How does Car weight impact braking?
06-15-2005, 03:39 PM
#1
Race Director
Thread Starter
How does Car weight impact braking?
That is the question.
How does car weight impact braking?
Couple assumptions
Assume the same chassis, brakes, and tires.
Only difference is one car is heavier than the other. Assume no aero down force. Also don't assume multiple stops. (We are not concern with fade here)
Also assume dry pavement with no abs (ABS just complicates things)
Some Background discussions
How does car weight impact braking?
Couple assumptions
Assume the same chassis, brakes, and tires.
Only difference is one car is heavier than the other. Assume no aero down force. Also don't assume multiple stops. (We are not concern with fade here)
Also assume dry pavement with no abs (ABS just complicates things)
Some Background discussions
Originally Posted by ColorChange
I think I am wrong about the lighter car stopping faster. I'm looking into it right now.
Originally Posted by M758
No you are right. Simply just think about brakes. Heavy car vs light car using same brakes and tires. Which will stop faster? Also think of a car towing a trailer. Which stops faster.
Light car right? It all as to do with energy. It takes more energy to stop a heavy object. That energy needs to go somewhere.
Light car right? It all as to do with energy. It takes more energy to stop a heavy object. That energy needs to go somewhere.
Originally Posted by jeeva
Brakes and tires are NOT the only variables...a few others that come to mind, rear engine with rear weight bias will stop faster, stiffer dampers should help stop quicker I think, etc., .....It's true that a lighter car will stop quicker than a heavier car, if all other variables including the chassis are the same,...only two identical cars can be that way, or only in computer simulations...
As a general rule I will never say all light cars are safer than all other heavier cars...simply not true.
EDIT: I may be wrong, the only reason I am voicing my opinion is to LEARN...
As a general rule I will never say all light cars are safer than all other heavier cars...simply not true.
EDIT: I may be wrong, the only reason I am voicing my opinion is to LEARN...
Originally Posted by ColorChange
Ughh!!!! I WAS WRONG!!!! A LIGHTER CAR DOES NOT STOP NOTABLY QUICKER THAN A HEAVIER CAR
I know this doesn't make intuitive sense but it's true. Very simply, the heavier car pushes down on the tires and achieves more stopping force to offset the additional weight. This doesn't work at rediculous extremes, but it is roughly correct.
Here is a nice link.
http://hyperphysics.phy-astr.gsu.edu/hbase/crstp.html
So, no more Elise. Ughhhh!!!! I hate making such big errors!
I know this doesn't make intuitive sense but it's true. Very simply, the heavier car pushes down on the tires and achieves more stopping force to offset the additional weight. This doesn't work at rediculous extremes, but it is roughly correct.
Here is a nice link.
http://hyperphysics.phy-astr.gsu.edu/hbase/crstp.html
So, no more Elise. Ughhhh!!!! I hate making such big errors!
Originally Posted by jeeva
WOW! stopping distance is totally independent of the weight, I never knew that, thanks for the link...a good trivia question, now that I know the answer for it 
...elise may be out, but the hunt for a safer track car is still on, Right ? (surely you are not going to rip you TT, to install a cage)
...elise may be out, but the hunt for a safer track car is still on, Right ? (surely you are not going to rip you TT, to install a cage)
Originally Posted by macnewma
Here is another explanation of the relationship between stopping distance and coefficient of friction:
http://www.msgroup.org/TIP125.html
Basically, if you want to stop faster, get stickier tires so long as your brakes are powerful enough to lock up your current tires.
http://www.msgroup.org/TIP125.html
Basically, if you want to stop faster, get stickier tires so long as your brakes are powerful enough to lock up your current tires.
Originally Posted by M758
Tim,
I looked at the link you posted. I am trained mechaical engineeer. The equations presented are correct, but not accurate. Reason is the friction does not really apply the form stated. It is in fact more complex. That same reasoning would also tell you that in cornering weight is irrelvant. We know that is not the case however. The way friction and grip work is more complex.
Just think real world examples. Pickup truck empty stops in one distance. Now load it with a bunch of crap, but don't overload it. It WILL take longer to stop. The reason is that the friction that goes into slowing the car is not the mu x normal force which is the basic friction model.
"How to make your car Handle" by Fred Puhn covers this at least with respect to cornering.
I looked at the link you posted. I am trained mechaical engineeer. The equations presented are correct, but not accurate. Reason is the friction does not really apply the form stated. It is in fact more complex. That same reasoning would also tell you that in cornering weight is irrelvant. We know that is not the case however. The way friction and grip work is more complex.
Just think real world examples. Pickup truck empty stops in one distance. Now load it with a bunch of crap, but don't overload it. It WILL take longer to stop. The reason is that the friction that goes into slowing the car is not the mu x normal force which is the basic friction model.
"How to make your car Handle" by Fred Puhn covers this at least with respect to cornering.
Originally Posted by macnewma
Joe, could you explain how it is different? Should we open a separate thread? I think this is a very interesting concept and I would like the explanation of engineers although in relatively layman's terms. Thanks!
06-15-2005, 03:48 PM
#2
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The effect of vehicle weight is quite small/negligable. This is a good link explaining it.
http://hyperphysics.phy-astr.gsu.edu/hbase/crstp.html
Here is some car data: from 90 km/hr and 120 km/hr
Honda Integra GS-R 42 74.4
Audi A4 43.5 80.7
BMW Z3 (2.8) 36.9 64.5
Ferrari 550 Maranello 33.6 59.7
Lexus ES300 42 73.8
Lexus LS400 45.3 78
Mazda MX-5 45.6 76.8
Mazda Protege 47.4 86.1
Mercedes C36 36 63
Mercedes SLK230 Kompressor 36 62.7
Nissan Maxima 42 72.9
Nissan 200SX 38.7 68.4
Saab 9000 Aero 36.6 66.3
Subaru Liberty RX 40.8 70.8
Toyota Camry V6 43.5 82.2
Toyota Corolla 55.8 95.7
Porsche 911 Carrera 4 37.8 66.9
There is a pretty big difference in weights but no consistent trend.
http://hyperphysics.phy-astr.gsu.edu/hbase/crstp.html
Here is some car data: from 90 km/hr and 120 km/hr
Honda Integra GS-R 42 74.4
Audi A4 43.5 80.7
BMW Z3 (2.8) 36.9 64.5
Ferrari 550 Maranello 33.6 59.7
Lexus ES300 42 73.8
Lexus LS400 45.3 78
Mazda MX-5 45.6 76.8
Mazda Protege 47.4 86.1
Mercedes C36 36 63
Mercedes SLK230 Kompressor 36 62.7
Nissan Maxima 42 72.9
Nissan 200SX 38.7 68.4
Saab 9000 Aero 36.6 66.3
Subaru Liberty RX 40.8 70.8
Toyota Camry V6 43.5 82.2
Toyota Corolla 55.8 95.7
Porsche 911 Carrera 4 37.8 66.9
There is a pretty big difference in weights but no consistent trend.
06-15-2005, 03:59 PM
#3
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Depends on coefficient of friction and area in contact between tire and asphalt. If we assume the tire is properly inflated for heavier weight contact area should stay the same. Coefficient of friction stays the same and only pressure changes. Assuming friction to be 0.8 heavier car slides longer.
If we were using track tires with co efficient of 1 distance would be the same. Greater than 1 heavier car would slide less and stop quicker.
IMHO
hrk
If we were using track tires with co efficient of 1 distance would be the same. Greater than 1 heavier car would slide less and stop quicker.
IMHO
hrk
06-15-2005, 04:04 PM
#5
Race Director
Thread Starter
Tim,
Remember that all of those cars have differente brake and tire combos as well as weight distribution.
As I said the actual relationship of friction is not simple mu x times normal force. In Fred Puhn's book he explains this when talking about cornering forces.
Firstly cornering forces are similar to braking except in different direction. However if you use your link above it would also state the in corner the weight of the car is not relevant. Reason is that the normal force increases as does the mass. Well the reality is that as mass increases the normal force in effect does not keep up with the mass incrase. So 10% more mass results in 9% more friction. Part of this may be due to changes in coeffiicent of friction.
I honestly forget the details and don't have the book handy. Even so it proves what our intution and real world testing will prove.
Simply physics can be a dangerious thing if the proper assumptions are not used.
Example... Simply physics says all objects fall to the ground at the same time from the same distance. It does not mater how big or heavy. They all accelerate at rate of 9.8 m/s or what we call "G". Well that therefore mean that parchutes are useless and people will fall like a rock.
Obviously not true since the above does not take into account air drag. Air when comparing a football to 150 lbs boulder is irrelvant. However with a Parachute the drag is so great as to radiacally slow the decent of a 150lbs person vs 150 rock.
So in that case physics does not lie, but make a wrong assumption and it can lead to you an improper conclusion.
Remember that all of those cars have differente brake and tire combos as well as weight distribution.
As I said the actual relationship of friction is not simple mu x times normal force. In Fred Puhn's book he explains this when talking about cornering forces.
Firstly cornering forces are similar to braking except in different direction. However if you use your link above it would also state the in corner the weight of the car is not relevant. Reason is that the normal force increases as does the mass. Well the reality is that as mass increases the normal force in effect does not keep up with the mass incrase. So 10% more mass results in 9% more friction. Part of this may be due to changes in coeffiicent of friction.
I honestly forget the details and don't have the book handy. Even so it proves what our intution and real world testing will prove.
Simply physics can be a dangerious thing if the proper assumptions are not used.
Example... Simply physics says all objects fall to the ground at the same time from the same distance. It does not mater how big or heavy. They all accelerate at rate of 9.8 m/s or what we call "G". Well that therefore mean that parchutes are useless and people will fall like a rock.
Obviously not true since the above does not take into account air drag. Air when comparing a football to 150 lbs boulder is irrelvant. However with a Parachute the drag is so great as to radiacally slow the decent of a 150lbs person vs 150 rock.
So in that case physics does not lie, but make a wrong assumption and it can lead to you an improper conclusion.
06-15-2005, 04:08 PM
#6
Race Director
Suspension design will potentially have a noticable effect as it can easily affect load transfer. Now, I'm not talking about springs/dammpers/swaybars. Those things do not, although ride height does, all other things being equal.
Don't under-estimate the effect of brakes that are easier to modulate. The bigger the sweet spot, the easier it is to stay in it.
Don't under-estimate the effect of brakes that are easier to modulate. The bigger the sweet spot, the easier it is to stay in it.
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06-15-2005, 04:26 PM
#8
Nordschleife Master
The effect of vehicle weight is quite small/negligable.
My understanding is that a vehicle with locked up brakes is under static friction. That is to say a steadily declining coefficient of friction. (or a sloping line on a graph) A vehicle with turning wheels is under kinetic friction, which is a straight line on a graph. As a car slows, it is in the kinetic region first, which is fairly high, but if the wheels lock up, it goes into static friction, which is even higher, INITIALLY, but then drops off very quickly. If you keep the wheels from locking up, you staying in the higher kinetic friction coefficient range (with more friction) longer. (ie, reduced stopping distance)
Before the Porsches I drove a 97 chevy cavalier. It had horrible brakes on it, from the factory. When I moved to college, I loaded that thing up WAY over the maximum allowed gross vehicle weight. It looked like the front of the car was jacked up by 2 feet. I drove it for 4 hours on over the mountain highways... let me tell you, there was DEFINATELY an increase in stopping distance!!!!!!
real life experience > classroom physics!
06-15-2005, 04:59 PM
#9
1. Brakes work by turning kenetic energy into heat... more mass means more KE means more heat. ("Friction" is simply a colloquialism for converting KE to heat.) Heavier car will require brakes with more heat dissapation capacity.
2. Tires are not liner..e.g. G load down does not turn 100% into G load laterally. For a given tire, more weight = less ultimate G load.
3. Weight tranfer impacts braking performance. For cars of equal mass the forward KE of the car is always the same, but the distribution of weight changes the load put on the brakes and tires... impacting performance (see above...four tires and brakes will always work better than two tires and brakes asked to do the same work)
4). A locked-up tire is not an efficient mechanism for converting KE into heat
5) A CAVALIER!? and you are willing to admit that in front of who knows how many people... talk about absorbing heat!
2. Tires are not liner..e.g. G load down does not turn 100% into G load laterally. For a given tire, more weight = less ultimate G load.
3. Weight tranfer impacts braking performance. For cars of equal mass the forward KE of the car is always the same, but the distribution of weight changes the load put on the brakes and tires... impacting performance (see above...four tires and brakes will always work better than two tires and brakes asked to do the same work)
4). A locked-up tire is not an efficient mechanism for converting KE into heat
5) A CAVALIER!? and you are willing to admit that in front of who knows how many people... talk about absorbing heat!
06-15-2005, 05:00 PM
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The weight of a car will have a direct effect on the force needed from the brakes to reach the friction threshold of the tires on the car.
If we assume that the car A and car B are identical with exception of weight (and it meats all of Joe's earlier stipulations). Both cars have identical brake systems that can generate enough force to reach threshold braking for up to 10,000lbs.
Car A weighs 2000 lbs.
Car B weighs 3000 lbs.
Does car A stop in a shorter distance than car B? If so, please explain how you know this.
Disclaimer: These questions are not rhetorical, I really haven't a clue.
If we assume that the car A and car B are identical with exception of weight (and it meats all of Joe's earlier stipulations). Both cars have identical brake systems that can generate enough force to reach threshold braking for up to 10,000lbs.
Car A weighs 2000 lbs.
Car B weighs 3000 lbs.
Does car A stop in a shorter distance than car B? If so, please explain how you know this.
Disclaimer: These questions are not rhetorical, I really haven't a clue.
06-15-2005, 05:07 PM
#11
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Thread Starter
Car A stops faster = Answer is in JCP911S 's #2
2. Tires are not liner..e.g. G load down does not turn 100% into G load laterally. For a given tire, more weight = less ultimate G load.
06-15-2005, 05:13 PM
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Ok, since I am not an engineer or physicist, could you explain this to me?
Thanks everyone. I like these kinds of discussions on the racing forum, I think it benefits all of us.
Thanks everyone. I like these kinds of discussions on the racing forum, I think it benefits all of us.
06-15-2005, 05:39 PM
#13
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They stop in about the same distance. The tires, while not perfectly linear as JPC points out, over most reasonable ranges, they are close to linear and the cars stop at about the same point.
06-15-2005, 05:50 PM
#14
Nordschleife Master
Tires do not have that linear relationship that we learned in basic physics.
Some have stated it...here is a picture to describe it (courtesey of danno).
It's really difficult to say heavier/lighter is better once you introduce varying tire sizes.
But for a given tire size, the lighter car will stop quicker because of this non-linear relationship.
Also, in some of the comparisons that have been made of loading up a car and it braking slower. Could you lock up the brakes at 50mph with the car empty? Could you still lock up the brakes at 50mph when it was loaded up? If not....then there's the error in your experience.
Some have stated it...here is a picture to describe it (courtesey of danno).
It's really difficult to say heavier/lighter is better once you introduce varying tire sizes.
But for a given tire size, the lighter car will stop quicker because of this non-linear relationship.
Also, in some of the comparisons that have been made of loading up a car and it braking slower. Could you lock up the brakes at 50mph with the car empty? Could you still lock up the brakes at 50mph when it was loaded up? If not....then there's the error in your experience.