Hello everybody,
I was trying to understand the very physical sense of most of the FWD cars being prone to understeer, but some things are getting too far.

http://www.suv-rollovers.com/a ... tions/understeer-01292010

According to this website:

A front-heavy vehicle with low rear roll stiffness (from soft springing and/or undersized or nonexistent rear anti-roll bars) will have a tendency to terminal understeer: its front tires, being more heavily loaded even in the static condition, will reach the limits of their adhesion before the rear tires, and thus will develop larger slip angles. Front-wheel drive cars are also prone to understeer because not only are they usually front-heavy, transmitting power through the front wheels also reduces their grip available for cornering.

To make things less complicated, let's talk about the car not accelerating or braking at all, just entering the corner. I've thought the higher the load on the front tires, the higher the grip. As it turns out, somebody forgot to mention (in my opinion) the important suspension settings factor.

In FXO, for example, the weight distribution is as follows: 57 front, 43 rear. When the car is turning right, the weight is going to the left more to the front of the car than, as the centrifugal force is higher, but at the same time the maximum friction the front can generate is higher than on the rear. We all know that when this max. friction is e.g. two times higher when the centrifugal force is two times higher. To prevent mentioned weight transfer, I set minimum anti roll on the rear and the maximum in the front. 57F and 43R on the paper doesn't seem to make a very big difference, however the car is still very understeery, as the first wheels losing grip are the front ones. Can anyone get me through this?

Quote from pajkul :

In FXO, for example

Quote from pajkul :

Demo racer

That's what I was afraid of, but no, this is not topic related.

What's more: when I drive RWD with very heavy rear and lock the tyre by braking in the corner, when I turn right, the one blocked is the front right. Even if the brake balance is set to 50%. This means that the load on rear tyres is higher making them more difficult to lock. No matter how many times higher is the centrifugal force generated on the rear of the car, the maximum friction should rise the same number of times, as it depends on the gravity force. I don't get it.

Mmk. To start off:
-there is no such thing as centrifugal force.

Now that I've told you that, lets get down to business:

Understeer is caused by something called momentum. When you enter a corner with too much speed, the momentum of your car is "breaking" friction and causing the car to slide straight in the corner. The key to proper cornering is limited body roll. The key to limiting body roll is a stiff chassis. Body roll causes weight shifts which disrupt the line that the car was taking. However, from my understanding, a slight amount of weight transfer onto the front tires does help because it increases the force of friction. Hence why if you understeer, you can recover, or at least attempt to, by pressing the gas and brake at the same time. That transfers enough weight to the drive tires so they can "pull" you out of the understeer. And it really does work, I've tried it. lol

Anywho, feel free to roast me. I used my understanding of physics (I'm in school to be a mechanical engineer) to attempt to answer his question.

Quote from roadrash17 :

Mmk. To start off:
-there is no such thing as centrifugal force.

Now that I've told you that, lets get down to business:

Understeer is caused by something called momentum. When you enter a corner with too much speed, the momentum of your car is "breaking" friction and causing the car to slide straight in the corner. The key to proper cornering is limited body roll. The key to limiting body roll is a stiff chassis. Body roll causes weight shifts which disrupt the line that the car was taking. However, from my understanding, a slight amount of weight transfer onto the front tires does help because it increases the force of friction. Hence why if you understeer, you can recover, or at least attempt to, by pressing the gas and brake at the same time. That transfers enough weight to the drive tires so they can "pull" you out of the understeer. And it really does work, I've tried it. lol

Anywho, feel free to roast me. I used my understanding of physics (I'm in school to be a mechanical engineer) to attempt to answer his question.

Why is it that there's no centrifugal force? In general sense of physics, this force exists. And I'm convinced this is one of the forces the car has to deal with when turning. From the car's perspective, this force is trying to beak the friction between the tyres and the road. If the loses grip, it means that there was some higher force pointed against the friction (how do I call this force)? Probably fictitious force, in this case - centrifugal force.

Quote :

In general sense of physics, this force exists.

No, it doesn't.
But there is a centripital force.

Or you could bypass all the mumbo jumbo and use XRT for the rest of your life. Taadaa. No more understeer.

Quote from Ingolf :

No, it doesn't.
But there is a centripital force.

http://en.wikipedia.org/wiki/Centrifugal_force

It does. In non-inertial reference frames, e.g. from the car perspective.

The force that causes the tires to slip isn't a force, but it's called momentum. Momentum describes a moving objects desire to resist a change in direction. When you drive, the car and everything inside the car has momentum. When you turn, the car WANTs to continue in it's straight path but the force of friction between the tires and the road, if high enough, causes the car to turn, but because of its desire to resist change in direction, the car leans and you feel like there is a force pushing you towards the outside of the corner when there isn't. That feeling is what people like to think of as centrifugal force. Now, if momentum is too great, then the tires break the force of traction and you slide. Better?

Quote from roadrash17 :

The force that causes the tires to slip isn't a force, but it's called momentum. Momentum describes a moving objects desire to resist a change in direction. When you drive, the car and everything inside the car has momentum. When you turn, the car WANTs to continue in it's straight path but the force of friction between the tires and the road, if high enough, causes the car to turn, but because of its desire to resist change in direction, the car leans and you feel like there is a force pushing you towards the outside of the corner when there isn't. That feeling is what people like to think of as centrifugal force. Now, if momentum is too great, then the tires break the force of traction and you slide. Better?

Yes, thanks. Can you tell me why is it so hard to eliminate understeer? The front is just slightly heavier than the rear of the car. Even if I set the anti roll to the extreme values, it doesn't help that much.
Just to add one thing: when the car is finally taking the turn, the only grip it has is through its tyres, so the centrifugal force exerts on the car just as on a person sitting inside the car. But the suspension is dimnishing the real effect it has on the car's stablitiy. Am I right? In described case, there is the acceleration vector pointed towards the center of the circle being the path the car drives. So the fictitious force is the centrifugal force.

Anyway, I'm pretty much sure that the momentum (momentum is a fictitious force used in non-inertial reference frames) you wrote about is the centrifugal force. I read about it in some car magazines and web articles written by allegedly smart people. So we have two different opinions, can anyone go into facts?

As I see it, there is understeer in three places, each with different (and sometimes conflicting) ways to deal with it.

While outright friction determines the overall understeer/oversteer balance while at the limit, the difference in cornering stiffness front and rear defines the balance while under the limit of adhesion. This is a direct property of the tyres, in response to load and slip. The higher load on the front tyres will make the front stiffness relatively less (cornering stiffness increases non-proprortionally with load), thus creating understeer on turn in.

At the initial point of turn in, before weight transfer begins, the only tool you have to control this is tyre pressures. More pressure will increase the stiffness, and thus the sharpness at this end. So you could increase front tyre pressure and reduce the rear, to reduce understeer at this point.

As turn in progresses and weight transfer begins, you can use the suspension to diagonally shift the weight transfer, and thus control the load distribution of the front and rear tyre pairs. The more evenly loaded the tyres (which axle has the relatively softer roll stiffness - relative to the supported mass, that is), the less they will need to slip in order to produce force, again reducing understeer at that end.

Finally, once settled in to the corner, with maximum weight transfer (I don't see the point of the whole centripetal/centrifugal force argument... the force from the tyres is torquing the car, which creates weight transfer as the suspension reacts to the body roll created), things get more complicated again. If the front and rear are exactly equal in outright friction (unlikely), then everything said this far still applies, and will control the balance of the car. If not, the more unbalanced the two ends of the car are, the more you will just feel this difference determining the handling, and the effect from he difference in cornering stiffnesses will become much reduced.

One last complication with FWD cars is that the engine is generally providing some torque to the wheels, causing longitundinal slip. This reduces cornering stiffness massively, and is responsible for the large feeling of understeer, even when all four tyres are still well within their traction circle.

Managing all of this is not easy and you can never remove all of the understeer, at all times, from a non balanced car, and FWD will always add understeer (much like RWD will always add oversteer).

"The higher load on the front tyres will make the front stiffness relatively less (cornering stiffness increases non-proprortionally with load), thus creating understeer on turn in."

Meaning this? I can't really understand this sentence.:

Quote from pajkul :

Anyway, I'm pretty much sure that the momentum (momentum is a fictitious force used in non-inertial reference frames) you wrote about is the centrifugal force. I read about it in some car magazines and web articles written by allegedly smart people. So we have two different opinions, can anyone go into facts?

Momentum isn't a force, it's a conserved quantity.

http://dictionary.reference.com/browse/centrifugal+force

Look at the second reference. It states it's a fictitious force. i.e. it's something we think is there but actually isn't. I have a college level physics book that states what I am telling you. Centrifugal force is something someone came up with to describe the feeling of being pushed outwards when going in a circle. The actual reason you feel like you're being pushed outwards is explained in Newtons First law of Motion: Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. That external force is the force of friction between the tires and the road.

Quote from pajkul :

Yes, thanks. Can you tell me why is it so hard to eliminate understeer? The front is just slightly heavier than the rear of the car. Even if I set the anti roll to the extreme values, it doesn't help that much.

There is a limit of how fast you can go around a corner with a car. you will never be able to fully eliminate understeer, but you can do your best to increase the top cornering speed of a car. I suggest playing with Camber, tire pressure, and anti-roll.

Quote from pajkul :

Just to add one thing: when the car is finally taking the turn, the only grip it has is through its tyres, so the centrifugal force exerts on the car just as on a person sitting inside the car.

The thing people call centrifugal force (actually Momentum, a conserved quantity) acts on all bodies in motion.

Quote from pajkul :

But the suspension is dimnishing the real effect it has on the car's stablitiy. Am I right? In described case, there is the acceleration vector pointed towards the center of the circle being the path the car drives. So the fictitious force is the centrifugal force.

The suspension is helping the car go around the corner by taking a portion of the kinetic energy (also a conserved quantity) that the car has and dissipating it by compressing the shocks on the outside side of the car as it corners. The acceleration vector does point towards the center of the circle and the fictitious force is, in fact, centrifugal force.

I've done my best to explain this concept to you...it's something that took weeks of schooling to fully comprehend to a point where I could use it and explain it to people. I hope I helped.

Quote from roadrash17 :

Momentum isn't a force, it's a conserved quantity.

http://dictionary.reference.com/browse/centrifugal+force

Look at the second reference. It states it's a fictitious force. i.e. it's something we think is there but actually isn't. I have a college level physics book that states what I am telling you. Centrifugal force is something someone came up with to describe the feeling of being pushed outwards when going in a circle. The actual reason you feel like you're being pushed outwards is explained in Newtons First law of Motion: Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. That external force is the force of friction between the tires and the road.



There is a limit of how fast you can go around a corner with a car. you will never be able to fully eliminate understeer, but you can do your best to increase the top cornering speed of a car. I suggest playing with Camber, tire pressure, and anti-roll.




The thing people call centrifugal force (actually Momentum, a conserved quantity) acts on all bodies in motion.



The suspension is helping the car go around the corner by taking a portion of the kinetic energy (also a conserved quantity) that the car has and dissipating it by compressing the shocks on the outside side of the car as it corners. The acceleration vector does point towards the center of the circle and the fictitious force is, in fact, centrifugal force.

I've done my best to explain this concept to you...it's something that took weeks of schooling to fully comprehend to a point where I could use it and explain it to people. I hope I helped.

I know this is a fictitious force, as I've mentioned before. It is actually the momentum in circular motions. So we have basically the same concepts, correct me if I'm wrong.

OK, looks like you're understanding it like said before, you can never entirely remove understeer...it's just impossible. but you can get it to where you corner pretty damn fast without understeering. :]

Pajkul have you considered the possibility that your steering input is too great this will quickly induce more under steer every time.

Maybe post a replay so we can see if your driving style is not smooth enough etc.


(IIRC I was taught that centripetal is a real and measurable force and can be accounted for externally to a given system, while centrifugal force is non existent and only describes the feeling you would experiance if you were inside the system.)

SD.

Quote from pajkul :

"The higher load on the front tyres will make the front stiffness relatively less (cornering stiffness increases non-proprortionally with load), thus creating understeer on turn in."

Meaning this? I can't really understand this sentence.:

Essentially, yes. The net result is that the front tyres need to slip more to create the same (in this case, lateral) force as the rears, which is the definition of understeer.

so, have we established how a demo user is driving a FXO? my post has been deleted so i can only assume that we have confirmed that this user has not cracked lfs. according to a mod, i did not post in this thread. strange things are afoot.

Quote from dadge :

so, have we established how a demo user is driving a FXO? my post has been deleted so i can only assume that we have confirmed that this user has not cracked lfs. according to a mod, i did not post in this thread. strange things are afoot.

I didnt read that he was driving a FXO while being a demo user. He just put the FXO as an example.

Quote from TurboBlaster :

I didnt read that he was driving a FXO while being a demo user. He just put the FXO as an example.

Yes, however he also said how he set up the ARB on the FXO

An explanation is in order

SD.

i'd also like to know why a completely legitimate post i made previously regarding this matter has been removed.

Because banging on about software piracy and posting 4chan crap really isn't helpful, or nice to read for the rest of us. If you suspect something, report the post, and leave it at that.

i'm sorry. i didn't realise that having a bit of fun on the forum was against the rules. so a crime that isn't reported is a crime not committed?
if you don't like my posts, put me on your ignore list. but don't remove my posts so that you can talk with a user who has pirated LFS. that kind of behaviour will only encourage more cracker/hackers to post here.
and since when have i been banging on about software piracy? i think that's the millionth time you've exaggerated this week. <<< sarcasm, i only hope you get it.

I'm a mod, so I can't actually put anybody on an ignore list. The whole point is that I'm supposed to see everything so I can take appropriate action. In theory I could justify removing any off topic post but obviously that becomes unmanageable. Don't feel like you're being picked on, there's just too much rubbish on the forums for us to get everything, but it stood out a lot in an otherwise interesting thread.