Vincent's power & talk

[johnnyboy-wonder57]

I had a problem, I once read a Vincent Black Shadow had (perhaps in the later D series) 56bhp, it was in my Uncle Ken's 1951 Daily Mail "Choose your Mount" Motorcycle Gazetteer, along with the price of £325 & a quoted top speed of 125mph!  I dreamed of such a machine, but they had gone out of production before I was born, the Golden Age was at an end!
I use to lie in bed as a youngster and read the stat's on each machine, from 35bhp BSA Golden Flashes to 18bhp Velocette Macs,  29bhp Dominator's &4.5 bhp D1 Bantams etc!
Stat's again, they get into your head & your stuck with them for the rest of your life, Vincent's were lightish, but not that light, so why I was to think, later on in life,  was more & more power bring consumed  in later machines to get a similar speed or four,  five  or six miles faster, where was the extra power going, how was it being lost?

A Rocket Three 58/60 bhp slight increase in power, but  probably slightly slower top-speed to a Vincent;
Norton Commando 58 bhp, ditto;
Honda CB750 67 bhp, same story,
Z900 82 bhp, 26 more bhp six miles faster!
Z650 64 bhp 120ish;
BMW K75 75 bhp 131 mph, only in the late 70s and early 80s was the spell broken when 90 bhp Laverda Jota's & 100bhp GSX Suzukis & their ilk, finally add 15 to 20 mph to the score of 125mph, but with significabnt power increases to boot.

So, I know:
Air resistance/Streamlining is crucial; Doubling  the speed takes approximately 4 times the power!
Weight is a factor;
Engine characteristics Torque X RPM is crucial too;
As are the gearbox and final drive ratios;

But I cannot help but think, even considering correction factors,  that the horses measured by the British companies in the past , where perhaps different horses to those measured elsewhere by other manufacturers, I may be wrong & often am, but I sit & ponder whether an Imp engine putting out depending on spec' 42 to 62ish bhp @ the crankshaft, in a Feather-bed frame, to be built when my  personal economic downturn ends and with everything as right as can possibly be, will be enough to propel me to the dizzy heights experienced by Vincent owners on equal or lesser horse power!

Or would I have to be brave resort Voodoo or  to Somander Singh's grooved cylinder heads to find that factor X, that Vincent's possessed!


Cheers

John

JBW

[Rex]

Two points to consider there, Johnny-
1) The Law of Diminishing Returns, explained to me by the case of Bantam tuning vs Z1 tuning. It would be relatively easy and cheap to increase the Bantam's output by 10%, but next to impossible to do the same with the Z1 as Kawa have already done all the obvious and easy mods.
Therefore a good design in a lowish state of tune (MAC for example) is better than an OK design highly tuned (Z1) and despite what MCM may have spouted endlessly in the 1970s/80s the old British factories' engine designers were just as savvy of the principles of power as any from Japan.

2) Had the Japs etc really tuned the UJMs it wouldn't necessarily have been great for reliability, longevity, or even rider comfort. Not everyone rides at ten tenths, and most never do; that's why a true Goldie was such a pain in the arse on the road way back when, and why Golden Flashes were far more popular..

[Bomber]

Go ahead, build the Norton Imp, they are great fun... here's mine

[johnnyboy-wonder57]

Where?

Have you pic's?

John

[johnnyboy-wonder57]

Memory going checked my old book & Velocette Mac was 14 bhp!

[R]

There is also the minor matter that air resistance increase with the CUBE of the speed.  So a doubling of the speed requires 8 times the power.

Which explains some of the massive outputs of modern bikes, with what seems like not a large speed increase...

[johnnyboy-wonder57]

Yep,
Doubling the speed increases drag, theoretically by a factor of 4, which is where I got confused with my figures, but you are right according to physics, doubling the speed of a vehicle will take the cubed root, 2 x 2 x 2 = 8 times or an 800% increase in power, however,  it doesn't quite work out like that, say 25 bhp motorcycle gave you 80 mph, in theory then to do 160 mph you would need 200 bhp!  So something else must come into play  than just drag co-efficients.

A Vincent  Black Shadow was geared in top gear to do 22 miles per 1000 rpm,  taking into consideration bhp per litre, max bhp capability & rear wheel size; it is possible from max rpm,  to work out the theoretical max speed, obviously the engine has to put out the power to deliver the theoretical max, not sure what the max torque on a Vincent Black Shadow was I can't find a figure anywhere, but I presume it was a pretty effective engine in terms of efficiency of pushing the steed along. Indeed someone stated that the Vincent 1000 held the crown of the world's best performing production 4 stroke,  until the advent of the Z,1 903cc beast producing 82 bhp,  which incidentally came along 17 years after production of the Vincent had finished!


Cheers


John

[R]

Yep,
Doubling the speed increases drag, theoretically by a factor of 4, which is where I got confused with my figures, but you are right according to physics, doubling the speed of a vehicle will take the cubed root, 2 x 2 x 2 = 8 times or an 800% increase in power, however,  it doesn't quite work out like that, say 25 bhp motorcycle gave you 80 mph, in theory then to do 160 mph you would need 200 bhp!  So something else must come into play  than just drag co-efficients.

8 times the power to double the speed, due entirely to air resistance,  is actually a pretty good basepoint for these things.

It takes about 10 hp to do 60 mph, and the first 20 or 30 mph don't really count in the wind resistance stakes - so just 3 hp will do 40 mph. And a healthy 30 bhp to do 100 mph.

A 55 hp Vincent does 125 mph, so to do 250 mph it would need 440 bhp. Which is obviously off the planet, but so is a 250 mph naked Vincent.  Rollie Free with his bathing suit showed there is more to life than horsepower though.

Once you add a full fairing, things improve - and a Honda MotoGP bike with pneumatic valve springs making 270 bhp @ 19,000 rpm gets about 345 kph on the longest straight in MotoGP - thats approx 215 mph.

The Aprilia 250 GP bikes are currently getting ~440 bhp/litre, so a 1000cc version of one, if that were possible, should get that 250 mph easily. Except that this year is the last year for them, next year they go 4 stroke Moto3 spec.

And the Gatsos get ya if you wind it on in 3rd gear. Or even 2nd...

Cheers.