[Vwdiesel] Fwd: mileage

[Erik Lane]

oops. I sent this to just James on accident.

On Sun, Nov 29, 2009 at 7:42 PM, James Hansen <jhsg at sasktel.net> wrote:
>>
>> My statement of it being pure kinetic is just because it's relatively
>> easy to lock up all the brakes and that is more of fair comparison
>> between the different vehicles. It's just a simpler way to test them,
>> is all.
>
> Except that is difficult to attain on a fully loaded semi.  In practical
> tests (collision avoidance) we have killed 2 tires, and spun 2 rims on the
> older trailer with the clamp on style rims.   I have NEVER seen a car able
> to break the bead loose from brake application, even on race applications
> where there is much more brake and traction.

That's not something that ever occurred to me. I've never seen it
happen, or even heard of it. That would make it a little hard to apply
full braking force... :)

>
> Wheel hop on a semi occurs because of loading of the suspension during brake
> application while the wheel is still turning but breaking.  When the tire
> eventually loses traction, the suspension unloads, driving the wheel forward
> and down.  It achieves a harmonic frequency almost immediately.  It's nearly
> identical to braking wheel hop in a car, but far more dramatic.

Do you happen to know if the hopping changes the braking distance?
Yes, there are times when the wheels aren't even touching, but when
they are touching there is even more normal force than if the wheels
stay in constant contact with the pavement. I haven't tried hard, but
I was wondering how that changed things.

>
> I once watched a fully loaded concrete mix truck wheel hop the rear tires
> when an air line broke and the maxi's went on. (safety feature when there is
> air loss)  This is spring applied by a giant spring in the air brake pot.
> He spun some of the lead tandem tires, (they went flat after), and broke a
> control arm. I was behind him at a safe following distance.  I used all my
> brakes to not hit him.  It stopped fast, remarkably so. The hopping was so
> intense, it unloaded all the trinkets that a concrete truck carries, like
> hose, troughs, shovels, etc...


It is very interesting to hear that even with the hopping that the
truck stopped darned fast. Whether or not it's identical to the
braking distance, it at least helps that when the wheels are in
contact there is so much extra force from having to reverse the
momentum of the axle and whatever weight it is bearing.