Oversteer - Tail Wagging the Dog

[John&Vicki]

When you see the tree you're going to hit, that's called understeer. If you can only hear and feel it, it's oversteer.

There's another active thread right now that discusses the dangers of towing on sharp declines. If speed isn't controlled carefully, especially when entering a curve at the end of a decline, the trailer can take over with disasterous results, such as a roll-over. It appears to me to be very similar to oversteer, something I'm familiar with.

Early Porsches, with the engine in the rear, are notorious for oversteer. Ralph Nader pretty much got the "unsafe at any speed" rear engined Corvairs off the road. Porsches, while having much better handling, are every bit as bad.

When you drive an early Porsche at high speeds, such as on the track, you learn to scrub speed before entering a curve: slow in, fast out. Hitting the brakes, or even lifting on the throttle, too late in the middle of a curve is a strict no-no and can result in your car going by you. An exception is that experienced Porsche drivers use oversteer to their advantage by throttle steering through curves: go in hot, lift a bit in the middle which kicks out the rear end, then re-apply the throttle when you're pointed the direction you want to go.

So who cares - this is an Airstream forum.

I haven't seen oversteer discussed here, but I think we potentially have a big case of it when towing a trailer. Unlike a Porsche engine which weighs a few hundred pounds, we have thousands of pounds of inert weight back there. I suspect that oversteer may be a factor in many accidents involving down grades and curves. I know trailers have brakes which helps a lot but consider the following scenario:

A driver has been on a long downgrade and has used his brakes a lot to control his speed as opposed to downshifting. Trailer drum brakes are primitive and are probably hot and fading at this point, while the more sophisticated car brakes are still working fine. He enters a curve too fast and hits his brakes - the car slows down and the trailer keeps going. Now the tail is wagging the dog.

On downgrades I think the key to prevent this is to go slow and control the speed with the engine as much as possible by down-shifting. And on twisty roads the key is slow to the correct speed before entering the curve, not midway through.

Cheers,
John

[Alphonse]

John, great point. You are correct that we don't see much discussion about it. There are a couple of folks here who I have seen speak up about it and give some good advice.

As you point out, slow and easy down the hill or this oversteer stuff can come violently into play when the trailer tries to pass you. We often forget that the kinetic energy of it all is proportional to the square of the speed. Too fast equates to a system with big energy in it to take you down.

[John&Vicki]

A crollary to this is countering trailer sway. Again, when the trailer starts swaying the reaction is to lift on the accelerator and step on the brakes. That just encorages that beast in the back to try and pass you. Just as with a Porsche in a curve you shouldn't brake and you shouldn't lift on the throttle. Just gently maintain speed and use the brake controller manually to slow the trailer and restore the tension between it and the tow vehicle. So much of this is counter intuitive. I'm really grateful that I've had so much seat time in a vehicle that grossly oversteers. Kind of like practice for towing a trailer.

Cheers,
John

[pteck]

Thank you for starting this thread. It's definitely a topic that is not well understood and worth more discussion on.

I like the Porsche analogy as it resonates with me but perhaps may not with others. The key here is that the rear engine Porsche's carry a high percentage of weight at the rear, and under trail braking conditions in a turn, the rear has a propensity to pass the front end.

This dynamic is even more serious in a articulated vehicle.

Specifically, this is the condition where the tow vehicle is contributing the majority of braking (i.e. engine braking, badly setup trailer brake bias, bad trailer brakes). In this state, stability will be compromised in a turn or evasive maneuver.

Once an angle is introduced to the coupling, the momentum of the trailer will be translated into yaw on the tow vehicle. This induced yaw, will add additional angle onto the coupling, which adds additional yaw leverage on the tow vehicle. A situation that can quickly spiral out of control if not recognized by the driver.

Like the Porsche example, braking may exacerbate the situation. I say may because that depends on all the equipment working and the brake bias in the brake controller to be configured correctly.

The safer response is to apply manual trailer brakes via the brake controller or even accelerate.

[DHart]

Great topic!

A related side note... as a motorcycle rider, one learns quickly that the time to scrub speed is prior to entering the curve and then to accelerate through it. If you find yourself too fast and you're in the curve... MUCH better to just hold steady on the throttle and lean into the curve even harder, than to grab the brakes and throw all the dynamics into a tizzy. And then, don't make that mistake ever again! Motorcycle curve techniques and dynamics are very much like driving a 911 (which I drove for a few years in the mid 1970's).

In addition, well developed skills in situational awareness is something that serves the motorcyclist VERY well, and it applies well to driving all manner of much larger vehicles, as well. Not that I would advise EVERYONE learn to ride a motorcycle, but it imparts valuable learning which I have found applicable to other vehicles including driving a 44' Class A motor coach, pulling a 5th wheel, and a trailer.

[Silver Otter]

Originally Posted by DHart

Great topic!

A related side note... as a motorcycle rider, one learns quickly that the time to scrub speed is prior to entering the curve and then to accelerate through it. If you find yourself too fast and you're in the curve... MUCH better to just hold steady on the throttle and lean into the curve even harder, than to grab the brakes and throw all the dynamics into a tizzy. And then, don't make that mistake ever again! Motorcycle curve techniques and dynamics are very much like driving a 911 (which I drove for a few years in the mid 1970's).

In addition, well developed skills in situational awareness is something that serves the motorcyclist VERY well, and it applies well to driving all manner of much larger vehicles, as well. Not that I would advise EVERYONE learn to ride a motorcycle, but it imparts valuable learning which I have found applicable to other vehicles including driving a 44' Class A motor coach, pulling a 5th wheel, and a trailer.

Great comment, Dhart. As an old Motorcycle Safety Foundation Instructor, I feel I'm a better truck/trailer driver for my training in Bike handling as described above.

[Wingeezer]

Originally Posted by Silver Otter

Great comment, Dhart. As an old Motorcycle Safety Foundation Instructor, I feel I'm a better truck/trailer driver for my training in Bike handling as described above.

I agree whole heartedly. Apart from being more aware of the physics involved,
I think that the years I have spent on two wheels serve to make me much more of a defensive driver when on 2 wheels or 4, and even more when towing.

You just seem to automatically expect the other guy to screw up and constantly assess what you would do when he/she does, what would be your options and escape route etc.?

It also makes you aware of things such as staying out of other people's blind spots, and not driving too close behind large trucks where your forward visibility is severely limited and you may suddenly be faced with a huge chunk of tire lying in the road in front of you with insufficient time to react.

Brian.

[A W Warn]

Originally Posted by John&Vicki

go slow and control the speed with the engine as much as possible by down-shifting. And on twisty roads the key is slow to the correct speed before entering the curve, not midway through.

Cheers,
John

Excellent advice!

[John&Vicki]

Thanks for the post pteck. Your technical explanation of oversteer really adds to the discussion and my basic comments. And great input from those experienced with motorcycles. It's not that common to have experience with oversteer as almost all vehicles have engines in the front with a tendency to understeer. I think it's valuable for those of us who have experienced oversteer by happenstance to share our experiences. A good understanding of this topic should help to make us all safer out there, regardless of our trailer, TV and hitch. Physics trumps everything.

Cheers,
John

[gecko]

John--great thread! Three comments: (1) in curves, (and especially downhill) ALL vehicles should be driven like Porsches and motorcycles--just makes sense for all the great explanations here (2) it makes sense to set the trailer brakes to be just s bit stronger than the tow vehicles. In the event that you actually do have to brake downhill on a curve (elk, anyone?), this will let the not overtaxed (because you haven't been riding them, of course) trailer brakes to make trailer serve as a sea anchor,slightly holding back the lead vehicle instead of pushing it and (3) with the combo of EARLY engine braking and EARLY vey light straight brake application well before the curve, you can easily control your speed while in the vulnerable downhill mode.

[tjdonahoe]

Every one is still talking about the Bozeman pass accidents, well it is a cake walk if you pay attention, we been traveling it up and down in our big trucks and with our travel trailers for over thirty years with no problems, with very little service braking,the jake brake and exhaust brake doing most of the work.....

[Lumatic]

Trailer Brakes First! https://video.search.yahoo.com/video...sigi=11v52jveh

[m.hony]

Everything we ever wanted to know about towing a trailer (40 feet of train!) we can learn from Lucille Ball and Desi Arnaz, or from Clinton Twiss, the author of the book.

[switz]

Those yellow speed caution signs with lower speed numbers than the posted speed limits are there for a reason. I would suggest that when towing, the driver should drop speed to that caution number or even lower when on steep grades.

We have a challenge on every trip when heading North or East by going through Salt River Canyon east of Phoenix. The posted speed limit when transiting the actual canyon is 35 while some turns are cautioned at 25. The grade is steep and the turns are tight with the mountain right at the edge of the road.

On a motorcycle, this is great sport as long as no loose gravel is on the road from a recent rain....

I keep a longer than normal gap between me and the vehicle head and let the engine brake hold the speed at or below posted or the yellow sign speed. I grab the trailer disc brake controller to apply some braking action if the speed starts to creep upwards sort of like releasing a drag chute. That keeps the rig in line.

The longer gap allows me to watch all the brake lights light up on the vehicles ahead and just let up on the throttle to keep the distance. Whenever this is a small pull off, I let the speed demons pass as they like to be about 10 feet off the trailer bumper.

[JCWDCW]

A trailer pushing the TV is akin to trying to push a string or a chain...it buckles. Using the manual application of trailer brakes only, pulls the string taught. So does acceleration, but if you run out of speed before you run out of hill then the eventual disaster is multiplied big time.
I've noted in previous posts, my 1500 Jimmy and 23' Award were a monster combination. I've had vehicles around me dive for cover when the Award started wagging. Even with the friction sway control up tight if there was any play in the linkages we would be fighting downhills. One day when I had no room to correct, it hit me...Manual trailer brakes!!!!

I never had that problem with my 25s but I am reading shorter trailers are experiencing it. The Hensley or Pro-pride effectively lock the rig into one single inflexible unit when the trailer starts to push. Friction type anti-sway tries to do the same but as I discovered on my Award even 1/16 to 3/32 of play was enough to allow the trailer to take over. Just before the end of a long wearing trip I got out the wrenches and tightened everything up and the whole rig was suddenly docile again!
JCW

[handn]

This is a good explanation of the dynamics of how to get in trouble driving a vehicle towing a trailer but the Porsche and motorcycle analogy takes you only so far.
If you are going too fast in your rear engine Porsche, you can slow down in a hurry provided you do so before entering the corner. Those big brakes are one of the things you pay for. It takes a lot longer to scrub off speed towing a trailer as the braking particularly going down hill is laborious. That's why the do as the truckers do admonition is helpful--maintain a speed slow enough so that you can easily make the caution limit before entering the corner.
I have seen videos of Porsche Cayenne's whipping an Airstream through a slalom course on flat land. That picture would be ugly at the if the course were at the end of a long downhill. Instead of the tow vehicle pulling the trailer swiftly through the curves, it would be the trailer pushing the rear of the tow vehicle into an overstreer inducing skid. If Airstream is going to market the trailers' sports car like handling, they should vastly upgrade their brakes.

[polarlyse]

A few years ago we were on I-70 towards Denver from the west. The posted signs took the trucks down to 25 mph. Now that's pretty serious on an interstate highway. Towing our 30' AS I found it comfortable at 35 mph. Coming from the east we just don't see hills of that magnitude. Having travelled the rockies before it was nothing new to us. Moral of the story: as has already been mentioned beware those speed warning signs. Sometimes they really mean it.

[cwf]

I 'wish' you could experience that feeling.....once... the one where you realize you have a bit too much 'inertia'/ kinetic energy...and it is trying to PASS you.... and survive unscathed.... Just once...

When traveling in 'unknown' areas, we decided to be 'over-prepared'. It was a smart move... because since my traumatic brain injury, I am a real nin-com-poop....when 'judging' speed, distance, etc... and why I no longer ride MC... but I still take 'right seat' rides when a friend offers to take me flying..

[Jacob D]

Good thread every driver should read. I humbly add some thoughts from the Vintage A/S angle. My "67 has original Henschen axles with 12 inch brakes. Replacement axles have 10 inch brakes. Even though the 10 inch brakes are slightly wider, it will take higher pressure to get the same stopping power with consequently greater heat generation and brake fade. I have just learned that Inland RV can supply Dexter axles for the vintage trailers with 12 inch brakes which in my mind is a must for the reasons discussed in this thread.