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Old 17th July 2013, 21:29   #121
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Default re: Evasive manoeuvres & Rollovers?

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Originally Posted by Guna View Post
But in reality, when the tipping point is reached, it is the car with the stiffer suspension which is likely to topple (2 wheels up in the air). Car with the softer suspension, even though it leans dangerously, will still have all 4 wheels grounded for 'longer period' (till the tipping point is reached). Softer suspension allows part of the mass to shift while the stiffer suspension makes it behave like a single unit(relatively).
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Originally Posted by Sutripta View Post
For same suspension travel?
Quote:
Originally Posted by san_jayd View Post
Nope, for the same speeds.
Can't get what you are saying. What will happen at the same speed?

Quote:
Both stiff and soft suspension setups have their advantages and disadvantages. It depends on what the manufacturer thinks is important for their vehicle.
Agreed. But was not commenting on suspension design philosophies. In this thread an (absolute) statement was made, and a (very well documented) counterexample was offered.

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Sutripta

Last edited by Sutripta : 17th July 2013 at 21:30.
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Old 18th July 2013, 12:07   #122
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Default re: Evasive manoeuvres & Rollovers?

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Originally Posted by DerAlte View Post
I am sure you will look at it differently if you look at the maths of dynamics of suspension. Suspension design is not done by hit and trial, right?
Most assuredly not. And with the same sentiment, I'll assure you again that no matter what the stiffness of a spring, or any component for that matter, the deformation will start at the same time if the force acts at the same time. The only difference stiffness makes is magnitude of the deformation.

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Originally Posted by DerAlte View Post
And why not? The physical rules / engineering stay the same, no?
Yup. But here we are talking only about suspension stiffness I think. And I've not worked on any race/rally prepped vehicle, but I think they make a lot of changes besides changing the suspension. For starters, just by observation you can tell that they use different size wheels, tyres, and wheel angles (castor, camber, toe-in, toe-out). That is one thing which will make an effect. Plus they throw out any unneeded trims from the car, reducing the weight. So in order to compare a modified vehicle, we'll need to know what all changes they make. I unfortunately do not know it. If you do, I'd appreciate the additional information.

And additionally, race car drivers would almost always want better steering response, regardless of whether it roughs their ride or makes the vehicle susceptible to toppling. They are professionals after all, and one would expect that they know well how to operate their vehicle to the best of its ability without going over the limit.

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Originally Posted by DerAlte View Post
Paris-Dakar Rally contestants use SUVs. And SUVs don't get significant down force at the speeds they travel.
True, interestingly. I've only very rarely watched Dakar Rallies. That too as cut up highlights on the television. As I said before, I cannot comment on SUVs or trucks or buggies or whatever vehicles which compete in those rallies because I just do not know what all changes they make.

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Originally Posted by DerAlte View Post
Do you know how much a Moose / Elk weighs, and how tall it is? If one is travelling at 100Kmph on the secondary highways in Scandinavia (that is the average speed on those highways) and hits a Moose / Elk straight on at 60Kmph (after braking), the moose - hit near the knees by the bonnet - will crash into the cabin through the windshield. One will certainly be crushed in the driving seat - airbag won't help. Quite different from hitting a dog, small deer or a donkey. Did you think the Scandinavians are dumb to conduct the Moose Test?
Mate, the last part was just my personal opinion. Curiously, I did not know what how much the moose weighs exactly. Turns out 360 Kgs on an average. An average sedan on the other hand, usually weighs around 2000 Kgs. So yes, I'd still prefer slamming the brakes and hitting the stationary moose as opposed to crashing into a possible car coming from other side at same speed I'm at. It is a matter of drivers' choice I suppose.

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Originally Posted by mallumowgli View Post
Dont you think a shift of CG is detrimental to the dynamic stability of the vehicle? For a vehicle to be stable the CG needs to stay lower and also towards the centre of the axis the movement of vehicle. Else the moment of inertia will carry the vehicle off the line the curve. Basic physic

Consider the shifting of CG of a softly sprung vehicle. The CG will shift pull the weight of the vehicle towards the outside of the curve thus making the inside wheels to have a tendency to lift off the road

The only difference (as per the point made by you and Guna) is that a softly sprung vehicle would appear to topple in slow motion, whereas a stiff-suspension set-up turn turtle as a single mass!!
Okay, here's a question. What is the purpose of suspension?

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Originally Posted by mallumowgli View Post
Really surprised to see this comment. Do you mean to say that softly sprung cars can take more speed into corners than cars with stiff suspension?? Can you elaborate? People from companies like Blistein would be interested in that.
Without toppling, yes. I'm sure people from Bilstein already know why. And in that sense, I'll answer my own question which I posted above. What is the purpose of suspension? To absorb road shocks, and isolate the occupant cabin from forces which the wheels are subjected to, primarily. A softer suspension is able to absorb smaller forces as compared to stiffer suspension, which usually transmits the smaller forces directly to the occupant cabin.
So in this condition, what we have is:
Source: Road undulations.
Transfer path: Wheel -> suspension system -> Vehicle body.

Now in current scenario, what we are talking about (toppling) is exactly the opposite. Large magnitude of centrifugal forces are generated by the vehicle body itself (and whatever's inside). These forces then transfer through the suspension, into the wheel and axle components. The moment which axle is subjected to causes it to lift from one side.
So in the current condition we have:
Source: Centrifugal force generated by vehicle body and trims.
Transfer path: Vehicle body -> suspension system -> Wheel, axle components.

In both conditions though, part played by the suspension is the same. Absorb forces. A rigid suspension will absorb very little portion of the forces compared to a softer one.

Consider an ideal suspension. One which absorbs all forces transferring through it. What you'd have is a vehicle body getting very noticeable tilted while the wheel and axles have no unwanted forces acting on them whatsoever, and hence staying on the ground.

It is simple concept really. Even in today's PC simulation games which one might play (the ones where you get to customize your cars and whatnot), they mention that stiffening the suspension will increase the tendency of your car to flip over when cornering.

Quote:
Originally Posted by Sutripta View Post
Can't get what you are saying. What will happen at the same speed?
Apologies, I believe you were asking Guna about the tipping point. A stiffer suspension will travel much lesser compared to a softer one at the same speed is what I think he was saying. If you were asking something different then ignore my interruption.
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Old 18th July 2013, 12:49   #123
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Default re: Evasive manoeuvres & Rollovers?

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Originally Posted by san_jayd View Post
... the deformation will start at the same time if the force acts at the same time. The only difference stiffness makes is magnitude of the deformation. ...
Strange! You seem to be recognizing only the statics, not the dynamics (d/dt dependence). There is a shock absorber there, but maybe you don't consider it a part of the suspension equation?

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Originally Posted by san_jayd View Post
... So in order to compare a modified vehicle, we'll need to know what all changes they make. I unfortunately do not know it. If you do, I'd appreciate the additional information. ...
We would be far away from the discussion!

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Originally Posted by san_jayd View Post
... And additionally, race car drivers would almost always want better steering response ...
Ditto.

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Originally Posted by san_jayd View Post
... As I said before, I cannot comment on SUVs or trucks ...
The discussion is about SUVs, in case you haven't noticed.

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Originally Posted by san_jayd View Post
... last part was just my personal opinion. ...
Doesn't absolve us from getting data before expressing the opinion, does it?

BTW, a moose cow weighs that; moose bulls can weigh upto 700+Kgs. No matter what the car size or weight is, please calculate what damage that weight can do if it enters through the windshield at 60Kmph on impact. Just the kinetic energy of 500Kgs @ 60Kmph released onto the occupants. Go ahead.

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Originally Posted by san_jayd View Post
... It is a matter of drivers' choice I suppose. ...
I thought you earlier said "Professional driver will never tell you to swerve your car (be it SUV or otherwise) if a moose wanders in front"? That doesn't amount to driver's choice but a recommended practice, if so. It isn't, fortunately.
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Old 18th July 2013, 14:04   #124
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Default re: Evasive manoeuvres & Rollovers?

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Originally Posted by san_jayd View Post

Now in current scenario, what we are talking about (toppling) is exactly the opposite. Large magnitude of centrifugal forces are generated by the vehicle body itself (and whatever's inside). These forces then transfer through the suspension, into the wheel and axle components. The moment which axle is subjected to causes it to lift from one side.
So in the current condition we have:
Source: Centrifugal force generated by vehicle body and trims.
Transfer path: Vehicle body -> suspension system -> Wheel, axle components.

In both conditions though, part played by the suspension is the same. Absorb forces. A rigid suspension will absorb very little portion of the forces compared to a softer one.
I can understand your line of argument -but not convinced about stiffer suspension toppling earlier.

Your line of argument goes that vehicle with stiffer suspension travel will reach its maximum lean (maximum suspension travel) earlier and then it will tend to lift the inside wheels whereas the softer suspension will take longer time to reach the maximum lean or max suspension travel

I would like to see some back up for the data - reason being the following

Let us take two cars/SUVs - one with a stiff suspension (A) and the other with softer set-up (B)

Now car A travelling at 100kmph leans by around, say, 15 degree from the centre line

Car B travelling at the same 100kmph being a softer suspension will lean more, (than 15 degree) Correct?

Now there will be a maximum angle to which the vehicle can lean before toppling over due to the shift in CG or the mass or centrifugal force or whatever - the softly sprung car will reach this point faster compared to the stiffer one

But if the maximum suspension travel is reached before reaching the above speed then the vehicle reaching this point will topple first (Assuming all other factors to be the same)

Now I think only empirical evidence can prove this!!

Whatever may be the result - just shows that I am sitting in office without any work whatsoever!!

(FYI ramming into a large animal is never recommended - reason already explained. Just check the web for camel hits happening in middle east. Few months back there was a similar accident with an elephant where the people in the car died when the elephant fell on the car)
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Old 18th July 2013, 14:27   #125
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Default re: Evasive manoeuvres & Rollovers?

Recently I happen to watch a Fifth Gear episode which demonstrated the use and need of ESC (Electronic Stability Control) in vehicles. I attribute much of the toppling sort of accidents under high speed maneuvers to lack of such safety system in majority of our cars. Europe has made it mandatory to have ESC systems in all vehicles sold from 2014. Hope our Govt. open its sleepy eyes and save some lives by making certain safety kits a must have in all cars sold here.

Fifth Gear episode ( Watch from 8th min for ESC related content) :
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Old 18th July 2013, 16:29   #126
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Default re: Evasive manoeuvres & Rollovers?

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Originally Posted by roy_libran View Post
Most people I see around me, end up buying an SUV / SUV-like vehicle for the wrong set of reasons.
For this the manufacturers are, to a large extent, to blame. A lot of SUV advertisements promote wrong usage and driving patterns for SUV's

Absolutely right on this one. I used to drive a '99 Santro. It did not have a powersteering. Once while taking a 'significantly' sharp left bend at a speed of around 40, i felt the right side wheels go up in the air. Since then i resolved never to drive a 'tall boy' hatch or a UV (I have a Bolero which i dont consider to be a SUV ) at speeds above 90 in a straight line and above 30-35 while taking curves.

The SUV Advertisements reminds me- does anybody remember the Safari Petrol Advertisement- the one with two guys in the parking lot of a circus/entertainment park who did not raise an eyebrow in a scary ride but completely freaked out while gunning the Safari. Thats misleading. You inveigle people to drive their SUV's at high speeds and they continue to do so without understanding the weight of the chassis, the engine and the fact that you havent equipped the 'SUV' with rear disc brakes
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Old 18th July 2013, 16:53   #127
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Default re: Evasive manoeuvres & Rollovers?

I think everyone who know how to drive a car properly should just try to feel and sense the feedback which is coming from the road to the steering and then should manouvre the car on corners. For eg: the swinging effect due to high ground clearance is clearly felt and one should just reduce the speed at the time when the car begins to loose its line on the corners. Thats why they say- be gentle on the curves. There is nothing one can do when an obstacles is seen or comes up suddenly. You can just try to steer away using the brakes and downshift as quickly as possible (engine braking) and leave the rest to car's dynamic abilities and safety features.
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Old 18th July 2013, 17:29   #128
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Default re: Evasive manoeuvres & Rollovers?

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Originally Posted by san_jayd View Post
The suspension will deform by a larger extent. And by deform, I mean it'll go down on one side. Hence slightly lowering the CG.
So, while one side's going down and lowering your CG, is the other side laying flat? Or is that side lifted, lifting your CG?

Quote:
Originally Posted by san_jayd View Post
What is the purpose of suspension? To absorb road shocks, and isolate the occupant cabin from forces which the wheels are subjected to, primarily.
What about the part the suspension plays to keep the tyres on the road? Is that not important?

Quote:
Originally Posted by san_jayd View Post
It is simple concept really. Even in today's PC simulation games which one might play (the ones where you get to customize your cars and whatnot), they mention that stiffening the suspension will increase the tendency of your car to flip over when cornering.
Most of your theories are having large holes due to flaws in the fundamentals. Please take the time to study from the bottom up, rather than postulating from the top down.

Cheers.
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Old 18th July 2013, 20:21   #129
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Default re: Evasive manoeuvres & Rollovers?

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Originally Posted by san_jayd View Post
Apologies, I believe you were asking Guna about the tipping point. A stiffer suspension will travel much lesser compared to a softer one at the same speed is what I think he was saying. If you were asking something different then ignore my interruption.
Hi,
Understood (or think I did) what Guna was saying. Didn't agree with it, and instead of getting into long winded 'technical' discussions, gave a counterexample.

Would still like to understand what you were saying.

Regards
Sutripta
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Old 19th July 2013, 12:42   #130
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Originally Posted by DerAlte View Post
Strange! You seem to be recognizing only the statics, not the dynamics (d/dt dependence). There is a shock absorber there, but maybe you don't consider it a part of the suspension equation?
Purpose of a shock absorber is to dampen the vibrations which will generate in a suspension system after external forces have been removed. Here I believe we are discussing a situation where a vehicle is taking a turn, and lateral forces are very much continuously existent.

And I'm not considering the delta (t) dependence because we're comparing two cars having different suspension setups with respect to each other, not with respect to time. Delta (t) in both cases will have to be same for us to compare them properly.

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Originally Posted by DerAlte View Post
We would be far away from the discussion!
Exactly my point!

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Originally Posted by DerAlte View Post
The discussion is about SUVs, in case you haven't noticed.
Cutting up my sentences is going to cause misunderstanding. You can be certain that I've read every post in this thread, and it was originally started for discussing standard SUVs, not the ones used in Dakar Rallies I think. As you said yourself, we would be far away from the discussion if we got into that.

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Originally Posted by DerAlte View Post
BTW, a moose cow weighs that; moose bulls can weigh upto 700+Kgs. No matter what the car size or weight is, please calculate what damage that weight can do if it enters through the windshield at 60Kmph on impact. Just the kinetic energy of 500Kgs @ 60Kmph released onto the occupants. Go ahead.
Thank you for the weight correction! Ask yourself this though, why doesn't Euro NCAP conduct simulation tests of moose hitting the windshield if they are so lethal? On the other hand, it does conduct plenty of tests regarding car to car impacts. And, it also conducts tests of cars impacting pedestrians. The point of those tests is to make sure no vital part inside a car bonnet will get damaged in case the car hits a pedestrian and his head bumps on the hood with high velocity. Eerily similar, although scaled down.

Came across an interesting statistic today while reading a CCIS pdf that 8/10 frontal crashes above 90 Kmph resulted in deaths and 8/10 side impacts above 70 Kmph also resulted in the same (studies were done of seat-belted drivers). Am on work computer right now, so have access to very few websites hence can't give a link. But would be able to after I reach home if any of you wishes to take a read. My only reason for putting this up was to highlight that car-to-car impacts are not something to be taken lightly.

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Originally Posted by DerAlte View Post
I thought you earlier said "Professional driver will never tell you to swerve your car (be it SUV or otherwise) if a moose wanders in front"? That doesn't amount to driver's choice but a recommended practice, if so. It isn't, fortunately.
I'd hope that professional drivers have enough sense to not recommend average drivers to swerve their car suddenly. I'd hope that a professional driver tells an average drive to slow his car down on roads where "Moose Crossing" signs are put up. That is their purpose after all.

If we are to ignore safety signs, then we might as well conduct "Wrong-side test" which determines the maneuverability of a car in case a driver feels like ignoring the "One-way traffic" sign post. Sudden swerves/lane changing is best left for the movies methinks.

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Originally Posted by mallumowgli View Post
I can understand your line of argument -but not convinced.....

..... the people in the car died when the elephant fell on the car)
Quote:
Originally Posted by gthang View Post
So, while one side's going down and lowering your CG...
......
Cheers.
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Originally Posted by Sutripta View Post
Hi,
Would still like to understand what you were saying.
I've cut up all your posts if you don't mind, otherwise mine will be too long! Going to reply simultaneously.

First things first: some hard data. There is a certain component of suspension system called a sway bar or anti-roll bar which most SUVs these days incorporate. Assuming that you don't mind reading walls of theoretical texts (since we're all posting in the 'Technical Stuff' section as gthang had aptly pointed out before), I would suggest going through a usual Wikipedia page on "Sway bars", and a page titled "Anti-roll bar" on a website called www.turnfast.com for a bit of background theory. I would have tried to find some videos, but am at work as mentioned before. Net access is very much restricted. Thankfully they keep team-bhp on!

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Originally Posted by Thilak29 View Post
Recently I happen to watch a Fifth Gear episode which demonstrated the use and need of ESC (Electronic Stability Control) in vehicles....
Watched it too! It was a brilliant demonstration of why Europe had made ESC mandatory. One would certainly want it to be made mandatory everywhere else. The difference between a vehicle with and without it was just phenomenal.
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Old 19th July 2013, 14:09   #131
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Default re: Evasive manoeuvres & Rollovers?

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Originally Posted by san_jayd View Post
... Delta (t) in both cases will have to be same for us to compare them properly. ...


Quote:
Originally Posted by san_jayd View Post
... Exactly my point! ...
And that is ...?

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Originally Posted by san_jayd View Post
... Ask yourself this though, why doesn't Euro NCAP ... My only reason for putting this up was to highlight that car-to-car impacts are not something to be taken lightly. ...
It would be better if you avoid asking an unrelated question instead of giving an answer as is conventionally expected. And, inundating others with unconnected data (however 'correct', 'true' or 'right' it is, it is still unconnected) is considered trolling. Are you studying, or have studied, for an MBA by any chance?

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Originally Posted by san_jayd View Post
... I'd hope that professional drivers have enough sense ...
What made you assume that your hypothesis of hitting an obstruction instead of swerving is correct / safer option? Your 'hope' will be tangible only if you have a rational basis. 'Hit the blessed animal and hope for the best' is not very rational - the physics is to the contrary.

And 'Moose crossing' is not a speed reduction instruction, it is a warning to drivers to be cautious. But does 'being cautious' obviate the possibility of an accident?
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Old 19th July 2013, 15:23   #132
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This thread has lost it's focus I feel. I personally don't understand where is the discussion heading to? Perhaps even Mr. Lotfi Zadeh would find it difficult to dish out "crisp" information from such arguments.

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Are you studying, or have studied, for an MBA by any chance?
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Old 19th July 2013, 15:31   #133
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Default re: Evasive manoeuvres & Rollovers?

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Originally Posted by san_jayd View Post
Purpose of a shock absorber is to dampen the vibrations which will generate in a suspension system after external forces have been removed.
Do you mean vibration or oscillation of the spring? Completely different things.

Quote:
Originally Posted by san_jayd View Post
First things first: some hard data. I would suggest going through a usual Wikipedia page on "Sway bars", and a page titled "Anti-roll bar" on a website called www.turnfast.com for a bit of background theory.
So was the anti-roll bar not there when you were suggesting that lowering of one side will reduce the CG?

Wish you had gone through the website as well. Quotes from above site:
Quote:
"Aside from bump absorption, the spring also contributes to the roll stiffness of the car--the ability to resist dive under braking, squat during acceleration, and body roll during corning. The anti-roll bars also play a roll in this, and the two combined create the total roll stiffness of the car. Stiffer springs will resist body roll more, reduce change in the suspension geometry, and maintain a more consistent tire patch size.
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Originally Posted by san_jayd View Post
And, it also conducts tests of cars impacting pedestrians. The point of those tests is to make sure no vital part inside a car bonnet will get damaged in case the car hits a pedestrian and his head bumps on the hood with high velocity. Eerily similar, although scaled down.
Are you sure about this? Is reduction of injury to pedestrians not the priority?
http://www.euroncap.com/Content-Web-...rotection.aspx

Car to car, car to wall, side impact, etc., are controllable tests. You can only test the capability of the vehicle to avoid an Elk/Moose. How can you do impact tests when the outcome is random. You think the moose will fly the same way every time?

Most times you will not see an Elk/Moose till it's too late because their eyes dont shine like other animals, and the natural reaction will be to swerve.

Like I said before, a little background study might help before postulating.

Cheers.
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Old 19th July 2013, 17:12   #134
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Default re: Evasive manoeuvres & Rollovers?

@Spike : I'll add a little further to the obtuse angle of the thread

Check this study

ftp://ftp.demec.ufpr.br/disciplinas/...5_rollover.pdf

Particularly the below conclusion

Quote:
Simulations performed for vehicle 3 (with increased roll stiffness and roll damping) and vehicle 4 (with modified suspension geometry) indicate that neither of them can be rolled over in J-turn or fishhook maneuvers regardless of the amplitude of steering angle. Thus relatively minor changes in suspension design can improve rollover resistance of vehicle with marginal static stability factor. These improvements of resistance to maneuver induced rollover for vehicle 3 and 4 were predicted by the simplified model. The results of simulations in the case of fishhook maneuver for vehicles 3 and 4 as compared to the baseline vehicle 1 are shown in Figures 10 and 11, respectively
Few other interesting points within the study

Quote:
in dynamic maneuvers, the roll angle of vehicle body may exceed (overshoot) the steady-state value. The amount of overshoot depends on the type of maneuver, but for a given maneuver it is related to the roll damping of suspension as well as suspension stiffness and the body moment of inertia about the roll axis
Quote:
During cornering maneuvers on smooth roads vehicle body is usually subjected to vertical forces, often referred to as “jacking” forces, which tend to lift the vehicle center of gravity above the static location
Quote:
During cornering maneuvers, progressive characteristic of suspension permits smaller deflection in compression of the outside suspension than deflection in extension of the inside suspension. As a result, height of vehicle center of gravity increases. This effect is highly dependent on the particular stiffness characteristic, so it is difficult to capture in a general approach
Quote:
The second jacking effect is a result of forces in suspension links. Lateral forces generated during cornering maneuvers are transmitted between the body and the wheels through relatively rigid suspension links. In general these members are not parallel to the ground; therefore the reaction forces in these elements have vertical components, which usually do not cancel out, resulting in a vertical net force, which pushes the body up
PS : For those who do not have patience to read through the study, don't misunderstand the term 'relatively rigid suspension links' for a comparison to a softer suspension set up. It is just a reference to say that a suspension, though is a softer component in the whole set up, may still be considered rigid!!

Anyway, am off this thread - I feel am replying to a troll. How is anti-roll bar related to this discussion?
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Old 19th July 2013, 20:55   #135
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Originally Posted by mallumowgli View Post
Anyway, am off this thread -
Please don't. Im relying on you, as you'll find out in a moment.

@pratheesh: would you have searchable (ie not scanned) pdf's of Milliken, and Gillespie.


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Originally Posted by san_jayd View Post
I've cut up all your posts if you don't mind, otherwise mine will be too long! Going to reply simultaneously.

First things first: some hard data. There is a certain component of suspension system called a sway bar or anti-roll bar which most SUVs these days incorporate. Assuming that you don't mind reading walls of theoretical texts (since we're all posting in the 'Technical Stuff' section as gthang had aptly pointed out before), I would suggest going through a usual Wikipedia page on "Sway bars", and a page titled "Anti-roll bar" on a website called www.turnfast.com for a bit of background theory. I would have tried to find some videos, but am at work as mentioned before. Net access is very much restricted. Thankfully they keep team-bhp on!
Am sure technical wiki articles is going to be heavy weather for laymen like me. Maybe you could point out what is relevant in that, and explain that in terms we could understand.

In the meantime, can't understand
a) your original statement of 'at the same speed'.
b) relevance of sudden appearance of antiroll bars in current discussion.

Lets return to the original premise that harder suspension -> 'more likely' to topple. ('more likely' is unquantified as of now)
Let us start with two cars, one with harder suspension than the other, but otherwise identical. (esp. identical with respect to ride height, suspension travel, suspension geometry, and weight distribution). And then apply mallumowgli's line of thought. Since his and yours conclusion are different, could you point out the fallacy in his reasoning.

Regards
Sutripta
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