Wheel Pairs on Steel Coasters

This may have been already discussed here at some point, but a search of the forum topics didn’t turn up any matches, so I’ll ask the question: why do steel coaster wheels come in pairs? My guess is that because modern steel coaster wheel carriers independently pivot, they are basically castors and are therefore subject to castor flutter (think of a shopping cart), and pairing up the wheels, with one ahead of and one behind the pivot point (yaw-axis), helps prevent flutter. Also, I’m guessing that articulated wood coaster trains can use single wheels because of beam steering (Great Coasters) or Ackermann steering (The Gravity Group), where in the latter case the pivot point is not directly in line with the wheel path but is instead inboard of the carrier. This is my best guess, so I’m hoping that someone else has a more definitive answer. Thanks!

Paging Dave Althoff Jr... Mr. Dave Althoff Jr...

Yeah I really should just get his phone number.

Really, I am not that hard to track down...

Bakeman31092, are you referring to the road wheels, or the guide wheels? Doesn't matter; I have excuses for both...

If you look at the earliest steel coasters, there were only single road wheels. The wheels are mounted on spindles just as on wood coasters, with a single road wheel at each corner of the car. Arrow's earliest design had to be able to navigate narrow curves, and it had to run relatively quietly and smoothly, because the ride was constructed inside a building. As a consequence, they built a car with axles that could steer at both ends, in a design that persisted almost to their very last coaster.

A critical difference between wood coasters and steel coasters is the use of polyurethane or nylon tires on the wheels. Those tires give a smoother, quieter ride than a steel wheel would, and they do far less damage to the track. But at least in part because the tire is flexible, it can carry less weight than a plain steel wheel can. For the early coasters this wasn't such a big deal. But in the mid-1970's, Arrow came up with a design that required more flexibility, which led to a fully articulated coupler between trailered cars. Trailering means only one axle per car, which means each axle has to carry about double the weight of the old design. Put too much weight on a polyurethane tire, and you have to worry about excessive heat and premature failure.

But doubling up the wheels cuts the weight per wheel in half! So using the double wheel carrier on the trailered car keeps the weight down to something reasonable in the same way that it brings down the axle weight on a road trailer.

The guide wheels, which are not load bearing, are another story...and yet, even the earliest modern steel coaster has the doubled guide wheels. So what's going on here? Well, remember that we're talking about axles that can steer. The simplest design is probably the Arrow Runaway Train axle, where the whole axle is a rigid assembly with one road wheel on each side and two guide wheels on each side. If you have an axle with a design like that and a single guide wheel, as we see on wood coasters, one of the first things that is going to happen is that the axle will turn sideways in the track and derail. Oops!

Putting guide wheels both ahead of and behind the road wheel makes it so that when the axle approaches a curve, the outboard lead wheel will start to steer the axle from the outboard side, but at the same time, the trailing guide wheel on the other side will also make contact with its rail and limit the axle rotation. Basically, we're steering the axle from both sides.

When you switch from a swivel axle to independent wheel carriers, you don't have to worry about twisting the axle between the rails. But again, by steering the wheel from both ahead and behind, you can steer it both when it approaches a curve (with the leading guide wheel) and when it straightens out again (with the trailing guide wheel). You can accomplish much the same by using a tie rod (and in fact that's what PTC did on their junior coaster cars with the flanged wheels) but now you are relying on the steering action of the wheel on the opposite side to bring the outboard wheel out of the curve.

You can see this on the Lost Coaster of Superstition Mountain. Each car has two axles, and each wheel carrier can steer. On that train, though, the guide wheel is directly below the road wheel, in the usual position for a wood coaster train, and a pilot wheel sits ahead of the axle and connects the two wheel carriers with a tie rod. The front pilot wheels are ahead of the front axle road wheels, and the back pilot wheels are behind the rear axle. The problem with that design is that while it can handle tight curves with ease, because the wheel carriers are only steered from one end, they don't manage straight track very well at all, because there is nothing to hold the axles in compliance without a curved rail. It's not a problem on Lost Coaster, because that ride doesn't have any straight track. But on a typical steel coaster, the train needs to be able to handle both straight and curved track. Steering the axles and wheel carriers from both ends accomplishes this.

I hope that makes some kind of sense!

--Dave Althoff, Jr.

Thanks for the response, Dave.

If you have an axle with a design like that and a single guide wheel, as we see on wood coasters, one of the first things that is going to happen is that the axle will turn sideways in the track and derail. Oops!

I've come to that realization before, but it seems to only apply to cases where the guide wheels are on the inside. If they're on the outside then they're basically hugging the track, which prevents the axle from turning inward. Wouldn't a single axle, trailered car fixed wheel carriers be able to navigate a turn just fine if the guide wheels were on the outside?

Also, doesn't GC use a trailered, single axle, fixed carrier design? If so, how do they prevent the axle from twisting inside the track? My guess would be that the coupling between cars is rigidly fixed to one of the cars, in the same manner as a standard boat trailer. If each axle could yaw about the coupling element (like if a boat trailer's wheels were attached to their own solid axle that could yaw relative to the rest of the trailer), then you'd have the problem of twisting inside the track.

But, doesn't the standard trailer design limit how the cars navigate the track? If you instead allowed each axle to pivot about the coupling between axles and placed the guide wheels on the outside to prevent axle twist, it seems like the cars could handle curves better. In the first case, the lead car can't remain perpendicular to the track (with the wheels running parallel to the rails) because the coupling beam is fixed to the lead axle. However, in the second case, both cars can remain perpendicular because both axles can yaw about the coupling member.

It doesn't matter if the wheels are on the outside or inside. They'll steer just the same.

But outside vs inside matters in terms of derailment. It's hard to describe without a picture but I'll give it a shot. If you just consider a single axle with road and guide wheels and forget everything else, such an axle with the guide wheels on the inside can slip right off by turning sideways, as Dave said, but with guide wheels on the outside it has nowhere to go. In other words, the rails prevent the axle from yawing if the guide wheels are on the outside.

I think derailing in this sense is more about being misaligned with the track than simply sliding off. Wooden coasters can get away with having fixed wheels that don't pivot. Steel coasters need to because of the way they grip the track. Having only one guide wheel on a tubular steel track becomes succeptible to a very slight pivot that will bind up the wheel and stop the ride.

There are upstop wheels in either case. Derailment has nothing to do with anything.

For the case I’m describing, upstop wheels wouldn’t help. But regardless, I’m not trying to claim that any current design can derail.

Millennium Flyers have wheel carriers that are rigidly fixed to the car. Each car attaches to the one in front of it via some kind of hitch joint, which is in the center of the axle of that next car. However, this means that both axles cannot be perpendicular to the rail when the trail goes through a curve, and therefore the road wheels have to skid just a little bit.

One solution to this would be to give each axle the ability to yaw relative to the coupling member. The problem then becomes the potential for derailment, because if each axle can yaw independently then there’s nothing stopping them from turning inward (and again, upstop wheels have nothing to do with this). The common solution, as Dave described it, is to pair up the guide wheels. I’m arguing that placing the guide wheels on the outside accomplishes the same thing, because now the rails are preventing the axle from turning inward. This arrangement also appears to be self-steering, because the only way for the axle to fit on the track is if it’s perpendicular to the rails with the road wheels pointing straight forward.

I think my description was too graphic. This is a derailment not in a safety sense of the thing coming off the rails so much as in being unable to follow the track. No time right now, but I'll clear this up later today.

--Dave Althoff, Jr.

Think "proper alignment" instead. Having two guide wheels prevents the road wheel from being misaligned from the track. It doesn't matter if the assembly is on the inside or outside of the rails because the inside left side looks exactly like the outside right side. Independently the assemblies work either way.