Question:
When you get out of truck school, you'll spend more times driving just your tractor on streets and highways sometimes.
You would think, without the trailer, it would handle and perform more like a car or light truck.
Wrong!
The rear-end only weighs about 8,000 pounds. That's only 1,000 on each of the hard (high pressure ) tires. That sounds like a lot, but because of the tire's stiffness, it's pavement "footprint" is tiny and floats above loose grit and dirt on the road.
The rear suspension, designed to carry some weight, doesn't really absorb any roughness in the road. So those eight wheels are basically bouncing continously as the weight's shifted from one rock-hard tire footprint to another.
And here's the clincher: The brakes only work on the first drive axel.
The second drive axel brakes are operated through the trailer brake system when a trailer is attached and pressured-up.
So when you're bobtailing, you've got a skittery rear-end bouncing around, only half your brakes work, and they'll lockup and skid with even light braking pressure.
Bobtailing is EXPECIALLY dangerous on wet or slick roads, since it only compounds existing traction problems.
So leave lots of following distance and watch your throttle on turns. The rear-end is just barely keeping traction with the best road conditions. On wet or slick roads, your drive axel's will skid at the slightest opportunity.
Note: Take a couple seconds to wipe-off your the rear bobtail lights before taking it on the road. They pick-up a lot of dirt under a trailer. Without the trailer, those dirty lights are all you've got back there to show yourself at night, or communicate your turns and braking, etc. So wipe them off and and make sure you're as well seen as you would be with a big'ol trailer back there.
[This message was edited by Shuffler on May 26, 2003 at 14:43.]

Answer:
Shuffler
Thanks for the post.
Had wondered 'bout this. Just got CDL Friday and be looking for work soon. (Graduate in June). Scanty mention of bobtail in school 'though info was on safety films.
Frankly this info is the kind that a newbie needs but may overlook even when receiving the info in class 'cause they're concentrating on regs, driving, and backing skills.
-->
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Answer:
I don't think I would get paranoid about bobtailing. It really isn't dangerous to do. A little care on turns and acceleration and there's no problem. Rainy roads can turn you around quickly on turns but not a problem with slow acceleration. One thing for sure, most trucks are quite fast without a trailer attached so watch your speed. Don't want any tickets.
The last truck I drove had antilock brakes. Most newer trucks will come with it on them. Antilock brakes require more brake pressure to stop when you are bobtailing than conventional brakes. From reading the owners manual and doing it I found it to be true. Don't be alarmed the first time you bobtail and the truck seems hard to stop. Just apply more pressure and you'll be alright.
Bobtailing can be fun. I always enjoyed pulling straight into a convienance store parking lot to park. Plus it's easier to keep up with that particular car you were able to see into at the last light.
"Life, Liberty and the Pursuit of all who threaten it"
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Answer:
One thing I disagree with:
And here's the clincher: The brakes only work on the first drive axel.
The second drive axel brakes are operated through the trailer brake system when a trailer is attached and pressured-up.
Unless the tractor you're currently driving is really unique and the axle raises when the tractor is not loaded.
Many tractors only have a maxi-cam on one axle, the maxi-cam serves as the parking brake. However, the brakes work on all axles when the brake pedal is pushed.
If a tractor is bob-tailing and the brakes didn't operate during a roadside inspection the tractor would be placed OOS.
Brakes must work on all axles per:
§393.42 Brakes required on all wheels.
(a) Every commercial motor vehicle shall be equipped with brakes acting on all wheels.
Part 393.42
During a MCSAP inspection there is the 20% rule (except for steer axles) 5 axles = 10 brakes = 2 brakes out-of-adjustment or non-operational before the vehicle is placed OOS. This does not include air leaks in the system.
In closing, all brakes must operate all the time, unless it is a tag axle and can be raised when not in use.
Be safe.
John Q.
Mothers all want their sons to grow up to be President, but they don't want them to become politicians in the process.
John F. Kennedy (1917-1963)
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Answer:
John Q. is right. The parking brake is usually only on one drive axle. But the service brakes must be operational on both drive axles and on the steer axle.
"Life, Liberty and the Pursuit of all who threaten it"
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Answer:
Stuffiu, I don't know where got your information, but anti-lock brakes are no harder to apply bobtail than with a trailer.
What you are thinking of is the "Bobtail Proportioning Valve" This was used prior to the advent of Anti-Lock brakes. What this valve did was put most of the braking pressure on the steer axle when bobtailing, thus severely limiting the brake power on the drives. This valve was the cause of many bobtail accidents because the driver was used to the brakes with a trailer attached.
When the anti-lock brakes first came out, they still used the proportioning valve for awhile, but then eliminated it.


Answer:
PTD, got my info from the Ford truck owners manual and experience as I stated above. Could be a Ford thing cause that truck needed more pressure to stop. The freightliner I drive now seems to need a bit more pressure but not as much. I do know that braking is different on a tractor alone than it is a tractor trailer.
"When the anti-lock brakes first came out, they still used the proportioning valve for awhile, but then eliminated it." Now I have to wonder why you have to wonder where I got my information. Where did you get yours -->
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Answer:
You're correct about the braking. The parking brake - not the service bake - splits the drive axels. The "brakes" do "work", I said it wrong.
But....one axel carrys drag (or acceleration) from the drive train, the other rolls free. Adding service brakes can cause the already (engine) braked axel to lose traction first -- half your rear brakes. The whole thing is good with a heavy weight, but overkill prone to skidding when bobtailing.
My service brakes are touchy when I bobtail, to keep all my traction back there over here in rainy Seattle.

Answer:
Stuffiu, I got my info from my 93 379 Pete and my 96 W900L. The 93 had the proportioning valve, meaning it took alot of foot pressure to stop bobtailing. ( The majority of braking was tansfered to the steers) My 96 was one of the 1st trucks with anti-lock brakes, but it still had the proportioning valve. That didn't make sense to me since the anti-locks were supposed to eliminate the locking up problem that the proportioning valve took care of. Kind of Redundant. My 99 Pete had anti-locks without the valve, as does my 01 Pete. Haven't really noticed needing any extra pressure needed to stop bobtailing.
Also, alot has been written about the problems with the proportioning valve in the Truck magazines a few years ago;
INFO
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Answer:
"The 93 had the proportioning valve, meaning it took alot of foot pressure to stop bobtailing." PTD sounds like what I was referring to. You had to put the brakes on hard with the truck I drove. I thought it was an anti lock thing, guess it's not. I do know it scarred me the first couple times I went to stop. I needed to stop and it didn't seem to want to. I thought at first that I had a mechanical problem until the owner pointed it out in the manual.
I never did have a problem with skidding with anti locks. They never did. Just had to apply more pressure to them to stop.
So let me rephrase what I said. On some older trucks with Antilock brakes require more brake pressure to stop when you are bobtailing than conventional brakes. -->
"Life, Liberty and the Pursuit of all who threaten it"
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Answer:
Let me re-phrase my response; It appears that the Ford you were refering to most likely had a proportioning valve as well as anti-locks. The new trucks with just anti-locks might need a little more pressure to stop, but not as much as the ones with a P' Valve.
Regardless, with or without anti-lock brakes, one should still use extreme caution applying brakes bobtailing, especially with wet roads. Most tractors have 4 channel anti-lock systems, the steer axle and the rear axle. The front drive axle is not ABS, so it will still lock up.
Boy, I am glad we got this straightened out, Its past my bedtime! -->


Answer:
Bobtailing can be fun. I always enjoyed pulling straight into a convienance store parking lot to park. Plus it's easier to keep up with that particular car you were able to see into at the last light.
Yeah, but were they wanting the attention from YOU?

Answer:
Bobtails are harder to stop. The reason is simple. For one you have 6 brakes stopping you instead of the 10 brakes you have when pulling a typical dual axle trailer. Also everything on a truck is designed to work under a load, so when the load is not there the brakes do not work as well, the ride is rougher, less traction, etc. Even shifting a bob tail is somewhat differnt than shifting while under a load.
As Shuffler pointed out. Take it easy while bob tailing. Pay close attention how your truck is handling. Bob tailing is not really hard it is just a slightly different animal to deal with than you may be used to if you are always yanking that trailer around.
I will always be a mutter trucker at heart.
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