Inside/outside frame power

I'm no expert, but I'd wager it had more to do with stability.

Rusty

Brian Solomon in his book Steam Locomotives (page 109 -- "Narrow Gauge Locomotives") says that prior to 1900 most narrow gauge engines were just smaller versions of standard gauge machines, and were built with the same arrangement of inside frames. The narrow inside frames, however, limited the size of the firebox and thus the power of the engine. Starting in 1903 D&RGW solved this problem by fitting outside frames to their new Mikados, which allowed the use of a larger firebox supported by a trailing truck. The larger firebox made these engines more powerful than an equivalent locomotive fitted with narrow frames and a smaller firebox. 

Here's some additional information from Guide to North American Steam Locomotives (pages 158-159): " The difficulty of scaling down a standard gauge locomotive was that the power output was similarly scaled down, and the most severe limitation was firebox size. A firebox between the frames of a narrow gauge engine couldn't be much wider than a coal shovel; placing a firebox above the frames helped, but it was still limited by the back-to-back spacing of the wheels. Moreover, space between the frames was too restricted for easy maintenance of journals and valve gear."   .....   "By 1903 locomotive technology had progressed so that Baldwin could build 2-8-2s for D&RG with twice the pulling power of the Consolidations. The locomotives were Vauclain compounds, and they had outside frames. The trailing truck supported a wide firebox entirely behind the drivers, and the wider spacing of the frame rails made the locomotive more stable than an inside-frame model."

So outside frames offered a variety of advantages. But accommodating a wider firebox was still the basic reason for adopting outside frames.

So the answer was the requirement for a larger firebox to generate more heat, thus more steam, ultimately more drawbar pull. Makes sense to enlarge frame and tuck wheels inside bearing blocks and put driving appartus external to bearing blocks.

Thanks!!

IIRC, didn't the D&RGW have some engines that were original built as standard gauge, but were converted to 3' gauge by making them outside frame engines?

wjstix posted:

IIRC, didn't the D&RGW have some engines that were original built as standard gauge, but were converted to 3' gauge by making them outside frame engines?

Thoses are the K-37 class 2-8-2's.  They started life as standard gauge C-41 2-8-0's, and were rebuilt in the Burnham shops using new frames.  The original tenders were reused, and the wheels on the tender trucks were regauged by moving the wheels inboard on their axles.

Stuart

Kelly Anderson, great answer!

Thanks,