Lets Talk Physics - Rotating Mass v's Dead Weight

BilletGarage

BilletGarage

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Alright, it is no secret that most drag racers wished their vehicle was lighter and had more horsepower (obviously resulting in quicker ET's). But, I am interested in learning the difference and/or relationship between rotating mass (ie: Tire/Wheel Weight) and Dead Weight (curb weight). Basically what difference in ET is made by shaving 1lb of curb weight v's shaving 1lb of rotating mass (ie: Tire/Wheel combo). I realize this can be tricky since on the rotating mass side you have to consider where the weight is concentrated on the wheel/tire. But for sake of argument lets assume best case scenario and the shaved weight is at the furthest point from the hub on both the wheel and tire.

With that said, anyone got any cool formulas or calculators??? How about good explanations on the web??? Personal experience shedding pounds???

I have heard that shedding 1lb of rotating mass is roughly equal to shedding 8lbs of dead weight? What say you?
 
You're thinking about these things at THIS hour of the morning? Omg... no more protein shakes for you boy.
 
I have been told by a local chassis shop that 1 lbs. of rotating weight is equal to 9.25lbs. dead weight.
 
I don't think there is enough info. You need to know the diameters, gear ratios etc, etc.

Something to remember the velocity of the tire at the contact point of the pavement is zero(assuming no slip)
 
F=MA - (translational acceleration)

T=I*alpha - (rotational acceleration) I for a circle is (pi*r^4)/4

We need to know all the T's, F's, and r's....

Its a long road ahead.....

Ben
 
Im Not Sure About The Actual Weight Saved, But I Do Know A Couple Of Guys Racing Their Hemi And 6.0 Chevys (gasoline Stuff) Going From Factory 20"s Down To 17" Wheels And Tires And Picking Up In Both 60 Foot Times And Quicker Ets. I Would Assume That Would Be More Towards The Changing Gear Ratio But It Would Save Weight As Well
 
In the motorcycle world, (Rotating Mass / Unsprung Weight) is what they always try and reduce for better performance....Of course, superbikes are a LOT lighter and have WAY less Torgue so that may make it more important....

I'm confident that if you had two setups - One with 30" tires (regardless of wheel size) that weighed 70lbs each and Another setup that had 30" tires that weighed only 60lbs each.....The 60 lb. set would have a quicker E.T. and Possiblly higher mph.....

Reducing a combined 40 lbs of Unsprung weight VS. 40lbs of Sprung weight........BIG DIFFERENCE!!!!!!!!!!!!

Just try and do BOTH!!!! That'd be the ticket!!!
 
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I'm an old gasser racer, and the big issue with rotational weight (unsprung) is getting more HP to the ground. Less rotational mass equals more HP hitting the pavement. The terms used are sprung (body/chassis/etc) vs unsprung (rotational mass of the driveline) weight.

Found a bunch of garbage about fuel savings, etc... but no hard formulas for improved ET based on sprung vs unsprung weight. If I remember correctly (which I don't often do) 100lbs of sprung weight is good for .01 in the quarter mile from the days I raced. The same unsprung weight was good for closer to a .1 in the quarter mile.
 
I don't have any of my good textbooks with me, but think of the rotational resistance to acceleration (mass moment of inertia) as CSM mentioned above. Think of trying to get a bicycle tire spinning, then think how much more torque would be necessary to get a similar diameter wheel/tire combo spinning if its tire were filled with lead.

Found a handbook with some formulas:
Ix (mass moment of intertia) of a hollow circular cylinder :
Ix = (pi * density * width) * (Ro^4 - Ri^4)/2

also Ix = mass *(Ro^2 + Ri^2)/2

So the diameter plays a 2nd power role.

Probably added more confusion than clarity ;)

Regards,
Michael Pliska
 
Sooooo....basically you guys are saying I've lost alot of HP when I put my 41's on?.....lol

I probably lost a teeny tiny bit more because of my wind resistance......hehehehe
 
Awesome guys/gal! Good stuff. What's got me thinking about this is I am racing my truck with factory forged wheels (unknown weight) but I have 33" tall 10 ply E Load rated tires which have got to weigh 65lbs a piece alone. My idea is to buy an additional set of forged wheels and mount Toyo Proxies on them for the track which are only 31.5" in diameter and weigh 43lbs a piece. I would imagine I could shave 20lbs a wheel plus reduce my diameter by 1.5" for better gearing. Any guesses (educated) on what kind of ET difference this will make if I shave 20lbs per wheel? My truck weighs 7,650lbs right now with me in it.
 
We've seen up to .4s off a 1/4mi pass stepping from 325/50/18's on chrome clad aluminum to a 245/75/16 on factory alloy wheels.
 
joefarmer said:
We've seen up to .4s off a 1/4mi pass stepping from 325/50/18's on chrome clad aluminum to a 245/75/16 on factory alloy wheels.
Click to expand...

Wow! If I could shave 4/10ths by switching wheels/tires I would be a happy camper! Even 2/10ths would make me smile. Thanks for that info Joe!
 
joefarmer said:
We've seen up to .4s off a 1/4mi pass stepping from 325/50/18's on chrome clad aluminum to a 245/75/16 on factory alloy wheels.
Click to expand...
Almost the same here. Back in the day, going from a 325/60/18 (88lbs just for the tire) to stock 265/75/16 shaved .5 off my 1/4 mile time. Now I run a 285/60/18 tire which is a lot closer to the stock tire height and weight.

Big heavy tires rob big power!
 
Tell me about it...

Last year at TS, I was running 285/70R19.5's, about 150lbs each, and 35.5" tall...ran about a 16.0.

Then, as some of you may know, for IRP I swapped in a set of 285/40R17's, at about 55 lbs each, and 26.5" tall.

No other changes to the truck, and dropped over 1.5 seconds.From a 16.1 to consistent 14.47-47.48's.

No I run 285/65R18's, and with an entirely new setup I ran a 14.3x at TS last weekend. These are a good compromise for street and track duty for me!
Chris
 
Double J said:
Almost the same here. Back in the day, going from a 325/60/18 (88lbs just for the tire) to stock 265/75/16 shaved .5 off my 1/4 mile time. Now I run a 285/60/18 tire which is a lot closer to the stock tire height and weight.

Big heavy tires rob big power!
Click to expand...

Wow!!!

Signature600 said:
Tell me about it...

Last year at TS, I was running 285/70R19.5's, about 150lbs each, and 35.5" tall...ran about a 16.0.

Then, as some of you may know, for IRP I swapped in a set of 285/40R17's, at about 55 lbs each, and 26.5" tall.

No other changes to the truck, and dropped over 1.5 seconds.From a 16.1 to consistent 14.47-47.48's.

No I run 285/65R18's, and with an entirely new setup I ran a 14.3x at TS last weekend. These are a good compromise for street and track duty for me!
Chris
Click to expand...

AND WOW!!!!!!!!!!!!!
 
straight Newtonian Mechanics will show that to accelerate a vehicle has to include total mass plus accelerating the tires which will be a function of moment of inertia.

You could have the same weight tires but if the weight was situated on the outside of the tire the moment of inertia would be much greater and reduce your acceleration, similiar to if you had an egg carton with two eggs and you put them at the center of the carton and tried to swing it side to side and if you put both eggs on either side of the carton and did the same. you could feel the resistance to acceleration. Lowest moment of inertia tires will yield fastest acceleration all things being equal.

It does tie into plane on a runway to be sure

If you took two masses of the same mass and rolled them down an incline, one being a sphere and the other being formed into a ring, the sphere would smoke the ring as it has less moment of inertia. that would just be gravity doing it
 
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