Where is a 911's fore/aft balance point?

[Jack Olsen]

I know it won't be the same for all 911 sub-models, or even be consistent from one identical year car to another (assuming they've been modified). I'd always assumed it was the jack pad receiver tubes, but when I think about it, my gut says that would let the nose come down.

Does anyone have a good idea of where the 911's 50/50 balancing point would be -- how far ahead of the jack pad receiver it would be?

I know -- I could take a couple of jacks out to the garage and test it. But I've also got a second question that's related:

How far forward would this point move with the car's engine and transaxle removed?

[haycait911]

sounds like you've got 'A' covered. anyone with a roller to check on 'B'? I could do mine but with all the f 'glass and a SWB to boot the balance wouldn't be a good comparison.

[DRACO A5OG]

Hi Dr Jack!

Wouldn't the amount of gas cause the point to shift, besides all the junk in the trunk?

Your Humble ACRA Track Student

Jim

[winders]

Jack,

Get some scales and find out. Do you have the corner balance numbers for your 911? It pretty easy to figure this all out if you do. Well, until you remove the transaxle. Then you have to weigh it again.......

Scott

[BK911]

When I jack my car up at the jack tube, the front goes higher than the rear. That would indicate the 50/50 point is behind the jack tube. For my car anyway.

[Wil Ferch]

I would imagine this Jack--->

- for a fully functional 911, the typical weight distribution is 40/60 ( maybe 38/62, but we're splitting hairs). Roughly....starting your measurement from the rear, if you move 40% of the car's lengh ( distance) toward the front, you should get close to the balance point or CG.

- with the engine removed, the "Shell" is roughly equal mass throughout....so maybe close to 50/50 of the length measurement will get you there as to balance point. Maybe not if there is a gas tank with some/full amount of fuel. It that case, it would be forward-biased a bit ....and maybe measure a point ( starting from the rear for consistant reference)...maybe 52-55% distance measurement from rear-to-front...to find this new balance point.

Right?...wrong?.....

[JeremyD]

what Wil said.

also depends on the amount of gas.

On my G-50 car - even with a full tank of gas - the balance point was behind the jack pad receiver.

On my 1974 - it's forward. It moves to being pretty equal with the receiver with an empty tank. That's with a mag case transmission and mag case engine.

21 gallons of gas at 6 lbs per gallon = 126 lbs - and in the nose makes a big difference

[sc_rufctr]

In my 78 I've always guessed it's just behind my hips... About were the kidney bowels meet the sill. (I still carry a spare) It's always felt like the car steers from there. Defiantly in front of the jack point.

With the engine and gearbox removed? I would guess about half way (or a little bit forward) between the front and rear wheels.

[Jerome74911S]

What about taking the driver's weight into account (including the fact that legs stick forward and weigh quite a bit)? The balance you ask about needs to consider this, it seems to me, if you are ultimately talking about handling dynamics.

[Wil Ferch]

I don't think he's talking about handling dynamics if one of his two questions deals with a bare chassis and no motor and trans. I think he is purely and simply asking where is the balance point with engine and without. Jack?........

[aston@ultrasw.c]

on my 86 the balance point is very close to the receiver

in fact before i removed the engine last time, I filled up with gas to make the shell a tad nose heavy

the time before that i stacked a couple of spare tires in the trunk to keep the schnozz down

[911pcars]

Jack,
What's the objective if I may ask? Are we going to see a transformation to a mid-engined layout, for example?

Sherwood

[tctnd]

Find the results of a couple cars corner balancing sessions. That will give you the total front and rear weights at the wheels. From there its simple to calculate the balance point. If you do this with several cars, you should get a pretty good idea of the typical range.

regards,
Phil

[Jack Olsen]

Thanks for the replies. I don't have any plans to change my car's weight distribution. I'm thinking about a way to put a lift in my garage and am trying to work out what would work best in terms of placement and configuration.

[winders]

Jack,

Do you have your car's corner balance numbers?

Scott

[Bill Verburg]

Originally Posted by Jack Olsen

Thanks for the replies. I don't have any plans to change my car's weight distribution. I'm thinking about a way to put a lift in my garage and am trying to work out what would work best in terms of placement and configuration.

The stock side jack point is near the fore/aft center of gravity, and not by accident.

my lift has a target on it, which can be seen in this pic, they want the Center of Mass of the car to be close to where the target is, but mour cars are so light that there is quite a bit of leeway

[porschenut]

One thing you could do for your individual car is just jack it up on both sides simultaneously and see which wheels remain on the ground. If it's the rear, the center of gravity is aft of the jack tubes. Then see how much effort is needed to lift the heavier end off the ground to estimate how much fore/aft of the jack tubes is the real CG.

[Wil Ferch]

Porschenut.....I may have to gently disagree as the fronts and rears may not see "air" at the same point of suspension travel.

If you have total front weight vs total rear weight....or if you have corner weights.....you should be able to see if the jack point is "it" or not.

Bill V, I think, has got it right as no matter how much heavier our cars got over the years.....the fr/rr for NA 911'a were always about 40/60 and the Turbo's were always about 38/62, with very little variation. Jack's car had a lot of mods and even altered suspension *geometry* , so I can understand his asking since his car might not follow the norm.

[Jerome74911S]

Originally Posted by Wil Ferch

I don't think he's talking about handling dynamics if one of his two questions deals with a bare chassis and no motor and trans. I think he is purely and simply asking where is the balance point with engine and without. Jack?........

Ah. I maintain my perfect score of missing the obvious point.

[Fishcop]

Jack, I would think given the age of your garage you couldn't be sure of what pressure that old concrete is rated for. I would think your best option for manoeuvrability would be a scissor lift like in Bill's photo.