Racing Techie question regarding braking performance
12-15-2015, 02:43 PM
#46
According to Google.
https://www.physicsforums.com/thread...weight.120420/
The answer is "It depends"..
Do I get a cookie?
https://www.physicsforums.com/thread...weight.120420/
The answer is "It depends"..
Do I get a cookie?
12-15-2015, 03:02 PM
#47
Rennlist Member
Thread Starter
Don't lump me in with Mark on this!!
I made no calculations. All I said was that due to "tire load sensitivity", if you take two theoretically identical cars and just add weight to one without changing weight distribution, the ultimate cornering and braking performance of that car MUST suffer. How much depends on how much weight you add. This is indisputable fact.
I made no calculations. All I said was that due to "tire load sensitivity", if you take two theoretically identical cars and just add weight to one without changing weight distribution, the ultimate cornering and braking performance of that car MUST suffer. How much depends on how much weight you add. This is indisputable fact.
Sure, since the plane's engines are pushing/pulling through the air to accelerate the plane, what happens at the conveyor belt is inconsequential to the plane taking off. The plane still moves forward and takes off. If the plane's tires did the acceleration, then we would have a plane just standing still on the runway.
remember all the HP vs torque discussion..... why is it that the constant friction force more of a burden at speed on the conveyor vs on the normal runway?
Yeah, we have a brief off season here in Northern California. It is welcomed too. It gives me a chance to get the transaxle refreshed. It also gives me time to rebuild the CV joints, R & R the clutch, and a myriad of other race car projects.
I am changing springs at the rear and camber at the front which should improve both ends of the car. So I will need to do some early season testing before the race season starts to make sure the car is right. Lots of work ahead but I can't wait!
I am changing springs at the rear and camber at the front which should improve both ends of the car. So I will need to do some early season testing before the race season starts to make sure the car is right. Lots of work ahead but I can't wait!
.... so it was a welcome excuse to change both. now, ready for action ..... cant wait!According to Google.
https://www.physicsforums.com/thread...weight.120420/
The answer is "It depends"..
Do I get a cookie?
https://www.physicsforums.com/thread...weight.120420/
The answer is "It depends"..
Do I get a cookie?
Of course, Peter Krause, things heavy cars all corner and slow at the same rates as their counterparts... just he will never post the data.... Peter? something simple?????? just a decel rate comparison a turn approach from a track near you???????
(heavy vs light.. heck, just the same tires, doesnt even have to be the same car! )BUT, thanks for posting.... the more physics description that are posted, the smarter everyone gets. Plus, the comments put what is in writing, and applies it to our sport.
Last edited by mark kibort; 12-15-2015 at 03:20 PM.
12-15-2015, 03:04 PM
#48
Rennlist Member
I just like the fact that the physics forum guy showed his detail math...
I still think I deserve a cookie...
12-15-2015, 03:15 PM
#49
Rennlist Member
Thread Starter
so, you get a cookie... just because you are discussing like good guy!
you dont like my detail or math??
Two cars , CAR A (light car at 3000lbs) vs Car B (heavy car 500lbs heavier at 3500lbs). They get the point that of the exact same initial braking point, the two cars with the same 400HP are running:
car A 120mph (53.64 m/s)
car B 114mph (50.9 m/s)
the simple part of this example is that we can just plug in the numbers if we agree that the heavy car will slow at 8% less deceleration rate. we will call this 1g vs .92g respectively. this is because of the changing of the mu for the tires as weight is added to them. (weight on the tire, is the total weight/4 plus weight transfer weight/2 based on deceleration rate)
This means in 4 seconds the light car slows to 33mph (15m/s)
this also means in 4 seconds, the HEAVY car slows to the same speed
keeping it really simple. the KE at the start is:
1,991956 J Light
2,085,914 J Heavy
the KE at the end at the final same exact speed is:
153,405 J light
178,977 J heavy
this ends up with the lighter car dissipating a total of 1,838,551 for the light car
and 1,906,937 J for the Heavy car. heavy car dissipates more energy by 3.7%
not anywhere near the increase that the 15% increase of weight would first indicate
So,
Because the lighter car slows to the same speed in 4 seconds (3.94secs) and the heavy car slows to this same speed in 4 seconds too the RATE OF KE DISSIPATION is HIGHER for the heavy car by about 2%. no surprise here
625HP/sec for the light car ( example : 1,838,551 J /4 sec (3.94actual) /746watt =hp/sec)
639HP/sec for the heavy car ( example : 1,906,937 J /4 sec /746watt =hp/sec)
(only about 10hp/sec greater dissipation rate)
BUT, since we know the turn in point will happen at a slightly lower speed for the heavy car (13.5ms or 30mph), the result is: . Because now we are slowing at the same rate of .92g but for 4.15 seconds. NET NET 626hp ave dissipation rate....... the heavier car has the same of KE dissipation as the light car
The heavy car:
2,085,914 J at 50.64/s (114mph)
144,971 J at 13.5m/s (30mph)
Total KE dissipated of 1,940,942. / 4.15 seconds = 626hp ave dissipation
The rate of heat dissipation for the HEAVIER CAR is the same vs the light car for its decel rate and its lower top speed at the moment of braking (at the same spot on the track) and slightly lower target speed upon brake release for turn in.
the interesting thing here is also, with both cars activating their brakes at the same point, they end up at the same spot before turn in, 320f vs 316ft, if you use the decel rates of 1g vs .92g .
Therefore, this shows that adding weight, under these normal conditions, actually doesn't change the burden on the braking system.
car A 120mph (53.64 m/s)
car B 114mph (50.9 m/s)
the simple part of this example is that we can just plug in the numbers if we agree that the heavy car will slow at 8% less deceleration rate. we will call this 1g vs .92g respectively. this is because of the changing of the mu for the tires as weight is added to them. (weight on the tire, is the total weight/4 plus weight transfer weight/2 based on deceleration rate)
This means in 4 seconds the light car slows to 33mph (15m/s)
this also means in 4 seconds, the HEAVY car slows to the same speed
keeping it really simple. the KE at the start is:
1,991956 J Light
2,085,914 J Heavy
the KE at the end at the final same exact speed is:
153,405 J light
178,977 J heavy
this ends up with the lighter car dissipating a total of 1,838,551 for the light car
and 1,906,937 J for the Heavy car. heavy car dissipates more energy by 3.7%
not anywhere near the increase that the 15% increase of weight would first indicate
So,
Because the lighter car slows to the same speed in 4 seconds (3.94secs) and the heavy car slows to this same speed in 4 seconds too the RATE OF KE DISSIPATION is HIGHER for the heavy car by about 2%. no surprise here
625HP/sec for the light car ( example : 1,838,551 J /4 sec (3.94actual) /746watt =hp/sec)
639HP/sec for the heavy car ( example : 1,906,937 J /4 sec /746watt =hp/sec)
(only about 10hp/sec greater dissipation rate)
BUT, since we know the turn in point will happen at a slightly lower speed for the heavy car (13.5ms or 30mph), the result is: . Because now we are slowing at the same rate of .92g but for 4.15 seconds. NET NET 626hp ave dissipation rate....... the heavier car has the same of KE dissipation as the light car
The heavy car:
2,085,914 J at 50.64/s (114mph)
144,971 J at 13.5m/s (30mph)
Total KE dissipated of 1,940,942. / 4.15 seconds = 626hp ave dissipation
The rate of heat dissipation for the HEAVIER CAR is the same vs the light car for its decel rate and its lower top speed at the moment of braking (at the same spot on the track) and slightly lower target speed upon brake release for turn in.
the interesting thing here is also, with both cars activating their brakes at the same point, they end up at the same spot before turn in, 320f vs 316ft, if you use the decel rates of 1g vs .92g .
Therefore, this shows that adding weight, under these normal conditions, actually doesn't change the burden on the braking system.
acceleration calculator used to determine the time intervals to match the g rate of deceleration and starting and final speeds
http://www.smartconversion.com/unit_...alculator.aspx
http://www.smartconversion.com/unit_...alculator.aspx
12-15-2015, 03:22 PM
#50
But none of his detailed math included formulas accounting for the "tire load sensitivity" phenomenon. In other words, it was basic physics which often does not deal with the real world very well. You know, it is the same physics that says objects accelerate at the same rate when dropped due to gravity.
12-15-2015, 03:25 PM
#51
Rennlist Member
But none of his detailed math included formulas accounting for the "tire load sensitivity" phenomenon. In other words, it was basic physics which often does not deal with the real world very well. You know, it is the same physics that says objects accelerate at the same rate when dropped due to gravity.
Ok.. ill give it back....
But the point is that barring the small "tire load sensitivity" factor, all else being equal, the car will stop the same..... That is a revalation to most folks.... The only thing left is to quantify "Tire load Sensitivity" which your wiki article defines as .6 - .8 (from memory).. Plug that into the equation above and you have your answer....
But I would postulate the "tire Load Sensitivity" value varies with the surface, the tire compound, temperature, temperature gain (from the act of braking), not to mention the phase of the moon etc... now we are back to "It Depends"
I want my cookie back....
12-15-2015, 03:48 PM
#53
Rennlist Member
Thread Starter
Viking and Peter, look.... I bring these things up because they are commonly talked about , not only at the track, but publicized. Mosty, i just like to think and figure out the real answers or the conditions where those conditions are true. Its helped me in some big ways on the track and with others ive helped on and off the track.
The reason is this. when someone says that they dropped 2 lbs off each tire or rotor, and gained 1second a lap improvement, or whines that his car is too heavy and he is burning his brakes up (for example) . We can use physics, especially with the race car data when available , to prove that the gains they made, were not due to the modification , but to the driving characteristics or other condition changed... Ive shown that many times on this list. Not only does this become educational, it is a good time for some to discuss. if you dont like the discussion. you dont have to.. it's your choice.
The great thing for me, is even when i know the subject mater reasonably well, i always pick up some thing and learn a little more through the exercises.
I asked the question here , because i have never seen any thing published in detail of the tire loading sensitivity, other than the charts Ive posted and the link Scott provided. I dont have a lot of data to analyze , and i thought you (peter) could share with us , two cars , near the same at different weights and show the same decel rates, discounting the theory that Scott and I showed and spoke of. again, there are factors in the driver that can give those results. remember, correlation doesnt equal causation.
The reason is this. when someone says that they dropped 2 lbs off each tire or rotor, and gained 1second a lap improvement, or whines that his car is too heavy and he is burning his brakes up (for example) . We can use physics, especially with the race car data when available , to prove that the gains they made, were not due to the modification , but to the driving characteristics or other condition changed... Ive shown that many times on this list. Not only does this become educational, it is a good time for some to discuss. if you dont like the discussion. you dont have to.. it's your choice.
The great thing for me, is even when i know the subject mater reasonably well, i always pick up some thing and learn a little more through the exercises.
I asked the question here , because i have never seen any thing published in detail of the tire loading sensitivity, other than the charts Ive posted and the link Scott provided. I dont have a lot of data to analyze , and i thought you (peter) could share with us , two cars , near the same at different weights and show the same decel rates, discounting the theory that Scott and I showed and spoke of. again, there are factors in the driver that can give those results. remember, correlation doesnt equal causation.
12-15-2015, 04:01 PM
#56
Rennlist Member
Thread Starter
Ok.. ill give it back....
But the point is that barring the small "tire load sensitivity" factor, all else being equal, the car will stop the same..... That is a revalation to most folks.... The only thing left is to quantify "Tire load Sensitivity" which your wiki article defines as .6 - .8 (from memory).. Plug that into the equation above and you have your answer....
But I would postulate the "tire Load Sensitivity" value varies with the surface, the tire compound, temperature, temperature gain (from the act of braking), not to mention the phase of the moon etc... now we are back to "It Depends"
I want my cookie back....
But the point is that barring the small "tire load sensitivity" factor, all else being equal, the car will stop the same..... That is a revalation to most folks.... The only thing left is to quantify "Tire load Sensitivity" which your wiki article defines as .6 - .8 (from memory).. Plug that into the equation above and you have your answer....
But I would postulate the "tire Load Sensitivity" value varies with the surface, the tire compound, temperature, temperature gain (from the act of braking), not to mention the phase of the moon etc... now we are back to "It Depends"
I want my cookie back....
Heck, even i fell into that basket, where i just assumed, all heavy cars would brake and corner worse. its not the case, for the most part.... Just as the heavier cars being harder on brakes is another misnomer, for near some of same reasons and a few others.(unless Peters Krause's "observations" are true for all cars under all conditions, and his universe, that doesnt follow Newton, is a place i want to visit someday!
)
12-15-2015, 04:02 PM
#57
Rennlist Member
Thread Starter
12-15-2015, 04:17 PM
#58
Race Car
Unless you're a materials engineer working in the tire industry, it would be virtually impossible to know the variables necessary for solving Pacejka's magic formula. For those who don't know - and apologies to all who already do know - it's basically a sin of arctan function with about 30 different coefficients and scaling variables. Each one of these variables is unique to each and every brand, make, and size of tire. I think that Pacejka solutions are influenced by internal construction and tire dimensions, as well. Furthermore, the forces generated laterally are almost always different than those generated longitudinally, so it doesn't help to compare cornering force of different masses to braking, etc.
So, without knowing ANY of these magic numbers that tire manufacturers spend hundreds of millions to discover [mostly by persistence and luck], hence working vigorously protect, any calculation would equate to guessing and almost certainly be incorrect. As a proxy, one can collect acceleration data for analysis. Pro Coach is probably the user in best position to some answers, assuming he has a dataset for same pro driver, same day, same car with xxx lbs more ballast.
So, without knowing ANY of these magic numbers that tire manufacturers spend hundreds of millions to discover [mostly by persistence and luck], hence working vigorously protect, any calculation would equate to guessing and almost certainly be incorrect. As a proxy, one can collect acceleration data for analysis. Pro Coach is probably the user in best position to some answers, assuming he has a dataset for same pro driver, same day, same car with xxx lbs more ballast.
12-15-2015, 04:23 PM
#60
Rennlist Member
Thread Starter
Unless you're a materials engineer working in the tire industry, it would be virtually impossible to know the variables necessary for solving Pacejka's magic formula. For those who don't know - and apologies to all who already do know - it's basically a sin of arctan function with about 30 different coefficients and scaling variables. Each one of these variables is unique to each and every brand, make, and size of tire. I think that Pacejka solutions are influenced by internal construction and tire dimensions, as well. Furthermore, the forces generated laterally are almost always different than those generated longitudinally, so it doesn't help to compare cornering force of different masses to braking, etc.
So, without knowing ANY of these magic numbers that tire manufacturers spend hundreds of millions to discover [mostly by persistence and luck], hence working vigorously protect, any calculation would equate to guessing and almost certainly be incorrect. As a proxy, one can collect acceleration data for analysis. Pro Coach is probably the user in best position to some answers, assuming he has a dataset for same pro driver, same day, same car with xxx lbs more ballast.
So, without knowing ANY of these magic numbers that tire manufacturers spend hundreds of millions to discover [mostly by persistence and luck], hence working vigorously protect, any calculation would equate to guessing and almost certainly be incorrect. As a proxy, one can collect acceleration data for analysis. Pro Coach is probably the user in best position to some answers, assuming he has a dataset for same pro driver, same day, same car with xxx lbs more ballast.
In your first paragraph, yes, i agree.... the differences probably are as numerous as there are conditions .... but, i think its generally accepted, that the mu will go down by some amount with added weight from what you might have today. How much?.... well, that was my original question. Not simple i know and it would be interesting to see circumstances from peter that show the same decel rates for cars that have 500lbs (or some substantial value) added too them.