Corner weight.

[Al Weyman]

Still need a car under 600kgs for it to work.

[GORDON STREETER]

The answer when using bathroom scales on a heavy car is not to place them directly under the wheels but to use a simple rectangular frame made from metal tube that you put one end under the wheel and the other end on the scales. Depending on how long the frame is the ratio will be altered to suit the max weight of the scales. As long as everything is measured and marked you can get repeatability. But I wouldn't use digi scales as they are too fragile and as said do switch off. But is that only when they have no weight on them ?

[johnlear]

Originally Posted by JamesH

You would need 4 bathroom scales, not 2.....

I meant that the car would need all of it's weight being carried on all of it's wheels, i.e. I was considering any flat level surface the car was sitting on to be 'ground', whether the car was resting on one, two or four scales. It can be done with less than four scales, it just gets a lot harder with two and a lot more harder with only one (and less and less accurate).

If the car is made tripodal by any means (take off or lift a wheel Citroen style, or jack it up in the middle at one end...) then any adjustment to spring heights etc will cause no change on the scales, only a ride height change. When the car is back on four wheels the corner weights could be just about anywhere.

[jedrinck]

I understood the first time, it was logical and well explained and I became aware of my thinking error. I shall cover my head in mud.

[dtype38]

While we're on this subject... can I ask about "flat floors" (the car setup variety, not the bottom of the car variety).

Two simple questions: For a basic clubbie racer just how flat and how level does the floor need to be to get a reasonable shot at a setup. I deliberately separate the idea of "flat" from "level" bacause the floor in my garage is reasonably flat (ie a smooth plane), but is a little bit inclined.

So what are we talking? Flat within 1mm? 5mm? 10mm? Level within 0.1deg? 0.5deg? 1deg?

Please don't answer with "as flat and level as possible" or I'll ask you to identify exactly which parts of which British race tracks are that flat and level!

[GORDON STREETER]

Personally in the workshop I have made sure that the base is level in both directions by using sheets of plywood. As to whether it makes that much difference I wouldn't know. If it was an F1 team I expect it would absolutely flat.
I would doubt if most of the drivers on 10/10ths would be able to tell the difference anyway, even if it was an inch or so different !

Don't forget a full or empty fuel tank will alter the readings if its not central.

[jedrinck]

I would assume it depends very much on the stiffness, suspension travel, tires and spring rates of the car. If you wanted to set up a Range Rover you could probably do that well enough on a not very level floor that would be intolerable for a single seater or so.

[phoenix]

Many years ago (when the world was in black and white) I saw Ferrari setting their corner weights in the paddock at Brands Hatch using aluminum discs in place of the wheels. If course they had a perfectly flat and level surface to work from too.

[jedrinck]

Was that because the world was still flat then?

[phoenix]

Originally Posted by jedrinck

Was that because the world was still flat then?

Almost certainly because the world was flat - but the paddock at Brands hasn't been so - not since before the last ice age anyway...

[david-]

bathroom scales is working very well if the floor is flat
but don't buy the cheapest ones.. they don't like it in the long run.
the accuracy I don't know.. +/- 5 kg perhaps?
I use it for placing things in the build.. for fine adjustments I wouldn't use bathroom scales..

[dtype38]

Thanks for the input guys, but I'm hoping that someone can give me some actual values. I take your point jedrinck that it's gonna depend on the stiffness and suspension travel.

What I'm thinking is it should be possible to calculate something. Eg if the floor is flat but inclined, then I'm guessing that the weight distribution error is caused by the actual centre of gravity being slightly offset from the correct position on the floor. So there'll be a small redistribution of weight on the tyres towards the direction of the offset. Knowing the weight of the car and the height of the CoG should produce something like X deg of incline gives Y increase in kg on one side of the car and decrease on the other.

Similarly for flatness, knowing the spring rates should be able to produce some calcuation that would say X mm out of flat would give Y kg increase in weight at that wheel.

Then you would just need to say that I want the corner weights to be within say 2kg of exactly accurate and backtracking would give a number of degrees of incline or mm of out of flat which can be tolerated.

Trouble is, there'd have to be a few assumptions and if's and but's and I'm not sure my maths is up to it.... so if anyone's got a rule of thumb? Pretty please?

[GORDON STREETER]

Er sorry dtype. I belong to the flat earth society
http://en.wikipedia.org/wiki/Flat_Earth_Society

[Al Weyman]

Originally Posted by david-

bathroom scales is working very well if the floor is flat
but don't buy the cheapest ones.. they don't like it in the long run.
the accuracy I don't know.. +/- 5 kg perhaps?
I use it for placing things in the build.. for fine adjustments I wouldn't use bathroom scales..

Whats the point in weighing this car in its current state?

[GORDON STREETER]

Al read his last line !

[Al Weyman]

I still dont quite get it as i would have thought you get things to the rear or central of the car as best you can then set it up when finished but I guess if you have the time and knowledge some agvantage could be gained but I could'nt be asked to be honest so probably why I dont get too far!