What causes excessive nose dive in an emergency stop? Is it not enough front or rear brake, springs rates, weight distribution? How can you reduce it???

 

Nose dive is caused by weight transfer. Under heavy braking, a large portion of the vehicle weight gets transfered to the front suspension. Stock springs are roughly 300lb/in springs. For each 300 lbs place on the springs they compress 1 inch. As more weight shifts to the front suspension, the front springs compress more (and conversely, as weight shifts away from the rear suspension, the rear springs expand) All this causes the nose to drop and the tail to rise, resulting in what is called nose dive.

The easiest way to eliminate nose dive is to run stiffer springs. If you were to slap some 600lb/in springs on the front, they would only compress half as much under the same load as the 300lb/in springs.

 

Some also say an aftermarket torque arm and more rebound on the rear shocks helps.

 

Nose dive isn't a bad thing, but it feels wierd and I don't like it. The stock suspension will compress until the front is on the bumpstops, that's what limits the front dive. Stiffer front springs will help to reduce it as well as stiffer rear springs. Increasing the rear shock rebound will slow the whole process down as well. Lowering the car will also decrease the amount of angle the car can get before it contacts the front stops.

 

So this has nothing to do with front/rear brake balance?

 

Mostly inertia.

 

Proper brake bias just keeps the braking balanced front to rear, so one axle doesn't lock up way before the other does and both the front and rear brakes can operate at their full potential.

Nose dive is affected by spring rates, shock valving, and suspension geometry. The first two are the easiest to change, the third can get a little more involved.

Stiffer front springs and higher rebound damping in the rear shocks were already mentioned. Increasing the front shock compression damping also helps. Aftermarket torque arms help as well.

With suspension geometry, there's a parameter called anti-dive that controls the amount of suspension deflection during forward weight transfer. The load transfer is a function of wheelbase, deceleration rate and center of gravity height (another reason why lowering springs are effective). Anti-dive controls body motion/suspension movement as a result of this load transfer. If the anti-dive is set to zero, the shock and spring rates are the only things that will control this motion. As anti-dive is dialed in, the suspension arms begin to resist the load transfer along with the springs/shocks. If you were able to set anti-dive to 100%, the suspension arms would resist all of the load transfer and nothing would go through the springs. Since no load is resisted by the springs, there wouldn't be any suspension deflection due to braking even though there is still load transfer to the wheels, so there would be no brake dive. Changing this is usually reserved for race cars with a lot of suspension adjustability, though.

 

I did forget about the lower center of gravity helping to reduce it. My bad.

I also wanted to make a point that a lot of people don't realize. Stiffer rear springs will reduce nose dive as well.

As far as brakes go, they have more to do with weight transfer and then weight transfer contributes to nose dive. Instead of thinking of the brake bias or brakes in general, think more about the deceleration rate of the vehicle. The greater the deceleration rate, the more weight transfer.

Brake bias just effects the deceleration rate. Just like the pads and tires. As the deceleration rate goes up the more nose dive you get (to a limit, which is the front bumpstops). Switching to grippy R compound tires will increase nose dive, as would grippier pads.

The only thing an improperly balanced brake bias would do is limit the cars ability to decelerate which is like the opposite to having grippy pads or tires. Think what would happen if you only had the rear brakes working. You would have only 30% or so of the cars normal braking ability. That would greatly reduce the nose dive, but do you really want to do that? No.

So to answer your question, yes, brake bias can effect nose dive. Poor brakes can reduce the dive, but you want to avoid that as it is the greater of two evils. Get the brakes working as well as you can. Get the grippiest tires and pads you can and THEN worry about the brake dive.

Sometimes people think the nose dive is bad because it can happen so fast and it feels wierd. I know I don't like it. I have the Koni SA all around with 550/165lb springs, Carbotech Panther+ pads, lowered ride height and 275/315 R-compound tires. My car will still dive under braking, but it does it at a much slower rate and I have complete control. It truly doesn't bother me any more.

FastKat, if you are running the stock shocks and springs, you are probably experiencing a lot of dive right as you stomp the pedal. Just a shock change will make the car feel a whole lot more controlled because their low speed valving is what controls the body movement. Good shocks will slow the dive down so that it doesn't happen all at once. Stiffer springs will also help to slow down the body movements. You will never get rid of the dive, but slowing it way down is almost as good.

I don't know much about anti-dive geometery so I'll leave that to TooSlow02 and others. I CAN tell you that I drove a Buick LaCrosse not too long ago and as you can imagine they ride real soft and comfy. The thing I really noticed is that although they had this really soft car ride, they had almost no nose dive under braking. I couldn't believe it, no dive! It's just odd. I figure it had to be the suspension design with a lot of anti-dive built into it. I don't know much about it, but I wish our cars had some more anti-dive built it.

 

Oh yea, weight distribution does play a small role, but unless you mount the engine in the back seat or something there's not a lot you can do to really make a change in brake dive. That's one of the reasons the Porsche 911 is known to be a great braker. The rear engine design helps to distribute the cars weight much more equally to all 4 wheels. Our cars tend to be more like 70%-80% on the fronts under hard braking. The 911 family are also good off the starting line for that same reason. The rear mounted engine puts more weight on the rear tires.

So they are good under braking as well as accelerating, so why not make all performance cars like that you say? Trailing throttle oversteer. It's killed many a driver before. It basically means when you get off the throttle in a corner the weight shifts to the front and the rear gets loose. The heavy engine in the back gets the car sideways in a hurry. Say you go into a corner too fast, you let off the gas to slow down and boom, there you go into the wall. If you go into a corner too fast and stay on the gas you might end up understeering and the car will plow straight. So your damned if you do and damned if you don't. The point is don't go into a corner too hot in a rear engined car.

Anyway, I like our front engined cars, they are fun and relatively easy to drive. Once you get the suspension balanced you can make the front slide or the rear slide, depending on what you want to do.

I'm starting to ramble, but I hope that gives you a better idea of how weight transfer effects the vehicle, etc...

 

Good shocks are a great idea. I just replaced my junk DeCarbons with Koni SA fronts/DA rears. It made a big difference, even with stock springs. If my car didn't have the ride height of a Jeep I might have kept it this way.

I'm not a big fan of brake dive either, it's uncomfortable at best. You won't be totally rid of it but at least you can make a few changes to reduce it to a tolerable level. It helps to inspire more driver confidence IMO. And that's always a good thing.