Goost,Originally Posted by Goost
My quantitative interpretation of a "small" effect is that it is one, that if ignored in a calculation, still leaves the calculation within whatever error margin you expect.
By contrast, anything with a "large" effect makes a real mess of your numbers whenever you ignore it. Much as the Matlab generation love to quote numbers to five significant digits (or 8?, or 12?), most engineering calcs of the above type are doing well to be accurate in the first digit (ie. within 10%). If you ignore a "large" effect, then even that first digit will be way off.
IMO good engineers spend as much time considering how big their errors are, as they spend actually calculating the numbers.
(Edit: Of course, the modern fix for bad calculations is simply to include a "global grip factor", or some such, that can be tweaked AFTER you know the correct result, so that the calculated result now looks remarkably accurate!)
~~~o0o~~~
My "real data" comes from my "real life".... you rarely if ever provide real data for justification.
In this case, I think the estimate of 20% slip is nearly double the actual amount during peak braking.
I have attached a longitudinal force vs slip curve from real data...
In this case, my real data is the screaching noise my tyres frequently make when I stomp on the brakes of my NON-ABS equipped car. Stupid young person (or maybe really old person!) pulling out of a side street, or dog running out onto road, etc.
How many FSAE cars have ABS? Without ABS, or a really skillful driver, what chance is there of the longitudinal-slip remaining at its "optimum" value of, let's say, 10.846234%? Or, for that matter, anywhere near that peak?
Bottom line here, anytime the brakes are close to lock-up, the tyres will be absorbing more energy than the brakes.
~~~~~o0o~~~~~
The 27% error is what happens when "large" effects are treated as if they are "small" (ie. negligible).do you have any comments or insight on angel_aso's equations? that's actually the topic of concern in here.
His measured temps were (both) about 27% lower than the simulated ones.
We could attribute that to a lot of things, but until we nail down some real numbers for use in his simulation it's not going to improve his model's accuracy nor clarity.
More details of angel_aso's equations would help here, but it seems that he (she?) is assuming that the only way that the car's kinetic energy is dissipated is via the brakes.
As noted before, longitudinal-slip "heat-into-the-tyres/road" is a "large" factor.
And slip-angle drag is also significant (and not only during cornering, but also in a straight line if the car has significant toe-in or toe-out).
Smaller, but still worth including, is tyre-rolling-drag (higher for racing tyres than road tyres, but very roughly 1% of Fz).
And, of course, engine-braking drag.
And, of course, the everpresent AERO-DRAG.
And, in the spirit of the Second Law of Thermodynamics, you should expect that any other so far unaccounted factors will also have the same sign as those above, so they make life a little easier for the brakes by helping to slow the car down...
And then there is the whole business of that "h" coefficient.....
Z
 |
|
|
Reply With Quote