Dutton Kit Cars and their owners
P1050908b
Weighing the car one corner at a time. I put blocks under the other wheels to keep the car level. It weighed 623Kg in total without occupants. The scales were £4.50 each from Tescos.
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Comment by James Doulton on February 9, 2013 at 14:22
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I eliminated tyre deflection by measuring from the top of the wheel rim to the bottom of the wheel arch. Is there any chance that I just have the wrong springs? They look right to me in that they fit the fixings etc, but they are 4 leaves and some I have seen on-line are single leaf. Also they must be different on each side because the bodywork is over 30mm higher on one side, so maybe they are both wrong. All help welcome.
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Did I read that right, you think that its wrong, BV?
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Comment by Big Vern on February 9, 2013 at 13:53
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Wrongity wrong wrong wrong, wronger than a wrong thing thats very wrong! Your expectation of 140lb/in is about what I would have expected. (the nearest I could get with q562fdd was 150lb/in). Trouble with your method is tyre deflection. Also the inherant problem with leaves is stiction.grit and corrosion gets between the leaves and then they don't 'slide' giving what appears to be a massive spring rate. Its a little better if you can add weight rather than lifting the weight off the spring, I know Allan Staniforth suggests the static deflection method by jacking up the car but it tends to lead you right up the garden path, especially with leaves.
As a yardstick the rally boys are still using less than 200lb/in in many cases based on what springs you can buy so I don't think 380lb/in is right.
Over to you Andy S. what do you reckon?
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Well, I have now measured my rear suspension and it is NOTHING like what I imagined. Both springs have the same rate but are different shapes. The rate looks like being about 380lb/in whereas I was expecting a rate of 146 or so. Total rear suspension movement between wheel just touching the ground and the car fully supported by the springs is only 21 mm. That means that the front and rear rates are completely different even though the weight distribution is close to 50:50. Now what to do? I know, the brakes!
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The rear will be a little more difficult especially with leaf springs. I too had mismatched rear springs. My 'partner in crime' used his experience here and cut one of the leaves off with an angle grinder to soften the rear - I would suggest taking the leaf out in a way that it could be re-used if needed.
You can look at all the spreadsheets and run all the macro's but 'nothing beats putting it on the car and go drive it' as my old boss used to say, a comment aimed at engineers he thought were spending too much time in the office. Now car suspension is mostly developed on CAD models in an office, engineers moan they never get to go out!
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Yes, I can get carried away with the engineering for a time. Sometimes the theory is not nearly as useful as experience and gut feel. But I do like to get some 'facts', so that there is some basis for my decisions and for discussions. That was why I weighed my car in the first place - you can guess and guess and after a while you can believe your guesses, if you know what I mean. Although my measurements may not be as accurate as they could be, they are a million miles better than my guesses.
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BV you are making me look like an amateur! Oh, but I am an amateur - so that's okay :-)
It had never occurred to me to undo any bolts.
What I find most surprising about my little experiment is how straight the line is on my graph, it almost looks too good to be true. I am also surprised that your measured unsprung weight is such a good match to my graph.
Maybe I will repeat the experiment with my rear suspension tomorrow, maybe I can work out what is going on there.
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I measured a motion ratio of 1.42 on q562fdd which had the original dutton lower arms, measured by removing spring refitting damper and measuring the wheel movement vs damper movement at the damper rod. No other real way to measure it accurately. Unsprung weight measured at around the same time by supporting the chassis at the correct ride height then lift the wheel assembly up and slide the scales underneath. Need to have the other side front damper free as well or the arb screws everything up. As it happens I had 39.3kg for front unsprung. For the parasitic load issue I usually loosen the fixings to ensure the parasitic load is minimal. These fixings should be torqued at normal ride height so the parasitic rate only really has an effect at extremes of travel ie when getting close to full bump or full rebound.
To keep your data repeatable and reliable use some blocks of wood that are the same thickness as the scales/wood you have in the picture, that way the car should be level. Ideally you would drive the car up ramps onto the scales but that takes quite a lot of construction. I used to do that but now have access to proper corner weight scales at work :-)
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The total jacking movement was only just over 2 inches, so I don't think that weight transfer would be significant. I was more concerned about the suspension geometry causing the track to change as I lowered the car - I noticed that the wooden plank slipped sideways over the scales as the car descended as the track increased.
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I measured the leverage at 1.47 based on measurements and CAD layout. But I don't think that the measurements warrant 2 decimal places, so 1.5 would be close enough.
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