View Single Post
Old 12-22-2009, 07:17 AM   #83 (permalink)
CapriRacer
Tire Geek
 
CapriRacer's Avatar
 
Join Date: May 2008
Location: Let's just say I'm in the US
Posts: 796
Thanks: 4
Thanked 393 Times in 240 Posts
Quote:
Originally Posted by Old Tele man View Post
...would be nice (better!) if those charts had X- and Y-gridlines to make it easier to "eyeball integrate" the straightness/curviness of those plots.

...and, since those were only "single-point" data collections, those "dips" might actually be from data "round-off" errors, etc. and not really "dips" per se.
A couple of thoughts:

1) The paper is almost 30 years old. The tires tested are a bit different than they are now - including the tire sizing methodology.

2) The traction part of the test is a "Braking" test - typically performed using a trailer specifically designed for this purpose. Unfortunately, lots of tire testing - including braking traction - are highly variable. The paper doesn't say, but usually there is a minimum of 3 different tires tested to get the data. I suspect they only did a single tire for each data point (The tires get destroyed during the test). I would take the "dips" as being based on real data (and not round off), but factor in the variable nature of collecting this data.

And one last thought to help the discussion:

Unlike the classical friction theory where there is a difference between the "static" friction force and the "sliding" friction force, tires develop their highest grip when sliding in the 10% to 20% range. Notice that the braking test data has "PEAK" values and not "Static".

So if we look back at the diagram posted earlier (the one with the rubber on top of the peaks), what happens is that the rubber is torn away by the peaks. That is what is generating the higher values and why it takes relative motion to generate the maximum grip in a tire.

Question: If you were to double the vertical load - and therefore the amount of penetration of the rubber below the peaks - what would happen to the amount of sliding force? Put another way, does the location of the tearing action change the force needed to tear the rubber off?

Barry
  Reply With Quote