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Old 03-21-2005, 06:12 PM   #1
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Question Jerkiness when you take your foot off gas

Hey all,
I've got a manual '8 and have noticed that when I take my foot off the gas, the car instantly jerks back. This is especially apparent in first and second gears.
This is my first manual transmission that I've owned, so I want to ask: do they all do that? To this extent? It really is beginning to get annoying. I can't let off the gas without jerking back and forth every time. If it's NOT typical, what could be the problem (before I take it in)?
Thanks! Take it easy
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Old 03-21-2005, 06:19 PM   #2
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i noticed it too and have had several manual cars.....i think its just the way the trans is....normal i guess
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Old 03-21-2005, 06:24 PM   #3
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Are you easing off the gas?
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Old 03-21-2005, 06:24 PM   #4
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I think it's normal if you're at low speeds for that gear.

Once you get to like 10mph in 1st it shouldn't be jerky. Or like say 15+ in second.

I've noticed that too.
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Old 03-21-2005, 06:28 PM   #5
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This is normal for a stick in low gears at low speeds. Especially for a high rpm responsive engine like the RX8 has. I usually use 2nd in stop and go trafic to avoid it.
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Old 03-21-2005, 06:49 PM   #6
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It probably also has something to do with the fact that the engine has a relatively low amount of moving mass, so less stored energy to "soak up" throttle changes. The flip side is you get better response when you *push* the pedal
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Old 03-21-2005, 08:20 PM   #7
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yeah it's perfectly normal. you shouldn't be jerkin it around so much though. you should definitely be slowly easing your foot off the pedal.
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Old 03-22-2005, 03:20 AM   #8
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the jerkiness gets annoying... im use to of just popin the clutch off in a 2-3rd shift in all my other manual cars.. but not this one.. it jerks even when shiftin in 3rd..and sometimes 4th... i can understand in lower rpms but mine does it even in powershifting durin higher rpms... it feels as if the fuels cutting off or somethin..??

need to finesse my skills at this point... i guess it could be the 8s pressure plate is weaker?? doesnt clamp on as instantly as other cars..
i dont know just guessin..
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Old 03-22-2005, 03:23 AM   #9
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Ditto on the flipside......planet that is.

Tried to be smooth, just a quirk I guess.
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Old 03-22-2005, 07:40 AM   #10
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I have noticed this on the AT as well, but maybe my problem is a completely different one... it seems to stutter more when it's cold out
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Old 03-22-2005, 08:05 AM   #11
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1st gear if you let your foot off the gas it will jerk around esp. if the rpms are low..

don't pop the clutch off, ease off and everythign will be much much smoother.. i find i can "pop" the clutch or let off of it faster once i pass the 3rd gear.. Everything started to get much smoother after 10,000km as well.


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Old 03-22-2005, 08:56 AM   #12
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This is a rotary engine guys....as it spins at low revs, especially in first or second, without clutching the torque transferred to the wheels fluctuates more than piston engines, hence the jerking. Perfectly normal, but my advice is to use the clutch more. When driving at slow speeds, constanly use the clutch to smooth it out. For instance, in heavy traffic, I leave her in second and clutch contstantly, keeping the revs smooth and therefore the car smooth. It's a new experience if you've never driven a rotary like the renesis, but once you get used to it, the extra "work" makes you feel like you're really driving this car
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Old 03-22-2005, 09:12 AM   #13
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The car will "buck" any time there is a sharp transient input to the driveline. This includes sharp throttle tip-in and tip-out. The reason this happens is that the lowest torsional natural frequency of the driveline is excited by what is essentially a step input, resulting in low-frequency, high-amplitude translational speed fluctuation (called bucking). EVERY car does this to some extent. The reason it is less pronounced in automatic transmission vehicles is that the unlocked torque converter provides significant viscous damping. Another reason that the RX-8 exhibits more sensitivity to throttle tip-in/tip-out is that there is very little damping on the electronic throttle. The engineers wanted to make this a VERY responsive car, so when you press on the accelerator you get IMMEDIATE response. This is bad for vibration and comfort, but great for performance feel. It's a trade-off--if you add damping to the throttle, you will reduce the vibration at tip-in/tip-out, but you will also add a slight delay to the throttle response (say 0.1 seconds between the time you press/let off the accelerator and the time that the throttle plate actually starts to open/close). To reduce the bucking, you need to learn to modulate the clutch to provide a gradual torque increase/reduction to the driveline. Under certain conditions, like low-speed, low-RPM crusing, the bucking is pretty much unavoidable unless you ride the clutch. It's the price you pay for owning a sports car.
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Old 03-22-2005, 09:15 AM   #14
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^ Great posts Genesis and Buck.
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Old 03-22-2005, 09:22 AM   #15
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Quote:
Originally Posted by Genesis
This is a rotary engine guys....as it spins at low revs, especially in first or second, without clutching the torque transferred to the wheels fluctuates more than piston engines, hence the jerking.
Hmm, interesting thought, but I'd have to disagree. What's your reasoning behind this? I've always heard that one of the advantages of the rotary engine is that there is less dynamic torque due to the lack of reciprocating components (a piston engine has addional inertial torque contributions from the reciprocating components). This is one of the reasons the rotary is silky smooth all the way to redline, while piston engines sound and feel like they're going to explode at redline.

I believe the reason for the bucking is what I described in my previous post. However, your post did remind me of one other thing--the flywheel in the RX-8 is probably much lighter (less inertia) than those found in cars with piston engines. A light flywheel allows more dynamic torque to be transferred to the driveline, thus more torsional vibration response.
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Old 03-22-2005, 09:38 AM   #16
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Quote:
Originally Posted by RX8_Buckeye
Hmm, interesting thought, but I'd have to disagree. What's your reasoning behind this? I've always heard that one of the advantages of the rotary engine is that there is less dynamic torque due to the lack of reciprocating components (a piston engine has addional inertial torque contributions from the reciprocating components). This is one of the reasons the rotary is silky smooth all the way to redline, while piston engines sound and feel like they're going to explode at redline.

I believe the reason for the bucking is what I described in my previous post. However, your post did remind me of one other thing--the flywheel in the RX-8 is probably much lighter (less inertia) than those found in cars with piston engines. A light flywheel allows more dynamic torque to be transferred to the driveline, thus more torsional vibration response.
Don't mind you to disagree Buck...your post is great....I was only commenting on actual "feel" of a rotary vs what I've experienced with manual piston engines. Torsional vibration response in the flywheel? Beyond me, but yer comment about silky smooth all the way to redline doesn't happen in the 8 in the lower revs, unless of course you clutch often to smooth it out. Once it's over 4k rpm then I agree.
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Old 03-22-2005, 09:40 AM   #17
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Amen to the more clutch useage.
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Old 03-22-2005, 10:10 AM   #18
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Very interesting posts, thanks. I guess this is indeed a common issue. In which case, it doesn't bother me as much.
I've been trying to minimize clutch wear but I guess it's unavoidable...
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Old 03-22-2005, 10:26 AM   #19
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Quote:
Originally Posted by Genesis
Don't mind you to disagree Buck...your post is great....I was only commenting on actual "feel" of a rotary vs what I've experienced with manual piston engines. Torsional vibration response in the flywheel? Beyond me, but yer comment about silky smooth all the way to redline doesn't happen in the 8 in the lower revs, unless of course you clutch often to smooth it out. Once it's over 4k rpm then I agree.
Okay, I understand what you're saying. What I'm pointing out is that the reason the RX-8 seems to "buck" more than other vehicles at low revs is due more to the properties of the entire driveline system than it is to the torque excitation from the rotary engine. In all cars, it is not the "smoothness" of the engine that has the greatest impact on the bucking, but the inertial, stiffness, and damping characteristics of the entire driveline system, including engine & flywheel, clutch damper, transmission, driveshaft, diff, axle shafts, wheels & tires, and vehicle mass.

Its a bit hard to explain if you don't have some knowledge of vibratory systems. Torsional vibration is simply a fluctuation in the rotational speed of a component. For example, at a given vehicle speed the driveshaft may rotate at say 500 rpm. In actuality, the rotational velocity of the driveshaft is fluctuating between 490 and 510 rpm, with an average velocity of 500 rpm. The dynamic velocity is said to be +/- 10 rpm, while the mean velocity is said to be 500 rpm. The bucking of the vehicle happens when the wheels have a significant dynamic rotational velocity (torsional vibration). This fluctuation in rotational velocity translates to a fluctuation in vehicle translational speed, and we perceive a bucking effect (repetitive speeding up and slowing down of the vehicle).

To reduce torsional vibration from the engine, a flywheel is used in combination with a clutch damper. The clutch damper is basically the springs that are seated around the clutch disk. Those springs help to isolate the driveline from torsional vibration input from the engine. Ever heard of a dual-mass flywheel? The entire purpose of the dual-mass flywheel is to further reduce torsional vibration transmission to the driveline. This is achieved by an extra spring that is added between the two flywheel masses. The reason that the springs cannot eliminate vibration at low engine speeds is that they cannot be made soft enough (for practical applications) to isolate these frequencies. By slipping the clutch, you are essentially adding another very soft spring between the engine and transmission, and thus the vibration is reduced.
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Old 03-22-2005, 11:37 AM   #20
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Yes, great post dude. It wasn't until your last paragraph that I knew what the heck you were talking about
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Old 03-22-2005, 11:52 AM   #21
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Quote:
Originally Posted by Genesis
Yes, great post dude. It wasn't until your last paragraph that I knew what the heck you were talking about
Thanks. It's difficult to describe in non-mathematical terms. My Master's thesis was related to driveline torsional vibration .
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Old 03-22-2005, 06:16 PM   #22
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And here I always thought it was driveline windup!
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Old 03-22-2005, 06:30 PM   #23
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And here I always thought it was driveline windup!
Yes, you could describe it that way--winding up and unwinding of the driveline due to torsional inputs. There's a bit more to it than that, but that's the general idea.
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Old 03-22-2005, 06:36 PM   #24
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Just like my '58 chevy pickup in 1965!
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Old 03-23-2005, 05:04 AM   #25
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now i can understand the jerking in low RPMs.. but what about high rpms.. my car jerks in high rpms too .. say when doing a 0-100 mph run..even wen shifting from 3-4th sometimes?? first it jerks then the delay and then the power kicks in... all this happens in like .5 sec-1 sec.. is this normal ??
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Old 03-23-2005, 05:04 AM
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