RV Hauler Axle and hitch placement

[beyerjf]

My only thought on the whole singling short thing. When I see a fifth wheel, any flavor mounted some distance( 48" or more behind the centerline of the rear axle, I believe you call it the "moment arm"), I see a lever with oscillations( up and down) that is compounded by 2 dimensions, the length of the moment arm itself, and wheelbase of the tractor. Worse case scenario, fairly short wheelbase tractor( 180") and a fairly long extension behind the axle(60"). And I have seen those dimensions on more that a few HDT's.

Since my tractor is a short wheelbase(168"), with a sliding 5th wheel that actually goes about 12" aft of the rear axle, to handle certain specialty trailers I have experienced first hand the degraded handling characteristics that this configuration is prone to, ranging from annoying to outright dangerous. All of those are caused by out of whack axle weights, like 8K on the front axle, 30K on the tractor tandems and 30K on the trailer.

As has been mentioned, if the weights are all as they should be, the handling is going to be fine, but the oscillations remain. Might be safe, but hard on the equipment(trailer). Hence the plethora of "bouncy " hitches out there.

Since I am not an engineer, I am sure one of you slide rule types could easily check to see how these dimensions affect this characteristic. If nothing else put some kind of a strain gauge measuring g forces up and down and see how it does.

I would go the more expensive route, stretch the frame or single long and keep the 5th wheel as close to the center line of the rear axle as possible. Every configuration has it's drawbacks.

 

There has been a mention of a common commercial application, car haulers with a "stinger" low mounted fifth wheel some distance( 48") behind the tandem axles. Not really a good comparison because the wheelbase of the tractor with a stinger is usually about 300", carrying 3 or more cars on the header racks, plus the fact that the stinger itself is 12" above the pavement and the trailer wheelbase is optimized to minimize the oscillations. In addition, the entire center of gravity is as low as possible with barely 6" clearance under the entire length of the trailer.

[Av8r3400]

 

Yes, that was a bare bone configuration, I was ferrying the truck. Bought it in Salt Lake City, had it singled by the Volvo dealer there and drove it 3,000 miles to New Hampshire. The problem with singling short is that you remove about 2,000 pounds from the rear (an entire axle), so it goes down from about 8,500 to 6,500 and becomes a real "light ass truck" and a real short base truck. Actually the first truck conversion I did was a truck that was singled long (mine only had a hitch in frame rails at that time). I was spooked by my truck at that time, so I wanted to see what a "bare bone" singled long truck would do bobtail. The difference was night and day, I couldn't "drift" the rear end even under heavy braking.

 

 

In this "bare" configuration, what is the weight difference on the rear end weather singled short, mid or long? The weight of the additional frame left on the truck only?

 

Looking at this with an eye toward weight and balance, the difference (weight on the axle) would be negligible.

[phoenix2013]

I would tend to agree, frame rails are typically formed fro 5/16 tempered plate and are probably about 10 pounds per foot, so even 10 extra feet can'r be more than 200 pounds, which is nothing compared to the weight of an axle, four tires, rims, brakes and suspension components that you remove singling.

[Av8r3400]

Sorry, I don't mean to be thick in the head, BUT...

 

Just the process of singling the truck makes it handle terrible? Does it make any difference in handling to single mid or long?

 

I"ve been contemplating a "short" single, with the (ET ) hitch up as close as possible to the rear end, in order to keep the truck maneuverable and to keep the OAL less than 65'. I have no want for a Smart bed.

[Steve from SoCal]

Any truck that is not laden with "some" weight on the back is going to be touchy. A tractor with a 5th wheel and no load on the rear axle/s has the weight of the axles, 5th wheel and perhaps 10-15% at most weight transfer from the rest of the chassis. A 17K tractor with tandem rear axles has about 6K on the back empty, remove the front axle and related stuff loose 2500-3000 pounds. So the truck has very disproportionate weight distribution, traction on the rear axle is limited further by the dual wheels spreading the weight. S0 on say 36 inches of tread you have 4000 pounds or just over 100 per inch of tread. That is why a bare singled truck is so dicey.

 

Steve

[Jack Mayer]

Sorry, I don't mean to be thick in the head, BUT...

 

Just the process of singling the truck makes it handle terrible? Does it make any difference in handling to single mid or long?

 

I"ve been contemplating a "short" single, with the (ET ) hitch up as close as possible to the rear end, in order to keep the truck maneuverable and to keep the OAL less than 65'. I have no want for a Smart bed.

That is exactly what I had. It handled fine with a body on it. Put the ET as close to the axle as space allows. You will have no problem with that. Determine your axle placement based on your needs. My wheelbase was 182". That is short. I drove that truck as a daily driver for over three years, as a full time RVer. And many miles. Overall, I put 125K miles on that configuration, with trailers ranging from 36'/17K lbs to 42' and 23K lbs. I NEVER had a handling issue with it. I know of many others, back in that era before smarts that did identically.

[Alie&Jim's Carrilite]

Sorry, I don't mean to be thick in the head, BUT...

 

Just the process of singling the truck makes it handle terrible? Does it make any difference in handling to single mid or long?

 

I"ve been contemplating a "short" single, with the (ET ) hitch up as close as possible to the rear end, in order to keep the truck maneuverable and to keep the OAL less than 65'. I have no want for a Smart bed.

PM me your e-mail and I'll send you pics of our truck. We're singled short at 183".

The only issue if you can call it an issue, is if the road is wet and you're at a stop light. When you accelerate past the white line, the drive tires have a tendency to "spin", break loose- loose traction on that strip of paint. When we are hooked up to the RV and have the extra 4850# of pin weight it doesn't happen.

 

Unloaded- No RV

Front axle- 10200

rear axle- 8125

 

Loaded-

Front- 9825

Rear- 12950

 

Front axle unloads 375lbs

Pin weight is 4825 (25-30 gallons of water on board) Water tank in half way between front trailer axle and pin.

Front axle is a 12k axle, rear is 19k.

Technically, I have enough weight room to carry a Smart car on the front axle of the truck but I can't find a big enough basket to put it in .

 

Our truck also has the "Baby" M11 Cummins that weighs less than the D-12 Volvo engine.

[DamianF]

My old KW is 255" wb to center of tandems, and the air hitch is directly over the rear axle. Pin weight roughly a shade under 5k, trailer nearly 21k as it sits today. With all my crap on the truck, plus the trailer weight (1 trailer at the moment), we are about a half of a fully loaded semi rig. From personal experience which isn't a lot compared to some, but still well over a million miles commercially, fair caution is required to get around on wet, snowy and icy roads, especially over a pass. As Jim described slipping tires on the white painted line, that issue doesn't really get fixed by adding 5k more on the hitch. Makes it a little better yes, but far from decent in my opinion. The last two times I've driven over Raton Pass between Colorado and New Mexico, weather was terrible despite forecasts otherwise. Personally, with the total weights involved here, the last thing I'd want is a hitch any significant distance behind the rear drive axle from a control perspective. I'm already light on the drives, and unless I add tons of steel, it'll always be that way. To this point I trust my experience to know when to give up the road for a rest stop.

[Ronbo]

Jack i sent you a spreadsheet to use. The numbers are from my rig 2014.

[The Few]

Not really much more on a semi than the heavier RV haulers. A semi is going to push the front end whenever loaded (as a generalization). My front, loaded, is pretty heavy. An RV Hauler is still going to be running between 9400 lbs (absolute worst case measured) and 11K+ on the front (more typical). That is not going to affect anything noticeably. And that is proved out in usage scenarios. Yes, they run more weight, but not that much more. And the other forces acting on the geometry are way more extreme.

 

As to a short singled truck, it really is not an issue. Or at least it has not proven to be, as LONG AS you have enough pin weight and do not have a lot of cantilevered force acting at the back of the trailer. Where I would worry about a short singled truck - or really any RV hauler - is when there is a very light pin weight relative to the total trailer weight, AND there is a lot of weight in the back of the trailer. Specialized Toyhaulers carrying many thousands of pounds in the garage have proven to be an issue. But that is not related to a short singling, but to the massive movement of force from the rear of the trailer acting on the front pin area. Set that in motion and you can have some "issues". In practice Henry and I have seen a few pretty severe examples of poor handling in that situation.

 

On the other hand, with a conventional trailer at 21+K lbs and 5000+ lb pin weight on a 182" wheelbase singled short Volvo 610 there were zero issues. Even running a heavier trailer at 23K and 6K+ pin I had no problems. That is over 120+K miles and 10+ years of fulltiming with those rigs. That tractor handled excellently in heavy winds and on poor condition roads. So given that other parameters were in normal ranges I see no issue with short singled trucks.

Hey Jack, thank you for answering my question, and I'm impressed of your knowledge.. ( Wish I was as clever)

 

However, I am not 100% convinced about your theory stemming from my question.. (single short/long stinger/frame)

Plausible deniability... I conciser myself to be (somewhat) clever, and I have also have also served as a Marine Corps Motor Transport Chief for Many, Many years.. Not that my Occupational title means anything, or I feel I am the duty expert on the subject, but I am absolutely a fact checker!

 

I do not just listen to people, and automatically assume they are right, but when it comes to me giving people advise, especially on a forum and creating the assumption that I know what I am talking about, I am at risk of giving other people the impression that whatever I say is solid gold, or it must be right..

 

I am very careful what I tell people, and how they perceive what they hear from me, because all it takes is someone taking my advise the wrong way, and end up being in a situation that was unplanned for that day! Tht is something I absolutely could NOT live with and have a clear conscience!

 

What I am trying to say ( in the most humble way), is that I am a fact checker, having knowledge and experiences myself, but my words and experiences work for "ME", and no one else.

I feel that we have a difference of opinion on the single short theory, and that is absolutely fine! I just don't like the single short and hitch behind the wheel too far to the rear theory, because in my opinion and experiences, it poses a very real threat..

 

Thank you for your video, I felt it was entertaining and had great insight!!

 

(excuse all of my grammatical errors...LOL)

[MrSeas]

opps.......I almost stepped on to my soapbox and made a big non PC mistake.....MY BAD!

[Dollytrolley]

Well Jack indeed this thread seems to pick some interesting view points among forum members.

 

Sometimes I try to remind myself that behind that LED lights and wiz-bang-cams and too many computers that the actual bones of these heavy trucks are little removed from the diesel truck of the 50's and 60's......just under the shinny skin these rigs are fairly well related to many older trucks.

 

Now of course New trucks have far more complex subsystems of emissions, ABS, Craeature comforts, Air Suspension, Auto-manual Transmissions, Etc, Etc...........However you can still obtain a "New" glider configured almost identical to a 1955 Kwopper and it will move a large down the road smartly in spite of very low tech.

 

 

What am I getting at here......well the deal is that OFTEN heavy trucks are ......"out-of-balance" because they are built to haul a HUGE range of loads over various types of conditions.

 

These trucks are........TRUCKS......lets not kid ourselves here.....when you strap one of these rigs on you need to put your big-boy panties on and consider that these things are NOT the latest and greatest in handling and starting and stopping and NEVER EVER should forget that you are a BIG dog among a swarm of little puppies that often seem to not know how much room we really need to operate safely.

 

Some months back Jeff mentioned that sometimes he needs to move loads that tax the limits of balance of his rig and during those times he draws on Decades and Millions of miles of experience to exert the skills and caution need to get the job done.

 

These trucks are NOT cars and they can be a dream come true .......but..... sometimes we need to move them when they are not a dream and during those time we need to adjust our operations to fit the conditions at hand.

 

Short, Long, Mid, hitch here and hitch there..... it's still up to the operator at the end of the day to be that safety factor that really matters......

 

Ok now I gotta have my prune juice with a shot of shine for a night cap .......rant over.....maybe....

 

Drive on........(try to keep some ....balance)

[Jack Mayer]

 

I feel that we have a difference of opinion on the single short theory, and that is absolutely fine! I just don't like the single short and hitch behind the wheel too far to the rear theory, because in my opinion and experiences, it poses a very real threat..

 

Thank you for your video, I felt it was entertaining and had great insight!!

 

 

No argument from me....I think we agree.

 

If you have too much "arm" on the cantilever you are going to have issues. And that has to take into account the overall wheelbase. I'm just saying that for a conventional hauler layout with 56" overhang and a relatively normal pin weight you should not experience that. But you have to take into account ALL of the parameters for the individual truck and circumstances you are dealing with. I've seen plenty of singled short 610/630 that handled just fine with a 56" cantilever. Not just my truck, I've probably driven a half dozen of similar ones. But you have to work the numbers to see if you are unloading the front too much. And we defined "too much" in our view. The typical RV hauler does not push the boundaries. But I've sure seen a few that would/should.

[The Few]

No argument from me....I think we agree.

 

If you have too much "arm" on the cantilever you are going to have issues. And that has to take into account the overall wheelbase. I'm just saying that for a conventional hauler layout with 56" overhang and a relatively normal pin weight you should not experience that. But you have to take into account ALL of the parameters for the individual truck and circumstances you are dealing with. I've seen plenty of singled short 610/630 that handled just fine with a 56" cantilever. Not just my truck, I've probably driven a half dozen of similar ones. But you have to work the numbers to see if you are unloading the front too much. And we defined "too much" in our view. The typical RV hauler does not push the boundaries. But I've sure seen a few that would/should.

 

Very well said sir!

[Refuzn-To-Grow-Up]

 

 

These trucks are........TRUCKS......lets not kid ourselves here.....when you strap one of these rigs on you need to put your big-boy panties on and consider that these things are NOT the latest and greatest in handling and starting and stopping and NEVER EVER should forget that you are a BIG dog among a swarm of little puppies that often seem to not know how much room we really need to operate safely.

 

Some months back Jeff mentioned that sometimes he needs to move loads that tax the limits of balance of his rig and during those times he draws on Decades and Millions of miles of experience to exert the skills and caution need to get the job done.

 

These trucks are NOT cars and they can be a dream come true .......but..... sometimes we need to move them when they are not a dream and during those time we need to adjust our operations to fit the conditions at hand.

 

Short, Long, Mid, hitch here and hitch there..... it's still up to the operator at the end of the day to be that safety factor that really matters......

 

I agree and well said. Each time I step into the cab of my truck, I accept full responsibility for the fact that I am piloting a very heavy rig that does not handle and/or stop like a car.

 

Ok now I gotta have my prune juice with a shot of shine for a night cap EWWW!. I do NOT agree.

 

[peety3]

Any truck that is not laden with "some" weight on the back is going to be touchy. A tractor with a 5th wheel and no load on the rear axle/s has the weight of the axles, 5th wheel and perhaps 10-15% at most weight transfer from the rest of the chassis. A 17K tractor with tandem rear axles has about 6K on the back empty, remove the front axle and related stuff loose 2500-3000 pounds. So the truck has very disproportionate weight distribution, traction on the rear axle is limited further by the dual wheels spreading the weight. S0 on say 36 inches of tread you have 4000 pounds or just over 100 per inch of tread. That is why a bare singled truck is so dicey.

 

Steve

We've already beat that horse into the ground. Traction does not change by dual wheels vs. single. It does not change by over-/under-inflating the tires. It changes by switching to a tire with a different rubber compound (that changes the coefficient of friction). When it's time to step on the GO pedal, a major axle weight imbalance will impact your traction, as the drive axle(s) are trying to launch 100% of the vehicle weight while only getting the traction of X% of the vehicle weight (where X = axle weight / total weight). When it's time to step on the STOP pedal, ABS will overcome the weight imbalance. Same goes with throwing more weight on the truck in the winter: it'll help you GO (by changing the driven/undriven weight distribution), but has zero effect on how quickly you can STOP.

[NoDirectionHome]

Wow! Wish I could break the laws of physics like most of you... Where's that darn wizard hat?

[MrSeas]

Will this "horse" ever DIE!

[phoenix2013]

Well Jack indeed this thread seems to pick some interesting view points among forum members.

 

Sometimes I try to remind myself that behind that LED lights and wiz-bang-cams and too many computers that the actual bones of these heavy trucks are little removed from the diesel truck of the 50's and 60's......just under the shinny skin these rigs are fairly well related to many older trucks.

 

Now of course New trucks have far more complex subsystems of emissions, ABS, Craeature comforts, Air Suspension, Auto-manual Transmissions, Etc, Etc...........However you can still obtain a "New" glider configured almost identical to a 1955 Kwopper and it will move a large down the road smartly in spite of very low tech.

 

 

What am I getting at here......well the deal is that OFTEN heavy trucks are ......"out-of-balance" because they are built to haul a HUGE range of loads over various types of conditions.

 

These trucks are........TRUCKS......lets not kid ourselves here.....when you strap one of these rigs on you need to put your big-boy panties on and consider that these things are NOT the latest and greatest in handling and starting and stopping and NEVER EVER should forget that you are a BIG dog among a swarm of little puppies that often seem to not know how much room we really need to operate safely.

 

Some months back Jeff mentioned that sometimes he needs to move loads that tax the limits of balance of his rig and during those times he draws on Decades and Millions of miles of experience to exert the skills and caution need to get the job done.

 

These trucks are NOT cars and they can be a dream come true .......but..... sometimes we need to move them when they are not a dream and during those time we need to adjust our operations to fit the conditions at hand.

 

Short, Long, Mid, hitch here and hitch there..... it's still up to the operator at the end of the day to be that safety factor that really matters......

 

Ok now I gotta have my prune juice with a shot of shine for a night cap .......rant over.....maybe....

 

Drive on........(try to keep some ....balance)

 

I used this picture on another thread in another context, but I am using it again to illustrate another concept.

 

Technical threads, although immensely useful, can become tiresome when all the points of view have been exhausted but there remains just "one more thing" that needs to be "defended". After the readership has been sufficiently beaten into a bunch of sheep carrying it beyond makes one look like an ass. The reason I quote Dolly above because he contributed the best summation.

 

These trucks are........TRUCKS......lets not kid ourselves here.....when you strap one of these rigs on you need to put your big-boy panties on and consider that these things are NOT the latest and greatest in handling and starting and stopping and NEVER EVER should forget that you are a BIG dog among a swarm of little puppies that often seem to not know how much room we really need to operate safely.

 

I don't know what I strap on, but it certainly isn't what I normally wear. I started wearing it in the mid 70's,

 

 

when I strapped an 11 foot slide-on onto the back of my pickup, approached an intersection in a "normal fashion", put the brakes on when the light turned yellow, watched it turn red and finally stopped 3/4 into the intersection 10 feet from the other side. "Wow, this SOB doesn't stop like a regular truck"!!!!!!!!!!!!! Second time I almost needed a change of what I was wearing, first time was when I was 16, got my license and attempted my first stop with the snow on the street.

 

Yes, whatever you do on the road you have to approach it with the "right attitude", then it doesn't matter, what the road condition are, how much you weigh, what your length is, or where the hitch sits. When I had pickups I drove them like regular cars (empty), the minute I strapped that slide-on on my attitude changed, it would change more with a 32 footer behind and even more with a 36 footer. It became thoroughly "unpleasant" with a 40 footer behind a pickup. It became "more pleasant" with an MDT and thoroughly pleasant with an HDT. But I am always weary of the fact that even though I "might be OK", because of my weight and length I can cause a massive damage to others and it doesn't matter if it is because of my stupidity or theirs.

 

Few years back I was asked to help ferry a 40 footer behind a Volvo "singled short", with a hauler body and a hitch "behind the rear axle".

The owner asked me to do it since he never had done anything like this (he did RV in "smalls things"). It was a good thing that the ferry distance was 3,000 miles. He caught on pretty quick but it took that distance to instill in him the "right attitude", which included, "take this turn wide, no wider, go all the way into the opposite lane, screw the guy coming up, he'll have to stop short and wait". "Don't do jackrabbit take offs, everything is flying in your fifth. You worried about the guy behind you is getting impatient? He's driving a shitbox, you just payed over $100K for your rig". "Look at this idiot coming on from the intersection, look, look, HORN, HORN!" "Those speed signs on the off ramps, they are for you and you are not in a car. I did not enjoy the feeling of your fifth being on two wheels on that last off ramp". The last one was a habit hardest to change, finally I got pissed and told him, "I used to do aerobatics in an airplane, nothing scares me, that last off ramp scared the crap out of me". The last 1,000 miles we were OK, HE GOT THE ATTITUDE!

Regardless, trip like this really showcases the beauty of our coutry and what RVing is all about. We started in Utah.

 

 

 

Went through Rockies and Colorado

 

 

 

Midwest

 

 

And finally in Massachusetts I pushed the rig into a 300 foot driveway,

 

 

 

and it's final parking spot. When it comes to backing there is nothing like those Volvo mirrors.

 

 

We accomplished the "attitude change" and the trip was a blast.

[Star Dreamer]

Will this "horse" ever DIE!

I actually think this is a very good topic. Jack, the video was very good.

 

We are one of the singled short trucks with a long overhang.

 

When we bought out truck, the hitch was 12" behind the axle but sat too high as it could not be lowered because of equipment for the axle. This raised our trailer up too much in front and caused it to be un-level. We had 9880# on our front axle and 5700# on the rear axle unhitched. 63% on front and 37% on rear. When hooked up, our pin weight was pretty light at only about 3000# and only increased the rear axle and did not off load the front axle much. Our biggest issue at that time was using the Jake brake in the rain once and only once because we never used it after that in the rain before the bed was built. We now can use it on low without any issues.

 

When we designed the bed, we had a couple of criteria we wanted to be able to do, one was have the space for a smart car if we ever went that way, second was to have the space to put our van on the back to go to shows that we did not need the trailer for. That meant we needed a bed which is 13'10" long to get the vans tires on the bed (the back of the van would overhang). This also put the hitch at 50" behind the rear axle which is the closest I could go and have clearance with the lower part of the trailer and bed in corners. After we built the bed the truck weight when trailer was unhitched went up by 3300# and due to the overhang the front axle weight went down to 9340# and the rear axle went up to 9540# without much stuff in the storage boxes. we were now at a what i feel is a good 50-50% weight distribution. Hitched up the trailer and our van the back of the trailer, our loads were now at 8740# on the front axle, 14600# on the rear axle, which included about 4400# pin weight which also included full water tanks of 130 gallons on the trailer.

 

Our pin weight changes dramatically based on what we have in the trailer and where it is located. When we weighed at the RV Rally in Hutchinson in 2013, Truck without the trailer was 9450# on front axle, 10250# on rear axle (more items in our storage boxes on the bed) and hooked with the van in back of the trailer but only about 20 gals of on board water, front axle was 8250#, 36% (about 1200# off loaded) and 14650# 64% on rear axle which included a 3200# pin weight, 18% of trailer weight. We were close to overloading our rear axle on the trailer but it was still below but by only 50#. I set about the task of trying to shift some weight to the truck.

 

At the ECR rally this year, we reweighed but with a car in the back of the trailer. I did manage to shift some weight to the truck, unhooked we were 9525# on the front axle and 10675# on the rear axle. Hooked up we were 8725#, 36% on front and 15825#, 64% on rear axle with 4350# pin weight, 25% of trailer weight. Surprisingly with the lighter car in the trailer, I lightened up the two front axles on the trailer but not the rear one and added more pin weight.

 

We have not experienced any unusual handling conditions (except for the Jake brake issue in the rain which was easily solved by turning it off). These trucks are tall and heavy and they are not sport cars.

 

With our set up, I can move the hitch rearward a maximum of 85" from the rear axle. If I go to carry a smart (older style) I would need to move the hitch back at least 8" if I remove my side fairings and no drom box. If I did not want to remove the fairings, I would have to move the hitch approximately 23" back or a total of 73" from the axle. I do not plan to move the hitch all the way back to 85" but maybe I will experiment if I get spare time once I retire.

 

Since we are in the middle of designing a new trailer, our biggest challenge will be to keep the weight weight in a good percentage at the same time not increasing it so much that the truck offloads the front axle too much. We will be crunching the spread sheets to do our best to make it all work as I hate to have to buy another truck.

 

We also know that how you drive your truck is a big factor on if it feels unsafe. We keep our speed to around 63 MPH (mainly because of our trailer tires) and while I will pass on the other trucks on the way up the mountains or hills, they will pass me on the way down as I keep it to the slowest possible speed without using brakes and just using the Jake brakes as much as possible. Our trailer sits much lower than the normal RV trailer so that may help some by keeping its center of gravity lower.

 

Dave

[noteven]

In Alberta the heavy truck hitch offset to rear dimension allowed for stiff (non wagon) trailers is 1.6m = 63 inch from the centreline of the rear axle or axle group. This is covering units where the trailer typically weighs about the same as the truck, and have low relative coupler ("pin") weight in relation to their gross weight. Common one would be a tandem drive tank or dump truck 16,000lbs front, 37,500lbs drives, and 50,725lbs on the trailer tri axle set.

 

So looking at a 18,000lbs trailer being pulled by a 18,000lbs truck (or less) I am thinking offsets in the same range might lead to good behaviour steering and front braking wise...