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Thread: Fifth Wheel Position: Science vs Opinion

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    sdloe is offline Rookie
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    Default Fifth Wheel Position: Science vs Opinion

    Without respect to weight distribution (because this changes the whole topic), I see sliding the fifth wheel forward as far as possible because:

    1. Reduces the turn radius, important for us city-delivery drivers.
    2. Improves fuel economy (research shows).
    3. Here is the opinion vs. science part: By placing more weight on the steers, this places more stress on the front springs thus reducing 'stiffness' and absorbing bounce. Yet so many have the opinion that sliding the fifth wheel back improves ride. What say you? When in a sleeper & loaded heavy, I always slid it forward as much as I could get away with and never had issues with the smoothness. On my present daycab, hauling LTL, sliding forward seems to roughen the ride & bounce more, though the other benefits outwiegh the ride. Opinions?

    Steve

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    1) agreed.

    2) I use Air Tabs to help with aerodynamics of having more gap between tractor and trailer

    3) I tend to keep the front in about the 12,000 to 12,100 # range. Nice balance of ride and weight distribution. I have not noticed any real problem with component and tire wear keeping weight up at this level. I am not able to "suck" the trailer up to the tractor that much though, since I have 150 gallon fuel tanks and my Intl 9400 is a little heavier anyway than a lot of aerodynamic tractors. I am of the opinion that the aerodynamic trucks with set back front axles tend to ride better with more weight up front whereas the classic style trucks with traditional set forward front axles tend to ride better when weight reduced up front. Of course, in places like Chicago, who can really tell.

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    allan5oh is offline Senior Board Member
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    Running the 5th wheel up to reduce turning radius.... Hmmmm....


    I`ll add some spin to that. Say the 5th wheel was on top of your steer axles. Your trailer will start turning faster, because it reacts faster to the steer axle. If it`s all the way to the back, it does the opposite, reacts slower. I think having the 5th wheel further back is actually better for going around corners.

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    I can't say that I've noticed much difference in the turn radius with the fifth wheel forward as opposed to back. The biggest factor is the position of the trailer tandems.

    I've heard that the closer the trailer is to the back of the cab the better the fuel mileage due to less wind resistance. I'll buy that. Just make sure its far enough back that you dont hit the cab with trailer if you go over an angled driveway or parking lot or turn on a hill at an angle. If you take your hand, make a fist and put it against the back of the cab your elbow should not touch the front of the trailer. if it does you are too close and should be very careful.

    As for ride? I find that I get less bounce if I keep the steers near 11500 or so. Seems to ride nicer but the trailers and trucks are all air-ride and its been some time since i've dealt with springs.

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    sdloe is offline Rookie
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    Default Turning Radius

    Don't have any exact science here, but my theory is as follows: A shorter wheelbase (steering box ratio ignored) equals a shorter radius. I've played with this some myself, and for our routes it really doesn't make a big difference but I still notice 99% of the 'city trucks' set up the same way. An extreme example is the new style of long framed trucks we see, they are hilarious to watch when they have a tight corner. The wheelbase refered to is the entire length of the unit, front axle to trailer tandems. Make sense?

    Steve

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    allan5oh is offline Senior Board Member
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    The wheelbase stays the same. The wheelbase is considered from the kingpin to the rear tandem. What you are modifying is where the front of the trailer attaches to the truck.

    Consider this, you are half way through a turn (the truck is). Now, consider if you had the trailer attached right at the front of the truck. The trailer would be turning earlier then it should be. If you had it attached at the rear, the trailer "follows" later, taking a wider berth. If you had it attached to the front axle, the truck would be impossible to manouver.

    But you also have to consider the drive wheels don't follow the steering exactly. Hmmm...

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    ok, I'm sure there is some geometry thing here we could use to explain this out but I'll let someone else deal with it.

    I'm just saying that the distance you can move your fifth-wheel on an adjustment might make a little difference but pales in comparison to the amount you can shift the trailer tandems. The wheelbase on the tractor makes more of difference. I've driven setback axle trucks and the turn radius is much better and I prefer them but I still never had much of a problem with most areas I've been in either way. I think that unless you have one of those monster spread out trucks you shouldnt worry too much about the tractor and concentrate on the trailer.

    If you want a comparison of the difference the wheel base on the tractor makes watch a yard-goat sometime. The difference in his wheelbase and yours is something like the difference in your standard truck and one of those stretched trucks. But among standard trucks....I'll take shorter every time.

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    When you do city driving with a tandem axle tractor and a long trailer, you will corner better with the 5th wheel as far forward as possible because the overall length of the unit is shorter then if you have the 5th wheel all the way back. To verify this, next time your in a gravel parking lot with some spare time, try making a 90 degree corner with the 5th wheel all the way forward and then all the way back. Then measure the distance at a right angle from the tractor tandem tracks to the trailer tandem tracks in both tests. You will find that the trailer will track farther away from the tractor when the 5th wheel is back then it does when it's forward. It's basic geometry, the longer the overall unit is, the more room it will require to corner.

    As far as the ride of the vehicle, I think it's more personal preference but most trucks have a 12,000 pound front axle in them so exceeding that could cause problems at a scale. I generally preferred to run the the 5th wheel about 6 to 10 inches forward of the center point between the tandems. I found that running it farther back then that causes the steers to slide in a turn under power.
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    allan5oh is offline Senior Board Member
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    Quote Originally Posted by Shnerdly View Post
    You will find that the trailer will track farther away from the tractor when the 5th wheel is back then it does when it's forward.
    If it tracks further away, then the trailer will go around corners easier. You WANT the trailer to be further back. Now think of it this way, what if you had a 10 foot extension off the back of the truck and attached the trailer 10 feet back? Have you ever driven a vehicle where the trailer attaches much further back then the rear axle? The tail swing works to your advantage. When pulling a boat with my parents motorhome, the motorhome has so much tail swing that the boat wheels follow the rear axle of the motorhome almost exactly. It's like you're not even driving with a trailer.

    It's basic geometry, the longer the overall unit is, the more room it will require to corner.
    This is oversimplifying things. Besides, triple 28's will corner better then one 53.

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    allan5oh - "If it tracks further away, then the trailer will go around corners easier. You WANT the trailer to be further back."

    It depends on what your goal is. Making the corner easier too me means less torque on the trailer tandems. Thats not what I'm talking about. I'm talking about making a corner in a tight area. The longer your overall unit is (5th wheel back), the farther you have to go into the intersection with the tractor before you start your turn in order not to go up the curb. The farther the trailer tracks away from the tractor, the more room it requires to make the corner.

    allan5oh - "Besides, triple 28's will corner better then one 53."

    I agree however in most states, multiple trailers are only allowed on the interstates and routes to and from. I got the impression that this thread was primarily about driving in the city.
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    allan5oh is offline Senior Board Member
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    I don't think it much matters, the difference would be small.


    But I still think I'm right

    I'll try to come up with something tonight.

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    as far as smooth ride on the steers... if you have flat spots, or even just bad trailer tires your front can bounce & feel like the prob is in the steers.

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    Truck drivers thinking.... what great entertainment!
    The reason I'm a narcissist is cause everyone else is so lame.

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    Quote Originally Posted by sdloe View Post
    Without respect to weight distribution (because this changes the whole topic), I see sliding the fifth wheel forward as far as possible because:

    1. Reduces the turn radius, important for us city-delivery drivers.
    2. Improves fuel economy (research shows).
    3. Here is the opinion vs. science part: By placing more weight on the steers, this places more stress on the front springs thus reducing 'stiffness' and absorbing bounce. Yet so many have the opinion that sliding the fifth wheel back improves ride. What say you? When in a sleeper & loaded heavy, I always slid it forward as much as I could get away with and never had issues with the smoothness. On my present daycab, hauling LTL, sliding forward seems to roughen the ride & bounce more, though the other benefits outwiegh the ride. Opinions?
    Chances are, if you are running with the fifth wheel as far forward as possible, you are greatly exceeding the maximum load rating of your steer tires, if not the axle itself. The most common steer tires have a load rating of 6125 pounds, for a total of 12,250 lbs. I run with my fifth wheel one notch forward of center and that puts me around 12,300 loaded and about 11,800 empty with full fuel tanks.

    As others have noted here, turning radius refers to how tight of a circle (a full 360 degrees) the TRACTOR is able to make with the steering wheel turned to “lock” one way or another. Fifth wheel position has no bearing on this. It does affect total length of the combination vehicle, but the difference would be negligible as far as maneuverability of the entire vehicle is concerned. You are talking about a difference of perhaps 18 to 24 inches in the total length of a 65 – 70 foot-long (give or take) vehicle so the difference could be as much as two or three percent… not enough IMO to overload your tires or steer axle.

    Fuel economy is certainly affected when the gap between the back of the tractor (or end of the cab extenders if you have them) exceeds 36 inches. The shorter the gap, the less the wind resistance will be as it whips around the back of the tractor and impacts the front of the trailer.
    Last edited by Musicman; 12-29-2008 at 05:52 PM.
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    Musicman's Avatar
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    Quote Originally Posted by allan5oh View Post
    The wheelbase is considered from the kingpin to the rear tandem.
    I guess you MIGHT be technically correct, given the fact that sdloe seems to be talking about the whole combination vehicle, but isn't the term "wheel base" generally applied to the tractor only? I can't recall ever hearing anybody using that term when talking about overall distance between trailer axles and tractor axles or the kingpin. But if we were to apply "wheel base" to the whole combination vehicle, wouldn't we want to measure from the center of the steer axle to the center of the rear trailer tandem?
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    allan5oh is offline Senior Board Member
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    No, because it's an articulating vehicle.

    "wheelbase" for the trailer is kingpin to middle of rear tandem set.

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    Quote Originally Posted by allan5oh View Post
    No, because it's an articulating vehicle.

    "wheelbase" for the trailer is kingpin to middle of rear tandem set.
    I agree, for the trailer, but not the whole combination vehicle.
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  19. #18
    can-do is offline Rookie
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    I set my fifth wheel dead center about three yrs ago and haven't moved it since with no problems to speak of. I keep decent rubber on both tractor and trailer. I have no weight issues that can't be resolved by sliding the trl tadems. Good smooth ride ( as smooth as it gets )on 05 fl columbia.
    Last edited by can-do; 12-30-2008 at 02:31 PM.

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    Lngtaltxn is offline Rookie
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    Quote Originally Posted by allan5oh View Post
    No, because it's an articulating vehicle.

    "wheelbase" for the trailer is kingpin to middle of rear tandem set.
    Actually, "wheelbase' is measured between pivot points, which for a tractor would be from front spindle to the centerpoint between the rear tandoms which is where the rear pivots.

    Lngtaltxn

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    i got mine set in the center, seems to ride the best there with the leaf spring trailer behind it. plus i store vee boards on the catwalk so i make sure i have enough swing to clear them.

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