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E46 M3 (2001-2006) Engine: S54 - Max Hp: 333 hp at 7,900 rpm / 262 lb/ft at 4,900 rpm
Total Produced: 45,000+ - Years Produced: 2001 to 2006.


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Old Sat, Mar-21-2015, 04:58:01 AM   #71
oldss72
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Default Re: Perfered tire size for OEM 19's ?

Realize newton's second law is more accuratly net force = mass x acceleration.

If you had a means to create a constant torque load at a given speed and could change the inertia (not diameter) while keeping speed completely constant (no acceleration) you would measure the same torque, therefore the same power.
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Old Sat, Mar-21-2015, 05:09:11 AM   #72
terraphantm
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Default Re: Perfered tire size for OEM 19's ?

Quote:
Originally Posted by oldss72 View Post
Realize newton's second law is more accuratly net force = mass x acceleration.

If you had a means to create a constant torque load at a given speed and could change the inertia (not diameter) while keeping speed completely constant (no acceleration) you would measure the same torque, therefore the same power.
True. Just because the forces cancel out does not mean the engine isn't making any power. Though since a dyno measures after the wheels, it will "see" an inertia difference as a power loss.

And now that I think of it a dyno will probably register some torque and power at constant speed because they do place a known load on the driveline. The reading will be lower than full torque/power.
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Old Sat, Mar-21-2015, 03:57:51 PM   #73
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Default Re: Perfered tire size for OEM 19's ?

Quote:
Originally Posted by oldss72 View Post
Realize newton's second law is more accuratly net force = mass x acceleration.

If you had a means to create a constant torque load at a given speed and could change the inertia (not diameter) while keeping speed completely constant (no acceleration) you would measure the same torque, therefore the same power.
Right. Imagine driving up a very long hill with a constant incline at the same engine/vehilcle speed, zero acceleration.

Are you not producing power in this case, sufficient enough to push the vehicle up the hill?

Further, inertia wouldn't mater in this scenario - a heavy or light wheel gives identical power ouputs.

So, I'm still confused as to how, exactly, rotational inertia has an affect on rwhp.
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Old Sat, Mar-21-2015, 04:15:14 PM   #74
oldss72
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Default Re: Perfered tire size for OEM 19's ?

Torque= inertia x acceleration.

Power = torque x speed.

When you measure power, you are accelerating the dyno, therefore the power measure is engine power - inertial losses- driveline losses Ect ect.
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Old Sat, Mar-21-2015, 04:24:35 PM   #75
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Default Re: Perfered tire size for OEM 19's ?

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Right. Imagine driving up a very long hill with a constant incline at the same engine/vehilcle speed, zero acceleration.

Are you not producing power in this case, sufficient enough to push the vehicle up the hill?

Further, inertia wouldn't mater in this scenario - a heavy or light wheel gives identical power ouputs.

So, I'm still confused as to how, exactly, rotational inertia has an affect on rwhp.
The engine is producing power. It is being cancelled out by friction, drag, and gravity. Essentially power at the wheels is 0.

Newton's second law can take a few forms. One such example is T = Iα (T = torque, I = moment of inertia, α = angular acceleration). For a point mass, moment of inertia is mass * radius^2. For more complex shapes you essentially have to integrate that expression, which can get pretty nasty for really complex shapes. But in general, the moment of inertia increases with mass and the square of the radius.

To get back to the power reading -- the dyno is measuring at the wheels, not the crank. The dyno has no idea what the inertia is for the driveline. The amount of power the engine puts out is fixed. With heavier and/or larger wheels, more of that power is being used to spin the wheels, so the dyno will measure less power
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Old Sat, Mar-21-2015, 04:32:17 PM   #76
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Default Re: Perfered tire size for OEM 19's ?

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The engine is producing power. It is being cancelled out by friction, drag, and gravity. Essentially power at the wheels is 0.

Newton's second law can take a few forms. One such example is T = Iα (T = torque, I = moment of inertia, α = angular acceleration). For a point mass, moment of inertia is mass * radius^2. For more complex shapes you essentially have to integrate that expression, which can get pretty nasty for really complex shapes. But in general, the moment of inertia increases with mass and the square of the radius.
Totally - the moment of inertia as it relates to diameter makes perfect sense.

I guess I'm just having trouble with the basic example I outlined above. The math says we are putting zero power to the wheels going up hill at constant speed. This is hard to grasp conceptually since the wheels are pushing the car against a constant force at speed.

If power = torque * speed it seems that we are making power since we are exerting toque at speed.

I guess it's the "net" I'm having trouble with.
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Old Sat, Mar-21-2015, 04:37:05 PM   #77
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Default Re: Perfered tire size for OEM 19's ?

The engine is trying to push the car forward, friction in its many forms is trying to push the car backwards. When the car is moving at constant speed, force from the engine and force from friction are equal. So you are putting torque to the ground. It's only enough to overcome the forces acting against the car. Thus net torque is 0. And if net torque is 0, so is net power.
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Old Sat, Mar-21-2015, 05:59:32 PM   #78
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Default Re: Perfered tire size for OEM 19's ?

Makes sense.....good convo!
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Old Sun, Mar-22-2015, 07:37:14 PM   #79
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Default Re: Perfered tire size for OEM 19's ?

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Originally Posted by EricSMG View Post
Right. Imagine driving up a very long hill with a constant incline at the same engine/vehilcle speed, zero acceleration.

Are you not producing power in this case, sufficient enough to push the vehicle up the hill?

Further, inertia wouldn't mater in this scenario - a heavy or light wheel gives identical power ouputs.

So, I'm still confused as to how, exactly, rotational inertia has an affect on rwhp.
For a vehicle to maintain a constant speed, power must be produced to overcome friction (drivetrain and aerodynamic). If you take your foot off the gas pedal, the car will slow down.

If you drive a vehicle up an incline, additional power must be produced to overcome gravity. (The vehicle is being accelerated against gravity.) If you encounter an incline and don't depress the pedal further, the vehicle will slow down.

If you are purely maintaining constant speed in either of these scenarios, rotational inertia doesn't come into effect.

To illustrate how rotational inertia comes into effect, consider an inertial dyno. If you increase the mass of the drum, it will take more power to effect a given increase in the rpm of the drum. The effect is exactly the same in the drivetrain, which is why the mass of the tires/wheels, flywheel and pulley ratios have an effect on power to the wheels.
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2002 (12/2001) M3 6MT Coupe Carbon Black on Black - ZCP/CSL Wheels, 245/35/19 Mich PS2 & 285/30/19 Conti ExtremeContact DW, H&R 15 mm Front Spacers, CSL F/R Brakes, EBC Redstuff, TMS SS F/R Brake Lines, PSS9, TMS F/R Sway Bars, TMS Rear Control Arms, TMS Underdrive Pulleys, TMS/Carbonio Intake, Eisenmann Exhaust, AutoSolutions 30% SSK, BMW Illuminated Shift Knob, BMW Performance Pedals, BMW Performance 4.10 R&P, CDV Delete, Flossmann GTR Carbon Fiber Hood/Trunk, Carbon Fiber Rear Diffuser, Euro Front Signal Lights, Euro Rear Tail Lights, Angel Eyes, Homelink

1997 328is 5MT Coupe Madeira Violet on Black - Staggered M Forged Double Spoke Wheels, Bilstein Sport, H&R Sport, M Tech Rear Spoiler (Sold)
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Old Sat, Apr-04-2015, 06:53:22 AM   #80
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Default Re: Perfered tire size for OEM 19's ?

I installed 285/30/19 Continental ExtremeContact DW tires on the back today. I'm very happy with they way they fit. They are definitely not too wide for the ZCP/CSL wheels. The sidewalls are square and the tires don't look like rubber bands, the way the 265/30/19s do. I had to remove the 10 mm spacers, but they clear the wheelwells nicely, even at the lowered ride height. I had intended to replace the PS2 with PSS, but there are no 285/30/19 except for run flat, which I didn't want. I sent an email to Michelin that I have bought their tires if they had the size I wanted.
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2002 (12/2001) M3 6MT Coupe Carbon Black on Black - ZCP/CSL Wheels, 245/35/19 Mich PS2 & 285/30/19 Conti ExtremeContact DW, H&R 15 mm Front Spacers, CSL F/R Brakes, EBC Redstuff, TMS SS F/R Brake Lines, PSS9, TMS F/R Sway Bars, TMS Rear Control Arms, TMS Underdrive Pulleys, TMS/Carbonio Intake, Eisenmann Exhaust, AutoSolutions 30% SSK, BMW Illuminated Shift Knob, BMW Performance Pedals, BMW Performance 4.10 R&P, CDV Delete, Flossmann GTR Carbon Fiber Hood/Trunk, Carbon Fiber Rear Diffuser, Euro Front Signal Lights, Euro Rear Tail Lights, Angel Eyes, Homelink

1997 328is 5MT Coupe Madeira Violet on Black - Staggered M Forged Double Spoke Wheels, Bilstein Sport, H&R Sport, M Tech Rear Spoiler (Sold)
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Discussing Perfered tire size for OEM 19's ? in the E46 M3 (2001-2006) Forum - Engine: S54 - Max Hp: 333 hp at 7,900 rpm / 262 lb/ft at 4,900 rpm
Total Produced: 45,000+ - Years Produced: 2001 to 2006. at BMW M3 Forum.com (E30 M3 | E36 M3 | E46 M3 | E92 M3 | F80/X)