Quote:
Originally Posted by dhanushs ... I guess what DerAlte meant was the not just Mu, but Mu X Area ... |
No, I meant for a particular car (unique make+model), the primary dependency is Mu, since Area (tyre footprint) will be constant in all tests / comparisons. Ignoring Area (actually assuming Area constant), one can say braking distance is proportional to Mu. Engineering language, not precise unambiguous mathematical equation - which would be the long form including wheel camber, toe, moisture content of surface, contaminants etc. etc.
In a crazily inhomogenous world, engineering makes assumptions to stay sane.
Quote:
Originally Posted by FlatOut ... using the tyre sidewalls as friction and developing such a deep wedge of gravel or sand that it becomes a high wall of resistance. ... |
Actually we meant resistance due to a material build-up transverse to the car, not longitudinal. The material build-up is due to the pushing of loose sand much like a bulldozer would. Sidewalls do contribute to braking, but since they are not exerting much force against the sides of the channel, that is not significant. Unless one angles the wheels, that is.
Quote:
Originally Posted by FlatOut ... If gravel or snow is so deep you can 'gouge-in' as you brake, the chances are you will have more trouble maintaining any speed than slowing down. ... |
By "trouble maintaining any speed" do you mean control over the car? The objective in this situation is to slow down by hook or by crook. And slow down it does, very similar to snow-boarders and skiers turning while stopping, to enable the larger area presented by board / skis in building up snow in front. 'Gouge in' as expressed by @ss-traveller meant the same, not longitudinal gouging.
Quote:
Originally Posted by FlatOut ... the tyre may reach the underlying tarmac quickly when a wheel brakes, but it is the wedge effect of a build-up of sand, gravel or snow in front of the tyre which often performs most retardation ... |
Misinterpretation - I didn't make supposition here. If the underlying surface is hard, sand / gravel / snow acts as a lubricant, deteriorating braking. Snow is even worse - it melts and then water reduces the Mu further. On black ice, the front tyres make the situation worse for the rear tyres. Reduced Mu = reduced retardation = less reduction in momentum, which increases the chances of the tyres simply sliding over whatever wedge was built up.
On a soft surface (obviously the same material going down deeper), this wouldn't happen as the transverse wedge will build-up faster than Mu reduction.
Quote:
Originally Posted by FlatOut ... 'Gravel' encompasses stones from 4mm to 64mm. ... |
In India, gravel is coarse river sand (usually reddish or yellowish brown) with particle size 0.5-2mm!
Sand is finer, and funnily not the color of gravel - it is usually grey in North India. In South India, gravel and sand are both the same color, with gravel being the coarse particles seived out of the river sand.
Quote:
Originally Posted by FlatOut ... What happens when this gravel is rounded as opposed to sharp-edged, what happens if this gravel is of various sizes and shapes? ... |
Unless the stones have something to grip into, they all roll no matter angled or rounded. In cities like Mumbai, where sloping surfaces like on overpasses are a nightmare in the rainy season, the road maintaining agencies embed 'sized stones' (that's what they are called in English here; in Hindi it is 'roDi' or 'roDa' depending on size) 10-15mm dia into the tarmac to increase Mu.
Quote:
Originally Posted by FlatOut ... This wedge effect is what causes aquaplaning ... |
The main difference is 'how much' material is there between the tyre surface and a surface that can cause planar separation in the material. The harder the substrate, the tougher it is for wedge buildup (and the easier for the material to act like liquid / lubricant). In deep enough water, planar separation doesn't occur causing the bow-wave buildup = wedge. If sand / gravel / snow is deep enough, the same will happen. That's what I expressed earlier.
Quote:
Originally Posted by FlatOut ... What becomes clear is how, although careful theoretical mechanical analysis of this hazard is beneficial, ... |
Correct. In informal discussion we tend to be empirical.
Quote:
Originally Posted by FlatOut ... sometimes it makes sense to switch it off and let the wheels lock. ... |
For cars where an ABS override is provided intentionally (most cars in India don't have that), the owner / driver is usually instructed at the time of purchase of the car why it is provided and under what circumstances should one use it. One wouldn't have the presence of mind to switch ABS off in mid-slide, so forewarned is forearmed.
Quote:
Originally Posted by FlatOut ... In an age where people think technology and science can provide every answer, it is still the individual human brain which works best, when used. |
+10^6!