| Re: [GMCnet] Trip Repport (in segments) [message #343170] |
Tue, 07 May 2019 15:58  |
emerystora
Messages: 4442
Registered: January 2004
Karma: 13
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Senior Member |
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Ken
I don’t think I anyone is expecting 4 tires to apply more retarding force to the road than can 6 tires.
If one is braking properly in he same distance, they should use exactly the same amount of retarding force. We should not be concerned with the maximum retarding force.
As long as there is adequate retarding force to quickly stop the vehicle that is our concern. The problem is that the tires might slip so as to apply less stopping force.
As long as the tires don’t slip there is no problem. .
The no slip condition is
𝑣=𝑟𝜔
v
=
r
ω
, where 𝑣
v
is the linear speed of the centre of the tire/wheel/car, 𝑟
r
is the radius of the wheel and 𝜔
ω
the angular speed of the wheel, will result in the frictional force between the tire and the road at the point of contact being zero on a horizontal road.
When the engine tries to make the wheel rotate faster there is the potential of relative movement between the tire and the road at the point of contact so a static frictional force acts on the tire which increases the linear speed (and angular speed) of the tire/wheel/car.
This is the static frictional force acting in the forward direction.
When the brakes try and slow the rotation of the wheel down again a static frictional force acts on the tire which reduces the linear speed (and angular speed) of the tire/wheel/car.
That static frictional force acts in the opposite direction to the motion of the car.
So the static frictional force between the tire and the car always tries to make sure there is no relative movement between the tire and the road at the point of contact.
If there is slipping between the tire and the road (either during acceleration or braking) then kinetic friction takes over to try and reduce the relative motion between the tire and the road at the point of contact.
So in simple terms your have an number of forces acting on the car including the weight of the car, the normal reaction of the road on the car, the frictional forces between the tires of the car and the road and the frictional forces between the air and the car.
As long as there is adequate friction between the tire and the road so that the tires don’t slip it does’t matter if there are two tires, 4 tires, or more.
That is why I tried panic stops in my testing to make sure my tires weren’t slipping.
I really don’t care to get into any arguments with anyone about my use of whatever braking system I desire or how it compares to whatever anyone else wishes to use.
All I care about is whether it works for me. If it does then I am satisfied. If it didn’t then I would make further modifications until it did. If your system works for you that is all that matters but please don’t anyone tell me that I am wrong because of what I choose to use. This is beginning to sound like the tire wars of old. Do what you want to do and let me do what I want to do.
Emery Stora
77 Kingsley
Frederick, CO
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| Re: [GMCnet] Trip Repport (in segments) [message #343179 is a reply to message #343170] |
Wed, 08 May 2019 00:04  |
Richard Denney
Messages: 920
Registered: April 2010
Karma: 9
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Senior Member |
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Somehow we invaded Scott’s thread, or so it appears on the Gmail app on my
phone.
As Albert said, the reason for balancing front and rear brakes isn’t
necessarily to shorten braking distance, but to keep the rear of the coach
behind the front. No brakes at all on the back will do that, but the front
wheels will reach the limit of adhesion more quickly because their contact
patches are carrying a greater retarding force for the same stopping G’s.
Loss of grip on the front can cause uncontrolled understeer in a panic
stop. That means a loss of steering control. But it’s better than oversteer
which can cause the rear to pass the front.
Braking two rear wheels is an option, as is braking all four. Braking four
wheels really requires solving the tendency of the coach to climb over the
front bogies during braking, which is the point of the reaction rod. And it
requires front/rear balance. Brake balance is always hard with rear drums.
The servo action makes the rear brakes respond nonlinearly, while disks
respond linearly. So the front/rear braking balance changes with braking
force.
But the one-ton kit puts bigger brakes on the front, and it makes sense to
match that with a similar increase on the back to prevent locking the rears
up before the fronts. Four disks with reaction rods does that, compared to
the stock setup. And with rear disks with reaction rods, the front/rear
balance will only change with overall weight transfer. The stock setup was
adequate, but balance is upset by the same weight transfer, plus drum-brake
servo action and the unweighting of the rearmost wheels by bogie windup.
The stock setup will cause rear skidding compared to the fronts in an
emergency stop, and the combination valve is designed to minimize the risk
of that in a panic stop.
I suspect GM designed for the common case and then tested alternative
stopping scenarios to check for acceptable operation. They would err on the
side understeer—locking the fronts up before the rears. But there are too
many variables so the front brakes would have been over-designed. With more
linear rears, we can aim for a closer balance.
Rick “stopping force at limit of adhesion is weight times contact patch
times coefficient of friction” Denney
On Tue, May 7, 2019 at 9:46 PM Emery Stora via Gmclist <
gmclist@list.gmcnet.org> wrote:
> Ken
> I don’t think I anyone is expecting 4 tires to apply more retarding force
> to the road than can 6 tires.
>
> If one is braking properly in he same distance, they should use exactly
> the same amount of retarding force. We should not be concerned with the
> maximum retarding force.
> As long as there is adequate retarding force to quickly stop the vehicle
> that is our concern. The problem is that the tires might slip so as to
> apply less stopping force.
> As long as the tires don’t slip there is no problem. .
>
> The no slip condition is
> 𝑣=𝑟𝜔
> v
> =
> r
> ω
> , where 𝑣
> v
> is the linear speed of the centre of the tire/wheel/car, 𝑟
> r
> is the radius of the wheel and 𝜔
> ω
> the angular speed of the wheel, will result in the frictional force
> between the tire and the road at the point of contact being zero on a
> horizontal road.
>
> When the engine tries to make the wheel rotate faster there is the
> potential of relative movement between the tire and the road at the point
> of contact so a static frictional force acts on the tire which increases
> the linear speed (and angular speed) of the tire/wheel/car.
> This is the static frictional force acting in the forward direction.
>
> When the brakes try and slow the rotation of the wheel down again a static
> frictional force acts on the tire which reduces the linear speed (and
> angular speed) of the tire/wheel/car.
> That static frictional force acts in the opposite direction to the motion
> of the car.
>
> So the static frictional force between the tire and the car always tries
> to make sure there is no relative movement between the tire and the road at
> the point of contact.
>
> If there is slipping between the tire and the road (either during
> acceleration or braking) then kinetic friction takes over to try and reduce
> the relative motion between the tire and the road at the point of contact.
>
> So in simple terms your have an number of forces acting on the car
> including the weight of the car, the normal reaction of the road on the
> car, the frictional forces between the tires of the car and the road and
> the frictional forces between the air and the car.
>
> As long as there is adequate friction between the tire and the road so
> that the tires don’t slip it does’t matter if there are two tires, 4 tires,
> or more.
>
> That is why I tried panic stops in my testing to make sure my tires
> weren’t slipping.
>
> I really don’t care to get into any arguments with anyone about my use of
> whatever braking system I desire or how it compares to whatever anyone else
> wishes to use.
> All I care about is whether it works for me. If it does then I am
> satisfied. If it didn’t then I would make further modifications until it
> did. If your system works for you that is all that matters but please
> don’t anyone tell me that I am wrong because of what I choose to use. This
> is beginning to sound like the tire wars of old. Do what you want to do
> and let me do what I want to do.
>
> Emery Stora
> 77 Kingsley
> Frederick, CO
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
--
'73 X-Glacier 230 "Jaws"
Northern Virginia
Offlist email: rick at rickdenney dot com
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