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Author Topic:   Trailer Geometry 101
kingfish posted 02-24-2002 05:19 PM ET (US)   Profile for kingfish   Send Email to kingfish  
I'm shorting out on this one and I think I need some help from one of our engineer contributors (or at least from someone who paid better attention in Geometry than I did).

While in the process of renovating the brakes and tuning the keel rollers on my tandem trailer, I am taking the opportunity to cure an inadequate transom support condition by moving my Outrage 22' forward on the trailer frame.

My question is, how do I calculate the relationship between the distance I move the boat forward and the distance I need to move the tandem axles forward if I want to maintain the same approximate tongue weight? I just know those two dimensions aren't the same - that would be too easy...



Tom W Clark posted 02-24-2002 05:57 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

For all intents and purposes, moving the axle forward the same amount would be fine. If you really want to be picky, then yes, I would agree the relationship is not exactly linear, the axle would move forward some small amount less than the boat.

If you do move the axle forward the same amount as the boat then there would be more trailer hanging off the back and less in front than previously. This would change the net balance of the rig. You could calculate exactly, but why? "Use the Force, Luke..." i.e., let your intuition guide you. If you're moving the boat forward 6" then move the axle forward 5".

kingfish posted 02-24-2002 06:53 PM ET (US)     Profile for kingfish  Send Email to kingfish     
Thanks, Tom-

Sounds like my instincts, though hazy, are still operating in the right direction. Believe it or not, I've moved the boat forward 15" - I'm thinking of tying the axles down 12" forward of original.


Tom W Clark posted 02-24-2002 08:42 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

Great minds think alike.

triblet posted 02-24-2002 09:39 PM ET (US)     Profile for triblet  Send Email to triblet     
A good first-order estimate would be say,
OK, the boat, motor, etc. is 85% of the total
weight of the rig. So I'll move the axle
85% of the distance I moved the boat. I've
a suspicion that will land you more like 13"
or 14" instead of 12" for the 15" boat move.


Tom W Clark posted 02-24-2002 10:11 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
If you extrapolate my 5” of axle movement for every 6" of boat movement that would yield 12.5" of axle movement for the 15" of kingfish's boat movement.

If you use Chuck's 85% estimate you get 12.75" of axle movement. As I was saying about great minds...

Actually this is an algebraic problem more than a geometric one. We would need to know the weight of the trailer per unit of length, e.g. 100 lbs per foot of trailer length. We would also make the assumption the this weight per unit of length is constant over the area of potential axle movement.

Tom Byrum posted 02-24-2002 10:13 PM ET (US)     Profile for Tom Byrum  Send Email to Tom Byrum     
You need to move the axles to a position that puts around 350 pounds on the tongue.
triblet posted 02-25-2002 10:43 AM ET (US)     Profile for triblet  Send Email to triblet     
I could calculate it precisely with the following
Current weight on the trailer wheels loaded.
Current weight on the tongue.
Weight of the trailer, or weight of the boat+motor+...
Distance from c/l of wheels to tongue.


Jerry Townsend posted 02-25-2002 03:07 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Kingfish - getting exact numbers require that you get your boat and trailer to a commercial scale which may not be easily done for yourself. Knowing your boat weight, engine weight, select dimensions of your trailer and applying a little engineering knowledge, we can come sufficently close without getting your boat/trailer on a scale. Of course - having exact weights would be nice - but not absolutely necessary. I'll play around with some preliminary numbers - but I think this will work.

Incidently, and with all due respect, the weight distribution is important from a stability and safety aspect and I suggest not "guessing" on how far to move the trailer axles. A 22 Outrage tangeling with pavement would not be pretty.

Let me know if I can help you. ---- Jerry/Idaho

kingfish posted 02-25-2002 06:16 PM ET (US)     Profile for kingfish  Send Email to kingfish     
Firstly, as always I am impressed by (and grateful for) the thoughtful and thought-provoking responses that questions like these generate here. Thank you all.

Secondly...I don't know about you guys, but I really hate it when someone asks me a question that takes something more than just a "yes" or "no" or "it's in the third drawer down on the left" to answer, so I engage myself with the problem and develop a response, only to find that the guy who asked me the question has gone ahead on his own and figured out the answer. (I guess I don't really hate it, but you know what I mean...)

Well, with all due respect and gratitude for everyone who has been kind enough to offer advice, opinion and support so far, I have done got engaged and intrigued by my own question (hope I don't be hung by my own petard), and I think I have figured out the answer. Here is what I've come up with and why; please check my numbers and logic.

TRAILER - 25' (300") long from hitch to end of frame. Weighs 1200 pounds. For convenience sake I am going to call the weight of the running gear (or the unsprung weight), i.e., rims, hubs, brakes, tires, springs, shackles, etc. to be 300 pounds (it's probably actually closer to 400 pounds but 300 works better in my scenario).

BOAT - 2050 lb. + motor @ 500 lb. + fuel @ 500 lb + batteries and oil reservoir and radar arch and electronics and anchor and addt'l teak etc., etc.-another 500 lb, for a grand total of 3500 pounds max.

So here's *my* story:

The axles were originally centered 20'-2"(242") back from the hitch. I've moved them forward 15" to 18'-11"(227") from the hitch. So remove the unsprung trailer weight, leaving 900 pounds to be divided by the length of the trailer (not necessarily accurate I know, the front of the trailer is lighter than the rear of the trailer, but not a lot and I had to start somewhere); that gives us 3 pounds for each inch of trailer length. Then remove twice the weight of the length of trailer cantilevered to the rear of the axle center (weight of cantilevered portion is balanced against an equal length of trailer in front of axle center, cancelling each other out). That leaves 552 pounds of sprung trailer weight between the hitch and the axle center, divided by 2 equals 226 pounds of tongue weight on an empty trailer before I moved anything.

Now. I moved the axle center forward 15" so we double that due to the balanced cantilever effect and we have 30" x 3 ppi (pounds per inch)or 90 pounds by which we have reduced the weight of the sprung portion of the trailer between the hitch and the axle center. So we take our original 552 pounds, subtract the 90 pounds for 15" of reduced length, and we have 462 pounds of sprung weight between the hitch and the axle center, which we again divide by two and get a new tongue weight of 226 pounds. Subtract the new tongue weight from the old tongue weight and we have reduced the tongue weight by 45 pounds by moving the axle center forward by 15 inches.

We're closing in on it now.

We've got a reduction of 45 pounds of tongue weight to compensate for by moving the boat ahead differentially from the distance we moved the axles.

The new distance between the hitch and the axle center is 12'-10" , or 154", so we divide the boat weight of 3500# by 154 and we have 22.73 ppi of boat weight per inch of travel.

45# of reduced *trailer-generated* tongue weight is divided by 22.73 pounds per inch of travelling boat weight and we see that in order to maintain the same overall tongue weight we had before this move, we would move the boat 1.98 inches further forward than the axle center was moved.

Since in the real world what I am trying to accomplish is to move the boat forward 15", and move the axles forward only whatever amount I need to maintain the original tongue weight, I will round things off and move the axles 2" less than the boat, or 13" forward of their original position.

Whew!! Bar is open and smoking lamp is lit - how do those numbers sound to you guys?



kingfish posted 02-25-2002 06:31 PM ET (US)     Profile for kingfish  Send Email to kingfish     

That scwag of yours at 85% comes in mightily close to what I got after all that horsing around (15" x 0.85 = 12.75") - pretty fine-tuned instincts!


lhg posted 02-25-2002 07:02 PM ET (US)     Profile for lhg    
JCF: I hate to tell you this, but I'm not sure you're right, although I had a hard time following your math. Basically you are dealing with a structural engineering problem, involving moments (ft/lbs), which involves calculation of both approximate continuous loads (trailer and boat) and actual concentrated loads (engine, batts, spare tire, winch stand) times distance acting, resulting in an uneven balance and putting 400 actual lbs on the hitch. With a boat, it's a fairly complicated calculation to be highly accurate (depending how the boat's hull weight is distributed over it's length), but in it's simplest form is a "teeter-tawter" problem, with the trailer wheels being the fulcrum. My guess is that you moved the wheels too far forward at your 15" calculation.

If you think of a simple teeter-tawter situation, you will get an idea of the concept of "moments" (force x distance). A heavy person, sitting on the short end of a board (the part of a rig behind the wheels), may not even be heavy enough to counter the weight of a child, sitting on a much longer end of the board. You actually want the long end of the board (in front of trailer wheels) to require an upward resistance force of 400 Lbs (tonguw wieght) to balance the cantilevered moment behind the wheels.

I think the original information you got was correct. If there was no trailer, moving the boat forward on a fulcrum would would require the fulcrum be moved also, same distance. But since the trailer weight is now not in the same relationship to the boat, the wheels would be moved forward less. I would guess about a 3/4 relationship.

Tom W Clark posted 02-25-2002 07:20 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

Here's what we need to do: I hope you have not moved anything on your trailer. Measure what your tongue weight is now. I mean measure it accurately with whatever fuel load and gear you "normally" trailer with. Remember, if the fuel tank is not exactly centered over the axle ta varying fuel load alone will change the weight distribution.

Go ahead and move the axle and boat until you get it balanced with the same tongue weight. Now you will have the correct answer to your question and it will be interesting to see how it compares to the answers offered here including your own.

I think lhg makes some good points. There are more variables here than have been mentioned, but I still think the estimates made here (none are guesses) will be very close.

For the record, tongue weight should NOT necessarily be 350 lbs. It SHOULD be 5%-10% of gross trailer weight. The whole rig will tow more smoothly if the tongue weight is closer to 10% but in the case of large trailers this gets to be a bit of a burden so typically a lower figure close to 5% is used.

In the case of kingfish's rig and based on the figures he's presented here (4700 lbs, gross), his tongue weight should be 235 lbs to 470 lbs.

If you have a trailer with a relatively high center of gravity (kingfish's boat does not) then the 10% figure should be used as the weight distribution will vary much more depending upon grade and acceleration/deceleration.

Perhaps we should make this a contest, best estimate gets a ride in kingfish’s boat. I’ml still sticking with my estimate of 12.5”

kingfish posted 02-25-2002 09:14 PM ET (US)     Profile for kingfish  Send Email to kingfish     

Yeah, I veered off from Trailer Geometry 101 and got into a little of The Mathematical World According to Flook, and complicated that even further by trying to describe one medium of expression while using another. Confused my own self...

I'm pretty solid about my theory of how much the empty trailer tongue weight is at various axle center distances from the hitch. Where I'm in uncharted waters is how and by how much the boat weight affects the tongue weight as it is moved forward from the axle center.

I think what I needed to do differently to calculate the variable effect of moving the boat's center of gravity forward from the axle center was to apply the percentage of the boat's weight that was equal to the percentage that the distance the boat was moved forward was of the distance between the hitch and the axle. If I moved the boat forward from the axle center 2", and the distance from the hitch to the axle center is 227", then 2" divided by 227" is 0.0088 times 3500 pounds or 30.83 pounds. If this is accurate, then everything else being equal, it would take three inches of boat travel from axle center to evenly compensate for shortening the hitch to axle center distance by 15 inches. Or roughly speaking, if I moved the boat forward 15 inches, I'd only move the axles forward 12 inches (which is what my instinct told me to begin with). I'm hoping that Chuck or Jerry or you or somebody can take the data I've provided and see if I've managed to stumble anywhere close to accuracy...

1. loaded wt. on trlr. wheels - 4700#
2. tongue wt. - 300#
3. trlr. wt. - 1200#; boat wt. - 3500#
4. whl. c/l to hitch before move - 20'-2"
Desire = move boat forward 15" and maintain same tongue weight; how far to move axles?

Jerry and Tom - The fat is in the fire - fenders are off, rims are off, boat has been shifted forward 15", axles are off and spring shackles are loose; short of rigging up some kind of temporary mechanical fulcrum and figuring out how to measure an accurate tongue weight in this condition, I won't be able to accurately measure actual tongue weight until I put the whole thing back together. I will keep accurate measurements and weights recorded, though. And any of you guys who want a ride in Outre' can have one anytime you want, whether you nail the calculations the closest or not. All you gotta do is get yourself here...


triblet posted 02-25-2002 11:21 PM ET (US)     Profile for triblet  Send Email to triblet     
The numbers don't add up.

boat+trailer = 3500+1200 = 4700
wheels+hitch = 4700+300 = 5000

These two should be the same.


Jerry Townsend posted 02-25-2002 11:39 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Kingfish - based on the numbers you provided - namely 3500 lb boat, 4700 lb total weight, 300 lb hitch weight, 300 inch trailer length, initial 242 inch distance from the center of the axles to the hitch, moving the boat forward 15 inches and keeping the same hitch load - then move the axles 16 inches forward.

Calculated by knowing the bending moment at the transom is zero (by definition) and simply writing the summation of moments about the transom location. Further, the bending moment of the boat about the transom is the same for the previous trailer configuration and your modified configuration. Incidently, the calculated center of gravity of the boat is about 98.6" forward of the transom - which is reasonable.

Questions? let me know. ---- Jerry/Idaho

Jerry Townsend posted 02-26-2002 12:08 AM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Kingfish - using 5000 lbs total weight instead of 4700 doesn't make any difference - well, it decreases the axle displacement by less than 0.1 inch.

Again - any questions, let me know --- Jerry/Idaho

lhg posted 02-26-2002 12:28 AM ET (US)     Profile for lhg    
Jerry, your calculation of moving the axles MORE than the boat surprises me, but I am now assuming that moving the concentrated load from the winch stand, and presumably a spare tire, forward also, changes the weight distribution of the trailer itself, requiring a greater than 15" move forward? I agree, and "velly intellesting"!
kingfish posted 02-26-2002 09:56 AM ET (US)     Profile for kingfish  Send Email to kingfish     

I misunderstood your question-

The *entire* weight of the whole rig, trailer and all, if it were all hung from a single point and weighed, is 4700#. The original tongue weight was 300#, so the weight left to be imposed on the ground at the tires is 4400#. For the purposes of my calculations I chose to remove the "unsprung" weight of the running gear from the weight of the trailer, being that the running gear could be conceptualized as staying in one place while the rest of the trailer is moved in relation to it. I would guess the "unsprung" weight to be between 300# and 400#; if your calculations differentiate between weight on the tires and weight on the ground at the tires, then the 4400# number would be reduced by the above to, say, 4050#.


I am seriously flummoxed now - I have no grounds of any sort to question your results, and have all possible respect and gratitude for your calculations, but I'm going to have to work on this one for a while before my dead reckoning will be able to absorb the notion of moving the axles forward *more* than the boat. My head is already hurting...

How in the world did you determine where the center of gravity of my boat is? I would have paid a long dollar for that statistic while I was wrestling with those numbers yesterday.

Thanks all,


Tom W Clark posted 02-26-2002 11:38 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
You guys, you're making this about a thousand times more difficult than it needs to be.

Jerry, you've got it backwards. Look at it from another angle: The boat is going forward a certain amount, in this case 15". If you move the axle forward 15" as well what happens? The boat is in the same position relative to the axle and the tongue weight it REDUCED not increased. This is because there now is more trailer hanging off the behind the axle than there is in front of the axle. Therefore, in order to compensate for the lessened tongue weight, the axle needs to move back just a little bit thus putting a bit more BOAT in front of the axle to provide some more tongue weight (and a wee bit of trailer weight as well).

kingfish posted 02-26-2002 11:51 AM ET (US)     Profile for kingfish  Send Email to kingfish     
I hope you're right, Tom - that's how I conceptualize it. If you're wrong, I'm going to have to deal with a headache for a while as it all sinks in-
Jerry Townsend posted 02-26-2002 12:55 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
John, LHG - lets see if I can explain this calculation - this message may be a tad long though. I placed (assumed) the transom at the back of the trailer. Denoting the length of the trailer as X3, the distance from the transom to the center of the wheels as X1, the distance from the center of gravity to the transom as X2, the total weight of the boat/motor/trailer/ as Lt, the weight of the boat/motor as Lb and the hitch load as Lh.

Now, the bending moment at the transom (back of the trailer) must be zero by definition. Then, writing the summation of moments about the transom (defining clockwise as positive) gives:

X3(Lt) + (Lt - Lh)X1 - Lb(X2) = 0

and the same basic equation for the modified configuration - same loads but different distances as:

Y3(Lt) + (Lt - Lh)Y1 - Lb(Y2) = 0

Notice the load on the wheels is written as (Lt- Lh).

Now, from the transom, the bending moment of the boat/engine will not change - or namely, Lb(X2) = Lb(Y2)- so strictly speaking, we don't require the location of the center of gravity of the boat/engine. Remember that everything is referenced to the transom.

Solving each of the above equations for Lb(X2) or Lb(Y2) - remember, these are the same - and equating those equations gives:

LbX2 = X3Lh + X1(Lt - Lh)
LbY2 = Y3Lh + Y1(Lt - Lh)

and equating these equations gives:

X3Lh + X1(Lt - Lh) = Y3Lh + Y1(Lt - Lh)

and solving for Y1 (the distance from the transom to the centerline of the wheels) in the modified configuration gives:

Y1 = [X3Lt + (Lt-Lh)X1 - Y3Lh]/ (Lt - Lh)

which, when rearranged gives:

Y1 = [Lt(X3-Y3) + (Lt-Lh)X1] / (Lt - Lh)

and with X3 = 300, Lt = 4700, Lh = 300, X1 = 58, Y3 = 285 i.e. (300-15 : from moving the boat forward 15 inches) gives:

Y1 = 59 inches. Which is the distance from the transom to the centerline of the tandem axles in the modified configuration.

Note, the original distance from the transom to the centerline of the tandem axles was 58 inches (300 - 242). Therefore, the tandem axle should be placed 59 inches forward of the transom - or in other words, moving the boat forward 15 inches should be accompanied by moving the centerline of the axles 16 inches forward.

The above calc considers the transom at the end of the trailer - a few inches one way or the other will not make much difference.

Calculating the center of gravity of the boat/engine/ can be obtained by solving one of the initial equations, or:

X3Lt + (Lt-Lh)X1 - LbX2 = 0

and solve for X2 as:

X2 = [X3Lt + (Lt-Lh)X1] / Lb

which gives: X2 = 98.63 so the boat/engine center of gravity is 98.63 inches forward of the transom.

Everyone should, as desired, check the above calculations. Should you have questions, let me know. I have, rounded some number off - for example, the above 59 inches is actually calculated as 59.024 inches.

Now, regarding some of the wags/guesses etc, remember that the boat is the controlling component at 3500 pounds of the total 4700 - so if I were NOT making calculations, I would move the axles the same distance as I wanted to move the boat.

Hope I haven't confused anyone - and if you want to discuss this thing further with me - I welcome any comments. E-mail me if you want - ---- Jerry/Idaho

Tom W Clark posted 02-26-2002 01:51 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

1) Why must the bending moment, by defenition, be zero at the transom?

2) How do you reconcile the fact that moving the axle forward 16" and the boat forward 15" will result in a tongue weight less than with the axle at 15" (when moving the axle forward 15" will already have resulted in a reduced tongue weight) and your assertion that 16" is the corrct answer?

I'm not trying to start an argument but rather trying to understand your assertion.

kingfish posted 02-26-2002 03:41 PM ET (US)     Profile for kingfish  Send Email to kingfish     

I don't know if your calculations started with the transom being flush with the end of the trailer or if they ended there, but be aware that in the original configuration, before I moved the boat forward, the transom overhung the end of the trailer by 15" +/-. Moving the boat forward by 15" has placed the transom such that it is now about flush with the end of the trailer.

John (who hasn't had so much fun in some time!)

Jerry Townsend posted 02-26-2002 07:57 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
A couple answers to Tom and then results of a refined calculation follow.

Tom - The bending moment at the free end of any structure is zero. Now, of course, in the middle of the trailer or other supporting structure, the moment will be, in general, non-zero. That is the reason that I wrote the equations about a free end - the hitch works as well.

Regarding the reconciliation of the apparent decrease in hitch load - I initially thought that this was caused by the initial hitch load not actually being 300 pounds. But realized later last night that I had, in fact, set the initial load to 300 pounds as well. The only remaining item was the trailer being more of an influence that initially thought.

And as Kingfish pointed out - his initial position of the boat had the transom about 15 inches aft of the end of the trailer whereas I had placed the transom at the end of the trailer.

Then, making another calculation correcting the initial position of the boat relative to the trailer and factoring in the trailer is in order.

For the trailer, I took the tire and wheel weight of 40 pounds (my ST205/75D14 boat trailer spare weighs 37.5 pounds), a hub, spring and mounting hardware weight of 35 pounds - or 75 pounds for each of four tires. I also considered 75 pounds for each of the two axles. This 450 pounds (4 ea @ 75 + 2@75) was concentrated at the location of the wheels. The balance of 750 pounds (1200 -450) was assumed placed midway on the trailer frame. The 750 pound trailer frame weight over 25 feet means that the frame weighs about 30 pounds per foot - which is heavier than I would have thought - I would have estimated about 15 pounds per foot (a 6 x 3 box section with 1/8 inch wall thickness). But then, I don't know how Kingfish's trailer is built. The 30 pounds per foot seems heavy, but it is not completely out in left field.

The hubs, springs and axles I have in my garage (left over after building two horse trailers) are much heavier than used on the boat trailers - so the weight of the springs and hubs used in the calculations are guesses

And, as mentined before, the initial location was corrected to where the transom was initially 15 inches aft of the end of the trailer. Therefore, the initial length (hitch to transom)of 300 inches was changed to 315 and the transom was located 73 inches aft of the axle centerline.

The recalculation with these changes gives a transom to axle centerline distance of 71.3 inches or moving the boat forward 15 inches requires moving the axle centerline forward 13.3 inches. This is more in line with what our intuition and gut says.

Kingfish - I hope that I have the configuration right this time - sorry for the assumption of placing the transom over the end of the trailer. Let me know if I need to make any more corrections. Incidently, the cg of the boat/engine is calculated to be 74 inches forward of the transom. -------- Jerry/Idaho

Tom W Clark posted 02-26-2002 08:10 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

Well done. I think we should make you the official "ContinuousWave Engineer".

Can you answer a few more questions (assuming an initial tongue weight of 300 lbs):

1) What would the tongue weight be if the axle moved only 12.5" instead of 13.3"?

2) What would the tongue weight be if the axle moved the full 15"?

kingfish posted 02-26-2002 10:14 PM ET (US)     Profile for kingfish  Send Email to kingfish     

No apologies necessary - I should apologize tio you for not making clear what the transom/end of trailer condition was.

My trailer main frame is a 2" x 6" section, I think; I'd have to traipse out through some new snow to confirm that but I think it is so. I'll check in the morning just what it is and look up the weight per foot in my structural steel manual at the office tomorrow. Anyway, there's one running down both sides and there are four cross members with a 4" x 4" section and wall thickness that I think is in excess of 1/8". And I am *really* embarassed to say that as I sit here writing this it dawns on me that I have added two of those four cross members at 75# +/- each and 8 ea. 12" roller assemblies that with brackets, bolts, rollers and pins must go 10# each, a spare rim, tire and bracket at say 50#, and extra lights, brackets and guide posts that probably add up to another 25#, for a total adder of 305# +/- of sprung weight to the original 1200# trailer weight.

It is I who owe you an apology.

Aside from my inadvertantly leaking this information out to you a piece at a time, I need to remark that when I made my initial request for help with a formula, I naively thought there would be a simple one-line calculation into which I could plug two or three numbers and Voila!, have my answer. It never occurred to me that it would take as complicated a set of calculations as it has, and I want you to know that I am grateful to you for your time and interest. I hope you have enjoyed this as much as I have.

Tom, it sounds like our instincts (and estimates) are vindicated.

Chuck, you gonna stand pat with your 85% scwag, or take a run at a calulation? Inquiring minds...


Landlocked posted 02-26-2002 10:39 PM ET (US)     Profile for Landlocked  Send Email to Landlocked     

Are you guys a bunch of engineers or what? thought I had stumbled into the wrong site for a minute. Interesting, but my head is killing me. I've got an idea. Move it the 12" pull it down the road. If it doesn't feel right, move it some more.

Sorry - I usually avoid being a wise-#$%@ but I'm having flashbacks to remedial college math (which I failed by the way) (twice).


kingfish posted 02-26-2002 10:53 PM ET (US)     Profile for kingfish  Send Email to kingfish     

Anybody could do it that way...I figure if we keep at it hard enough, pretty soon it will be Spring and we can all do some *real* boating. (The virtual kind ain't bad for now, though.)


Landlocked posted 02-26-2002 11:00 PM ET (US)     Profile for Landlocked  Send Email to Landlocked     
Well - I guess you have a point....I don't understand it, but I'm sure there is a point in there somewhere.

Math and me never got along. I think back to my ACT test Scores went something like 30, 25, 24, 12. Guess what the 12 was in.

I'll leave you guys to your ciphering. Gota go dream about that new motor. Don't stay up too late.


Jerry Townsend posted 02-26-2002 11:17 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Tom - answers to your questions.
1) Moving the axles only 12.5 inches would give a hitch load of 314 lbs.

2) Moving the axles forward 15 inches would give a hitch load of 270 lbs.

The 300 lbs is a little over 6% of the total load and as such, it is in the ball-park, but another 50 pounds or so wouldn't hurt anything. One thing you don't want is for the hitch being dynamically lifted while going over rough roads or changing contours (such as entering onto a road at a slightly higher elevation.

Hope this gives you information you wanted. Any more - let me know. ----- Jerry/Idaho

triblet posted 02-27-2002 12:47 AM ET (US)     Profile for triblet  Send Email to triblet     
The downside of a higher hitch load is that
it transfers truck weight from the front
wheels to the rear wheels. This can cause
the front wheels to lock way first under braking,
and then you can't steer. When I first got
my Montauk, I had this problem. I was at
about 10% tongue weight. I dropped it to
6%, and the problem went away, and it was
just as stable on the road.

You want the fronts to lock JUST before the
rears. That maximizes braking effect, and
keeps you from swapping ends (when the rears
lock first). If the fronts lock early,
you don't stop as well.

Chuck (who spent a misbegotten youth
getting trophies in sports cars)

Jerry Townsend posted 02-27-2002 11:42 AM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Chuck - with all due respect, I offer a couple of comments. Lockup of any brake system is bad - you lose control - and that is the reason that Detroit developed anti-lock braking systems. Those without anti-lock brakes should avoid braking to the extent that one or more wheels lock up.

The vehicle designs incorporate a braking system distribution. On the heavier duty vehicles, the rear brakes are designed to provide more braking than the front. On 3/4 ton 4x4 pickups, it used to be 60% on the rear and 40% on the front. I am not sure about cars, but would assume that pickups, suburbans, are designed with about the 60/40 ratio.

I would suspect that this ratio is incorporated, in part - because you want the principal force (braking) to be BEHIND the cg of the vehicle. When the cg of the vehicle is behind the principal forces, the vehicle will, by definition, tend to establish a stable configuration by swapping ends should the cg be displaced from the axis of motion. This subject can be extensively discussed - but is not really germane here.

SuburbanBoy posted 02-27-2002 03:49 PM ET (US)     Profile for SuburbanBoy  Send Email to SuburbanBoy     
There is a lot of cipherin' going on here. I say move it forward about 12-13" and balance it with a cooler full of beer and ice (~50 pounds) in the bow or midships.


kingfish posted 02-27-2002 04:17 PM ET (US)     Profile for kingfish  Send Email to kingfish     
Now there is some engineering we can all get warm and fuzzy with...I like it.
jimh posted 02-28-2002 01:00 AM ET (US)     Profile for jimh  Send Email to jimh     
I find this thread very interesting, but I do think the subject should be changed to "Trailer Geometry 424"--clearly this is an advanced course.

I hope to re-work most of this into a REFERENCE article, as I think it will be interesting and with the aid of a few illustrations much easier to follow.

I like Jerry's analysis and I look forward to John's careful application and measurement of it on his trailer!


kingfish posted 03-05-2002 11:29 AM ET (US)     Profile for kingfish  Send Email to kingfish     

Having come to recognize through the course of this educational exercize that accurately assessing the empty trailer weight and distribution thereof plays a large part in answering a question like the one that I initially posed, I have gone back with the original build-up specs from EZ Loader, my AISC Steel Construction Manual, a measuring tape and calculator and have developed an estimate of the weight and distribution of virtually every item that is a part of the trailer.

Starting with that, I have run calculations that with my limited exposure to and knowledge of the sort of math you are using, still seem right to me (and I've checked my calcs for errors numerous times; didn't find any the last two passes so I stopped there). The result is that my numbers show a much greater difference between the two dimensions than yours show - moving the boat 15" forward only allows moving the axle center forward about 8" according to my calculations.

I have zeroed in a lot more accurately on a lot of the data which I had inadvertantly mis-reported to you. If you are not too tired of this whole thing yet, I'd be interested in e-mailing to you the more accurate data to plug in and re-run your calculations, to see if your final figures come closer to mine, or if I'm still out in left field.

Under any conditions, I am going to proceed with the project of measuring tongue weight at various axle-center locations before I put everything back together. I have now finally relocated all the cross members, bunks and keel rollers to accomodate the new boat location, and the new axles and disk brake system has arrived. I'll install the new coupler, then remove the springs and shackles so there is nothing left but the main trailer body and the boat. Then with a system of jack stands and two cheapo bathroom scales I picked up at WalMart for 5 bucks each, I'll have the combined capacity for measuring up to 600 lbs tongue weight without the need of any reductive levering at the tongue. I'm real curious to see just what the scales will show.

My thinking is that the absent wheels, hubs, axles and brakes won't affect the final tongue weight because they are what the boat and trailer essentially fulcrums on when I'd measure tongue weight when the project is complete; the absent springs and fenders won't affect anything because they both distribute their weight evenly before and behind the axle center. It seems to me that the test weighing will be close enough for my purposes.

Jerry Townsend posted 03-05-2002 11:55 AM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Kingfish - go ahead and e-mail your stuff to me ( and I will review it and get back to you.

Regarding neglecting the weight of the tires, wheels, hubs and axles - the weight of these components, per se, is satisfactory - but you can't neglect the upward force via those, because that is where 90 to 95 % of the weight is carried. It would be nice if the summation of moments were zero at the axle load point - but unfortunately, this is not the case.

No, I am not tired of this thing. In fact, and in view of the suggestions, comments, I noticed regarding your questions, I am writing a brief (I hope) discussion regarding the experimental and analytical approaches to balancing the loads on the trailers.

I look forward to seeing your e-mail and calculations. ===== Jerry/Idaho

DanT posted 03-05-2002 01:29 PM ET (US)     Profile for DanT  Send Email to DanT     
I'm not sure if what I have to add here will help solve this problem but perhaps someone will find this useful. I stumbled across a procedure for weighing a boat on a trailer using a bathroom scale. I'm restoring a 1966 Eastport and was concerned that the hull may have absorbed some water. Using this procedure I determined the hull weighs about 565 pounds. If anyone finds a flaw in this procedure, please let me know. Here it is.

1. Level the trailer and move the boat back
several inches. In my case I moved the
boat back about 6 inches.
2. Weigh the tongue. This is W1.
3. Pull the boat forward using the winch.
Accurately measure this distance. This is
distance d.
4. Weigh the tongue again. This is W2.
5. Measure the distance between the weighing
point of the tongue and the wheel axis.
This is distance s.
On my single axis trailer I used the
center line of the axle.

Formula Boat weight = (W2-W1)*(s/d)

Accurate measurements are important. I repeated this procedure twice and each time the calculated boat weight was within 10 pounds of the other. My hull was empty so I only had to subtract the weight of the motor (known), boat cover (known) and the rails (estimated). I hope someone finds this information useful.


triblet posted 03-06-2002 10:17 AM ET (US)     Profile for triblet  Send Email to triblet     
Jerry, my problem was that the fronts locked up
way before the backs. Once the tires lock,
they don't stick nearly as well as when they are
rolling (High School Physics: the static
coefficent of friction is ALWAYS higher than the
dynamic (sliding) coeficient.). If I wanted to
avoid flat spotting the tires, I couldn't brake
very hard. So I reduced the tongue weight and
all was well. This was on a '95 Pathfinder which
did not have four-wheel antilock.

But, absent anti-lock brakes, lockup happens.
It can happen because of an imbalance in load,
or an imbalance in friction, or because you
panic (been there). If you are
right on the verge of lockup, you will get
a very brief lockup as one wheel or another
crosses a paint strip.

The trick is to have all four lock up at about
the same time, with the front's just barely
before the rears so you go straight instead
of swapping ends. Most vehicles have a brake
balance valve in the hydraulic system to
handle this. It's tricky on a pickup becuase
the weight distribution can vary so widely
depending on load. Some have a load (height)
sensing valve in the hydraulics.


Jerry Townsend posted 03-06-2002 12:06 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
DanT - Your equation

BoatWeight = (Wh1 - Wh2)*X1/X2

Wh1 = hitch load, initial
Wh2 = hitch load, boat displaced
X1 = distance, hitch to axle
X2 = distance, boat displacement

is theoretically correct taking moments about the axle. However, I have a couple of comments :

1) the cg of the boat may initially only be an inch or two in front of the axle(s) so moving the boat back rearward of the axle, the boat and trailer would tip with the tongue high. This could cause safety or damage related problems.

2) The calculation may not give accurate results because you are taking the difference between two weights obtained via a bathroom scale and then amplifying any error in those readings many times (perhaps 25 to 100) by multiplying by the hitch to axle distance and dividing by a small number (1 to 6 inches). For example, if the weight difference is off by 2 pounds, with a hitch to axle distance of perhaps 16 feet and moving the boat 4 inches would give an error of 2 x 16 x 12 / 4 or 96 pounds. But, the calculation does get you in the "ball-park".


Tom W Clark posted 03-06-2002 12:38 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

565 pounds seems a bit light. Perhaps you could varify the weight with a trip to a certified scale. We would then see how accurate your estimate is.


I really like DanT’s formula and I am glad you have confirmed the validity of it’s theory. You have also answered my two questions about it too. In order to get any result at all you have to have a positive tongue weight in both positions in order to be able to weight the darn thing.

Your point about compounding errors is good as well, though if the cg of your boat is only an inch or two forward of the axle, you have far to little tongue weight to start with.


More tongue weight is a good thing in most cases because transferring weight to the tow vehicle usually helps the braking balance rather than making it worse (Your Pathfinder being an exception here). I mostly have towed with pickup trucks as many others do as well. Pickup trucks almost always have a huge tendency to lock up the rears well before the fronts because they have so little weight over the rear axles. Putting the tongue weight closer to 10% of gross will definitely IMPROVE to braking ability of the whole rig.

Other benefits include the fact that any weight carried by the tow vehicle is that much less weight carried by the trailer. My own experience is that it will track better as well. The weight added to the tow vehicle will also make a huge difference at the launch ramp where (in the absence of four wheel drive) a 100 lbs or 200 lbs extra on the back of the truck will make the difference between getting up the ramp while spinning your wheels or not.

Your Pathfinder is a (blessedly) modest sized SUV with a relatively short wheel base so this may be why these comments do not apply to you. The Pathfinder is, however a nice tow vehicle. I know, I towed my 18 Outrage with a 1990 3 liter V-6 (145 hp, I think?) Pathfinder for three years. It was more than adequate.

lhg posted 03-06-2002 06:25 PM ET (US)     Profile for lhg    
Jerry & Kingfish: I think you guys should get together and start the "Classic Whaler Trailer Company", designing and manufacturing keel roller trailers exclusively for Classic Boston Whalers.
kingfish posted 03-06-2002 08:17 PM ET (US)     Profile for kingfish  Send Email to kingfish     
Hey, welcome back, Larry-

I think when we're done here we'll have enough data for a text on the subject.

If Jerry (and Tom) and I go into business, the first thing we'll have to do is figure out how to make a profit and still give our labor away, because if we charged for our labor, nobody could afford to buy the first trailer...

I wound up going with Tie-Down Engineering stainless steel disk brakes, as well as Tie-Down's disk brake actuator and hollow spindle axles. Should be a "whale" of a trailer when we're done.

Stay tuned-


Jerry Townsend posted 03-06-2002 09:11 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Hey wait a minute you guys - you have to realize that I am retired - well, from everything except fishing, golf, having fun, photography In this case though, we are having fun. ---- Jerry/Idaho
triblet posted 03-06-2002 09:51 PM ET (US)     Profile for triblet  Send Email to triblet     
Tom, don't say more tongue weight is a good
thing in MOST cases. I agree that it may
well be with a pickup, but SUVs are
outselling pickups. And most folks that I
know that tow with a PU have 300 pounds of
dive gear in the back.

The best 200 pounds to put in the back for
more traction is your dive buddy. Have him
stand on the hitch. Don't let the cops see

Your 1990 PF was 170 HP, IRRC (I had two PFs
with the 3.0L).


Tom W Clark posted 03-07-2002 12:42 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

I will say: more tongue weight is a good thing in MOST cases. Actually I should be more precise. A tongue weight closer to 10% of gross trailer weight rather than 5% is a safer thing even if it is awkward having a heavy tongue, though if you are in that range you will be OK.

Even if SUVs are outselling pickups (an erroneous assertion), Pathfinders are NOT outselling pickup trucks. You could no more argue that the Pathfinder is representative of the size, wheel length and weight of the average SUV than you could argue that you and your dive buddies with your 300 lbs of gear are representative of the average pickup owning trailer boater.

I totally agree with you that you do not want your front wheels to lock up before your rear wheels but this just does not happen in most cases (yours excepted).

For getting up the ramp you are correct, a guy on the step bumper or hitch will be a tremendous help. My brother's one and only responsibility when I pull my boat out at the ramp is to get his butt back there.

kingfish posted 03-08-2002 05:29 PM ET (US)     Profile for kingfish  Send Email to kingfish     

E-mailed you *all* the data, and actual tongue weight measurements taken in five different notated locations of tandem axle center line.


rubadub555 posted 06-08-2003 01:31 PM ET (US)     Profile for rubadub555  Send Email to rubadub555     
What tongue weight are Montauk trailer users finding satisfactory?
My new Continental trailer seems to ride a bit skittish/light.

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