Bathroom Scale Cross Weighting

[wret]

I believe I have developed a reliable cantilever device that can be used with a home bathroom scale. After initial use it seems to deliver repeatable results. Most of the materials were scrap I had laying around. Additional hardware was less than $20. It is a very low profile design, so I didn’t have to lift the car onto huge piles of scrap wood. I know a little about scale design (a lot, actually), so I know it works. It has a 4x reduction so 1000 pounds of weight from a front wheel only weighs 250.

Here is a drawing. If there is any interest, I’ll write up some simple instructions.

 

[terefic181]

That's great, let us know what your complete dimensions were.

The cantilever type devices utilizing a bathroom scale do work. They have been around for quite a while and is published in one of the earlier suspension setup, books. Someone was selling them fairly inexpensively if I recall correctly. I built one a couple of years ago from square tubing and an aluminum plate that worked on a 3:1 ratio, and it worked quite well for getting cross weights and corner weights setup for my car.

When I find the book I'll add an edit to include it.

(edit) The publication was Carrol Smith's, "ENGINEER TO WIN", ON PAGE #219.

I see you have a swivel device on the tire platform. Can you drive up onto this scale?
Scratch the above , I see the swivel is in the wrong direction.



Tom

 

[wret]

The pivots are necessary to prevent changes in position of the load or tire angle (camber) from affecting the reading. Without them impossibly precise alignment would be required to get repeatable readings.

I’ve read about people trying this but I was not able to come up with any actual photos or plans. I got the impression that most dismissed it as being fundamentally inaccurate.

Here’s picture of the scale in use.

 

[MNGSX]

In the book they were called rug(g)les scales or something...

 

[terefic181]

Heh! Wret how goes it,

The Pivots: The way the one in the book is set up, and I set mine up, it uses angle iron on end to give a smaller point contact for more accuracy instead of square tubing.

MNGSX,

Yes it's the name of the guy that I guess originally came up with the idea, or so says Carrol. The name of the Company is Jpr Development an they are as far as I know at:

Jpr Development
1747 E Avenue Q
Palmdale, CA 93550
661-274-1652

I hope I'm not taking any buisness away from you Wret

Tom

(edit) also re: accuracy. If you can reproduce the reading it doesn't make to much difference about what the scale shows as far as corner and cross weight are concerned. The total weight may not be correct but the reference between each corner and the percentages will be.

 

[wret]

I hope I'm not taking any buisness away from you Wret .

I think it may be the other way around. I just gave it away.

 

[wret]

In the book they were called rug(g)les scales or something...

Ahhh, more info with that term. Here's a neat calculator for the math impaired:

http://www.rennlight.com/cgi-bin/balance.cgi

 

[terefic181]

WRET:> I’ve read about people trying this but I was not able to come up with any actual photos or plans. I got the impression that most dismissed it as being fundamentally inaccurate

That's OK, I'll bet the wheel is re-invented every day. The thing is you came up with it yourself from scratch. All I did was copy something someone else had done.

You Da Man!!!

Tom

 

[GTM]

That's OK, I'll bet the wheel is re-invented every day. The thing is you came up with it yourself from scratch. All I did was copy something someone else had done.
You Da Man!!!
Tom

When I worked as tech inspector for SCCA we just used a block of wood supporting one end with an angle iron unattached, a 2"x12" x 6' plank. The other end was placed on the scale, the plank was scribed at intervals defining if it was 1/4, 1/3, 1/2, 3/4. We could get within 5 lbs accuracy for each wheel.

Cheers,
GTM

 

[mitsugsx95]

Excuse me for the dumbness, but do you have to use this to weigh one wheel at a time, or do you need to make 2, both for the front then do the rear?

 

[wret]

You could make more than one, but I wouldn't. There inherent inaccuracies in both the scales and the cantelever device. These inaccuracies have minimal impact as long as they are the same for each wheel. It's a little tedious, but you must move the scale around to each corner, make adjustments, then check them all again.

 

[terefic181]

I agree with Wret. ^

I picked up 4 scales but never made additional Ruggles devices for them. The calibration of the different scales, and the devices themselves as Wret said, can be different and thus give you different readings from one to another. Using a single scale device eliminates one error factor and will give you more accurate readings.

It would certainly be much easier and faster, to set a car up, utilizing four scales, especialy if your going to be jacking weight from one corner to another, but it would be difficult to get all of them reading correct weights.

If your going to do this and want the most accurate readings, be sure to have the car on level ground and block the remaining three wheels up to the same height as the wheel on the scale device.

Tom

 

[mitsugsx95]

If your going to do this and want the most accurate readings, be sure to have the car on level ground and block the remaining three wheels up to the same height as the wheel on the scale device.

Tom

Thats what i was trying to get at. So, you must have the car somewhat level when the one wheel is on the scale, or the reading is going to be off, correct?

Also, will that design change with the size of the wheel? I have 17" wheels with 225/45/17 tires. Just wondering.

 

[terefic181]

Yes, the car should be level at all four corners and tire pressures set as you will run it, when you take your readings. If they're not, you are inadvertently jacking weight around the car.

I'm not sure what your trying to accomplish (corner weights and percentages or just getting the weight of your car), but here is an article that might answer some questions if your doing a complete setup.
http://www.grmotorsports.com/news/012005/understanding-corner-weights.php

Wret is much more knowledgable in this area than I, so maybe he'll jump in and give you some additional info.

Tom

 

[wret]

Thats what i was trying to get at. So, you must have the car somewhat level when the one wheel is on the scale, or the reading is going to be off, correct?

Also, will that design change with the size of the wheel? I have 17" wheels with 225/45/17 tires. Just wondering.

"Somewhat level" is good enough if you want "somewhat accurate" corner weights. A driveway might not be good enough. A garge floor will normally be pretty good.

Wheel size doesn't matter.

 

[wret]

I had an IM request for a parts list. Here it is for all:

Parts list:

1 - 8’ 2x4
1 – 1” x 3/16” steel bar cut into 2 – 6.5” pieces
1 – ¾” steel pipe cut into 2 – 20” pieces.
4 – ¾” pipe hangers with screws
Scrap ¾ plywood approx. 8” x 12” and 4” x 12”
2 - Scrap wood pieces 1” x 1.5” x 5”
2 - Scrap wood pieces 1” x 1.5” x 3”
8 – 5” x ¼” bolts with nuts and washers
4 – 3.5 or 4” dry wall or wood screws
1 – pc scrap wood 20” long with a half round or triangular cross section for the floor contact piece. This piece does not really have to span the width of the device. You could use two 1.5” pieces if it is sufficiently strong material to take the mounting hardware without splitting.
Wood scraps-enough to buildup under the other 3 wheels to the exact height of the wheel “pan” on you device.

Tools:

-Saw- could be done with a hand saw. I used a table saw.
-carpenter square
-wrench for the bolts/nuts
-drill with 1” spade bit for holes
-screwdriver
-Drill, preferably a drill press to drill through the cross section of 2x4
-1/4 metal drill for holes in steel bar and holes in 2x4

The dimensions are laid out in the drawing. Measurements do not have to be exact. If you want the weights to be a meaningful ratio, be sure to keep the distance between the floor pivot and the tire bar exactly 1/3 of the distance between the tire bar and the scale bar. It will still work as a corner weighting scale but it will be difficult to calculate the true weight of the vehicle. In other words you cannot simply multiply your weight readings by 4, you must find the new ratio and multiply by 3.87956 or some other unwieldy number.

 

[mitsugsx95]

Thanks for the parts list.

Any tips on adjusting it onto the scale or where to position it? Also, any recommendations on the scale, or just one like in the picture (i know obviously that you wont want a digital one ).

 

[wret]

A digital scale may work though mine is just a cheap spring scale. Position is not critical. Try to position it so the device rests near the center but not covering the gage. Look for a scale with at least 300lb capacity. Adjust the scale to zero with the device resting on it.

Be sure to center it under your tire so the tire doesn't contact the support beams and rests only on the pan. Mine was a little too narrow and I had to add a thickness of plywood on the scale pan to prevent it. I added an inch to the width of the drawing above. Be sure that you stack Wood under each will to the same height of the wheel pan.

The first time you lower the full weight of the car onto it, slide a piece of paper under the beams (especially where the bolt heads are) to be sure that no part of the device is in contact with the floor, except for the floor pivot itself.

 

[mitsugsx95]

Ok, ive got all the parts cut and layed out, but I'm still a little sketchy on the alighnment of the pivot < tie/middle 2x4 > scale platform. How high do you want the pivot off the ground, or does it not matter.

*** Correct me if im wrong, but i think that the 20" longer side section above has to be longer than the 20" indicated in the diagram. I noticed that in both the other pics that there was a noticable space between the platforms, where as i have the side planks at 20" and i have barley any space between the 2 planks and the middle tie bar. Not trying to criticise, but i just want to make sure I am following the directions corretly and make it right the first time

 

[wret]

Yup, you are right. That 20" was meant to be the distance between center of the floor pivot and the scale bar. Oops. Overall beam length is 24".

I'll see if I can find the drawing and correct it.

Just center the holes for the scale bar in the 2x4. My floor pivot is only half an inch thick. You won't be able to go any thinner than that but a little thicker is ok.

Placement of the middle connector 2x4 isn't critical. Keep it a couple inches away from your tire plate. Even and square.