mpw

You need to consider that those semis also have high compression diesel motors, so engine braking is going to be more effective, in addtion to the lower gearing that Clayton mentioned. Not too many diesel cars on the track.

- Matt

gnosis

Oh yeah, trucks have higher compression too.

That must explain why I can outbrake all those 944T drivers with their 0.0:1 compression ratios.

But I never compression brake. It must be something else that does it.

Oh, I know! It's the superior dynamic capabilities of the normally aspirated 968!

Clayton

Danno

"but anyways, explain to me how where the jacking force in corning comes from. If it isn't a part of the cornoring force, as Carroll Smith seems to show (correct me if I'm wrong) in his books, where DOES it come from??"

It's from the suspension geometry. The body-roll and jacking is the result of weight-transfer conering-forces (Gs), not the other way around, that is, body-roll causes weight transfer. You can weld up a suspension so that it doesn't move at all, then go around a corner. If you go around that corner at the same speed as before, you'll have exactly the same amount of weight-transfer, just with zero body-roll now.

Rich is also correct about a secondary effect in weight-transfer. That is the lateral displacement of the C.O.G when the body rolls. This causes a small amount of the lateral weight-transfer as well, but it's insignificant compared to conering-G.

Back to the jacking effect. Take the case of a lateral swing-arm suspension like in some British cars. Lateral cornering forces at the tread-patch will actually uncompress the suspension and cause a positive camber gain as it raises the body. On the other extreme, you can have a trailing-arm suspension. This suspension would be mimimally affected by cornering forces and lateral weight-transfer. But it the bad side-effect of zero chamber-change with body-roll resulting in positive camber for relatively small amounts of body-roll.

turbo944

Well, for those of you driving a 951, forget engine braking.....unless of course you have swapped and put in an NA engine or something. I've noticed this phenom myself after accelerating and letting off the gas (not while engine braking as I don't do it). The 951 has a low compression engine and once you let off the gas and the engine no longer has that load, it loses boost and suddenly you are driving a car which is not going to slow itself much from engine compression. Drive down a long hill in the 951 and in another car which has a higher compression engine. I always build more speed without using the gas in my 951 than in the other car. Don't expect this to help you on street or track by any big margin as it's much less effective than in an NA.

As for engine braking in general, it does greatly increase the wear on the engine and if you are braking fast enough, you are losing speed faster than the engine braking could ever hope to decelerate you, so it's not needed. I saw somewhere that a 928GTS made like 3000 hp from its brakes or something. That really is "brake" horsepower! If you are using engine braking while decelerating, you aren't using your brakes enough.

As for trucks, well, they are using some drum brakes to stop a 30,000 pound truck. Figure out the momentum for it and how much energy it would take to stop that truck. If they leave it in a higher gear and go down a hill, it will accelerate. And accelerate out of control as there is enough force to cause it to accelerate. So there are several reasons that they do it. You can REALLY smell those brakes at the bottom of many of the mountains around here where they've gotten hot. This isn't a problem in your car unless you're driving an 80s Mustang with stock brakes! I know as I used to have one...they are terrible!

To help settle the car once you get it into a corner, use trail braking into the corner, which means you are still lightly braking into corner entry and that can often help stabilize the car. If you are driving very smoothly though, there should be no reason to engine brake into the corner as it's trying to force your rear tires to go slower, not your front, and that could induce a spin and is probably slowing you at a point where you want to be back on the throttle anyway, at least a neutral throttle (not accelerating or decelerating, using all of your grip for cornering) and balance the car.

Now, my tips for those people who want to learn out there. Always look ahead of your car. Not 2 car lengths in front of you, but all the way as far as you can look ahead of where you are going. Take ALL of the road in from there to your car. I see people trying to drive fast who are only watching where their car is at and consequently are always thinking about a corner when they go into it, not in advance of getting there. Thus they are in very poor control of their car with jerky motions, braking too late, and in general is very unsmoothly driven and in the end, not fast. However if you are driving like this and behind the wheel or a passenger, as someone else mentioned, it feels like you're flying, braking at the last possible second, etc. I kind of see this as like driving in the fog on a road you don't know very well with a lot of curves. Constantly having things thrown at you to deal with. By looking as far ahead of you as you can you actually slow down the sensation of speed and you'll see things happening up the road that you'll have to deal with and prepare for them approaching. This sounds like common sense, but behind the wheel I see people do this all the time, even on a racetrack we see this when two drivers concentrate on each other and where they are rather than preparing for the next turn and both are way too hot into the corner and don't brake until its well and truly too late. That is sometimes trying to outbrake, but sometimes it just follows the above, losing concentration on the track and the job at hand, getting your car to the end of that lap and driving the next.

It's already been mentioned about shifting, Mustangs of the 80s and early 90s are famous for bent 3rd gear linkages from people shifting too fast and hard. I never had that problem once. If you go too fast, you can bounce off the gate and in general you are sloppy. Learn to shift right and make it quick smoothly.

Same with anything you do in the car. Never slam on the gas, never slam on the brakes, never jerk the wheel. Make your motions smooth. Weight transfer will be less abrupt, meaning less chance of having to catch a snap spin.

Seating position is also very important as it how your arms are when you grip the wheel. Some drivers get away with perfectly straight arms and get really good, most don't. With your hand at the top of the wheel you should still have a slight bend in your elbow as you need leverage to steer and straight armed you lose a lot of that, plus you force yourself to drive with your shoulder muscles, which will cause fatigue. When you do steer, push the wheel with the hand of the side of the body into the direction of the turn, don't lift the wheel with the other side. It's more precise. Don't take your hands off the wheel, but the direction of the turn is the hand you should use to make your primary hand in setting where you want the wheel and on turning it out of the turn, obviously you will use the other hand. This is one thing you can try and do on the street and if I'm in a car with the right seating position, except for changing gears or very sharp turns, I can leave both hands on the wheel (at 3 and 9 o'clock) all the time.

Also, be nice to your equipment....smoothness is easier on it and if you take care of your equipment, it will have a much higher chance of taking care of you.

jc22

Engine braking in trucks is completely different than with a car. Jake brakes actually force the exhaust or intake valves closed and won't allow them to re-open until you take your finger off the button. Diesel engines do not have throttles so there is really no compression braking without some type of exhaust brake or Jake brake because there is no limit as to the amount of air entering the engine. They can only create a few hundred hp of braking where brakes can produce thousands. The only time compression braking should be used is when descending a steep grade where you would have to ride the brakes the whole way down to maintain speed. This scenario is usually not applicable in cars or trucks not towing large loads.

Just wondering why you say this? I've been told by experts that all this does is wear out your gearbox and upset the balance of the car under braking. I was told to shift once and once only, before turn-in to the proper gear. What is the point in shifting into third when you really want to be in second? I know a lot of people do this but I don't understand why. Thanks.

mpw

An easy way to determine if you're in the "correct" seating position is to sit with your shoulders against the back of the seat and put your arm on top of the steering wheel; your wrist should be what's touching the wheel. This is probably going to feel awkward to people who are used to sitting like they are in an easy chair, at least it did for me.

- Matt

Ken

From what I remember from reading the Skip Barber book, there are two main reasons.

1. It's a more consistent downshift. The time it takes to move from 5-4 or 4-3 is pretty similar but the time to move from 5-3 is longer. You must then account for this in the timing of your downshift. They claim it's better to have one consistent timing pattern of the downshift. Especially since different corners require different amounts of shifting.

2. It's safer for overrevving and missing gears. When you follow the pattern the chances of missing gears is less. Take the 5-3. If you shift 5-4 then 4-3, the chances of missing 3rd are very small. Doing it in one motion increases the chance you'll accidentally put it into 1st. Especially on the track where the adrenaline is pumping.

Also, There's a large speed difference that you must account for. Say you're travelling 120mph in 5th.(I'll use the gearing for my car as that's all I know.) 4th redlines at 115 and 3rd at 85. So if I want to be in 3rd for the upcoming turn. I can brake, at around 90-100 make the downshift to 4th. Then around 60-70 I make the downshift to 3rd. Doing all that with consistant timing. If you're going directly from 5th to 3rd. You need to hold off the downshift until you're going 60-70. It becomes, brake, wait, wait, wait, wait, then shift. Now different corners require different amounts of waiting. If you mess up and shift too early, you can easily overrev then engine. Again, this is just a timing issue. When you keep you're timing consistant, less problems occur.

FSAEracer03

Compression is not a factor in engine braking, it's actually engine internal friction (I'll supply a source if need be when I get home tonight).

I have references for the jacking force stuff, and will post them when I get the chance as well... I'll even post a little 951 photoshop diagram I have an urge to make later on, lol

-Kevin

FSAEracer03

Danno, I think we're on the same page really... "Body roll uses some of the sideward corning force and translates it to a jacking force" I'll scan my little sketch to once again better clarify what I'm saying, body roll is from the cornering force and suspension geometry, and jacking is from body roll and cornering force... right?? actually, no... don't comment on that until I post my sketch, lol I'm horrible at explaining things!

As for the Andretti quote... what I believe he was refering to was when you engine brake in a RWD car, the rear wheels are the only wheels being slowed down. That gives more stability to the front because front brakes take the majority of the braking load... when you slow the rears down through engine friction, keeping the same total braking force requires less front brake pressure, giving more stability... once again, I'm not explaining this very well... but think of this: Imagine throwing your front brake bias 100% to the front. Lock them up and the car doesn't want to stay straight. Reverse the situation and the car will stay straight as long as you dont move the wheel.

more on that later... i need to get back to work, and i need more sleep, lol

-Kevin

poindex30

Do formula one cars really have 3000 lb. springs?

I don't know if this has already been mentioned because I don't have time to read all of the posts but when I moved my seat forward as far as I could comfortably, I was able to detect and control oversteer to a much greater extent.

What is the best setup for an S2 for autocrossing?

Thanks,

Mickey

Friendan

Thanks for the kind words Clayton. I plan on autoxing this year, though as soon as I can get some new tires and the weather clears up I'll be out there!

Z-man

Using the engine to slow down in such manner will more likely cause the rear wheels to LOCK UP rather than stabilize the rear end. The potential for a spin is there.

Ever miff a heel-toe-downshift? Rev's not high enough when you let out the clutch? Guess what happens? The rear locks up, or the ABS kicks in. Been there, done that. (If you properly match rev's on a heel-toe downshift, you won't cause the engine to slow down the car)

There's no way that 'engine braking' will stabilize the rear end. TRAIL BRAKING will in some cases (Ex: Entry to Hog Pen turn at VIR), but downshifting won't.

-Zoltan.

ninefiveone

Actually, if you lock the rears, you are more likely to have the rear come around and spin. You don't want any wheel to lock up but the fronts should lock before the rears. If you lock the fronts, the car will continue to go in the direction it's already going.

You don't put the feathers on the front of the arrow. You put them on the back.

M758

Comments on engine braking and downshifting.
I have noticed a settling effect from compression braking on my 944 race can and turbo street car. I do not use it slow down as much as to settle the rear of the car. One thing to remember is that stock proportion valves typically have strong front brake bias so the fronts will lock-up long before the rears. So a little added braking like 5% will typically not take the tires beyond their limits.

Even so using the engine to slow the car is not correct. I have leared to heel and toe, but cannot do so fast enough to hit each gear on the way down. So I will go from 5th 2nd or more often on my tracks from 4th to second. The biggest issue here is that this puts more load on syncros and requires you do down the downshift much later. Doing too early results in overrevvs. Now when I do my heel and toe downshift I most always want to be in a straight line since any small error can cause as spin otherwise. I also do quite a bit a trail braking. So a common routine for me is this

1) Brake straight & slow the car - At braking limit
2) heel & toe downshift just before turn in typical engine revs are 4000 to 5000 - At braking limit
3) continue braking and begin turn (note that there is some engine braking occuring as I am slowing the car with car in gear and the clutch out). - Tranistioning from braking limit to cornering limit.
4) Ease of brake and transtion to throttle while turning still. Revs should be 4000+ (no lower than 3500) - Cornering limit
5) Reduce turning as more throttle is applied. - Combined power & Cornering limit


Now...
Suspension stiffness. 1st balance is the key. Next stiffer springs allow a car to react faster than one with softer springs. Car with soft springs require you to wait a bit after you make an input as it takes a bit of time for the car to take a set. Of course body roll does have a negative impact on camber.

Also a stiffer car allows you to run a lower ride height. If you have soft suspension for the same bump or curve you will have more suspension travel. This means you need more ride height to prevent bottoming. Double the stiffness and you have less suspension travel for the same bump or corner and this can lower the car. Lowering the car DOES reduce weight transfer.

Also a stiffer car is required for cars generating downforce as softer sprung cars will be pushed down to their bumpstops from the downforce generated.

As stated however all of this at the price of traction. If are too stiff your car will not proper contact the road and this cause a loss of traction. What you want is compromise. For most road cars 2 or 3x stiffness increase from stock will greatly improve the car. Beyond that it becomes much more complicated.

Go slower to go faster.
I did just that in fact at my last track day. I actually went slower into a couple corners. I applied the brakes earlier and feel like I was going slower at my turn in points, but when I came out of the corners I had more RPM's and this was going faster. Overall my lap times improved. Going too fast into a corner can cause you to spend the entire duration of the corner working to make it around vs gathering speed for the next straight.

The corellary to this is don't out brake your self. I have driven with many students that just out brake themselves at each corner. They wait so long and stand on the brakes so much that two things happend. First is that the completly miss the turn in point thus requiring them to slow even more to make tighter corner. Second they lose so much speed that work forever to gain it back. Just because you can stop the car in 40ft from 110 mph does not mean you should.

Driving a low hp car i have learned to slow the car as little as possible, but still mantaining controll is important.

mpw

Please do, I'd like to understand this. It would seem to me that it would take more energy (perhaps not the correct term) to compress the air in a diesel cylinder than in a gas motor cylinder because the compression ratio is much higher. How does compression ratio not factor in?

- Matt