UK Monster Owners Club Forum » .: Technical :. » Engines, Clutch, Gears » Anyone remember thrust curves?

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Old 22-03-2020, 11:05 PM   #1
Luddite
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Anyone remember thrust curves?

I thought I'd start a new thread for this post rather than clutter up FrakenDesmo's http://www.ukmonster.co.uk/monster/s...d.php?p=573930

Quote:
Originally Posted by Flip View Post
It’s nice to know it reaches maximum revs reasonably cleanly but pressing on by revving it to 9k through the gears is kind of missing the point of the old 2v 900 as that’s way past peak torque which the wave you want to be riding to enjoy the bike at its best.
I'm with Flip on when to change up - the main thing that appeals to me about the 2v motor is that fat midrange. I really enjoy surfing that wave of torque and rarely venture past 6.5k.

Having said that, when I had my old S2R 1000, I made a point of finding out what the actual optimum gear change points were.

Now, who remembers the thrust curves that used to appear in service manuals back in the day and even some owners' manuals (that's when manuals used to contain useful information, not just an endless list of health and safety warnings!)?

They used to show the effective thrust applied at the rear wheel against the road at various road speeds in each gear and were designed to show the best speeds to change gears (in the days before revcounters were common).

Anyway, bearing in mind that the raw graph of torque at the rear wheel for the S2R 1000 looks like this:


Then this is what the resulting road thrust graph looks like for each gear:



You can see from these curves that the optimum gear-change point is where the curve for one gear intersects with the next. It is actually after the peak torque in any particular gear because, even though engine torque is dropping off in the lower gear, rear wheel thrust is still higher than that produced by the next gear for another couple-of-thousand revs.

So, despite the fact that peak torque is produced between around 5,500-6,200, rear wheel thrust in first is higher than that in second (for an equivalent road speed) until around 8,700, when second gear produces more and changing up is beneficial.

This is because of the multiplying effect of the various gearbox ratios, (the lower gear multiplies the engine's torque more than the higher gear), and it also explains why there's not one recommended shift limit for all gears.

Even knowing all that, as I said earlier, I still prefer to change up around the 6k peak torque - it just feels right.
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Old 23-03-2020, 04:48 AM   #2
350TSS
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I am sorry I think I disagree with your peak change points in the second graph above. If you hang onto the gear until 8.7, 8.5, 8.3, 8.0 and 8.0 you have lost the best acceleration available, i.e when the graph is rising at its steepest.

Increase in speed on the graph happens fastest when the graph is rising steeply. Therefore on your graph 2 above effectively the length of the line between gear changes is equivalent to elapsed time.

If you change gear at the highest point of the graph for each gear, when the next gear is engaged you continue your ride on that part of the graph that is still climbing steeply and you reach the next peak sooner, you are gaining speed faster.

Your seat of the pants feel - change at about 5750/6000/6250 is absolutely the best (fastest) way to make progress

Last edited by 350TSS; 23-03-2020 at 05:05 AM..
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Old 23-03-2020, 07:33 AM   #3
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I love the big, fat mid range of Ducatis, I'm much happier than when revving the what-nots off an IL4, probably why I like diesels aswell - the current daily is a 5 cylinder one.

I don't tend to look at the rev counter when I'm out and about and I change up when I feel the shove tailing off, it's more accurate that trying to see what the wobbly rev counter is saying anyway !

My ZXR750J1 also had loads in the middle (and it's one of the few vehicles that made less outright power than its predecessor but it did however have more torque) and in the real world, not a lot could keep up. The GSXR600WV that replaced it though was ridiculously 'busy' as it had to be revved hard everywhere and together with the ART race can, made it completely anti-social and not much fun.
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Old 23-03-2020, 07:38 AM   #4
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You don’t have the same problem with an early M900. There’s no rev counter to distract you from what “feels” right.
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Old 23-03-2020, 08:13 AM   #5
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Yes, I agree with the concept of relatively-short-shifting, which keeps you in the rev-range of the most enjoyable features of Ducati engines and also goes some way to reducing high-rev engine wear too because valve clearances stay static much longer. It may come as a surprise to die-hard air-cooled Ducatisti that the 4V motors have massive mid-range too, and I find I don't need to even approach double figures on the S4R's taco to proceed in an alarmingly antisocial manner!

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