wheels for looks, not performance
11-19-2014, 10:23 PM
#31
Drifting
as far as your "rolling down the hill "tests, I don't really understand the comparison. two identically weighted wheels with one with weight concentrating on the outside vs inside, yes the lower inertia wheel and tire combo would be better... (i.e. higher percentage of its weight toward the center = lower inertia). again, this ONLY applies to acceleration.. steady state speed (constant velocity) there is no difference with the force needed to keep constant velocity,
I don't have a dog in this fight...I just think it's interesting. Concentrating mass to the outside of a large diameter wheel will make it much harder to twist.
11-19-2014, 11:28 PM
#32
Rennlist Member
Sure...it's always the accelerating that eats up the fuel. And concentrating the mass to the outside of a large diameter hollow spoke wheel makes for a wheel that resists acceleration (it will also resist slowing down once rotating). I've seen lightweight flywheels with mass drilled out in the center that help reduce the overall mass of the car, but retain desired rotational inertia. I had a Porsche mechanic tell me for best acceleration and fuel economy I'd be happier sticking with the stock wheels. This is what set me thinking about the effects of rotational inertia. It makes it harder to rotationally (not translationally) accelerate the wheel.
I don't have a dog in this fight...I just think it's interesting. Concentrating mass to the outside of a large diameter wheel will make it much harder to twist.
I don't have a dog in this fight...I just think it's interesting. Concentrating mass to the outside of a large diameter wheel will make it much harder to twist.
your flywheel story makes no sense. if you drill out the center of the flywheel, you lose inertia. the MASS is NOT maintained. its reduced. its a way to get the mass (weight) out of the flywheel, without drilling holes in the contact surface. this weight helps with acceleration , not with deceleration. different than wheel and tire weight, its tied to the engine. its effect is dependent on which gear you are in. for 7lb savings, it might be worth 200lbs in the car in 1st, and 15lbs in 5th.
Again, fuel economy will not be effected and you can tell your "mechanic" that. why, because if there is no change in speed (acceleration) there will be no downside effect of the heavier wheel, other than what it would cost if it was in the car (x 1.5 ).
Now,what do you mean? " It makes it harder to rotationally (not transitionally) accelerate the wheel." please tell me what this means?
11-20-2014, 03:00 AM
#33
Rennlist Member
First, I did not expect the additional weight on the 944 Turbo to make the car handle better. The timers happened to be left on and someone else noticed I had gotten the same time. I was quite surprised. I just happened to look at the speedo at the end of the fastest straight and noticed I was 10 mph slower.
It is difficult to see the speedo with the position of the wheel in a turn so I don't glance there when cornering. My technique is to slow lift off the brakes on turn-in to start the car rotating to setup the 4 wheel drift, then I add throttle just enough to stop the rotation when the car is headed for the apex but still maintain a 4 wheel drift. Then I throttle steer the drift into the thru the apex. Then I add throttle and straighten the wheels to carry the car to the outside of the turn until once again going straight at full throttle. And rest assured every corner was 4 wheel drifted in the same fashion for both the FTD and passenger laden FTD. That's where the fun is for me and why people want to ride with me.
If the straights were slower the difference to make the same lap time had to come from somewhere!
On several occasions I had track instructors ride along on one of my laps. The only thing they could offer to make my laps faster was to get stickier tires. A couple of them even commented that I was doing better than they were and that they picked up a couple of tips from riding with me.
It is difficult to see the speedo with the position of the wheel in a turn so I don't glance there when cornering. My technique is to slow lift off the brakes on turn-in to start the car rotating to setup the 4 wheel drift, then I add throttle just enough to stop the rotation when the car is headed for the apex but still maintain a 4 wheel drift. Then I throttle steer the drift into the thru the apex. Then I add throttle and straighten the wheels to carry the car to the outside of the turn until once again going straight at full throttle. And rest assured every corner was 4 wheel drifted in the same fashion for both the FTD and passenger laden FTD. That's where the fun is for me and why people want to ride with me.
If the straights were slower the difference to make the same lap time had to come from somewhere!
On several occasions I had track instructors ride along on one of my laps. The only thing they could offer to make my laps faster was to get stickier tires. A couple of them even commented that I was doing better than they were and that they picked up a couple of tips from riding with me.
Last edited by RKD in OKC; 11-20-2014 at 03:22 AM.
11-20-2014, 07:58 PM
#34
Rennlist Member
First, I did not expect the additional weight on the 944 Turbo to make the car handle better. The timers happened to be left on and someone else noticed I had gotten the same time. I was quite surprised. I just happened to look at the speedo at the end of the fastest straight and noticed I was 10 mph slower.
It is difficult to see the speedo with the position of the wheel in a turn so I don't glance there when cornering. My technique is to slow lift off the brakes on turn-in to start the car rotating to setup the 4 wheel drift, then I add throttle just enough to stop the rotation when the car is headed for the apex but still maintain a 4 wheel drift. Then I throttle steer the drift into the thru the apex. Then I add throttle and straighten the wheels to carry the car to the outside of the turn until once again going straight at full throttle. And rest assured every corner was 4 wheel drifted in the same fashion for both the FTD and passenger laden FTD. That's where the fun is for me and why people want to ride with me.
If the straights were slower the difference to make the same lap time had to come from somewhere!
On several occasions I had track instructors ride along on one of my laps. The only thing they could offer to make my laps faster was to get stickier tires. A couple of them even commented that I was doing better than they were and that they picked up a couple of tips from riding with me.
It is difficult to see the speedo with the position of the wheel in a turn so I don't glance there when cornering. My technique is to slow lift off the brakes on turn-in to start the car rotating to setup the 4 wheel drift, then I add throttle just enough to stop the rotation when the car is headed for the apex but still maintain a 4 wheel drift. Then I throttle steer the drift into the thru the apex. Then I add throttle and straighten the wheels to carry the car to the outside of the turn until once again going straight at full throttle. And rest assured every corner was 4 wheel drifted in the same fashion for both the FTD and passenger laden FTD. That's where the fun is for me and why people want to ride with me.
If the straights were slower the difference to make the same lap time had to come from somewhere!
On several occasions I had track instructors ride along on one of my laps. The only thing they could offer to make my laps faster was to get stickier tires. A couple of them even commented that I was doing better than they were and that they picked up a couple of tips from riding with me.
Yes, you were driving better with the extra weight, its just that simple.
added weight doesn't help lap times. Different days, means potential different potential time.
you really should post a lap that you drove in both Autox.... I can usually detect things pretty easily by watching film. if you see the slower straight line speeds..(10mph is kind of high for what you are doing too).
its not possible for 300lbs to cause that kind of speed change, (you can run the KE numbers, but you can easily see that 10mph is way too much of a change for the amount of weight you changed...... but even saying its true
and you went though the turns faster, the reason was because you drove better, not because weight helped, as it CANT!
300lbs is like 30hp loss in your case, and up to 120mph its possible to lose 10mph but not to 60-70mph from 30-40mph.
post some video so we can see what you are doing.
one thing GREAT about physics is that you can see what is possible and what is not. It helps drivers get better by nailing down where they do well and bad. in your case, if you added weight and went faster, you did better some places than before. ........ if you ran heavier rims and ran 3 seconds slower, you drove worse, especially if you were able to run only 2 seconds slower with removing 80lbs, because the acceleration effects of the 32lbs rotating would only be near 50lbs as if it was in the car.
Mk
11-20-2014, 11:01 PM
#36
Drifting
Mark - I looked up some pictures of "lightweight" flywheels. I was wrong about where the mass was drilled out. It's actually removed inside and outside of the clutch contact surface. Many appeared to have more of the mass in the center. So the lightweight flywheels would have less rotational inertia, more demanding for the driver to make clean shifts.
A wheel has inertia whether it is moving or not, but we cannot sense the inertia if the wheel is at rest or moving with constant velocity. But if an object is being accelerated linearly the inertial effect becomes apparent (it's what we feel and how we know acceleration is happening) -- this "translational" inertia refers to simply the inertial resistance of the wheel (or any object) in a linear direction (as if the wheel was not rotating and the inertia is just due to the mass of the wheel). Rotational inertia is a separate type of inertia that is more of an "effect" due to rotating an object. The odd thing about rotational inertia is the distribution of the mass relative to the axis of rotation is more important than the actual mass. The radial distance matters most because rotational inertia increase is directly proportional to the square of the radial distance from the axis of rotation to where the mass is concentrated. This is easier to explain with a YouTube demo.
Think of gravity as providing force through a drivetrain to two wheels, each with different rotational inertia. Which "car" (with equal masses) would have greater acceleration?
A wheel has inertia whether it is moving or not, but we cannot sense the inertia if the wheel is at rest or moving with constant velocity. But if an object is being accelerated linearly the inertial effect becomes apparent (it's what we feel and how we know acceleration is happening) -- this "translational" inertia refers to simply the inertial resistance of the wheel (or any object) in a linear direction (as if the wheel was not rotating and the inertia is just due to the mass of the wheel). Rotational inertia is a separate type of inertia that is more of an "effect" due to rotating an object. The odd thing about rotational inertia is the distribution of the mass relative to the axis of rotation is more important than the actual mass. The radial distance matters most because rotational inertia increase is directly proportional to the square of the radial distance from the axis of rotation to where the mass is concentrated. This is easier to explain with a YouTube demo.
Think of gravity as providing force through a drivetrain to two wheels, each with different rotational inertia. Which "car" (with equal masses) would have greater acceleration?
11-21-2014, 01:44 AM
#38
Rennlist Member
hey, we are all friends here.... you know I might be right..(and I say , might because all I have quoted is theory, but its good theory) (well, some empirical data of my own as well...... and over the last 10years, I'm pretty consistent at the tracks I go to as well. AND, those laps are many. 1000s of laps... I've done the heavy vs light wheel, sticky vs non sticky rubber, passenger or no passenger, ballast weight vs no ballast , etc. with me, it all ads up to what the physics say.
Just suppose I am right..... you can even get faster times if you focus on why you were as fast with the extra weight, or why you slowed with 8lbs of wheel weight. If you video tape the session , I might be able to catch something you didn't.... that's how we get better.. competitive? maybe....just trying to always improve? yes.
the physics is pretty clear on the change of speed result. you can get the kinetic energy change for your car at 35mph and then at the top speed point of 70mph. then, do the equation for that same start speed and 10mph slower. I guarantee over the time that would take.... about 4-5 seconds, 300lbs/ (or +/-30hp) added wouldn't come close to having a 10mph lower/higher speed impact.
I have said this all along. I have never talked about the same mass, I've talked about rotational inertia. two same weight (mass) wheels are not the same rotational inertia. Same mass with the mass near the center more, has less rotational inertia.
you remove ANY weight from a flywheel and it will have less inertia and mass. obviously, more from the outer portions will have a greater effect.
Im still trying to figure out your point.....
Transitional would be a flywheel in the car sitting idle, but moving in the car.
the moment of inertia changes with the additional weight on the wheel or tire, so that the effect on the transitional inertia is 50-100% greater depending on where the weight is distributed (on tire or wheel) Does that make it more clear?
The inertia is not a factor in our discussion (of fuel efficiency) unless there is acceleration. the only thing that more mass in the car or on the wheels will do, is increase rolling friction if there is no acceleration
as I mentioned, the effect of the greater moment of inertia of a wheel, is 50% that if that exact weight as in the car (if its at the wheel) and 2x if its on a greater diameter , like where the tire is.
Just suppose I am right..... you can even get faster times if you focus on why you were as fast with the extra weight, or why you slowed with 8lbs of wheel weight. If you video tape the session , I might be able to catch something you didn't.... that's how we get better.. competitive? maybe....just trying to always improve? yes.
the physics is pretty clear on the change of speed result. you can get the kinetic energy change for your car at 35mph and then at the top speed point of 70mph. then, do the equation for that same start speed and 10mph slower. I guarantee over the time that would take.... about 4-5 seconds, 300lbs/ (or +/-30hp) added wouldn't come close to having a 10mph lower/higher speed impact.
Mark - I looked up some pictures of "lightweight" flywheels. I was wrong about where the mass was drilled out. It's actually removed inside and outside of the clutch contact surface. Many appeared to have more of the mass in the center. So the lightweight flywheels would have less rotational inertia, more demanding for the driver to make clean shifts.
A wheel has inertia whether it is moving or not, but we cannot sense the inertia if the wheel is at rest or moving with constant velocity. But if an object is being accelerated linearly the inertial effect becomes apparent (it's what we feel and how we know acceleration is happening) -- this "translational" inertia refers to simply the inertial resistance of the wheel (or any object) in a linear direction (as if the wheel was not rotating and the inertia is just due to the mass of the wheel). Rotational inertia is a separate type of inertia that is more of an "effect" due to rotating an object. The odd thing about rotational inertia is the distribution of the mass relative to the axis of rotation is more important than the actual mass. The radial distance matters most because rotational inertia increase is directly proportional to the square of the radial distance from the axis of rotation to where the mass is concentrated. This is easier to explain with a YouTube demo.
Rotational Inertia Demonstration - which is faster? - YouTube
Think of gravity as providing force through a drivetrain to two wheels, each with different rotational inertia. Which "car" (with equal masses) would have greater acceleration?
A wheel has inertia whether it is moving or not, but we cannot sense the inertia if the wheel is at rest or moving with constant velocity. But if an object is being accelerated linearly the inertial effect becomes apparent (it's what we feel and how we know acceleration is happening) -- this "translational" inertia refers to simply the inertial resistance of the wheel (or any object) in a linear direction (as if the wheel was not rotating and the inertia is just due to the mass of the wheel). Rotational inertia is a separate type of inertia that is more of an "effect" due to rotating an object. The odd thing about rotational inertia is the distribution of the mass relative to the axis of rotation is more important than the actual mass. The radial distance matters most because rotational inertia increase is directly proportional to the square of the radial distance from the axis of rotation to where the mass is concentrated. This is easier to explain with a YouTube demo.
Rotational Inertia Demonstration - which is faster? - YouTube
Think of gravity as providing force through a drivetrain to two wheels, each with different rotational inertia. Which "car" (with equal masses) would have greater acceleration?
you remove ANY weight from a flywheel and it will have less inertia and mass. obviously, more from the outer portions will have a greater effect.
Im still trying to figure out your point.....
Transitional would be a flywheel in the car sitting idle, but moving in the car.
the moment of inertia changes with the additional weight on the wheel or tire, so that the effect on the transitional inertia is 50-100% greater depending on where the weight is distributed (on tire or wheel)
Does that make it more clear?The inertia is not a factor in our discussion (of fuel efficiency) unless there is acceleration. the only thing that more mass in the car or on the wheels will do, is increase rolling friction if there is no acceleration
as I mentioned, the effect of the greater moment of inertia of a wheel, is 50% that if that exact weight as in the car (if its at the wheel) and 2x if its on a greater diameter , like where the tire is.
11-21-2014, 03:43 AM
#39
Rennlist Member
I have been autocrossing for over 10 years myself, and while I don't have thousands and thousands of laps on the same few courses over and over and over with a track record to compare my times to. I have had to setup my cars and driving style to be fastest on a wide variety of courses where you only have 8 laps each 10 to 15 minutes apart to put in your fastest run then you are done, next event next course. No two course exactly alike. I did however, have the lap times of myself and all the other drivers from every event to use for comparisons. You could even tell the general course setup by time differences in high hp cars and small better handling cars. Puts being consistent on a whole different level.
Also, the suspension setup on my 944 Turbo was not the "Physics Dictated," "Everyone else is running it" cup car setup. It was DIFFERENT. I went with the softer spring rates Andial recommended and had to call Porsche Motorsports to find them. Everyone else tired to talk me into the cup car setup. Interestingly enough Porsche Motorsports immediately knew EXACTLY what I was asking for.
Once set up it was not only the fastest car at the autocross, but I was trading laps at a local track DE with the guy that was winning the local club races in his RUF CTR. He even came over after the session to find out how come my 944 Turbo was so fast.
Incidentally, I found DE's kinda boring just doing the same lap after lap after lap with no timing or racing.
So, you will have to excuse me when your physics and driver consistency comments just go in one ear and out the other.
It's all moot anyway. Since my foot operation I have not been able to heel and toe downshift. It's like they cut the throttle blip part of my foot off. I've tried and tried and practiced and practiced and just end up smashing the brakes harder instead of blipping the throttle. So, unless I get something with PDK, driving in anger would just be frustrating.
-
Back to OP...
While my right foot was recovering I drove my 928 GTS from Oklahoma City, to Rolla MO. 400 miles up I-44 with 200 miles of it being limited access turnpike. Because of my foot I used the cruise control almost exclusively while on the highway. There was very little time that I could just cruise. I was always having to use the deccel or accel on the cruise control to slow down or speed up due to traffic.
Made a similar trip a couple of weekends ago in the Cayenne. Straight down I-35 from OKC to Austin. Also not able to use the cruise control and maintain a steady speed due to traffic.
Just an observation, did not keep track of gas mileage numbers, make any wheel changes, or make video on either of those trips to offer any sort of data. Just noticed I could not maintain a steady speed due to traffic.
Also, the suspension setup on my 944 Turbo was not the "Physics Dictated," "Everyone else is running it" cup car setup. It was DIFFERENT. I went with the softer spring rates Andial recommended and had to call Porsche Motorsports to find them. Everyone else tired to talk me into the cup car setup. Interestingly enough Porsche Motorsports immediately knew EXACTLY what I was asking for.
Once set up it was not only the fastest car at the autocross, but I was trading laps at a local track DE with the guy that was winning the local club races in his RUF CTR. He even came over after the session to find out how come my 944 Turbo was so fast.
Incidentally, I found DE's kinda boring just doing the same lap after lap after lap with no timing or racing.
So, you will have to excuse me when your physics and driver consistency comments just go in one ear and out the other.
It's all moot anyway. Since my foot operation I have not been able to heel and toe downshift. It's like they cut the throttle blip part of my foot off. I've tried and tried and practiced and practiced and just end up smashing the brakes harder instead of blipping the throttle. So, unless I get something with PDK, driving in anger would just be frustrating.
-
Back to OP...
While my right foot was recovering I drove my 928 GTS from Oklahoma City, to Rolla MO. 400 miles up I-44 with 200 miles of it being limited access turnpike. Because of my foot I used the cruise control almost exclusively while on the highway. There was very little time that I could just cruise. I was always having to use the deccel or accel on the cruise control to slow down or speed up due to traffic.
Made a similar trip a couple of weekends ago in the Cayenne. Straight down I-35 from OKC to Austin. Also not able to use the cruise control and maintain a steady speed due to traffic.
Just an observation, did not keep track of gas mileage numbers, make any wheel changes, or make video on either of those trips to offer any sort of data. Just noticed I could not maintain a steady speed due to traffic.
11-21-2014, 10:41 AM
#40
Pro
I seem to remember a Mythbusters episode where they tested a dirty car vs a clean car for MPG. They also put clay all over the car and tested it smooth and with golf ball divots. They stated that since they were doing steady state gas mileage, they traveled a couple of miles in a straight line maintaining 60mph, no curves, acceleration or deceleration, that the weight did not matter enough to take into account. The golf ball divoted heavier test got the best gas mileage, then smooth clay and clean, then dirty, with 29, 26, 26, 24 respective MPG results.
When i got my 2001 BoxsterS it came with 18in lightweight turbo twist hollow spoke wheels. I changed to some 18 in Roock USA wheels for LOOKS. The Roock wheels were .5in wider and 8 lbs per wheel heavier with same tires. While I did not notice much gas mileage difference, the Heavier Wheels were 3 seconds a lap slower autocrossing. In autocrossing there is a LOT of 30 to 70 mph acceleration and braking.
I later removed a bunch of stuff from the car to try to reduce the weight to get my 3 seconds back. I took out a little over 80lbs measured on a scale Including the passenger seat and a bunch of trim stuff and the engine covers under the covertible top. I only got 1 second a lap back instead of the full three I lost to the heavier wheels even though I removed more sprung weight than the 32lbs gained in unsprung ROTATING weight.
I was surprised because with my previous 944Turbo adding a 300 lbs passenger while reducing my top speeds on the straights did not increase my overall FTD lap time.
I really liked the look of the Roock USA competition wheels, so put the seat and all the trim back and just resigned myself to being a bit slower than I could be. Note: I still set FTD for street tires.
When i got my 2001 BoxsterS it came with 18in lightweight turbo twist hollow spoke wheels. I changed to some 18 in Roock USA wheels for LOOKS. The Roock wheels were .5in wider and 8 lbs per wheel heavier with same tires. While I did not notice much gas mileage difference, the Heavier Wheels were 3 seconds a lap slower autocrossing. In autocrossing there is a LOT of 30 to 70 mph acceleration and braking.
I later removed a bunch of stuff from the car to try to reduce the weight to get my 3 seconds back. I took out a little over 80lbs measured on a scale Including the passenger seat and a bunch of trim stuff and the engine covers under the covertible top. I only got 1 second a lap back instead of the full three I lost to the heavier wheels even though I removed more sprung weight than the 32lbs gained in unsprung ROTATING weight.
I was surprised because with my previous 944Turbo adding a 300 lbs passenger while reducing my top speeds on the straights did not increase my overall FTD lap time.
I really liked the look of the Roock USA competition wheels, so put the seat and all the trim back and just resigned myself to being a bit slower than I could be. Note: I still set FTD for street tires.
They walk among Us ... they really DO!
not sure if serious. since you kinda went and disproved each of your points with exceptions. passengers (especially my fat friends) change fuel economy. weight - sprung or unsprung - changes fuel economy. the almost always wider rubber on taller rims increases rolling resistance and lowers fuel economy. i don't quite understand why you're so excited to be so incorrect.
On a hot day the trye pressure will rise a little (= less rolling resistance) etc etc. Should I go on?
The most dangerous ppl are those who have absolutely no idea of how little they really know without realising it.
Back to OP...
While my right foot was recovering I drove my 928 GTS from Oklahoma City, to Rolla MO. 400 miles up I-44 with 200 miles of it being limited access turnpike. Because of my foot I used the cruise control almost exclusively while on the highway. There was very little time that I could just cruise. I was always having to use the deccel or accel on the cruise control to slow down or speed up due to traffic.
Made a similar trip a couple of weekends ago in the Cayenne. Straight down I-35 from OKC to Austin. Also not able to use the cruise control and maintain a steady speed due to traffic.
Just an observation, did not keep track of gas mileage numbers, make any wheel changes, or make video on either of those trips to offer any sort of data. Just noticed I could not maintain a steady speed due to traffic.
While my right foot was recovering I drove my 928 GTS from Oklahoma City, to Rolla MO. 400 miles up I-44 with 200 miles of it being limited access turnpike. Because of my foot I used the cruise control almost exclusively while on the highway. There was very little time that I could just cruise. I was always having to use the deccel or accel on the cruise control to slow down or speed up due to traffic.
Made a similar trip a couple of weekends ago in the Cayenne. Straight down I-35 from OKC to Austin. Also not able to use the cruise control and maintain a steady speed due to traffic.
Just an observation, did not keep track of gas mileage numbers, make any wheel changes, or make video on either of those trips to offer any sort of data. Just noticed I could not maintain a steady speed due to traffic.
11-21-2014, 12:04 PM
#41
Rennlist Member
You sire, obviously do not autocross on a regular basis.
11-21-2014, 01:04 PM
#42
Rennlist Member
I have been autocrossing for over 10 years myself, and while I don't have thousands and thousands of laps on the same few courses over and over and over with a track record to compare my times to. I have had to setup my cars and driving style to be fastest on a wide variety of courses where you only have 8 laps each 10 to 15 minutes apart to put in your fastest run then you are done, next event next course. No two course exactly alike. I did however, have the lap times of myself and all the other drivers from every event to use for comparisons. You could even tell the general course setup by time differences in high hp cars and small better handling cars. Puts being consistent on a whole different level.
Also, the suspension setup on my 944 Turbo was not the "Physics Dictated," "Everyone else is running it" cup car setup. It was DIFFERENT. I went with the softer spring rates Andial recommended and had to call Porsche Motorsports to find them. Everyone else tired to talk me into the cup car setup. Interestingly enough Porsche Motorsports immediately knew EXACTLY what I was asking for.
Once set up it was not only the fastest car at the autocross, but I was trading laps at a local track DE with the guy that was winning the local club races in his RUF CTR. He even came over after the session to find out how come my 944 Turbo was so fast.
Incidentally, I found DE's kinda boring just doing the same lap after lap after lap with no timing or racing.
So, you will have to excuse me when your physics and driver consistency comments just go in one ear and out the other.
It's all moot anyway. Since my foot operation I have not been able to heel and toe downshift. It's like they cut the throttle blip part of my foot off. I've tried and tried and practiced and practiced and just end up smashing the brakes harder instead of blipping the throttle. So, unless I get something with PDK, driving in anger would just be frustrating.
Also, the suspension setup on my 944 Turbo was not the "Physics Dictated," "Everyone else is running it" cup car setup. It was DIFFERENT. I went with the softer spring rates Andial recommended and had to call Porsche Motorsports to find them. Everyone else tired to talk me into the cup car setup. Interestingly enough Porsche Motorsports immediately knew EXACTLY what I was asking for.
Once set up it was not only the fastest car at the autocross, but I was trading laps at a local track DE with the guy that was winning the local club races in his RUF CTR. He even came over after the session to find out how come my 944 Turbo was so fast.
Incidentally, I found DE's kinda boring just doing the same lap after lap after lap with no timing or racing.
So, you will have to excuse me when your physics and driver consistency comments just go in one ear and out the other.
It's all moot anyway. Since my foot operation I have not been able to heel and toe downshift. It's like they cut the throttle blip part of my foot off. I've tried and tried and practiced and practiced and just end up smashing the brakes harder instead of blipping the throttle. So, unless I get something with PDK, driving in anger would just be frustrating.
the problem with judging if you had really lost 3 seconds due to wheels, and then gained it back by removing interior, seats, and other things (probably done at home), is that the courses are never the same, and you probably are not going out on stickers every time either. (which is the most dominant factor)
you can say you are fast guy of the day, and that still is impressive, but its hard to say how to compare that if the cars and drivers are varied .
Racing for many years, you know the drivers, cars and tracks well. you start to learn that most any one, with enough time can spin a fast lap time. its the little things that set the victor apart in equal cars. the cars themselves, especially the ones that have been shaken down and don't change much, react very predictable to HP and weight changes. always, if you have the right information , they follow physics and why wouldn't they.
so, you will not lose 3 seconds a lap by adding 8lbs per wheel on a normal weighted race / street car. you will gain about 1 second by taking off 80llbs though on most longer courses .
wheel weight as far as acceleration or decel, has the effect of no more than 2x that if that weight was in the car, PERIOD! its a fact and is indisputable! the ONLY other factor that extra weight ont he wheel would have is in its gyroscopic effect and ability for the shocks to control up and down motion. on a smooth course, this is not much of a factor, and would be more of a factor on an autocross, but not 3 seconds a lap. you could have tried to get some validation by quickly changing back to the light rims and see if you could have gained back that 3 seconds in one session.
The problem as I see it, and most see it, over many years of racing and autocrossing, on different days, folks see all sorts of different times with NO changes to their cars. its racing ...... too many other factors to count, not even counting the driver!
put a camera in your car, tape the run.... its very easy to see and time segments in the lap to see where the difference is found. for example, there is no way the 300lbs in the other car, would allow for 10mph slower top speed , unless you were much slower coming onto the straight. that is just not physically possible for the time/ speed and distance you traveled.
you ran as fast as before , with no change in your fast lap.... with 300lbs ...... and 10mph lower top speed. it means you were going around turns faster, also pointing as to how impossible it would be to go 10mph slower top speed. (it narrows that speed range gap of 35 to 70mph to maybe 45 to 60, if you are 10mph slower, because you HAVE to be going faster in the turn, right? )
Anyway, its an interesting discussion, and I would have no problem paying for your flight to come out and prove me wrong on any of this, using a test car here , in a DE or time trial environment. but I only pay if you are right!
The weight thing sprung, unsprung or rotating mass is supposedly negated when maintaining a steady speed on level ground. By my observations, which are obviously suspect due to lack of proper data acquisition and not being a controlled environment, in the Real world steady speed on straight and level roadway does not happen.
for MPG concerns, steady speed, increasing weight on wheels or tires or the car is all the same. and 50lbs added to a car, over a MPG test is not going to effect the outcome, even going up hills because you have to go down them too, where it assists.
(as long as you start and stop the test at the same elevation. ) cruise control can sometimes screw that data up, unless its being used on a flat test... if you notice, cruise control speed correction for hills is usually very bad and full throttle's the car to make slight changes in speed which wastes fuel.
quick bit of physics for the KE calculation for how much power you would have to lose to lose 10mph for a 3 second straight at an autox.
car weight: 3300lbs and then 3600lbs with passenger
time for the straight : 3 seconds
speeds range for the two situations: 35mph to 70mph and then with 300lbs extra: 40mph to 60mph
assumptions: you have to go around the turns at least 5mph faster to get the same lap time, so if you saw, even once, a 10mph diff at the end of the fastest straight (i.e. 70mph to 60mph), then here is the HP needed to be "lost " to do that. 300lbs passenger just wouldn't cause that.
40-60mph in 3 sec is 141hp with the extra weight of 300lbs..... without it, its 129hp
35-70mph in 3 sec is 246hp
either way, to lose 10mph its going to cost you 120 to 100hp.... 300lbs doesn't effect Hp/weight ratio that much. its more like 30hp...... So, with 300lbs extra in the car, plan on about 1/3 of that change in terminal velocity. If you go for the KE numbers, it all works out pretty clearly.
the only logical explanation, is that you came round the turns much faster (to keep overall lap time the same vs without the weight) , and took a long time to gather up the turn exit causing you to be very late on applying the gas, giving the lower top speeds.... this would give the same lap time and lower straightline speed.
if you want to play with the KE numbers, you can use lbs and MPH, for KE, but divide by 11.2 to get Joules. (instead of Meters/sec and Kg) Then take the joules and divide by 746 to get HP-seconds. And then just divide by the time you are accelerating to get how much hp you used or is required for the job.
1 J = 1watt-second and there is 746watts to a HP.
KE = 1/2MV^2
cool autox video
Jeff Kiesel SCCA autocross in car video 3/1/09 El Toro - YouTube
and here is an example of Fastest lap of the day.... doesn't mean a whole lot..... this guy is all over the place.
2015 Subaru WRX Autox POV (fastest car of day) - YouTube
11-21-2014, 03:23 PM
#43
Rennlist Member
Yep just too bad all the stuff with the 944 Turbo and then the BoxsterS was pre-gopro/or smart phones.
Removing the weight on the boxsterS was done same day same event. There were two run groups each taking turns doing an individual lap for 4 laps, then the other group would do theirs. The the first group would go again for another 4. I removed everything between my groups runs while the other group was doing their laps.
The straight I noticed the 10mph drop on was a long one and the top speed sans passenger was 100mph.
Even though the courses are typically designed so drivers are not supposed to get over 80mph, with that 944 Turbo I often got to over 100 topping out 3rd gear. Where the 944 Turbo accelerates best.
Removing the weight on the boxsterS was done same day same event. There were two run groups each taking turns doing an individual lap for 4 laps, then the other group would do theirs. The the first group would go again for another 4. I removed everything between my groups runs while the other group was doing their laps.
The straight I noticed the 10mph drop on was a long one and the top speed sans passenger was 100mph.
Even though the courses are typically designed so drivers are not supposed to get over 80mph, with that 944 Turbo I often got to over 100 topping out 3rd gear. Where the 944 Turbo accelerates best.
11-21-2014, 04:14 PM
#44
Rennlist Member
Yep just too bad all the stuff with the 944 Turbo and then the BoxsterS was pre-gopro/or smart phones.
Removing the weight on the boxsterS was done same day same event. There were two run groups each taking turns doing an individual lap for 4 laps, then the other group would do theirs. The the first group would go again for another 4. I removed everything between my groups runs while the other group was doing their laps.
The straight I noticed the 10mph drop on was a long one and the top speed sans passenger was 100mph.
Even though the courses are typically designed so drivers are not supposed to get over 80mph, with that 944 Turbo I often got to over 100 topping out 3rd gear. Where the 944 Turbo accelerates best.
Removing the weight on the boxsterS was done same day same event. There were two run groups each taking turns doing an individual lap for 4 laps, then the other group would do theirs. The the first group would go again for another 4. I removed everything between my groups runs while the other group was doing their laps.
The straight I noticed the 10mph drop on was a long one and the top speed sans passenger was 100mph.
Even though the courses are typically designed so drivers are not supposed to get over 80mph, with that 944 Turbo I often got to over 100 topping out 3rd gear. Where the 944 Turbo accelerates best.
If you are hitting 100mph on an autocross course, that would be interesting to see..... generally , they are not long enough. as you know, most 1/4mile trap speeds are between 100 and 110mph for decently fast street cars.
anyway, even if you saw 100mph sans the passenger, it would take more than 300lbs to knock that down to 90mph, just based on common KE calculations for a 3000 to 3300lb car and 300 to 400hp range.
so, if you were able to drop the weigiht and find that 1 second, that sounds reasonable. but the 3 seconds lost for the wheels doesn't, and that was on a prior day, correct? or did you have both wheels changed out during the day? either way, 32lbs of rotating wheel weight is only like 50lbs in the car, so you should have seen that kind of effect... . the fact that you had 300lbs in a car and then removed and ran the same lap time, should be an alarm to what really was going on. you just ran faster as a driver with the heavier weight, its that simple. the trade offs would be seen immediately with some video..... that you don't have........ post a video of a passenger vs no passenger autox in the near furture so we can see some of that driving!