j4k3

Is everything tight? Lugs, suspension bolts, tie rods, etc etc?

PAWESOME

Good question, haven't had time to really go over it, but I will in the morning. looking for a new manual steering rack if any one has one

Fabrik8

You'd know it a lot quicker than a highway lane change if something important wasn't tight.

PAWESOME

Yeah... there's no rattle, just a REALLY loose feeling once you get above about 65

vbspec

What do you mean by loose? If you are not lifting off the throttle or hitting the brakes I dont see how the car would be loose. If you ever do a track day you will find that if the rear starts to move with a FWD car the proper fix is to reduce your steering input or add throttle. If it were a suspension issue you would feel it more on the on and off ramps. Even at speed limit speeds you are transfering weight much much more on those ramps than a simple lane change of 6 feet while going straight. Physics are Physics. Now a grooved road with an out of alignment car can make it pull and that will feel weird but is not loose. The only other thing to check would be tire pressures and again this would show up 10X more on on ramps at half the speed.

Fabrik8

Steady-state cornering on an off or on ramp at lower speeds isn't necessarily going to have the same results as transient events like a lane change. I understand what you're trying to say, but I don't think you can make the correlations that you're talking about.

vbspec

I would like you to elaborate on this. As I only have the seat time while driving and instructing at speed well over the stated 70mph and not the engineering schooling you have. I don’t understand how a FWD car will become loose at any speed while simply changing lanes. As I see it in a simple lane change the throttle stays the same and minimal steering input needed to move a car 6-8 feet and will not transfer weight forward thus making the backend "loose". When negotiating an off ramp there is first a deceleration lane (braking zone) when weight is transferred forward then the corner will begin and much more input will be required to make the turn. This is where the car should get "loose". In my almost 10 years of autoxing, tracking, instructing and racing in a fwd cars if the car gets light in the rear its because of a lift or hitting the brakes. These are the only way to get weight to the front.

Fabrik8

I'll use what you're saying as an example. If you're cornering under steady state conditions at the limit of lateral grip, and then you lift off the throttle or hit the brakes, you change the amount of force in different places and create acceleration that wasn't there before. That acceleration creates enough extra force to upset the rear (if you're already at or near the limit of lateral grip), which is why the rear starts sliding. Actually, the weight transfer has to do with the acceleration too, because any time you change velocity there is an acceleration (which is the definition of acceleration). You can't just jump from one velocity to another instantaneously.

Any time you're turning along a curved path, there is a tangential acceleration (tangent to the curve) and a radial acceleration (pointing toward the center of the curve radius). If that curve is constant (a circle) and you're at a constant speed, the tangential accel is zero. You still have the radial accel. Anyway, the combination of those accelerations is part of what makes up the cornering force. So if you change speed, or the curve changes (you turn more, or turn less), then the acceleration changes and the cornering force changes also. At the most basic, you're familiar with F=m*a, force = mass * acceleration. If that force is right at the limit of what the tires can handle, and then the acceleration becomes greater, you overload the tires. The weight transfer that you're talking about factors into that also, as a change in the mass term of that equation. There's actually a slightly different equation for circular motion but the concept is the same.

Going back to the beginning, those acceleration components are what causes the weight transfer that you're talking about. If you aren't turning, you don't have any acceleration, and you don't have any lateral weight transfer.

If you change lanes, you have an acceleration when you start moving in a different direction, and when you stop moving in a different direction. You're also changing curvature (you were going straight, then the car turns, then you go straight again) so that's part of it also.

I absolutely agree with what you're saying, and there shouldn't be any reason that the rear end should lose grip during a lane change unless you're really doing it fast or something is wrong. However, if there is an alignment problem that is already making things sketchy or making the tires track funny, a quick lane change at speed could be just enough to upset things. I'd imagine that it would have to be something like incorrect toe, or something happening dynamically to the suspension geometry under weight transfer (something is loose, etc)..

I'm still trying to think of a clearer way to explain this; I'm not doing a good job of this so far..

vbspec

I mean you have explained simple weight transfer in a corner. I still dont get how a car can get loose when simply changing lanes. I see it this way. If you are going straight at a constant speed the suspension will have settled and you will have close to equal load at all 4 corners (i did say close to equal). With a simple input of steering you will not unload or over load the rear enough to push even the worst street tires past their "max lateral grip".

An alignment issue would have to be really jacked up to make a car get loose while mainting a constant speed and making a simple 12-1 steering input. And I see no way that said alignmnet issue would not make itself noticable at any speed and in all driving conditions.

Fabrik8

I agree, but something out of the ordinary is obviously going on. I can't picture what it would be. I can see what could happen, but I can't see the conditions being extreme enough to make it happen.


And going straight, you don't have close to equal load on all 4 corners, unless you have close to perfect front/rear weight distribution (and then there's the aero loads, etc..). Obviously there is no lateral load on the tires until you start turning, so they produce zero lateral grip in a straight line. Still, none of that explains why the rear would get loose, just like you said.

I still think it's a alignment setting that's making the rear feel loose. I can't really say if it's actually losing traction, but it may feel like it based on a really horrible toe setting.