Lets Talk Physics - Rotating Mass v's Dead Weight

jponder said:
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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Makes perfect sense! So, a smaller diameter (light) wheel and a light tire is the order of the day. Keeping the concentration of weight closest to the hub for a lower moment of inertia. Of course that tire has to be load rated for how heavy my truck is which adds weight v.s a similar sized tire for a car application.

I am debating between another set of 17X7.5" factory forged rims like I have now with 275/60/17's (30" Diameter) mounted on them or jumping up to 18X8 inch '08 Factory forged wheels and mounting 285/60/18's (31.5" Diameter). The weight savings on the 17's is 4lbs (43lbs v.s 39lbs) but I would be giving up .5" of tread width. Another option is mounting 285/60/17's (30.5" Diameter) on the 17X7.5" rims above but the tire is rated for a minimum 8" wide rim so I'm not sure about that.

Anyway, any suggestions?
 
there are several things..

tires obviaously light smaller spin eaiser in a accelartion race.

stiffer axle that dont twist as much,
stiffer dirveshaft.. cant findit saw a vid of a DS with a line paint down it. on a launch it twisted 1/4-1/3 of a revolution between u-joints im sure more power would result in more twisting

axles do the same thing

it is all classified as parasitic lose in power
 
When I got new tires on my old gas truck I noticed a big difference in acceleration. Went from a 285 to a 265 all with a 3:55 axle ratio.
 
The formula for our sprint car we use was taught to us by a guy years ago who would win alot.His theory was for every 1lb of rotating weight in the drivetrain you remove you gain 7hp.Its something we have gone by for years and is easy to see nowadays with all the lightweight materials being used in drivelines in all types of competition.The man who showed all of us years ago was none other than Kenny Weld when he tore up the local circuit driving Weikerts Beefmobile......Andy
 
Hammer said:
The formula for our sprint car we use was taught to us by a guy years ago who would win alot.His theory was for every 1lb of rotating weight in the drivetrain you remove you gain 7hp.Its something we have gone by for years and is easy to see nowadays with all the lightweight materials being used in drivelines in all types of competition.The man who showed all of us years ago was none other than Kenny Weld when he tore up the local circuit driving Weikerts Beefmobile......Andy
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Wow! That's awesome.
 
Do the experiement yourself.

Next time you are out figure skating go into a spin on one skate. Tuck your arms in for a dramatic high speed spin. Then extend out them for a fast slowdown. Let the crowd see your big smile. The you can tuck your arms back in and zip back up to high speed again. You notice a huge change in rotational speed just by moving your arms in or out and you will really "WOW" the crowd.

Fairly good example of a constant mass and a changing moment of inertia I would think.

Jim
 
NoSeeUm said:
Do the experiement yourself.

Next time you are out figure skating go into a spin on one skate. Tuck your arms in for a dramatic high speed spin. Then extend out them for a fast slowdown. Let the crowd see your big smile. The you can tuck your arms back in and zip back up to high speed again. You notice a huge change in rotational speed just by moving your arms in or out and you will really "WOW" the crowd.

Fairly good example of a constant mass and a changing moment of inertia I would think.

Jim
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Me? Ice Skate? :umno: :badidea: :wrong: But great example!!! :hehe:
 
Well, this weekend at the track showed me that you have to remove ALOT of dead weight to make a noticable difference. Best time first time out was 14.55 @ 89.44. This time I took off the tailgate, hitch, 2 12" subs and box, amp and bed rails, and went 14.575 @89.34 :bang
 
BigB82789 said:
Well, this weekend at the track showed me that you have to remove ALOT of dead weight to make a noticable difference. Best time first time out was 14.55 @ 89.44. This time I took off the tailgate, hitch, 2 12" subs and box, amp and bed rails, and went 14.575 @89.34 :bang
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What about fuel level. Were you empty last time and full this time? You can save 200+ lbs depending on fuel level....I like to be right at where my fuel light comes on (approx 7-8 gals) everytime race, just to be consistant on weight.
 
Just make sure you don't start drawing air on acceleration! I know it's apples to oranges, but my GMC Typhoon, with its 1.82 60' times, MUST have half a tank of fuel to keep from drawing air in on a dragstrip pass.

One minor tire issue to consider is that taller front tires will increase rollout, which can give you more of a "rolling start" if you stage truly shallow.

Regards,
Michael Pliska
 
I did have a full tank this time, in preparation for PRP, but that didn't happen. I'm not sure about my last time, but it was maybe 1/2 or above.
 
Update on Tire/Rim Selection!

So I decided to buy a few more of my current factory forged wheels and mount Toyo Proxes on them in the interest of cost. I was very surprised to find that the factory rims I have are only 23.2 lbs each. Very light for an 8 lug wheel. I then weighed three different configurations for comparison.

- My current wheel/tire combo - Nitto Terra Grappler 285/70/17: 78.2 lbs each
- Factory wheel/tire combo - BFG Rugged Trail 265/70/17: 73.0 lbs each
- Race wheel/tire combo - Toyo Proxes S/T 285/60/17: 61.6 lbs each

I'm pretty happy with a reduction in weight of 16.6 lbs per wheel/tire and almost 2.5" reduction in Diameter overall. I can't imagine this wouldn't have a significant impact on track times/speed. I'll let you know!

Wheels are from lightest to heaviest - Left to Right

IMG00003.jpg
 
Nice Ant, I can't wait to see what a difference it makes. Are you going to the track next week??
 
Another thing to consider is the width of the tire. I know that you need some width for a decent contact patch and to achieve enough load capacity, but the wider the tire, the more rubber=weight it has at the furthest point from the central axis, meaning you could have a slightly taller tire that is skinner but of equal weight to the shorter wider tire and have different moments of inertia.

Due to your weight/load bearing limitations, expense limitations etc, sounds like you did what was most feasible. Let us all know how it goes as far as ET's.
 
Big Blue24 said:
Another thing to consider is the width of the tire. I know that you need some width for a decent contact patch and to achieve enough load capacity, but the wider the tire, the more rubber=weight it has at the furthest point from the central axis, meaning you could have a slightly taller tire that is skinner but of equal weight to the shorter wider tire and have different moments of inertia.

Due to your weight/load bearing limitations, expense limitations etc, sounds like you did what was most feasible. Let us all know how it goes as far as ET's.
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Good point! I think the width is the same as both have a section width of 285mm. The factory tires are only 265mm in width but 1" taller. I debated between 285's and 275's but I think the extra 10mm of rubber on the pavement is more beneficial than the reduced drag. We shall see!
 
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