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Old 17th March 2005, 16:02   #1
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Default Question on Clutch Slip!!!

During our last meet, me, Psycho and Iceman were discussing top speeds being calculated using gear ratios.

Now according to Psycho, the top speed of a car cannot be determined by looking at the engine rpm, gear ratio, final drive ratio and wheel circumference. He says that the clutch starts to slip once the wind resistance increases (more like 150kmph and above).

At first me n iceman disagreed but, finally, thought that it would be best to post this on the site and see what really happens.

I still feel that wind resistance at 185kmph (top speed of my car) will not have enuf force to make the clutch slip. But, i ain't betting on this since Psycho has been working with cars for a long time.

Can someone find some info on this, on the net and please post it here?

The fact that my tyres light up at the slightest hint of high rpm launch means that my clutch isn't slipping enuf for it to affect the top speed. I can understand such problems with wornout clutches but once a good clutch clamps on, it takes a lot of force to make it slip.

Shan2nu

Last edited by Shan2nu : 17th March 2005 at 16:04.
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Old 17th March 2005, 16:20   #2
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Well, without looking too deeply into it, I don't see this as being true. That is, I don't think wind resistance at speeds as low as 150-200kmph would cause a clutch to slip. This may only be a case at really high speeds, something our cars are not capable of. And, to get to those speeds, the vehicle would have some serious horsepower, consequently have a clutch tough enough to handle that power.

To bring a real world scenario into the picture, when I was driving around a battered Nissan in Canada, for kicks, a friend in his old accord and I went up against each other in a sort of head to head pushing contest. Both of us locked front bumpers and tried to push the other guy. Well, all that happened was our wheels started spinning. So that means the clutch was strong enough to withstand the pushing force of another car under full throttle. That pretty much clears this subject for me.
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Old 17th March 2005, 16:32   #3
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Agreed most cars in India dont have the power to reach such speeds where the clutch might slip due to wind resistance. But like rtech pointed out, when a car has a lot of power...the manufacturer will provide with a suitable clutch.

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Old 17th March 2005, 16:33   #4
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To bring a real world scenario into the picture, when I was driving around a battered Nissan in Canada, for kicks, a friend in his old accord and I went up against each other in a sort of head to head pushing contest. Both of us locked front bumpers and tried to push the other guy. Well, all that happened was our wheels started spinning. So that means the clutch was strong enough to withstand the pushing force of another car under full throttle. That pretty much clears this subject for me.
Thats what i was thinking. Can that sort of force be compared with a car doing 150, 200 or even 250kmph? Most of the high performance cars are highly aerodynamic, which reduces drag, so even at 250kmph the wind resistance on that car would be equal to that of a bus at 100kmph.

Even if i tie a parachute to my car (the drag ones) and rip the car, i don't think the clutch is gonna give up that easily, the car maynot even cross 100kmph and if at all something gives up, it'll be the adhesive force of the tyres.

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Old 17th March 2005, 16:36   #5
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But like rtech pointed out, when a car has a lot of power...the manufacturer will provide with a suitable clutch.
Point noted.

A powerful car capable of 300kmph with a clutch not suited for 300 will slip but, manufacturers aren't that ignorant as to do such a thing.

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Old 17th March 2005, 16:40   #6
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Quote:
Originally Posted by Shan2nu
Now according to Psycho, the top speed of a car cannot be determined by looking at the engine rpm, gear ratio, final drive ratio and wheel circumference. He says that the clutch starts to slip once the wind resistance increases (more like 150kmph and above).
Well Shan2nu,

Those were two points that I had mentioned that affect the top speed of a vehicle, to requote them again

1) The Drag Coefficient of the car causes the car go more slowly when the wind resistance is higher.

2) This may lead to clutch slippage at higher speeds than 150kmph. Also you cannot say that there is no transmission loss due to clutch slippage as the clutches do not necessarily transfer 100% of the engine power to the wheels (well if that was the case why would we have race clutches with better pads and more powerful pressure plates on them)

These 2 points together got me to a conclusion that the top speed of a car cannot be determined just by looking at the engine rpm, gear ratio, final drive ratio and wheel circumference.

Also here is an informative thread that talks about transmission losses.
http://www.eng-tips.com/viewthread.cfm?qid=81815&page=4

Last edited by Psycho : 17th March 2005 at 16:46. Reason: Added a Link
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Old 17th March 2005, 16:48   #7
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Dunno if this is right but here is my theory...

Wind resistance (force) is given by Cd * density of air * area * velocity

the clutch is designed to take a torque load of approximately 1.4 times the max torque of the engine. Say this torque be "T"

Now for the clutch to slip, the wind resistance must exceed the torque the engine produces.

since torque and force cannot be equated we convert it to power. So we equate the work done by the wind resistance against the moving car and the work done by the engine against the clutch.

since the clutch can already take the work done by the engine, we can find the theretical limit at which the clutch will start to slip.

work done by the engine against the clutch per second is equal to the power of the engine. which is 107 * 736 watts or 78752

now the work done by the wind resistance against the car per second is the product of wind resistance and the velocity of the car.

so work done = Cd * density of air * area * velocity * velocity

Cd is what ??? .7 for city??? lets assume it is 0.7
density of air is approx 1 so it can be neglected
velocity in this case is 150 kmph = 41.67

now according to out formula, for the clutch to slip at 150 kmph,

0.7 * 41.67 * 41.67 * frontal surface area of car = 78752 * 1.4 (factor of safety the clutch)

or frontal surface area of car = 90.79 square metres

i am not sure how much the surface area is going to be. but this looks to be a pretty big amount. My guess is the only areas that might cause drag are the front bumpers. the bonnet and windshield may not cause that much drag.

If my theory and calculations are right then it can be inferred that the clutch will slip at 150 kmph if and only if the frontal surface area is equal to or more than 91 square metres.



I have neglected transmission loss here. I think the clutch slip will be around 0.8??? right psycho? not sure... but still considering a factor of 0.8 means the frontal area works out to around 72.8 square metres still.

if some one can come out with a approximate value for the area then i think we can find at what speed the clutch might slip due to aerodynamic drag alone

Last edited by Harrie : 17th March 2005 at 16:55.
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Old 17th March 2005, 16:49   #8
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Quote:
These 2 points together got me to a conclusion that the top speed of a car cannot be determined just by looking at the engine rpm, gear ratio, final drive ratio and wheel circumference.
Thats correct. Simply putting these down on paper without compensating for the small loss of power due to friction, heat, wind, etc would not give you an exact figure. But, it would give you a decent ballpark figure.

Conditions are rarely ideal I guess. The same car may clock different speeds at different times of the day. Thats why records always call for 3 runs within a certain time before it can be considered a record.
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Old 17th March 2005, 16:51   #9
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Quote:
1) The Drag Coefficient of the car causes the car go more slowly when the wind resistance is higher.
Agreed........

Quote:
2) This may lead to clutch slippage at higher speeds than 150kmph. Also you cannot say that there is no transmission loss due to clutch slippage as the clutches do not necessarily transfer 100% of the engine power to the wheels (well if that was the case why would we have race clutches with better pads and more powerful pressure plates on them)
Performance clutches help launch the car from higher rpms (without clutch slip), it's not directly related to top speeds. How many cars at the drag event hit top speed within the 1/4 mile? Not even one.

I never said the clutch doesn't slip, it will slip for a fraction of a sec if i revv my engine to 7000 rpm and dump the clutch but, that sort of force cannot be compared to the aerodynamic resistance a my car faces at 185kmph.

My car with a M 800 clutch will surely slip at 185kmph but my stock clutch wont. It just can't. My clutch might start slipping at over 230kmph, but my car wasn't built for that.

So, i think i can safely calculate my top speed based on the engine rpm, gear ratios and wheel circumference.

Shan2nu
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Old 17th March 2005, 16:53   #10
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If my theory and calculations are right then it can be inferred that the clutch will slip at 150 kmph if and only if the frontal surface area is equal to or more than 65 square metres.
So a football field travelling over 150kmph would have its clutch slip! Good to know

Harrie, are you a maths/physics proff or something?
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Old 17th March 2005, 16:55   #11
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If my theory and calculations are right then it can be inferred that the clutch will slip at 150 kmph if and only if the frontal surface area is equal to or more than 65 square metres.
Well, what you're forgetting is that the CD of the City woud be around .31-.34 and not .7. The NHC has a CD of .29.

So, it's highly impossible for the clutch to slip at 150 or even 185kmph.

And the City doesn't have a frontal surface area of 65 square meters. LOL

Shan2nu

Last edited by Shan2nu : 17th March 2005 at 16:59.
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Old 17th March 2005, 17:01   #12
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he he it would be nice watching a foot ball field travel at 150 kmph.

Quote:
Harrie, are you a maths/physics proff or something
well i did my mechanical engineering.

ya shan i assumed the worst case scenario here. it is unlikely i think. whatever clutch slip we have at 10 kmph will be there at 180 or 190 kmph also i think.

dom is the right person i suppose.
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Old 17th March 2005, 17:01   #13
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Another point we were talking about Icemans Zen doing over 200 kmph and not a stock car, I agree where everyone has said that each car gets the right clutch and Drag coeff (aerodynamics) from the manufacturers for the given top speed.

Well also theoretically you have to see the figures below for a good laugh coz I really dont think that my car can do it.

Gear Km/h per 1000 RPM Km/h @5000 RPM Km/h @6000 RPM
--------------------------------------------------------
1 8.21 41 49
2 15.63 78 94
3 22.49 112 135
4 30.67 153 184
5 39.58 198 237

Gear Change RPM drop (change @5000) RPM drop (change @6000)
-------------------------------------------------------------
1 -> 2 -2373 (to 2627) -2847 (to 3153)
2 -> 3 -1526 (to 3474) -1831 (to 4169)
3 -> 4 -1334 (to 3666) -1601 (to 4399)
4 -> 5 -1125 (to 3875) -1350 (to 4650)

Gear Ratios:
Final Drive: 3.727
1st Gear: 3.736
2nd Gear: 1.963
3rd Gear: 1.364
4th Gear: 1.000
Overdrive: 0.775

Tyre Specs: 205/55/15
Dia: 606.50
Circumference: 1905.38


This is based on the the same calculations that you have mentioned (gear ratios for the conti and the tyre specs).

Well you are welcome to come over and set the conti speed record if you want.

Also on your point
"Performance clutches help launch the car from higher rpms (without clutch slip), it's not directly related to top speeds. How many cars at the drag event hit top speed within the 1/4 mile? Not even one. "

Please go through the link I have provided for transmission losses I think it will help clear a few more doubts (even for the 1/4 mile).
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Old 17th March 2005, 17:05   #14
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well, i agree with rtech and drifter on this. the slipping of the clutch is not related to wind.also, in contradiction to what psycho said, the speeds of a car are indeed, determined by the overall gearing, drive ratios, circumference of the driven wheel, and finally, the power belted out by the engine. but even the power of the engine cannot do much if there are mechanical losses, viz;inefficient powertrain (incorrect gear ratios to harness the power).
also, if what psycho says is true, then at least theoretically, automatic transmissions should be more efficient, as there is no clutch, and power is transferred through fluid flywheel.as said above, manual transmissions should lose their advantage after a certain velocity.
but this is not true, as you can see in case of the new honda cr-v, as an example. both the manual and automatic versions of the car have 5-speed ratios, but still, the manual manages a better top speed, and is more efficient (note:i didn't mention acceleration figures, as they are not of concern here: we are discussing loss of power transmission through clutch at high speeds).
also, there isn't gap between the clutch plate and the flywheel while travelling at speeds,and the clutch plate is in full contact with the flywheel except while shifting or when the clutch pedal is pressed. the friction coefficient of the clutch, which is an altogether different topic , only aids in better power transmission.
besides, the gearbox bellhousing houses the flywheel completely and there isn't enough gap for strong air turbulence.
also the coefficient of drag (C.D) is indicative ratio of wind resistance, viz;the lesser the C.D, the better. and also, if a car with a C.D of 0.31 and another with 0.27 employ the same powertrain, they will suffer the same mechanical losses, if both employ the same clutch, but the latter will attain a better top speed due to it's lower C.D. what say guys?

Last edited by veyron1 : 17th March 2005 at 17:23.
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Old 17th March 2005, 17:14   #15
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Quote:
Well also theoretically you have to see the figures below for a good laugh coz I really dont think that my car can do it.
Quote:
This is based on the the same calculations that you have mentioned (gear ratios for the conti and the tyre specs).

Well you are welcome to come over and set the conti speed record if you want.
That's true, your car is geared for 237kmph in 5th. Whats wrong with that. My car is good for 248kmph in 5th gear.

So what's your point?

Question is, does you car hit 6000 rpm in 5th gear? Cos mine doesn't redline after 3rd gear. My top speed is aerodynamically limit and not due to clutch slip.

If i had clutch slip, my 4th and 5th gears would have redlined with the top speed still staying at 185kmph (which doen't happen).

Shan2nu

Last edited by Shan2nu : 17th March 2005 at 17:16.
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