Originally Posted by Burlington Route:
Lee, what's wrong with the other semis? Maybe adding a tad of weight over the drive axles...
Are the trailers detachable?....
I suppose it depends on what you mean by "wrong." I don't and won't have pictures today and they wouldn't help a lot, but this explains it:
The first 18 wheelers I made took an obvious approach: I used shortened but otherwise stock 'Streets chassis as the tractor. I added a second unpowered dummy axle, that could swing side to side in curves, trailing just behind the stock drive axle to simulate the tractor's third axle, then made a dual-axle swiveling "truck" underneath the trailer. With the shortened wheelbase and swiveling axles, these had the geometry to stay on D21 and D16 curves even though they were lengthy. Two problems: first, with only one drive axle they had limited traction: they would go through flat, smooth D21 curves well enough if they had a running start but slowed and spun their wheels badly on D16. They didn't like grades at all. Second, like all stock 'Streets vehicles, they would not really run slow, smoothly: dragging all the weight of the trailer exacerbated this problem. Putting a lot of weight in the tractor (filling it with lead shot) and making the trailer as light as I could (had to have some weight to keep its flanges on the rails) helped, but not much. Adding electrical pickups to all wheels and center pickups under the tractor's axles helped a lot with smoothness and lack of stalling, however.
Just after this, I build my series of city buses (corgi bodies shortened a bit): custom chassis, monster motor (full size flywheel can motor as used in big diesels) and a 3:1 or 4:1 gearbox to reduce speed. These were entirely successful in meeting my needs: the three I have run very smoothly and steadily and never stall, they go very slowly (scale 3 mph if set just right) and climb up to 12% grades. One of them has over 600 hours of running at 10-12 mph back and forth around my downtown, with no problems.
Generation 2 of my 18 wheelers used this same drive approach as in the buses - sort of. I turned the monster can motor vertically and put it just over the tractor's rear axle set, sticking straight up, with 3:1 reduction gears, powering both rear axles of the tractor. The motor stuck upwards above the height of the top of the cab, but then the box trailer fit down over it and hid it. They looked good. Electrical contacts through all wheels and a center pickup under each axle. Performance was disappointing given all the work. They had more traction but still not enough, and ly ran a bit better - acceptable of flat D21 curves. And there was this: with the motor vertical, the tractor had a high center of gravity, and tended to lean a lot in corners, even when weighted, etc. and they did not like to pull up hills.
The third generation 18 wheeler, the one in the pictures above, has a big can motor, 4:1 reduction gearing, but located in the trailer. the motor is not vertical but turned horizontal (lengthwise) to lower center of gravity. Only the lead axle of the trailer's two axles is powered, but it has traction tires on both those wheels. This powered axle is fixed (does not swivel) and the axle behind it is just a trailer than can swing from side to side and bears no weight on it (thus hee single rear axle with tractiontires supports all the weight of the trailer's rear part). The tractor is thus an unpowered, three-axle chassis with cab that simply connected to the trailer at a pivot point (as do real tractors). the trailer pushes the tractor.
This arrangement almost works well enough. It will push up 5% grades and negotiate D21 curves well enough (both at the same time) with just a trace of wheelspin. But I still have some experimentation to do. The tractor in the pictures above is built so its dual rear axle set swivels in curves. This seemed like a good idea, but it seems to run better when I pin them so they can't pivot. I've got to play with that some more to understand why and what of that. Also, the motor, though horizontal, is too high and the trailer leans in curves too much: I think I can lower the motor more and get the center of gravity down even lower. With those changes, and some experimenting with weight added or not to trailer and tractor, I'm fairly confident the next generation will smoothly and dependably for hours at a time.
