Rear axle and GVM to GTM

KevinJ · Senior Member

Rear axle and GVM to GTM

yobarr · Guru

Looks interesting,Kevin! After visiting another Grey Nomad this morning,to help him with his weights,I have been working under my own van all day,so bit tired,but I will have a closer look when I can. Well done though,and I am sure that your charts will help others to better understand weights,and how they can change as the fuel load reduces.My car has 180 litre fuel tank capacity,with 90 litres behind the rear axle,so it is critical that I use that tankful first.Gives me a range of 1000km,so big changes in weight distribution over that distance.Cheers

Gundog · Guru

My only issue there is a big discrepancy in all you calculations there is no mention of front axle weights and I assume that TBD is actully TBW.

I would also like to see actual weighed GCM as a confimation of GVM and GTM calculation.

iana · Guru

Couple of things, vehicle payload, including passengers ?, would be a shared load over both front and rear axles, two passengers, the load would be shared more for the front axle, same situation regards fuel, depending on where the tank is positioned.
As the vehicle became lighter, then the vehicle height would raise slightly. This would in turn raise the tow ball, which in would effect the center of gravity. How much would depend on the vehicle set up. But this in turn could effect tow ball weight. All these different parameters Gee!

yobarr · Guru

iana wrote:
Couple of things, vehicle payload, including passengers ?, would be a shared load over both front and rear axles, two passengers, the load would be shared more for the front axle, same situation regards fuel, depending on where the tank is positioned.
As the vehicle became lighter, then the vehicle height would raise slightly. This would in turn raise the tow ball, which in would effect the center of gravity. How much would depend on the vehicle set up. But this in turn could effect tow ball weight. All these different parameters Gee!

Ian,I would suggest that even if the towball rose 100mm,the difference in towball weight and centre of gravity would be miniscule,particularly with load-sharing suspension on the van.Kevin's figures are designed,I believe,to help the average Joe who has absolutely NO idea about weights and physics,but is happy to argue with those who do know what they're talking about.He should be applauded for his efforts.Cheers

-- Edited by yobarr on Saturday 14th of August 2021 04:53:25 PM

KevinJ · Senior Member

Gundog, My only answer to that is that as weight comes off the rear axle, that weight goes onto the front axle and I was under the impression that this is not a bad thing and in fact can only improve steering. If that is not the case, I am happy to improve the software if you can provide the specs for me. In respect of TBD (Tow Ball Download) yes it is the same. What is the correct terminology, TBD or TBW? It's an easy change to the software. I don't mind being challenged on these things.

Gundog · Guru

KevinJ wrote:
Gundog, My only answer to that is that as weight comes off the rear axle, that weight goes onto the front axle and I was under the impression that this is not a bad thing and in fact can only improve steering. If that is not the case, I am happy to improve the software if you can provide the specs for me. In respect of TBD (Tow Ball Download) yes it is the same. What is the correct terminology, TBD or TBW? It's an easy change to the software. I don't mind being challenged on these things.

When weight is applied to the towball generally weight also comes off the front axle in some situations, hence some vehicle require the use of a weight distribution hitch. There are a lot of metrics that come into play before deciding wether a WDH is needed or not.

An example of this is my Ranger where a WDH is required if towing in excess of 1800kg, my 2600kg van after setting up the towball and coupling heights, when the van is connected the rear axle weight increases by the TBW plus 65kg that has come off the front axle, when the WDH is engaged that 65kg is returned to the front axle and about 15kg is returned to the caravan axle group all coming off the rear axle.

KevinJ · Senior Member

Iana, thank you for your comments. You are absolutely correct is saying that the load is shared over both axles. The software takes that in to account. There is also a distance parameter in the calculator for the position of the centre of gravity for the fuel tank. You are also correct regading the tow ball height but no, that is not in the calculator. Neither is whether you are climbing a hill or descending a hill. The impossible I can do but miracles take time. Even the Lift formula for an aircraft has a variable called "Coefficient of lift" where they park all the bits they can't nail down. Please keep challenging my formulas as I have no problem admitting I am wrong. See the attached document for examples showing the distribution of weight between the axles including the fuel tank. Hopefully that will explain it better than words.

yobarr · Guru

Gundog wrote:
KevinJ wrote:
Gundog, My only answer to that is that as weight comes off the rear axle, that weight goes onto the front axle and I was under the impression that this is not a bad thing and in fact can only improve steering. If that is not the case, I am happy to improve the software if you can provide the specs for me. In respect of TBD (Tow Ball Download) yes it is the same. What is the correct terminology, TBD or TBW? It's an easy change to the software. I don't mind being challenged on these things.
When weight is applied to the towball generally weight also comes off the front axle in some situations, hence some vehicle require the use of a weight distribution hitch. There are a lot of metrics that come into play before deciding wether a WDH is needed or not.
An example of this is my Ranger where a WDH is required if towing in excess of 1800kg, my 2600kg van after setting up the towball and coupling heights, when the van is connected the rear axle weight increases by the TBW plus 65kg that has come off the front axle, when the WDH is engaged that 65kg is returned to the front axle and about 15kg is returned to the caravan axle group all coming off the rear axle.

Graham,I am somewhat puzzled by your maths.Assuming 260kg towball weight,there is no way known that only 65kg is transferred from your car's front axle to the rear axle. No way. The actual figure would be in the vicinity of 110kg,depending on how much your WDH extends your TBO.(Towball overhang, or distance from rear axle to hitch point).You appear somewhat confused,as no weight is ever "returned" to the van's axle group,for the simple reason that none was ever removed from that axle group in the first place. Certainly there is weight ADDED to the van's axle group by the use of a WDH,but there is no way known that you should have your WDH tensioned so tightly that the total weight transferred from your car's rear axle to your car's front axle,and the van's axle group,is greater than the weight originally transferred from the front axle. Oversteer looms! And you can forget the antiquated methods of measuring wheel arch height,and the like. Cheers

P.S Kevin,the generally accepted terminology is towball weight,or TBW,but it hardly matters if it is understood?

-- Edited by yobarr on Saturday 14th of August 2021 04:43:16 PM

Gundog · Guru

yobarr wrote:
Gundog wrote:
KevinJ wrote:
Gundog, My only answer to that is that as weight comes off the rear axle, that weight goes onto the front axle and I was under the impression that this is not a bad thing and in fact can only improve steering. If that is not the case, I am happy to improve the software if you can provide the specs for me. In respect of TBD (Tow Ball Download) yes it is the same. What is the correct terminology, TBD or TBW? It's an easy change to the software. I don't mind being challenged on these things.
When weight is applied to the towball generally weight also comes off the front axle in some situations, hence some vehicle require the use of a weight distribution hitch. There are a lot of metrics that come into play before deciding wether a WDH is needed or not.
An example of this is my Ranger where a WDH is required if towing in excess of 1800kg, my 2600kg van after setting up the towball and coupling heights, when the van is connected the rear axle weight increases by the TBW plus 65kg that has come off the front axle, when the WDH is engaged that 65kg is returned to the front axle and about 15kg is returned to the caravan axle group all coming off the rear axle.
Graham,I am somewhat puzzled by your maths.Assuming 260kg towball weight,there is no way known that only 65kg is transferred from your car's front axle to the rear axle. No way. The actual figure would be in the vicinity of 110kg,depending on how much your WDH extends your TBO.(Towball overhang, or distance from rear axle to hitch point).You appear somewhat confused,as no weight is ever "returned" to the van's axle group,for the simple reason that none was ever removed from that axle group in the first place. Certainly there is weight ADDED to the van's axle group by the use of a WDH,but there is no way known that you should have your WDH tensioned so tightly that the total weight transferred from your car's rear axle to your car's front axle,and the van's axle group,is greater than the weight originally transferred from the front axle. Oversteer looms! And you can forget the antiquated methods of measuring wheel arch height,and the like. Cheers
P.S Kevin,the generally accepted terminology is towball weight,or TBW,but it hardly matters if it is understood?

-- Edited by yobarr on Saturday 14th of August 2021 04:43:16 PM

Those figures are only an example because I never quoted the actual TBW because my model Ranger has a max TBW of 250kg which is displayed on the towbar and in the owners manual.

In reality I have made signifant weight reduction for the van and transfered some of that weight to ute.

As we are static in NQ during the pandemic I have no idea of my actual weights and correct WDH settings prior to hitting the road in the future everything will be weighed checked and reset.

yobarr · Guru

Gundog wrote:
yobarr wrote:
Gundog wrote:
KevinJ wrote:
Gundog, My only answer to that is that as weight comes off the rear axle, that weight goes onto the front axle and I was under the impression that this is not a bad thing and in fact can only improve steering. If that is not the case, I am happy to improve the software if you can provide the specs for me. In respect of TBD (Tow Ball Download) yes it is the same. What is the correct terminology, TBD or TBW? It's an easy change to the software. I don't mind being challenged on these things.
When weight is applied to the towball generally weight also comes off the front axle in some situations, hence some vehicle require the use of a weight distribution hitch. There are a lot of metrics that come into play before deciding wether a WDH is needed or not.
An example of this is my Ranger where a WDH is required if towing in excess of 1800kg, my 2600kg van after setting up the towball and coupling heights, when the van is connected the rear axle weight increases by the TBW plus 65kg that has come off the front axle, when the WDH is engaged that 65kg is returned to the front axle and about 15kg is returned to the caravan axle group all coming off the rear axle.
Graham,I am somewhat puzzled by your maths.Assuming 260kg towball weight,there is no way known that only 65kg is transferred from your car's front axle to the rear axle. No way. The actual figure would be in the vicinity of 110kg,depending on how much your WDH extends your TBO.(Towball overhang, or distance from rear axle to hitch point).You appear somewhat confused,as no weight is ever "returned" to the van's axle group,for the simple reason that none was ever removed from that axle group in the first place. Certainly there is weight ADDED to the van's axle group by the use of a WDH,but there is no way known that you should have your WDH tensioned so tightly that the total weight transferred from your car's rear axle to your car's front axle,and the van's axle group,is greater than the weight originally transferred from the front axle. Oversteer looms! And you can forget the antiquated methods of measuring wheel arch height,and the like. Cheers
P.S Kevin,the generally accepted terminology is towball weight,or TBW,but it hardly matters if it is understood?
-- Edited by yobarr on Saturday 14th of August 2021 04:43:16 PM
Those figures are only an example because I never quoted the actual TBW because my model Ranger has a max TBW of 250kg which is displayed on the towbar and in the owners manual.
In reality I have made signifant weight reduction for the van and transfered some of that weight to ute.
As we are static in NQ during the pandemic I have no idea of my actual weights and correct WDH settings prior to hitting the road in the future everything will be weighed checked and reset.

All is good,Graham.Just didn't make sense.I too,am in FNQ,but looking to soon head to Normanton,then Cloncurry,then?? Cheers

Plain Truth · Guru

iana wrote:
Couple of things, vehicle payload, including passengers ?, would be a shared load over both front and rear axles, two passengers, the load would be shared more for the front axle, same situation regards fuel, depending on where the tank is positioned.
As the vehicle became lighter, then the vehicle height would raise slightly. This would in turn raise the tow ball, which in would effect the center of gravity. How much would depend on the vehicle set up. But this in turn could effect tow ball weight. All these different parameters Gee!

Could you please explain how as the vehicle gets lighter,"this in turn could effect the tow ball weight"

KJB · Guru

Plain Truth wrote:
iana wrote:
Couple of things, vehicle payload, including passengers ?, would be a shared load over both front and rear axles, two passengers, the load would be shared more for the front axle, same situation regards fuel, depending on where the tank is positioned.
As the vehicle became lighter, then the vehicle height would raise slightly. This would in turn raise the tow ball, which in would effect the center of gravity. How much would depend on the vehicle set up. But this in turn could effect tow ball weight. All these different parameters Gee!
Could you please explain how as the vehicle gets lighter,"this in turn could effect the tow ball weight"

Change in TBW , if any, would be so minute it would not be worth worrying about......(as paint is chipped off the van's draw bar by stones , the TBW probably decreases as well ........ these things are all too all too pedantic in the real world....) KB

iana · Guru

Fuel used. If for instance the fuel tanks are to the rear of the tug. Weight would be removed from the rear of the vehicle, because of fuel consumed. If the van is level with the tow vehicle when fully loaded, if the rear of the tow vehicle comes up, then the tow hitch/ball will come up. The van pivoting about an axis will have the effect of moving the center of gravity back. And so weigh will come off the tow ball. How much, as Yobar says maybe very little.

travelyounger · Guru

iana wrote:
Fuel used. If for instance the fuel tanks are to the rear of the tug. Weight would be removed from the rear of the vehicle, because of fuel consumed. If the van is level with the tow vehicle when fully loaded, if the rear of the tow vehicle comes up, then the tow hitch/ball will come up. The van pivoting about an axis will have the effect of moving the center of gravity back. And so weigh will come off the tow ball. How much, as Yobar says maybe very little.

Hi then you put the fuel in again and you are back to where you started same with water and the food and beer that we consume does it really matter at the end of the day over little bits of weight transfer

Cheers