weighing rear wheels

[davet]

I want to weigh both rears and adjust rear weight so I can get equal numbers on both sides. I have 2 scales that are exactly the same make and model. I place them side by side and put one wheel on each with the front DFW setting on a spacer to keep car level. The rear wheel opposite the raised wheel is always lighter even if I switch scales side to side. However, one of the scales always reads a little heavier no matter which wheel it's under. Both scales calibrate perfectly and both give exactly the same weight for the entire car when used individually. Is there a factor I'm missing?

[Stan Pope]

I want to weigh both rears and adjust rear weight so I can get equal numbers on both sides. I have 2 scales that are exactly the same make and model. I place them side by side and put one wheel on each with the front DFW setting on a spacer to keep car level. The rear wheel opposite the raised wheel is always lighter even if I switch scales side to side. However, one of the scales always reads a little heavier no matter which wheel it's under. Both scales calibrate perfectly and both give exactly the same weight for the entire car when used individually. Is there a factor I'm missing?

Are the scale pans exactly flat? The pans on my black HF Pocket Scales are depressed in the middle. If I am not careful to place the wheels exactly at the bottom of the depressions, the rear of the car tips left or right. This causes a shift in weight from one wheel to the other.

As a means of simplification, I have plywood panels that just fit the edges of the scales and saddles into which the wheels rest. That takes all the variance out of the readings!

[davet]

Stan, both are flat. They are giving me readings of .20 oz difference. Once it was only .1 oz. In the past when weighing the DFW I've raised and lowered the front 3/8" from the back and still the difference was only .02 oz. When I get a chance I'll level the table and level both scales side by side then level the car and see what I get.

If I flip the car upside down (raised wheel is now resting on the front spacer and rear wheels have swapped scales) the higher weight shows up on the other rear wheel so I know my thinking is correct that rear wheel opposite raised wheel is lighter but need an accurate measurement. I suppose I could just weight it once right side up then flip it upside down and weigh it again. Whatever discrepancy I have I could just split it and call it close enough.

[Stan Pope]

Good! You have avoided the most common pitfall!

Let's check the scales. Gather a sample weight that is about the same weight as one of your rear wheels is showing on the scale. Then use that sample weight on each scale to confirm that the scales are reading the same in that range. They should be, but .. maybe not.

If the sample weight weighs the same on both scales, then let's repeat your trial after
(1) assuring that the scale pans are level, and
(2) assuring that the rear wheels are at the extreme outer end of the axles. (This avoids another pitfall. The distribution should be measured with the wheels where they will be when the car is running!)

[davet]

I did not check to see if both wheels were at the heads. Also, in prev post I said flip car over, re-weigh and split the difference. I meant to say swap scales and re-weigh then split the difference.

[sporty]

I will add. A summing that both are equel.

I have seen right handed people or left handed people. When they drill the other side. Sometimes they tend to naturally not do it just like the natural side related to there dominant hand they use.
Odd. But seen it and did this myself.
How did you drill the axle wholes ?
And same o.d. on the wheels ? No bent axle ? Very true ?
You bending the rear as at all or drilling can't ?
This all may be non pertinent questions and comments.
But thought I'd ask.
Also. I have never felt that weight was the way to go to adjust the rear weight equel loads. If something is off.
Meaning not to fix a issue. Where there is more can't or off on the same height for the axle wholes.
The scales work for me to see if I have a issue and to try ensure my weight placement is close to balance on the rears as possible.
But not for correcting the other issues I mentioned. Because it is just adding more friction to one wheel. Masking the real problem.
Hope I explained this well enough for you to comprehend what I mean.
You might buy another scale of same make and model. To rule out overall scale difference between the two you have now.
I had at one time 4 of the same make and model and I paid like $6 total for each of them.
But I have seen that scale issue. Too. But also wanted to mention the other stuff too.

[sporty]

Added til bit. I have also used pin gauge same size. Two of them. Inserted same distance into axle wholes and placed rear on scales. To check weight. Balance. I do check mine in grams or the oz reading with the finer readout of numbers.
I tried axles before. But dang making sure the axle heads were the same too. Was a pain and I would loose them and seemed to take better care and storage of my more costly pin gauges.

Also when trying to achieve equel loads. I found often. My axle whole drilling was off. Like .003 to .008 and it applied the error and was my most common issue of Un equel weight loads. Next to canting not being the same.
Gets pretty picky. So if it's close and not dead on match. No worries. Had plenty of fast cars that were not perfect.
I also seen changes just from my imperfect weight pockets I cut out or the slop of gluing them in not being equel.
Hope this all gives perspective to your issue.

[davet]

I'm not sure how any of those slight differences could cause .2 oz variance. When I weighed my DFW I tested my numbers by raising the front 3/8" higher than the rear then lowering the front 3/8" from the rear. Even though the wheelbase is 5 9/16" as opposed to roughly 1 7/8" between my rear wheels the difference was only.02 oz. I can't see how an axle bent 1 degree more than the other could give me .2 oz variance.

I'm running bent rears at 1.5 degrees but right now straight axles are in the holes. The race axles and wheels are in a wheel tube. I have 2 of the exact same scales.

I'm not trying to correct a problem. My thinking in now equalizing the side to side weight is to hopefully reduce friction on the rears during a very long flat section for our Council race. I want to keep an aggressive COM yet have more weight on the DFW so I can run less drift. Less drift I think will keep us ahead of the pack. Just a theory and after reading againstthegrain's last thread I think it sounds good.

[davet]

I see I didn't answer how I drilled the axle holes. I used a ProBody jig and a hand drill.

[davet]

I'm not sure why this is but I did like you mentioned and pulled the wheels and rested the rear on the axle heads on the scales. This actually made the rear contact patches far enough apart that I could face both scales in the same direction. Now I have the same readings within .01 on both sides no matter which scale is on each side. The rear DFW side corner is much heavier but at least my numbers are accurate now.

I don't get it but next time I'll go to u guys first. I spent 2 hrs trying to get it figured out prior to pulling the wheels. Thanks guys.

[Stan Pope]

Equal weight when resting on the axle heads may not be your intended target. The car will actually rest about 1/4" inboard from the axle heads because it rests on the inside of the tread. Since your CM is offset, moving the support 1/4" toward the CM on each side will INCREASE the amount of weight on the DFW side and DECREASE on the other!

You can understand the result by looking at the leverage equations.

[FatSebastian]

...I could face both scales in the same direction. Now I have the same readings within .01 on both sides...

Assuming your platform is rectangular, try measuring a test weight by placing it on each of the four corners, and then in the center. Does it give the same reading each time? Not every scale will.

[davet]

You guys aren't making this easy for me.
Both of my scales give the same readings with a small weight in each and every corner of the pad and in the center of the pad.
I may not be able to get a good reading after all because in order for me to place the scales facing the same direction rather than both pads resting up against each other I need to keep the nails out about 3/4" so they both touch the scale pads. Even if the weights aren't accurate is the difference between the 2 sides accurate?

If I position the scales so both pads are facing each other I get different readings. The non DFW side will weigh .7 oz heavier then when I turn the car to face the other way and the non DFW side is other scale it will be only .2 oz heavier. If I swap scales I get the same thing.