Is it possible that a car can be quicker while “wasting” energy slipping the clutch?

[weedburner]

I put this short article together to help illustrate benefits of using controlled clutch slip. I hope it gives those manual trans guys who are "radial curious" a little re-assurance that the more efficient radials can be a viable choice. I know many make more power than this example, but this stuff generally scales nicely as long as there is enough clutch capacity to match engine output.

How is it possible that a car can be quicker while “wasting” energy slipping the clutch? Truth is simply that an engine can burn more fuel spinning at a higher rpm. This makes it possible to produce a surplus of energy, beyond that which is absorbed as heat in the clutch assembly, net result being MORE power applied to the track, not less. Same reason why it's possible for an automatic car to be quicker in spite of installing a looser converter that produces more slip, it's all about producing energy faster than you are "wasting" it.

To illustrate, here's some feedback on our ClutchTamer from an east coast NMRA Factory Stock racer. Powered by a 302ci crate engine, stick cars like his are required to run a diaphragm clutch. This guy was having a rough time, as he was a slick racer switching to radials. With a typical 4300 launch, the radials bogged the engine to 2300rpm and 1.7x 60's. Stepping up to a 4800rpm launch in an attempt to eliminate the bog, this is what happened to his faceplated TKO...



He repaired the transmission, and I sent him an in-dash version of the ClutchTamer to try. He installed the ClutchTamer, made a few test hits in the driveway to get familiar with it, then went to the track. His immediate results were dead hooked radials and back to back 1.45 60's. This graph is from a 1.42 run...



Couple months later, he's still putting down 1.4 60's at class weight with no transmission failures from 5200rpm launches. This increase in durability is due to two things-
1- a reduction of engine rpm loss before clutch lockup.
2- spreading rpm loss over a longer time period.
Both are beneficial on launch as well as after the shifts, as they team up to reduce shock to the drivetrain and tires.

Here's the same graph w/ a couple lines added to help illustrate those benefits. His graph is fairly easy to understand, as there is very little wheelspin to confuse things...



The added orange line is a rough representation of the engine's ability to gain rpm in 1st gear.
The 1st added vertical black line represents the launch, or beginning of clutch engagement.
The 2nd added vertical black line represents the point of clutch lockup.
The distance between the two vertical black lines represents the time it took for clutch lockup to occur.

Clutch slip duration was roughly .7 seconds, engine rpm at lockup was about 5100.
…If clutch lockup had occurred at .4 seconds, engine rpm would have been pulled down to appx 4200 on the orange line.
…If clutch lockup had occurred at .25 seconds, engine rpm would have been pulled down to appx 3500 on the orange line.

This is a bit of a simplified explanation. Reduced engine output at lower rpms would also reduce the engine's ability to gain rpm, that added power loss is not reflected here. The basic point is- the earlier clutch lockup occurs, the lower the rpm point on the orange line that the engine will have to accelerate from.

Anyone wonder why that orange line on the graph aligns with 2700rpm at launch instead of zero rpm? It's because a line representing rate of acceleration is actually even steeper before the clutch locks up. This happened because no power was used to accelerate the rotating assembly prior to clutch lockup, so more power was available to accelerate the car. Here's the same graph, with a red line added to represent acceleration rate before clutch lockup...



See how much steeper the car's acceleration rate was before clutch lockup?

This launch could have reached it's shift point even quicker if the clutch had slipped longer, as the car would have rode the trajectory of that steep red line to a higher point before switching to angle of the orange line. Same logic applies to clutch slip after the shifts, a car can simply accelerate quicker before the clutch locks up. Generally the longer you delay clutch lockup, the longer you ride a steeper acceleration rate.

The lightest clutch assy may not necessarily be the quickest when it comes to exploiting clutch slip, as the clutch needs to have enough thermal capacity to absorb the slip without overheating/warping. Having plenty of clutch capacity for the task is the 1st requirement, then it's a matter of controlling the application of clutch pressure to match engine power.

Just trying to make this subject easier to understand. Any feedback, good or bad, is greatly appreciated.

Grant
ClutchTamer.com

[TheSilverBuick]

That's nifty. I've dead hooked the drag radials on my car and it bogs the engine pretty good (or it just lights the tires up), and this year the plan is for me to do my best to manually ease the clutch out on launch, which is likely a whole lot more inconsistent than the device there.

[squirrel]

doesn't MS handle that, Randal?

[TheSilverBuick]

It handled the 2-step rev limiter tied to the clutch pedal Both the gas and clutch pedals on the floor, rpm's at ~3,000rpm, light turned green dumped the clutch and haze the tires as the rpms screamed towards redline, lol. Probably wasn't the best for the 8.5" either Hopefully the 9" is going to get a work out this year and see if I can get the 60ft below 1.9.

[Barry Donovan]

too much drama on the launch needs a gear change to shorter.

I just chuckled at the thought of launching a truck in 4 lo like that.
my 3/4 ton transfer would need a 5 ton military upgrade.

and dog teeth.. do I see dog teeth? plain silly.

[BigAL]

Top Fuel does it.

[anotheridiot]

Top Fuel does it.

pretty sure part of the reason top fuel slips the clutch is to keep it from welding itself together.

[Deaf Bob]

From the web.."why top fuel cars slip the clutch"



Bullet
Nitro Member
Let me caveat the following with the statement that I am not an engineer, nor I am I a clutch guy. I have never worked on a Fuel Car clutch or even really touched the components for that matter. The following is what I have learned from looking, reading and listening mixed in with a little mechanical knowledge.

A Top Fuel clutch is basically a centrifugal clutch, that has a pedal. There are a number of "fingers/levers" on the pressure plate that are weighted differently and are different height and lengths. When the RPM of the engine goes up and the clutch speeds up with it the centrifugal force on the fingers, which are mounted like a lever apply more pressure to the pressure plate reducing the clutch slip. The higher the RPM goes, the more counterweight force is transferred to the pressure plate.

There is a "ram" that stops the fingers from releasing and applying pressure, this ram is controlled pneumatically (I think is bleeds air pressure) and is activated when the throttle is hit on the line. Keep in mind that when a Top Fule/Funny Car is on the line fully staged, the clutch pedal is already out and the clearance of the clutch plates allows it to slip and the driver can hold the car still on the brake.

When the driver hits the throttle the RPM of the engine goes up drastically and the fingers/levers on the pressure plate that are not stopped by the ram apply more pressure to the clutch and drive the car forward. The ram moves at a predetermined rate and allows more and more fingers to apply pressure as the car goes down the track causing it to eventually be at 1:1 with the engine.

In simple terms, it is like taking off in 5th gear with a standard transmission, but instead of gradually releasing the clutch pedal with your foot to apply more and more pressure to the clutch, the slippage is controlled by the "ram" allowing more and more pressure to be applied to the clutch.

Keep in mind that they have to let the clutch slip so that it doesn't spin the tires as they have enough power even to do this at a 1:1 drive ratio (same at top gear), where as in a car you would slip the clutch so that the engine didn't stall as it doesn't have enough power to overcome the load on the engine.

I am not sure that I have heard the term “5-stage” before when referring to the clutch, but it is most likely to refer to the 5 disc clutches that they teams use. There are 5 clutch plates and floaters between the plates.

“Slipping the Clutch” is a reference to a state where the engine RPM is greater than that of the clutch output RPM. This is at any point during the run before the clutch has completely locked up and the engine and clutch output RPM are the same.

The entire clutch assembly rotates with no parts being “fixed” with the exception of the bell housing. When the car is sitting on the line fully staged the clutch plates are not spinning, they are sitting on the spline of the output shaft. The clutch stand is rotating at engine speed at all times as it is connected to the crankshaft.

Coupled with the application of the clutch the fuel system needs to be set up so that the engine has just the right amount of fuel to support the application of the clutch but not too much so that it drops a cylinder then loses power and the clutch pulls the engine down etc.
 
#3Bullet, Jul 15, 2009 Last edited: Jul 15, 2009

[Deaf Bob]

I can see slipping the clutch to acheive better launches..
Getting it consistant seems to be the key..
Even offroad in mud or on big rocks, clutches slip to get moving..

[Polyhead]

Yep, and I have personal experience with this on motorcycles. With two stroke bikes it's really common to slip the clutch to keep the engine on the reed.

btw they make hydraulic clutch dampners for consistent drag launches. You press the clutch to the floor and the damper lets the peddle close at a consistent rate. How do you people not know about these?

[Polyhead]

pretty sure part of the reason top fuel slips the clutch is to keep it from welding itself together.

Not even close. It's the clutch slippage that causes them to weld together. They do it to control traction. A slipping clutch is a hot clutch.

[weedburner]

More to add...

The big thing with a radial is that you basically have a line that you don't want to cross when it comes to shocking the tire. Anything you can do to smooth out power delivery is going to make it easier to operate closer to that line. Any torque peaks that you have in your power delivery need to be shaved down, which basically allows you to elevate the average amount of power you can deliver to the tire...make sense?

Counterweight style clutches produce an RPM drop trace that looks more like a backwards "J" with a hook at the bottom. Their sharp, near vertical drop indicates a quick loss of rpm (intense discharge of inertia energy), which transitions into a gradual "hook" near the bottom when the clutch begins slip more. That hook area at the bottom of the "J" is where most of an adjustable's slip actually occurs. The intense vertical drop is something you have limited control over, as a centrifugal design dictates that rpm must come down before clamp pressure can be reduced. This is a big reason why SoftLoc's and such are only marginally effective when trying to run radials, as they still have a relatively intense discharge of inertia energy just after the shift, until rpm comes down enough for the bulk of their slip to occur.

A diaphragm PP combined with a ClutchTamer makes it possible to build a slipper clutch that has a more diagonal/linear rpm drop trace than the typical centrifugal. This softer engagement in the instant just after the shift helps keep those radial tires stuck.

If you have a clutchless transmission, the basic ClutchTamer will not give you any slip after your shifts, so your only current solution would be to use a traditional adjustable PP that has a centrifugal component. That gets you at least some form after shift slip, even though the "J" shaped rpm drop is less than optimal. The future will bring a system much like a fuel car, which will use some form of pneumatics or hydraulics to get the control needed to change that.

The ClutchTamer is just a brutally simple way to control a clutch. I'm sure myself and others will soon come up with more sophisticated/expensive means to get basically the same job done, you will have to wait if that's what you are looking for.

[silver_bullet]

Little was known about Garlits' Dart II, but I heard that it broke 200 because Emory Cook couldn't take his foot completely off the clutch making for a faster pass...

[STINEY]

btw they make hydraulic clutch dampners for consistent drag launches. You press the clutch to the floor and the damper lets the peddle close at a consistent rate. How do you people not know about these?

Some of us know about them and run such setups. In my application they lessen transaxle driveline breakage by lessening the shock loading dramatically.

Ace Controls part # MA225 on a simple fabricated bracket against the throwout bearing arm, can be adjusted for more or less resistance - or a faster or slower engaging clutch if you will.

[Polyhead]

Some of us know about them and run such setups. In my application they lessen transaxle driveline breakage by lessening the shock loading dramatically.

Ace Controls part # MA225 on a simple fabricated bracket against the throwout bearing arm, can be adjusted for more or less resistance - or a faster or slower engaging clutch if you will.

There are a few setups that are just a hydralic ram that rides on the top of the clutch peddle. I had such a setup and it was a peach really. So the why my particular setup worked was that there was adjustment on the ram length, so that when you side stepped the clutch peddle it slapped into the top of the ram and that set your initial clutch engagment then there was a valve you could adjust to slow the final close rate of the clutch. Now it should be said the car wasn't quick, only ran 13's, but it was a 13 second slant six rig that in itself is something. I broke a lot fewer crank shafts with that setup than I did trying to get the launch right on tire pressure alone. Only having 4 main bearings kiiiiiind of sucks.

I'm still pretty shocked people don't know about these things. Seriously, I was just a 17 year old kid in Indiana when I learned about these dudes... then again, I got to hang around events like the Mopar Nationals and US nationals, so maybe I was in an area that sported more racing technology. Makes me wonder though, really, how helpful is the internet really? Looks like getting your ass to a track and hanging around guys laying down the same number EVERY, STINKIN, PASS, EVERY, STINKIN, WEEKEND is worth something. Next I'll find out there are people here that can't read spark plugs. What is the world coming too?