Wanted Ducati 250 Clutch

[Rocla]

Hello everybody

Many thanks for your explanations, Bob. My appraisal was around 90 km/h at 1000 rpm, I would be atonished if it is about 82 but...

I've recalculated everything with an other method, from the reduction ratios that are mentionned in the owner manual. I put that on an excel sheet with such data, from the first column to the last one :
Engine speed (rev/mn)= 1000
Reduction ratio between engine and primary shaft=0,473709143 (2,111/1)
Reduction ratio of the 5th speed=1,030927835 (1/0,97)
Exit gear box sprocket nb of teeth (C)=15
Crown rear wheel nb of teeth (D)=57 (that is probably not the genuine one)
Reduction ratio for secondary chain (C/D)=0,263157895
Rear wheel rotation speed (at 1000 rpm/mn)=128,5157739
Rear wheel diameter (cm)=76
Perimeter (cm)=238,64
Motorbike speed at 1000 rpm (km/h)=18,4

The problem is that my motorbike is still at the Ducati garage. I've entered 76 cm for the wheel diameter but is is an appraisal from an other motorbike I have.
If I enter the right diameter in order to get 96 inch circumference (243,8 cm) you mentionned, I obtain almost the same speed you wrote, that means we have the same global reduction ratio calculation. Maybe you are right about the speed for 1000 rpm, unless there is something special in my rear tyre diameter... I am expected to get my Ducati back tomorrow and I will measure the exact diameter of the wheel to get the exact speed at 1000 rpm in 5th speed...
If you are right about this diameter, I think my 350 can afford a 16x40 combination given its very strong torque at low engine speed and its final power. Well, the maximum power is get arount 8500 rpm (!)(24 hp) and if the reduction ratio is too long, the maximum speed won't never reached in 5th gear. Consequently, in order to get the maximum speed, in 5th, the best combination is probably to reach 135/138 km/h around 8500 rpm, that means (if no surprise about the tyre diameter), a 15x47, that is to say the current reduction ratio 15x47 of my Scrambler ! But for cruising on the road (and motorway), to get 110 km/h at 5000 in 5th speed seems to be absolutely realistic, that would mean a 15x35 combination. You recommended 15x40 (95 km/h at 5000) but it seems low. In fact, it depends on the engine power curve. At 5000 rpm, may be it is already above 20hp ?

[DewCatTea-Bob]

[quote= Rocla ...
" Many thanks for your explanations, Bob. "

____ You're welcome !
However there are now some discrepancies concerning some things which you have-not taken into proper consideration...



" My appraisal was around 90 km/h at 1000 rpm, "

____ Well that speed@RPM (56MPH/1k.RPM) would be IMPOSSIBLE no matter how high your sprocket-gearing, as your rear-wheel would then have to be nearly 6-times LARGER than a normal 18in.wheel !
So you MUST've more-likely meant '90-km/h at 10,000-RPM', or 9-MPH at 1k-RPM,, either of which is of-course much-more of an actual possibility.



" I would be atonished if it is about 82 but... "

____ Well at the 5k-RPM, nearly 82-k/h is what 'the-math' actually results at, with the 57/15t.sprocket-gearing (and-with a 83"/211cm rear-wheel) ! _ So you really ought-not be astonished with that speed-result, using such a very-low sprocket-ratio (of 3.8:1) !



" Crown rear wheel nb of teeth (D)=57 (that is probably not the genuine one) "

____ Did you not personally count the actual number of teeth on your rear-sprocket ?
UPDATE - My initial-thought inspired by your chosen-wording, was that your 57t.count may-not be the genuine/actual number of teeth on your r.sprocket,, but now, I'm thinking that you had probably/actually meant that you suspect that your 350's rear-sprocket is not the same as the original/stock-sprocket. _ And that is no-doubt a valid-conclusion, as I'm fairly-sure that the original r.sprocket was 45t.
____ Your written-English is pretty-good, but you don't use it in exactly the same manor which Americans usually do.



" Reduction ratio for secondary chain (C/D)=0,263157895 "

___ That's the 'inverse' of 3.8, (which is that of your stated 57/15t sprocket-combo).
Gear-ratios are considered to-be 'lowered' as their numeral-values are 'raised'.



" Perimeter (cm)=238,64 "

____ That means that the circumference-length of your rear-wheel (as expected by YOU), is 94-inches,, and I'm rather quick to much-doubt that it's rolling-range could actually be so far as THAT MUCH length-distance, for merely-just exactly only ONE / 1.0 rotation ! _ That wheel-circumference has just-got to be TOO-high of a figure to be even close to reality !
__ If your Scrambler happens to have an unworn 18x4.25" tire, then it might possibly have an unloaded rolling-distance of as far as 87-inches (FULLY pressurized !),, but certainly-NOT so EXTRA-far as 94-inches !



" Motorbike speed at 1000 rpm (km/h)=18,4 "

____ That seems nearly 12% too-high, with a more reasonably-sized tire ! _ I'd rather expect only about 16km/h at 1k-RPM, (with the rear-wheel being 83 rather than 94").



" I've entered 76 cm for the wheel diameter "

____ That's a whopping 30-inches in height ! _ You really shouldn't expect any greater than nearly 27-inches, AT MOST !
__ Besides, you ought-not use wheel 'diameters' within your calculation-figuring, as doing-so will most-likely result with incorrect calculation-results !
RATHER, you should instead only use circumference-figures, (cuz the rolling-distance better provides real-world calculation-results) !
__ OR, you could instead go-by the ACTUAL-radius (from axle-center to ground-level, which ought-to actually be somewhat shorter than half of the 'diameter' [BECAUSE the tire compresses slightly, under weight]) !



" If I enter the right diameter in order to get 96 inch circumference (243,8 cm) you mentionned, I obtain almost the same speed you wrote, "

____ I had only mentioned that (incredible) wheel-circumference because that's what the-math came-up with in-order to fulfill the speed@RPM-specs which you had given !
__ I really expect that in reality, your rear-wheel is between 12 & 16% less in rolling-circumference length/size (than you've allowed-for).
__ Anyhow, at-least it seems that we have 'checked' each-others math-figuring results, as having been correctly-done.



" that means we have the same global reduction ratio calculation. "

____ Actually, I don't use any established formula,, rather, I just simply do all the-math.



" Maybe you are right about the speed for 1000 rpm, unless there is something special in my rear tyre diameter... "

____ My results of about "10.1-MPH" at 1k-RPM, is correct,, providing that I've luckily guessed-right that your rear-wheel is 83 to 83.5 inches in 'rolling-circumference' -(which is of-course a bit less than 'actual-circumference' !).
__ Your "rear tyre diameter" figure-amount, is no-doubt a fault in your calculation-figuring.



" I will measure the exact diameter of the wheel "

____ I much-suggest that you NOT bother to do THAT !
Rather instead,, with your rear-tire filled-up to no-greater than normal-pressure, you should roll your Duke -(motorbike) on a flat/hard-surface and-then measure the actual-distance (on the ground) that it-takes for it's rear-wheel to complete 1.00/FULL-rotation.
As only in that manor, will you then obtain an ACTUAL circumference-figure to work-with.



" If you are right about this diameter, "

____ Again, I do-NOT work with 'diameters' ! _ But rather only with MEASURED-circumferences of rear-tires.
__ I had GUESSED your tire to be as-if 83-inches in 'circumference' but,,
however, I would-not be surprised to learn that it's actually as small as only 81-inches.



" I think my 350 can afford a 16x40 combination given its very strong torque at low engine speed and its final power. "

____ I do-not disagree. _ But if your rear-wheel's rolling-circumference is more than 87-inches, I'd then suggest that you choose 40/15t instead.
However if it's actually under 82-inches, then the 40/16t-combo may be better-suited for you.



" Well, the maximum power is get arount 8500 rpm (!)(24 hp) and if the reduction ratio is too long, the maximum speed won't never reached in 5th gear. Consequently, in order to get the maximum speed, in 5th, the best combination is probably to reach 135/138 km/h around 8500 rpm, that means (if no surprise about the tyre diameter), a 15x47, that is to say the current reduction ratio 15x47 of my Scrambler ! But for cruising on the road (and motorway), to get 110 km/h at 5000 in 5th speed seems to be absolutely realistic, "

____ I will get-back to this aspect of thought/figuring AFTER you've discovered your rear-wheel's actual 'rolling-circumference'.
__ If you knew exactly what RPM your engine produces it's MAX.HP, then you'd choose to 'gear' your Duke's overall-ratio so-that it can't exceed that RPM in 5th (or 4th) gear.
Cuz if you can exceed that-RPM, then the gearing is not geared-up high-enough for obtaining max.top-speed.



" that would mean a 15x35 combination."

____ The smallest #428 rear-sprocket is 38t (I believe).
__ You ought-to consider a 17t front/drive-sprocket, because the smaller sprockets are tougher on chain-life and-thus consume more power !



" You recommended 15x40 (95 km/h at 5000) but it seems low. "

____ Firstly,, I did-NOT recommend the 40/15t sprocket-combo ! _ Rather, I had merely meant to state that it was the stock ratio-set employed by Ducati's 350 non-SCR.models.
__ Secondly,, those of my figure-results which had been based on YOUR ("95 km/h at 5000" [with your 57/15t-combo]) posted figure-specs,
have actually already been indicated as found to-be about 15%-off, [likely due-to a tire-size variance]).
And also, it was ACTUALLY the 49/15t-combo which I had brought-up, that would be required in-order to provide the '95k/h at 5k-RPM' results (with a normal 18"tire) ! _ (NOT the "15x40".)
The 40/15-ratio should raise the speed up-to about 116k/h at the 5k-RPM eng.speed, (which is the FIRST-time I've stated any results for THAT sprocket-combo !).



" In fact, it depends on the engine power curve. At 5000 rpm, may be it is already above 20hp ? "

____ What country are you in ? _ As it may make a considerable difference, if YOUR 350-Scrambler actually has a real 'scrambler-cam' -(the White-cam), or a 350M3/F1-cam -(Green&White-cam) !
__ Anyhow,, that aspect which you've now brought-up, is likely better addressed by someone in your country who has dyno-tested 350Scrambler-models from-within YOUR-own country.



____ I hope you understand everything I've gone-over here.
__ It will only become significantly more fruitful to continue-on forth-ward, AFTER you've come-up with a trustworthy figure for YOUR rear-wheel's ROLLING-circumference.
So until-then.......

Hopeful-Cheers,
DCT-Bob

[Rocla]

Hello,

Many thanks for your help, you are both an Ducati engineer and an american langage teatcher, the top

Well, you didn't reply to my questions about the power curve but they are crucial. Of course, my cam is a genuine one, that I've just replaced. I can confirm it is a Scrambler one (otherwise, my previous wording would have been different). As you know, the engine has a lot of torque (even if the max power is annouced around 8500 rpm). That means it can afford a small rear crown. The power curve is determinent for choosing the number of teeth, and the second element that is as much determinent is the kind of usage the owner is aiming... Calculation methods (that I know too) are only to serve this goal, i.e. finding a trade off between what the engine is capable of, and what the driver is waiting for, it is not the contrary of course.

On the Amici dello Scrambler provider website, there are 35 teeth crowns (presented as for 428 models) but I've asked yesterday if it is compatible with the 350 chain threat or not. I am waiting an answer (is seems that 428 Scrambler and the lower models are not compatible with the chain thread). Otherwise, I have a friend who have many sprockets and crown dimensions (maybe for racing) and I can order from him if necessary.

My objective remains to cruise at 110 km/h at 5000 rpm. I think it is a normal expectation from a 350cc with a 24hp output, especially if the power curve is relatively flat*. Do you have such curve for the Scrambler ?

*With my 2 stroke 250 MZ ETZ deeptly modified (you can see all the mods on the crankshaft, ignition and clutch here : http://strv.pagesperso-orange.fr/motodesign/UKETZ.htm ) the genuine power output is around 21 hp at 6000 rpm with a very low torque in comparizon with the 350 Scrmabler, and though the genuine reduction ratio for this 250 enables 108 km/h at 5000 rpm (at this speed, the 250 MZ ETZ has a 19 HP output). I am almost sure that at 5000 rpm, the 350 Scrambler is more powerfull than the 250 MZ. Here attached the 250 MZ ETZ genuine curve. Reason why I am looking for the 350 Scrambler curve...

[img]G:\Claude\Motos\250MZPowerTorqueCurve.jpg[/img]

[DewCatTea-Bob]

Hello,

Many thanks for your help, you are both an Ducati engineer and an american langage teatcher, the top

Well, you didn't reply to my questions about the power curve but they are crucial. Of course, my cam is a genuine one, that I've just replaced. I can confirm it is a Scrambler one (otherwise, my previous wording would have been different). As you know, the engine has a lot of torque (even if the max power is annouced around 8500 rpm). That means it can afford a small rear crown. The power curve is determinent for choosing the number of teeth, and the second element that is as much determinent is the kind of usage the owner is aiming... Calculation methods (that I know too) are only to serve this goal, i.e. finding a trade off between what the engine is capable of, and what the driver is waiting for, it is not the contrary of course.

On the Amici dello Scrambler provider website, there are 35 teeth crowns (presented as for 428 models) but I've asked yesterday if it is compatible with the 350 chain threat or not. I am waiting an answer (is seems that 428 Scrambler and the lower models are not compatible with the chain thread). Otherwise, I have a friend who have many sprockets and crown dimensions (maybe for racing) and I can order from him if necessary.

My objective remains to cruise at 110 km/h at 5000 rpm. I think it is a normal expectation from a 350cc with a 24hp output, especially if the power curve is relatively flat*. Do you have such curve for the Scrambler ?

*With my 2 stroke 250 MZ ETZ deeptly modified (you can see all the mods on the crankshaft, ignition and clutch here : http://strv.pagesperso-orange.fr/motodesign/UKETZ.htm ) the genuine power output is around 21 hp at 6000 rpm with a very low torque in comparizon with the 350 Scrmabler, and though the genuine reduction ratio for this 250 enables 108 km/h at 5000 rpm (at this speed, the 250 MZ ETZ has a 19 HP output). I am almost sure that at 5000 rpm, the 350 Scrambler is more powerfull than the 250 MZ. Here attached the 250 MZ ETZ genuine curve. Reason why I am looking for the 350 Scrambler curve...

[img]G:\Claude\Motos\250MZPowerTorqueCurve.jpg[/img]

____ I'll likely respond to your post with more details later,, but for now, I ought-to explain that the 350Scrambler-models which were imported here into the U.S. of A, did-not come-stock with the 'scrambler-cam' !
But I understand that some-OTHER countries received 350Scrambler-models only with the REAL 'Scr.cam', which is color-coded with a dab of white-paint (found on threaded-end of cam-SHAFT.
Have you noted the color-code of YOUR camshaft ?
__ I-myself do-not have a torque/power-curve to post for you, but my guess is that peak-power for the White-cam is around 6800-RPM (+/- 500RPM),, while p-p. for the G&W.cam is between 7500 & 8500 RPM, (depending-on whether M3 or F1 version).


Tillater,
-Bob

[Rocla]

I've check the cam I've replaced (by the same but not damaged): no color but it is ingraved on : "2.5 SCR". With that, is it possible to guess the model, the max power and the relative engine speed ? What you wrotte sounds right, I was very atonished to read everywhere on websides that the 350 Scrambler max power was 24 HP at 8500 rpm, moreover 24HP is very weak for a 350 displacement engine, it is not necessary to reach high speed engine to get it...

[Rocla]

Hello

I didn't get my motorbike back from the garage so I didn't check the rear tyre dimension yet. But a friend of mine sent me specifications tables and it seems that the 250 SCR engine has his maximum power at 6500 rpm and its maximum torque at 4500 rpm. It is quite the same for the 450. Despite nothing is written in the 350 SCR column, it is likely to be the same specifications. It confirms what you said, and my feeling. Given torque and power are easily available, if you're right about the tyre dimension, the best trade off, for my own usage, would be 17 x 40 teeth (my friend tell me that he has 17 teeth sprockets compatible with 350 chain)... 107 km/h at 5000 rpm sounds correct for a 4 stroke with 350cc displacement... With 16 x 40, it would be only 100 km/h at 5000 rpm. By the way, why am I considering 5000 rpm? For me it is like a psychological threshold that is not to be exceeded when cruising...

[DewCatTea-Bob]

[quote= Rocla ...
" I didn't get my motorbike back from the garage so I didn't check the rear tyre dimension yet.

____ I've been putting-off a response-post until you got your rolling-circumference measurement corrected, and-also have been hoping that someone-else would chime-in with some trustworthy torque/power figures first.
But your new-post here has inspired me NOW.



" a friend of mine sent me specifications tables and it seems that the 250 SCR engine has his maximum power at 6500 rpm and its maximum torque at 4500 rpm. "

____ For a 250SCR, that seems rather low to me, unless it's with a somewhat restrictive muffler. _ Cuz otherwise, I would've expected max.power to be more around 7200-RPM.



" It is quite the same for the 450. "

____ That seems more believable, since the 450 can't breath well enough to fill it's cylinder.



" Despite nothing is written in the 350 SCR column, it is likely to be the same specifications. "

____ That would be fairly sensible IF the 250's spec is really at as low of a RPM as the 450's. _ But, all else being the same -(carb, cam, muffler, etc.), I find this hard to accept ! _ As I'm quite sure that the 250's max.specs -(T & P) would occur at a higher RPM !



" It confirms what you said, "

____ Not that those stated spec.figures were actually correct.
__ I don't clearly recall any specific power-spec for any Duke-model, but it seems to me that the 350-Sebring's was said -(by some cycle-mag) to-be 24-HP at 6800-RPM, while the 350SSS & 350Mark-3 were 27-HP at 7600-RPM & 28-HP at 7800-RPM -(the slight difference probably being due-to their varied mufflers). _ But while these figures are likely not spot-on, they are none-the-less pretty-close to actual -(at crankshaft, not rear-wheel).



" if you're right about the tyre dimension, the best trade off, for my own usage, would be 17 x 40 teeth "

____ That seems rather HIGH.
So-then, what exactly is YOUR intended "usage" supposed to be for in particular ? _ As I HAD gathered that you wanted to make-sure that your chosen-gearing would be set so-as to align max.power against max.wind-resistance (for obtaining max.top-speed).



" 107 km/h at 5000 rpm sounds correct for a 4 stroke with 350cc displacement... "

____ To obtain THAT speed@RPM in 5th-gear, you'd then need a 49/17t.sprocket-combo.



" With 16 x 40, it would be only 100 km/h at 5000 rpm. "

____ I don't know how you came-up with that result. _ Do you mean in a lower-gear than 5th-speed ?
Because "100 km/h" is nearly 24% lower than actual for 5th-gear ! _ That is, unless you expect your rear-wheel to be ONLY 67-inches in circumference.
As I've already calculated the 40/16t.combo in 5th-gear at 5k-RPM for you, in my prior-post,, and my stated result was 124k/h .
__ The 40/17t.combo (in 5th-gear @ 5k-RPM) would result with a speed of about 131k/h .


" By the way, why am I considering 5000 rpm? For me it is like a psychological threshold that is not to be exceeded when cruising... "

____ I guess I find that to-be a reasonable reason. _ However, I don't think that there's anything actually magical about that precisely-particular 5000-figure,, so rather instead, I really think you should consider the closest RPM to that 5k-figure which fairly-much calms-down eng.vibes, so that you may thus-then choose sprocket-gearing that'll match that smoother-RPM with the speed-limit of the highway-roads which you most-often travel-on.


Hopeful-Cheers,
-Bob

[Rocla]

At last I've got my motorbike back, and I've got some additionnal pieces of information :
- the diameter of the rear tyre is 66 cm, that means with 15x47 the speed you spoke about ! You were right !
- the new came that is mounted on my bike is a white color, the same than the old one which is engraved 2.5 SCR on, that means 24HP it seems
- today, I've met a 250 Scrambler owner with a 40 teeth rear crown, with no reduction ratio difficulty feeling, that doesn't amaze me at all

So, in a nutshell:
- with the 15x47 reduction ratio, when I am at 5000 rpm/mn (in 5th speed, I specify to avoid any additionnal question as you said, I am only at 80 km/h !
- my 250 MZ before modifications, with the genuine 21 HP, was at 107 km/h at 5000 rpm without any difficulty (of course it is the genuine ratio)(of course, 5000 rpm is not a magical threshold, I could choose 4977,2 rpm, but you understand the advantage of 5000 it is like for financial markets, simple and round figures are attractive and easier to memorize and to monitor)(for my car, which is a 2000cc displacement, my threshold was different when I moved the 3rd, 4th and 5th speed sprockets, and I chosed them shorter)

- so, as I see it, I can easily afford a 17x40, that could mean a max speed somewhere between 125 and 135, probably with a maximum below the maximum power speed, it is to say a max speed a little bit below the potential, but with a better comfort for cruising, and good acceleration too... But of course, it could be easier to decide with the power curve For sure such curve exists... But where to find it?

[DewCatTea-Bob]

[quote= Rocla ...
" the diameter of the rear tyre is 66 cm, "

____ That's 26-inches which means it's full-circumference is 81.6-inches,, however the actual rolling-circumference will be even still less ! _ So it ought to actually be measured as I've suggested.



" that means with 15x47 the speed you spoke about ! You were right ! "

____ Yes I was "right" BUT, only for 57/15-gearing, and with an 83-inch rear-wheel working-circumference ! - (I'm assuming that your '4' in "47" must be a typo-error, as you've previously stated "57" !)
__ Now we know that your rear-wheel is even smaller than even I-myself had guessed it may be... Which means that all my previous figuring-results were off by at-least 1.7 %,, and-so to adjust my past result-figures, they thus now need to be multiplied by '.983'. _ Which means that my previous stated "82-k/h at 5k-RPM" is now just 80-k/h ! - (Which is exactly 10.0-MPH per 1k-RPM [and NOT "10.1", as I had stated before].)



" the new came that is mounted on my bike is a white color, the same than the old one which is engraved 2.5 SCR on, "

____ Would STILL like to know what country your 350 was originally imported into, because this news tends to confirm a suspicion I've had as to why 350Scrambler-models imported into the USA did-not come stock with a scrambler-cam.
__ Anyhow,, this means that your 350 has the superior low-end & mid-range power (and even high-end power, [over 350s with the G&W.cam], when running with a muffler).
So if you're not intending to go 'racing', then you luckily have the better-choice of camshaft-model.



" that means 24HP it seems "

____ Actually, since the 350-Scrambler has larger carb, port, valves, and slightly wilder cam than the 350-Sebring, I'd thus expect that your 350 produces that 24-HP at the rear-wheel (rather than merely at the crankshaft).



" today, I've met a 250 Scrambler owner with a 40 teeth rear crown, with no reduction ratio difficulty feeling, that doesn't amaze me at all "

____ Well that of-course stands to good-reason,, cuz as I've already pointed-out,
the 250-motor has a LOWER primary-ratio ! _ Which makes it's overall-gearing 18.5% lower than your 350's !
So-thus, a 40t.rear-sprocket on a 250 is about the same as a 47t on a 350 !



" with the 15x47 reduction ratio, when I am at 5000 rpm/mn
as you said, I am only at 80 km/h ! "

____ Is that "47" a typo AGAIN ?
Anyhow,, the 80-k/h.speed at 5k-RPM is indeed CONFIRMED with the 57/15t.gearing !
__ However, "47"/15t.gearing would result with 97-k/h @ 5k-RPM !



" (of course, 5000 rpm is not a magical threshold, I could choose 4977,2 rpm, but you understand the advantage of 5000 it is
simple and round figures are attractive and easier to memorize and to monitor) "

____ Well of-course having the resulted-speed for each 1k-RPM , (such-as your 16.1-k/h per 1k-RPM), is of-course quite logical to have, as you've indicated,, however the "5000", not so particularly as much though.
But-however,, if you happened to find that your motor-engine vibrated any less at about 4975-RPM, when traveling at your posted speed-limit,, then you ought-to shoot-for sprocket-gearing that came closer to [b]that]/b] RPM, rather than the "5000".



" so, as I see it, I can easily afford a 17x40, that could mean a max speed somewhere between 125 and 135, "

____ That would be a cruising "max speed" of 129.2-k/h @ 5k-RPM -(80MPH), actually.
And with your 350's gearing being set so high as that, it's somewhat rather doubtful that your 350-engine would produce enough power to hold-at 130-k/h at that low of RPM.
__ I agree with choosing the 17t,, however in combo with the 40t, you may rather wish for 2 to 5 additional teeth.
At what speed do you prefer to cruise at ?



" probably with a maximum below the maximum power speed, it is to say a max speed a little bit below the potential, but with a better comfort for cruising, "

____ I believe I understand what you're meaning to convey...
You're realizing that the 40/17t.gearing is going to be too-high to reach high enough RPMs to make enough power so-as to obtain the absolute top-speed that's possible with your particular engine,, but yet, you prefer to accept the trade-off for the resulting improved-comfort of lessened eng.vibes while running at cruising-speeds.
__ And if you happen to choose your sprocket-gearing just-right, you may then end-up being able to attain the top top-speed in 4th-gear.
____ I have a 350-Sebring of-which I once raised it's sprocket-gearing up-to 2.0:1 ! _ And I next found that the overall-gearing was then raised so very HIGH, that it made 5th-gear absolutely USELESS !
So I had to replace the rear-sprocket with another which had 3-more teeth, (thus-then providing a 2.2:1 ratio), and that made 5th-gear useful once-again,, so I could then really enjoy the lower-revving engine at cruising-speeds !
__ BTW, your considered 40/17t.sprocket-gearing has a ratio of 2.35:1.


Hopeful-Cheers,
-Bob

[Rocla]

Hello Bob,

Your answer is interesting and raises some remarks and questions :

-diameter or cirumference, it is the same source, given you know the formula to calculate the second from the first one. I guess, if you emphasize on the second, is it because it is more specific and reliable to measure?
- my Scrambler comes from Barcelona in Spain, and it is in theory the same model than the ones we can see in France (I am French) and UK but there are so many combinations, it is a big mess, Mototrans, that was the maker in Barcelona, happened to modify many things, and not only on the body, but on the engine too. Except for the 1974 models, because Mototrans was the only maker for all the Scrambler engines, including for Italian market ! So, I have a kind of certainty that my engine is the same of every 1974 Scrambler in Europe. I would be prone to add "and for American market too" but I don't really know, may be there are some legal or cultural constraints?
- the current combination is 47x15 and, when I came from Barcelona (I was brave to tempt coming back with the bike just after buying it, but it worked perfectly) my feeling was, if I rely on my ears, to run 90 km/h at about 5000 rpm. Maybe there was a confusion between 47 and 57 but it is 47 teeth on the rear and 15 teeth on the front sprocket. I don't catch anymore. Could you confirm the speed at 1000 rpm in 5th speed with a 47x15 and 350 Scrambler genuine engine? Is it about 16 or 18km/h?
- I don't have the sufficient knowledge to introduce air penetration forces but your top speed estimation seems OK to me, and it won't be so far from the genuine top speed (130 !). My objective is to be somewhere between 105 and 110km/h at 5000 rpm. The only information I need to order new sprocket and crown is to know my actual speed at 1000 rpm (or 5000)... By mails, it is not easy to communicate, but at last we will find the solution
- I introduce an another topic, to take benefit from your knowledge: I've noticed that the exhaust seal at the head cylinder exhaust port is with a lower diameter than the port itself (4mm less). I will enlarge it and the beginning of the exhaust pipe too. I think, in addition with the conical air filter, thanks to that, I can grab some additionnal power without lossing torque. I don't want to modify the ports timing and moving the cam for a more agressive one, it is the best mean to loose torque (given the number of miles on the clock, 44 000 km, high speed engine is not recommended any more). Do you think the conical filter plus the increase in the exhaust port diameter could help the engine a little bit?
- another additionnal topic: I was told that all the revel single Ducati engines were noisy. Is it because the conical sprocket on the top cylinder? I am wondering about sticking a rubber layer inside each head cylinder lid. Rubber is very efficient for noise isolation
-last question: do you think electronic ignition is significantly profitable for a 12v Scrambler performance?