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Old 03-02-2016, 12:18 PM   #61
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yeah tire diameter fluctuates all over the place pretty often
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Old 03-02-2016, 03:06 PM   #62
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lol yes i know how it works. I just dont have any of my own atm and i'm not running my adaptronic so i can take a log.
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Old 03-02-2016, 03:11 PM   #63
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Quote:
Originally Posted by Savington View Post
In theory, yes. In practice, I've spent lots and lots of time comparing RPM-calculated "speed" to GPS-verified speed, and they never, ever match. At high speed, the difference is substantial (5%+). RPM-calculated speed always overestimates actual ground speed. My hypothesis is deformation of the tire at high speed.
I am perfectly aware that I am going to disagree with the MT.net gods here. But:

Tires increase in diameter at higher speed (centrifugal force - see Topfuel Dragster tires). So the speed calculated based on RPM should be underestimating at high speed, not overestimating, assuming that it was close at lower speed.
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Old 03-02-2016, 03:17 PM   #64
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Quote:
Originally Posted by stefanst View Post
I am perfectly aware that I am going to disagree with the MT.net gods here. But:

Tires increase in diameter at higher speed (centrifugal force - see Topfuel Dragster tires). So the speed calculated based on RPM should be underestimating at high speed, not overestimating, assuming that it was close at lower speed.
In theory, sure. Top Fuel cars work this way. In practice, 100% of my data convincingly shows the opposite to be true. Theseus had a 6sp, 3.909s, and a 7400rpm limiter, which on paper indicates a top speed of ~154mph. In practice, GPS Vmax was consistently 146mph while banging off the limiter in 6th.
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Old 03-02-2016, 03:58 PM   #65
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Savington, what were you using for tire circumference in that calculation?

I ask because, the effective circumference is a fair bit less that what you get if you just calculate using the nominal tire dimensions.

Example:
I just picked a 224/45-15 Toyo R888 as an example, using the nominal dimensions I get a diameter of 22.97 inches
Per tire rack toyo claims the tire is 22.8 in diameter, so slightly shorter than it should be.
Using the 22.8 diameter calculating the revs per mile, I get 885 revolutions per mile, but again per tire rack the revs per mile is 911, indicating about 2.5% shorter yet.

I think this makes perfect sense because the tire is always squished on the bottom a little bit, I'm guessing by almost 2.5%

If I screwed up the math I'm going to be embarrassed.
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Old 03-02-2016, 06:09 PM   #66
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Tire treads flex, and rubber deforms - at higher speeds under no acceleration wind resistance is present in not-insignificant quantities - tires are still doing work. At the base of the equation then, the aerodynamic drag of the vehicle is pushing rearward on the drive tires as they spin forward. Since tires are not a rigid solid, the treads flex and compress while in contact with the ground, and then spring backwards and expand when contact ends. Since ground speed is lost due to the initial flex/compression at contact, but no speed is gained with the ground contact ends, it makes sense that under light loads, the tire must spin faster than the external circumference of the tire would indicate. The effect would be magnified under acceleration or with more flexible treads. The circumference of the tire should not see an increase of any significance if we are using steel belted radials since steel is so wonderfully inelastic under tension.
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Old 03-02-2016, 06:39 PM   #67
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Quote:
Originally Posted by MrJon View Post
Savington, what were you using for tire circumference in that calculation?

I ask because, the effective circumference is a fair bit less that what you get if you just calculate using the nominal tire dimensions.

Example:
I just picked a 224/45-15 Toyo R888 as an example, using the nominal dimensions I get a diameter of 22.97 inches
Per tire rack toyo claims the tire is 22.8 in diameter, so slightly shorter than it should be.
Using the 22.8 diameter calculating the revs per mile, I get 885 revolutions per mile, but again per tire rack the revs per mile is 911, indicating about 2.5% shorter yet.

I think this makes perfect sense because the tire is always squished on the bottom a little bit, I'm guessing by almost 2.5%

If I screwed up the math I'm going to be embarrassed.
That car ran on 225/45 NT-01s, which have a published diameter of 22.95. Punching that into a gearing calculator with a 7400rpm redline, 0.843:1 6th, and 3.909 gears, you get 153.32mph. To get down to the GPS-indicated 146mph, you have to bring the tire diameter all the way down to ~21.9".

I don't really have a good hypothesis beyond "it flexes a lot" when it comes to explaining that difference, but the data was really consistent.

Last edited by Savington; 03-04-2016 at 12:55 PM.
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Old 03-04-2016, 10:56 AM   #68
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Kind of late to the party here. Looks like Andrew and others have it covered.

Here is another way to look at acceleration from a very simple first principles approach. Neglecting aero drag and rolling losses, etc.

Consider:
F=m*a (force = mass*acceleration)
P=F*v (power = force*velocity)

Combine and solve for acceleration:
a=P/(v*m) (acceleration = power/(velocity*mass) )

This shows two important things:
1) Acceleration depends on power, and that engine torque is completely irrelevant.
2) Acceleration is not constant with vehicle speed even with constant power. It dramatically reduces with speed, due to the 1/v component of the equation. This is a hyperbolic function.
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Old 03-04-2016, 12:04 PM   #69
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Stumbled on an interesting (and relevant) video today, and remembered seeing this thread a few days ago. Thought I'd share.

It mirrors a lot of what has been said in this thread, but with the benefit of more visuals.

https://www.youtube.com/watch?v=UIQj...ature=youtu.be

Last edited by nick470; 03-04-2016 at 12:18 PM.
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Old 03-04-2016, 12:28 PM   #70
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Quote:
Originally Posted by speedengineer View Post
Kind of late to the party here. Looks like Andrew and others have it covered.

Here is another way to look at acceleration from a very simple first principles approach. Neglecting aero drag and rolling losses, etc.

Consider:
F=m*a (force = mass*acceleration)
P=F*v (power = force*velocity)

Combine and solve for acceleration:
a=P/(v*m) (acceleration = power/(velocity*mass) )

This shows two important things:
1) Acceleration depends on power, and that engine torque is completely irrelevant.
2) Acceleration is not constant with vehicle speed even with constant power. It dramatically reduces with speed, due to the 1/v component of the equation. This is a hyperbolic function.
Let me just ask a simple question . . .

How is "F" created on most cars?
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Old 03-04-2016, 01:39 PM   #71
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Quote:
Originally Posted by hornetball View Post
Let me just ask a simple question . . .

How is "F" created on most cars?
Torque to the wheels is how. The torque that goes to the wheels creates the force that contacts the road.

Like I said in post 2, maximize torque to the wheels at all times.
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Old 03-04-2016, 02:35 PM   #72
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Not sure if retard or troll.

Quote:
Originally Posted by speedengineer View Post
1) Acceleration depends on power, and that engine torque is completely irrelevant.
On the first half of this point, we can agree, on the second half of this point, we can disagree.

Engine torque is far from irrelevant. Acceleration depends entirely on wheel torque. Wheel torque depends on power (which is engine torque and engine speed) plus gearing.

Quote:
Originally Posted by speedengineer View Post
2) Acceleration is not constant with vehicle speed even with constant power. It dramatically reduces with speed, due to the 1/v component of the equation. This is a hyperbolic function.
Acceleration is independent of vehicle speed and dependent on wheel torque. Acceleration will indeed dramatically reduce with engine speed *if power is constant*, because in order to keep power constant while engine speed increases we have to dramatically reduce engine torque. Unfortunately for you, the *if power is constant* thing is irrelevant, because generally *torque* is the more constant line on a dyno plot which forces power to increase.

We can also say that generally as engine speed increases, acceleration dramatically reduces *relative to power*
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Old 03-04-2016, 03:15 PM   #73
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Originally Posted by fooger03 View Post
Not sure if retard or troll.
That's kind of harsh. I vote drunk-posting.
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Old 03-04-2016, 10:20 PM   #74
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Quote:
Originally Posted by fooger03 View Post
We can also say that generally as engine speed increases, acceleration dramatically reduces *relative to power*
Why? Power is still increasing. Gearing hasn't changed. What would cause the rate of acceleration to decrease?
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Old 03-04-2016, 10:26 PM   #75
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Because torque is dropping.

The premise of your assertion above is what this whole thread is here to disprove.
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Old 03-04-2016, 10:41 PM   #76
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if it was only about torque and nothing else wouldn't all dragsters be diesel powered?
why do top fuel drag cars rev so high?
why not just make 3,000tq at 3k like a diesel and gear it accordingly?
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Old 03-04-2016, 10:48 PM   #77
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Are you being dense on purpose?
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Old 03-04-2016, 10:53 PM   #78
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I'm asking legitimate questions.

You can either 1) ignore 2) answer

Those are your only options
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Old 03-04-2016, 10:55 PM   #79
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Before this becomes 3 pages of semantics, let me ask start with this.

You were being sarcastic in your above post, and you dont legitimately think we should all run diesels with 3000ftlb and 50hp, right?
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Old 03-04-2016, 10:57 PM   #80
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Quote:
Originally Posted by nitrodann View Post
Before this becomes 3 pages of semantics, let me ask start with this.

You were being sarcastic in your above post, and you dont legitimately think we should all run diesels with 3000ftlb and 50hp, right?
You seem to have resorted to asking questions that "you know the answer to". I'm doing the same thing for the sake of discussion.

Your question about my motive is irrelevant. So is whether I know what's wrong or not wrong with that premise.

Anyone want to take a shot at the question?
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