How important is GVWR?

[JaSAn]

Again, the GVWR number on the door post of your truck is a legal definition; you are not authorized to change it irregardless of what improvements, modification, or upgrades you make to the vehicle. You may decide to take the (obviously good) chance that you will never have an issue with your slightly overweight truck, but it is not a slam-dunk.

Beefing up one component just moves the weak link in the dynamic handling of the vehicle to another. And without a lot of testing you cannot know how that affects your vehicle in non-normal situations (like a panic stop going downhill on a rain slicked, off camber road).

jim

 

[rruff]

A lot of people arguing with ego instead of with logic in this thread.

Indeed... but I'm not one of them... It's reality and common sense. Is that not logic? There are many MANY legal vehicles on the road that handle like pigs and have a worse stopping distance than your typical expedition style overloaded 1/2 ton. Stating that the pickup is dangerous merely because the GVWR has been exceeded is ridiculous.

Again, the GVWR number on the door post of your truck is a legal definition; you are not authorized to change it irregardless of what improvements, modification, or upgrades you make to the vehicle.

Obviously it isn't a legal definition for safety. How could it be, when huge RVs are deemed "safe" that have 3x the stopping distance?

Beefing up one component just moves the weak link in the dynamic handling of the vehicle to another. And without a lot of testing you cannot know how that affects your vehicle in non-normal situations (like a panic stop going downhill on a rain slicked, off camber road).

These trucks are driven off-road hard and fast for many thousands of miles, in conditions that are much more challenging (panic stop going downhill on a mud slicked off camper trail, for instance). They are going to fair *much* better in challenging conditions on the road than your typical RV.

 

[DaveInDenver]

Indeed... but I'm not one of them... It's reality and common sense. Is that not logic? There are many MANY legal vehicles on the road that handle like pigs and have a worse stopping distance than your typical expedition style overloaded 1/2 ton. Stating that the pickup is dangerous merely because the GVWR has been exceeded is ridiculous.

It is indeed. The number given to you on the door sill is just a number, maybe some SAE standards, maybe some engineering SWAG, some lawyer involvement. But it's not an absolute number unless all those things have been done using worst cases with poor cargo management, foul weather, component variance (e.g. they'd have to assume the OE springs and shocks are at their EOL), bad road conditions, limited lifespan and reliability.

Point I'd make is GVWR must assume some typical conditions and if those conditions are not applicable or you've addressed the deficiencies then you adjust your GVW up or down accordingly. The listed GVWR isn't explicit that GVW is safe on worn tires in the snow, so why shouldn't it be implicit that GVW may exceed GVWR if you have fresh load range E-range (assuming the OE were P-rated, since this is in the context of Toyotas) tires and upgraded suspension on dry roads? It's up to the individual to know what the original rating meant, how it applies to their specific vehicle and conditions and how any changes impacts it.

Obviously it isn't a legal definition for safety. How could it be, when huge RVs are deemed "safe" that have 3x the stopping distance?

Usually it's implied to be a legal restriction but I have not seen anyone cite the specific law or regulation that indicates an non-CDL individual operating an under 26,000 lbs vehicle must do anything beyond operate a "safe" vehicle. And what that means varies widely. In my home state a light truck registration requires a vehicle under 16,000 lbs empty and unless it's US DOT registered you do not have to stop for weigh stations. The only supposition usually made is were you to get in an accident a lawyer *may* try to argue you have liability for exceeding the GVWR, but they'd have to demonstrate that doing so was negligent and the cause of the accident. But they don't argue that you escape liability if you're under it but fail to account for wet roads. It would still be your fault.

 

[Ozrockrat]

How could it be, when huge RVs are deemed "safe" that have 3x the stopping distance?.

Once again you are playing with alternative facts. The big RVs are required by law to be able to stop at full GVWR from 60mph in 250 feet. So now you are saying a Tacoma (let alone a modified over weight one) will stop in less than 84 feet.

The RVs I am most worried about are not the big ones on truck type chassis but the C class ones that are built on the puny domestic cab chassis pickup type platforms. Even the E450 based ambulance I had was at the limits of its capabilities in standard configuration and then I added bigger wheels and the braking capacity went way down.

How about you provide some definitive testing for us. There are plenty of apps for measuring acceleration/deceleration. Put your spare standard wheels on your Tacoma and do a brake test. Then put your oversized wheels on and do it again.

As a reference point I have done this with an underweight expedition vehicle (land cruiser 75 6 wheel rig) but not by choice. In Australia (QLD) you have an inspection called a road worthy certificate. Part of the testing is dynamic braking capacity. With 33” wheels on my truck it would not pass. So I put a set of 31” wheels on and it did pass. (They only gave me a pass/fail not a rating). So it officially went from being unsafe to safe by reducing the tire diameter. That was unsafe in a vehicle that was under the GVWR.

Once again real world experience versus gut instinct/ I think / I feel.

 

[DaveInDenver]

Does a road worthy test require the truck to be loaded typically? I understood these were inspections and not tests but I also admit to not being knowledgeable about rules down under. I also think they require engineer's certifications for modifications. Curious about the standard they would be testing to. Does a modified vehicle have to stop within the same distance or rate as the original vehicle did or is there a standard criteria (I presume it would be the ADR)?

 

[Ozrockrat]

Does a road worthy test require the truck to be loaded typically? I understood these were inspections and not tests but I also admit to not being knowledgeable about rules down under. I also think they require engineer's certifications for modifications. Curious about the standard they would be testing to. Does a modified vehicle have to stop within the same distance or rate as the original vehicle did or is there a standard criteria (I presume it would be the ADR)?

The roadworthy does not require the truck to be loaded and it used to be just an inspection. But a couple of years ago they changed it to an actual on road deceleration test (rolling road or physical road). They do have different values for different weight vehicles and also test the emergency brake the same way.

The braking value is a stated standard criteria and not based on what was in effect when the vehicle was produced. I.e. 3.8 m/s/s for up to 2.5t and 2.8 m/s/s for 2.5-4.5t.

Yes they do require engineering certification for modifications outside ADR limits. For example you can lift a vehicle up to 75mm (3”) above original design height without an engineering certificate. That is 75mm total including the tires and suspension. Vehicles that are not 4WD can only increase the tire diameter by a maximum of 15mm and a 4WD can only have a maximum of 50mm over original.

 

[bkg]

Comparing a Toyota to an RV is simply stating “yeah, but there are worserer stuff out there, so I’m fine.” Doesn’t hold water.

Has to be a logical clmparison is - is an underloaded Toyota safer than overloaded?

 

[rruff]

The big RVs are required by law to be able to stop at full GVWR from 60mph in 250 feet.

Can you link the regulation? Google just gave me pages stating that RVs require a lot more time to stop than a typical car, with 400-500ft (plus reaction time) being typical from 65 mph.

 

[DaveInDenver]

Comparing a Toyota to an RV is simply stating “yeah, but there are worserer stuff out there, so I’m fine.” Doesn’t hold water.

Has to be a logical clmparison is - is an underloaded Toyota safer than overloaded?

I know your question is rhetorical, but actually it may not be that simple. An underloaded vehicle may handle poorly, too. Design assumptions are made to select spring and damping rates such that being under damped or over sprung can create the reciprocal problem to being undersprung for too much weight.

I gotta think with all your experience you've had to have come across trying to control an under laden mini truck with OME heavy or Deaver 10-leaf packs over washboard. A truck like that has a mind of its own.

The target is having the intended weight that went into the design calculations, changing anything negates those numbers in either direction.

 

[bkg]

I know your question is rhetorical, but actually it may not be that simple. An underloaded vehicle may handle poorly, too. Design assumptions are made to select spring and damping rates such that being under damped or over sprung can create the reciprocal problem to being undersprung for too much weight.

I gotta think with all your experience you've had to have come across trying to control an under laden mini truck with OME heavy or Deaver 10-leaf packs over washboard. A truck like that has a mind of its own.

The target is having the intended weight that went into the design calculations, changing anything negates those numbers in either direction.

Now your nitpicking. ?. I should have said “generally speaking, a loaded Toyota loaded under the mfg GVWR is likely ‘safer’ than one loaded over the mfg GVWR.”

 

[DaveInDenver]

Now your nitpicking. ?. I should have said “generally speaking, a loaded Toyota loaded under the mfg GVWR is likely ‘safer’ than one loaded over the mfg GVWR.”

Just part of my argument that we get focused on increasing GVWR that the basis for it goes beyond how much squat the back end of your truck had sitting static in a parking lot. What's safe is continuously variable up and down.

Just taking the top off a 1st gen 4Runner changes the way it handles and not necessarily making it better and that was true even 100% stock otherwise.

 

[Ozrockrat]

Hard to link off my phone but it’s NHTSA 49 CFR 571.121 Standard 121 Air Brake Systems specifically S5.3.1 Stopping Distance.

I will also add that since 1999 these vehicles have been required to have ABS.

Somewhere out there the NHTSA also published the results of their testing of air brake vehicles. I am too lazy to chase it down and I have already read it. So use your own google.foo if you are interested.

Also just to clarify. The 250’ stopping distance is for fully loaded GVWR and it is 235’ for lightly loaded.

One other question I have is do these lift kits come with anything to reset the brake proportioning valve. If you move the axle away from the body the OEM setup is not going to work anymore. I know I had to rework that with my 6 wheeler before engineering would certify it in Australia but I have never seen it mentioned in the USA.

 

[Clutch]

Comparing apples and squirrels.

What kind of apples?

...and which Squirrels, fox, grey, red, flying, ground? Or are we talking SE Asia squirrels? 'cause... man, that is a whole other ball of wax...

perfect. amazing how many guys that can change oil & shocks know more than teams of credentialed engineers.

Load 'er up!

 

[s.e.charles]

this thread is getting a bit nuts

 

[Clutch]

I seen it mentioned whilst reading install guides and reviews.
Of course meaningless in a country what mostly allows any ’ol Wrenchpotato to cobble up something then put it on the road.

It will be fine...just toss an extra axle under tharrrrr....couple ratchet straps, and we're good! See ya'll at Jellystone!