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How was a steam locomotive’s top speed calculated?

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alexl92

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In the ‘Golden Age of Steam’ - or even just before computer modelling became a thing, how was a steam locomotive’s top speed calculated? Was it calculated, or did they just run tests to see how fast they could get them to go safely?
 
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krus_aragon

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One could make calculations as to the expected performance of a new locomotive design, but their actual performance could be markedly different to expectations. (The compound engines that Webb designed for the LNWR are an example of how reality might not always match up to the drawing board.) You also had all sorts of innovators coming up with new ideas for locomotives; everything from the Ffestiniog's Fairlies to this eclectic list. Add to this the fact that most steam locomotives wouldn't have a speedometer, and things get rather nebulous.

The sure-fire way to know for sure was to hitch up a dynamometer car to the train, and tell the driver to go for it.
 

The Lad

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You could calculate when bits were likely to fly off or suffer Mechanical derangement and add in a safety factor to allow for tolerances and wear but I suspect it was mostly more empirical than that.
 

John Webb

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I think that the piston speed was the limiting factor, particularly when lubrication was more of a 'hit or miss' affair than it eventually became. If the piston moved too fast and lubrication was poor there was a risk of the piston and cylinder being damaged through metallic contact. But I've no idea what maximum piston speed was considered safe!
 

chorleyjeff

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I think that the piston speed was the limiting factor, particularly when lubrication was more of a 'hit or miss' affair than it eventually became. If the piston moved too fast and lubrication was poor there was a risk of the piston and cylinder being damaged through metallic contact. But I've no idea what maximum piston speed was considered safe!

Were there actually any speed restrictions on locos pre BR ? I read somewhere that Fowler 2-6-4 tanks were restricted to 80mph and I have read that 9Fs were supposed to be limited to 50mph,although that was BR, but the 2MTs with the same diameter wheels but shorter cylinder stroke had no such limit and cruised comfortably at 70mph between Preston and Ormskirk.
 

70014IronDuke

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I think that the piston speed was the limiting factor, particularly when lubrication was more of a 'hit or miss' affair than it eventually became. If the piston moved too fast and lubrication was poor there was a risk of the piston and cylinder being damaged through metallic contact. But I've no idea what maximum piston speed was considered safe!

Yes, I've certainly read that piston speed was a limiting factor. You could certainly calculate the forces on the piston rod as it flew one way and then reversed. I suspect your point regarding lubrication is also valid.

Were there actually any speed restrictions on locos pre BR ?

I don't know, but I get the feeling that generally there were not, even well into BR days.
Most schedules did not need passenger trains to do more than 75 to a max of 80 mph up to about 1960. Of course, there were speeding spots down banks (where the track layout allowed high speeds) such as down Stoke Bank and towards Bedford from both directions on the Midland, but although there were reliably recorded speeds well into the 90s at such places, I don't think these were every day events - more when needing to make up time, especially if that meant getting an hour in the pub.

Perhaps the lack of special attention to maximum speed was two-fold, viz a) the trains didn't need to go above 80 mph, and to do so meant you'd only be brought to a stand for signals once ahead of schedule (and so your fireman would have wasted a lot of effort shovelling) and b) steam locomotives could be very rough, so pushing them over 85-90 was just plain uncomfortable.

As high-speed train for its day, the Bristolian was one exception, and numerous logs show Castles doing 80 - 85 mph for mile after mile between Swindon and PAD, but - from what I've seen, they rarely ventured into the 90s. (Someone will soon tell me if I'm wrong on this one.)

I read somewhere that Fowler 2-6-4 tanks were restricted to 80mph and I have read that 9Fs were supposed to be limited to 50mph, although that was BR, but the 2MTs with the same diameter wheels but shorter cylinder stroke had no such limit and cruised comfortably at 70mph between Preston and Ormskirk.

The standard 2MTs had 5'-0" diameter drivers, the LMS ones had 4'-8 1/2". 70 mph? Seems a tad to fast to me for such locos, but maybe.
 
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John Webb

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Were there actually any speed restrictions on locos pre BR ? I read somewhere that Fowler 2-6-4 tanks were restricted to 80mph and I have read that 9Fs were supposed to be limited to 50mph,although that was BR, but the 2MTs with the same diameter wheels but shorter cylinder stroke had no such limit and cruised comfortably at 70mph between Preston and Ormskirk.
Bear in mind that before BR there were still limits on speed set by the type of train, so loco designers tended to make locos for handling different types of train at the lowest possible costs, both capital and running.
Regarding the BR 9Fs, the suggestion to limit their speed to 50mph was based on the wheel diameter of 5ft, and followed an occasion on the East Coast main line when a train with senior BR officials on board was given a 9F en-route for London when the original allocated loco was used for another train. The 9F ran the train on time to Kings Cross, but some consternation arose when the said officials found out that the 9F had hauled the train and touched 92mph on Stoke bank. The proposal to limit locos to a top speed related to wheel diameter was quietly dropped when it was realised that many express locos would be restricted to well below the speed needed to keep to the then existing timetables! (See page 59 of "Riddles and the '9Fs'" by Colonel H C B Rogers, Ian Allan 1982.)
 

30907

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By the 30s, well-designed express locos could comfortably exceed the speed limits set by the Chief Civil Engineer - which were cautious and allowed for locos not being fitted with speedometers! - but as well as the track, there was also coal and water consumption to consider, and loco maintenance issues. Then there were signal sighting and braking distances to be considered - a considerable number of distant signals had to be relocated as speeds increased generally.

To go back to the OP's question, I don't think maximum speed was generally the subject of tests, though of course there were the high-speed stunts of the 30s.
 

alexl92

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Were there actually any speed restrictions on locos pre BR ? I read somewhere that Fowler 2-6-4 tanks were restricted to 80mph and I have read that 9Fs were supposed to be limited to 50mph,although that was BR, but the 2MTs with the same diameter wheels but shorter cylinder stroke had no such limit and cruised comfortably at 70mph between Preston and Ormskirk.

Didn’t one 9F manage somewhere between 70 - 90mph on an express on one occcasion?
 

Taunton

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Speed limits were not only set by the civils, but by what was comfortable on the loco. With two cylinders, each piston thrust gave a slight lifting moment to that side, at speed there could be an uncomfortable sensation as the loco, if not actually lifting its wheels off, at least was relieving the suspension on each side alternately many times a second, a phenomena known as "boxing" (like the fighter, alternate punches). Balance weights were there to alleviate this oscillation, but did not do so wholly. You can't, with just two cylinders, have both pistons thrusting simultaneously, as the loco could stop with both on dead centre, and be unable to get going again.

The GWR working (they called them "service") timetables stated in the front pages for some sections of main line "the speed may be as high as required". They were not fitted with speedometers. High speed GWR locos were generally four-cylinder, which avoided most of this oscillation, and was a further reason why the crews despised the BR Britannias on expresses.
 
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