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Craig 930 RS's Avatar
 
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Exclamation * CORNER balanced - numbers from this...and the weight

My 79SC, no options, Recaro PP seats, front carpet adios, but NO other lightweight tricks............on the corner balance scales it reads:

2405 lbs w/ 1/4 tank of gas.

Pretty light.

With my fat *ss it is 2607

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Old 03-25-2005, 10:52 AM
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Hey Craig,

Please forgive my ignorance, I know very little about this, but is that a good reading on the rear? It seems that there is over a 100# difference?

Front is very close at 5#......

??

-Chris
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Old 03-25-2005, 11:00 AM
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Nice and light! Did you remove the sound deadening material or the rear seats from the interior?
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Old 03-25-2005, 11:02 AM
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Craig,
Your numbers appear like this:

LF 514
LR 845
RF 509
RR 739

It looks like your LR is carrying a little too much weight. Does it sit higher on that corner as well?

The way it appears now, the LF/LR proportion is 61% and the RF/RR is 69%. You should aim for approximately the same L vs R side. Once the side-to-side proportions become closer, the diagonal weights will follow.

Try lowering the LR to transfer some weight to the other corners, then continue adjusting ride heights until you get closer. I know, resetting rear height is a b*tch.

What are the current corner heights?

Sherwood
some corner balance info and related links here

Last edited by 911pcars; 03-25-2005 at 11:21 AM..
Old 03-25-2005, 11:18 AM
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The corner balance numbers are just where we want them.

There is no way to get equal left to right numbers with a driver, unless you balast the car, or have a car that is otherwise right side heavy to begin with. There is also no way to get more equitable side to side numbers and diagonals at the same time without physically moving weight inside the car.

The diagonals are indeed important, and usually there is a slight compromise when trying to reach the other objective particular to 911 chassis, nearly equal front weights. At threshold braking, this helps alleviate any funny pulls to one side or another, or single wheel lockup. In this case, the diagonal is within 2%, more than acceptable in a noodly production chassis with radial tires.

The sway bars were disconnected, the tire pressures set exactly and the scale pads in platens levelled to within .1 degree of each other.

The corner heights are between 23 7/8 and 24 1/2" and are appropriate to yield about a 1 to 1-1/2 degree rake.
Old 03-25-2005, 11:46 AM
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"There is no way to get equal left to right numbers with a driver, unless you balast the car, or have a car that is otherwise right side heavy to begin with. There is also no way to get more equitable side to side numbers and diagonals at the same time without physically moving weight inside the car.
"


Randy,
I totally agree with you.

My numbers computed the front-to-rear weight distribution on each side of the car which is a target to achieve. This is NOT the same as LF+LR = RF+RR.

The archives are full of discussion on this topic. Try the automated weight balance calculators linked on my site.

Sherwood
Old 03-25-2005, 12:05 PM
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These numbers can be better.....

Some of us understand that there is a left bias. Look at archives under discussions that Chuck Mooreland and I have had,... I used a complicated formula but found out that Chuck's short form (which asks... is LF/LR=RF/RR ?) works just as well.

So let's use this:

LF/LR= 514/845 = 0.6083

RF/RR= 509/739 = 0.6888

Can usually get second significant figure close.....

Wil

EDIT:

The surprising thing for most of us is that as you try out various corner balance changes....the net result is that ANY TWO ADJACENT wheels will always total out to the same number ( LF+RF...or LF+LR...or LR+RR...or RF+RR).
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Last edited by Wil Ferch; 03-25-2005 at 12:13 PM..
Old 03-25-2005, 12:08 PM
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http://rennlight.com/cgi-bin/balance.cgi?name=Craig&car=&notes=&LF=514&RF=509&LR=845&RR=739
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Old 03-25-2005, 12:11 PM
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I just had my 1972 911 done and the numbers are.

LF 515
RF 469
LR 778
RR 730

Front Weight = 984 = 39.6%
Rear Weight = 1508 = 60.4%
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Old 03-25-2005, 12:13 PM
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Can't you balance the car with the driver's weight in it, or am I missing something?

That's the way mine was balanced at Speedware, so the total weight was the weight of my car plus the driver balast, and I have to subtract my weight from the sheet to get the car's weight. (2550 with a little less than a 1/2 tank, a porker, even though I have quite a bit pulled out)
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Old 03-25-2005, 12:26 PM
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You want to set up the car as it will run, including assuming a certain fuel load. Of course you want to do the corner balance with the driver, and ideally this means you will even do the alignment with the actual driver or a representative weight. Even representative weights are compromised however because of the way a body spreads it's weight out. One exception is if a person will always have a passenger, then sometimes it's better to scale without a driver or representative ballast.

The thing is though, most production car chassis are so flexible that the margin of error is quite large. Tube frame fabricated race cars are an entirely different beast, where chasing a few pounds or percentage points in cross will actually be discernable in balance.

FWIW, we did do the balance with Craig in the car, that's the principle reason the RR corner is heavy.

It's true that there are as many opinions about the scaling methods and objectives as there probably are about tires or brake pads. I just happen to feel that compromising the cross equity for even front weights has practical benefits in 911s in particular, with their static and dynamic weight distribution biases.

Many of the formulas don't account for the objective of trying to equalize the front weights to promote maximum braking capabilities. On the face of it, it would seem that a heavy RR would destabilize the chassis under acceleration, but the G sum that can be reached on the brakes will always be vastly higher than any acceleration the engine can produce.

Just my 2 cents.

Last edited by Randy Blaylock; 03-26-2005 at 07:37 AM..
Old 03-25-2005, 12:45 PM
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By the way...it is not a reasonable target to say you want equal weights on the front...this runs counter to your point that we can't expect equal left to right balance because of left weight bias. True enough. So you can't target equal front weights as "good" for the same reason !

Equal sums-of-diagonals also is not proper as we're not talking about a symmetrical "Formula" type car. We're talking a left-bias tin can car.

Whether or not swaybars were un-connected is moot if they're not adjustable ( afterwards). I don't recall if these were stock ( non-adjustable) sway bars or aftermarket with adjustable drop links.

Wil
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Old 03-25-2005, 01:05 PM
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I am confused, as usual. There was a thread recently showing the weights of Moses' car done at TRE I think. They were very even left-right. Both at the front and the rear. Was there not weight in the car when it was done?

Jeff
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Old 03-25-2005, 01:29 PM
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Here were my numbers from my recent corner balance.
1969 911 E race car with fiberglass fenders, bumpers, hood, decklid. I am saving $$$ up for doors and a lexan windshield.

These numbers are with me in the car:
LF 398 RF 359
LR 680 RR 641

Front weight 757 (36.4%)
Rear weight 1321 (63.6%)

Total weight of car without driver 1904 lbs (3.5 gallons of gas)
Total weight of car with driver 2078 lbs (3.5 gallons of gas)

Steve at Suspension Specialties did great work.
I was VERY happy with the results.
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Old 03-26-2005, 07:24 AM
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Wil,

I don't follow the logic that having an objective of nearly equal front corner weights is congruous with a left/right balance.

Please correct me if I misinterpreted your post, you define side to side balance or equity as a sum of front and rear weights, compared to each other.

These are two separate states. I have already articulated that I am willing to accept a RR bias as part of a compromise package that includes the objective of even front weights and diagonals. I target nearly equal front weights for a specific reason that I also articulated, not related to the objective of equal side to side, front and rear sum equality.

I also articulated that there is a compromise between my two objectives of equal front weights and diagonal equity. The inherent flexibility in a production chassis helps mitigate the margin of error of this compromise. It's a matter of opinion, I guess, about the priority of objectives, I happen to see these two things at the top of the list.

Any chassis with weight jacked into it diagonally will handle differently in right turns versus left turns. The degree is dependant upon chassis stiffness and resistance to torsional bending of course, so naturally a production chassis will not exhibit the tendency as much as say a late model stock car or F1 car. A chassis with a LF/RR bias will almost always turn left better, and the same is true for a RF/LR bias.

I agree with your point about sway bars, of course the bars were adjustable and subject to attachment without preload after balancing.
Old 03-26-2005, 08:02 AM
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Just had mine done at Johnsons in Torrence (nicest guys in the world) Didn't take a picture but my 82SC with full tank of fuel with a milllion deleates and upgrades came in at 2382 lbs.
What a difference bettween factory settings and running .75 neg front and 1.5 neg back. Off course the new 21/26 and carrara sway bars help
Steve
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Old 03-26-2005, 08:19 AM
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Randy:

Sigh.....this comes up *every* time a new poster posts results and thereafter asks questions "why"...and I end up in a long, protracted discussion as to "why"...and it is challenged although reference material and dialogue exists in the archives...by people who asked the same questions before.

Allow me this....please look up this topic along with companion keywords "Chuck Mooreland" and "Wil Ferch"....read it over thorughly..if you would please. Then contact me here or via PM and we can discuss further. I would be most glad to go over fine points that still may be unclear.

Basically ( in short-form summary) it's like this:
You want each corner to actually carry its apportioned weight that already exists on the car. For US cars, the steering wheel, battery, etc, are on the left, and this imparts a left-bias....as built. Similarly, the engine is in the rear and carries about 60% of the weight. You pretty much say the same thing. Now....knowing this....why would you "target" equal front weights? Doing so imparts a "weight-jacking" that does not correlate with the inherent right/left difference that exists due to how the weight is apportioned in the car. Same for front/rear. You wouldn't ( for example) "target" a 50/50 front-rear balance, when the "Actual" weight distribution is 40/60. See?

For best cornering and best braking performance....target each corner to its apportioned weight.

Wil
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Old 03-26-2005, 10:32 AM
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Wil, I've met Randy and trust me he is plenty knowledgeable and an accomplished driver. (ex Pro I believe)

There is a second valid school of thought on corner balance; It says that for maximum braking performance, you want the two fronts to be equal in corner weight.

Randy is simply striking a balance between the two schools, fully recognizing that improved braking performance is at the expense of equal cornering performance. Everything has a compromise.
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Old 03-26-2005, 11:04 AM
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I have no intention of "talking-down" to Randy or anyone else on this board. Long time posters here understand my history in this regard.

Randy, I apologize if I came across in a seemingly difficult manner.

I recall the *exact same* type of dialogue that preceded Alan DuBoeff's refusal to come back to a Rennlist discussion board....whereby he quoted his "expert" garage as wanting to set equal front weights ...for the same reason......and getting substantial push-back that this isn't the best way to go. Unfortunately....got nasty...and nobody wins under such circumstances.

If indeed two schools of thought are likely on this topic...let's examine each one using a "crazy" (extreme) example. Lert's say the inherent weighting of a car puts fully 55 % of the "front" weight on the LF...as if a reasonably sized guy were always sitting on your LF fender. Now we'll look at these two schools of thought:

Method 1--> setting up the car so the corner balance "matches" this LF bias.
Method 2--> setting up the car with equal L-R front weights....

During heavy, threshold braking....which method will result in severe one-wheel, wheel lock ??

I'm open to dialogue on this.....

Wil
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Old 03-26-2005, 11:19 AM
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Pardon my ignorence but in a purely mathlematical sense, if the contact patches of each tire are carring the same weight and breaking under the same loads neither one should have a tendancy to lock up before the other, correct? If one is carring sbstantually more load it would lock up first right? always thought that sitting on a scale the car doesn't know how it was built just that it wants to be balanced side to side,right, wrong?
Please don't jump down my throat, just my 2 cents
Steve
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Old 03-26-2005, 11:43 AM
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