[Lemonlocators]

Does anybody know the formula for figuring out what 1 pound of rotating weight is equal to in HP or chassis weight? I use to race motorcycles and 1 pound of wheel weight (17") equaled to 7 pounds of chassis weight. I was told this anyway.

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[ds968]

<!--quoteo(post=68356:date=Mar 11 2009, 03:16 PM:name=Lemonlocators)-->QUOTE (Lemonlocators @ Mar 11 2009, 03:16 PM) <{POST_SNAPBACK}><!--quotec-->Does anybody know the formula for figuring out what 1 pound of rotating weight is equal to in HP or chassis weight? I use to race motorcycles and 1 pound of wheel weight (17") equaled to 7 pounds of chassis weight. I was told this anyway.<!--QuoteEnd--><!--QuoteEEnd-->


I was told it's roughly a factor of 4X on automobiles.. as far as wheel / tire weight deltas are concerned.
No idea if flywheel weight has a greater or a lesser impact.

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[flash]

it's not x pounds equals x horsepower - it's a function of percentage of weight - the effective weight of wheels or tires is 4 times the actual weight

here's how it works:

weight of car / (weight of car - (weight difference of tire/wheel *4) *4) * hp of engine - hp of engine = effective hp change

for example:

3086lb car
4lb per wheel change
240 hp engine

3086 / (3086 - (4*4*4)) * 240 - 240 = 5.08 hp

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[rxter]

at the crank or the wheel [img]style_emoticons/<#EMO_DIR#>/wink.gif[/img]

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[Lemonlocators]

<!--quoteo(post=68359:date=Mar 11 2009, 06:45 PM:name=flash)-->QUOTE (flash @ Mar 11 2009, 06:45 PM) <{POST_SNAPBACK}><!--quotec-->it's not x pounds equals x horsepower - it's a function of percentage of weight - the effective weight of wheels or tires is 4 times the actual weight

here's how it works:

weight of car / (weight of car - (weight difference of tire/wheel *4) *4) * hp of engine - hp of engine = effective hp change

for example:

3086lb car
4lb per wheel change
240 hp engine

3086 - (3086 - (4*4*4)) * 240 - 240 = 5.08 hp<!--QuoteEnd--><!--QuoteEEnd-->


So if I were to change all of my tires to a lighter tire . say a total of 12 pounds I would be looking at roughly 15HP gain or 3.5HP? I think 3.5HP... I need to get some food...hard time thinking clearly tonight.

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[rxter]

3 lb per wheel change = 12 total, so yes a 3.5 effective hp change.

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[flash]

3.8 hp

crank

i also corrected the example formula where i typed "minus" at the beginning where it should have been "divided by"

p.s. - this is a close approximation - there are factors that play into this a bit - tire size, compound, alignment geometry, yada yada, all play into the friction component - whether it is hub weight, rim weight, or tire weight also plays in - the farther out, the larger the difference

but this is a good tool for figuring out the effects - you can feel it when you drive too

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[biotechee]

Couple of questions/comments-

1) I thought our HP was 236 for SAE and 240 for DIN. If so, would you not want to use the 236 number to make things copacetic with the weights in pounds?

2) Bob- can you revise the formula so I can type it in Excel? I have tried to type into Excel as many ways as I can think and I only end up with either 15.9 or .004something something. Can't get the spreadsheet to spit out 3.8. Probably a simple change to the order of operations, but I'm missing it.

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[flash]

1 - not really important what number you use, in fact, modified engines will use different numbers - the percentage change is the same though regardless of measuring method - a quick ratio of before to after will allow you to convert to whatever measuring standard you want

2 - sorry - i'm not an excel guru - i really don't use that for anything but budgets - the formula protocols always mess me up too

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[rxter]

<!--quoteo(post=68376:date=Mar 12 2009, 06:35 AM:name=biotechee)-->QUOTE (biotechee @ Mar 12 2009, 06:35 AM) <{POST_SNAPBACK}><!--quotec-->Couple of questions/comments-

1) I thought our HP was 236 for SAE and 240 for DIN. If so, would you not want to use the 236 number to make things copacetic with the weights in pounds?

2) Bob- can you revise the formula so I can type it in Excel? I have tried to type into Excel as many ways as I can think and I only end up with either 15.9 or .004something something. Can't get the spreadsheet to spit out 3.8. Probably a simple change to the order of operations, but I'm missing it.<!--QuoteEnd--><!--QuoteEEnd-->

=((3086)/(3086-(4*4*4))*240)-240

excel whiz to the rescue

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[biotechee]

I cut an pasted your formula directly into Excel, then changed the 4 lb difference to 12 lbs, and.... I still get 15.9, not 3.8. Which makes sense since in the original example, a 4 lb change yields 5.08 HP, a larger difference (such as 12 lbs) in weight should yield a larger difference in the output of the equation, hence the 15.9 I see....

For reference, when I plug in your formula and use the 4 lb difference, I get the 5.08 HP as shown in Flash's original equation. So, a 12 lb difference at the wheels must be equal to 15.9 HP!!!! [img]style_emoticons/<#EMO_DIR#>/ohmy.gif[/img]

What am I missing? Am I stupid? (DON'T ANSWER THAT! [img]style_emoticons/<#EMO_DIR#>/laugh.gif[/img] )

EDIT- Wait, I got it. The 3.8 HP is PER wheel in the 12 lbs total example. ALL IS WELL!!! I think I hurt my brain with this whole thing. [img]style_emoticons/<#EMO_DIR#>/dry.gif[/img]

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[gryphon]

So, if I put four of those little collapsible spare tires/wheels on my 968 will it be hella fast?

Tom

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[rxter]

<!--quoteo(post=68413:date=Mar 12 2009, 01:26 PM:name=gryphon)-->QUOTE (gryphon @ Mar 12 2009, 01:26 PM) <{POST_SNAPBACK}><!--quotec-->So, if I put four of those little collapsible spare tires/wheels on my 968 will it be hella fast?

Tom<!--QuoteEnd--><!--QuoteEEnd-->

Shich yea in a straight line - get the Z-rated spares. And look out for that .99G turn.

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[Lemonlocators]

Has anyone tried the General Eclaim UHP's? I would loose total of 34 pounds compared to my current tires. The generals are super light!!!! 20 front and 23 rear. Pounds each tire.

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[biotechee]

I have them on my daily driver (2004 Jetta GLI VR6, 200HP) and I like them. I'm not a crazy driver but I do find myself driving, shall we say, a little bit faster than the posted limits on occasion.... I would buy them again in a heartbeat.

Now, I do not know how they would fare on a car with staggered sizes and a bit more braking/cornering ability (check that - a lot more ability!)... If I hadn't picked up my cab which has nearly new tires, my previous coupe was due for new tires and the UHPs were going on.

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