 chassis required holes to be drilled in the brass tube to let the super-glue in, then the tube and gear have to be glued to the axle as it is passed through the chassis - difficult to do, with a high risk of gluing the axle solid!

So overall the Mk2 version of this chassis is much easier to assemble and should be more durable and easier to service in the future.


Paul B. said...

Ah, so thats how they differ! The Mk2 does seem to be much improved, I shall have to find an excuse to build one.
Thanks for posting this.

Christian said...

I can only agree, it looks far superior.

I tried to build one of these as a 9mm gauge chassis for a KB Scale Ruston a while back, and managed to botch it up. That slimmer motor and neater chassis looks far batter.

I agree about other chassis length options - as I remember there's not much at the non-motor end, so a few extra holes would mean you cut cut off the end of the chassis for super short locos, or just leave the extra length for stability on larger ones.