Preliminary design for Homemade wheel scales - any thoughts?

[sporqster]

I'm actually working on this with a 1400lb max working weight (per wheel)- so you could use this to set up your pickup truck if you wanted to. The load cell is basically a surplus industrial single-acting hydraulic cylinder with a gauge stuck in the port. That is then inside a piece of tubing with just a slightly larger ID than the OD of the cylinder. The weight of the wheel functions like a scissor jack to compress the cylinder, resulting in a pressure reading.

Based on what I've gathered from talking to the guys in the lab where I work, I should be able to get around +- 5% readings on the weight, most of the error in the cheap pressure gauge I'm looking at.

I'm eyeballing the total cost of this to be about $50 each, $200 for the set of 4, in materials. Not uber cheap, but heckuvalot cheaper than a set of longacre's.

Has anyone seen or attempted a rig like this? feedback? experiences? any reason you can think of that this is a really dumb idea? I'm sure someone has thought of doing something like this before, and I tend to think there must be a reason nothing like this is commercially available (to my knowlege).

[Shawn 357]

It sounds like a cool idea but it would depend on what you do with the car that you corner balance on that scale.
assuming a car ways 2500lbs and 60% of that weight is on the rear tires (911) and the scales are off by 5% then each rear scale could be off by about 40lbs and the fronts about 25lbs. This is of course making a lot of assumptions like the weight being evenly distributed between the two rear tires etc. For a street car I think that is accurate enough. I personally prefer more accuracy for a track car. Keep in mind that my math may be off and the rest of it is my personal opinion
Still a really cool alternative to paying gobs of money on a set of scales especially if you could make them more accurate than 5% Maybe you could build one and test how accurate it is before you build the other three.

[sporqster]

Originally Posted by Shawn 357

For a street car I think that is accurate enough. I personally prefer more accuracy for a track car.

This is for a street car occasionally driven on the track (not vice versa). I'm basically looking to get in the ballpark -

I might be able to do better with a better hydraulic gauge, but only the prototype will tell...

[DW SD]

I would be concerned about friction within the scisscors throwing off of the measured numbers. I know my old scissor lift had plenty of friction in the mechanism. In fact, it wouldn't drop, if it had no weight on it (though the setup itself weighed hundreds of pounds. I think your 5% measurement error assumes no friction or equivalent amongst all four. I don't think that is a safe assumption.

Can't you do something simpler and more accurate with a balance beam and maybe a spring with known force constant? Or maybe something with a known counterweight? How about something like the scales at the doctor's office?

[j_mancini]

Originally Posted by DW SD

I would be concerned about friction within the scisscors throwing off of the measured numbers. I know my old scissor lift had plenty of friction in the mechanism. In fact, it wouldn't drop, if it had no weight on it (though the setup itself weighed hundreds of pounds. I think your 5% measurement error assumes no friction or equivalent amongst all four. I don't think that is a safe assumption.

Can't you do something simpler and more accurate with a balance beam and maybe a spring with known force constant? Or maybe something with a known counterweight? How about something like the scales at the doctor's office?

Definitely a concern - all those pins and links are going to add drag on the mechanism and skew the results. You may be able to calibrate such a setup, however, but the best system will be something with the fewest moving parts. I'd agree with DW SD - something like a simple balance beam supported on a single fulcrum would provide the least internal friction. Making that stout enough to support a car...that'd be a trick. Thinking of a commercial weight system, each wheel might rest on a pad that is supported on a load cell.

[Dixie]

I'd be concerned about several things.

• It looks like it would be a PITA to setup and use.
• You still have to solve safely getting under the car.
• You still need to have a level surface.
• 5% accuracy perhaps, but what about repeatability?

[J P Stein]

I used a set of the cantilevered bathroom scales to corner balance.....they are cheep, but even cheeper when borrowed from a buddy.
I did the job before the start of the season and finally got onto some electronic scales at an SCCA National Tour event in late August. The overall weight was damned close & the corner balance was still good.

I've seen these scales around for 2-300 bucks on evilbay....dunno who makes em', but there's no need to reinvent the wheel, I figure.

[David]

http://www.aircraftspruce.com/catalog/topages/rugglescale.php

[sporqster]

I was counting on the fact that the assembly doesn't move (cylinder full of hydraulic fluid) to help with the friction issue. Assuming a car on top of the assembly should break the static friction (a lot bigger than dynamic friction by an order of magnitude) and remove the assembly slop. As for set up, there is no set up- assuming you have all the bolts in the thing holding it together (obviously don't have all those details in the assembly yet). I was also going to slot the top 2 1" steel tube components and hold the top joints apart with LH and RH threaded rod ends so that I could calibrate the assembly.

Getting the car up on there - the whole thing is all of 3" high, so a little ramp would do it (that's a 225-45 15 sitting on it in the pic.). Or you could just lift the car and put them under, but then you'd have to jostle the car around to get the suspension to settle on it.

[sporqster]

Originally Posted by 125shifter

http://www.aircraftspruce.com/catalog/topages/rugglescale.php

Am I reading that right?
$310 for 3 scales? I wonder how much for 4....

Hey JP - I'd love to borrow some scales from a buddy.... if I had a buddy

Trouble is I just don't know anyone hardcore enough to own scales well enough to ask a favor!

[TimT]

We now bring scales to the track. Prior to that we used a wheel load checker from Speedway it was handy to chek the cars balance after we would make changes