My recently produced model above, happens to be custom run from Berwyn’s Toys & Trains but is mechanically identical to other recent MTH Premier Mikados. It had a problem derailing its front drivers at a perfectly level Ross O72 switch, on its curved leg which is in the middle of an O72 180 degree curve. No dips/humps or anything that would account for it in the track. It also had occasional pilot truck derailments, so I did not try softening that spring as a way to increase front driver weight; actually had to shim the pilot truck spring with a single flat washer to cure that.
Diagnosis revealed a few things that contribute to driver derailments: Center of gravity is behind the third driver axle, and the blind drivers have equal diameter with flanged so all drivers ride on the rails. The 8-coupled driver wheelbase is long, making a sharper attack angle for the front drivers against the outside rail of a curve. And the short driver diameter places the pull of the drawbar about 3/8” (guesstimate) higher than the driver axles, so pulling hard on a train places a lifting leverage at the fulcrum of the rear drive axle with its traction tires, taking additional weight off the front drivers. On any hump, all of that places all driver weight on the 3rd and 4th driver sets, with combined effects taking the front drivers off the railhead.
Lou, the MTH repair person for Berwyn’s, confirms that a few others have had similar problems on their layouts, unique to each, hard to duplicate, and hard to pin down why more people do not have the problem with these Mikados. Unweighted, or almost unweighted even on level track, worse when pulling, with the long driver wheelbase and relatively sharp “attack angle” of those front driver flanges against the outside rail of a relatively tight curve (even O72, at least on a switch, in my case), the outside front driver climbs over the railhead often enough to not be acceptable behavior.
I don’t use smoke. So, I separated boiler from frame and removed smokebox front, and managed to wiggle the smoke unit out by first removing screws attaching it to its bracket (accessed screw heads through open front of smokebox) and wiggling bracket out of bottom of boiler past the smoke unit body, and then the smoke unit. Electrical connectors for cable between boiler and frame, and the front-most 4-wire connector on the board in the boiler, were disconnected to make room to get smoke unit out past the front of that board. Two connectors for the smoke unit were taped over.
I had a 16 oz fishing weight of the drop type with a loop in its top that is slightly smaller than the inside diameter of the boiler. Cut to length for the available room it made a 12 oz slug giving a net increase of 10 oz after removing the 2 oz smoke unit. Using threaded hole that was smoke unit bracket attachment point as a clearance hole, fixed weight in place with a 4-40 machine screw threaded into the lead. This moved the COG to in front of the 3rd driver axle, not quite centered but definitely placing weight on the middle two axles now.
After shimming the pilot truck spring with a single flat washer, the loco can now pull a 7 car train of Atlas and MTH 60 foot heavyweight passenger cars, through curves and that O72 switch, which is after a long O72 S-curve so it’s pulling hard on the drawbar (unweighting front drivers a bit in that process). So far in multiple circuits of the mainline, no more derailments. Lou at Berwyn’s was not surprised to hear of the problem, and might have taken the same approach if softening the pilot spring had not solved it (not a good choice for my loco, as noted above.). The long driver wheelbase (attack angle issue), all drivers riding on the rail unlike many other blind driver locomotives, small diameter drivers giving drawbar leverage when pulling, and rear-heavy weight distribution have combined to cause trouble for a few folks like me.
Will be interested to learn about the experience of others with these Mikados.
Don
