BBMGT3

Am not sure I understand your post, you are saying that your 996 was quicker than your 997, due to lower unsprung mass, but that the "ratio" is 1:1? That doesn't make sense...

mdeleeuw

Providing that adequate strength and safety factors are maintained, reduction of unsprung mass is always high on the list. Higher Unsprung mass will reduce the suspension natural frequency and generally reduce handling responsiveness. Any "extra" mass will also create additional inertial loads in the suspension as a result of road surfaces and require larger (heavier still!) components to resist this force. Body mounting points need to be reinforced to handle the higher loads. It becomes a never ending horror story...

The true benefit of lower unsprung mass is not the actual and effective reduction of total vehicle mass, but the improvement in handling dynamics through reduced inertia (not rotating inertia, but translational) of the suspension itself. One reason you see inboard brakes and inverted dampers in dedicated racing cars - no less total mass, but located for better dynamics of the wheel. Being that wheels are the outermost (farthest from the chassis) component with the highest dynamics, any weight savings there pay more dividends in handling than for instance a lighter weight spring or lower control arm.

Mike

NJ-GT

996 with actual weight on scales at 2,770 lbs and 380Hp: Fast

997 with 3140 lbs minus 92 lbs of unsprung weight reduced (wheels/brakes) and 415Hp: Slow

Using a factor of 4:1 or even worse 6:1, my 997 would be so much lighter than my 996 and perform so much better, not a chance.

I use a 1:1 ratio, that's it. 92 lbs reduced on unsprung weight is 92 lbs reduced.

BBMGT3

Just to clarify

1) I assume both cars are running on similar tires and dialled in suspensions?
2) Your 996 is running with HEAVIER wheels and brakes than your 997? 2770 lbs is very light, so I find it unlikely that you've got more unsprung mass on your 996 than your 997...

LuckyP

Here's a funny thing:

Had a puncture recently on a std corsa shod rear RHS (24kg with tyre) 1 mile from home. Got a lift back to fetch a BBS shod with MPSC (21kg with tyre). Did the swap amd gingerly drove home. On the way back, I pulled some mild acceleration- no break of traction, and the car lunged off to the left each time!

BBS wheel and tyre was 3kg lighter, different tyres I grant you (and possibly a slightly different roll circ??) and offset 1mm difference I think.

Wonder if that lurch to the left was the weight difference?

Hmmmm....

aj986s

Unsprung weight is not just about acceleration. There's also a handling element to all this. Less unsprung weight means the suspension can react faster to road irregularities, enabling better contact and traction. I agree, the ratio is not a hard science, but removing a pound of unsprung weight should yield better overall results, as a combination of acceleration & handling, than a pound of sprung weight.

NJ-GT

Nope, just a tire diameter difference. Due to the presence of a LSD, both rear tires need to have the same height. Happens to any RWD car with a LSD.

I used to run Pocono North course in Pennsylvania, most of the turns are right turns. If I spent a full day there, my car was slightly pulling to the left on the way back home. After I learned this, I started swapping the wheels left-to-right and right-to-left in the middle of the day.

NJ-GT

I prefer 1 lbs saved from unsprung weight than 1 lbs saved from sprung weight.

However, these ratios of every pound of unsprung weight being the equivalent to 6 lbs (or more) of sprung weight are just not right.

If I remove 100 lbs on unsprung weight, my car doesn't feel like a 600 lbs lighter car, not in acceleration, braking or turning. This is why I'm fine using a 1:1 ratio.

NJ-GT

1) it has nothing to do with unsprung weight
2) buy a postal scale, put parts on scale, write weights on paper, sum, then compare. Tip: my 997 ran 996 PCCB rotors

BBMGT3

When comparing apples to apples, its important to have as few variables as possible.

So

1) Is important for ruling out external variables (re: tires and suspension)
and
2) Your 996 and 997 are not a suitable comparison unless your super-light 996 has not had weight reduction in wheels&brakes or other unsprung components. It DOES make a difference, if you're going to compare the performance of two cars particularly when considering dynamic weight to try and compare the same things.

Yorkshireman

I hate to invoke the Math god, but Newton's law of motions says that F (force) = m (mass) x a (acceleration). So, igoring losses like aero and friction, acceleration is force divided by mass. So you take mass out, any mass, and it improves acceleration. So, the ratio is 1:1.

Now there are other effects that are special to removing unsprung mass rather than just any mass. Unsprung and non-rotational mass, like A-arms, reduce suspension inertia and allow it to work better. Serge944's comments on the effect of rotational mass like wheels, brake disks, flywheel etc are spot on, in that rotational mass also imroves the engine's ability to spin up faster.

So, I think the whole "what is the ratio" question has been safely "busted". Removing ANY mass will improve acceleration. Unsprung mass will also positively affect the ability of the suspension to respond and rotational mass (thanks Serge944) will improve the motors ability to spin up faster.

One last point. Just one more reson to love those PCCBs! I think that they have a much larger effect on both unspring and rotational mass than any other mod, including the beloved RS flywheel mod. LOL.

Carl.

911SLOW

Glad to see that two weeks of drinking and clubing in Eastern Europe, haven't caused irreversible damage to you brain cells and that money spent for your degree are not wasted.


Yes, in a straight line it, removing weight from either, feels to be the same..

Agreed

Agreed

PCCB ftw!


And now that I 've agreed with everybody let me quote Olaf Manthey:

“Removing 33 pounds of unsprung weight at the wheels is equivalent to losing 198 pounds from the body of the car, as a 1:6 factor has to be applied when the car is moving and that weight becomes mass. In fact, we are conservative with the 1:6 ratio, as Porsche considers it to be 1:7."

link

Nice..?

BBMGT3

The SLOW has spoken...

NJ-GT

That's so wrong from Mr. Manthey, and even worse from Porsche.

Case in point, my 997 GT3 RS at 3,075 lbs with 3 gallons of fuel. Carbon GT2 seats, LW battery, Sharkwerks center muffler bypass pipes, no rollbar, everything else standard. Assume same tire brand, model, width.

Car1: 3,075 lbs 2007 GT3 RS

Now I replace the rotors with 996 GT3 PCCB and the wheels with BBS 9"/11"x18 Magesium wheels, for a total unsprung weight saving of 90 lbs.

Car2: 2,985 lbs 2007 GT3 RS (PCCB+BBS Mg)

So, this German company and this other German guy claim that Car2 has the equivalent to a weight loss of 540 lbs or 630 lbs? Car2 has the equivalent to a weight loss of 630 lbs to get down to 2,445 lbs. 2,445 lbs? really?

Car2 should be massively faster than Car1, Car2 would actually beat a GT3 Cup.

Biggest BS I have read in a long time. I stick to my 1:1 ratio.

Car3: 2,675 lbs 2007 GT3 RS (400 lbs lighter than Car1) but still carrying the heavy brakes and wheels as stock.

Manthey and Porsche say Car2 would be faster (I mean lap times, not drag racing), I say Car3 beats them all by a mile.

911SLOW

As I said, I agree with you in straight line acceleration terms it feels (and reads) the same, but removing unsprung weight helps with everything else..
Let's call it a 1+something:1 ratio. : )

But on the other hand, I had to quote Mr. Manthey..