How do wheel/tire size affect speed
06-18-2009, 04:14 PM
#31
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http://en.wikipedia.org/wiki/Rolling_resistance
Here is a good definition and some relative values of rolling friction
It has a 2200lb car on pavement as being 300N of force needed. thats about 70lbs.
Take that 70lbs of force (resistance) and divide it by a gear ratio near you.
at 100mph, you might be at around 4.5:1. so, that total rolling friction cost in ft-lbs on the engine would be about 15ft-lbs. thats the total rolling friction. You can see how this value changes for different types of surfaces like grass or sand too. So, callit 15ft-lbs of torque at the flywheel of the cars engine.
(using 24" rear tires for ease of caculation, or 12"/1ft radius). If you increase the contact patch by the 1 to 1.5", you might increase friction by 10%. That still would only be an increase of near 1.5ft-lbs lost and and that would be under 2hp as well. So, now we have worst case scenareo of 2hp for the weight and 2hp for the increased friction. 4hp doesnt make 6mph straight changes.
However, it is unlikely that widing the tires can increase rolling friction by a proportional amount, but you get the idea.
Here is a good definition and some relative values of rolling friction
It has a 2200lb car on pavement as being 300N of force needed. thats about 70lbs.
Take that 70lbs of force (resistance) and divide it by a gear ratio near you.
at 100mph, you might be at around 4.5:1. so, that total rolling friction cost in ft-lbs on the engine would be about 15ft-lbs. thats the total rolling friction. You can see how this value changes for different types of surfaces like grass or sand too. So, callit 15ft-lbs of torque at the flywheel of the cars engine.
(using 24" rear tires for ease of caculation, or 12"/1ft radius). If you increase the contact patch by the 1 to 1.5", you might increase friction by 10%. That still would only be an increase of near 1.5ft-lbs lost and and that would be under 2hp as well. So, now we have worst case scenareo of 2hp for the weight and 2hp for the increased friction. 4hp doesnt make 6mph straight changes.
However, it is unlikely that widing the tires can increase rolling friction by a proportional amount, but you get the idea.
06-18-2009, 04:40 PM
#32
Three Wheelin'
I'm not going to get involved in all the rotating mass/inertia crap goes I really don't care enough to do any "accurate" calculations. Either way its probably higher.
My contention is that the dynamic rolling radius is what you're seeing. Motec calculates speed from wheel speed, and therefore your ground speed is calculated out. If the rolling radius is not quite right then the error will be pretty significant once you get into higher speeds. I've seen 2%-3% errors in rolling radius become several mph off at speeds above 120mph. A simple way to check will be to roll your car in a straight line thru the paddock and count out 10 wheel rotation. Then take a measuring wheel and measure the exact distance travel. You may find that the static radius you've input into the Motec doesn't match what you measure from rolling the car.
My contention is that the dynamic rolling radius is what you're seeing. Motec calculates speed from wheel speed, and therefore your ground speed is calculated out. If the rolling radius is not quite right then the error will be pretty significant once you get into higher speeds. I've seen 2%-3% errors in rolling radius become several mph off at speeds above 120mph. A simple way to check will be to roll your car in a straight line thru the paddock and count out 10 wheel rotation. Then take a measuring wheel and measure the exact distance travel. You may find that the static radius you've input into the Motec doesn't match what you measure from rolling the car.
06-18-2009, 04:45 PM
#33
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I'm tired of all of the math mumbo-jumbo. 
I can't tell if you're right or not. Time for a real world test.
Hey Brinkley, why don't you do a roll-down test. Find a fairly level spot and at a defined starting point going 80 mph, push in the clutch and measure exactly how far you coast. Do it again with the narrower tires. That should give you some clue as to whether or not rolling resistance has some factor in this.
Till we get more facts, this isn't worth debating anymore.
I can't tell if you're right or not. Time for a real world test.Hey Brinkley, why don't you do a roll-down test. Find a fairly level spot and at a defined starting point going 80 mph, push in the clutch and measure exactly how far you coast. Do it again with the narrower tires. That should give you some clue as to whether or not rolling resistance has some factor in this.
Till we get more facts, this isn't worth debating anymore.
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Larry Herman
2016 Ford Transit Connect Titanium LWB
2018 Tesla Model 3 - Electricity can be fun!
Retired Club Racer & National PCA Instructor
Past Flames:
1994 RS America Club Racer
2004 GT3 Track Car
1984 911 Carrera Club Racer
1974 914/4 2.0 Track Car
CLICK HERE to see some of my ancient racing videos.
06-18-2009, 04:47 PM
#34
Rennlist Member
I assumed that his comparison of rolling diameter was done before hand.
manufacturer specs are generally off a little, and yes 2-3% errors would be big at top speed. (they would be 2-3% off 150mph would actually be greater than 3mph error for 2% . However, 2% error would have to be a 25" tire really being 1/2" off. Pretty good size error. But, that certainly could be it.
In fact, Just mounted up some Hoosiers 315 x30s. Ill see what they actually are as far as diameter.
manufacturer specs are generally off a little, and yes 2-3% errors would be big at top speed. (they would be 2-3% off
150mph would actually be greater than 3mph error for 2% . However, 2% error would have to be a 25" tire really being 1/2" off. Pretty good size error. But, that certainly could be it. In fact, Just mounted up some Hoosiers 315 x30s. Ill see what they actually are as far as diameter.
I'm not going to get involved in all the rotating mass/inertia crap goes I really don't care enough to do any "accurate" calculations. Either way its probably higher.
My contention is that the dynamic rolling radius is what you're seeing. Motec calculates speed from wheel speed, and therefore your ground speed is calculated out. If the rolling radius is not quite right then the error will be pretty significant once you get into higher speeds. I've seen 2%-3% errors in rolling radius become several mph off at speeds above 120mph. A simple way to check will be to roll your car in a straight line thru the paddock and count out 10 wheel rotation. Then take a measuring wheel and measure the exact distance travel. You may find that the static radius you've input into the Motec doesn't match what you measure from rolling the car.
My contention is that the dynamic rolling radius is what you're seeing. Motec calculates speed from wheel speed, and therefore your ground speed is calculated out. If the rolling radius is not quite right then the error will be pretty significant once you get into higher speeds. I've seen 2%-3% errors in rolling radius become several mph off at speeds above 120mph. A simple way to check will be to roll your car in a straight line thru the paddock and count out 10 wheel rotation. Then take a measuring wheel and measure the exact distance travel. You may find that the static radius you've input into the Motec doesn't match what you measure from rolling the car.
06-18-2009, 05:00 PM
#35
Rennlist Member
Math mumbo jumbo? 
Hey, just trying to help with some real world info!
the coast down is kind of a crazy test. say you end up 10 feet further, what would that mean??
We use dynos to measure power. We trust their output and its easily correlated at the track. What Im giving you is the same exact type of calculation that a dyno would use. If you have a known value, you can then rule it out if it doesnt match the characteristic of the problem. the only thing really in question here is toe in and rolling friction. Give me the speed at the exit, and the top speed before and after and I can give you some ball park power required to accelerate the car to that speed with aero drag for both top speeds. Im sure there are others here that have seen 5-10mph speed increases of top speed down their favorite straight. Lets hear of what changes they have made. My 8mph cost near 60hp. Anyone else?
Hey, just trying to help with some real world info!
the coast down is kind of a crazy test. say you end up 10 feet further, what would that mean??
We use dynos to measure power. We trust their output and its easily correlated at the track. What Im giving you is the same exact type of calculation that a dyno would use. If you have a known value, you can then rule it out if it doesnt match the characteristic of the problem. the only thing really in question here is toe in and rolling friction. Give me the speed at the exit, and the top speed before and after and I can give you some ball park power required to accelerate the car to that speed with aero drag for both top speeds. Im sure there are others here that have seen 5-10mph speed increases of top speed down their favorite straight. Lets hear of what changes they have made. My 8mph cost near 60hp. Anyone else?
I'm tired of all of the math mumbo-jumbo. I can't tell if you're right or not. Time for a real world test.
Hey Brinkley, why don't you do a roll-down test. Find a fairly level spot and at a defined starting point going 80 mph, push in the clutch and measure exactly how far you coast. Do it again with the narrower tires. That should give you some clue as to whether or not rolling resistance has some factor in this.
Till we get more facts, this isn't worth debating anymore.
I can't tell if you're right or not. Time for a real world test.Hey Brinkley, why don't you do a roll-down test. Find a fairly level spot and at a defined starting point going 80 mph, push in the clutch and measure exactly how far you coast. Do it again with the narrower tires. That should give you some clue as to whether or not rolling resistance has some factor in this.
Till we get more facts, this isn't worth debating anymore.
06-18-2009, 05:09 PM
#36
Rennlist Member
I think I found part of the problem thanks to 2BWISE. I just so happened to have just mounted up a set of 315x30x18s on a set of 10" rims. (I didnt know that was common practice. glad to hear someone else has tried this) Anyway, with a bubble level and measuring tape. guess what the diameter was???
you got it. 26" . You are near 1/2" off, and that would make a 2.5mph at top speed near 150mph, (1.5%) depending on how fast you were going.
you got it. 26" . You are near 1/2" off, and that would make a 2.5mph at top speed near 150mph, (1.5%) depending on how fast you were going.
06-18-2009, 05:09 PM
#37
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If math had all the answers then there would be no need for testing.
Real world info would be data and not a bunch of equations and dyno tests.
06-18-2009, 05:09 PM
#38
Race Director
My 944 on full tread tires does not accelelerate as well as it does on tires worn done to the cord. Weight difference is 1-2 lbs per tire just from lost vs fresh rubber.
This does not have a massive impact on lap times, but you can tell the car just does not accelerate quite the same.
06-18-2009, 05:17 PM
#39
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Exactly. Do the roll-down test with a GPS data logger and you can plot the deceleration curve where it matters most, probably from 80 to 60 (at a guess). That 10 feet that Mark theorized could be all the difference in the world. If there were no difference in rolling resistance, I'd expect the distances to be practically the same.
06-18-2009, 05:18 PM
#40
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Math will help you point in the right direction. Never said real world testing is not needed . You use real life situations to see emperical testing. You use math to find answers and explain what you find or have overlooked.
So, Data like what we are going on here on this discussion??? Did you just see the wrench tossed into the gears of "real world data". Friggen diameter used in the computer is WRONG. If thats wrong, what else is wrong??
Math is use in simulators. Did you know that most airline pilots never even sit in a real airliner before they fly their first passenger laden trip? yeah, math, simulators, are useless . Yeah the earth is at the center of the universe too base on what you can prove and predict based on what is seen with the naked eye.
So, Data like what we are going on here on this discussion??? Did you just see the wrench tossed into the gears of "real world data". Friggen diameter used in the computer is WRONG. If thats wrong, what else is wrong??
Math is use in simulators. Did you know that most airline pilots never even sit in a real airliner before they fly their first passenger laden trip? yeah, math, simulators, are useless . Yeah the earth is at the center of the universe too base on what you can prove and predict based on what is seen with the naked eye.
06-18-2009, 05:19 PM
#41
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I have been racing my boxster (3.6L 996) with 17 inch wheels for many years. Recently I purchased a set of 18 inch wheels and have noticed that my top speed and rate at which I get to top speed has been adversely affected by the 18's. Now before you think gear effect take a look at the wheel and tire specs below.
17 inch wheels: Front is 7" wide 38lbs w/ 225 Hoosier R6
while the Rear is 9" wide 44lbs w/ 275 Hoosier R6.
Hoosier Outside Diameter Front 24.7"
Hoosier Outside Diameter Rear 25.5"
18 inch wheels: Front is 8.5" wide 40lbs w/ 245 Hoosier R6
while the Rear is 10" wide 47lbs w/ 315 Hoosier R6.
Hoosier Outside Diameter Front 24.7"
Hoosier Outside Diameter Rear 25.6"
So, overall diameter (gearing effect) between the two sets is not the answer, So I'm looking at the difference in weight: 2lbs front and 3lbs rear per tire. I can't believe that 10 lbs total can cause a difference of up to 5-6 mph at the top end of a straight? Now if you can image, as you look at 2 laps of data acquistion, the area under the speed curve as I accelerate out of a corner, the seventeen's quickly attain a higher speed vs. the 18's and that higher speed increases down the straight. This causes a large loss in comparative lap time. Another fact, I carry higher speeds all through the corner with the 18's and get on the throttle earlier with the eighteens, as you'd expect with more rubber on the road. You would think faster corners and earlier on the gas would equal faster down the straight. It doesn't. The 17's are always faster. Also, my shifting RPM's are the same to insure I am keeping the engine in the same HP band. All data acquired by Motec.
What do you guys think is the mathematical or driver related problem here?
17 inch wheels: Front is 7" wide 38lbs w/ 225 Hoosier R6
while the Rear is 9" wide 44lbs w/ 275 Hoosier R6.
Hoosier Outside Diameter Front 24.7"
Hoosier Outside Diameter Rear 25.5"
18 inch wheels: Front is 8.5" wide 40lbs w/ 245 Hoosier R6
while the Rear is 10" wide 47lbs w/ 315 Hoosier R6.
Hoosier Outside Diameter Front 24.7"
Hoosier Outside Diameter Rear 25.6"
So, overall diameter (gearing effect) between the two sets is not the answer, So I'm looking at the difference in weight: 2lbs front and 3lbs rear per tire. I can't believe that 10 lbs total can cause a difference of up to 5-6 mph at the top end of a straight? Now if you can image, as you look at 2 laps of data acquistion, the area under the speed curve as I accelerate out of a corner, the seventeen's quickly attain a higher speed vs. the 18's and that higher speed increases down the straight. This causes a large loss in comparative lap time. Another fact, I carry higher speeds all through the corner with the 18's and get on the throttle earlier with the eighteens, as you'd expect with more rubber on the road. You would think faster corners and earlier on the gas would equal faster down the straight. It doesn't. The 17's are always faster. Also, my shifting RPM's are the same to insure I am keeping the engine in the same HP band. All data acquired by Motec.
What do you guys think is the mathematical or driver related problem here?
As for reasons for difference.
I think it's been alluded to here a couple of times mostly by Larry, as he said it's probably a number of small things. But you may also be fighting a setup that is better for the 17's than the 18's as well.
So I go back to the part that I bolded in your post. It probably should read. "With the current setup the 17's are always faster"
There very well could be a setup where the 18's are faster as well. Yes the diameter and circumference are nearly the same. However you now have a very different sidewall height which has implications on they way it behaves dealing with lateral forces as well as the 'spring rate' that it has as well.
06-18-2009, 05:23 PM
#42
Rennlist Member
Well, I hate to tell you, based on well known physics, it will be well beyond your abiltiy to sense any change. 1lb per tire, would be 4lbs change for the total car's weight. (like 2 quarts of gas). Oh yeah, its rotating on the outer edge of the tire, so multiply it by 2. (based on accepted physics equations)
SO, what you are seeing with a 1lb lighter tread, is effectively 8 lbs as if it was in the car. So, if you can tell if 1 gallon is missing from your tank or not. You got a pretty sensitive butt dyno. This is a very straight forward answer. It is correct, there is no way that it is wrong and what you are feeling is the effect of 1 HP difference.
SO, what you are seeing with a 1lb lighter tread, is effectively 8 lbs as if it was in the car. So, if you can tell if 1 gallon is missing from your tank or not. You got a pretty sensitive butt dyno. This is a very straight forward answer. It is correct, there is no way that it is wrong and what you are feeling is the effect of 1 HP difference.
Yes.
My 944 on full tread tires does not accelelerate as well as it does on tires worn done to the cord. Weight difference is 1-2 lbs per tire just from lost vs fresh rubber.
This does not have a massive impact on lap times, but you can tell the car just does not accelerate quite the same.
My 944 on full tread tires does not accelelerate as well as it does on tires worn done to the cord. Weight difference is 1-2 lbs per tire just from lost vs fresh rubber.
This does not have a massive impact on lap times, but you can tell the car just does not accelerate quite the same.
06-18-2009, 05:28 PM
#43
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I think there are a lot of little factors that are contributing to this. A higher moment of inertia is one. Wind resistance may be one. Stiffer tire walls on the 18" may result in less overall grip as the spring rate is increased. Differences in unsprung weight. Variances in circumference due to manufacturer tolerances. Variances in circumference due to differences in heat transfer to the tires. All of these could be contributing incrementally to the result. The only real change you can make is to adjust suspension and tire pressures to see if there is a sweet spot to the 18" wheels that makes them equal or better than the 17" wheels. You may even find that one works better just at some tracks.
06-18-2009, 05:29 PM
#44
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Math will help you point in the right direction. Never said real world testing is not needed . You use real life situations to see emperical testing. You use math to find answers and explain what you find or have overlooked.
So, Data like what we are going on here on this discussion??? Did you just see the wrench tossed into the gears of "real world data". Friggen diameter used in the computer is WRONG. If thats wrong, what else is wrong??
Math is use in simulators. Did you know that most airline pilots never even sit in a real airliner before they fly their first passenger laden trip? yeah, math, simulators, are useless . Yeah the earth is at the center of the universe too base on what you can prove and predict based on what is seen with the naked eye.
So, Data like what we are going on here on this discussion??? Did you just see the wrench tossed into the gears of "real world data". Friggen diameter used in the computer is WRONG. If thats wrong, what else is wrong??
Math is use in simulators. Did you know that most airline pilots never even sit in a real airliner before they fly their first passenger laden trip? yeah, math, simulators, are useless . Yeah the earth is at the center of the universe too base on what you can prove and predict based on what is seen with the naked eye.
Here is exactly what you posted a few posts back
Hey, just trying to help with some real world info!
As for the diameter thing you're talking about. Why do you think that's the problem? Someone just mentioned that it could add to the confusion of the differences and now you are jumping on the bandwagon that it's been the problem the whole time.
And as for your last paragraph. WOW, just wow. Way to much inaccuracy and arm waving going on to even find a place to start parsing out all the issues there.
06-18-2009, 05:42 PM
#45
Rennlist Member
real world information IS math and physics. Do you know what the definition of physics is??? Ok then
The diameter thing does explain the chance of an error doesnt it? He has seen 5mph difference at the end of the straight. Hmmmm. so, in REALITY, if the the tire (I JUST MEASURED ) is .5" larger in diameter. I think that is part of the problem because the computer doesnt know how fast he is going unless it is based on GPS. IF it is based on wheel diameter and RPM input then certainly you can see the 1.5% error . Does anyone know if the motec use accelerometers or RPM signals and tire diameter inputs for accurately measuring speed?
Here is exactly what you posted a few posts back
However all you've contributed to this thread is a bunch of math equations and some subjective experience. Neither of whihc are real world info that I'd consider helpful.
As for the diameter thing you're talking about. Why do you think that's the problem? Someone just mentioned that it could add to the confusion of the differences and now you are jumping on the bandwagon that it's been the problem the whole time.
And as for your last paragraph. WOW, just wow. Way to much inaccuracy and arm waving going on to even find a place to start parsing out all the issues there.
The diameter thing does explain the chance of an error doesnt it? He has seen 5mph difference at the end of the straight. Hmmmm. so, in REALITY, if the the tire (I JUST MEASURED ) is .5" larger in diameter. I think that is part of the problem because the computer doesnt know how fast he is going unless it is based on GPS. IF it is based on wheel diameter and RPM input then certainly you can see the 1.5% error . Does anyone know if the motec use accelerometers or RPM signals and tire diameter inputs for accurately measuring speed?
Here is exactly what you posted a few posts back
However all you've contributed to this thread is a bunch of math equations and some subjective experience. Neither of whihc are real world info that I'd consider helpful.
As for the diameter thing you're talking about. Why do you think that's the problem? Someone just mentioned that it could add to the confusion of the differences and now you are jumping on the bandwagon that it's been the problem the whole time.
And as for your last paragraph. WOW, just wow. Way to much inaccuracy and arm waving going on to even find a place to start parsing out all the issues there.