* CORNER balanced - numbers from this...and the weight

[Randy Blaylock]

The numbers are indeed small, but measurable, and far from an anomaly since most, if not all cars aren't square as you stated.

The challenge is that this fact doesn't totally support the example used for instructional purposes to draw an important distinction between jacking weight and physically moving weight, an important concept that is usually the first major hurdle in understanding scaling cars.

Larry's scale figures are valuable for basic biases and gross weight, but the individual corner weights will be inaccurate in my opinion unless the floor the pads are sitting on has been proven to be dead level. Making suggestions for adjustments is a fruitless exercise in the case where they just threw the pads down and jacked the car up and dropped it on the pads. You also want to have slip plates or a platform of similar height to roll on and off the scales a few times to unbind the suspension.

FWIW, the model of apportioned weights is as old as 1981 as far as I can tell. One very popular book that promotes this method also has many other techniques and conclusions that have been proven over time to be inaccurate and over-simplified, and in some cases actually completely off base. Surely there are contemporary models that have evolved the technique of corner balancing in the past almost 25 years since then, based on trial and error, meticulous testing and some pretty big budgets.

I have to admit that it is appealing as a method that is applicable to a wide variety of different chassis and applications, but I haven't been able to find any testing data or other conclusive evidence to support it as the preeminent method for reasons other than those mentioned above.

Testing and data collection does, and has supported corner balancing using weight jacking to acheive varying objectives,
the "balanced chassis" being one of the most common. Most people these days define a balance chassis not by what the raw corner weight values are, but by how the chassis handles and what tire data looks like.

It is because of the fact that cars are almost never square that corner weights will never be equal, it's just a question of what is appropriate to meet one's goals. Apportioned weight seems to be nothing more than a theoretical approach, perhaps a starting point from which corner weight adjustments can be made, once again, to meet certain objectives. In this context, as a practical method it does have value as a basic principle, but I would suggest that there is also something to weight jacking that needs to be considered.

[Wil Ferch]

Randy:

I do think "yee protesteth too much ! "....

I believe you and others can see that I have taken a more broad view of this whole matter since my very first response. I have seen the value of purposeful weight jacking to promote the concept of better braking, for example, by targeting equal front weights. This, however is still a compromise...one I'd be willing to take more readily with a heavy-rear weight biased car like a 911....than ( for example) a front-biased FWD car. A FWD car set up this way would likely steer differently left and right...and would screw up rear brake balance.....

Nonetheless...unless you have some very specific data or testing that supports weight jacking beyond what was presented here....I offer the opinion that ( regardless if the concept goes as far back as 1981 or not...I think the law of gravity is even older, but no less valid)....coming into a shop with a badly screwed up car... and going out of a shop with each corner properly "apportioned" based on where the car's masses are located.....is not necessarily a bad thing to target.

Doing anything more is under the heading of "advanced concepts"....and goes beyond either a true corner balance or an attempt to get a fine edge on braking using equal front balance.

As a further point of clarity ( or confusion ! ) ...note that the factory manuals from Porsche only require ( as I recall) that left-right weights of any wheel not be "off" by more than 20 lbs. Even this simplified approach has its merits and debits.....but it at least gets you away from a grossly mismatched car...like sitting on a 4 legged stool with three long legs...or even a diagonal pair that is long !

Wil

[Craig 930 RS]

My brain is full, but I will add this note I received from a friend:

"My last CB at ******'s was closer to your CB balance than the perfect cross weights. My fronts are within 12lbs of each other...
Non ABS cars NEED close front weights for better braking!"

This is also what I surmised re front braking

Looks like another race shop uses Randy's methods -

[patkeefe]

We need a blackboard.
Wil, I see what you meant…the before and after adjacent totals don’t change. I had to think about LF/LR = RF/RR for it to make sense. I didn’t pay attention well.

Going back to the center of gravity thing, I was looking at the corner balance net results from a geometric point of view. The resultant actions being taken by either of these methods effectively alters the center of gravity. Just a couple of different ways to get there.

This discussion has been very educational for a novice such as myself, so thanks to all of the participants for the enlightenment!

Pat

[dickster]

Wil,

since you directed me here i blame you for my "unqualified" post!

Method 1--> you corner balance such that the LF actually carries 1000 lbs at rest , and the RF carries 500 lbs. My opinion is that during threshold braking...the heavy loaded LF is "properly" loaded..and the RF ( although lighter) is also properly loaded....and one brake will not be prone to lockup before the other, all else being equal. The opposing view will say that regardless of this.....the RF wheel will lock up first because it's normal ( downward force vector) force is less and therefore will lock up first. Friction is a function of normal ( downward) load.

This is the crux. I understand what you are saying but I am unable to come to the same conclusion. If the measured weight under the LF is 1000lbs and the RF 500, surely the RF WILL be prone to lock up first. I dont get your logic!

You stated what you beleive to happen but not why?

That seems to be like taking a car with perfectly equal corner weights (and even braking?), shoving a 1000lb weight in one corner (LF) and expect it to handle great!

Sorry if anyone has answered/responded to this already - i did try and read it all.....

[Wil Ferch]

Dickster:
I think I've come around to see that equal front loading ( regardless of where the static weight "is")..will promote a finer edge on braking performance. For a while...I was thinking that an apportioned weight basis might be better....but clearly...a lightly loaded ( by design or otherwise) wheel will lock first.

Pat Keefe:
I'm glad you're getting the hang of this...but unfortunately, no...the center of gravity remains *constant* for a given car unless actual mass is moved withing the car...like moving a battery ahead of time, or moving the engine front-to-back with different engine mounts.

I think the nuance I've learned from this episode is that a compromise using matched-corner weighting...will result in unequal front wheel loadings...which severely compromises braking performance. OTOH....a compromise setting that purposely weight-jacks the car for equal front wheel loading....optimizes front ( "important" end) braking perfomance, perhaps at the expense of L-R turning equality and rear braking. For a rear-heavy car, where rear brakes seldom lock...this is the better compromise.

Open note to Randy:
for those of us who have:
- fairly stock car ( no cage...therefore "flexible" chassis)
- stock or slightly stiffened suspension and bushings,
- no "specific" tuning for a particular track

....what sort of corner-weighting targets would you suggest? Would it default to the equal-front method used for Craig?

Wil

[Randy Blaylock]

Yes, equal front in conjunction with equal diagonals. Usually the diagonal equity will have to suffer a bit to accommodate the front corner equity. I usually see a bias around 2% in the diagonals in 911 chassis, something the chassis flexibilty masks. If a person finds more than 2-3%, then there is something going on with the pan, possibly a defect or collision damage.

Craig's percentages and biases represent what the I have found to be effective under the circumstances you describe.

There is one important distinction remaining to consider, and that's if you will be driving alone usually, or have a passenger.

If you will be driving alone, then scale the car with the driver, or a representative weight in the driver's seat and a little on the floor ahead of the seat (legs), to simulate the way the mass of the body distributes it's weight in the chassis.

If you will typically have a passenger, then you have to assume a range of weights in the passenger seat, and it's most practical to just scale with no weight in either seat.

[Wil Ferch]

Randy:

Thank you for this answer.....

Question: .... How is 2% or 5% cross-diagonal bias determined? Are you saying that one diagonal sum should be no less than 98% of the other diagonal sum, if you intend to abide to a 2% bias? Is that what you're saying?

Lastly.....you may have missed this nugget from previous work I did in this area, regarding the effect of a single driver, placed on a car seat. Here it is again:
...the effect of a 150 lb weight ( in the driver's seat) of a 911 results in THIS effect on the original corner weights:
LF= Adds 61 lbs
RF= Ads 11 lbs
LR= Adds 50 lbs
RR= Adds 28 lbs

Different weights ( other than 150) can be scaled up or down accordingly.

- Wil

[Randy Blaylock]

The 2% is a margin from the ideal 50/50 diagonally, so a 48/52 diagonal difference either way seems to make no distinguishable difference in handling left to right, under the criteria we have been using, true race car chassisor caged production chassis not included.

Thanks for the reference.

[Wil Ferch]

Randy:

For clarity....is this the way you're calculating "within 2%" ?

Example:

With Craig sitting in his car, his diagonals are:
1354 LR-RF
1253 LF-RR

Sum of diagonal is then 2607 ( no surprise..total weight!).

1354/2607 = 51.9%
Other diaginal is 1253/2607= 48.1%

So each is compared to "50%" ???

- Wil

[ttweed]

Originally posted by Wil Ferch
I think the nuance I've learned from this episode is that a compromise using matched-corner weighting...will result in unequal front wheel loadings...which severely compromises braking performance.

Severely? We are splitting hairs on this thread. All setups are a compromise, and have to be altered for each individual track if you are truly interested in getting 100% out of the car, which I dare say only happens with professional race teams with full time engineers! Are you changing your setup when you run a track in the opposite direction, other than putting the least worn tires on the outside? Probably not. How much difference is 20 lbs. of downward force going to make in the wheels traction when it has 400-800 lbs. acting on it? A few percent, at most. This may be barely perceptible in absolute, straightline, threshold braking, but I would suggest that most drivers would not be able to feel the difference or modulate the brake pedal that finely, and if there was the slightest amount of turning involved in the brake zone, the dynamic difference in loading of the front tires has already changed by more than that amount (i.e. , any time you are trail braking.) How much difference in straight line braking capacity is the camber you are running in your front tires making? Probably more than a few percent, but you do it because you want the grip in the corners. It's all a compromise.

I say get the corner weights in the ballpark and drive that pig. We are not pros here, and I would bet there's more wrong with the nut behind the wheel than with the handling of most of our cars with 20 lbs. of weight distributed incorrectly on one corner!

It's too easy to get caught up in the theory and black art of suspension tuning and strive for some unreachable perfection of the machine, when the truth is most of us are not Michael Schumaker and we are not on the ragged edge of the friction circle at all times during a lap anyway. There's more time to be found in eliminating driver error than there is in 20 lbs. of corner weight, unless you are on a level of competitive driving that I will never reach.

YMMV,
TT

[Randy Blaylock]

Affirmative Wil.

[Randy Blaylock]

Well said TT. In fact that's generally my nature as my friends will attest. In fact, I need to get the hell off this desk and get something done in the shop. Thanks.

[Wil Ferch]

Tom:

I think I say much the same thing in an earlier post on this thread where I openly question if ( for example) the forward weight shift during hard barking "overshadows" the numbers were chasing here. I'm quite sure I have already opened that "door".

The word "severely" may have been over-stated, but it serves to illustrate the different approaches Randy and I took going-in on this discussion.

No real argument, Tom, but I'm surprised at the aggressive tone of your letter.

For you to say ( "just get it into the ballpark").....is exactly the point of this whole discussion! Just what * IS * ballpark??? Would most of us here even have a clue ? I think I gave one possibility ( apportioned corner weights) and Randy promotes yet another way...probably better...using equal front wheel loading.

At least others ( me too) have some idea now...reading this... what "ballpark" is ....the next time we get up on the scales....

I guess I would turn the tables on you Tom and ask, why is trying to (at least) *understand* an issue "bad"...even if the dicussion takes a turn toward a fine-cut on the numbers ??? I, for one, find technical aspects interesting and have come away from this with a new-found appreciation of "another view".

- Wil

[Randy Blaylock]

By the way, my car is for sale over on the cars for sale site;

Street/Track 930

It handles o.k.

[Craig 930 RS]

Anyone who wants a turbo and passes up on this car need their head examined. Objectively speaking.

[Randy W]

Randy Blaylock is right (he is also very well respected here in the PNW). Greg Fordahl also told me that equal weight balance from RF to LF is most important, and rear weight balance sometimes has to be compromised as a result. Greg should know since he has set up suspensions for many Grand Am, LeMans and Daytona class winning 911s. Changing the weight balance between the fronts by 40 lb is pretty easily felt under threshold braking - not sure about 20 lb. though.

[Steve@Rennsport]

Randy & Wil:

Great thread and honestly, one of the best ones I've EVER seen here,.... Both cases were well stated, educational and informative. Its refreshing to see civility and professionalism maintained in an energetic and lively debate.


Tom:

Thanks for the reality check,.... Given that one is always in the pursuit of perfection, its easy to lose perspective. As Randy said, these cars are simply not all that stiff (without a good cage) and one must make compromises. Nevertheless, we all strive to provide the best handling and well-behaved car we can possibly do,...


For the record, I've tried both philosophies of corner weighting over the past 30 years and from a practical aspect, Overall, Randy's techniques have been more successful for me as I'll spend whatever time necessary to get a (race) car as close as humanly possible.

[Wil Ferch]

Thanks Steve.....

Well, that's 3-4 very well qualified votes for Randy's method ! .......

(The sound you hear is another fold created in my grey-matter. Learned something new today...)

- Wil

[911teo]

Gentlemen thanks to everybody. A fantastic thread.
Instinctively I was on Wil's camp but knew that I wouldn't been able to add anything useful to the discussion so I just kept reading.

My car will be corner balanced in the next 10 days... Now I know what BULLPARK figures we should aim for.


PS
Randy I wished you had decided to sell your car at the end of december. When I first saw your ad in the classified section I thought hard about going ahead with my project vs buying your car... But I had already bought the brakes and the engine... I was in too deep... Good luck with your sale!

Again thanks everybody