Help me settle a debate

[RF Lighting]

NO!

 

[WDIK]

ok. because forward propulsion was dependant upon appling a forward force to a static medium ie the road, when the static surface arrives at the same speed as the forward motion then at that point the actual acceleration becomes the forward motion at null ie it matches the new medium, plus the previous forward motion so .......

 

[Space cat]

Driving up a ramp into a moving truck is just like driving into a stationary one but with one added complication; as soon as your driving wheels hit the ramp you'll find yourself in the wrong gear. You can get round this by dropping into neutral at the last minute.

Your velocity relative to the truck should, ideally, be just enough to get you up the ramp. To a first approximation, this is given by v = sqrt(2gh) where h is the height of the truck floor above the road. Front wheel drive is best because you can get first gear engaged and get the car under control on the ramp earlier. With rear wheel drive your relative velocity is more critical. If it's too low, you'll roll back off the ramp before your driving wheels reach it.

 

[empip]

Watch it ... Especially the - undriven - front wheels, when just on the ramp...
http://www.youtube.com/watch?v=xsg5V01FhxI

 

[breezer]

empip, edward current posted a link to that clip (different name, same clip) i mentioned earlier that edwrad current posted a link to that clip, so i dont see what all the fuss is about

 

[blondini]

Front wheel drive is best because you can get first gear engaged and get the car under control on the ramp earlier. With rear wheel drive your relative velocity is more critical. If it's too low, you'll roll back off the ramp before your driving wheels reach it.

I still think RWD would be easier.

With FWD you lose forward propulsion as soon as the front wheels hit the ramp. Only the front of the car is on the ramp and quick work then needed to get the power back on and haul the rest of the car on board if there's not enough inertia. You can't hit the brakes until the back wheels are on.

With RWD you could park the front wheels on the ramp and then keep driving along happily at the same speed as the truck if you wanted . Point is you can get further up the ramp before the critical transition of the driven wheels and once they are on that's the whole car on the ramp, you can just hit the brakes and declutch.

To put it more succinctly, at the point where you lose forward propulsion due to transition of the driven wheels, the FWD car only has one axle on the ramp, but the RWD car has both axles on the ramp. It is on the ramp.

 

[megawatt]

I don't think it makes much difference because, as soon as the front wheels ride up the ramp, they will have traction with the ramp and continue to pull the vehicle up it and, at this point, the road surface plays no further part as the rear wheels are simply freewheeling like the non-driven wheels of the coach.

MW

 

[Space cat]

With FWD you lose forward propulsion as soon as the front wheels hit the ramp. Only the front of the car is on the ramp and quick work then needed to get the power back on and haul the rest of the car on board if there's not enough inertia. You can't hit the brakes until the back wheels are on.

You don't actually lose all forward propulsion UNLESS you declutch before hitting the ramp but you really should do this anyway. If you don't, your front wheels, which are driving at road speed, will quite likely spin on the ramp and you could slide off sideways.

In a front wheel drive car, I would close on the truck at a relative velocity of about 10 mph. This is enough to lift the car no more than one metre off the ground. I would declutch just before contact, engage first gear then let the clutch up if the car showed any sign of rolling off the ramp.

With RWD you could park the front wheels on the ramp and then keep driving along happily at the same speed as the truck if you wanted . Point is you can get further up the ramp before the critical transition of the driven wheels and once they are on that's the whole car on the ramp, you can just hit the brakes and declutch.

This has the advantage that you can approach the truck slowly and nudge your front wheels onto the ramp. The question is, do you have enough torque from the back wheels to finish the job? You should only attempt this if you know that you can climb steep hills in a high gear.

And finally --

Nobody has yet considered that the truck might not be travelling in a straight line. If there's any chance that it might veer off the straight and narrow, I would definitely want my driving wheels on that ramp as soon as possible!

 

[blondini]

In a front wheel drive car, I would close on the truck at a relative velocity of about 10 mph. This is enough to lift the car no more than one metre off the ground. I would declutch just before contact, engage first gear then let the clutch up if the car showed any sign of rolling off the ramp.

That sounds like a good plan, what would you do with a RWD car?

 

[tim west]

That sounds like a good plan, what would you do with a RWD car?

Drive up the ramp backwards

 

[blondini]

Great idea Tim Rather you than me though

 

[anobium]

As a layman in these matters could somebody explain to me the following viz,
How come then that when I walk along an escalator or moving walkway I increase my speed significantly greater that those who just stand still on the escalator or moving walkway.
Would the same results apply to a vehicle with either front or rwd?

 

[Space cat]

That sounds like a good plan, what would you do with a RWD car?

I would have to calculate the required relative velocity more accurately and judge it more precisely. Too slow and the back wheels won't make it onto the ramp. Too fast and I run out of stopping distance in the truck.

PS: Nobody has yet mentioned four wheel drive, which presents a problem all of its own. The front-rear differential, if there is one, will be subjected to some extreme forces, possibly in excess of anything the designer envisaged. And if the isn't one ---

OH SH****T!!!

 

[Space cat]

How come then that when I walk along an escalator or moving walkway I increase my speed significantly greater that those who just stand still on the escalator or moving walkway.

You are looking at the problem the wrong way. The escalator isn't moving at all; the rest of the building is moving backwards! Did you assume that the building was stationary? In which case why is the rest of the universe rotating around Earth's axis?

The simple fact is that there is no possible way of working out what's moving and what isn't. Although you are walking up the escalator at normal speed, the building is coming down towards you and this creates the illusion that you are walking a lot faster.

 

[Softus]

As a layman in these matters could somebody explain to me the following viz,
How come then that when I walk along an escalator or moving walkway I increase my speed significantly greater that those who just stand still on the escalator or moving walkway.

I'm baffled as to why you don't understand how walking makes you move faster than standing still.