Thread: Confused - D3/D4/RRS Towball Weight

Originally Posted by Tryanything

So a 3.5 tonne van being towed by a 2.5 tonne 4wd with a 5.7% (200kg) tow ball load? Please let me know what stretch of road you are on cause I will steer clear thanks.

Seriously, there is a actually bit of theoretical physics and experimental evidence behind the towball load recommendations. Too low and the rig will likely snake, too high and it will be more stable 99% of the time but could jacknife without warning.

Personally, I would not tow anything exceeding the weight of the loaded D3 (before towball load is addd), and even then would keep it below 90km/hr.

Do you know more about towing than the engineers who apply design vehicles around tow ratings?

No I dont. But I have researched this subject quite a bit (too much probably) before I bought my D3 because of all the scare mongering around about the Landrovers not being able to use a weight distribution hitch.

Given my plan is to use my new D3 for towing duties with my family on board I have obviously concluded that the inability to fit a WD hitch is not a problem. In fact I found quite a bit of evidence to suggest that the common practice of using a WDH to "relevel" the tow vehicle will lead to much higher chance of the tow vehicle oversteering at quite low cornering forces.

However, I did come across quite a lot of research which suggests that a towball load as low as 5.7% is not good for sway stability, and it is really not a good idea to tow a van which is materially heavier than the tow vehicle. Instability can occur as low as 100km/hr and the outcome can be nasty.

It has been proven that there is a marked difference in how a towing combination performs depending upon van characteristics such as its yaw inertia, cornering stiffness of its tyres, distance from tow hitch to van axle and of course towing speed.

These van related factors are obviously outside the control of the LR engineers and I can't speak for what they assumed when they calculated a load rating of 3.5T and 350kg ball load. But I can tell you that in general, a van weighing nearly 1.5 times the tow vehicle mass, with just 5.7 % towball load will have a MUCH lower stability threshhold speed.

The attached slide was prepared by Richard Klein, an SAE contributor and Automotive Engineer of more than 30 years experience in the field, who has pretty much dedicated his professional life to the study of trailer towing instability. His response when I mentioned a 2.5 tonne vehicles towing 3.5 tonne van at speeds approaching 100km/hr was that that was "VERY dangerous".

I do believe HE knows more about towing stability than the LR engineers...

What assumptions about the trailer drawbar length have been made to produce that graphic?

Not sure, and there are a number of other assumptions that have been made about the vehicle and van. You will see the notes actually say that each combination will have a somewhat different result. But I've read research papers by Richard which have considered eveything from family sedans to large US made pickups and the general conclusions remain the same - be careful with towball load and avoid towing vans materially heavier than the tow vehicle.

From what I have read a longer drawbar ( or longer distance between hitch point and van axle) would have the effect of moving the lower diagonal line to the left ( ie it has a favourable effect on sway stability).

And to move the upper diagonal to provide more stability:

Increase the wheelbase of the tow vehicle;
Reduce the distance between rear axle and the towball;
Increase stiffness of tyres by raising pressures or use LT tyres for increased sidewall stiffness, and

I am sure that suspension stiffness would also play a part.

Spot on. Although I think those things also help with the lower line as well.

Also any sloppiness in the steering will also hurt it as well as it reduces the effective cornering stiffness of the tow vehicle.

From what I can tell the upper diagonal is associated with loss of understeer of the tow vehicle. Research on this refers to it as divergent instability, meaning instability happens abruptly with no warning signs (jack knife). This can happen if the rig receives a side load at speed such as a passing truck or road defect.

The lower line is the snaking/oscillatory instability which can start at quite low speeds and gets progressively worse with speed until the natural damping of the rig is overcome (since damping is being reduced by speed whilst at the same time the sway forces are increasing).

In both cases, speed is the enemy.

The term "typical" in the attached slide and notes gives a fair bit of leeway. The EAS LR, being a dynamically suspended vehicle, is not typical when compared to a static suspension system and its unsprung mass. Hence the non-requirement for WDH's. There is no drooping of the rear axle under load and the associated assumption of a constant spring rate.

I'd imagine that in this application, yes - the LR engineers probably know more. They have some reasonably qualified automotive engineers working for them you know. Regardless, it's a bit of a strawman issue - LR do not indicate that 5.7% towball load is ideal.

Cheers,

Gordon

Originally Posted by Tryanything

No I dont. But I have researched this subject quite a bit (too much probably) before I bought my D3 because of all the scare mongering around about the Landrovers not being able to use a weight distribution hitch.

Given my plan is to use my new D3 for towing duties with my family on board I have obviously concluded that the inability to fit a WD hitch is not a problem. In fact I found quite a bit of evidence to suggest that the common practice of using a WDH to "relevel" the tow vehicle will lead to much higher chance of the tow vehicle oversteering at quite low cornering forces.

WDH's are something I would never use either. Fine if you only travel on flat land with no corners, but a serious concern if you involve hills and/or sharp corners.

Originally Posted by Tryanything

However, I did come across quite a lot of research which suggests that a towball load as low as 5.7% is not good for sway stability, and it is really not a good idea to tow a van which is materially heavier than the tow vehicle. Instability can occur as low as 100km/hr and the outcome can be nasty.

Are you aware that heavy trucks tow heavier trailers with the only nose weight being the weight of the drawbar?
Stability is about the entire vehicle and trailer interaction, it cannot be expressed in simple percentages as it is far more complex than that.
Zero ball weight can be perfectly safe and stable, where trailers with the often touted 10% static ball weight can be a danger to everyone.

Originally Posted by Tryanything

It has been proven that there is a marked difference in how a towing combination performs depending upon van characteristics such as its yaw inertia, cornering stiffness of its tyres, distance from tow hitch to van axle and of course towing speed.

Don't forget axle/suspension configuration. It's one of the biggest in trailer stability.

Originally Posted by Tryanything

The attached slide was prepared by Richard Klein, an SAE contributor and Automotive Engineer of more than 30 years experience in the field, who has pretty much dedicated his professional life to the study of trailer towing instability. His response when I mentioned a 2.5 tonne vehicles towing 3.5 tonne van at speeds approaching 100km/hr was that that was "VERY dangerous".

I do believe HE knows more about towing stability than the LR engineers...

Unfortunately that slide alone means nothing. If you were to include the SAE paper the presentation was about and the full power-point presentation then perhaps we could draw some conclusions. Such a graph can only beproduced relating to a specific case and we have no information about that specific case.