Severe Understeer

hobbes80 · May 26, 2009

Not yet... aside from not having the $$$ to spend on BFG KDs, they seem to have stopped making them in 275/40 17" size... which is irritating, means I need to make another choice.

I'm trying to decide between the g-Force T/As, Nitto 555R Extreme, and Falkien 615s...

Definitely starting out with just fronts, so they need as much, if not more, grip than the back, but they also need to at least pretend to have traction on them for when it is wet. While I typically don't drive in the rain, there are those days when I get caught out and need to be able to limp home.

007 · May 26, 2009

hobbes80 said:
Definitely starting out with just fronts, so they need as much, if not more, grip than the back, but they also need to at least pretend to have traction on them for when it is wet. While I typically don't drive in the rain, there are those days when I get caught out and need to be able to limp home.

You should try KDW2s up front. I cant believe how much dry traction I have, and they are awesome in the wet too. I had KDW1s before and they werent very good. Also had Michelin Pilot Sports ($$$$) and the KDW2s were WAY better.

I'm running them on all four corners and couldn't be happier. 295 18s out back and 265 18s up front. Masses of grip.

hobbes80 · May 27, 2009

They don't sell KDW2s in 275/40R17.... I could get it in 255/40R17.... the question is, would the speed be off 8%, or would it remain the same since the back sizing hasn't changed? how well does a 255 fit a 17x9 rim?

I like the tread pattern and have heard nothing but good things, but don't know if i want to reduce my width up front that much.

40oz · May 27, 2009

Flghtmstr1 said:
The weight distribution of a vehicle has a large effect on every dynamic motion. The excess weight (combined with Ford's factory suspension tuning) IS what causes understeer. Adding a stiffer rear swaybar will cure it, but it does so by limiting lateral traction in the rear, not by addressing the underlying issue. Removing weight from the front will not only help the car's handling, but will increase acceleration and fuel economy as well.

As for your question: I have modified my car to be a jack of all trades. I enjoy autocross, occasional drag strip passes, and just tearing it up on the street. Those three disciplines necessitate compromises when it comes to suspension tuning, and my suspension handles all of these counter-productive goals admirably: competitive in autocross (with my buddies, not in my class), great weight transfer at the strip (1.8 second 60' time on my regular street tires), and a balanced, taut feel for the street. However, as the saying goes: "Jack of all trades, master of none." There are inevitable compromises that must be made if I want to take my car in a particular direction, and those are compromises I don't want to make.

Not to hijack this thread with an internet argument, but there are many factors that affect understeer. Nobody claimed weight distribution was irrelevant, just that it is hardly the most important or insurmountable.

Basically you are telling the OP to put a couple hundred pounds of sandbags in the trunk before an autocross and his car will magically stop understeering. I'm sure you do the same yourself, right? Just load up the trunk to get that perfect 50/50 spread and watch out world!

You laugh, but that is EXACTLY what you are claiming when you say your car understeers because of weight distribution.

Corvettes don't have 50/50, Porsche weight spread is faaar from 50/50, not even Ferrari makes everything 50/50. BMW is the only maker I know of that brags about 50/50 - can you say "marketing?"

A couple percent more weight up front isn't the end of the world. Unless you think it is.

Flghtmstr1 · May 27, 2009

Alrighty, I suppose we better finish this up. In steady-state turning, the most important thing that affects understeer is the roll stiffness, so you would be correct in saying that using a stiffer rear swaybar would help cure understeer. However, in the real world, and especially at an autocross, there are very few steady-state turns.

When a car must negotiate a course requiring different steering angle and throttle inputs, weight distribution becomes very important. The reason for this is that the car has momentum, and will naturally attempt to continue traveling in whatever direction it was before the control input. Therefore, a bunch of weight over the nose will make the nose more likely to plow ahead in whatever direction it was already traveling.

It is very difficult to measure things like roll stiffness and understeer percentages in the real world. Ideally, you would find an ideal set of spring rates and swaybar rates that would result in a very slight amount of understeer during steady-state turning, followed by weight distribution tweaking to improve transient responses.

40oz · May 28, 2009

Flghtmstr1 said:
Alrighty, I suppose we better finish this up. In steady-state turning, the most important thing that affects understeer is the roll stiffness, so you would be correct in saying that using a stiffer rear swaybar would help cure understeer. However, in the real world, and especially at an autocross, there are very few steady-state turns.

For the record, I didn't say the OP's solution is a stiffer rear swaybar. That was part of the solution in my old Civic, but since it was front wheel drive it needed the rear to slip more than would be practical or helpful in a RWD car. In the OP's case, better tires and a softer front bar would be more helpful, IMHO.

I can't say his driving is a factor since I don't know what he is doing inside the car. But I do know that improper use of braking and gas will exacerbate any tendency to understeer at the start of a turn. One needs to use the brake to transfer weight to the front immediately prior to a sharp turn in otherwise obscene plowing occurs. This is related to mass distribution, but hardly specific to Mustangs. Poor technique will lead to a conviction the car is a pig. Good technique will allow better response and control.

hobbes80 · May 28, 2009

40oz said:
For the record, I didn't say the OP's solution is a stiffer rear swaybar. That was part of the solution in my old Civic, but since it was front wheel drive it needed the rear to slip more than would be practical or helpful in a RWD car. In the OP's case, better tires and a softer front bar would be more helpful, IMHO.

I can't say his driving is a factor since I don't know what he is doing inside the car. But I do know that improper use of braking and gas will exacerbate any tendency to understeer at the start of a turn. One needs to use the brake to transfer weight to the front immediately prior to a sharp turn in otherwise obscene plowing occurs. This is related to mass distribution, but hardly specific to Mustangs. Poor technique will lead to a conviction the car is a pig. Good technique will allow better response and control.

I'm inclined to agree that practice and technique are the most powerful tools that are brought to any driving scenario.

That being said, with the nose planted firmly into the ground, the car plows forward on the tires, whether it was me driving, or the instructor driving my car. The goal of this thread was to learn the various things that affect a car while turning heavily and things that cause understeer, the result is that I've learned a lot of things.

I found this great write-up on mustang suspension that has really helped me understand what may be affecting my turning abilites:
Late Model Mustang Suspension Basics

What this has led me to believe is that the inherent "flaws" in the design of the front end of the car will promote understeer in a situation in which the front suspension is severely compressed as it effectively changes the camber of the front wheels. While being "front heavy" is not necessarily the reason why the car understeers, the overall mass of the motor will add to my inertia and further compress the springs, exacerbating the existing "flaw" in the design.

The SFC will limit the ability of the car to roll into the turn, which will create a better balancing effect between the front and the back of the car. Basically if the car can start to roll into the suspension a little at a time, it can build quickly as each little bit adds mass to the inertia of the turn. Whereas if the car is more rigid, there has to be enough momentum to "pull" up the full weight of the car, essentially helping it to become more "balanced".

So basically, you are both correct. A car with a balanced 50/50 curb weight isn't necessarily going to be a better handling car than one that is 60/40, but a car that has a stiffer frame is going to transfer weight in a more linear fashion, allowing for more consistent and predictable turning in a more balanced fashion.

This is my take on it based on all the reading I've done in the last few days and my extremely limited experience actually driving that aggressively. I could be totally off, but it feels like is at least mostly right.

40oz · May 29, 2009

Flghtmstr1 · May 29, 2009

Good show, 40oz :SNSign:

007 · May 29, 2009

hobbes80 said:
They don't sell KDW2s in 275/40R17.... I could get it in 255/40R17.... the question is, would the speed be off 8%, or would it remain the same since the back sizing hasn't changed? how well does a 255 fit a 17x9 rim?

I like the tread pattern and have heard nothing but good things, but don't know if i want to reduce my width up front that much.

255s are too small for 9s if your looking for grip. I used to have 275s on 9 all round and thats great on a 9 inch rim. The 275s may or may not rub very slightly at full lock. My 265s dont. Do they make 265s KDW2s like I have in 18s for 17 inch rims? That'd be great.

I think I have now setlled on a perfect setup really. 265s on 9s up front, 295s on 10s out back. It all fits perfectly.

Maybe just get the 275 KDs up front dude....

Flghtmstr1 · May 30, 2009

I kinda hate to dive back into this, but staggering your tires won't help with the understeer at all, unless you plan to swap the big tires to the front for autocross.

hobbes80 · Jun 1, 2009

Flghtmstr1 said:
I kinda hate to dive back into this, but staggering your tires won't help with the understeer at all, unless you plan to swap the big tires to the front for autocross.

Understood. That being said, I already own 17x9s in the front, 17x10s in the back (may even be 17x10.5.....

Ideally, I should probably run 18x9s all the way around to autocross, but realizing that 4 new rims (or even 2x new 17x9s) are not in the immediate budget, and also realizing that my original 17x8s were sold a few years ago, I'm left with determining the best solution for my existing rims.

I'd prefer not to buy new tires for the back until I need to, which leaves just buying fronts.

The don't make KDW 2s in 275/40 17, and I can only find one site that has KDs in that size, and they only have left side, and they are on clearance with 4 left, BFG says they are no longer making it.

That brings me back to my original point of deciding on the right rubber to get the best grip I can in dry in the front, while still being street legal and allowing me to drive home safely if it is raining, even if it means going 15mph the whole way.

Currently, I'm looking at the Nitto 555R Extremes, Falkien 617s, BFG g-Force T/As, and the original KDWs (though the wear rating of 400 makes me leary)

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