Rotational weight

[bobtile]

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[Budnicks]

Someone had mentioned to me about setting up some better air flow on the front of my 68rr. I think some better air flow under thr front end may help. He also mentioned the wheel wells also. Looking at that but thats a tough one not sure just how to get that to flow smoother. I am doing disc brakes in the front as we speak and that should save a good amount of weight. On my 04 viper the whole bottom is an al. sheet.

That was no doubt me...
Redirection of air going into the front end tapper it inward, to go thru the radiator
& not under hood or underneath the front end...
A front splitter or air dam/spoiler of some type, to redirect air from under the car,
would probably help some too...
My main point was/is;
Installing basic inner fender-well panels also...
Just keeping air from making it a big parachute like effect,
grabbing air while going down track, that will slow you down...
Even just a simple flat alum. panels, like were/near the orig. inner fender-wells were,
or from frame rail to top on diagonal bars {if it has a cage, front bars},
then a 40*-45* bend {what ever angle} to go back up-to the fender lips...
That would help keep air out from under the front end somewhat better,
anything is batter than, a completely open engine compartment,
With inner fender-well panels, then only the wheel wells would catch the bulk of the air,
going from the front end that way...
A belly pan {sheet closing off the air from under the engine bay},
that could also help some too, but the heat & stacked/trapped air needs to go somewhere...

sorry didn't want to hi-jack Bobtile's thread,
but I kind of wanted to answer, clarify his question/statement...

 

[bobtile]

That no doubt was you

 

[4406]

Good luck with your winter upgrades.



Ray

 

[HT413]

I've read lots of people talk about 1lb rotating is like 10lb static and that's a bunch of crap - it's too complicated to use a simple rule like that as far as straight line acceleration goes. Now I'm not doubting when someone says they picked up 0.05 for removing 10 lbs of rotating weight (claiming it is equivalent to 50lbs static) but there is always some other factor like better track prep / better weight transfer / better tire etc that went into that result. like budnicks states above, rotational weight is complicated and depends on a few things:

-Weight (mass really) if the piece
-How far it's mass is from the axis of rotation - a tire is way farther from the rotating axis (axle obviously) than a driveshaft is. A wheel is complex because while most of its weight is near it's radius, there's a lot that's close to the rotating axis as well.
-Rotational speed - thos brings a impact of a driveshaft much closer to the impact of a tire since it rotates (in your case) 4.1 times for each tire rotation.

You can calculate it by hand OR you can read this piece that coveres the topic nicely. You can ignor the physics and go straight to the calculators. Because each shape has a different rotational inertia, the page has different calculators for the different components like brakes, wheels, tires, driveshafts, even lug nuts lol.

http://hpwizard.com/rotational-inertia.html

 

[bobtile]

I was waiting for your answer. thanks

 

[dvw]

Good info! How much does the car weigh?

3340 with me and full of fuel. I was pleasantly surprised As many people say that a high torque combo wouldn't benefit much. I was pretty sure it was worth something. After the first trip to the track I installed the A&A billet drum which weighed over 10 lbs. The car was never as quick. This spring I made the drum/sun shell swap with no other changes.
Doug

 

[HT413]

3340 with me and full of fuel. I was pleasantly surprised As many people say that a high torque combo wouldn't benefit much. I was pretty sure it was worth something. After the first trip to the track I installed the A&A billet drum which weighed over 10 lbs. The car was never as quick. This spring I made the drum/sun shell swap with no other changes.
Doug

How's the rpm of the front drum compare to the rpm of the motor?

- - - Updated - - -

Reason I ask is in 1st gear, the motor turns 2.45x as fast as driveshaft and with 4.10's, 10.8x as fast as the tires, so if that drum turns about the same rpm as te motor, I bet that 7lbs from the front drum really was like 70lbs on the wheels, and over 100lbs static in 1st gear, with the benefit reduced as you shift gears.

I'd bet your 60ft times improved quite a bit, but in 3rd gear, the trans is only going the same speed as the driveshaft, so the benefit diminshes as you go down the track.

 

[HT413]

I took 7lbs of rotating weight out of the trans this spring and picked up about .10 & 1 mph in the 1/8th. No mph gain in the 1/4 though.
Doug

Now that we look at the numbers, makes sense you saw so much improvement in the 1/8th, but I wonder do you mean your 1/4 time was the same as with the heavier drum? Because that means you actually slowed down the during the last half of the 1/4.

I would have thought going 0.10 faster in the 1/8th would have carried over to the 1/4.

 

[mopar 3 B]

So what I take from this is the enertia of the heavery part is of no value once rotating at its potential and just a loss of energy to the source of original energy.

 

[dvw]

In 1st gear the high reverse drum and sunshell (they're splined together) rotate in the opposite rotation to the engine/driveshaft. At this time they're speed is less than engine rpm. During the 1/2 shift they have to stop completely. 3rd gear, they must accelerate back to engine rpm.
Doug

 

[HT413]

So what I take from this is the enertia of the heavery part is of no value once rotating at its potential and just a loss of energy to the source of original energy.

I'd say that's correct. It's all about how hard it is to accelerate the piece, which is why it matter so much in drag racing.

Think of it like this:

Say you had 600 ft lbs of torque available. Now if you have a rim that puts 10 lbs of weight 1 foot from the axis of rotation (just for simple numbers) it would take 10 ft lbs to get it moving, so you now have only 590 ft lbs available to accelerate the car (and to get the the rest of the pieces rotating).

Once it's all rotating and headed down the track, now you're just fighting friction and air resistance.

- - - Updated - - -

In 1st gear the high reverse drum and sunshell (they're splined together) rotate in the opposite rotation to the engine/driveshaft. At this time they're speed is less than engine rpm. During the 1/2 shift they have to stop completely. 3rd gear, they must accelerate back to engine rpm.
Doug

Whoa. Lol, and THAT is why these rules of 5:1 or whatever just don't hold up. Too dang complicated.