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Old 1st June 2014, 23:40   #1
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Exclamation Can cooler weather cause a grabby clutch?

Its been raining occasionally(causing a slight drop in temp. as well) in Kolkata these days. I noticed something minor but peculiar over the past few days . On days it has rained or the weather is cooler my clutch feels grabby. By 'grabby' I mean that when I start pulling away from a stop in 1st gear the clutch seems to 'bite' a bit earlier than usual, causing shuddering(just like if you let the clutch go way earlier without giving much accelerator input). If the day is warm or the clutch pedal has been pumped a few times it works perfectly like it should.

I have an Innova (72000 kms, regularly serviced at A.S.S. ,driven with care and no clutch riding) The car is mechanically very sound.

Now I thought about it and I came up with something.

Lets get technical :
The Innova has a hydraulic clutch. Hydraulic fluid is essentially a special kind of oil. Now we know that with a drop in temperature, viscosity of a fluid increases(naturally,density increases as well) & vice-versa.

Physics says : Mass = Volume X Density.
Assuming there are no leaks, the hydraulic system should have a constant mass of fluid in it.
With an increase in density, volume decreases(Mass being constant like in this case).

Now when I press the clutch pedal I'm transferring the hydraulic fluid from the clutch master cylinder to the clutch slave cylinder under pressure. So, let's suppose that the clutch pedal is held in a fixed position. One of the following cases should happen :

Case 1 : If the fluid is warm(lower density, higher volume) - Then pressure inside the slave cylinder should me more because of a greater volume of fluid entering the slave cylinder. Anyone who knows the clutch mechanism will know that more pressure in the slave cylinder means more clutch disengagement. Thus in this case, the pressure plate should disengage the clutch centre plate a bit more.


Case 2 : If the fluid is cold(higher density, lower volume) - Then the opposite of Case 1 i.e. Lower pressure inside the slave cylinder hence a little less disengagement.

In my situation Case 2 is applicable because due to the rains I've got lower atmospheric temperatures thus a colder clutch fluid. So when I pull away from a stop in first gear, because of a cold fluid my disengagement is lesser. Thus the clutch has a earlier 'bite' point due to more engagement - Hence the shuddering.

Note : I said earlier that the clutch works perfectly when it's warm outside or the clutch pedal has been pumped a few times. If its warm outside, Case 1 is applicable, and if I've used the clutch pedal a few times the fluid would warm up(because of Compression of the fluid, Friction of the pistons on master and slave cylinders against the cylinder walls) and have similar consequences of Case 1. This might explain why the clutch works normally after sometime.

Am I right people?
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Old 2nd June 2014, 07:50   #2
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Default re: Can cooler weather cause a grabby clutch?

Originally Posted by sr_1445 View Post
Am I right people?
No, you are not, for several reasons:

Although, of course, the viscosity of the hydraulic oil is influenced by the temperature, the influence of the ambient temperature is, in practice, very little. If anything the oil reservoir would be under the hood and will be more influenced by whatever temperature it is under the hood. But again, you really don't need to worry about this at all.

The working of your hydraulic clutch depends on volumetric displacement not pressure. The volumetric displacement is irrespective of the temperature and or viscosity of the hydraulic oil.

With the clutch pedal not pressed down, the master cylinder is open to the oil reservoir. When you start pressing the clutch pedal the movement of the plunger inside the cylinder at some point early on, will shut off the connection from oil reservoir. At that point in time you have a give fixed volume of hydraulic oil in the circuit consisting of master + slave + all tubing. Irrespective of temperature and or viscosity.

Shuddering has nothing to do with the hydraulic mechanism. Shuddering is more likely to be caused by something in the clutch (plate) itself or the (thrown) bearing. Other than external and internal leaks, it is very unusual for anything to go wrong with the hydraulic system of a clutch. It is virtually maintenance free. With the one exception that it is advisable to flush the whole system once every 2-3 years. Hydraulic fluids are hydroscopic, so over time they absorb water moisture, which can cause some problems.

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Old 18th June 2014, 18:23   #3
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I also have a 2012 Innova. Last monsoon also I faced the same issue. Unfortunately couldnt explain it exactly & replicate the same also.

Now in my second monsoon with Innova, I have observed it again few days ago at Kodaikanal where it was raining.

As suggested by Jeroen, can it be that the fluid may be bad? But I doubt if the fluid is required to be changed so early.

Request if anybody can suggest any other points to look for.
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Old 18th June 2014, 19:02   #4
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Default Re: Can cooler weather cause a grabby clutch?

it is because of the condensation on the flywheel and clutch components. once the frictional components become warm, the shuddering effect will not be there.
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Old 18th June 2014, 19:11   #5
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Default Re: Can cooler weather cause a grabby clutch?

Originally Posted by star_aqua View Post
it is because of the condensation on the flywheel and clutch components. once the frictional components become warm, the shuddering effect will not be there.
I agree. The same phenomenon is also felt in drum brakes just after rains and early in the mornings due to condensation. It takes only a few applications of the brake or clutch to dry the friction material due to heat.
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