Originally Posted by Gearhead_318
You would think your clutch would start to slip in lower gears, but it doesn't. Don't know why, probably voodoo.
Plenty of good clutches out there that will easily hold the powa.
LOL - good one!!
Let's work through some of the detail here:
As we all know, clutches usually "wear out" a little bit before they actually start to slip on a regular basis, if your clutch looks new when you pull it out, consider that the "wear" of the clutch plate has actually made it thinner, this means that the spring that actually puts pressure on the clutch is under less flex when it is actually clamping down on the clutch material, and springs that are under less flex will always apply less pressure -> translation = a worn clutch will hold less torque than a new clutch - You'll probably only ever notice this change though if you're very near the torque holding capabilities of your clutch in the first place - most users will have a clutch with capabilities well in excess of available crank torque - they'll start to notice clutch failure when the friction disk has worn out.
So lets go into the gears portion of today's lesson:
You will first notice your clutch begin to slip when your engine is producing peak torque - since cold air is naturally denser than warm air, it is generally expected that your car will produce a somewhat higher peak torque in the winter months - I theorize that clutch failures are therefore much more common in winter months than in summer months - case in point: the OP.
Now let's examine the difference in behavior of peak torque between gears:
For science's sake, lets make an assumption: The peak torque on our science car occurs at 5500 RPM.
As your clutch begins to fail, it doesn't fail in 5 FT-LB steps, it fails very slowly (until the point that it actually fails, but more on that in a moment) so your first clutch slip is going to happen at exactly 5500 RPM - the clutch will hold just fine at 5499 RPM, and it will hold just fine again at 5501 RPM. Chances are, the first few times your clutch slips, you won't even notice it, it will slip very briefly and very slowly, maybe a few degrees of rotation at first, and eventually by entire rotations.
Again, for science's sake, I'm going to briefly ignore the difference between static and kinetic friction - which is really a bigger deal than I'm making it out to be - we'll explore later.
As the clutch slowly begins to fail, the RPM range at which it fails will slowly grow - soon enough, the clutch can't hold 100% of power from 5490 to 5510 RPM - a range of about 20 RPMs.
Consider how long you spend in the failure range of 5490-5510 RPM in first gear.
Now consider how long your engine operates within that same RPM range in 4th or 5th gear. Not only do you have to stay within that range longer purely because of gear ratio, but wind resistance will also hold you into that RPM range for a longer period of time.
So, lets translate those extended times to differences in RPMs
While in the failure range, your engine will be accelerating faster than your tranny input shaft, this means that the longer you are in the failure range, the faster your engine will spin relative to the input shaft. In first gear, because you are only in the failure range briefly, your engine may only be spinning at 10 RPMs more than the input shaft by the time you exit the failure range - what is the chance that you are going to notice those 2RPMs? Now consider what you're doing in 5th gear. By the time you exit the failure range in 5th gear, your engine may be doing well over 15 rpms faster - again, you probably won't notice that difference, but consider that the failure range is only 20 rpms - your engine has increased 20 rpms, but your car has only increased speed as if by 5 rpms. Now lets re-introduce the difference between kinetic and static friction into this equation...
Static friction means that "when your clutch is holding, it's more likely to continue holding"
Kinetic friction means "when your clutch starts slipping, it's more likely to continue slipping"
Once you exit the small failure zone in first gear, your clutch is still slipping, though because you've passed the failure zone so quickly, we'll say the difference in rotational speed is 2 RPMs, the difference will continue to increase after you have left the failure zone until your engine torque falls off to a point where it no longer exceeds the kinetic friction - at this point, your engine RPMs will stop decreasing briefly until your car "catches up".
See how that friction thing works? Static friction in - Kinetic friction out - so even though your clutch may hold 200 ft-lb of torque *before* it starts slipping, it might only hold 180 ft-lb of torque *during* the actual slip - once it recovers from the slip, the holding potential will again be 200 ft-lb.
Now lets again investigate what hapens in 5th gear: Your car has increased speed by an equivalent of 5 RPMS, but your engine has increased rotational speed by fully 20 RPMs, at this point, your engine torque falls below the static friction threshold of 200 RPMS, but just like in the 1st gear example, your engine will continue to accelerate relative to the car until the RPM where it produces less than the kinetic friction of 180 ft/lb, at which point the speed difference between the engine and the car will begin to equalize.
So to summarize, consider these facts:
During both 1st and 5th gear, the clutch will begin slipping once the engine has strengthened to 200 ft/lb of torque, and it will continue to accelerate until the engine has weakened to 180ft/lb. At 180 ft/lb of torque, the engine will stop accelerating until the car catches up.
In 1st gear, the time spent between 200 ft/lb on the up swing of the torque curve, and 180ft/lb on the down swing might be as little as a half-second, and your engine may only increase speed by 50 RPM during that half-second
In 5th gear, the time spent between 200 ft/lb on the up swing of the torque curve, and 180ft/lb on the down swing might be closer to ten seconds, does this mean that your engine should only jump 500 RPM? not exactly.
Once the clutch starts to slip, it begins building heat VERY QUICKLY. In first gear, since it's spinning slowly and re-catches quickly, there's not a whole lot of heat generated - but in fifth gear, since it begins slipping and quickly escalates to high speeds, it gets super hot - this heat further reduces the torque holding capability of the clutch, so that in 5th gear, you're likely to go straight to red-line soon after the clutch starts slipping, while in first gear, you still don't notice the slip.
Need to save up money to replace your slipping clutch?
Then don't test your clutch beyond what you need in order to convince yourself that it is slipping. The very act of slipping will quickly ruin what is left of the friction material. Baby the car until you have the time and money to swap in the new parts.