How do wheel/tire size affect speed
08-13-2009, 11:36 AM
#106
Three Wheelin'
I don't disagree with your conclusions above, but just want to clarify our discussion a bit here. Ultimately wheel weight doesn't matter. What matters is the moment of inertia. You even showed it above. The larger mass has the larger moment of inertia for the same radius. If we have equal mass at an increased radius then again the moment of inertia increases.
Your conclusion from the trace is the same as mine, but even a 2hp change can be significant. I would kill for 2hp in my car. In Brinkley's case its a drop in the bucket, but perspective is everything.
Your conclusion from the trace is the same as mine, but even a 2hp change can be significant. I would kill for 2hp in my car. In Brinkley's case its a drop in the bucket, but perspective is everything.
08-13-2009, 12:49 PM
#107
Rennlist Member
Ok, lets clarify it all a little bit more. Ultimately hp to weight is what matters, and inertia just multiples the effect of weight as if it was sitting in the car. I provided the equation for that equivilance too. Basically, if its spinning on the tire diameter, its like 2x the weight as if it was sitting in the car. Take a 3000lb car with 300hp and each 10lbs is worth a cost or advantage of about 1hp. If its spinning 10lbs is more like 20lbs in the car and that has the effect of near 2hp.
No argument on the effect of intertia based on change of radius for a given mass.
Yep, the effect of 2hp is relative, but in Brinkley's case, its probably far less. Im glad you were able to see where his speed differences really came from. The plots are very telling. Plus, the same calculation is in how a dyno measures HP as we used above. I put a huge amount of slack in the calculations for Brinkley's situation. I used constant acceleration, (i.e. constant torque) and in actuallity, his torque would have been going down with speed, almost proportionally. In otherwords, any loss in power due to the intertial effects would be MUCH greater in the slower speeds vs the higher speeds because the rate of acceleration would be much lower. lower acceleration, lower power to accelerate any mass, rolling or not. So, that is the point of all this. Although it's great to try and get as much weight out of the car to maximize acceleration, you have to keep it in perspective. 3lbs on a tire, (x4) will have the effect of 24 lbs as if it was sitting in the car. (i.e.about 4 gallons of gas in the tank
) . These are the types of things that give fractional MPH changes down most straights.
It would be great if the next time Brinkley gets to the tracks again, he trys real hard to get on the throttle off those pre-long straight turns, so we can see the effects down the main straights. Those are some great plots. even for us that dont have the expensive data aq systems, it puts in our minds how easy it is to lose speed down long straights.
No argument on the effect of intertia based on change of radius for a given mass.
Yep, the effect of 2hp is relative, but in Brinkley's case, its probably far less. Im glad you were able to see where his speed differences really came from. The plots are very telling. Plus, the same calculation is in how a dyno measures HP as we used above. I put a huge amount of slack in the calculations for Brinkley's situation. I used constant acceleration, (i.e. constant torque) and in actuallity, his torque would have been going down with speed, almost proportionally. In otherwords, any loss in power due to the intertial effects would be MUCH greater in the slower speeds vs the higher speeds because the rate of acceleration would be much lower. lower acceleration, lower power to accelerate any mass, rolling or not. So, that is the point of all this. Although it's great to try and get as much weight out of the car to maximize acceleration, you have to keep it in perspective. 3lbs on a tire, (x4) will have the effect of 24 lbs as if it was sitting in the car. (i.e.about 4 gallons of gas in the tank
) . These are the types of things that give fractional MPH changes down most straights. It would be great if the next time Brinkley gets to the tracks again, he trys real hard to get on the throttle off those pre-long straight turns, so we can see the effects down the main straights. Those are some great plots. even for us that dont have the expensive data aq systems, it puts in our minds how easy it is to lose speed down long straights.
I don't disagree with your conclusions above, but just want to clarify our discussion a bit here. Ultimately wheel weight doesn't matter. What matters is the moment of inertia. You even showed it above. The larger mass has the larger moment of inertia for the same radius. If we have equal mass at an increased radius then again the moment of inertia increases.
Your conclusion from the trace is the same as mine, but even a 2hp change can be significant. I would kill for 2hp in my car. In Brinkley's case its a drop in the bucket, but perspective is everything.
Your conclusion from the trace is the same as mine, but even a 2hp change can be significant. I would kill for 2hp in my car. In Brinkley's case its a drop in the bucket, but perspective is everything.
08-13-2009, 06:56 PM
#108
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Since the original post I went back to Sebring and used the 18's on Saturday and the 17's on Sunday. I told myself that I must push myself to carry more speed and get on the throttle earlier with the 18's to insure a higher top end. This worked as I laid down my best lap time ever at 1:25.8. But then, on Sunday I put the 17's on and told my self the same thing, higher cornering speed and earlier to throttle.
I beat my best time ever by .8 tenths! 1:25.0
Below are some traces, the BLACK trace is the 17's wheels. The top trace is the "varience" above the zero line and the RED trace is faster (18's) / if below the zero line the 17's are faster. Early in the lap the 18's are .3 seconds faster but then by the end of the lap the 17's are .8 seconds faster.
What do you think?
I beat my best time ever by .8 tenths! 1:25.0
Below are some traces, the BLACK trace is the 17's wheels. The top trace is the "varience" above the zero line and the RED trace is faster (18's) / if below the zero line the 17's are faster. Early in the lap the 18's are .3 seconds faster but then by the end of the lap the 17's are .8 seconds faster.
What do you think?
Last edited by Brinkley; 01-26-2013 at 10:47 AM.
08-13-2009, 07:18 PM
#109
Rennlist Member
Great screen shots.
Now, we all now know the effects of ONLY changing the rotating mass. interesting that the first part of the lap is faster, with the 18s. Then, I was going to say, you probably got to driving a little harder near the end. (and often, my second race is .5 seconds faster too) but your traces show that you got off that critical turn that leads to a critical straight at about the same speed and position. so, what is different. well, you accelerate near the same with the 18s, but shift earlier with the 17s. then the 17 line out accelerates the 18 graph line. why? Is it due to the tire size or weight? well we already know the diameters measure out to almost identical and you provided the weights of near 3lbs per wheel and tire diff. so, can it be due to tire? well, if it did, the acceleration at the earlier speed is 2x what it is later in the trace, so if it was due to the tire, it would have been a much greater gain earlier than later.
could have been air temp? were you drafting someone, or were they both clean laps. again, what you are seeing of near 5mph of speed down the straight absolutely cannot be explained by wheel and tire weight alone.
the last graphs were more telling, with the throttle coming on 25 to 50ft later. This time, it is really puzzling. The great thing about the physics of it, is that it does guide you to look at other things, if you KNOW the effects of that single change and it shows in values much higher than possible.
Im going to look some more to see if I can see other clues. no chance of having RPM plots? was the speed GPS? (i think you said yes)
Thanks for sharing.
Now, we all now know the effects of ONLY changing the rotating mass. interesting that the first part of the lap is faster, with the 18s. Then, I was going to say, you probably got to driving a little harder near the end. (and often, my second race is .5 seconds faster too) but your traces show that you got off that critical turn that leads to a critical straight at about the same speed and position. so, what is different. well, you accelerate near the same with the 18s, but shift earlier with the 17s. then the 17 line out accelerates the 18 graph line. why? Is it due to the tire size or weight? well we already know the diameters measure out to almost identical and you provided the weights of near 3lbs per wheel and tire diff. so, can it be due to tire? well, if it did, the acceleration at the earlier speed is 2x what it is later in the trace, so if it was due to the tire, it would have been a much greater gain earlier than later.
could have been air temp? were you drafting someone, or were they both clean laps. again, what you are seeing of near 5mph of speed down the straight absolutely cannot be explained by wheel and tire weight alone.
the last graphs were more telling, with the throttle coming on 25 to 50ft later. This time, it is really puzzling. The great thing about the physics of it, is that it does guide you to look at other things, if you KNOW the effects of that single change and it shows in values much higher than possible.
Im going to look some more to see if I can see other clues. no chance of having RPM plots? was the speed GPS? (i think you said yes)
Thanks for sharing.
Since the original post I went back to Sebring and used the 18's on Saturday and the 17's on Sunday. I told myself that I must push myself to carry more speed and get on the throttle earlier with the 18's to insure a higher top end. This worked as I laid down my best lap time ever at 1:25.8. But then, on Sunday I put the 17's on and told my self the same thing, higher cornering speed and earlier to throttle.
I beat my best time ever by .8 tenths! 1:25.0
Below are some traces, the BLACK trace is the 17's wheels. The top trace is the "varience" above the zero line and the RED trace is faster (18's) / if below the zero line the 17's are faster. Early in the lap the 18's are .3 seconds faster but then by the end of the lap the 17's are .8 seconds faster.
What do you think?
I beat my best time ever by .8 tenths! 1:25.0
Below are some traces, the BLACK trace is the 17's wheels. The top trace is the "varience" above the zero line and the RED trace is faster (18's) / if below the zero line the 17's are faster. Early in the lap the 18's are .3 seconds faster but then by the end of the lap the 17's are .8 seconds faster.
What do you think?
08-13-2009, 07:26 PM
#110
Rennlist Member
I think i may have found it.
EDIT: I think it might be a HP thing. Notice that in the initial gear, you get a jump on throttle and out drag the 17s , then you shift earlier in the 18s, the next shift, you shift even earlier, at near the same speed, but a different position, showing that the 17 plot is quicker. I think it has to be a HP thing. (aero, or HP, but too big of a differnce for the effects of 3lbs x 4 wheel weight of near 1hp at the highest speeds of near 130mph)
The part that holds the largest clue of the plot, is the first half of the lap. you have some almost as long or fast straigths, yet the 18s are faster.
My guess, you were either in traffic with the 17s or 18s. (traffic can work for and against, depending on where you are) or your engine just wasnt putting out the power that last part of that lap. (could be timing change due to a knock sensor, engine temp, etc ) bottom line, it wouldnt be that hard to prove that the 3lbs of increase greater rotating mass couldnt provide 5mph greater top speed on that 130mph straight. did you see the time it takes to go from 60mph to 130mph??
Its about 20 seconds for 3000ft. ( well over the equation time of 7 seconds for rate of acceleration from 70mph to 140mph.) In otherwords, over this time period, the HP effects on acceleration are insignificant.
Another reason it looks like engine hp being the culprit, is notice the gains. they grow with the faster speeds, just the opposite of the effects of rolling mass. aero could do this as well, but its not the tires.
EDIT: I think it might be a HP thing. Notice that in the initial gear, you get a jump on throttle and out drag the 17s , then you shift earlier in the 18s, the next shift, you shift even earlier, at near the same speed, but a different position, showing that the 17 plot is quicker. I think it has to be a HP thing. (aero, or HP, but too big of a differnce for the effects of 3lbs x 4 wheel weight of near 1hp at the highest speeds of near 130mph)
The part that holds the largest clue of the plot, is the first half of the lap. you have some almost as long or fast straigths, yet the 18s are faster.
My guess, you were either in traffic with the 17s or 18s. (traffic can work for and against, depending on where you are) or your engine just wasnt putting out the power that last part of that lap. (could be timing change due to a knock sensor, engine temp, etc ) bottom line, it wouldnt be that hard to prove that the 3lbs of increase greater rotating mass couldnt provide 5mph greater top speed on that 130mph straight. did you see the time it takes to go from 60mph to 130mph??
Its about 20 seconds for 3000ft. ( well over the equation time of 7 seconds for rate of acceleration from 70mph to 140mph.) In otherwords, over this time period, the HP effects on acceleration are insignificant.
Another reason it looks like engine hp being the culprit, is notice the gains. they grow with the faster speeds, just the opposite of the effects of rolling mass. aero could do this as well, but its not the tires.
Last edited by mark kibort; 08-13-2009 at 08:02 PM.
08-13-2009, 07:34 PM
#111
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Speed is GPS.
The reason the 18's jumped out to a .3 second advantage is look just past the apex of turn 1. Notice the delay in getting back to throttle with the 17's? Major oversteer. If I kept the Lateral G trace up you could see it but I removed to save space.
RPM vs. GPS Speed
The reason the 18's jumped out to a .3 second advantage is look just past the apex of turn 1. Notice the delay in getting back to throttle with the 17's? Major oversteer. If I kept the Lateral G trace up you could see it but I removed to save space.
RPM vs. GPS Speed
Last edited by Brinkley; 01-26-2013 at 10:47 AM.
08-13-2009, 07:49 PM
#112
Three Wheelin'
There are a few very noticable trends.
17s out accelerate the 18s at Str5-7 and Fangio. The resolution is a bit tough, but it looks as though the throttle applies are nearly identical.
You're about even up to 10/11, but after that your braking is not nearly as good as with the 17s. At the hairpin, despite being down a few mph your braking occurs at the same speed. The 18s are slower but your braking waits longer and matches the braking performance of the 17s. At Fangio you brake early. At 10/11 you're actually braking earlier with the 18s and at a lower speed and that is when you finally start losing time to the 17s.
Time difference is even until the back straight. You're losing speed the entire straight and obviously that is what is killing your lap time. The percent difference between the 17s and 18s is consistent and shows that acceleration is nearly identical, but the issue must be at the preceding corner. With the 18s you brake sooner and have a lower apex speed, as well as a mid corner hesitation (correction) from heading back to WOT. At this point speed is equal, but 17s slope is steeper than the 18s. Even though you went WOT sooner with the 18s. You've gone to WOT at a lower speed than the 17s. This leads to your speed difference down the straight. If your Turn 15-16 was improved with the 18s (equal to 17s) your speeds down the straight would be equal.
Last note. After the straight your braking point into 17 is the same. You brake from a higher speed and more aggressively with the 17s.
Essentially, all your time loss with the 18s is due to your entry and mid corner speed from 15-16. It leads to the speed down the straight and the braking advantage the 17s have on this trace.
17s out accelerate the 18s at Str5-7 and Fangio. The resolution is a bit tough, but it looks as though the throttle applies are nearly identical.
You're about even up to 10/11, but after that your braking is not nearly as good as with the 17s. At the hairpin, despite being down a few mph your braking occurs at the same speed. The 18s are slower but your braking waits longer and matches the braking performance of the 17s. At Fangio you brake early. At 10/11 you're actually braking earlier with the 18s and at a lower speed and that is when you finally start losing time to the 17s.
Time difference is even until the back straight. You're losing speed the entire straight and obviously that is what is killing your lap time. The percent difference between the 17s and 18s is consistent and shows that acceleration is nearly identical, but the issue must be at the preceding corner. With the 18s you brake sooner and have a lower apex speed, as well as a mid corner hesitation (correction) from heading back to WOT. At this point speed is equal, but 17s slope is steeper than the 18s. Even though you went WOT sooner with the 18s. You've gone to WOT at a lower speed than the 17s. This leads to your speed difference down the straight. If your Turn 15-16 was improved with the 18s (equal to 17s) your speeds down the straight would be equal.
Last note. After the straight your braking point into 17 is the same. You brake from a higher speed and more aggressively with the 17s.
Essentially, all your time loss with the 18s is due to your entry and mid corner speed from 15-16. It leads to the speed down the straight and the braking advantage the 17s have on this trace.
08-13-2009, 07:53 PM
#113
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Here is a zoomed in look at the back straight, which is where the difference was .8 seconds. I went and looked up each shift point:
Black 17's:
1st shift 6836 2nd shift 6744
Red 18's:
1st shift 6891 2nd shift 6735
I shift based on RPM's not speed as I do not have the speed up on the MoTec Dash. Also to answer another question, both laps were clear of all traffic.
I don't follow you on your last post stating "if you move the shift points"? The real question is, if both laps have the same shift points (in RPM's) then how does one car get to that rpm faster (based on track position)?
Black 17's:
1st shift 6836 2nd shift 6744
Red 18's:
1st shift 6891 2nd shift 6735
I shift based on RPM's not speed as I do not have the speed up on the MoTec Dash. Also to answer another question, both laps were clear of all traffic.
I don't follow you on your last post stating "if you move the shift points"? The real question is, if both laps have the same shift points (in RPM's) then how does one car get to that rpm faster (based on track position)?
Last edited by Brinkley; 01-26-2013 at 10:47 AM.
08-13-2009, 08:33 PM
#114
Rennlist Member
Yep, saw that too.
I think if you look at the second longest straight inthe middle of the lap, there is no difference in acceleration rate and equal top speed. in theory the times should be identical, because the tires are the same, and the weight difference is an insignificant factor. If anything, it would cause handling issues, due to unsprung weight, but I dont think you are seeing much effects of that.
This is why those equations are so valuable, you can start to rule out stuff and look for the actual cause of the effect.
Im going to get some of this data aq. stuff! It is just too cool!
edit: one other thing to note, is that i have hours and hours of video of all of my races over 10 years. my shift points do not change much on any lap, even with slight HP changes. the only time it did, is when I added 50hp. so, at that point, I shifted 1-2 seconds earlier. Your last straigth has the net effect of shifting 1 second later, or near 200ft. (for same position and speed for a shift) that is a huge HP change from the prior day's lap. This could not be caused by 3 lbs of wheel weight, or 24lbs sitting in the car. (rolling mass equiv.)
Good topic!
mk
I think if you look at the second longest straight inthe middle of the lap, there is no difference in acceleration rate and equal top speed. in theory the times should be identical, because the tires are the same, and the weight difference is an insignificant factor. If anything, it would cause handling issues, due to unsprung weight, but I dont think you are seeing much effects of that.
This is why those equations are so valuable, you can start to rule out stuff and look for the actual cause of the effect.
Im going to get some of this data aq. stuff! It is just too cool!
edit: one other thing to note, is that i have hours and hours of video of all of my races over 10 years. my shift points do not change much on any lap, even with slight HP changes. the only time it did, is when I added 50hp. so, at that point, I shifted 1-2 seconds earlier. Your last straigth has the net effect of shifting 1 second later, or near 200ft. (for same position and speed for a shift) that is a huge HP change from the prior day's lap. This could not be caused by 3 lbs of wheel weight, or 24lbs sitting in the car. (rolling mass equiv.)
Good topic!
mk
Speed is GPS.
The reason the 18's jumped out to a .3 second advantage is look just past the apex of turn 1. Notice the delay in getting back to throttle with the 17's? Major oversteer. If I kept the Lateral G trace up you could see it but I removed to save space.
RPM vs. GPS Speed
The reason the 18's jumped out to a .3 second advantage is look just past the apex of turn 1. Notice the delay in getting back to throttle with the 17's? Major oversteer. If I kept the Lateral G trace up you could see it but I removed to save space.
RPM vs. GPS Speed
08-13-2009, 08:48 PM
#115
Rennlist Member
I think I corrected my post regarding "shift points" . you kind of have to get your mind around all the things that are going on. clearly, this straight result, is a power thing. Can you plot out 6000ft to 8000ft? (mid lap straight) that is an even longer straight, but shows near identical acceleration. why would the last straight be any different? 1 second different shift points tells me there is a serous power issue on that last straight. notice the 17 plot gets to the shift points much faster than the 18s, but only on that section of track, and subsequently, it gets to a 5mph higher top speed as well.
EDIT: also very tellng is that your shift points are 100ft apart for the first shift at near 100mph while the second shifts are near 200ft apart at a speed of near 115mph. This shows a major acceleration difference. Key point, why at the 6000 to 8000ft straight, mid lap, does the acceleration line up idenitcally. Now you need to get the motec to track engine timing, fuel mixture, EGTs, CHTs and fuel pressure and we can narrow this down a bit.
Now, poorman's "motec data aqu.". I monitor closely my redlnes with points on the track. basically, it gives me the same speed vs position data at only shift points, which is pretty useful. If I EVER go beyond my best lap shift points, i know something is up. either a major screw up on the prior turn or I have an engine problem. I had this happen only a couple of times when I did have a alternator start to go out. the car felt flat and I was loosing a bunch of power before the car started to miss.
EDIT: also very tellng is that your shift points are 100ft apart for the first shift at near 100mph while the second shifts are near 200ft apart at a speed of near 115mph. This shows a major acceleration difference. Key point, why at the 6000 to 8000ft straight, mid lap, does the acceleration line up idenitcally. Now you need to get the motec to track engine timing, fuel mixture, EGTs, CHTs and fuel pressure and we can narrow this down a bit.
Now, poorman's "motec data aqu.". I monitor closely my redlnes with points on the track. basically, it gives me the same speed vs position data at only shift points, which is pretty useful. If I EVER go beyond my best lap shift points, i know something is up. either a major screw up on the prior turn or I have an engine problem. I had this happen only a couple of times when I did have a alternator start to go out. the car felt flat and I was loosing a bunch of power before the car started to miss.
Here is a zoomed in look at the back straight, which is where the difference was .8 seconds. I went and looked up each shift point:
Black 17's:
1st shift 6836 2nd shift 6744
Red 18's:
1st shift 6891 2nd shift 6735
I shift based on RPM's not speed as I do not have the speed up on the MoTec Dash. Also to answer another question, both laps were clear of all traffic.
I don't follow you on your last post stating "if you move the shift points"? The real question is, if both laps have the same shift points (in RPM's) then how does one car get to that rpm faster (based on track position)?
Black 17's:
1st shift 6836 2nd shift 6744
Red 18's:
1st shift 6891 2nd shift 6735
I shift based on RPM's not speed as I do not have the speed up on the MoTec Dash. Also to answer another question, both laps were clear of all traffic.
I don't follow you on your last post stating "if you move the shift points"? The real question is, if both laps have the same shift points (in RPM's) then how does one car get to that rpm faster (based on track position)?
08-13-2009, 11:42 PM
#116
Rennlist Member
One of the things you can do with the formulas provided is see what it takes to accelerate a car of 3000lbs from the two speeds given, but to 5mph faster speeds. You will see that the hp will be in the 30 to 40hp range. (If I was to guess)
Think also about this. if you raced yourself, with the 18s and you having the 17s, you would race by your 18 alter-self at 5mph and shift 1 second earlier. Thats a huge difference, and something that can only be caused by some substantial HP differences. You got me thinking about what else it can be due to and Im running out of ideas. even running a wing at 15 degrees vs 5 degrees wouldnt produce enough drag to make this difference. (i.e. 300lbs of wing down force, equals about 30lbs of drag, divided by the gear box = 5ft-lbs at the engine). Im leaning toward some major knock sensor timing retarding on that one straight.
im guessing your car has 300rwhp and is near 2800lbs with you in it . close?
Think also about this. if you raced yourself, with the 18s and you having the 17s, you would race by your 18 alter-self at 5mph and shift 1 second earlier. Thats a huge difference, and something that can only be caused by some substantial HP differences. You got me thinking about what else it can be due to and Im running out of ideas. even running a wing at 15 degrees vs 5 degrees wouldnt produce enough drag to make this difference. (i.e. 300lbs of wing down force, equals about 30lbs of drag, divided by the gear box = 5ft-lbs at the engine). Im leaning toward some major knock sensor timing retarding on that one straight.
im guessing your car has 300rwhp and is near 2800lbs with you in it . close?
08-14-2009, 03:15 AM
#117
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I can’t take this anymore….. Mark, do your realize you have something like 47 posts in this thread so far, out of 116? And that most of them are fixated on an acceleration model that is grossly inferior to the GPS data possessed by the OP? Do you realize the differences in laptimes are being caused by what is happening in three-dimensional CORNERS on a racetrack?
Your formulas relate to straight-line frictionless acceleration of a mass in a vacuum. Such formulas are useful to roughly approximate drag-strip hypotheticals, or to save money by avoiding dyno or drag strip rental time. But we are not talking about those things.
We are talking about the dynamic interaction between a driver, a racecar at speed, and a three-dimensional racetrack. Those dynamics are far too complex to model mathematically unless you have the budget & computing power of NASA or Sony (and even then….you’d rather test it than model it).
I’m not trying to be negative. Your effort and enthusiasm are obvious. But this thread has become a monument to the idolatry of numbers. Easily 60%+ of this thread’s total word-count has been devoted to the spurious “accuracy” of mathematical models and personal anecdotes that have next to zero relevance to this problem.
I have nothing against mathematical models. They are useful for approximating physical outcomes that are too difficult or too expensive to test via experimentation.
But….from very early on in this thread we have had concrete data from an actual real-world test. The 17’s consistently enable lower laptimes.
Contrary to the pages and pages you posted above which claim it can’t possibly be true because the weight change is insignificant (in your model), the bottom line is that the time difference IS DUE TO THE WHEEL & TIRE package! The 17’s are allowing the driver to get through certain critical corners on a line, at a speed, and at a throttle position which, collectively, result in a lower elapsed time around the track. Factors causing this could relate to the 18” wheels’ effect on the vehicle’s bump-compliance over berms, steering feel, grip threshold, slip-angles, yaw rate, etc, etc… none of which are present in your models… and all of which could be affecting the driver’s perceptions about how the car is behaving in certain parts of the track and therefore impact his control inputs and racing line. Those two things in turn affect the vehicle’s acceleration mid-corner, differences in which become exaggerated all the way down any straight which follows.
Therefore, the driver should decide whether he can afford to diagnose and adjust the car’s setup to eliminate the current downsides of the 18’s, or just continue using the 17’s and enjoy the lower laptimes at this track, knowing that the outcomes may be different at other tracks.
Sorry for the long rant. I mean no disrespect to anyone. Just trying to clear the air a bit.
Your formulas relate to straight-line frictionless acceleration of a mass in a vacuum. Such formulas are useful to roughly approximate drag-strip hypotheticals, or to save money by avoiding dyno or drag strip rental time. But we are not talking about those things.
We are talking about the dynamic interaction between a driver, a racecar at speed, and a three-dimensional racetrack. Those dynamics are far too complex to model mathematically unless you have the budget & computing power of NASA or Sony (and even then….you’d rather test it than model it).
I’m not trying to be negative. Your effort and enthusiasm are obvious. But this thread has become a monument to the idolatry of numbers. Easily 60%+ of this thread’s total word-count has been devoted to the spurious “accuracy” of mathematical models and personal anecdotes that have next to zero relevance to this problem.
I have nothing against mathematical models. They are useful for approximating physical outcomes that are too difficult or too expensive to test via experimentation.
But….from very early on in this thread we have had concrete data from an actual real-world test. The 17’s consistently enable lower laptimes.
Contrary to the pages and pages you posted above which claim it can’t possibly be true because the weight change is insignificant (in your model), the bottom line is that the time difference IS DUE TO THE WHEEL & TIRE package! The 17’s are allowing the driver to get through certain critical corners on a line, at a speed, and at a throttle position which, collectively, result in a lower elapsed time around the track. Factors causing this could relate to the 18” wheels’ effect on the vehicle’s bump-compliance over berms, steering feel, grip threshold, slip-angles, yaw rate, etc, etc… none of which are present in your models… and all of which could be affecting the driver’s perceptions about how the car is behaving in certain parts of the track and therefore impact his control inputs and racing line. Those two things in turn affect the vehicle’s acceleration mid-corner, differences in which become exaggerated all the way down any straight which follows.
Therefore, the driver should decide whether he can afford to diagnose and adjust the car’s setup to eliminate the current downsides of the 18’s, or just continue using the 17’s and enjoy the lower laptimes at this track, knowing that the outcomes may be different at other tracks.
Sorry for the long rant. I mean no disrespect to anyone. Just trying to clear the air a bit.
08-14-2009, 03:42 AM
#118
Three Wheelin'
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I'm curious about this also, published tire dimensions don't mean much. Brinkley, maybe you could put the 18s on the rear, mark the pavement and sidewall at the same spot and roll the car forward 3 rotations of the rear wheels and then mark that distance and measure. Then do the same thing with 17s on the rear...
08-14-2009, 10:02 AM
#119
Mr. Excitement
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Mark,
Like it or not I KNOW when I have lots of rubber on the tires and when I don't. When I have less rubber the car more responsive. That is combination of weight and the smaller diameter on the worn tire. Strangly I don't notice a big difference on fuel weight, but I can sense the tire weight. Fresh tires feel like heavy shoes when old worn tires feel like light weight track shoes.
This is the best I can describe it.
Like it or not I KNOW when I have lots of rubber on the tires and when I don't. When I have less rubber the car more responsive. That is combination of weight and the smaller diameter on the worn tire. Strangly I don't notice a big difference on fuel weight, but I can sense the tire weight. Fresh tires feel like heavy shoes when old worn tires feel like light weight track shoes.
This is the best I can describe it.
The 17s do look to accelerate faster, but only slightly as we've expected. I think the bigger deviations seen are driver inducted. A few quick notes.
Tower Turn: definitely at WOT sooner with 17s which gives you the extra speed on the straight.
Back Straight: You've applie more throttle earlier with the 18s and for the first half of the straight the difference is small. It is noticeable that the 17s are slightly quicker, but the difference is marginal. The shift is what gets you. It much quicker on the trace with the 17s.
Exit of Turn 17: Again you've gone WOT sooner with the 17s and that gives them the advantage in speed at the end of the straight.
Overall it looks as though you're probably still learning the 18s. There going to be subtlely different and it looks as though we're seeing it in the trace as well. For most corners it looks as though you're back to the throttle stronger and sooner with the 17s. Although, the 18s do appear to have a serious advantage is some spots, after Turn 1 and Turn 15-16, where you get to WOT with the 18s where you hadn't with the 17s. You give up some straigh line acceleration with the 18s but they do look to have some advantage and with some time you'll probably we able to get quicker laps.
Tower Turn: definitely at WOT sooner with 17s which gives you the extra speed on the straight.
Back Straight: You've applie more throttle earlier with the 18s and for the first half of the straight the difference is small. It is noticeable that the 17s are slightly quicker, but the difference is marginal. The shift is what gets you. It much quicker on the trace with the 17s.
Exit of Turn 17: Again you've gone WOT sooner with the 17s and that gives them the advantage in speed at the end of the straight.
Overall it looks as though you're probably still learning the 18s. There going to be subtlely different and it looks as though we're seeing it in the trace as well. For most corners it looks as though you're back to the throttle stronger and sooner with the 17s. Although, the 18s do appear to have a serious advantage is some spots, after Turn 1 and Turn 15-16, where you get to WOT with the 18s where you hadn't with the 17s. You give up some straigh line acceleration with the 18s but they do look to have some advantage and with some time you'll probably we able to get quicker laps.
08-14-2009, 10:23 AM
#120
Three Wheelin'
One other thing to consider from the lastest traces. Turn 15-16 anc Back Str. It is clear the vehicle accelerates out of the corner sooner with the 17s. It looks as though there's a bobble with the 18s. This leads to a 1-2 mph advantage of the 17s out of the corner. Now, make the assumption that acceleration is equal between the two. That 1-2 mph difference in exit speed is equal to __% difference. If acceleration is equal that __% difference will be the same down the entire straight, which means the speed difference at the end of the straight percentage wise is equal. That means that your speed difference at the end of the straight is now 5-6 mph. Which describes the above trace.
Brinkley, I'm confident that if you can match the exit speed of the 18s with the 17s then your time loss over the last section of the lap will be zero. This is probably a portion of the track you'll want to focus on as elsewhere in the lap the differences are rather subtle.
Brinkley, I'm confident that if you can match the exit speed of the 18s with the 17s then your time loss over the last section of the lap will be zero. This is probably a portion of the track you'll want to focus on as elsewhere in the lap the differences are rather subtle.