B&M Train Steering

Tuesday, October 27, 2009 10:14 PM

I'm mulling over some train questions in my head so I figured I'd just lay them out and see if anyone could answer for me.

1. I seem to remember B&M trains with tie rods... I understand why tie rods are necessary, but I also see how that can increase the need for near perfect track gauge (within the travel of the spring pads on the guide wheels.) My question is, do the trains without tie rods steer (on the outside rail) exclusivly by the spring load on the rear guide wheel?

2. If the answer to the above is yes, why do any B&M trains have tie rods, and what makes the inverteds not need them?

3. Could a true trailered train function properly with fixed wheels (one road, one guide, and one upstop)?

I think the answers to questions 1 and 3 are yes and kind-of, respectively, but i'm not totally sure as I have zero experience in dealing with this.



Tuesday, October 27, 2009 11:26 PM

Griffon is without tie rods as well.

I would think that the more sloppy the track gauge can be, the more you need tie rods to avoid the individually steering wheel sets bouncing all over the place. Wood coasters in particular.

Wednesday, October 28, 2009 12:29 AM

This is a pretty interesting topic. I'm anxiously awaiting Dave's responses. :)

Paging RideMan...

Wednesday, October 28, 2009 7:12 AM

Jeff, cool pic of Griifon's underside!

Wednesday, October 28, 2009 8:41 AM

So shiny..must touch now..


Wednesday, October 28, 2009 8:48 AM

If that pic ain't coaster porn, I don't know what is! :)

Wednesday, October 28, 2009 9:59 AM

Jeff, do wooden coaster cars have wheel assemblies with independent movement? I always thought they were rigidly fixed the chassis.

Wednesday, October 28, 2009 10:27 AM

The 3-bench PTCs (like on Blue Streak) have wheel assemblies that are fixed, that much I know for sure. Dave has always said that PTCs cannot "steer," so I'm pretty sure the same holds true for their 2-bench trains.

Wednesday, October 28, 2009 11:07 AM

Gosh, I can't remember the last time I saw under a PTC train, so I don't remember. I don't think they steer. But you know, when I mentioned wood coasters, I was thinking of these awesome new trains "designed in the modern times." ;)

Wednesday, October 28, 2009 11:30 AM

Actually the back axle of the "newer" articulating PTC trains is able to rotate a few degrees in each direction. They started building these trains around 1990 when they had to rebuild the trailed trains they introduced in 1988. The first brand-new two-bench articulating train was built for Mean Streak (1991), and not much has changed since then.

Last edited by Jeffrey Seifert, Wednesday, October 28, 2009 11:33 AM
Wednesday, October 28, 2009 11:31 AM

I know the Gerstlaurers are all apart of one big metal frame for the entire car, but I don't know what goes on with the Millennium Flyers.

Wednesday, October 28, 2009 12:19 PM

Fun said:
Jeff, do wooden coaster cars have wheel assemblies with independent movement? I always thought they were rigidly fixed the chassis.

I've worked on Cedar Point's Mean Streak all last year, and I can include in that the wheel assemblies are fixed on.

Wednesday, October 28, 2009 12:38 PM

Did you actually work in maintenance on Mean Streak? Because according to PTC the back axle is supposed to move a few degrees on the articulating trains.

Wednesday, October 28, 2009 12:43 PM

I was one of the ride operators.

Wednesday, October 28, 2009 12:54 PM

I worked on mean streak in 2000. Fun ride to work, bad to ride.

Wednesday, October 28, 2009 5:11 PM

Just a few degrees of movement? They might as well be considered fixed, then. And Jeff, I thought you might be referring to the new Gravity Group trains, and maybe Millennium Flyers (don't know if the assemblies are independent, though the way Thunderhead felt, they almost have to be). But I know the 3-bench PTCs better, including from the underside (a few hours at entrance observing the trains as they hit the brake run and you'd have a solid image in your mind as well ;) )

Wednesday, October 28, 2009 7:25 PM

A side note to the original topic, Griffon doesn't even have spring loaded guide wheels, although the upstops are spring loaded (this makes sense). So I guess my original hypothesis is wrong or semi-wrong at least. I don't suppose the concave wheels are enough to steer the wheel assembly although right now that's all I can gather as far as the outside rail is concerned....


Wednesday, October 28, 2009 10:43 PM

The guide wheels do the steering.

Wednesday, October 28, 2009 11:20 PM

Yes, but on the outside rail of a turn (when the guide wheels are outside of the rails), the wheels are going to tend to go straight... in other words, there is no track imposing the forward motion of the guide wheels, forcing them to guide the wheel assembly in the right direction. This would typically be accomplished by the tie rod... ie, the wheel assembly on the inside rail is forced to turn by the track against the guidewheels, and it, in turn, pulls the outside wheel assembly into the correct position. Without a tie rod, there is nothing to direct the guide wheels around the turn.

If there was only a single guide wheel, at this point the wheel assembly would no longer track with the rail and would reach a point where friction would drag it toed out down the rail. Because there are two guide wheels, and the rear guidewheel is behind the pivot point, this is impossible... so what's really steering the outside wheel assembly? Nothing... it's just being held against the track by the train, which is being steered by the inside wheel assembly guide wheels.

I think in trying to explain my question better, I answered it. Although it seems kind of hokey, from an engineering standpoint, it does make the most sense... by not having it do anything, minor fluctuations in the track gauge during a turn won't affect it one way or the other... and it's free to bear it's load without having any unwanted lateral effects on the train... so I guess you could say that the inside rail really steers both wheel assemblies, with or without a tie rod, so long as there are two guide wheels.

Which, in turn, answers my other question... on a trailered train with one set of wheels per side, they must be fixed, with space between the guide rails and upstops and the track... if there's no gap between the wheels and track, they must be free to pitch and yaw, or the train will bind. If you don't use two guidewheels per side, you need tie rods as well.

So I figured most of it out just now... but why do some B&Ms have tie rods and some don't? That's all I don't understand.


Wednesday, October 28, 2009 11:48 PM

Sorry I am late to the party...it's been a busy day ( cat >/dev/null :( if you must know). So let me take us all through a bit of Coaster Steering 101.

First, let me plug my ancient essay on "How Roller Coaster Cars Work (or don't)". There is a lot in there, but it should be noted that when that was written, B&M was new and the first Millennium Flyer train had not been built yet.

Let me start with the original questions, then move on to the subsequent questions. Steve Guilmette asked:

...do the trains without tie rods steer (on the outside rail) exclusivly by the spring load on the rear guide wheel?

The B&M wheel carrier assembly has six wheels on three carriers. I'm pretty sure that all of the wheels are mounted on pivoting assemblies that allow the position of the wheel relative to the track to dynamically adjust; a flexible bumper between the wheels maintains the tension. But for our purposes, this is really unimportant. The spring loading reduces minor bounce but it really isn't important for steering the train. Take away the springs and you'll get a less smooth ride, but tracking should be about the same.

The B&M wheel carrier assembly is mounted on a bearing which can yaw about the Z axis of the bearing...that is, it can steer. The trick is that the point of rotation is at the center point of the road wheel carrier. This means that when the track turns to the outside, the front guide wheel will be pushed outward by the rail. When that happens, the wheel carrier will yaw, causing the rear guide wheel to be pushed *inward*. That's well and good, but what if the track curves to the inside? The track won't make contact with the lead guide wheel, in fact it will pull *away from* the lead guide wheel. That's where the tie rods come in, right? To keep the wheel carrier from pulling away from the track and allowing the train to derail. But look more closely. The distance between the center of the front guide wheel and the rear guide wheel is far enough that when the track pulls away from the lead guide wheel, as the train is pulled laterally (with or without tie rods) the track will actually push out on the rear guide wheel on the inside. So in a nutshell, when the train hits a curve, the axle is steered by two guide wheels: the front guide wheel on the inboard side, and the back guide wheel on the outboard side. Since the wheel carrier is a rigid assembly, the whole thing rotates to follow the rail.

This also explains why the road wheels usually start with a flat surface even though they are running on a curved surface. When the wheel carrier rotates, there is going to be some error, where the road wheel does not run exactly parallel to the rail. But the distance between the front and rear road wheels on each wheel carrier is short enough and the curve is broad enough compared with that distance that the error is relatively slight. You will see a greater error on the Arrow multi-element train axle, which has the guide wheels further apart than on the B&M train...but I am getting ahead of myself here.

If the answer to the above is yes, why do any B&M trains have tie rods, and what makes the inverteds not need them?


Honestly, I am not sure how B&M decided whether to use tie rods or not. It may be instructive to note that the inverted coaster, which has no tie rods, was their first design. I don't have an answer to this one, though it may be related to the specific geometry of the guide wheel assemblies and the maximum or minimum allowable curve radius on a particular ride design. Yes, that's a complicated way of trying to duck out from beneath the green slime.

The old-style PTC junior cars, which have steerable flanged wheels, do not have guide wheels and have only one flanged road wheel on each corner of the train. These trains have tie rods, because the wheel flange is only steered from the outboard side, unlike the modern design that is steered on both sides.

Could a true trailered train function properly with fixed wheels (one road, one guide, and one upstop)?

The answer to this one is an unqualified "yes". The trick is that the hitch point has to be positioned correctly, which is to say, right in the middle of the axle. If you do it that way, the back of the car can sit with its axle completely fixed. The whole axle can yaw, staying centered and parallel to the rail. The hitch point, positioned in the middle of the axle, will rotate as the axle shifts, but most important it will not translate...that is, regardless of the angle of the axle, the hitch will remain in the same position. That means the next car can move with some independence, following the curve as it arrives. The Morgan wood coaster cars work in exactly this way, except that many of those don't have up-stop wheels because they were built for Traver-style track that doesn't have enough clearance for up-stop wheels. This is exactly the configuration used on Colossus, Dragon, Giant Dipper, Giant Dipper, Grizzly (CGA), and the Jack Rabbit (Seabreeze), among others. Those Morgan trains are pretty much crap, but they track pretty well, and they don't tear the track up much. That is also the configuration of the original Prior & Church trains, and it is basically the configuration of the GCI Millennium Flyer. The GCI train actually has a floating wheel carrier that dampens some of the vibration, but from a high-level perspective, it makes about as much difference as the spring assemblies on the B&M train: it makes a difference, but it doesn't fundamentally change the design.

FUN asked:

...do wooden coaster cars have wheel assemblies with independent movement? I always thought they were rigidly fixed the chassis.

It depends on the wooden coaster car you are talking about. The older PTC trains, and the trailered trains I just described, offer no movement in the wheel assemblies. The GCI trains have some motion, but it isn't significant to the way the trains track. The exception with the trailiered trains is that the lead axle on the Morgan, GCI, and Prior & Church cars can roll on its longitudinal axis. The wheels are in fixed positions on the axle, but the axle can *roll*. It does not steer, but by rolling it allows the train to enter a bank without lifting any axles. In fact all of the hitch points can roll, so all of the axles can roll relative to each other. The more recent PTC trains, the ones Jeffrey Seifert is talking about, have a rear axle that can roll about three degrees in either direction. So the front wheels are attached directly to the chassis, and the rear wheels are attached to a steel frame mounted on a pivot shaft under the back seat. For Gwazi, the back seat of the car was raised about an inch to give the axle a little more room to swing, and that same modification has become standard on the Gravity Group coasters. The extra inch gives the train about +/- four degrees of roll, but that's really quite a lot...that means that within the six feet or so from the front axle to the rear axle the track can roll four degrees. Four degrees in six feet means that in 540 feet you could do a complete barrel roll. :) More usefully, it means that when you see a 90-degree banked curve, you know that unless the train is lifting wheels, there is a minimum of 135 feet of running length between the flat track and the maximum bank angle on the curve, a little more than three full train lengths.

PTC puts the roll pivot on the back axle instead of the front axle because that makes the most sense. As the train starts to roll, the front wheels will follow the track, and the car body will roll to match, while the car's rear axle swings a little to stay behind the roll. It's counter-intuitive, but once you realize that the train is following the track, not the other way around, it makes sense. But it isn't a requirement; Morgan, GCI and Premier Rides allow the front axle to roll.

Also, it's worth noting that except for the GCI and Gravity Group trains, most wood coaster trains are basically copies of the PTC articulated design.

I think I have some photos somewhere that illustrate some of this...unfortunately some of them are photos that I promised I wouldn't spread all over the 'Net, so I'll have to make sure the context is suitably obfuscated, or get permission to share...

I think that covers pretty much everything covered so far. What questions are still unanswered?

--Dave Althoff, Jr.

Last edited by RideMan, Wednesday, October 28, 2009 11:51 PM

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