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Snappy Makes Me Happy


faffi
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I don't know about you, but I personally don't bother how fast a bike is, but how fast it feels. Probably because I have no races to win.

 

While riding the Honda CB400SF today, I were again annoyed by the lack of response from the engine. Whenever you open the throttle, it's like the little screamer says" Really? You want me to go faster? Purrrrleeezzz, give us a break!" It's alike a flaccid rubber band, or like a crank turning in molasses. The bike isn't slow (for a 400), but it feels gutless.

 

I've had a couple of other bikes that felt similar. One was a Kawasaki Z1300DFI. Horrible. Very fast, but felt very slow. I switched bikes with my brother's XL500S, and its engine felt ever so willing and eager. I expected them to be pretty similar 0-60. Hah! Have you seen cartoons when a fast car passes a slow car so the latters just spins like caught in a tornado? That's how it felt to be passed by the 1300. But the XL felt great, the 1300 so bad I sold the bike after only 300 miserable miles.

 

So what do you prefer, engines that pick up smoothly or that hits instantly and with hard initial acceleration the moment you open the throttle a little? One that wil snap your head backwards even from low rpm, or the one that schrieks and goes like hell when the rpm gets high enough, but still is soft in its delivery?

 

I know what I want; a snap, in every gear, at any rpm. I don't care about the clock, I just want instant, crisp throttle response.

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Slow throttle opening = soft response. Rapid throttle opening = big lunge forward. If the bike has sufficient power, both responses are available on tap - just depends on the rate of throttle opening. That's called good fueling, and the *rider* should be able to modulate it as s/he chooses. Too snatchy in response to minor throttle adjustments and you have a bike ill suited for the track IMO, ad annoying on the street. It *should* be able to throw your head back...but only when you ask it too!

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Slow throttle opening = soft response. Rapid throttle opening = big lunge forward.

 

That depends on tuning and power. Try to whack the throttle open from 1500 rpm in top gear on a bike with a 10k redline, and chances are it would run better with less throttle. At least if the bike has carbs.

 

Ideally, the throttle should work like a rheostat; half throttle = half power, full throttle= full throttle. But in reality, half throttle usually gives you 80% power in my experience. Also, if you don't have surplous power, you may find that the bike will climb a steep grade with maybe 1/5th throttle, but will not have power to accelerate even using full throttle.

 

Of course, those spoiled with modern fuel injected litre sized race reps will not know what I'm talking about :P

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I don't know about you, but I personally don't bother how fast a bike is, but how fast it feels. Probably because I have no races to win.

 

While riding the Honda CB400SF today, I were again annoyed by the lack of response from the engine. Whenever you open the throttle, it's like the little screamer says" Really? You want me to go faster? Purrrrleeezzz, give us a break!" It's alike a flaccid rubber band, or like a crank turning in molasses. The bike isn't slow (for a 400), but it feels gutless.

 

I've had a couple of other bikes that felt similar. One was a Kawasaki Z1300DFI. Horrible. Very fast, but felt very slow. I switched bikes with my brother's XL500S, and its engine felt ever so willing and eager. I expected them to be pretty similar 0-60. Hah! Have you seen cartoons when a fast car passes a slow car so the latters just spins like caught in a tornado? That's how it felt to be passed by the 1300. But the XL felt great, the 1300 so bad I sold the bike after only 300 miserable miles.

 

So what do you prefer, engines that pick up smoothly or that hits instantly and with hard initial acceleration the moment you open the throttle a little? One that wil snap your head backwards even from low rpm, or the one that schrieks and goes like hell when the rpm gets high enough, but still is soft in its delivery?

 

I know what I want; a snap, in every gear, at any rpm. I don't care about the clock, I just want instant, crisp throttle response.

 

I agree, I want it to GO when I open the throttle.

 

But, tuning (and gearing) can have a big impact on this. I've had a good mechanic take a couple of my bikes from crappy to snappy. :) My little 250 (carbureted engine), when, tuned properly, feels very crisp with an immediate response, and on my 600 a power commander made a WORLD of difference.

 

Even my YSR50 will wheelie when shifting to second gear... but on that one, to get a quick start from a standstill requires significant rider participation, a la Fred Flinstone. :)

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Even my YSR50 will wheelie when shifting to second gear... but on that one, to get a quick start from a standstill requires significant rider participation, a la Fred Flinstone. :)

LOL :D

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Well, Hotfoot, it's not that long ago that the 125s in GP racing had to paddle along to get off the starting line. Low power output, a narrow powerband tall gearing meant they needed to get up to speed before they reached their stride. Your 50 is a lot smaller than a 125, so no wonder it want som Flintstone assistance to get going :D

 

I agree that gearing and also a quick-turn throttle can help making an engine more responsive, but both the Z1300 and the CB400 lack any hint of snappiness regardless of rpm and gear; pick-up is always soft, as if the crank is too heavy to allow quick changes in rpm. I'm pretty sure that's part of the problem for the 400, but it was the fuel injection of the 1300 as my brother had a carbed version of the same bike and it was very urgent to rev.

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I've never actually bought that argument about a heavy crank preventing the motor from "spinning up" quickly. Sure it makes a difference on throttle blips, or engine braking when gearing down, but under acceleration it is the mass of the bike that is preventing the motor from increasing more quickly in rpm, not the weight of the crank or flywheel.

 

It's actually easy to see this in dyno graphs where people have added a lightened flywheel (common with Ducatis). In first gear, you can realize a small hp gain, because the motor is increasing in rpm very quickly when accelerating in low gear, so you are storing energy rapidly in the momentum of the engine internals. But in the higher gears it makes no difference at all because the increase in engine speed (and therefore kinetic energy stored in the flywheel) is very small anyway compared to the kinetic energy stored in the forward motion of the bike. Above first or second gear the dyno will show no perceptible increase in hp with a lightened crank.

 

I always laugh when people show off their lightened crank and flywheel by blipping the throttle with the bike in neutral.

 

"See how fast she spins up now?"

 

"Yeah....so?"

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You do not get more power with less flywheel, but it makes it much easier for the engine to spin up. Acceleration will be better, but as you say less so at high speeds.

 

When Honda made the NR500, its flywheel was so light that it wouldn't idle below about 10,000 rpm and closing the throttle would lock the rear wheel in just about any gear at any speed. A Moto Guzzi from 1970 has so much flywheel that chopping the throttle will feel like pulling the clutch. All this energy must come from someplace, and it does. It is felt in the form of less acceleration and soft throttle response.

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Okay, so this is becoming a Physics debate :) . That's fine with me - it is fun!

 

My mind is totally open on this, but based on my understanding I currently disagree with two things in your last post:

 

1. A lighter flywheel *will* give you more hp at the wheel when the engine speed is increasing rapidly (i.e., in low gear, as we agree), because the energy that is not being stored in the momentum of a heavy flywheel is instead available to accelerate the bike. That by physical definition is more power because you are delivering more energy (or, doing more work) in the same amount of time. You will also measure it as more RWHP on a dyno.

 

2. I still don't see how a heavy flywheel results in "soft throttle response" in any gear other than one so low that the heavy flywheel is substantially affecting acceleration. The engine is directly connected to the rear wheel via the clutch, transmission and final drive, so bike speed and engine speed increase in exact proportion. In other words, the engine can only "spin up" as fast as the bike accelerates, regardless of the weight of the flywheel. Either the energy needed to accelerate the flywheel is significant relative to the energy needed to accelerate the bike, or it isn't. In third gear or higher, it generally isn't, because the flywheel isn't accelerating very quickly. Even in second gear it is generally a pretty minimal effect.

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That's too bad. The CB looks like a neat little bike. We don't have that. Just from my ZX6 on the track vs my EX on the street, I like the snappy.

 

How's this for a new saying?

"An engine with smooth power delivery is for people who need to work on throttle control." wink.gif

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When Honda made the NR500, its flywheel was so light that it wouldn't idle below about 10,000 rpm and closing the throttle would lock the rear wheel in just about any gear at any speed.

 

I don't buy this one either. When you are in gear and moving, the whole bike is the flywheel. You would have to pull in the clutch, not merely close the throttle, for the weight of the flywheel to matter.

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When Honda made the NR500, its flywheel was so light that it wouldn't idle below about 10,000 rpm and closing the throttle would lock the rear wheel in just about any gear at any speed.

I don't buy this one either. When you are in gear and moving, the whole bike is the flywheel.

This is not precisely right. It's only the rotating parts that act as a flywheel. Sure the, say, swingarm has kinetic energy due to the speed of the bike, but it has no rotational energy stored in it.

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When Honda made the NR500, its flywheel was so light that it wouldn't idle below about 10,000 rpm and closing the throttle would lock the rear wheel in just about any gear at any speed.

 

I don't buy this one either. When you are in gear and moving, the whole bike is the flywheel. You would have to pull in the clutch, not merely close the throttle, for the weight of the flywheel to matter.

 

Me neither. A low flywheel weight would make it less likely to lock up the rear wheel. I'm thinking extremely high compression. Snappy throttle response ? Use the gearbox more or lower the gearing and or lose weight :P

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the cb400 is a commuter , if honda gave it a I4 screamer, the current chassis and suspension esp ones at the back isnt gonna cut it imho...

 

the one thing you dont want on a commuter naked bike is too much power as demostrated by pre 2011 ninja 650's... frame/ swingarm flex is NOT NICE

 

 

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When Honda made the NR500, its flywheel was so light that it wouldn't idle below about 10,000 rpm and closing the throttle would lock the rear wheel in just about any gear at any speed.

I don't buy this one either. When you are in gear and moving, the whole bike is the flywheel.

This is not precisely right. It's only the rotating parts that act as a flywheel. Sure the, say, swingarm has kinetic energy due to the speed of the bike, but it has no rotational energy stored in it.

 

 

What I meant was that the momentum of the bike moving forward, acting through the rear wheel and drive train, will keep the motor spinning regardless of any flywheel momentum. You don't really need a flywheel once you are in gear and moving. That's why shutting the throttle is not going to lock the engine even if the flywheel is really light.

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YellowDuck, I think you would find that the lightened inertia engine makes more power up to about peak power, give or take, then makes less than a an engine with lots of inertia as the latter starts to give back the energy it absorbed during lower rpm.

 

Another interesting consideration is that more flywheel effect will make the engine turn more smoothly, which should aid valve timing and hence give more power from being more precicely tuned.

 

As to the whole engine working as a flywheel; my XT 600 would simply lock up if revs got too low, locking the rear wheel. And it had a pretty hefty flywheel. Less flywheel would have made it possible for the wheel to be locked at higher rpm, a hundred pound flywheel would have made it nigh on impossible to lock up.

 

Anyway, the flywheel will affect how quickly an engine can change rpm, up and down, and although these effects will be lessened when pulling a vehicle, the effects will still be quite noticeable. Lighter flywheel should, all things else being equal, offer more engine braking and quicker acceleration with lumpier low-end power delivery and less top speed potential. The most noticeable effects will be in engine braking and low rpm running.

 

 

 

 

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YellowDuck, I think you would find that the lightened inertia engine makes more power up to about peak power, give or take, then makes less than a an engine with lots of inertia as the latter starts to give back the energy it absorbed during lower rpm.

 

Another interesting consideration is that more flywheel effect will make the engine turn more smoothly, which should aid valve timing and hence give more power from being more precicely tuned.

 

As to the whole engine working as a flywheel; my XT 600 would simply lock up if revs got too low, locking the rear wheel. And it had a pretty hefty flywheel. Less flywheel would have made it possible for the wheel to be locked at higher rpm, a hundred pound flywheel would have made it nigh on impossible to lock up.

 

Anyway, the flywheel will affect how quickly an engine can change rpm, up and down, and although these effects will be lessened when pulling a vehicle, the effects will still be quite noticeable. Lighter flywheel should, all things else being equal, offer more engine braking and quicker acceleration with lumpier low-end power delivery and less top speed potential. The most noticeable effects will be in engine braking and low rpm running.

 

We don't seem to be making much progress here. :) Not sure how I can better explain what I have already said. Let's try this:

 

1. The effect of a lightened flywheel has nothing directly to do with where you are in the power curve. The heavier flywheel will not be "giving back energy" when you pass the power peak, if the bike is still accelerating. The heavier flywheel will store energy when the engine *speed is increasing* (causing lower hp output), and give back energy when engine *speed is decreasing* (causing reduced engine braking). Since the engine speed is directly linked to the bike speed when you are in gear, the flywheel stores energy when the bike is accelerating, and gives it up when the bike is deccelerating. This effect is strongest in the lower gears because that is when engine speed (and therefore flywheel momentum) is changing most rapidly relative to the change in bike speed.

 

2. Flywheel mass has no effect whatsoever on top speed potential. At top speed, whether speed is aero limited or rpm limited, the vehicle speed (and therefore engine speed) is neither increasing nor decreasing, so the flywheel is neither storing nor giving up energy.

 

I think you are correct that a heavier flywheel aids low rpm running, but especially "low load" running where you are on and off the throttle.

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You forced me to start reading about this again, not just going by memory :D Perhaps no more progress will come, but this is a pretty good and easy to understand site that does explain pretty well the effect of lightening flywheels. As you stated, the effect is less in a tall gear than a low gear. But the effect is quite dramatic even on a heavy car.

 

http://www.uucmotorwerks.com/flywheel/how_a_lightweight_flywheel_works.htm

 

Just as lower compression tend to increase top speed compared to very high compression, I still believe that a heavier flywheel will allow a vehicle to - very, very slowly - be able to gain speed with a tailwind or downhill beyond what it would do with a light flywheel. Both at the expence of acceleration at lower speeds, however.

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this is a pretty good and easy to understand site that does explain pretty well the effect of lightening flywheels.

 

http://www.uucmotorw...wheel_works.htm

 

 

That is a good site, with some interesting calculations! Note that by the time you are in only third gear, the benefit of the lightened flywheel on acceleration is only about 20% of what you had in first.

 

I also agree with them that the most important effect of a lightened flywheel might be how it affects things like rev matching during throttle blips (i.e., how the engine spins up under no load), and how it decreases engine braking. Those things can really affect the "feel" of an engine in use....just not so much in upper gear acceleration.

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I don't know about you, but I personally don't bother how fast a bike is, but how fast it feels. Probably because I have no races to win.

 

Yeah, I am kind of thinking more that way myself. Not exactly wanting a bike that "feels fast" (without actually being fast), but wanting a bike that is exciting to ride. I think a 2-stroke supermoto will do nicely...

 

Even for people who buy a 600/1000cc sportsbike for track days etc., I'm sure a lot of them like to paper race with mates at the pub, but not many people would use anything close to the power of even the slowest 1000cc bike. If you could see everyones throttle use, I would say that only a handful of riders at any given track day would use literally 100% throttle. Therefore if you're not using 100% throttle, you don't need a bike with more power, right? Actually I need to keep an eye on that myself - thinking back I can't even remember putting the throttle to the stop on the straights... (don't laugh - my regular track has a short straight!) tongue.gif

 

How's this for a new saying?

"An engine with smooth power delivery is for people who need to work on throttle control." wink.gif

 

 

I couldn't have been happier when I got my bike back after a dyno tune - the power delivery was so smooth and it just felt like a different machine. But then I don't ride to such a high level that I can crack on the throttle and drive with the exactly right amount of rear wheel slip right from the apex, so I obviously do need to work on my throttle control. smile.gif

But that smooth power delivery doesn't mean the bike doesn't have good throttle response - snap that throttle open and things happen, fast! blink.giflaugh.gif

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Even for people who buy a 600/1000cc sportsbike for track days etc., I'm sure a lot of them like to paper race with mates at the pub, but not many people would use anything close to the power of even the slowest 1000cc bike. If you could see everyones throttle use, I would say that only a handful of riders at any given track day would use literally 100% throttle. Therefore if you're not using 100% throttle, you don't need a bike with more power, right? Actually I need to keep an eye on that myself - thinking back I can't even remember putting the

 

 

That's one of the reasons that I love my bike on the track. It is an air-cooled 2-valve twin, and probably makes about 90 hp. However, I get to use all of that on the main straight, and sometimes on the shorter straights between corners as well. I am at 100% throttle *a lot*. It is fun.

 

Sure, the inline 4's (even the 600s) often zip by me on the straight, but they have to work at it because given the same line and corner speed I can walk away from them at first on the corner exit with all of the low rpm torque I have available (about 70 ft-lbs). If they show me a wheel before the next braking zone I happily let them by, but it's not easy for them!

 

It's actually a pretty flattering bike for a rider with middling skills, and it really is fun to be able to use all of the motor on a regular basis.

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That's one of the reasons that I love my bike on the track. It is an air-cooled 2-valve twin, and probably makes about 90 hp. However, I get to use all of that on the main straight, and sometimes on the shorter straights between corners as well. I am at 100% throttle *a lot*. It is fun.

 

Sure, the inline 4's (even the 600s) often zip by me on the straight, but they have to work at it because given the same line and corner speed I can walk away from them at first on the corner exit with all of the low rpm torque I have available (about 70 ft-lbs).

 

This is one of the things that I don't understand about the "V-twin guys" (yes, they're all guys): this thing (I'd call it an obsession) about the so-called "low and mid-range torque" of a V-twin. My experience is that a V-twin have LESS usable RPM range and LESS low/mid-range RPM torque than an inline-4 R6. I guess they say this simply because of the total lack of top-end power of said bikes :P (OK, I'll keep Bullet's missile out of this).

 

Some years ago, a friend and I swapped bikes at a trackday, just for fun: my bike (at the time) was a 2001 R6 with 95 bhp,his bikes was a SV650. The R6 had about 50% effective RPM range; on the SV650 it was 33%. I found myself changing gears much more on the SV650 than on the R6.

This was on Anderstorp, a big fast track, so yes, I spent a lot of time at WFO throttle. I walked away with a healthy respect for my friends laptimes, since it was clear that he was limited by his bike (On the SV, he did 1:58, I did 2:02. On the R6 I did 1:55 at the time).

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This is one of the things that I don't understand about the "V-twin guys" (yes, they're all guys): this thing (I'd call it an obsession) about the so-called "low and mid-range torque" of a V-twin.

Kai;

As one of these "guys" I like the fact that I can short shift entering a corner and let the twin's torque carry me out and thru what would have been an awkward shift moment.

 

The best example I can think of is the Bowl (or T-5) at Loudon where you descend downhill into a 180 degree turn and then climb back up the same slope you came down to enter into this 180 corner. Where it gets interesting is a kink as you approach the last quarter of the climb out which is just before the bike typically requires an upshift. If you had downshifted as you entered the Bowl you end up leaned over, up against the rev limiter when you apex the kink. An instructor told me to forgo the downshift at the base of the hill where you enter the bowl and use the twins torque to climb all the way out past the kink. I didn't think it would work but once I tried it I understood better why low end grunt can pay dividends. YRMV.

Rain

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