[Diy_efi] OT: ABS retrofit into older car...

[Adam Wade]

--- Mos <mos at sydney.net> wrote:

> so a violent pedal stomp in this situation results
> in more braking force being applied at the
> front, which in turn can lock the front wheels up as
> the braking force can exceed the maximal traction
> force. Once the weight transfers, obviously
> there's more tractive force available.

Weight transfer happens very quickly, but yes, the
scenario you propose can and does happen.  Rubber
brake lines actually make that much less likely, as
they act like a capacitor by expanding during the
initial moment of braking, allowing some transfer to
happen before a huge bite is transmitted.

> So the co-efficient changes with load?

Yep.  Basic physics states that with solids, the
friction changes linearly with load, the coefficient
is constant.  Tires are a different animal, and we can
all be thankful for that.  ;)  They respond in a very
non-linear manner, mainly due to the ability of rubber
to not lose material easily when exposed to shear
forces.  That means that the coefficient of friction
of a tire when loaded laterally is far in excess of
that you would get with even most other "compliant"
solids, much less regular solids (what do you think
the skidpad ratings would be for a car with wooden
tires?).

> Obviously the tractive force varies with load (and
> coefficient) but I didn't realise the co-efficient
> itself changes.

Yep.  That's how you get a non-linear response with
loaded tires.  That's why racing cars run wings.  The
drag doesn't increase linearly, so if the tractive
forces increased linearly on tires, wings would hurt
performance more than they helped.

> In the high school mechanics classes there was never
> any mention of the co-efficient of friction changing
> with load - what materials does this happen with?

I'm not aware of any other compund in regular use
beyond the alloys they are using in tires, composed
largely of natural or synthetic rubber.  It's not
exactly my fiend of expertise, but it's something I've
come across a few times in my travels.  You can bet
that the research done by tire companies is pretty
highly guarded.

=====
| Adam Wade                       1990 Kwak Zephyr 550 (Daphne) |
|   http://y42.photos.yahoo.com/bc/espresso_doppio/lst?.dir=/   |
| "It was like an emergency ward after a great catastrophe; it  |
|   didn't matter what race or class the victims belonged to.   |
|  They were all given the same miracle drug, which was coffee. |
|   The catastrophe in this case, of course, was that the sun   |
|     had come up again."                    -Kurt Vonnegut     |

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