Tuesday, August 5, 2014

"Individual Results May Vary"

"After:" weight born by each wheel of the Mini after adjusting the suspension arms, with driver aboard.  The "before"
varied by 70 lbs. across the front wheels and by a lesser but still significant percentage across the diagonals.

Here's one of those things I'd not thought about, but now seems obvious.  A guy I know just finished full restoration of an Old Mini.  He decided to put it on a wheel scale before he aligned it and aimed the headlights.  It registered a 15% difference across the front wheels, and a smaller but significant difference across the diagonals.  Not good for braking and handling, especially in a light car.  (This Mini weighs about 1600 lbs., soaking wet.)

My first thought was "This is race car stuff--who needs to put a restoration on wheel scales?"  My second thought was surprise that there would be so much variation in a production car.  The restorer pointed out that cars come off the line with much closer tolerances now than they did 50 years ago.

That got me thinking about production tolerance variance.  There's variation in the chemistry of steel batches, variation in the wire-drawing die bench (for springs), variation in spring height, and variation in chassis assembly tolerances.  If the variables "stack" to one side of the ranges, it's not hard to imagine a 2-3% deviance from nominal.  If each side happens to "stack" on the opposite extreme, 4-6%.  On a 3000 lb. car with 60% front weight bias, that could be a 90 lb. variation--right off the assembly line.

For Minis, substitute the variability of elastic hysterisis in the rubber springs, depending batch chemistry and mold tolerance.  An Old Mini restoration website recommends replacement of the rubber donut springs every 5-7 years, due to loss of elasticity.  This is similar to the recommended interval for replacement of unworn rubber tires as they age.

So, is it surprising to find a 5-10% variation in static weight-per-wheel on a restored or even new car? Unusual, but not impossible.

1 comment:

Wannabe Hotshoe said...

The BEFORE varied by 70 lbs? Wow....I'm impressed that there was that much opportunity/capability in the stock(?) suspension components to weight-jack that much of a "correction" in the Mini. I'm trying to think through the same math problem as applied to my Scirocco ....... I'm not sure where I'd start! I'm envious of your friend's mechanical knowhow.......

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