T/T Gap
- Thread starter P51bombay
- Start date
It depends on the kingpin depth on the trailer. A trailer with a 36 inch pin can be set closer to your cab than a trailer with a 24 inch pin.
It can also depend on your 5th wheel height (yes height!). A taller 5th wheel will rotate more meaning the trailer can roll into the top of the cab more.
It also depends on the weight you have on your steer axle.
It depends on a bunch of stuff and is something that isn't always carefully thought out when things are spec'd.
Last group of trucks I bought were for a reefer fleet. Trailers had 36 inch pin depth. Tractors had a 215 wheelbase with a 58 inch bunk. 5th wheel was low mount 6.75 inch. With the reefer trailer loaded the "ideal" 5th wheel setting was 6 to 8 inches forward of the centerline of the rear axles, trucks had a 24 inch slide on the wheel. Memory serves me we ended up with a 20 inch gap between the extenders and the trailer front. It was the tightest I'd ever spec'd a truck and it paid off with good fuel mileage but it left little room for rookies to knife it into a dock.
It can also depend on your 5th wheel height (yes height!). A taller 5th wheel will rotate more meaning the trailer can roll into the top of the cab more.
It also depends on the weight you have on your steer axle.
It depends on a bunch of stuff and is something that isn't always carefully thought out when things are spec'd.
Last group of trucks I bought were for a reefer fleet. Trailers had 36 inch pin depth. Tractors had a 215 wheelbase with a 58 inch bunk. 5th wheel was low mount 6.75 inch. With the reefer trailer loaded the "ideal" 5th wheel setting was 6 to 8 inches forward of the centerline of the rear axles, trucks had a 24 inch slide on the wheel. Memory serves me we ended up with a 20 inch gap between the extenders and the trailer front. It was the tightest I'd ever spec'd a truck and it paid off with good fuel mileage but it left little room for rookies to knife it into a dock.
arkjarhead
Veteran Expediter
The main thing is make sure you aren't getting your axle weights all out of whack. Wouldn't want any tickets.
Sorry P, I'm not known for simple quick answers
.Let me put it this way,
A 102 inch wide trailer with a 24 inch deep pin will need 36 inches of space from nose to back of cab to allow the corner of the trailer to clear the cab when the trailer is at a 45 degree angle .
The same trailer with a 36 inch deep pin needs 27 inches of space.
These numbers are calculations. Please don't post back and hold me responsible for the muffler being wiped off the back of your truck!! These numbers are absolutes, I WOULD ADD 6 INCHES JUST TO BE SAFE until you and a buddy can check it out slowly in a parking lot.
greg334
Veteran Expediter
So Piper, I don't want to seem dumb about the subject but.....
I am out west and here flies by a KW looonnnnggg wheel base W900 that has been chopped and lowered with about a gap between the trailer and the sleeper the size of my truck (or seemed to be), what's up with that?
I know I could stick my Caravan in that spot.
I am out west and here flies by a KW looonnnnggg wheel base W900 that has been chopped and lowered with about a gap between the trailer and the sleeper the size of my truck (or seemed to be), what's up with that?
I know I could stick my Caravan in that spot.
Sometimes it's all about lookin' cool! yeeechhh.
There is the bridge law way of calculating max GVW'S and from what I remember in the "good old days" each state was allowed to do it however they wanted. Many states favored the idea of very long vehicles to spread out the weight. This is why you sometimes see dump trucks or cement mixers with "training wheels" tag axles stretched way out behind. I think AZ still favors this. The fed stepped in and said 80,000 and under you all gotta have the same rules to get fed funds, above 80k do whatever. MI for example likes lots of axles instead of length, hence the 9 axle 42 foot dump trailer!
One fleet I was at the owner wanted one of these stretched monsters, had a 12 foot gap on a KW cabover. Truck got a whopping 3.9 mpg! Ya....real cool.
There is the bridge law way of calculating max GVW'S and from what I remember in the "good old days" each state was allowed to do it however they wanted. Many states favored the idea of very long vehicles to spread out the weight. This is why you sometimes see dump trucks or cement mixers with "training wheels" tag axles stretched way out behind. I think AZ still favors this. The fed stepped in and said 80,000 and under you all gotta have the same rules to get fed funds, above 80k do whatever. MI for example likes lots of axles instead of length, hence the 9 axle 42 foot dump trailer!
One fleet I was at the owner wanted one of these stretched monsters, had a 12 foot gap on a KW cabover. Truck got a whopping 3.9 mpg! Ya....real cool
.
P51bombay
Expert Expediter
Sorry P, I'm not known for simple quick answers
Please don't post back and hold me responsible for the muffler being wiped off the back of your truck!!
:LOL That's actually very good, just what I was looking for. Is that with or without extensions or do they matter?
Which pin depth is more common?
Extensions don't matter till you get close to 90 degrees.
24 and 36 are both common. If your looking for a "set it and forget it" set it up for a 36 inch pin.
Just remember to watch your axle weights!
Trailer Aerodynamics
Trailer aerodynamics can be improved minimizing the
gap between the tractor and the trailer, to reduce air
turbulence. Specifying wheelbase and fifth-wheel
settings that position the trailer as close to the rear of
the tractor as possible can reduce tractor-trailer gap.
Reducing trailer gap from 45 to 25 inches could
improve fuel economy as much as 2 percent.
http://www.epa.gov/smartway/documents/aerodynamics.pdf
Wheelbase and fifth-wheel setting are important considerations. For aerodynamics,
get the trailer as close to the back of the cab/sleeper as possible.The narrower the
gap, the better the aerodynamics. For example, decreasing the trailer gap by 10
inches represents a 0.5% to 1% increase in fuel economy.
http://www.kenworth.com/FuelEconomyWhitePaper.pdf
Tractor-Trailer Air Gap
To minimize drag resulting from crosswinds and turbulent air, tractor-trailer gaps should be minimized or aeroskirts should be used to smooth the airflow. Beyond approximately 30 inches, every 10-inch increase in tractor-trailer air gap increases aerodynamic drag by approximately 2%. If axle weights allow, slide the fifth wheel forward to minimize the gap and reduce wind resistance.
Rock
http://www.everytime.cummins.com/every/pdf/MPG_Secrets_Whitepaper.pdf
Turbulence in the area of the gap creates a tremendous amount of drag on the
vehicle over and above the air that sucks back into that area hitting up against
roughly 72 sq-ft of flat trailer. Since closing the gap completely on all four sides is
impractical, the next best thing is to narrow or bridge the gap.
Tractor side fairings help, but a fairing kit that extends to the top of the cab helps
even more.The red T2000 in the photo has fairing extenders that run to the top, while
the white T600 does not. Kenworth’s principal engineer, Alec Wong, estimates the tall
extenders can improve aerodynamics by 2-3 percent.
The width of the gap is another issue: the wider the worse.Wong says any gap wider
than half the width of the trailer (96 in. to 102 in.) is a real problem. He suggests careful
attention be paid to spec’ing tractor wheelbases and trailer kingpin setting
http://todaystrucking.com/images/TT-Jan-06.pdf
Trailer Gap Reduced from 46” to 38” + 1.0% to 2.5%*
http://www.kenworth.com/brochures/FuelEfficiency.pdf
http://www1.eere.energy.gov/vehiclesandfuels/pdfs/hvso_2006/06_clarke.pdf
http://www1.eere.energy.gov/vehiclesandfuels/pdfs/hvso_2006/02_mccallen.pdf
Trailer aerodynamics can be improved minimizing the
gap between the tractor and the trailer, to reduce air
turbulence. Specifying wheelbase and fifth-wheel
settings that position the trailer as close to the rear of
the tractor as possible can reduce tractor-trailer gap.
Reducing trailer gap from 45 to 25 inches could
improve fuel economy as much as 2 percent.
http://www.epa.gov/smartway/documents/aerodynamics.pdf
Wheelbase and fifth-wheel setting are important considerations. For aerodynamics,
get the trailer as close to the back of the cab/sleeper as possible.The narrower the
gap, the better the aerodynamics. For example, decreasing the trailer gap by 10
inches represents a 0.5% to 1% increase in fuel economy.
http://www.kenworth.com/FuelEconomyWhitePaper.pdf
Tractor-Trailer Air Gap
To minimize drag resulting from crosswinds and turbulent air, tractor-trailer gaps should be minimized or aeroskirts should be used to smooth the airflow. Beyond approximately 30 inches, every 10-inch increase in tractor-trailer air gap increases aerodynamic drag by approximately 2%. If axle weights allow, slide the fifth wheel forward to minimize the gap and reduce wind resistance.
Rock
http://www.everytime.cummins.com/every/pdf/MPG_Secrets_Whitepaper.pdf
Turbulence in the area of the gap creates a tremendous amount of drag on the
vehicle over and above the air that sucks back into that area hitting up against
roughly 72 sq-ft of flat trailer. Since closing the gap completely on all four sides is
impractical, the next best thing is to narrow or bridge the gap.
Tractor side fairings help, but a fairing kit that extends to the top of the cab helps
even more.The red T2000 in the photo has fairing extenders that run to the top, while
the white T600 does not. Kenworth’s principal engineer, Alec Wong, estimates the tall
extenders can improve aerodynamics by 2-3 percent.
The width of the gap is another issue: the wider the worse.Wong says any gap wider
than half the width of the trailer (96 in. to 102 in.) is a real problem. He suggests careful
attention be paid to spec’ing tractor wheelbases and trailer kingpin setting
http://todaystrucking.com/images/TT-Jan-06.pdf
Trailer Gap Reduced from 46” to 38” + 1.0% to 2.5%*
http://www.kenworth.com/brochures/FuelEfficiency.pdf
http://www1.eere.energy.gov/vehiclesandfuels/pdfs/hvso_2006/06_clarke.pdf
http://www1.eere.energy.gov/vehiclesandfuels/pdfs/hvso_2006/02_mccallen.pdf
nightcreacher
Veteran Expediter
Guess your refering to a chrom shop mofia tr4uck,low rider with a long stretched out frame./being cool costs,thats the ultimate cool,unless you have to buy the fuel
P51bombay
Expert Expediter
This isn't the truck but it is looooonng. Factory 350WB.
YouTube - PULLING PROJECT 350 OUT OF TIGHT DOCK!