high mph then glide

[pushycruze]

You guys should remember that drag is exponential. Going twice as fast requires 8 times the power...so there is definately a point at which it would be extremely inefficient to schmob and glide.

[trebuchet03]

Originally Posted by derangedrover

You guys should remember that drag is exponential. Going twice as fast requires 8 times the power

It's exponential, but by a factor of 2 - not 3



Twice as fast yields 4x the resistance. Assuming the same distance traveled -- doubling velocity will require 4x the power to move a given mass But you're point is still valid (but finding the actual speed where it's not effective seems like a good exercise - I would hypothesize that it's faster than an intuitive guess).

[KARR]

Drag force, as you said, is proportional to the square of the speed. Power, however, proportional do the cube of the speed:

http://en.wikipedia.org/wiki/Drag_force

[trebuchet03]

Originally Posted by KARR

Drag force, as you said, is proportional to the square of the speed. Power, however, proportional do the cube of the speed...

Doh - You're absolutely right... and my apologies Looks like I was confusing Power with Work (as I mentioned constant distance).

But ya, you're right...

Work: F*d (not changing v)
Power being F*v (making the v^2 --> v^3)

Again, my apologies - thanks for setting things straight

[KARR]

Originally Posted by trebuchet03

Doh - You're absolutely right... and my apologies Looks like I was confusing Power with Work (as I mentioned constant distance).

But ya, you're right...

Work: F*d (not changing v)
Power being F*v (making the v^2 --> v^3)

Again, my apologies - thanks for setting things straight

No problem! I just recalled what James May said about power and speed when he droven the Veyron: the fast you go the more mother nature tries to hold you back.

[brucepick]

You guys hit the nail re. drag - speed hurts your FE with p&g just as with steady state. You just feel it differently when driving using p&g.

If you pulse up to a higher speed, you need extra power/energy to get there (say, up to 65 or 70 mph). And the increased drag at the higher speed causes that speed to deteriorate to a slower speed pretty quickly. So all that fuel used to get up to say 70 mph is wasted pretty quickly. OTOH, if you pulse only up to 60 and let it glide down to 50 the overall FE is better.

Of course sometimes you just have to average 60 or 65 mph to get somewhere on time. I'm pretty sure that p&g'ing to attain that as an average is still more efficient than maintaining it steady state. But I really avoid getting in that situation. I'd rather p&g between 50-60 or thereabouts.

[trebuchet03]

^^ That's why we can't just say it's bad solely based on theory.

Mass will have a huge role at higher velocities. Which is why we've theorized there is a "sweet spot" for a vehicle's mass for P&G. A heavier car will glide much longer than that same car with less mass starting from the same speed. The question is, at what point does the power for acceleration overcome the benefits of increased linear momentum (at higher velocities)....

[bowtieguy]

Originally Posted by trebuchet03

^^ That's why we can't just say it's bad solely based on theory.

Mass will have a huge role at higher velocities. Which is why we've theorized there is a "sweet spot" for a vehicle's mass for P&G. A heavier car will glide much longer than that same car with less mass starting from the same speed. The question is, at what point does the power for acceleration overcome the benefits of increased linear momentum (at higher velocities)....

off the subject a bit, but was wondering something. in DOT training of CDLs, the "idea" is taught that an empty truck takes longer to stop than when full. i'm no intellectual, but hands on experience tells me this is bull cookies. my truck loaded takes longer to stop than when empty. in fact, i can feel the extra weight in handling and slowing as well. please set me straight or confirm me.

[rh77]

Originally Posted by bowtieguy

off the subject a bit, but was wondering something. in DOT training of CDLs, the "idea" is taught that an empty truck takes longer to stop than when full. i'm no intellectual, but hands on experience tells me this is bull cookies. my truck loaded takes longer to stop than when empty. in fact, i can feel the extra weight in handling and slowing as well. please set me straight or confirm me.

Depends on the condition:

A "Bobtail" rig without the trailer has a tough time stopping since most of the braking action is designed for the rear wheels. Loss of control is likely when the drive-wheel brakes lockup (I've seen it and nearly got rear-ended by one sliding).

If loaded, tires tend to get better grip on both the trailer(s) and tractor during braking. Otherwise, the loss of friction occurs between the rubber and the road.

Now, if you're talking max payload, that's a bit less believable. Brakes can only do so much...

RH77

[trebuchet03]

I agree with rh77 - and it really does depends on conditions.

For braking ability - the max force you can place is related to the weight of your vehicle and the coefficient of static friction (which is related to your tire compound and road surface).

Considering the road doesn't really change (so your static friction coefficient will remain nearly constant) - increasing the weight of the vehicle increases the amount of braking force you can apply before the wheels start skidding.

Which brings me to... Bodies that are skidding have a different friction coefficient (coefficient of dynamic friction) that is ALWAYS lower than static friction. This is why it's so difficult to regain control of a skidding vehicle. This is why ABS will stop a vehicle much faster than a vehicle that's skidding even though the ABS vehicle won't have the brakes on full all the time.

I'd bet their comparing an emergency stop condition - where an empty truck will skid while a heavier truck will be able to maintain traction for a little longer. But has rh77 said - brakes will only go so far when it comes to large masses of stuff to stop, there's just a lot of energy/momentum there.