Practical Transmission Tuning: Clutches

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Clutches

10.24

00:00	When increasing engine output, in many applications, we can increase the hydraulic pressure clamping the clutches to help them hold extra torque.
00:09	That said, there are limitations to how high the pressure can go.
00:14	Examples of limitations include sensors which can only read so high and control systems that prevent operation beyond those sensor limits.
00:24	Physical limitation of the pump, preventing pressure increase.
00:29	Check valves which open above pressures deemed too high by the OE supplier, bleeding off clutch fluid and avoiding further pressure increase.
00:38	At a certain point, we simply need clutches that can hold more torque and an achievable clutch pressure.
00:43	Like most multi-plate clutches found in basic manual transmissions or dual clutch transmissions, the friction plates in an automatic transmission clutch are separated by steel mating plates.
00:55	The more friction and mating plates in a clutch pack, the higher the total holding potential for a given friction area and pressure.
01:03	If the stock clutch pack had 4 sets of plates and we upgrade to 8, we've doubled the torque capacity of the clutch.
01:12	In some cases, there may be room for a taller clutch stack to accommodate more of the discs.
01:19	But more often, the compromise involves using plates which are thinner than stock, so we can fit more of them in the same amount of space.
01:28	There are a few potential downsides to running a high clutch plate count.
01:32	Like any friction disc, if it's thinner, it's probably going to wear down to the point requiring replacement more quickly.
01:39	There's simply less material present to start with.
01:42	Thinner steel plates have less thermal mass, so while clutch assembly can handle more torque before it slips, when it does slip, it can more easily overheat to the point of the steel's warping or the friction material melting and transferring material to the steel's.
01:58	Given the compact nature of these clutch packs, material transfer or any warping can result in partial engagement of a clutch without applying hydraulic pressure.
02:09	This unwanted partial engagement, sometimes called dragging, can be a serious problem.
02:15	Even if warping or material transfer isn't causing dragging, anytime force isn't applied evenly across the discs, the clutch's total holding capacity suffers, which can result in more slip events and more damage.
02:30	Shift feel becoming rougher is another example of behavior we might notice after clutches warp or suffer material transfer.
02:39	Think of how pleasant it feels when your brakes pulse after melting brake pad material onto the rotors and you'll get the idea.
02:47	Another shift feel related concern comes from adding more plates.
02:51	As I mentioned, with more plates, the higher the clamping force for a given clutch pressure.
02:57	The force applied still varies with clutch pressure, so we still have a level of control, but the precision of control reduces when upgrading to a higher clutch plate count, and I'll explain why.
03:07	Let's say we're using a transmission with a maximum clutch pressure of 20 bar, so you can control it from 0 to 20 bar with a precision of about plus or minus 0 .4 bar.
03:19	Remember, there are solenoids, actuators, valves, and a pump trying to achieve the pressure we're requesting.
03:25	But this is a complex mechanical and electrical system, and despite best efforts, it can't perfectly maintain the pressure we request.
03:33	So, there's always a level of pressure target error inherent in the system.
03:38	So, if we have a clutch pack rated as 400 foot-pounds capable at 20 bar clutch pressure, and we want an initial holding capacity of 40 foot-pounds as we start a shift, 40 foot-pounds is 10% of the 400 foot-pound total clutch capacity.
03:54	To get that, we can target 10% of the 20 bar max clutch pressure, which is 2 bar.
04:01	Based on our expected variation in pressure, we'll actually end up getting pressure wiggling around in the 1.6 to 2.4 bar range.
04:10	So, we'll actually get 8 to 12% of the clutch's capacity.
04:15	I've done that in my head, but when the numbers aren't so convenient, the long hand would be 1.6 divided by 20, or actual divided by maximum pressure equals 0.08, which is 8%, or 2.4 divided by 20 equals 0.12, or 12%.
04:36	So, 8 to 12% of 400 foot-pound max capacity is 32 to 48 foot-pounds, a relatively small range of 8 foot-pounds in either direction.
04:48	The long hand on that math is 0.08 times 400 equals 32, and 0.12 times 400 equals 48.
04:59	That's how we get that range.
05:01	A few foot-pounds worth of clamping potential isn't going to make or break drivability, but to help understand the impact of a big clutch upgrade, let's do the math with an 800 foot-pound capable clutch created by using twice the plate count.
05:15	Because the clutch has twice the capacity, we need half the pressure to get the same holding potential.
05:21	So, instead of two bar, we only need one bar.
05:23	While the target pressure goes down, the amount of pressure target error inherent in the system remains plus or minus 0.4 bar.
05:31	So, now our pressure range is going to be 0 .6 to 1.4 bar instead of 1.6 to 2.4 bar.
05:40	Because the clutch has double the plates, that pressure variation now has double the impact on total clamping force applied.
05:46	So, while one bar of pressure would still get us 40 foot-pound holding potential, the pressure variation that caused plus or minus 8 foot-pound error now causes plus or minus 16 foot-pound error.
05:58	So, actual holding potential becomes 24 to 56 foot-pounds.
06:04	When the goal is 40 foot-pounds and we get 32 to 48 versus 24 to 56, you see there's a significant difference in accuracy.
06:13	If this was torque on a nut we were trying to tighten, we could be way off.
06:18	Another thing that compounds the concern is that applying too little force, which causes excess clutch slip, can damage high count upgraded clutch packs more easily than stock ones.
06:28	As I mentioned earlier, higher clutch plate counts are often achieved by using thinner components to fit more plates in the same location.
06:38	So, now we have a clutch which is stronger in the sense that it can hold lots of torque, but more fragile if we apply too little pressure and slip it.
06:48	This generally leads to applying a bit more pressure than we need, just to make sure the clutch doesn't get unnecessarily worn or damaged.
06:56	So, now if we wanted 40 foot-pounds, but know we have to have at least 30 or we'll slip the clutch, perhaps we target 55 instead.
07:05	That way, even with pressure target error, we will still have enough to avoid slipping the clutch.
07:12	In this way, we tend to end up with greater clutch force being applied more quickly to protect the clutch, which the driver and occupants will feel as more aggressive engagement.
07:23	My example involved an upgraded clutch with double the torque capacity, but in many cases a clutch with 20 or 50 percent more capacity, perhaps a five or six plate versus stock four plate, would have been plenty.
07:36	More modest plate count increases reduce the amount of precision control lost.
07:42	Combine that with the plates not having to be as thin as an eight plate would, and allows us to retain more stock-like drivability.
07:51	Another option to increase the torque capacity of the clutch is changing the friction material.
07:56	Some materials have a higher coefficient of friction than others.
08:00	For those who don't have the option of fitting more plates, this may offer just enough improvement to make the clutches hold.
08:07	Since the clutches in an automatic transmission are wet, meaning bathed in automatic transmission fluid, the fluid itself also has an impact on the friction generated.
08:18	While changing the friction material and transmission fluid may not have as large an impact as upgrading to a high count clutch pack, as anyone who's dumped coca -cola on a dry clutch to make it hold for one more pass will tell you, these things all add up and sometimes all we need is just that little bit more to finish a race.
08:36	In some cases, an additional option to increase clutch holding capacity is the use of a larger diameter clutch assembly.
08:44	The increased mass and greater surface area for contact between the friction discs and steel mating plates also improves the clutch's ability to manage heat.
08:54	While the various options to increase clutch holding potential tend to make shift feel more harsh, some drivers love that more sporty feel.
09:02	Just keep in mind it's not for everyone, so it's important to know your audience if you're tuning for someone else or what you like if it's your own vehicle.
09:11	This also tends to prompt a conversation about compromise because ultimately we have to select components that we can hold the torque with and survive in the manner the vehicle will be driven.
09:23	If the shifts feel a little rough but the car is reliable and quick, that's often acceptable.
09:28	But if we err a little too far on making shifts comfortable and the clutches quickly fail, that compromise likely wasn't worth it.
09:37	So, let's quickly review what we've learned about clutch upgrades.
09:41	Adding more plates to multi-plate designs increases torque holding capacity.
09:46	But if the higher plate count is achieved by reducing plate thickness, we need to be extra careful in avoiding overheating them during a clutch slip event.
09:56	If we only need a little more clamping potential, sometimes the right fluid can save the day.
10:03	Calculating clutch capacity change based on a plate count change or a clutch pressure change is relatively simple math that we can do to help us choose the right clutch and pressure for our needs.
10:15	Friction material type and clutch diameter also impact how much torque the clutch can hold.