What is a "GSS Clutch Slipper"?..
How can you make your car faster by installing a GSS Clutch Slipper?...
For our purposes here, let’s define a “torque spike” as the quantified difference in kinetic energy stored in the engine/flywheel at the rpm before the clutch is released, as compared to the kinetic energy stored in the engine/flywheel at the lower rpm where clutch lockup is achieved.
Knowing the above, here’s a simplified example of how you might get more power to the ground by using controlled clutch slip to minimize the spikes. Let’s assume your oem manual trans is the weak link, and you want to put thru it the maximum power to make your car as quick as possible. 1st order of business is to minimize the torque spikes that occur during launch and at the shift changes. Suppose your transmission has an ultimate limit of 1000ft/lbs into the gearbox before failure. The quickest/fastest plan to get the most power thru that trans would be to put the max 1000ft/lbs into it as quickly as possible, for the entire duration of the run, without exceeding that 1000ft/lb limit. The problem then becomes that when you launch or shift with a manual transmission, the speed of the crank/flywheel assembly must change (lower) to match the gearbox‘s ratio with regard to the vehicle‘s ground speed. Basic physics tells us that there is kinetic energy stored in a spinning crankshaft/flywheel, and if you reduce the speed of that assembly, physics also dictates that the energy stored will be less. Basically, the amount of energy stored in our crank/flywheel assembly spinning at 8000rpm (pre-shift) is quite a bit more than the amount of energy stored at 6000rpm (after shift). Before the crank/flywheel assembly’s speed can be reduced to allow clutch lockup to occur, that excess energy will have to be channeled somewhere else. That somewhere is typically heat absorbed into the clutch assembly itself thru a little slip, the rest dissipated in a “torque spike“ which is basically a hammer blow to the rest of the drivetrain!!! For this example, let’s say that torque spike would be 400ft/lbs if clutch lockup were to occur within .130 seconds. To accommodate this spike, maximum engine power would not be able to exceed 600ft/lb until after lockup (no flatfoot shifts), as the transmission would still need to be able to tolerate that additional 400ft/lb torque spike without exceeding the 1000ft/lb limit. Those numbers would work, but there is room for improvement. If you let the clutch slip for double that amount of time for clutch lockup at .260 sec, the torque spike would be reduced to 200ft/lbs which would allow you to increase the engine power to 800ft/lbs and still absorb the torque spike without exceeding the transmission’s 1000ft/lb limit. Increase the lockup time even farther to .520 sec, you could increase the engine’s power to 900ft/lbs and still have enough capacity to absorb the resulting 100ft/lb torque spike.
How does the GSS Clutch Slipper work?...
...Just a simple hydraulic cylinder installed on the clutch pedal, similar to those cylinders used on residential screen door installations. It allows tuning a bit of "slip" into your clutch's initial engagement, damping the peak shock loads as power is being transmitted to the rest of the drivetrain. It's adjustable for exactly where in the pedal travel that it becomes active, and adjustable for rate of release from that point on (it controls slip only during the final part of engagement). The cylinder is hydraulic (not pneumatic like most), with characteristics similar to those of a 90/10 shock, pulling the rod out is easy, only the return stroke of the cylinder is controlled. With our clutch slipper installed on a clutch pedal, only the final part of clutch pedal's release is slowed down, not the whole release cycle. The rest of the clutch pedal's travel works like normal. If you are using the clutch pedal during shifts, the slipper will soften drivetrain shock during the gear changes too. If your drivetrain seems to have a weak area (similar to a T5 that always breaks 3rd gear), a clutch slipper can help you get more power to the ground before reaching that breaking point.
...Here’s the bottom line, if your engine’s output exceeds the ultimate capacity of your trans/rear/half-shafts/etc (or the available traction), then no clutch is going to be able to magically get that excess power to the ground. To get that combination of parts to live, either you have to dial back the power input, or limit the available traction at the other end of the drivetrain. But just because you are breaking drive train parts or spinning your tires, that does not necessarily mean that your engine’s output is more than they can handle. When a typical automotive clutch is released, there are actually 2 different components to the power available to accelerate the vehicle- the power output of the engine itself, and the kinetic energy stored in it’s already spinning crankshaft/flywheel assy. This stored energy is actually separate from the engine’s output, and a portion of it is suddenly released each and every time the engine rpm drops! When added to your engine's output, it can often add up to broken parts.
Another relevant concept to keep in mind is that the time it takes for clutch lockup to occur generally has a direct effect on the intensity of the resulting torque spike. Example- if a particular combination has a clutch lockup time of say .130 sec, the resulting torque spike’s intensity can roughly be cut in half by doubling lockup time to .260 sec. Delay lockup further to .520 sec, the spike would be cut to roughly 25% it’s original value.
...Basically when the clutch pedal is depressed, it pulls the rod out of the clutch slipper's cylinder. When the clutch pedal is released, the pedal comes out unrestricted until the stop on the cylinder's shaft contacts the dash bracket. From that point, the rate of clutch release is controlled by an adjustable orifice inside the cylinder. The rate of return is adjusted by simply spinning the cylinder's shaft "inward" (clockwise) from 0 to 10 turns of the shaft. We attach a plastic knob to the end of the shaft to make the adjustment quick and easy. The body of the cylinder is prevented from rotating during adjustment as it is attached to the pedal with a clevis pin (see picture below)...
Here's a look at where the other end of the clutch slipper cylinder is anchored, with it's shaft extending thru the dash bracket (and Delrin slide bushing), with the rod's plastic adjustment knob attached...
How is it adjusted?...
Adjustment #1- Initial Clutch Hit...Adjusting the nuts on the threaded portion of the shaft changes the amount of initial clutch "hit", similar to adjusting the base pressure on an adjustable long style clutch. Initially we dial in 7 to 10 turns of "delay" (see adjustment #2), as this basically removes the secondary clutch application and allows focusing on just the initial hit. It is preferred that the clutch initially slips enough that the engine is not pulled down below it's "staged" RPM when the clutch is dumped. For this part of the adjustment we usually just make 30' to 60' test hits, no need to make a full run.
The basic idea is to tune your clutch's initial hit to your chassis and track conditions. That could mean different things in different situations...
......If your primary goal is protecting your transmission, rear diff, or drive axles from sticky tires, a less aggressive initial hit simply allows for a little more initial clutch slippage than optimum. This effectively spreads the stored inertia's application over a longer period of time, reducing the torque spikes that the drivetrain would otherwise see when the clutch is dumped. Not enough initial hit (too much slip) will result in excessive clutch wear and reduced acceleration.
......If your primary goal is maximum acceleration, too much initial clutch hit can result in violent chassis reaction, an overall loss of traction, and excessive tire spin. You could try to control this with shock settings alone, but it is rare for overly stiff shock settings to be optimum for gear changes down the track. Ideally, a little tire spin can be good, but wheel-speed should not exceed vehicle-speed by more than 10-15%.
Adjustment #2- Secondary Clutch Lock-up Delay...Secondary lock-up delay is used to control clutch slip while the vehicle speed is catching up with engine speed. Turning the cylinder's winged dash knob (on the right in the pic above) basically changes how quickly additional clutch pressure comes in. There are 10 turns of adjustment on the knob. At "0" turns (fully counter-clockwise) our clutch pedal was delayed .181 sec, barely noticable. At "10" turns, the pedal takes about a minute to return.
When racing, if a clutch grabs too quickly it can pull the engine RPM down too much, below it's peak power range. When adjusted for a well prepped track, a slipper can allow an engine to spend more time operating closer to it's power peak. During experimentation, it was confirmed that a car is typically much quicker when the clutch is slipping a little more than we liked. We found the optimum slip to be one that did not pull the RPM down during launch, but allowed the engine to go straight to it's power range and stay there without activating the shift lite prematurely. Always try to begin your clutch tuning from a starting point of being too "grabby". Excessive slipping can wear out a clutch pretty quickly, so it is best to sneak up on your adjustment. We like to use enough secondary delay to keep the engine's RPM up near it's torque peak until there is enough vehicle speed for clutch lockup.
UPDATE-...We now use a simple dial type adjustment with an internal detent. No more need to use wrenches, now it's just a simple turning of a dial. You can dial in/out initial hit without even unbuckling your belts! Here's a pic of the new version, it now has a notch milled into the threaded rod, while the round Delrin "Initial Hit" dial (center of below pic) uses a steel ball detent that's pre-loaded with a simple o-ring to keep the dial from free spinning...
Is a special clutch required?...
...Basically we are delaying lockup to allow more energy to be converted into heat so that less energy will be available to create the torque spike. A simple doubling of lockup time can usually be absorbed by the typical HD organic or Kevlar friction linings, as long as there is sufficient clamping force to achieve lockup. Stretching lockup out to .5 sec or more at full power will generally require a lining that can handle the extra heat. Ceramic and dual friction linings have been used, but iron seems to work the best. Lockup times up to 1.2 sec are no problem with a typical RAM diaphragm PP and 900 series iron disc. Damped hubs are ok on lower power applications, but a point is reached where a harmonic problem occurs with the damping during slip, so solid hubs are preferred.
Keep in mind that delaying lockup to .5 sec or more during normal street driving will have no noticeable effect on clutch durability, as typical release times without a delay often exceed 1 sec.
Results...
...Initial testing was done using our Shop Mule. It is 99.9% street car, and hasn't ever been taken to any track on a trailer. No roll bar and no additional chassis stiffening, it does not adhere to anyone's set of rules. Just a quick street car that gets plenty of exercise. Here's some TNT results using our Clutch Slipper...
...Ram 10.5" 3000# Diaphram pressure plate
...Ram 900 series sintered iron disc (damped hub)
...17# steel flywheel
...3.73 gears
...275/60-15 M/T radials
...GSS Clutch Slipper
= 1.304 60' on 275 radials using a Ford Toploader 4spd (small 1-1/16" input / 28 spline output). We also quit twisting the splines in the 28 spline 1310 Spicer slip yokes. We don't know of anyone else getting 700+rwhp thru a small in/out Toploader.
At one point, we wondered just how effective our Clutch Slipper really was. We dis-connected it by simply pulling the attachment pin and removing the cylinder from the clutch pedal, which only took a few seconds. Same staging routine as before, this is what happened on the very next pass...
We made the replacement driveshaft to the exact same spec's as the twisted one in the above pic. Even though we are now spraying more off the line, no additional failures to report after re-attaching the Clutch Slipper. Clutch disc wear has also proven to be minimal. Two seasons now on the same clutch disc, still .320" thick and re-installed at the beginning of this year (it's a nitrous car, pulls great from lower rpms).
Another great thing about the slipper is that it can easily be adjusted for a really low staging rpm, making it much easier to hook up on a marginal track or DOT radials. With a conventional clutch, rpm is going to dip quite a bit on launch, so you have to stage quite a bit higher so that the rpm does not dip too far, below your power range. With the slipper, you can stage at a much lower rpm, so the power input into the chassis is much smoother. The launch is much more like foot braking an automatic, and can easily be made to "flash" like a converter to get you straight into your power range. The slip only lasts for less than 1 second, so you don't see the heavy wear that comes from low base pressures with a race style slipper clutch.
Frequently Asked Questions...
Q- How does temperature affect your contraption? My screen door slams shut when it gets cold outside. I know what pulls my screen door shut but I could not imagine using anything that primitive to accomplish anything other than closing the door. I went out and flapped that door open and shut 20 times tonight and it never did the same thing twice. If your a career test and tuner, your design may get you by. If your a racer, you need a proper system.
A- When you said that your screen door slammed shut when it got cold, that told me that you likely had a pneumatic closer, not hydraulic. The hydraulic closers operate like a 90/10 shock, easy on extension, but the return stroke is controlled by oil passing thru an adjustable orifice. Generally when oil gets colder, it's increased flow resistance would result in your door closing slower, not slamming shut. A hydraulic closer is probably as affected by temperature swings as your shocks, except that in this application the closer is inside the car, likely a more controlled environment.
We conducted a test for the purpose of creating a graph of the cylinder's characteristics. During the test, the cylinder was connected to an actual clutch pedal in a car. The pedal was fitted with switches at the top/bottom of it's stroke, and was released from exactly the same spot each time, from against the pedal stop.
Here's some observed data from that test....
"0" turns = .181 sec delay (+/- .006 sec deviation over 10 strokes)
"1" turns = .194 sec delay (+/- .005 sec deviation over 10 strokes)
"2" turns = .224 sec delay (+/- .005 sec deviation over 10 strokes)
"3" turns = .285 sec delay (+/- .001 sec deviation over 10 strokes)
"4" turns = .395 sec delay (+/- .001 sec deviation over 10 strokes)
"4-1/2" turns = .673 sec delay (+/- .003 sec deviation over 10 strokes)
"5" turns = .811 sec delay (+/- .005 sec deviation over 10 strokes)
"6" turns = 1.708 sec delay (+/- .012 sec deviation over 10 strokes)
With the cylinder completely removed from the pedal, the deviation was .011 sec over 10 strokes. This was attributed to the pedal returning at a rate that was faster than the foot could be removed from the pedal, introducing a bit of human inconsistency into the results. Conclusion? it's possible that your car will actually be MORE consistent with a clutch slipper.
Q- I'm playing with a little Eagle Talon, and the clutches offered for it suck badly. Most people run a circle track style mini twin that locks up too quick and doesn't have enough inertia, so it either bogs or breaks stuff.
A- The 1st problem here is likely that the clutch just locks up too quick. More inertia may help, but unless the clutch can initially slip, that added inertia is going to be dumped into the drivetrain very quickly and create a huge torque spike.
Here's some low "2000 rpm launch" observed data from our combo. The low rpm launch is very easy on parts, but with the slipper delaying clutch lockup the rpm "flashes" higher instead of bogging, getting the engine into it's power range much quicker...
...without the slipper connected, lockup occurred at around .132 sec while pulling the rpm down to 1700 at lockup.
...with 6-1/2 turns of delay, lockup occurred at around .624 sec with an rpm "flash" to 2857 before lockup.
...with 7 turns of delay, lockup occurred at 1.01 sec with an rpm "flash" to 3944 before lockup.
In this mode, the clutch slipper is basically causing the clutch to act much like an automatic during a footbrake launch, much like "flashing" a torque converter.
Do YOU need a Clutch Slipper"?..
...If your car breaks drivetrain parts- a slipper can help soften the torque spike that results when the clutch is suddenly released.
...If your car dead hooks and pulls your engine down out of it's power range- a slipper can help keep the rpm up where more power is made.
...If you race on un-prepped surfaces- softening the initial hit might make your car quicker overall.
...If you would like to make your manual trans bracket car more consistent- a slight clutch pedal delay can help minimize human inconsistency.
The big advantage of our Clutch Slipper for a street/strip car is that it allows using a conventional pressure plate, and the low pressure and slipping last for only a short period of time. SoftLoc style racing clutches are designed to use very low base pressures which allow the clutch to initially slip, then grab, as the sintered iron lining heats up and the "Long" style pressure plate's centrifugal assist comes in. The problem with that on a street/strip car is that when you are cruising down the hiway at lower RPM in high gear or even overdrive, there is little centrifugal assist. The clutch lining also cools off, so the clutch will likely start slipping again as soon as you get into the throttle even a little. Getting under the car and adding base pressure is almost necessary before going back out on the highway to minimize additional wear. If you would rather just adjust your street/strip car's clutch from the driver's seat instead of climbing under it to crank the base pressures up and down to make the transition from street to strip, our GSS Clutch Slipper might be your answer.
Our Clutch Slipper works with cable clutches as well. One difference between some hydraulic and mechanical clutches is that some mechanical clutches (like the older musclecars) are not self adjusting, so they would need to monitor/adjust freeplay to keep things consistent for cutting a good lite. If your T5 is in a FOX, the stock self adjusting mechanism will work just fine.
If you have any questions, feel free to e-mail me