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Author: Subject: Corner Weighing - the wrong way!!
davidimurray

posted on 27/3/15 at 10:23 PM Reply With Quote
Corner Weighing - the wrong way!!

This may be a stupid question, so I'm ready to be shot down in flames .....

New engine in the car and I would like to check the relative corner weights - I,e, left to right and front to back.

Now I know the proper way is to get some proper scales, but there is no way I can afford some. Next option is some bathroom scales - which is feasible but then I hear comments about their accuracy.

Then I hit on a stupid idea (and had a quick try), why could I not simply jack up the car one wheel at the time, bathroom scale under a wheel then lower the car onto it. Read value, move scale to other side and then repeat. You have the same scales so no difference in accuracy, both wheels are lifted the same amount so I can't see why it shouldn't work left to right. I'm not convinced front to back will work. The absolute values are not right but the relatives should be.

What does everyone think - have I lost my marbles ?





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ken555

posted on 27/3/15 at 10:30 PM Reply With Quote
Thats how I was planning to do mine,

But I was going to put scale height blocks under each of the 3 remaining wheel to keep it level.






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gremlin1234

posted on 27/3/15 at 10:33 PM Reply With Quote
its a good indicator, but would be at lot better if each of the other wheels were raised the same height at as the scales, (just a bit of wood under each would help)
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leon51274

posted on 27/3/15 at 10:39 PM Reply With Quote
Just done mine. 8 bathroom scales 40 quid. Made some 1mtr long strips of chipboard at the same height asbthe scales so we could roll the car off to bounce up and down. Took bloody ages as you actually looking for the front to rear diagonal weight. Do a google for corner weight calculator.
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MikeR

posted on 27/3/15 at 10:49 PM Reply With Quote
if you're doing multiple scales, estimate how much the car weighs and then divide by the number of scales. Then get a fixed weight around that value on each scale and set them to all read the same. You'll be surprised to see some will weigh different amounts. This way they all will give the approx same value and therefore you can tune the weight.

The chipboard is a very good idea also, just make sure you only adjust settings on the chipboard as the scales aren't very good with side to side motion.

(ok, i've not actually done this, but its in my list of plans)

For the fixed weight your misses is usually a good bet (just don't let her see the scales that read light or you may find yourself with only 7 left).

If you want a good giggle or have too much time on your hands (delete as appropriate), number each scale and take a number of readings of fixed weights - eg type9, engine, your misses, yourself. You can then plot a graph of how the scales react over a set of weights to calibrate your corner weights even more ....

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daniel mason

posted on 27/3/15 at 11:05 PM Reply With Quote
It will not be accurate but will give a good indication. A fellow sprinter I know did the same on a flat floor. He go a reading of 421 kg. since then he's had it properly set up and corner weighted and it was 475 kg! That's a massive tolerance!
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leon51274

posted on 27/3/15 at 11:09 PM Reply With Quote
I must admit that i wasn't totally convinced on the accuracy but it will defo put it in the right ball park and if nothing else it was nice to have a tinker on a Friday afternoon!
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daniel mason

posted on 27/3/15 at 11:12 PM Reply With Quote
A full set up is only £150 by the best guy around! That s what I'd do, and what I did… and they seem to be pretty quick!
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leon51274

posted on 27/3/15 at 11:29 PM Reply With Quote
Was that procomp?
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daniel mason

posted on 27/3/15 at 11:43 PM Reply With Quote
No. Ian dayson at force racing! In my opinion there's no one better!

[Edited on 27/3/15 by daniel mason]

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leon51274

posted on 27/3/15 at 11:52 PM Reply With Quote
Where are they Daniel?
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daniel mason

posted on 27/3/15 at 11:57 PM Reply With Quote
WS15 4LD rugeley Staffordshire. Mainly sets up sprint and hillclimb cars but his knowledge is vast! You only need to see the cars he designs/ builds to see that. I'm lucky enough to have one!
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CNHSS1

posted on 28/3/15 at 01:09 AM Reply With Quote
Pretty sure Ian worked for Westfield in the dim and distant past so has experience of 7 types. He and his late father have been on the hills scene for donkeys years. His Dad Bob made the best 'unbrakable' driveshafts in the world





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mark chandler

posted on 28/3/15 at 08:14 AM Reply With Quote
I used clock bathroom scales as they can go over, before starting filled up 3 x 25 litre drums with water and zero'd the scales at 75kg so they should all be pretty close, worked a treat.

A few weeks ago someone mentioned using hydraulic jacks and psi gauges, if you paired two jacks to a gauge then you could just twiddle until they both had the same reading as actual weight is not relevant.

An alternative is to use a lever and spring gauge (or torque wrench with a bit of thought) and see how much effort it takes to lift each wheel by hooking on the rim.

[Edited on 28/3/15 by mark chandler]

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mark chandler

posted on 28/3/15 at 08:25 AM Reply With Quote
Thinking about it using a 1/2" torque wrench to pull down on a pivot would be very easy to make and would provide easily repeatable results. Just pump the tyres up really hard and lift so you can just slide a sheet of paper under the wheel.
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orton1966

posted on 28/3/15 at 10:05 AM Reply With Quote
Here's my 2p

Accuracy of the scales is almost irrelevant so "good" bathroom scales will be fine. The only thing that matters is the consistency of their reading i.e. If you place the same weight on the scales repeatedly, do you always get the same reading. Absolute accuracy is less important because corner weights are about finding the variance between each corner and the percentage of weight per corner will not be be effected by whether the scales over or under-read.

So I'd use one of these methods:

If using one scale.
Mark the four wheel contact areas on the floor, move car, check relative heights with a spirit level and shim with plywood or layers of hardboard or similar until equal.
Make three blocks equal to the height of your scales (when under load)
Check your car tyre pressures, fuel as desired, then roll it back into place, jack each corner and place blocks or the scale under each wheel, take readings and rotate the scales until all corners are weighed.
Do any changes regarding driver, passengers, fuel load and repeat.

If using four scales, as above but you don't need the blocks, however I'd rotate the scales and average the results, unless you know the scales do all give identical readings.

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steve m

posted on 28/3/15 at 10:52 AM Reply With Quote
Surely bathroom scales are not accurate ?

when I got married 32 years ago I weighed 9 stone, now im a around 12 stone, so where does the 3 stone anomaly come from ?
it must be the dirt and age of the scales, so I may have to buy a new one !








Thats was probably spelt wrong, or had some grammer, that the "grammer police have to have a moan at




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Slimy38

posted on 28/3/15 at 12:10 PM Reply With Quote
quote:
Originally posted by mark chandler
Thinking about it using a 1/2" torque wrench to pull down on a pivot would be very easy to make and would provide easily repeatable results. Just pump the tyres up really hard and lift so you can just slide a sheet of paper under the wheel.


I think it would be easier than that. The torque required to lift the car will increase until the wheel leaves the floor, but as long as it's a true vertical movement the torque will stop increasing and that will be your measurement. No need to worry about 'just enough for paper'.

I did also find this picture on the interweb;




It's probably not overly accurate, but it would be consistent enough to measure each corner.

[Edited on 28/3/15 by Slimy38]

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mark chandler

posted on 28/3/15 at 12:40 PM Reply With Quote
Damn, someone has stolen my idea before I had it

It's only maths to work out the weight, our little cars something like this would take up very little room for a track day using an axle stand as a support, get a garage at brands for instance the floors are planed smooth and level.

[Edited on 28/3/15 by mark chandler]

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ianhurley20

posted on 28/3/15 at 10:55 PM Reply With Quote
I think this was the jack with a pressure gauge to weigh with mentioned earlier - cost about £12

[Edited on 28/3/15 by ianhurley20]


[Edited on 28/3/15 by ianhurley20]





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orton1966

posted on 29/3/15 at 06:46 AM Reply With Quote
quote:
Originally posted by ianhurley20
I think this was the jack with a pressure gauge to weigh with mentioned earlier - cost about £12

[Edited on 28/3/15 by ianhurley20]


[Edited on 28/3/15 by ianhurley20]

The only issue is that you ideally want the tyre contact patch to rest on the scale but yes measuring pressure in a jack will work.
Again I'd follow the system outlined above i.e. 1 scale, use appropriate sized blocks under the other wheels and rotate, until all wheels are measured. If 4 scales, if you have any doubts about their accuracy, rotate until all scales have measured all wheels and average the results

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Slimy38

posted on 29/3/15 at 09:22 AM Reply With Quote
Can you expand on the need for it to be on the contact patch? Is that to make sure you're measuring at the correct point? I can imagine the normal place for a bottle jack on the chassis would be more than useless.

Re the torque wrench picture, they use the upper point of the wheel rim, is that the equivalent?

Ian, where did you get the £12 price from? I can't work it out to £12 even by buying the jack and the gauge separately and doing it locost.

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coyoteboy

posted on 29/3/15 at 12:38 PM Reply With Quote
So long as it's weighed at the wheel and not the chassis/arms you'll be close enough (car must be level, all suspension in the "rest" position, not partly jacked over by lifting one wheel at a time). As mentioned, repeatability (across the same point multiple times or multiple units if using bathroom scales) is important rather than absolute error. Personally I think 4x cheap(ish!) bathroom scales and 4x little wooden ramps to allow drive-on without damaging scales is going to be reasonable for non-race use. If you're THAT bothered about cornerweighting take it to a pro.

[Edited on 29/3/15 by coyoteboy]





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ianhurley20

posted on 29/3/15 at 09:50 PM Reply With Quote
quote:
Originally posted by Slimy38
Can you expand on the need for it to be on the contact patch? Is that to make sure you're measuring at the correct point? I can imagine the normal place for a bottle jack on the chassis would be more than useless.

Re the torque wrench picture, they use the upper point of the wheel rim, is that the equivalent?

Ian, where did you get the £12 price from? I can't work it out to £12 even by buying the jack and the gauge separately and doing it locost.



I bought the jack off ebay and inc postage it cost me £8.14. The 63mm 1000psi gauge cost me £2.93 off ebay as well. So my materials cost was £11.07. I have taps already as well as drill bits etc. I drilled a suitable hole after dismantling the jack for the thread to be cut for the gauge and a smaller drill bit to get into the pressure part of the jack, with a tap cut an appropriate thread, reassembled and replaced the hydraulic fluid. Job done.
For me it cost £11.07 it would be more if you have to buy the drill , tap etc.
IanR





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kj

posted on 30/3/15 at 09:38 AM Reply With Quote
I used 4 scales and wooden platforms rolling onto the scales after changing to bike engine, it handle better and drilves better too.





Think about it, think about it again and then do it.

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