Go Back   Team-BHP > Under the Hood > Technical Stuff


Reply
 
Thread Tools Search this Thread
Old 16th July 2013, 19:29   #106
Team-BHP Support
 
noopster's Avatar
 
Join Date: Feb 2010
Location: Pune
Posts: 8,665
Thanked: 9,194 Times
Default

While the original question is straightforward, the ensuing discussion raises the very important question: why go even near the limit of toppling your vehicle?

After driving in India, one loses sight of a simple number that can save your life: the speed limit. The fact is that speed limits in India are so low as to be unrealistic so we tend to ignore them. But when I was driving in the US recently, I realised how scientifically those limits are calculated, on every stretch of at least the major roads. You may be "safe" till at most ten over, but by and large it's a good bet that you go any faster and you will end up endangering yourself and others.

Of course your vehicle may not even be able to stay safe at the speed limit. In which case I don't think any number of equations and formulas will be able to save you!
noopster is offline   (4) Thanks Reply With Quote
Old 16th July 2013, 20:11   #107
Senior - BHPian
 
Join Date: Nov 2007
Location: Bangalore
Posts: 2,376
Thanked: 528 Times
Default re: Evasive manoeuvres & Rollovers?

Quote:
Originally Posted by DerAlte View Post
Aren't you contradicting yourself here?

Both the suspensions allow the CG to shift, the softer one earlier and the stiffer one later (with respect to the point of time that the bump or swerve triggered mass shift). The 'softer' and 'stiffer' refers to the damping.

If what you say were to be true ("the stiffer suspension makes it behave like a single unit(relatively)"), wouldn't rally cars - with stiff suspensions and virtually 0 body roll under normal circumstances - be spending more time on their sides or roofs instead of wheels? Stiffer suspension allows the wheels to be on the road longer, even if the body rolls. At the tipping point, the suspension would be at the end of travel (compression in the side to which mass shifted, extension in the side that the mass shifted from). At this point the chassis, suspension and body are a single rolling mass.
I was taking the extreme (stiff) example to prove the point that flexing of the suspension is what allows the mass to shift without the wheel loosing ground. In realty the rightly sprung hold better while the softer suspnesions (which make people scary) are not as dangerous as they feel (as long as the rolling is progressive)
Guna is offline   Reply With Quote
Old 16th July 2013, 21:29   #108
Distinguished - BHPian
 
Join Date: Jul 2009
Location: Kolkata
Posts: 3,506
Thanked: 2,071 Times
Default re: Evasive manoeuvres & Rollovers?

Quote:
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).
For same suspension travel?
What were the changes made to the A Class?

Quote:
Originally Posted by DerAlte View Post
The 'softer' and 'stiffer' refers to the damping.
??

Regards
Sutripta
Sutripta is offline   Reply With Quote
Old 16th July 2013, 21:34   #109
Senior - BHPian
 
Join Date: Nov 2007
Location: Bangalore
Posts: 2,376
Thanked: 528 Times
Default re: Evasive manoeuvres & Rollovers?

Quote:
Originally Posted by Sutripta View Post
For same suspension travel?
What were the changes made to the A Class?
A class was a tall boy and the stance itself was changed to make it low slung
Guna is offline   Reply With Quote
Old 16th July 2013, 21:37   #110
Distinguished - BHPian
 
Join Date: Jul 2009
Location: Kolkata
Posts: 3,506
Thanked: 2,071 Times
Default re: Evasive manoeuvres & Rollovers?

Quote:
Originally Posted by Guna View Post
A class was a tall boy and the stance itself was changed to make it low slung
By change of tyres.
ESP added.
Anything else?

Regards
Sutripta
Sutripta is offline   Reply With Quote
Old 16th July 2013, 21:42   #111
Senior - BHPian
 
Join Date: Nov 2007
Location: Bangalore
Posts: 2,376
Thanked: 528 Times
Default re: Evasive manoeuvres & Rollovers?

Quote:
Originally Posted by Sutripta View Post
By change of tyres.
ESP added.
Anything else?

Regards
Sutripta
No idea. I know the previous generation was a tall boy car and the current gen is a low slung hatch
Guna is offline   Reply With Quote
Old 16th July 2013, 21:46   #112
Distinguished - BHPian
 
Join Date: Jul 2009
Location: Kolkata
Posts: 3,506
Thanked: 2,071 Times
Default re: Evasive manoeuvres & Rollovers?

Quote:
Originally Posted by Guna View Post
No idea. I know the previous generation was a tall boy car and the current gen is a low slung hatch
Was talking of MBs response to the original infamous A Class.
Amongst other things, the suspension was made harder.

Regards
Sutripta
Sutripta is offline   Reply With Quote
Old 16th July 2013, 21:50   #113
Senior - BHPian
 
Join Date: Nov 2007
Location: Bangalore
Posts: 2,376
Thanked: 528 Times
Default re: Evasive manoeuvres & Rollovers?

Quote:
Originally Posted by Sutripta View Post
Was talking of MBs response to the original infamous A Class.
Amongst other things, the suspension was made harder.

Regards
Sutripta
Did they stiffen the suspension on the tall boy version itself or in the subsequent design?
Guna is offline   Reply With Quote
Old 16th July 2013, 21:55   #114
Distinguished - BHPian
 
Join Date: Jul 2009
Location: Kolkata
Posts: 3,506
Thanked: 2,071 Times
Default re: Evasive manoeuvres & Rollovers?

Quote:
Originally Posted by Guna View Post
Did they stiffen the suspension on the tall boy version itself or in the subsequent design?
Original version. The one which had flipped over during the Swedish Elk test.

Regards
Sutripta
Sutripta is offline   (1) Thanks Reply With Quote
Old 16th July 2013, 22:13   #115
BANNED
 
Join Date: Aug 2009
Location: Coimbatore / MENA
Posts: 673
Thanked: 973 Times
Default re: Evasive manoeuvres & Rollovers?

Quote:
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).
The key difference is when the tipping point is reached. It will be at a higher speed with a stiffer suspension as compared to a softer one.

For any vehicle with a certain C of G, there will be a max lateral "g" force (directly proportional to the speed and inversely to the radius of the curve) that can be applied. A stiffer suspension allows the C of G to remain in the "safe zone" for higher values of "g". A softer suspension needs only a lesser force to be applied before the C of G topples over to the "unsafe zone".

This why "sportier" variants of the same vehicle does better in skidpad testing. For e.g., a GT versus a base Mustang.

Aint nuthin but a "g" thang, baby. (Purely coincidental )

Cheers

Last edited by gthang : 16th July 2013 at 22:26.
gthang is offline   (4) Thanks Reply With Quote
Old 17th July 2013, 11:29   #116
Distinguished - BHPian
 
saket77's Avatar
 
Join Date: Dec 2012
Location: Ranchi
Posts: 3,189
Thanked: 4,211 Times
Default re: Evasive manoeuvres & Rollovers?

And here a Land Rover spins out of control in Bangalore. The guy has killed 4 people too. Bad news, but as long as people don't come out of the mirage that SUVs mean a lot of speed and can handle well, I am sure, we will unfortunately have many more such links in future too.

http://timesofindia.indiatimes.com/c...w/21113208.cms

Edit: Does not mean that I am supporting rash driving by a sedan or a hatchback. Its a menace & must be curbed.

Last edited by saket77 : 17th July 2013 at 11:37.
saket77 is offline   Reply With Quote
Old 17th July 2013, 11:41   #117
Senior - BHPian
 
mallumowgli's Avatar
 
Join Date: Apr 2010
Location: Palakkad/Coimbatore
Posts: 1,101
Thanked: 704 Times
Default re: Evasive manoeuvres & Rollovers?

Quote:
Originally Posted by gthang View Post
American car magazines like Motor Trend, Road & Track, etc., used to test a car's "Lateral Acceleration", and the figures were calculated in "g" values. Generally a car with a higher "g" value could corner better, and hence offered better stability through turns. A high C of G vehicle like an SUV usually returned low "g" values.

Indian magazines don't do such useful testing unfortunately. But if you understand the principle, and get an open road which can be used as a skidpad, one can test for oneself their car's limit's.

Try this for starters:

http://www.mrwaynesclass.com/circula...orner/home.htm

Cheers.
I can understand why this is not a destructive test, though in the hands of a novice tester it can become a self-destructive test

But the Mr. Wayne's class link you have provided gives the logic in a twisted or rather inverted fashion. If the formula v(square)/R for calculates the curve's centripetal acceleration, then the car's v(square)/R should be within the above calculated value and not above for safely negotiating the curve. Else as the speed goes up, it becomes more safe to take the curve!!

Actually the logic of the class is towards calculating the safe radius of a curve at a given centripetal acceleration (friction) and velocity of the car
mallumowgli is offline   Reply With Quote
Old 17th July 2013, 13:55   #118
Newbie
 
Join Date: Mar 2013
Location: Pune
Posts: 15
Thanked: 9 Times
Default re: Evasive manoeuvres & Rollovers?

Quote:
Originally Posted by Guna View Post
I also want to bring up another point related to high CG and rolling.
If you take 2 cars with identical ride height/CG, but with differently tuned suspension set ups, the car with softer suspension is (obviously)likely to roll more than the one with a stiffer suspension, and this is likely create some scare (with regards to the stability). 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).
Perfectly true. This is one pretty important point which was missing from the discussion till now I believe.

Quote:
Originally Posted by DerAlte View Post
Both the suspensions allow the CG to shift, the softer one earlier and the stiffer one later (with respect to the point of time that the bump or swerve triggered mass shift). The 'softer' and 'stiffer' refers to the damping.
Both will shift CG at the same time. The difference is not of 'earlier' or 'later'. It is of 'more' or 'lesser'. Obviously, the car with softer suspension will have more shifting of CG. But it is not just about CG positioning. A softer suspension will tend to absorb a larger component of the lateral forces acting on the car as opposed to a stiffer suspension which will just transfer most of the forces onto the wheel components.

Quote:
Originally Posted by DerAlte View Post
If what you say were to be true ("the stiffer suspension makes it behave like a single unit(relatively)"), wouldn't rally cars - with stiff suspensions and virtually 0 body roll under normal circumstances - be spending more time on their sides or roofs instead of wheels? Stiffer suspension allows the wheels to be on the road longer, even if the body rolls. At the tipping point, the suspension would be at the end of travel (compression in the side to which mass shifted, extension in the side that the mass shifted from). At this point the chassis, suspension and body are a single rolling mass.
It won't be appropriate to bring race prepped vehicles into this. All that matters to racing driver is how quickly their vehicle responds to the steering inputs. Which is why they always prefer stiffer suspension. Besides, their cars are also tuned to generate much more than average amount of downforce.

Quote:
Originally Posted by Sutripta View Post
For same suspension travel?
Nope, for the same speeds.

Quote:
Originally Posted by Sutripta View Post
Was talking of MBs response to the original infamous A Class.
Amongst other things, the suspension was made harder.

Regards
Sutripta
Both stiff and soft suspension setups have their advantages and disadvantages. It depends on what the manufacturer thinks is important for their vehicle.

Quote:
Originally Posted by gthang View Post
The key difference is when the tipping point is reached. It will be at a higher speed with a stiffer suspension as compared to a softer one.
Hmm, this is new to me. If you have any reference for this, I'd appreciate the share of info.

Quote:
Originally Posted by gthang View Post
For any vehicle with a certain C of G, there will be a max lateral "g" force (directly proportional to the speed and inversely to the radius of the curve) that can be applied. A stiffer suspension allows the C of G to remain in the "safe zone" for higher values of "g". A softer suspension needs only a lesser force to be applied before the C of G topples over to the "unsafe zone".
Ah but does it?

What is a "safe zone" for CG? Since we are talking about toppling, obviously as low a CG as possible is "safer". So ask yourself this, what will happen to a car with softer suspension and being subjected to lateral forces? 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.

Quote:
Originally Posted by gthang View Post
This why "sportier" variants of the same vehicle does better in skidpad testing. For e.g., a GT versus a base Mustang.
Again, I do not think it is proper to compare race cars or "sportier versions". They usually include a lot of changes to cumulatively give better grip and better steering response.
-------

Personally, I also do not agree with this Moose Test. Professional driver will never tell you to swerve your car (be it SUV or otherwise) if a moose wanders in front. You hit the brakes, and hope for the best. Swerving suddenly is a wrong driving practice. Especially if you don't know who or what is coming from the other side. Might end up saving the moose but killing a mother on her way to drop her kid to school. Best that you brake and hit the damn thing.

Just my opinions.
san_jayd is offline   (1) Thanks Reply With Quote
Old 17th July 2013, 14:50   #119
Senior - BHPian
 
DerAlte's Avatar
 
Join Date: Mar 2007
Location: Bangalore
Posts: 8,076
Thanked: 2,876 Times
Default re: Evasive manoeuvres & Rollovers?

Quote:
Originally Posted by san_jayd View Post
... Both will shift CG at the same time. The difference is not of 'earlier' or 'later'. ...
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?

Quote:
Originally Posted by san_jayd View Post
... It won't be appropriate to bring race prepped vehicles into this. ... Besides, their cars are also tuned to generate much more than average amount of downforce. ...
And why not? The physical rules / engineering stay the same, no?

Paris-Dakar Rally contestants use SUVs. And SUVs don't get significant down force at the speeds they travel.

Low slung Formula racing cars with purpose built aerofoils (not the garden variety show-off spoilers one finds on many cars) exert significant downforce when the car is travelling in excess of 200Kmph - yes, we are not talking of those.

Quote:
Originally Posted by san_jayd View Post
... Personally, I also do not agree with this Moose Test. Professional driver will never tell you to swerve your car (be it SUV or otherwise) if a moose wanders in front. You hit the brakes, and hope for the best. Swerving suddenly is a wrong driving practice. ...
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?
DerAlte is offline   (3) Thanks Reply With Quote
Old 17th July 2013, 21:03   #120
Senior - BHPian
 
mallumowgli's Avatar
 
Join Date: Apr 2010
Location: Palakkad/Coimbatore
Posts: 1,101
Thanked: 704 Times
Default re: Evasive manoeuvres & Rollovers?

Quote:
Originally Posted by san_jayd View Post
Both will shift CG at the same time. The difference is not of 'earlier' or 'later'. It is of 'more' or 'lesser'. Obviously, the car with softer suspension will have more shifting of CG. But it is not just about CG positioning. A softer suspension will tend to absorb a larger component of the lateral forces acting on the car as opposed to a stiffer suspension which will just transfer most of the forces onto the wheel components
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 physics

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!!

Quote:
It won't be appropriate to bring race prepped vehicles into this. All that matters to racing driver is how quickly their vehicle responds to the steering inputs. Which is why they always prefer stiffer suspension. Besides, their cars are also tuned to generate much more than average amount of downforce.
If you think about why vehicle with stiffer suspension responds faster to steering inputs, you would get the answer regarding why talking about racing cars is relevant here.

Quote:
Hmm, this is new to me. If you have any reference for this, I'd appreciate the share of info.
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.

Quote:
What is a "safe zone" for CG? Since we are talking about toppling, obviously as low a CG as possible is "safer". So ask yourself this, what will happen to a car with softer suspension and being subjected to lateral forces? 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.
As explained earlier, CG will not lower, but rather move towards the outer of the curve destabilising the vehicle
mallumowgli is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
GM cars, price details, & How do dealers make money? chaxy The Indian Car Scene 16 3rd May 2006 14:08
Want to own a piece of Car&Driver and Road&Track ? Revvmaster Shifting gears 10 11th September 2004 15:25


All times are GMT +5.5. The time now is 05:53.

Copyright 2000 - 2017, Team-BHP.com
Proudly powered by E2E Networks