How do they calculate towing weights limits?

[TheOtherLeft]

Does anyone know how the manufacturers calculate the vehicles tow weight limits?

Is it just based on the structural factors in the chassis, drivetrain and suspension?

Does it take into account the engine and it's ability to tow a specific weight over flat roads or inclines? I.e. just because a vehicle can tow 3T doesn't mean it can tow it uphill comfortably. Then there's engine longevity as well...

[Peter @ Aawen4x4]

They've got engineers who go into all those variables, and look at the things like the towing speeds, competition towing limits, etc and then some bureaucrat makes a decision based on something not at all related, just like any other bureaucracy!!

Generally they can provide support for whatever decision they've made but sometimes it does get a little political. The change mid model to the GQ towing limit is a classic. What bought about an increase of 1 tonne? The release of the 2nd mod state 80 Series with the better motor and increased towing limits. Either Nissan had the upgrade already in the pipeline, or they ignored all the potential issues and had the new limit out just about that afternoon!

Why do you ask? Are you concerned about the towing limit of your Grand Vitara?

[TheOtherLeft]

My GV towing 2T of caravan, I think not!!!!

I'm just wondering why say a Grand Cherokee can tow 3.5T from a 3l TD with 510Nm, a Disco2 can tow 4T (I think) from a 2.5l TD with 253Nm whereas a LC100 can tow 3500 from a 4.2TD with 430Nm.

So back to my query of will a LC100 tow (up a hill with ease all day every day) a 2T (tare) caravan more easily then the Grand Cherokee or Dsico 2 simply because it has the bigger engine - ie is less stressed and hence will last longer.

Does it also mean that the LC100 engine can be tweaked more for more power/torque then the other smaller engines?

[jg1982]

Just guessing here, but I think the manufacturers take all the factors into account (with strength of towbar & what it mounts to, and braking capacity being most important) and come up with a 'cover their ass' figure for max load and towball weight which is well within the safety margins for things such as not ripping off the towbar, not buckling the vehicle and being able to stop the thing (sometimes there's other additions necessary like heat shields to stop a stinking hot exhaust melting/burning stuff). I don't think engine power would be a great consideration of theirs as long as it can still take off up a hill (more of a gearing issue than anything). Also with a 'passenger car' you can only legally tow up to 1.5 times your vehicle's unladen mass (provided you have the appropriate braking setup), so there's no point quoting a higher figure than that.
Most cars can (illegally) pull and stop significantly more than their rated towing amount, but the manufacturer's safety margins have to cater for people that might be towing constantly etc.

[Peter @ Aawen4x4]

jg1982, that 1.5 times the vehicles unladen mass is only something here in Oz, not seen anywhere else, and I do think they'll actually take the engine power bit into account a lot more than you'd think, simply 'cos these days of 3 yr & 5 yr Warranty's with km limits up in the 100's of thousands, anyone whacking a trailer on that is going to severely stress the motor is most likely to kill it way earlier than the warranty runs out!! And above all else, these corporate guys know how to Cover their Arse's!!

So back to your question Ben, YES the L/C 100 WILL be less stressed than the Jeep or disco (Hmm? Maybe not the disco, but in principle it's correct .....) and it will likely tow that 2T van all day every day leaving the 100 less 'worn' than anything else covering the 'test'.

But then you asked: "Does it also mean that the LC100 engine can be tweaked more for more power/torque then the other smaller engines?" and to that the answer is a qualified NO! But the conclusion isn't logically related, the thought process that got you there is just a little muddy and obscure. Just because a larger engine will be better off hauling weight around town and up and down hills, doesn't neccesarily mean that it hasta have more potential hoot under the bonnet!

You could equally as well assume that since the engines designed power outputs are based upon a calculated warrantable figure, they are all able to be tuned up to achieve the same ultimate level. But in reality there will be differing states of tune to get to the condition that the cars are in right now. F'rinstance, the Jeep is likely a pretty agricultural state of tune?? Not likely that anything has asked too much of it yet!! Tweaking should be able to release stonking great gobs of power and torque that you'd NEVER get out of the Disco, simply 'cos the Disco started it's life in a higher state of tune!

That make sense? (it IS 4:00am!)

[TheOtherLeft]

Thanks for the great info.

With regard to tune, I thought a 4.2 TD from the LC100 (430Nm) would have a more conservative tune then the 3 TD from the Jeep (510Nm). To me it seems that Jeep have really pushed their tune to get the most torque that may or may not sacrifice on longevity.

[doogie]

"The release of the 2nd mod state 80 Series with the better motor and increased towing limits"

Could you expand on this please? I have a 93 80 series & it's towbar rating is 2500kg, did the later model ones have a higher rating? What's involved in getting a higher rating i.e. say you want to tow a caravan weighing 2700kg's?

cheers

[sarmajor]

Hi there I have a late '92 80 series Diesel with the 16 inch rims and bigger brakes that they fitted from August 92. My vehicle is rated to tow 3500Kg here in NZ.

I suspect that your vehicle is rated to tow the same amount by Toyota but if it is fitted with a towbar that is only rated to 2500Kg then you are a bit stuck.
I have a genuine Toyota towbar on my other '92 80 series Diesel and it is also rated at 3500Kg.

Cheers