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TC weight reduction and lap times

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TC weight reduction and lap times

Old 02-27-2019, 11:55 PM
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Originally Posted by gigaplex View Post
I still wouldn't consider that mass of the car. I'd wave my hands and call that an aero effect. And remind people that the physics isn't that simple.
It's a toy car....

Lets not overthink this stuff. One off and a slow Marshall and all the theorizing and optimization in the world counts for squat anyway!

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Old 02-28-2019, 03:54 AM
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Originally Posted by Nerobro View Post
Under sharp acceleration, I suspect all of the air in the near vicinity of the body, can be treated as mass of the car.
That's aerodynamic drag and dependant on velocity/speed, not acceleration.

When launching from a standstill or a very low speed mass matters a lot and aerodynamics does not since there is very little air moving over the body and therefore very little aerodynamic drag being produced. Acceleration from a standstill is all down to overall mass and forwards force produced by the torque at the wheels -> F=ma. Taller gear ratio, more powerful motor and/or lower mass yields higher acceleration.

The faster you go the higher the aerodynamic drag force becomes. When the car reaches top speed the forwards force produced by the motor is perfectly balanced with the drag forces which will be almost completely aerodynamic. If we consider F=ma, if there is no acceleration (a=0), the effect of increased mass is not felt as F=0 no matter what m is. In reality, the drag from the drivetrain (bearings) and tyre rolling resistance will go up slightly with increased mass, though if they are significant compared to aerodynamic drag there's something horribly wrong with your car.

So if we are interested purely in acceleration and top speed then we could say at low speeds (significantly lower than the cars top speed) mass matters a lot, and at high speeds (close to top speed) aerodynamics matters a lot. At moderate speeds they both matter.
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Old 02-28-2019, 04:49 AM
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Originally Posted by nbTMM View Post
That's aerodynamic drag and dependant on velocity/speed, not acceleration.
If you imagine that the air under a body shell is fixed to the car (ie it doesn't flow in or out of the shell) then it can be counted as mass for purposes of inertia. However there's only about 6 or 7 grams of air under the body, and much of that flows in or out so it's not going to make much difference.
Originally Posted by nbTMM View Post
When the car reaches top speed the forwards force produced by the motor is perfectly balanced with the drag forces which will be almost completely aerodynamic.
With single speed electric cars, the main limiting drag force at 'high' speed (60kmh, not actually high speed) is the magnetic drag in the motor.
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Old 02-28-2019, 06:48 AM
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Originally Posted by gigaplex View Post
I still wouldn't consider that mass of the car. I'd wave my hands and call that an aero effect. And remind people that the physics isn't that simple.
Definitely an aero effect but the car feels it as "extra mass" when something hits a tire. It's one of the reasons why WGT-R cars look so different from F1 cars going around the track. I think your explanation is "right" for most people.

Originally Posted by nbTMM View Post
That's aerodynamic drag and dependent on velocity/speed, not acceleration.
Acceleration can happen in any direction, or on any axis. The air influenced by the body, that i'm speaking of, is why r/c touring cars seem to "stuck" to the ground. I'm talking about vertical acceleration, not horizontal. This happens at all speeds, and all body shapes, and really only depends on body area. If you'd like a really good demonstration of this effect, put a paint stick on a table, with 3" hanging off, then put a large sheet of paper over the part that's on the table, hit it.. hard. The paint stick will break. (This is an exaggerated, example, but hey, good for demonstration purposes.)

Originally Posted by LzREngineering View Post
If you imagine that the air under a body shell is fixed to the car (ie it doesn't flow in or out of the shell) then it can be counted as mass for purposes of inertia. However there's only about 6 or 7 grams of air under the body, and much of that flows in or out so it's not going to make much difference.
I think the area of influence is greater, and includes at minimum all of the air in the turbulant flow zone around the car, and if the car is in motion, any of the near disturbed air. I'd also suggest that's it's non linear, a car who's body is 5mm off the ground is gonna have a whole lot more resistance to being kicked up (a higher "functional" mass) than one that's already in the air.

Why would I bring it up though? When a touring car, hits a curb, and stays stuck to the ground, these cars aren't going fast enough to generate a whole lot of downforce, but for an instant, they seem to have a couple extra kilos of effective mass, allowing them to compress those 2-3kg/cm springs and 35wt oil really damned fast. More than I think the 1450g of the chassis can explain.

... bah, now I need to break out the physics book and do the math.

With single speed electric cars, the main limiting drag force at 'high' speed (60kmh, not actually high speed) is the magnetic drag in the motor.
Yup.
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Old 02-28-2019, 06:53 AM
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Kinda late to this but there was talk about fwd earlier on the thread and wanted to add that I try to keep my fwd at close to 70-30 weight distribution as possible it seems to have been my golden spot.
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Old 02-28-2019, 07:16 AM
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If "traction limited" then more weight to bring up traction may be better.
If "power limited" then less weight to the point before being "traction limited" may be better.
And, if "weight reduction" then must also do "weight balance" to realize benefit.

Power limited < - - - - (-) Weight/balance (+) - - - - > Traction limited
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Old 02-28-2019, 07:20 AM
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Originally Posted by rccartips View Post
If "traction limited" then more weight to bring up traction may be better.
If "power limited" then less weight to the point before being "traction limited" may be better.
And, if "weight reduction" then must also do "weight balance" to realize benefit.

Traction limited < - - - - Weight/balance - - - - > Power limited
it is a good start...yes
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Old 03-01-2019, 09:33 PM
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Originally Posted by LzREngineering View Post
With single speed electric cars, the main limiting drag force at 'high' speed (60kmh, not actually high speed) is the magnetic drag in the motor.
Which can be ignored as it is the same regardless of changes to mass or aerodynamics. Think of the motor as a black box that produces mechanical power at the output shaft - that power is transferred to the wheels with very little loss. That power equals torque as power = torque * rpm. Torque on a wheel produces a force which propels the vehicle forward while aerodynamic drag produces a force which opposes it. At top speed those forces will be equal and the vehicles stops accelerating. The power of the motor falling off at high rpm is secondary issue - there will still be equality between motor mechanical output power and aero drag when top speed is achieved and a vast reduction in acceleration when travelling near top speed. If your top speed is 60kmh with a high FDR but with a low FDR and the same motor the car can do 120kmh then what you are experiencing with the high FDR is that at 60kmh there isn't much aerodynamic drag but there also isn't much power being produced by the motor as it is being pushed to high rpm and the motor power is taking a nose dive. The lower FDR allows the motor to be producing usable power between 60 and 120kmh to overcome the aerodynamic drag and continue accelerating.
If top speed was say 70kmh, then going to a much more aerodynamic body which improves the top speed to 85kmh should also see an improvement in acceleration around 50-60kmh. The acceleration from say 0-20kmh would still be near identical with the more aerodynamic body however.

Last edited by nbTMM; 03-01-2019 at 10:17 PM.
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Old 03-03-2019, 11:47 AM
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Originally Posted by nbTMM View Post
That's aerodynamic drag and dependant on velocity/speed, not acceleration.

When launching from a standstill or a very low speed mass matters a lot and aerodynamics does not since there is very little air moving over the body and therefore very little aerodynamic drag being produced. Acceleration from a standstill is all down to overall mass and forwards force produced by the torque at the wheels -> F=ma. Taller gear ratio, more powerful motor and/or lower mass yields higher acceleration.

The faster you go the higher the aerodynamic drag force becomes. When the car reaches top speed the forwards force produced by the motor is perfectly balanced with the drag forces which will be almost completely aerodynamic. If we consider F=ma, if there is no acceleration (a=0), the effect of increased mass is not felt as F=0 no matter what m is. In reality, the drag from the drivetrain (bearings) and tyre rolling resistance will go up slightly with increased mass, though if they are significant compared to aerodynamic drag there's something horribly wrong with your car.

So if we are interested purely in acceleration and top speed then we could say at low speeds (significantly lower than the cars top speed) mass matters a lot, and at high speeds (close to top speed) aerodynamics matters a lot. At moderate speeds they both matter.
Just nitpicking here. But normally a "Taller" gear ratio means a lower numerical gear ratio. A "Shorter" gear ratio is a higher numerical gear ratio and provides more tractive effort and higher acceleration.
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