What do bogies do?

They would have taken some of the total weight of the locomotive- the prototype generally had weight in excess of that needed for adhesion under normal circumstances, and axle-loadings would have to be limited to avoid damage to track and structures such as underline bridges. Brian

They do carry some weight. Obviously, this does reduce the amount of weight available for the traction wheels, but the heavier the loco then the less of a problem this is. The leading bogie carries the least weight, and is there primarily to help with guidance on curves - it makes for a smoother ride, particularly at higher speeds.

The trailing bogie is load-bearing (it usually takes the weight of the firebox and cab), but is only needed in cases where the loco is so large that the drive wheels alone can't carry it all. There's a limit to how much weight you can put on the drive wheels, because there's a maximum axle loading on each pair and, with coupled wheels, there's a practical limit to how many pairs you can have. So if the loco is heavier than the drive wheels can take, then the extra has to be carried by the bogies. There's also an issue with larger fireboxes (whicha re necessary for greater power), as they're too big to fit between the drive wheels and hence have to be carried behind them with the weight supported by a bogie. This is only necessary in larger locos, though - that's why formations of n-n-0, with a leading bogie but no trailing bogie, are more common than the reverse.

In cases where both overall power and maximum traction are important, then the solution is to use articulation so that you can go beyond the usual limits of coupled drive wheels. But this is very expensive and technically complex, so it's also pretty rare.

Mark

The leading one is sprung sideways. When it enters a curve this springing puts a sideways pressure on the fixed part of the chassis to steer it into the curve.

The trailing one is to carry a wide firebox. The GWR with its narrow firebox didn't need it.

Or, in cases such as 0-4-2 "tank" locomotives which carry a tank of water and fuel bunker back there, the trailing truck is needed to carry weight that is well behind the drivers.

Most 0-4-2 engines are rigid; it's not a trailing truck. Likewise

Otherwise the fixed wheelbase is too short and the engine rides badly, oscillating as it runs.

"Most" by number built or by type? I can think of 0-4-2s with radial axles back to the 1850s - Sturrock's tanks on the GNR and their near-sisters on the London, Chatham and Dover - and a good many 2-4-0s had radial axles: Hardwicke and all her sisters, for one type (all had the Webb radial axle). I can't off-hand think of a 2-2-2 with radial axles, but I bet there's been one.

Thanks all. I get it now.

David

Im surprised no ones made a wise crack about bogies yet?

Peckett and fleck it?

BOGIES!

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