Preface: I'm writing this for the people who still want to know, not the veterans on this board who have put up with my beating them over the head with it for the last 4 years. I receive questions like the one below via email practically every day. This time I thought I would spend a little the time to try to be as clear as possible. Each time I write about something, I get a little more clear (I think), which is what's in it for me. Also - this is not about the engineering merits of the M5 clutch. It is just about how clutches work.
Quote:
Hi Greg, I'm just another car enthusiast who stumbled upon your site. Your webpage has a wealth of information, and my friends and I appreciate the driving school tips.
There is one thing I still don't understand though, no matter how many times I watch your videos - when you say the clutch is "all hooked up" even though you maintained the position of your feet, what does that mean? During my admittedly limited seat time driving a manual, I never noticed anything like this - either the clutch was disengaged, slipping, or engaged, and it was all based on how I manipulated the left pedal. Did you mean that the slip will stop by itself after the RPM's fall under a certain point? If letting the clutch engage by itself as the revs drop is the best way to avoid clutch wear, why did you completely let out the cluch in your other videos?
I guess what I mean to say is that your useage of "all hooked up" and "fully engaged" has confused me and some of my friends, even a guy that has been driving manual for a few years now. I would be grateful for any clarification and tips, as I am interested in acquiring an E36 M3 sometime in the future, and I would like to avoid any clutch wear due to driver error. Thank you for reading this!
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This is in fact a common area of misunderstanding. Hookup can occur with the pedal half-way up, and slip can occur with it all the way up. There are many factors involved, but ultimately the driver is in control. Hookup happens when slip stops.
What is "hookup"
Hookup is when:
- The engine is turning
- The car is in gear
- The clutch is not slipping - i.e., the engine RPM is equal to the transmission input shaft RPM.
- The tires are not slipping - i.e., the car's road speed is equal to what RPM *gear ratio * rear-end ratio * tire diameter predict.
In other words, hookup is when the engine is directly connected to the street (with some gears in the middle.) The time we're concerned about is the "Time Before Hookup" or
TBH.
What happens during TBH?
Let's start with the example of a 1-2 upshift from redline. This example uses stock M5 numbers (see
this Excel sheet), but the concept is valid with any car)
- In 1st gear at 6500 RPM, our speed is 37mph
- In 2nd gear at 37 mph and with nothing slipping, the engine RPM is 3700 RPM
- During the shift, the car is coasting at 37 mph.
Now let's consider two extremes:
Case A: We're gonna cruise at 37 mph: Clutch in, lift-throttle, shift to 2nd, wait for the RPMs to fall to 3700, dump the clutch and add just enough throttle to maintain speed. The clutch and tires didn't have to slip at all. No power was transferred through the clutch during TBH. It was more like an on-off switch: connected/disconnected/connected. Instant hookup.
Case B: We're drag racing: Keep the throttle foot to the floor, speedshift as fast as humanly possible. Sidestep the clutch. The instant we engage 2nd gear, the transmission input shaft begins turning at 3700 RPM. Even though the clutch pedal is still to the floor, the wheels are turning it at that speed. (The clutch is between the engine and the tranny, not between the tranny and the wheels.) The engine, on the other hand, is still turning 6500 RPM. When you begin to let the clutch pedal up, TBH begins. It ends when hookup happens.
TBH can not be zero. Something has to slip, or break.
So What DOES Slip?
Either the tires slip, the clutch slips, or both. If the clutch isn't slipping, the only way the engine can spin 6500 RPM with the transmission in 2nd is if wheels are turning at 62 mph, even though the car is going 37 mph. If the tires don't slip during TBH, the clutch must.
Under normal conditions, this slip doesn't last very long. During TBH, the car accelerates
and the engine decelerates until hookup occurs. In our example, the car might accelerate to 43mph while the engine RPM falls to 4500 RPM.
How is hookup related to the driver's clutch and throttle control?
The driver is in control of the following 3 factors:
- The throttle - how much power is the engine dishing out during the time before hookup?
- The speed of the clutch/shift/declutch
- The clamping force at the clutch
The driver controls clamping force with the clutch pedal. However there is no magic. Maximum available friction is beyond our control and
is not infinite! It is a function of the clutch springs, the surface area of the clutch/disk/flywheel interface, and the friction materials involved. And heat.
Beyond normal operating range, the hotter the clutch gets, the less friction it develops.
All we can do with the clutch pedal is reduce clamping force from its present maximum.
Rest assured that clutch slip can occur, even with the pedal all the way up. (Consider the extreme of letting the clutch pedal up with the throttle floored and the tires clamped in a vise.) Clutches are more often ruined by not letting it up fast enough during hard acceleration, allowing slip for too long under power.
What is the driver's job?
Slip can be desirable, even a neccessity - for launches and smooth shifts - which is why we have a clutch pedal in the first place. Our goal is managing what I'll refer to as the TOTAL ENERGY ABSORBED DURING SHIFT ("TEADS"), which is the sum of acceleration, tire heat and clutch heat generated. There is no free lunch - so you have to choose. In Case A above TEADS=0. All energy is transferred prior to and after TBH. But it also wastes the energy stored in our 6500 RPM flywheel of an engine, and it takes time for it to slow to 3700 while the car coasts. Not fast, but your clutch will last forever. In Case B we're asking the clutch and tires to accelerate the car
AND decelerate the engine against its screaming 400 ft-lbs of torque. TEADS = HUGE. Hard on everything, but fast -
as long as the parts last.
Of course there are many points in between. All up to the driver. A "typical" shift uses a quick throttle lift, and perhaps even a blip before the pedal comes up. This allows the clutch to decelerate the engine easily, transferring some of that energy as acceleration and only a little as heat. You can shift pretty quick this way - but clearly total TEADS, including its acceleration component, is lower.
The best possible performance shift is the one that maximizes TEADS over the shortest possible time. Just remember though, SOMETHING HAS TO SLIP- at least for a small time. If the post-shift RPM will put the engine way out of its power band if we hookup too fast, we might
choose to let the slip go a little longer. Bad technique allows the slip to go on for longer, generating too much heat. The driver has to recognize this condition and LIFT!
I don't get it - I need to "feel" it.
Here is a safe and simple exercise to experience the feeling of clutch hook-up and its relationship to your left foot. We're going to completely eliminate one variable - the throttle. This keeps the energy levels so low that you won't hurt your clutch, even over extended periods of slip. Get out on a street without traffic. From a full stop, WITHOUT TOUCHING THE THROTTLE, let the clutch pedal out until you can feel it loading the engine and the car begins to move. DO NOT MOVE THE CLUTCH PEDAL FROM THIS POSITION. If the engine died, you need to work on your clutch foot sensitivity. Try again, and don't let it out so far. Feel the point at which it is enough to move the car and don't let that pedal budge another millimeter. The car will accelerate from 0 to 4 mph or so - the speed of your car in 1st gear at idle RPM. You will be able to "feel" that it is no longer accelerating. You may or may note be able to feel it, but the clutch is not slipping any more, either, even though your pedal isn't all the way up. There is enough clamping force + time to get hookup. Now let the pedal up the rest of the way, immediately. There should be no jerk whatsoever. If there was, the clutch wasn't hookup yet, and still had some accelerating to do. Lack of jerk proves that the clutch was already hooked up.
Extra credit: Still, without touching the throttle, clutch in, shift to 2nd, start to let the clutch out, HOLD it again at the point where it doesn't jerk you but gently accelerates the car. It will hook up again. You can let the clutch pedal all the way up - no jerk.
Note that the above demonstrates what hook-up is. It is NOT the way to drive.
Linear Mode
