hp cost of weight, rotating/non-rotating

There have been some questions as to the effect or hp cost of using larger heavier wheels, tires, rotors etc on our cars. I developed a spread sheet to address this issue. The short answer is that 10 additional lbs(all other variables remaining constant)on a 25" OD wheel/tire unit accelerated from 0 to 60 in 6 sec is ~1.8-2 hp depending on some assumtions which need to be made.
Copies of the spreadsheet are available(whenever Verizon gets my e-mail running again). Hopefully others can suggest ways to improve it.

That is interesting stuff, Bill.
The performance factors of rotating weight
was demonstrated to me when I put lighter (magnesium) wheels on my Ducati motorcycle. It felt like the bike had gained 10 HP!

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Doug
'81 SC Coupe (aka: "Blue Bomber")
Canada West Region PCA
members.home.net/zielke/911SC.htm

that doesn't take into account the negative effect on handling when one increases unsprung weight.

F=ma, .5mv^2+.5Iw^2. . .I know what you’re up to . .. .too cool!
A "library" of wheel weights, rotor weights, maybe even tire weights, would help here.

. . .and did Jim T. volunteer to do a spreadsheet on unsprung vs sprung weight?
Differential equations; cool.

I've been soliciting rotor and caliper weights for several years, not too much feedback yet though. The reason is that most people have the parts mounted on their cars(how inconsiderate). I have 930 rotors but can't weigh them for that reason.

Doug,
I know I toyed with getting Ti spokes for my new bicycle. but I wimped out and went with more fashinable black ss ones. The lower the hp and total weight involved the more significant the effect of rotating weight is.

Bill,

I would be curious to see the spreadsheet.

Power of course is a different unit of measurement than weight, so there is no way to simply say 1 lb = 1.8 hp. That is like saying 1 ft = 1 lb, apples and oranges.

I assume you mean that the impact of a 1 lb change in rotating weight to the power/weight ratio is equivelant to a 1.8 hp change to the power/weight ratio. This would only hold true for a specific set of power and weight numbers. So I assume you used the numbers for your car or some "typical" 911.



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Chuck - '86 Cab, '77 Targa, '85 toyota 4wd rain mobile http://www.cheaterswayside.com/911/

Chuck, Your right there is no way here to *simply* say 1 lb = 1.8 hp. Thats why a spreadsheet is in order.

Aside: Rotational has and extra "inertial cost" Not only do you have to push it forward (like the spare tire in the front boot) but it also uses energy to get it spun-up.
To get to how much more "inertial cost" reqires more math than I want to do in my head while eyeing some cool looking 18" rims. . . that would allow me to put some larger brake disks and . . .
I look fwd to this spreadsheet.

For non-rotating mass the only 3 independant variables in the calculation of hp are time mass and distance(speed, time and mass are isometric variants.
The calculation of rotating hp is dependant on the shape, composition, homogeneity and radius of the rotating part as well as the 3 variables listed in part 1, it requires some estimation(educated guesses) as I don't have a test bench set up(I have a few spare 3/4 hp electric motorss though, Hmm?)
I will send you each a copy when I get home. the sheet has been modified since my first posting to include real #'s (Dunlop SP9000)as opposed to theoretical tires it can easily be modified to do other bands from theirwebsite information

Following is a cut and paste from an earlier post I made about the benefits of saving rotational weight vs. non-rotational -

"I played around once trying to figure out how much benefit could be realized from saving a pound of rotating weight compared to saving a pound of non-rotating weight. The answer is - it depends. The calculation is complicated by the fact that weight is not evenly distributed over the radius of the wheel/tire combination. And each combination has a different distribution.
However if 100% of the weight of the wheel/tire is carried on the tread surface (a theoretical worst case), each pound of rotating weight would be worth two pounds of fixed weight. This for both acceleration and deceleration, but not cornering or unsprung weight.

But since the real world distribution of weight is not on 100% on the tread, the real benefit is less than 2:1. So eyeballing it for a typical combination 25 lbs tire (carrying most of its weight on the tread and all of it close to the tread) and 16 lbs wheel (carrying about half its weight on the rim close to the tread), I would approximate the benefit at around 1.7:1"


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Chuck - '86 Cab, '77 Targa, '85 toyota 4wd rain mobile http://www.cheaterswayside.com/911/