Wanted Ducati 250 Clutch

[Rocla]

Well,

Just to be clear, I don't know exactly the spectrum of 350 Ducati outputs distribution according to some details like carburetor diameter, cam versions, (and when you write, unless there are typo errors, you seem to mean it is a kind of ramdom distribution but you can trust me: with 24HP, it is impossible to reach 150 km/h except for in a perfect condition with a very good fairing and a jockey sized driver.
With a normal air penetration rate (no fairing, no special position), it is of course impossible, even with the right reduction ratio. I don't need any confirmation about that, for me, it is a certainty... but, as a result, quite the opposite, I am interesting in 350 output confirmation !
The problem is that we are not discussing on same theme: I red everywhere that the 350 Scrambler output is almost 24 HP and when I compare my 350 Ducati with my 250 MZ (maybe 22 HP), I don't see any difference except for the torque (the Scrambler is like a tractor). It could be interesting to measure the right output... Unless if I give you the carburetor diameter, the exhaust model, given you know the cam model (white model), do you have tables with a right power estimation? Anyway, we will get a partial answer when I'll get new sprocket and crown! But for the moment, I am rebuidling the wiring loom then I will re paint the frame. 2 or 3 weeks job

[StewartD]

Hi Rocla,

I am doubtful of your statement:

“but you can trust me: with 24HP, it is impossible to reach 150 km/h except for in a perfect condition with a very good fairing and a jockey sized driver.” (150 km.p.h. = 93.2 m.p.h.)

The ‘Cycle World’ articles ‘A Ducati 250 for racing’, parts 1 and 2 appeared in December 1965 and February 1966 issues. Part 3 of the series appeared sometime afterwards, but my copy, downloaded from this forum, does not have a date or page numbers showing. In these articles they report dynamometer testing their Mark III or Diana 250:

“Before starting this project, we had the Ducati engine dynamometer-tested, and the results were a bit surprising. With megaphone in place, and after setting the spark and changing jets, to get maximum output, we found ourselves with an honest 22.2 B.H.P. at 8500 r.p.m.” (part 2, p59).

In the part 3 of the series they state that the top speed in “standard trim” was 97 m.p.h. (156 km.p.h.) In the context of this article, their “standard trim” was in the same condition as when dynamometer tested, and therefore with only 22.2 B.H.P. on tap and without a fairing.

Motorcycle magazines are notorious for taking manufacturer’s horsepower claims at face value and printing them alongside road tests. The ‘Cycle World’ article highlights this in part 2, p59 as follows:

“There must be something very special in the air around the Ducati factory’s dyno room, for they claim 28bhp, net at 8000 rpm (at those revs, our engine delivered 21.3 bhp.) It becomes even more curious when one considers that the Ducati Diana is one of the fastest stock 250s, and virtually all of Ducati’s competitors claim much more than 22 bhp. Is it possible that somebody, or several somebodies, has been playing fast and loose with the truth.”

‘Cycle’ and ‘Cycle World’ magazine I always found to be highly credible on technical matters. Their writers Gordon Jennings, ( who wrote the quoted article), and Kevin Cameron I think are the best technical journalists of recent times. Many times over the years that I read ‘Cycle World’ and ‘Cycle’, these writers and others pointed out the discrepancies between manufacturer’s claims and the real Brake Horse Power output produced on dynamometers. Many other magazines simply print manufacturer’s claimed horsepower figures as ‘truth’.

Cheers,

Stewart D

[DewCatTea-Bob]

[quote= StewartD ...
" I am doubtful of your statement:

but you can trust me: with 24HP, it is impossible to reach 150 km/h except for in a perfect condition with a very good fairing and a jockey sized driver.

(150 km.p.h. = 93.2 m.p.h.) "
" they report dynamometer testing their Mark III or Diana 250:

we had the Ducati engine dynamometer-tested,
we found ourselves with an honest 22.2 B.H.P. at 8500 r.p.m.

they state that the top speed in “standard trim” was 97 m.p.h. (156 km.p.h.) "

____ It's certainly possible for all of the above to ALL be true. _ As of-course engine-power alone is not the only factor which sets the max.speed-limit ! _ Since the footprint-shape through the air makes a fairly equal factor, as well.
For example if a n-c 250Mark-3 and a 350-Scrambler both happened to produce the same 23.0-HP, (each at their very-own favored RPM),, then all-else being equal, it would be quite-likely that the slightly smaller footprint of the 250, would allow it to reach a slightly higher top-speed !



" Motorcycle magazines are notorious for taking manufacturer’s horsepower claims at face value and printing them alongside road tests. "

____ Of-course they had to do that whenever there wasn't any other more trustworthy source for such data.
And most-all manufacturers had sales-departments who tweaked the HP.figures to better appeal to prospective buyers.

There must be something very special in the air around the Ducati factory’s dyno room, for they claim 28bhp, net at 8000 rpm (at those revs, our engine delivered 21.3 bhp.)

____ Well that's quite possibly so, because not only are all dyno.meters not calibrated equally, but the air-pressure is LIKELY to vary not only from place to place, but also from day to day !
Still however, not really by such a great percentage of difference... as the 'b' in "bhp" could often be allowed to slip as-to it's actual/intended meaning, thus allowing HP.figures taken directly of-from the crankshaft, (or a trans.shaft), to be confused as-if possibly taken-from where the rear-tire meet's the dyno.belt, (at which point, actual-HP has then become fully diminished).

It becomes even more curious when one considers that the Ducati Diana is one of the fastest stock 250s, and virtually all of Ducati’s competitors claim much more than 22 bhp. Is it possible that somebody,
has been playing fast and loose with the truth.

____ No-doubt that's most-likely, however most-all of those 'other' 250s ALSO have considerably larger 'footprints' to have-to push-through the air, as well !



" ‘Cycle’ and ‘Cycle World’ magazine I always found to be highly credible on technical matters. "

____ I Agree !
__ I happen to have fairly-complete collections of those cycle-mags going-back to mid-65, (but I let my subscriptions run-out come the early-90s).
(I sure miss the days when a 'cycle' was called a "cycle", and not a 'bike' !)



" ‘Cycle World’ and ‘Cycle’,
pointed out the discrepancies between manufacturer’s claims and the real Brake Horse Power output produced on dynamometers. Many other magazines simply print manufacturer’s claimed horsepower figures as ‘truth’. "

____ Well those HP.claims could possibly be the truth, assuming that their figures were instead taken directly from the engine, rather than the rear-tire.


Dukaddy-DUKEs,
-Bob

[Rocla]

Hello everybody;
I believed the discussion was over but with you, it is always everlasting

Firts of all, never trust newspapers. There is a safety rule that consists in never believing the journalists, too many times articles are simply copy/paste of press files. I am working in marketing and advertising domain, and I saw so many times the exact contents of press files with the added signature of the journalist. You seem confident with the article you quoted but I don't know it... And we have to know exactly what power we are speaking about : at the wheel, or on the gear box?
Secondly, if you check all the features of the current motorbikes between 10 to 30 HP, you can find that the max speed is distributed on a curve that is asymptotic to the x axis, because the most powerful you are, the most additionnal power you need to increase your max speed. Because of air penetration forces. Here below the genuine figures of many motorbikes, the type and mark, then the power (HP), then the maximum speed (km/h):
125cc MZ TS 11 100
150cc MZ TS 12 115
125 Hyosung Comet 15 120
125cc Yamaha/Yamaha/Kawasaki 15 125
250cc MZ ETZ or TS 21 130
250 Honda CRF 23 139
250 Honda CBR (with fairing!) 26 150
250 Kawasaki R30 (with fairing!) 30 150
Please, enter such figures on excel (I did that but I can't copy the image result on this post) and, on the curve, you can gess at any power level the expected related speed. In order to demonstrate, the 2 last motorbikes of the list are equiped with fairings... I keep my position, and our disagrement raises a problem of definition. If the newspaper you quoted is right, for sure there is a concern on the power itself : at the rear wheel in this case, at the gear box or cranckshaft in all the others. But impossible to reach 150km/h with only 24HP engine as it is usually displayed by the motorbikes makers, there is no doubt on that! I swap on my Scrambler : I've mounted a 45 teeth crown, I've bought a 17 teeth sprocket (but not mounted yet) but I have to many works to achieve on the frame and the body so far. I will test my motorbike by end of June. You can check the progression on my website:

[StewartD]

Bob, Rocla,

I think we all agree that the higher figures, that are bandied about, are manufacturer's crankshaft horse power figures.

Rocla: Where did the new figures come from? I assume they are all manufacturer's claimed crankshaft horse power.

The figure of 26 HP and 150 kph for the 250 Honda and 30 HP and 150kph for the Kawasaki sort of make sense, if it is crankshaft horse power. The 'Cycle World' figures for the Ducati are 22.2 rear wheel horse power and 156 kph speed. Remember the article had said Ducati claimed 28 HP, right between the Honda and Kawasaki figures. The Ducati was unfaired; the Honda and Kawasaki had fairings

So if we make the big assumption that the three horsepower figures were obtained in a consistent way, then the Ducati had a higher top speed than the Honda because it's slightly higher power and slimmer profile outweighed the advantage of the Honda's fairing. The Ducati's higher top speed than the Kawasaki would have to be explained solely on the basis that the Kawasaki was much fatter and/or the fairing was not very efficient.

It strikes me that the difference between the rear wheel horsepower of the Ducati and their claimed power of 28hp is too much. 22.2/28 = 0.79 therefore 21% difference in power between crank and Ducati rear wheel is approximately what the transmission losses would be. Machinery's handbook gives efficiency of spur gears as 99% and roller chains as 95 to 97%. The Ducati transmits through the Primary drive helical gear pair, the gearbox spur gear pair and then through a roller chain final drive. The efficiency of this should be: (0.98* x 0.99 x 0.95** = 0.92) 92% or 8% loss.
*(I used 98% for primary gear pair as it is helical cut and slightly less efficient that a straight cut spur gear pair; Machinery's handbook does not give a figure for helical cut gears).
** I've used lower figure as exposed motorcycle chains get dirty and due to long centre/centre distance can easily suffer slight misalignment.

I recently read somewhere that the power loss between crank and rear wheel was about 12%. I believe this is a more realistic figure than my calculated 8%, but even this is way below the loss between the Ducati claimed horsepower at crank and the rear wheel Dynamometer figure (21%). I think all the claimed crankshaft horsepower figures are exaggerated. It's just the way the world is.

Cheers,

Stewart D

[DewCatTea-Bob]

[quote= StewartD ...
" I recently read somewhere that the power loss between crank and rear wheel was about 12%.
but even this is way below the loss between the Ducati claimed horsepower at crank and the rear wheel Dynamometer figure (21%). "

____ I'd be more-apt to point the finger at the various d.meters themselves, as having a tolerance-reading of 5 to as much as 20% off-actual.


Dukaddy-DUKEs,
-Bob

[StewartD]

Bob, Rocla,

We are straying from this topic a fair bit. I have started a new thread on Dynamometer accuracy as it quite interesting.

Cheers,

Stewart D

[Rocla]

Hello everybody,

OK for a new topic, very interesting one too... Back to (one of) the previous topics, the reduction ratio: I've just tried my Scrambler with a 45 teeth crown and a 17 teeth sprocket and I am atonished because the bike has still very good accelerations and is capable of reaching very easily 110 km/h (just below 5000 rpm in theory). I've compared with the 350 Honda's friend of mine, on the road. Above 5000 rpm, I don't dare speeding up for many reasons: the piston is a brand new one, normally in a runing in period, and I don't feel confident with the engine yet. But it is so easy to accelerate in the 5th speed even at 45 km/h...

[DewCatTea-Bob]

[quote= Rocla ...
" Back to (one of) the previous topics, the reduction ratio: I've just tried my Scrambler with a 45 teeth crown and a 17 teeth sprocket
and I am atonished because the bike has still very good accelerations and is capable of reaching very easily 110 km/h "

____ Thanks for your update-report !
It's-not surprising that you've confirmed that your acceleration-rate has-not changed, as only 1st.gear-acceleration from a near stop should be all that's notably-reduced (to-be less quick, with your new raised-higher overall-gearing).
__ Unless your 47t-crown was worn-out, it was likely-not the best-choice to purchase a new crown-sprocket with only 2-teeth less.
Cuz now you have the same 45/17t.gearing as a Sebring, which can't attain it's engine-power's capable top-speed because it's top-gear is TOO-high and it's 4th.gear is too-low to be able to 'match-up' it's engine's peak-power together against the particular top wind-speed which it's peak engine-power is actually capable of fighting-through.
However since your Scr.rear-tire is about 5% longer in circumference (than a Sebring's stock-tire),, you might possibly get fairly-close to your 350's true top-speed, whilst in 4th.gear.



" Above 5000 rpm, I don't dare speeding up for many reasons: the piston is a brand new one, normally in a runing in period, "

____ So-then you probably ought-to put-off trying to reach 'top-speed', until AFTER you've run your engine for at-least 2000k.meters.



" But it is so easy to accelerate in the 5th speed even at 45 km/h... "

____ You can thank your actual SCR.type 'White-camshaft' for that low-end power-output !
And if your 350 is ever matched-against another w-c.350 with a Green&White-cam, (all-else being equal), you can then expect to 'trot-away' from it, at any RPM under 6k.revs. _ (But above 7k.RPMs, the G&W.equipped-350 will-only merely-just 'inch-away' from yours.)


Duke-Cheers,
DCT-Bob

[Rocla]

Thanks Bob
I would like to know the actual power of my white camshaft 350 Scrambler... (I forgot to tell you that I increased the pipe exhaust section at the top cylinder exit, because the new pipe I've bought to Amici Dello Scrambler had a bottleneck. But the effect is probably marginal)