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Old 2nd July 2006, 09:12   #1
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Default Acceleration at low gears

Any car when accelerated at low gears increases speed upto a certain point, though the RPM of the engine will increase even beyond. In other words, while accelerating at low gears, speedometer stops increasing beyond a certain point, whereas the tachometer continues to increase.

My question is, where is this extra revolution of the engine (RPM) absorbed? The natural answer is, it must result in clutch slippage. Does it? If so, it may result in unnecessary wear and tear of clutch liners every time we over-rev the engine.
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Old 2nd July 2006, 09:36   #2
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Looks like you're suffering from serious clutch slip coz on a normal car, this should never happen.

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Old 2nd July 2006, 11:14   #3
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Quote:
Originally Posted by Shan2nu
Looks like you're suffering from serious clutch slip coz on a normal car, this should never happen.

Shan2nu
i second shan2nu.u are suffering from major clutch slip and this would never happen on any car with a good clutch.Get your clutch checked asap as this problem would lead to loss of mileage,acceleration and top wack of the car.
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Old 2nd July 2006, 12:55   #4
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Quote:
Originally Posted by zen-ith
Any car when accelerated at low gears increases speed upto a certain point, though the RPM of the engine will increase even beyond. In other words, while accelerating at low gears, speedometer stops increasing beyond a certain point, whereas the tachometer continues to increase.

My question is, where is this extra revolution of the engine (RPM) absorbed? The natural answer is, it must result in clutch slippage. Does it? If so, it may result in unnecessary wear and tear of clutch liners every time we over-rev the engine.
whether car or bike if u are having more engine rev thru the gears withhout moving much ahead as should have been is something u can sense. this indicates a poor worn out clutch. get it changed.

on the other hand if ur question is about rpm increase in each gear then ur above observation is right as this is entirely dependent on the gear ratios. tall and short design.
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Old 4th July 2006, 19:34   #5
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Thanks for your replies shan2nu, humyum and 2fast4u, but i feel I must clarify my question a bit more.

2fast4u, i think you got my question right. specifically, what in the gear's design gives rise to the following symptom?

1st gear - no matter how high the rpm goes (2000 or 3000) my speed can never go above 20 kmph (which has been reached at 1500 rpm itself!). So in effect the relation between rpm and speed is non linear, that is, beyond a point the revs continue to rise but the car is already going as fast as it can in that gear (maybe in the range when the engine is whining and the tacho is close to redlining?) and so will not go any faster. My question pertains specifically to this point. If the relation between rpm and speed IS non linear, then, are some rpms being wasted? OR rather, if the increased rpms do not contribute to increased speed beyond a point, then what are those rpms doing? where are they getting lost before transferring their rotation to the gearbox/driveshaft?

Kindly let me know if I need to clarify my question any further.
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Old 4th July 2006, 20:08   #6
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zen-ith i understand what u are trying to say.The problem that u are staying that in 1st gear the speedo reaches 20kmph at 1500 rpm only but then the rpm keeps on increasing without any increase in speed.The car with a good clutch will reach max rpm and max speed in that gear at the same time.There will be no visible loss of rpm.
The flywheel rototes the clutch and the pressure plate are in between and the gearbox on the other side.When u release the clutch pedal the flyhwheel tranfers the power to the clutch plate and the pressure plate pushes it against the flywheel.From the clutch plate power is tranfered to the gearbox.So if the cluch plate is worn out is worn out it will rub against the flywheel but not rotate at the same speed as the flywheel so there is rpm loss there and thus the gearbox wont rotate as fast as the flywheel.If the pressure plate is worn out it wont push the clutch plate onto the flywheel with enough pressure due to which again there will be rpm loss

Last edited by humyum : 4th July 2006 at 20:09.
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Old 4th July 2006, 20:09   #7
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What those extra rpms are doing is, screw up your clutch even more. Your clutch has lost it's ability to hold onto to the flywheel at high rpms.

On a normal car, the clutch (when engaged) should rotate at the same rpm as the flywheel. This isn't happening in your case, the flywheel is rotating way faster than your clutch plate.

Your clutch plate rotates wrt transmission and the flywheel rotates wrt engine speed.

This is causing the engine rpm to increase without increase in vehicle speed.

I suggest you log onto howstuffworks.com and read what they have to say about clutches and clutch slip.

http://auto.howstuffworks.com/clutch.htm

Shan2nu

Last edited by Shan2nu : 4th July 2006 at 20:11.
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Old 4th July 2006, 20:24   #8
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OK, i agree humyum, but even in a perfectly good car, i have noticed this non linearity. Using the same 1st gear example, I find that the increase in speed (say from 10 kph to 15 kph) for a certain rpm increase (say 1500 rpm to 2000 rpm) is much greater than the increase in speed when pushed from 3000 rpm to 4000 rpm (the speed may increase from 20 kph to 25 kph, at the most, though the revs have increased by 1000 rpms, as against 500 rpms in the first case). In other words, the increase in speed per unit increase in rpm is not a constant across all ranges of the rpm curve. I somehow have a feeling that this non linearity DOES exist, even if the clutch has no problem. Which is why, when we plot a graph of rpm versus speed, we find that the rising slopy curve eventually flattens off at the top right corner. Maybe I am mistaken. I am sorry if I am hammering the same misconception using different words.
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Old 4th July 2006, 20:57   #9
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Ok, thanks Shan2nu and humyum. I cleared my misconception now. My understanding was wrong. Further proof is available in Table 1: Road speed at various rpm in each gear of a '94 Harley big twin on this page, where the rpm and the speed appear to be pretty linear. I guess I was confusing the Torque Vs RPM curve, which is an entirely different issue altogether. Thanks and sorry.
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