Thread: Confused - D3/D4/RRS Towball Weight

Originally Posted by Tryanything

Are the heavy trucks you are referring to much heavier than the trailer they are towing? Is the hitch point hanging off the back of the truck or between the axles? Does the trailer have two well spaced axles? If none of these answers are yes, then I doubt the rig would be at all stable at near 0 % ball load.

Yes, trailers several tons heavier than the towing truck and load.
Yes hitch point well over a meter behind the centre of the rear axle group. Legislation here limits rear axis to tow-coupling at a maximum of 45% of the wheelbase.
Trailers from two axle rigid to 5 axle (3 rear axles, two on the dolly).

Close to zero ball (ring-feder) load. Only the drawbar weight is possible and these are sprung to sit level with no truck attached for ease of attachment.

Here is just one example:


Setups vary, but at the 44t max combination weight (without getting into overweight permits) the loaded trailers are about 4t heavier than the loaded trucks. Primarily because dimensions work out to greater load space in the trailer.
The official New Zealand road code for heavy vehicles - Information for heavy vehicle drivers

I am very interested in information on that SAE paper.

I figure this is a good time for this landrover promotional video.

I'd estimate the towed load at
2.3t of defender 110
700+kg of trailer.
So there is an easy 3 ton.

I can't recall whether the downforce on the hitch shown is 150 or 250kg. Either way, the nose load is a lot less than the 350kg spoken about here.

[ame=http://www.youtube.com/watch?v=5gyc4Dmgbew]Land Rover's "Trailer Stability Assist" for Range Rover, Range Rover Sport and Discovery 4/ LR4 - YouTube[/ame]

The analysis work I have seen done is for trailer setups where there is only one axle, or two close together near the centre of gravity. Trailer setups where there is an axle near each corner as per the photos are inherrently more stable.

I dont doubt the effectiveness of trailer stability programs on vehicles such as the D4 to improve stability. In fact I found a research paper for that too ( though not specifically for the D4 program). http://delphi.com/pdf/techpapers/2008-01-1228.pdf


Would be interesting to see how the D4 handled that rig with the trailer stability switched off?

Dougal - Ill try to send you a copy of the other papers by pm

As for those who are towing a very heavy van, the only warning I would give is that lack of oscillation is not necessarily an indication of stability. In general higher towball loads % will reduce sway oscillations, but as shown on the slide I posted it can lead to what is referred to a different kind of instability (divergent instabiliy), which is not associated with oscillation and can occur without warning if the rear tyre slip angles get too high ( due to for example a sudden side force)

I'm missing the point you're trying to make? The PDF paper is a second order approximation for articulated vehicles. What are you trying to state w.r.t. towing with less than 10% towball weight for a D4?

Cheers,

Gordon

My touareg with the wdh is a superior ride towing our 3.5 tonne triple axle horse float then my airbag equipped D4....period.

It is much harder to set up, and much harder to disconnect, with the toaureg, but the D4 is not as stable as the touareg with this kind of weight.

Even my wife picked up on the difference....and it's very noticeable on uneven road surfaces. Less pitching and wallowing compared to the D4. The touareg would be a good couple of hundred kilos lighter.

I was not expecting the D4 to be as stable, but closer, though it is a darn sight easier to setup and disconnect.....and for this reason alone we use the D4 more for heavy towing now.

Brett....

Originally Posted by gghaggis

I'm missing the point you're trying to make? The PDF paper is a second order approximation for articulated vehicles. What are you trying to state w.r.t. towing with less than 10% towball weight for a D4?

Cheers,

Gordon

Gordon

I paper I attached was in response to the post about the D4 active sway control using tow vehicle brakes. Thats what the paper deals with and concludes that it can help.

It does also show the effect of towball load on the stability of the rig. In particular, it considers a vehicle weighing approx 2.7 tonne towing a trailer of approx 1.2 tonnes. It considers three towball loading cases, one at 10.8%, one at 2.8% and one at -8.8% (ie tail heavy van).

It then calculates and verifies through testing using a vehicle and test trailer, the speed at which the Damping Ratio becomes negative. (Damping Ratio is simply a measure of how big subsequent sway movements are compared to an initial sway introduced by a steering movement for example). A bigger damping ratio means the sway movement diminishes quicker, a damping ratio of zero means the sway neither gets bigger nor smaller, and a negative damping ratio means the sway oscillations increase in size - ie kiss your tush goodbye.

It concludes that the vehcile with 10.8% towball load has a critical speed ( speed at which damping ratio becomes negative) >160km/hr. For 2.8% towball load, critical speed is 105km/hr. For -8.8%, you're toast at 72km/hr!

Ok so 2.8% towball load is a bit less than the 5.7% which was originally suggested, but the paper also highlights that the critical speed drops as the mass of the trailer increases relative to the vehicle mass.

So given the test was done with a vehicle of about he weight of a disco, (2.7 tonne), but with a 1.2 tonne trailer, I dont think towing a 3.5 tonne trailer with a 5.7 % towball load would be very safe ( especially if you dont have the D4 trailer stability program! I wish my D3 did!)

I have other papers on the subject of trailer stability and they deal with towball load amongst other factors, but as I mentioned they are materials which have been provided to me directly by the author and are otherwise only available for purchase via SAE.

Hence I cannot upload them but am happy to send to those interested, provided they commit to not further distributing them.

Regards
Scott

Brett:
Never towed with a Touareg, so can't directly comment - but I have with a LC200 + wdh, and the RRS was far more stable at high speed.

Scott:
The info is interesting, but I don't see the relevance to a D4 - which is not an articulated vehicle?

I don't believe that a D4/RRS would have significant problems towing 3 tonne as long as there was some reasonable towball weight. What "reasonable" is seems to be the issue here - rule of thumb is around 10%, and it would appear that LR automotive engineers design around this figure. There doesn't seem to be any evidence that 6% or so is _not_ safe when considering a twin-axle caravan of this weight on the back of a D4, all other things being equal.

Cheers,

Gordon