RaleighSS

Buddy says he drops the clutch to the floor say at brake marker 3 at turn 1 vir same time grabbing brakes big time then at the the perfect moment off brakes blipping throttle lift clutch out and into new gear... In a sense holding clutch in on the braking zone...


I have full power up to the brake zone at Marker 3 get on the brakes car slow enough then clutch in blip and shift keeping clutch in only for the moment it took to shift blip and grab a lower gear then back into throttle on corner exit ..


Any correct / wrong way here or just different styles? Some say with driveline disconnected you will slow faster others say the opposite. Maybe it is just a preference ...


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RX-Ben

You want to use the engine compression to slow you down. Clutch is only in for a moment (on our synchro'd cars), at that moment you will also be braking, blipping, and shifting.

Jason

Your way.

Short-Throw

Sounds like your buddy is keeping the clutch in way too long.

You should be getting your braking done first, then engage the clutch, blip, shift, disengage the clutch, all while continuing trail-braking and roll back into the gas at apex.



Absolutely not! This is a bad habit to get into and can lead to zinging a motor and destroying drivetrains.

The car comes with brakes to stop the car, use them, not engine compression.

Mike

Painrace

Remember, some time a guy is braking later going into a corner than you because he is going into the corner slower than you.

Jim

VetteDrmr

+1 Your way. Also, I wait until I'm done with my hard braking and into the modulation stage before I do my downshifting. That way I have the least amount of engine rev change and I'm way below the rev limits.

One of the biggest mistakes I see newbies do (especially autocrossing) is coasting with the clutch in.

HTH, and have a good one,
Mike

RacePro Engineering

Paul,

Aside from how most us are accustomed to doing the down shift(s), this is a facinating question that you raise. Naturally, the velocity of the car (its weight and inertia) are the same either way.

One could argue that, with the clutch engaged, there is more mechanical drag to help slow the car. And on the other hand, one could maintain that, with the clutch engaged, the brakes need to work harder, overcoming the rotating mass of the motor.

Has anyone here actually tested this? Maybe, how do drag racers (without chutes) "shut down" if they must do it quickly?

Great question!
Ed

CHJ In Virginia

The head instructor for NASA Mid Atlantic teaches the advanced HPDE III and IV students to threshold brake with the clutch in, blip the throttle and shift just before entering the turn in point. I have tried it both ways - riding engine compression down clutch out and clutch in braking. I now prefer the clutch in method. I don't really understand the physics of the situation, but it allows me to pick up about 1/2 a marker going into turn one at VIR. I used to hit the binders at 3 1/2 but now run all the way to the 3. It works for me but may not for you.

RaleighSS

Right ... that same instructor was at CMP and that's where my buddy picked it up ... he stands by his method that clutch in is best ... I have not tried it but only time my clutch is in is when the shifter is moving :-)

Also updated original question to make it more clear... Was tired when i typed it :-)

VTME13

I've had the same instruction, and having tried both ways and thought about this a bunch, I find I have more brake with the drive line disengaged. Here is my take on it:

Two things are going on with the driveline engaged and the brakes engaged:

1. The engine is slowing the car at a given rate via driveline rotational inertia and engine pumping losses.

2. The brake torque is slowing the car.

There is a fixed rate (albeit non-linear rate for both) at which both actions occur, the brake torque deceleration being the greater of the two rates. However, the overall deceleration is a combination of the two decel curves/actions because they are combined as a system.

Therefore, if you remove the engine decel component, you get pure brake decel at the higher rate.

What I've found is it screws up my timing and has required some practice to employ. Some places it's not really feasible because you wind up doing and having to think about more things in a shorter period of time. That will probably go away with more practice.

PAULY G

I think finding speed in any given turn on any given track is more an art form than a prescriptive science. If it works for you then it works for you. That being said, the only problem with applying that principle generally is that, if I understand you correctly, you run the risk of engaging the clutch while the front end is weighted in the turn, thereby (possibly) locking up the rear wheels and swapping ends. Inducing a sudden motor drag very late towards the apex could get hairy. But like I said, there's no one way of doing things.

VTME13

Agreed... more than one way. You also increase your chances of a spin by trail braking. I've actually gotten to the point in a few areas where I'm using the clutch engagement to aid in trail braking, using all three pedal at the same time. Fastest way, dunno. Riskier . Definitely. Fun too. :-)

hellrazr

I think the right style is what is fastest.

But, the whole discussion seems to unnecessarily complicate the problem. The tires have a limited amount of traction to slow the car. So whether that is applied via the brakes only vs brakes and driveline drag probably doesn't matter. I think the decision would be more about which technique is easier on the various components of the car, and this will depend on the component durability, component condition, how long the race is, etc. Also, the more reliable and repeatable method that can be performed by the driver may be the better way to go.

RacePro Engineering

It is not quite that simple. The brake torque deceleration, and the motor compression resistance, are ADDITIVE in slowing the car. So, when one removes the engine deceleration component, although he still maintains the higher RATE component of the brakes, the sum TOTAL of braking force has been reduced.

At this point, I am certainly not disagreeing with anyone that the brakes, alone, slow the car faster, than the brakes plus the compression of the motor. I am just hoping that someone here has actually tested the theories, and can provide some on-track insight.

Ed

RacePro Engineering

Well, it might matter. The real question is, "Is there a NET benefit to engaging the clutch while braking?" Most of us seem to think that by doing so, we are adding to the car's braking power. However, there is also a downside to engaging the clutch - now the rotational inertia of the driveline must also be overcome by the brake forces.

And, if we are over-complicating things at this point, remember we have not yet discussed the side benefits of generating additional heat in the rear tires (assuming rear wheel drive), in preparation for the upcoming turn. Obviously, leaving the clutch IN does nothing for us in this regard.

I do agree with you, though - "the right style is what is fastest."

Ed

RX-Ben

A lot of the braking depends on your clutch. A little Quartermaster setup with very little interia will certainly help in braking compared to the heavy stock setup.

At T7 at NJMP Lightning, if I do not downshift to 2nd, I need to brake a bit sooner in order to make the turn. I'll have the datalogger in action soon and will be able to compare some numbers.

Jason

Since I'm just a hack with a fast car, I'll copy the guys that get paid to do this stuff.

SouthernSon

Are you referring to the brake bias to the rear added by the engine deceleration component in addition to the braking system bias itself?

RacePro Engineering

You got it right on the money, Arte!

I was really just having a little fun in attempting to point out that Paul's question IS based on a quite complex set of dynamics, some of which are not readily apparent.

Many of the INDY car teams, with the equipment, the budgets, and the expertise to measure and analyze such things, are studying how to better balance the grip of the tires (heat and pressure) in the first "half" of the turn. The slip angles generated by the steering input (during turn-in) peaks the heat in the front tires, but until power is applied (later in the corner), there is no corresponding peak for the rears.

And to HellRazr's point, is any of this going to make a couple of seconds per lap difference? Probably not - but it still would be nice to know if there is something to be gained.

Ed

wtknght1

Here's a video I made a few years ago regarding heel-toe. Skip forward to the 3:50 mark to see the footwork better.

http://www.youtube.com/watch?v=rIivgrDsoUU

In short though, don't push the clutch in until you are ready to shift. When you hit the brakes initially, the engine compression helps to slow the car - use it. When you do shift, blip that throttle up to about 5-6K, downshift and get that clutch back out. Don't ride/slip the clutch out because that will burn it up. It takes practice, but once you get it, it'll be second nature to you...and it'll be nice and smooth!

Here's another video of the braking zone(s) going into turn 10 (3, 5, and 6) at Road Atlanta. You can hear me get off the throttle, see the braking zone and then do the heel-toe shift about 50-100 feet before I actually turn.

http://www.youtube.com/watch?v=FxM6MWjyp68