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PETER DIXON
Frequent User
Username: petenlinid

Post Number: 47
Registered: 8-2003
Posted on Sunday, 06 August, 2006 - 07:18:   Edit PostDelete PostView Post/Check IP

Can anyone advise where the approximate lateral point of balance might be on a 1980 SS2, with say half a tank of fuel.

Thanks
Peter
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David Gore
Moderator
Username: david_gore

Post Number: 634
Registered: 4-2003
Posted on Sunday, 06 August, 2006 - 15:04:   Edit PostDelete PostView Post/Check IP

Hi Peter,

If you have access to a weighbridge, you can work this out using the following formula:

1. Measure weight of car [M1] with front wheels only on weighbridge.

2. Measure weight of car [M2] with rear wheels only on weighbridge.

3. Measure distance [L] between mid-points of front and rear tyre contact with the ground [wheelbase].

4. Balance distance [B] from front tyre contact mid point is calculated using the following formula:

B = (M2 x L)/(M1 + M2)

Regards David
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Bill Coburn
Moderator
Username: bill_coburn

Post Number: 704
Registered: 4-2003
Posted on Monday, 07 August, 2006 - 00:02:   Edit PostDelete PostView Post/Check IP

There must be others who are wondering why Peter would want to know or more honestly what the Hell is it? So I asked. I am sorry but I got an answer from my son and heir. This follows:-

Simple answer: the lateral axis is the axis parallel to the axles, the longitudinal axis is the one parallel to the door sills. The lateral balance point is the point (or line) along which the car can be balanced without tipping left or right. If Peter Dixon has asked about half a fuel tank, this infers that the fuel tank is not mounted symmetrically on the lateral axis - which means there will not be equal quantities of fuel on each side of the car.

Another answer: He may also be referring to the neutral steering point (NSP). This is the imaginary point along the longitudinal axis at which a lateral force will not turn the vehicle. If the force is applied forward of the NSP, the car will turn in the same direction as the force and if the force is applied rearward of the NSP, the car will turn in the opposite direction.

A bit more background. A tyre generates lateral force by establishing a slip angle - the angle between the direction it is pointing and the direction it is going. The relationship between the slip angle and the lateral force the tyre generates is dependent on a range of factors such as sidewall stiffness, which is affected by things such as inflation pressure and load.

In a car with identical tyres, inflated to the same pressure and with the same load (50-50 weight distribution), the NSP will be half way between the front and rear axles. Adding more weight at one end will normally move the NSP in that direction.

The significance of the NSP is its relationship with the centre of gravity.
This will determine whether the car has a general oversteering or understeering characteristic. If the centre of gravity is in line with the NSP, the car will exhibit neutral steering. In practical terms, this means that the angle of the steering wheel will not have to be adjusted once the car has started cornering. If the centre of gravity is forward of the NSP, the car will understeer. In practical terms, this means that the driver will have to increase the steering angle once the car has started cornering.
Similarly, if the centre of gravity is behind the NSP, the car will oversteer. In practical terms, this means that the driver will have to reduce the steering angle once the car has started cornering.

This understeering and oversteering characteristic is only one of several factors that will determine if the car as a whole will understeer. For example, a roll steering geometry in the rear axle will reduce understeer and increase oversteer.

To calculate the NSP for a 1980 SS2, it is necessary to know the slip angle function and the load on each tyre.

Hope this helps.

Are you contemplating the spitting of the car Peter? Many years ago in the States an old friend had a heart attack while working on his Phantom II. No more bending was the medical direction. No problem, the whole chassis sans coachwork was indeed spitted and could be rolled over to any position required. I would love to do this with the Phantom which is at that critical height so that to remain in position one has to latch one's manboobs over the top of the valance and secure them with paper clips. It is not very comfortable.
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Patrick Lockyer.
Grand Master
Username: pat_lockyer

Post Number: 600
Registered: 9-2004
Posted on Monday, 07 August, 2006 - 02:05:   Edit PostDelete PostView Post/Check IP

Cripes Bill is it a CX and i thought it was safe to use the "A bar" with the prop disconected to recover the car as the Ackerman setup did the rest.
Help Richard refresh my memory i am getting confused.
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Bill Coburn
Moderator
Username: bill_coburn

Post Number: 706
Registered: 4-2003
Posted on Monday, 07 August, 2006 - 14:30:   Edit PostDelete PostView Post/Check IP

Patrick
Don't blame me I am just the poor hapless ignoramous. Interesting you never see 'A' bars in Australia. They were common in the States. The big camper busses towed a small car behind them for commuting.
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bob uk
Unregistered guest
Posted From: brig-cache-4.server.ntli.net
Posted on Monday, 07 August, 2006 - 03:26:   Edit PostDelete PostView Post/Check IP

I assume that by lateral front to back is meant.

The Shadow has 50/50 weight distribution so the balance point will be half way between the wheel base( about 5ft ) and at the same position of the jacking point.

If side to side is meant then again it is in the centre. The weight of the fuel is off set by the weight of the spare wheel and bearing in mind that the car weighs 2.3 tons then the weight of the fuel will make hardly any difference.


How does a Porsche 911 goes round corners with all that weight out side the wheel base and not oversteer big time ( within limits )

It is all to do with polar inertia moments.

An ice skater spins faster by bring in their arms so that less weight is outside the "wheel base" and spins slow by putting arms out so that more weight is outside the "wheel base".

Mid engine cars when they let go spin out very fast where as a Porsche 911 spins out much slower.---- and gives the driver a better chance of getting the car back in line. However the 911 is more likely to step out of line in the first place due to the pendulum effect. The experienced driver knows this and is ready to put in a tad of opposite lock just as ( before) the rear end starts to slide. Where as the mid engine jobbies can let go so quick that they cannot be caught and into the bushes you go.

Many years ago I was trying to road test a 911 on a wet road and was so scared of the 911 reputation that I was driving it at 30 mph. My boss took over and he explained by letting the car slide a bit and correcting it then driving a bit faster and doing it naturally in every corner, and the car ( as a passenger ) felt very well balanced and bloody fast. I didn't know cars could corner that fast, and the limited slip diff was working ok after I rebuilt it.

So Porsche Engineers are not so mad after all.

(Message approved by david_gore)
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David Gore
Moderator
Username: david_gore

Post Number: 635
Registered: 4-2003
Posted on Monday, 07 August, 2006 - 17:06:   Edit PostDelete PostView Post/Check IP

With all due respect to BBC's son, I suggest his interpretation is not correct; he is correct about the lateral and longitudinal axes.

However movement involving the lateral axis is best described as "pitching" where the front moves up as the rear goes down and vice-versa.

Movement on the longitudinal axis is best descibed as "rolling" where the left side goes up as the right side goes down and vice versa.

A practical illustration is the behaviour of a boat encountering waves:

A boat facing the waves will pitch up and down as the wave passes under the boat.

If the boat is side-on to the wave, the boat will roll from side-to-side as the wave passes under the boat.
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Simon Coburn
Unregistered guest
Posted From: cpe-58-166-46-36.vic.bigpond.net.au
Posted on Monday, 07 August, 2006 - 22:24:   Edit PostDelete PostView Post/Check IP

Bill Coburn's son here. I like the Porsche anecdote. It hadn't occurred to me that an "outboard" engine would increase the polar moment of inertia and provide some warning for loss of traction. Watched Top Gear tonight (we're a year behind the UK) and watched a new Cadillac doing four wheel drifts.

I gather from the correction that we're not talking about neutral steering points.

I agree with David Gore's formula except that it is for calculating the longitudinal balance point (ie. the point along the longitudinal axis). The same formula can be applied for the lateral axis by using the combined weights of the left hand wheels and the right hand wheels.

I just want to know what someone is going to do with the answer.

(Message approved by david_gore)
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bob uk
Unregistered guest
Posted From: brig-cache-4.server.ntli.net
Posted on Saturday, 12 August, 2006 - 21:20:   Edit PostDelete PostView Post/Check IP

The only reason I can think of for the question is that his drive is tight and he needs to put in a turntable.

We had a turntable in a garage I worked in and we used to put one rear wheel on a car on the turntable and when someone stepped on to the turntable we would drive the turntable round and give them a fright.

Of course one day someone over did it and someone got thrown off the table.

We all got a lecture about H&S.

If he is building his own turntable. put rollers around the circumferance as bearings then the weight distubution is not so important.

(Message approved by david_gore)
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PETER DIXON
Frequent User
Username: petenlinid

Post Number: 48
Registered: 8-2003
Posted on Friday, 18 August, 2006 - 18:25:   Edit PostDelete PostView Post/Check IP

Goodness me, what a to do I have caused with my query. My apologies for a slow response, my PC has been down. Thanks for all the responses. I was thinking building a low level (750mm) car lift similar to Kwiklift. As an alternative design I was considering something on a seesaw principle. Hence the need to know where the balance point is. If anybody has done something similar, or has any ideas and can let me have the benefit of their knowledge and/or experience, it would be greatly appreciated.

Cheers
Peter
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bob
Unregistered guest
Posted From: brig-cache-4.server.ntli.net
Posted on Saturday, 19 August, 2006 - 22:50:   Edit PostDelete PostView Post/Check IP

Get a second hand 4 post lift with wheel free .

Your overhead clearnce will dictate how high you can go.

4 post lifts can be weather proofed for out side use.

Local council may require planning consent.

4 post lift are about 500- 1000 used. Which is going to be cheaper than making your own and you know it will be safe. They are not differcult to install either last one took me 3 hours with one helper. Good solid floor already in place. TI- bainbridge.--- 3 phrase allready in place.

(Message approved by david_gore)