Author Topic: How did they figure horsepower in the early Douglases? (Read 14434 times)

Dave · « **on:** 09 Jun 2005 at 21:05 »

Up until about the mid-1920's Douglas described their models as 2-3/4hp, 3-1/2hp, 4hp etc. Then in the mid-1920's they appeared to have dropped this reference to horsepower and described their engines purely in capacity (cc).

I was wondering, how would they have calculated "horsepower" in those days - 2-3/4hp seems quite a precise calculation? Surely they didn't have dynamometers?

Also, has anyone come across any mention in any Douglas material, about this apparent change in the way engines were described?

Ian · « **Reply #1 on:** 09 Jun 2005 at 22:24 »

Dave, the horsepower figure have no relation to actual horsepower. I am not an expert on this but my understanding is it was an arbitary calculation which effected the way vehicles were taxed. I think it was originally based purely on capacity so 3.5 is a 500, 4 is a 600, etc. This was consistent across all manufacturers. At some stage they changed the way this was calculated and it was based on the bore size only which is why many english cars had very long stroke motors - to get round the tax laws. I have a 1928 Sunbeam car which is rated at 16.2hp but actually developed about 4 times that amount !!

Doug · « **Reply #2 on:** 10 Jun 2005 at 03:10 »

Dave,

Yes they did have dynamometers back then. However somehow at some point, it was determined in the industry that a 350cc machine would put out 2-3/4hp. (3-1/2hp for 500cc, 4hp for 600cc, and 6hp for 750cc.) And 2-3/4hp became synonymous with 350cc, even long after 350cc engines were putting out far more than that. As Ian says, the HP rating might have also been a taxation class early on, another reason to lie about the power

, not sure. Certainly it was so for cars, and there was a formula for calculating the taxable HP. This had little relation to the actual HP, and also accounted for many British cars having tiny bores and really long stroke to get a given displacement.

By 1922, Douglas were advertising 2-3/4hp machines as weighing less than 200lbs (175-199lbs depending on model) and so qualifying for a special 30 shilling tax. So for motorcycles anyway, it was based on weight and number of wheels but I do not know when this first took effect. A solo over 200lbs was taxed at £3, a sidecar or trailer cost more. This lightweight tax was extended to include machines up to 224lbs in 1931, still at 30s per annum. They stop mentioning taxes in catalog literature in 1932, though obviously they still applied.

Douglas seemed to hold on to the 2-3/4hp, 3-1/2hp, 4hp, and 6hp designations long after everyone else dropped it to denote displacement. Douglas finally dropped it with the introduction of the EW model in late 1925, though some of those were still called ‘the new 2-3/4hp’ in the press. I do not think Douglas much liked that, stressing it was a completely new design (by the new chief designer Cyril Pullen.) But by 1926 in the catalog they were completely switched over to 350, 500, 600cc displacement classes for all the models. Internal to the factory they still annotated some of the o.h.v. racing machine drawings as 3-1/2hp and 4hp class up to the 1927 season.

-Doug

Dave · « **Reply #3 on:** 10 Jun 2005 at 10:08 »

Thanks Ian and Doug, the 2-3/4hp description has been on my mind for a long time. But I can’t imagine what a dynamometer looked like in those days.

Doug · « **Reply #4 on:** 14 Jun 2005 at 01:53 »

Here we see a Heenan & Froude type DPX1 water-brake dynamometer. Inside the housing is an impeller running in water. The hand wheel controls sluices that increases or decreases the load or friction. The absorbed power is converted into heat and carried away by the water. Partially obscured by the large trumpet at the top is the dial for the spring scale that measures the torque reaction. (The trumpet is the air inlet for a Rover SA-1 gas turbine.) Not sure when the DPX series first came out, but Heenan & Froude’s first patent for a water-brake was in 1911.

On this side the black rectangular object is one of the torque reaction counterweights, there would be a further stack of additional ones on top of this fixed plate, temporarily removed. At the top is the hand screw for calibrating the scale. The white face dial is a Smiths tachometer for accurately measuring revolutions per minute.

The DPX1 is a rather small dynamometer, capable of absorbing up to 150 horsepower, so was ideal for testing motorcycle engines. I figure this one will be ideal for testing Douglas Dirt Track engines!

-Doug

Doug · « **Reply #5 on:** 14 Jun 2005 at 02:21 »

I located the formula used for British taxable horsepower in a 1929 book of automotive motoring:

Where "D" is the bore diameter and "N" the number of cylinders.

For bores measured in millimeters, a devisor of 1.613 was recommended.

-Doug

trevorp · « **Reply #6 on:** 14 Jun 2005 at 09:01 »

u wouldnt want to test for very long they must have used some sort of fan for cooling the engines
u can make your own test dyno with a water pump but seems like u would be converting the post war engines back to what they were before bikes water pumps
i used to have a small dyno made of water pump into 44 gallon drum with baffles to stop cavitation worked great on small under 20 hp engines but u must use a good coupling i had a few fail in dramatic fashion

trevor

Dave · « **Reply #7 on:** 14 Jun 2005 at 10:32 »

That machine looks like huge fun Doug - one could spend hours and hours, tinkering and tweaking.

Found a brief history of Heenan & Froude Ltd – http://www.miac.org.uk/heenan.htm including a picture of a monster dyno. It says “William Froude was born in Devon in 1810. He was one of the great Victorian's and in 1877 he invented the hydraulic dynamometer that was later to become one of the world renowned products of Heenan & Froude.” …1877 !!

But getting back to the horsepower, if we try out the formula on say, a 1924 TS (described by Douglas as a 2-3/4hp), with a bore diameter of 60.8mm, convert that to 2.3937 inches, then square it and multiply by 2 (cylinders) = 11.45961. Finally divide by 2.5 = 4.58 horsepower. (If we use the metric measurement of 60.8, that final divisor needs to be 1613 to come up with the same 4.58.)

I imagine some of the dirt track boys would have had their engines dyno'd, but I wonder if anyone tested say a “2-3/4hp” engine on a dyno? Or perhaps there’s a popular estimate of actual horsepower developed.?

I guess the 1929 formula may have been an update to an earlier formula and this produced an upward revision of the taxable horsepower from 2-3/4 to 4-1/2. Maybe this had something to do with the dropping of the horsepower from the model description.

Ian · « **Reply #8 on:** 14 Jun 2005 at 22:34 »

I will try to find an article I have here somewhere on all the british tax things - but am pretty sure the formula Doug mentioned was only for cars. With bikes I think the tax in the late 20'sw, early 30's was aimed at lightweights so must have had a weight parameter as well.

Doug · « **Reply #9 on:** 15 Jun 2005 at 02:16 »

Quote from: "Ian"

...but am pretty sure the formula Doug mentioned was only for cars.

Yes, the formula was for cars, from a 1929 automotive book as mentioned. It does not pertain to motorcycles.

Quote from: "Doug"

So for motorcycles anyway, it was based on weight and number of wheels but I do not know when this first took effect.

For motorcycles in the late twenties it was based on weight. A 1930s Pittman Manual for the Douglas I have has a generic chapter (Pitman made books for many other makes of motorcycle), how to ride, rules of the road, and licencing. It only mentions weight and two verses three wheels for taxation purposes, nothing about displacment. In the 1936 edition, information on taxation is omitted entirely though the rest of the chapter remains. A 1942 edition has much the same chapter except for the inclusion of some new road signs that came into effect. As tax rates also dissapeared from Douglas advertising in about 1932, that would also colaborate the rules changed in the early thirties.

Certainly they were still taxing registration of motorcycles, it is after all jolly olde England we are talking about. But the rules must have become too complicated in such a state of flux they decided not to include them. What they did for taxation assement before wheel and weight classification I do not know. You start to see mention of the 30 shilling weight tax in advertising circa 1922 (though available catalog information before that is spotty); pre- great war catalog information just talks about the machine specifications.

-Doug