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Author Topic:   Launch Ramp Physics: weight distribution and incline
jimh posted 06-26-2003 12:38 PM ET (US)   Profile for jimh   Send Email to jimh  
Boston Whaler boats are very often trailered boats and require launching, usually from a ramp, and there has been much discussion about how best to accomplish this.

I'd like to turn for a moment to discussing the weight distribution on the axles of the vehicle towing the Boston Whaler on the launch ramp. Specifically:

Does the weight distribution on the axles of the car or truck launching a boat at an inclined ramp change as function of the angle of the inclined ramp?

And this corollary question:

Does the weight distribution on the axles of the car or truck launching a boat at an inclined ramp change as a result of the pulling load on the hitch exerted by the boat?

jimh posted 06-26-2003 12:47 PM ET (US)     Profile for jimh  Send Email to jimh     
[Administrative post]
Tom2697 posted 06-26-2003 05:28 PM ET (US)     Profile for Tom2697  Send Email to Tom2697     
Without trying to hold an online physics course...yes, and yes.

For the Engineers out there:
The weight on the front axle is roughly 1/2 of the vehicle weight (assuming 50/50 normal weight distribution and that the center of gravity is inline with the axles) times the squared cosine of the ramp grade (F=W/2*cos^2(alpha)). The weight on the rear axle is found by subtracting this from the vehicle weight (R=W*(1-1/2cos^2(alpha)).

I know I am leaving the trailer out of the equation but it is time for me to go home....Somebody else can expand on this.

jimh posted 06-26-2003 05:58 PM ET (US)     Profile for jimh  Send Email to jimh     
Tom, Glad you responded. When the axle load changes, given the conditions you specified, does more weight shift to the front axle or to the rear?
Dr T posted 06-26-2003 06:54 PM ET (US)     Profile for Dr T  Send Email to Dr T     
To the rear. In order to visualize this consider:

The center of mass of the body will remain invariant. It will also be above the plane of the ramp. If you start at an incline of 0 and elevate the left side of the plane, the center of mass will move to the right, that is, towards the downhill side.

tds

Dick posted 06-26-2003 10:20 PM ET (US)     Profile for Dick  Send Email to Dick     
Trailers are set up with the boat prox 10 0/0 of the weight on the tongue for road travel. Shifting of weight distrubition on the launch ramp has nothing to do with trailering safety. And is irrevelent.
There is now way you can set up a trailer to match a launch ramp and expect it to be safe on the road.

jimh
Didn't expect something like this from you.

jimh posted 06-26-2003 10:25 PM ET (US)     Profile for jimh  Send Email to jimh     
Dick,

I really didn't expect something like that from you, either.

I am asking a question, and it has nothing to do with percentage of trailer weight on the tongue.

jimh posted 06-26-2003 10:30 PM ET (US)     Profile for jimh  Send Email to jimh     
Ignoring the trailer again, and assuming the initial weight distribution between front and read axles was 50/50 and each axle bears, say, 100 pounds, then:

If the incline has a slope downward of ten degrees, what will the new weight distribution be?

Jerry Townsend posted 06-26-2003 11:29 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Jimh --- First, in answer to your initial post - the answers are yes and yes.

But a specific answer to your questions is not possible as there are other variables of significance that must be factored in. That is, the center of gravity (C/G) of the vehicle does not change appreciably (the fuel shifting to the rear of the tank will have a minor effect) - but the elevation of the C/G comes into play now. The elevation of the C/G and the slope of the ramp will alter the loading of the axles.

Which leads me to a slightly off-topic but related point - those using a pick-up with a camper will want to make SURE that the projected C/G of the camper stays IN-FRONT of the rear axle when on a ramp.

And throwing in another wrinkle - the braking distribution designed into the vehicle's braking system is not necessarily uniform between the front and rear axles. That is, commercially rated vehicles (for example, 3/4 ton heavy duty pickups, et.al.) are designed to provide more braking from the rear axles/wheels. Therefore, backing down a ramp will change the load distribution on the axles/wheels.

Be advised that the normal loading of 'towing class' vehicle is not 50/50. One can see that by looking at the vehicle loading rating on the pickups and SUVs.

Now with a trailer - the elevation of the hitch is similarily important.

I'm not sure where you were trying to get to with this thread and I certainly have not answered your questions - but if you want to pursue this - e-mail me and we'll get this one worked out too. ----- Jerry/Idaho

Dr T posted 06-27-2003 12:24 AM ET (US)     Profile for Dr T  Send Email to Dr T     
Another minor effect will occur because the position of the CG will be shifted by the compression of the vehicle springs. The situation I described was an idealized abstraction.

Note to self: Remember that there are good mechanical engineers on the forum that are far better qualified to answer questions like these than pure mathematicians.

And Jerry, I would hate to see the angle of the ramp that would cause the camper situation you describe to occur. Does this really happen? I shudder to think of the consequences.

Whalerdan posted 06-27-2003 09:12 AM ET (US)     Profile for Whalerdan  Send Email to Whalerdan     

I think it's we all agree that the weight distribution changes. Seems to me the question is "Does this hurt/help anything?" To me the distribution would shift more toward the back wheels of the tow vehicle and would give you more traction. Seems like a good thing. I don't think for the short period that you're on the ramp it could cause any damage to trailer/vehicle since they were really designed for vertical loading and not the angle of the ramp. Besides, (not me since I live in the low country) we tow up hills that have near the same angles.

Danny

Dick posted 06-27-2003 09:58 AM ET (US)     Profile for Dick  Send Email to Dick     
I think what I was trying to say is why waste forum space on something that can't be changed and realy has nothing to do with performance. A launch ramp is a launch ramp. Some are easy others are difficult and there is nothing that will change that.
The most important things in launching a boat is the correct tow vehicle and practice.

Dick

Jerry Townsend posted 06-27-2003 10:43 AM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Larry - the possibility is there and I agree, the consequences would not be good. I use a GMC 4 X 4 pickup with a camper and there is one ramp in Montana (at Hebdon lake near the west entrance to Yellowstone NP) that I will not use because of the steep ramp slope. --- Jerry/Idaho
triblet posted 06-27-2003 10:51 AM ET (US)     Profile for triblet  Send Email to triblet     
It's important KNOW that it happens even if you can't change
it. Knowledge is power.

It's a bad assumption that the CG is in line with the
center of the axles. It can be below (race car) or way
above (macho honker 4x4).

If you don't believe that weight transfer occurs, try this
thought experiment: Make the ramp steeper and steeper. At
some (absurdly steep) point, the CG is directly above the
rear axle and the truck is balanced. Weight transfer HAS
occured (and has been occurring in greater and greater
amounts as the ramp got steeper).

But what no one seems to have mentioned is that traction
is a function of the force normal (nerd-speak for perpendicular)
to the surface of the ramp. As the ramp gets steeper, a
smaller and smaller fraction of the rear wheel weight is
normal to the surface.

This is all most relevant to those with front wheel drive,
where the weight gets transfered OFF the front wheels AND
the normal force goes down. And 2WD trucks with a front
hitch have the same problem.


Chuck

newt posted 06-27-2003 11:02 AM ET (US)     Profile for newt  Send Email to newt     
Here's a thought. Maybe Jim is curious about the use of a front wheel drive vehicle to launch a whaler on a steep ramp. If the weight distribution changes and lightens the load on the front tires, then the available traction is lessoned.

Regardless, why all the second guessing about the purpose of this thread?

I should add a couple of qualifiers to the conclusion that an incline will increase the load on the rear axle.

1. A rear hitch is being used. (I have seen people moving boats around with a front hitch).

2. The center of gravity of the vehicle is above the axles.

newt posted 06-27-2003 11:04 AM ET (US)     Profile for newt  Send Email to newt     
Chuck, looks like you posted while I was writing. We are on the same page.
Dr T posted 06-27-2003 12:31 PM ET (US)     Profile for Dr T  Send Email to Dr T     
Chuck,

Thanks for your more complete elucidation of my earlier point. (I was too lazy to get into the normal vectors so I just stuck to somewhere above the plane of the ramp. But it is nice to see it so clearly written down).

Jerry,

I remember that when driving a tractor with a big disk harrow on the back I have had the front wheels lift while going up an inclide. Tractors turning over backwards have killed lots of farmers and I understand your reluctance. The situation you describe is one case when putting a trailer hitch on the front for manouvering near the launch ramp makes a lot of sense (besides the improved visibility. I have seen quite a few pickups equipped that way.

Terry

jimh posted 06-27-2003 01:22 PM ET (US)     Profile for jimh  Send Email to jimh     
I'd like to move to part two of my question.

Looking at the trailer, let's assume a single axle trailer, you can consider the weight to be distributed between the hitch (tongue weight) and the axle. Assume that when on level ground, the trailer weight is distrubuted 10/90, with only 10-percent on the hitch.

When the trailer is hitched to the car (on level ground), the hitch load is transfered to the rear axle of the car, increasing the weight bearing on that axle.

When the trailer is backed down the incline ramp, if it behaves as the vehicle did, the weight will increase on its rear axle and tend to decrease on its hitch. At the same time, the trailer will begin to increase the draw bar load as gravity tries to pull the trailer down the slope of the incline.

I see that this is a complex situation. Where does the load from the draw bar pull go? To the front axle of the vehicle or to the rear axle?

Tom2697 posted 06-27-2003 02:39 PM ET (US)     Profile for Tom2697  Send Email to Tom2697     
First a note: I realize that what I posted is a GREATLY simplified analysis of the question. For our purposes here on this forum, assumptions will better permit the lay folk to understand the idea that we are trying to get across...The CG of most vehicles is above the axles (hence body roll when cornering, braking, etc). The axles are also above the ground (obviously). Both of these factors will increase the load distribution as the ramp grade increases. As for the 50/50 weight distribution, again this is a simplified assumption to simplify the analysis for the lay person. One of the beautiful aspects associated with Boston Whalers is that you don't need a Ford F-650 to tow them. From viewing many of the threads on this forum, many people use their family sedan to tow their boat. Most street vehicles are designed to be close to a 50/50 weight distribution. This criteria bears a direct effect on the "slalom speed" that is so often stated in all the auto tests. I can go on in a pissing match with anyone when it comes to vehicular design for two reasons, one is that I like pissing matches and two is that I partake in vehicular desing with my job. I don't beleive that is the purpose of this thread though. I believe it is to share the knowledge of the experienced, trained, or whomever, with those who are interested.

jimh, to answer your second question. The loading on the axles will behave similarly. The load from the draw bar should pull on the frame and increase the axle loading on both the front and the rear. However, the effect on the real axle will be much greater until the CG of the boat gets to the point where the trailer tries to flip backwards. Then, the loading on the rear wheel is obviously lessened.

If you posted this question is regard to towing with a front wheel drive vehicle, this is why it is not recommended. If you posted because you are concerned with over loading the GVWR rating of your tow vehicle, I wouldn't be too concerned unless your boat is full of water (increased weight) and you try to muscle the boat off the ramp before draining the water.

Whalerdan posted 06-27-2003 02:53 PM ET (US)     Profile for Whalerdan  Send Email to Whalerdan     

Find the Center of Mass of the vehicle. Draw a line from that center to the point of the ball hitch. Now you have two forces acting on the Center of mass. The first being gravity down, and the second being the boat pulling on the lever arm from the CM to trailer hitch ball. The total forces on the lever arm are acting to try and lift the front wheels and push down on the back. The force of gravity is pushing down on both wheels which is what keep the car from doing a wheelie.
Jerry Townsend posted 06-27-2003 03:28 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Terry - several people use a front bumper hitch - predominately for visibility and to eliminate having to back a trailer down a ramp. There are many who really have problems backing a trailer. The load transfer to the vehicle depends on the hitch location and specifically the elevation relative to the axles. That is, if the hitch is mounted above the axles, the front axle will take more load and some load will be taken from the rear axle. If the hitch elevation is below the axles the hitch load will be shared between the front and rear axles.

Jimh - basically correct, but on the level, hitching up the trailer will increase the load on the rear axle and decrease (slightly) the load on the front axle.

Now, backing the trailer down the ramp - the draw bar load distribution will be pretty well equally shared between the front and rear axles/tires. In actuality, the rear may take a bit more of the draw bar load because of the effect of the hitch load taking a bit of the load off the front axle. ------ Jerry/Idaho

Dr T posted 06-27-2003 07:16 PM ET (US)     Profile for Dr T  Send Email to Dr T     
An interesting point was raised: On a really STEEP ramp, the CG of the boat/trailer will move behind the axle of the trailer. When this happens, the net affect will be an upward force on the trailer ball as exerted by the hitch. However, it is going to take a really steep ramp to do it, so steep that the tow vehicle probably can't climb it.

Fortunately, my 13 is so light that all of this is only of acamdemic interest (except when I am moving the boat by hand).

tds

Jerry Townsend posted 06-27-2003 08:02 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Terry - depends on how far the boat's C/G is in front of the axle to begin with - and the size of the boat. Also, remember that the C/G of the trailer is in front of the axle. But yes, backing down ANY ramp will reduce the hitch load. ------ Jerry/Idaho
Dr T posted 06-28-2003 01:08 AM ET (US)     Profile for Dr T  Send Email to Dr T     
Granted. But at some point it has to happen unless the CG is in the same plane as the center of the receptical for hitch ball and the axle. (Note: It may be hanging nearly vertical by that time, but it will happen. This is a real problem with being a pure mathematician. You can usually imagine worst cases that would never occur in practice. This always made T-F and multiple choice tests hard.--tds)

On a practical level, when I have to move the 13 by hand, I get one or both of the daughers to hang off the transom so it is easier to get the tongue up. Once I get the tongue up high enough that the load is nearly balanced, it is much easier to move the boat and trailer around.

tds

Al_A_Buy posted 06-29-2003 05:59 PM ET (US)     Profile for Al_A_Buy  Send Email to Al_A_Buy     
Maybe I missed it but, are we talking about moving from the flat surface of the ramp on to the inclined surface? If so, it seems to my sun addled brain that there are many vectors involved here, possiably too many to effectivly describe in a text forum.

My 2 cents....

AL W.

jimh posted 06-29-2003 07:40 PM ET (US)     Profile for jimh  Send Email to jimh     
It sounds like it is possible to summarize in very general terms the following changes in weight distribution on the axles of a vehicle on an inclined ramp:

1. The weight distribution tends to decrease on the front axle and increase on the rear axle.

2. If there is a trailer involved, the effect of the trailer may be described in general terms as tending to decrease the weight on the rear axle of the vehicle, although this effect is generally small.

3. Overall, as a vehicle and trailer back down an inclined ramp weight tends to shift from the front axle of the vehicle to the rear.

Any substantial argument with those statements?

triblet posted 06-30-2003 11:39 AM ET (US)     Profile for triblet  Send Email to triblet     
Jimh, I think that's an accuate assessment. But your #1
and #3 say the same thing. I think #3 says it better.


Chuck

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