Very interesting! This gets into the same problem that street cars (trams) encountered in this country. It was desired to extend the wheelbase of two axle units to give less fore/aft pitching, and also to accommodate more passengers. Most cities had curves as sharp as 36 foot radius which meant that the was a limit of about 8 ft wheelbase for two axle cars. There were several ideas tried- one the Robinson Radial truck (bogie) a three axle unit which used the offset of the middle axle on a curve to steer the two end axles radially. Robinson's advertisements showed how the axles conformed to the curve perfectly--but they fought each other when passing from tangent to curved track. and vise-versa.
Another try was made by the Brill company which stayed with two axles, that could displace radially, but in doing so lifted the car body slightly because the axles moved up a slight ramp when they were other than parallel- thus the weight of the car tended to keep them parallel.
This was the crux of the problem-to allow the axles free enough movement to conform on a curve, and yet try to keep them from "hunting" on the straight.
At one point in time, my sons and I built a 7 1/4" gauge railroad behind the house. One curve was particularly sharp, out of necessity. A "diesel" very much like the current Ingersoll-Rand had no problems with this, pulling 40 (scale) foot cars. I got the itch to build a model of a Brill railbus- a gasoline powered unit with a bogie in the front, and a single fixed axle in the rear. I built a powered "mock-up" to see if there was a problem with this configuration, and was surprised, and pleased, to find none. When I built the chassis for the model I noticed that I had made the mock-up wheelbase one (real) inch too short, which I corrected, and the chassis would derail on the outside of the curve every time- the rear wheel would climb the outside rail. I cured the problem by making the front bogie steer the axle through linkage.
I wonder how Hornby has tried to keep the axles parallel? Seems like all there is is a pivot.
____________________ Fix it again, Mr Gates--it still works!"
Herb-my little Model T railcar had the same problem. It has a 2 axle lead truck and single rear driver which tended to derail to the outside of sharp curves (all there is on the MRy). The "solution" was to make the rear driver pivot slightly and that cured the problem. The driveshaft is similar to a Shay or Climax which has sliding box joints and universals. Now the supt. can inspect at will without jumping the track and racing (at 5MPH) across the scenery. Woodie
____________________ It doesn't matter if you win or lose, its' how you rig the game.
ebtm3 wrote: This was the crux of the problem-to allow the axles free enough movement to conform on a curve, and yet try to keep them from "hunting" on the straight.
All of this stuff is of great interest to me, for a couple of reasons.
I've mentioned about the Hornby model of a Class 142 "Pacer" railbus.
What I forgot to mention was that, in some parts of the UK, the real things became known as "skippers" or "nodding donkeys" - these names are actually far too polite, as they are notorious for poor ride quality.
This isn't helped by the fact that they run in pairs - so, even if one of them actually wants to run properly, it gets pulled out of true by its partner in crime.
I don't know who had the "brainwave" of putting pairs of hacked-about Leyland National bus bodies onto rough 4 wheel chassis - and thrashing the resulting contraptions along the same tracks as proper trains. All I do know is that I wish they'd keep their "bright ideas" to themselves in future.
Class 142s are also notorious because their wheels start squealing loudly the moment the things go anywhere near a curve. This is unfortunate - as they are often used in hilly areas, with lots of tight curves. They might look pretty, but you can hear them coming!
In the former East Germany, I believe that some 4 wheel railbuses acquired a nickname which roughly translates to "piglet taxi" - while their "modern" British equivalents just seem to squeal like pigs. I've been told this is called progress.
In view of my obvious dislike for Class 142s, you might wonder why I'm so keen to build a model of a Wismar railbus. Well, I'm not planning on (ab)using it in the same way as the real 142s. I'd just like to see 1 railcar trundling along sedately every now and then - maybe even just sat quietly somewhere - so I can look at it and think: "I built that". Strange perhaps, but I just like the way they look.
Personal opinions aside, the real reason this stuff interests me is that I'd like to avoid the problems that have been flagged in this thread - but I don't have space for large radius curves. I think I might be working on "test hacks" before I even start on a model. I'm glad I enjoy technical challenges.
The story continues... A fellow modeller asked me for a model of the railcar, and I felt like having another, different one for me too. Walls from teakwood maybe? Tempting... So I made the drawings and send them to my lasercutter. This is the result so far:
inner body with interior
side view, second cab still missing
The assembling was quite difficult: The birch plywood warped during the staining - although I treated both side - and there was no chance to hide a bodge or fill a gap with putty. So I reinforced the cabs with sturdy frames from .060 plywood and the sidewalls were stabilized by the inner body. The mitres were hand filed and adjusted to each other. Some edges need a bit of staining, but I'm pleased with the result so far. Now comes glazing, wiring, illuminating, the housings for the headlines will be made from styrene tube.