Maximum occupancy / cargo weight & Tire pressure

Vincent Venat submitted an interesting question this week on the 124Spider.org forum, regarding the maximum occupancy weight restriction shown on the B-pillar tire rating placard.  He had included a photo of that label (edited for clarity and shown above) to prove that indeed, the maximum combined weight of cargo and occupants is listed at 340 pounds, and he postured “How serious is this?”  My first thought, after realizing I’d not noticed this spec before (I really only paid attention to the 29 psi recommendation), was that one wouldn’t think there should be a concern regarding the weight of two “normal” people getting into our 124 Spiders, since:

• the cabin and trunk are relatively small, and there being only two seats – whoever/whatever can physically fit in there wouldn’t be an issue, at least regarding the tires
• the car is so lightweight due to Mazda’s gram strategy design, one would believe that there would be extra capacity for cargo and occupants vs. a normal 4-door vehicle using these same tires (perhaps these particular tires aren’t installed in any sedans though)

Oh, and that weight restriction includes cargo, which could easily approach 75 pounds for a weekend getaway, or a trip to a big-box store.  So the weight of two people shouldn’t exceed 265 pounds?  Maybe in Japan, the gram strategy applies to occupants as well, and that limitation didn’t seem unrealistic.  🙂  What margin of safety is Fiat assigning to both the tire load capabilities and the GVWR?

I can imagine that in many instances, the total actual weight could realistically easily hover around 500 pounds!  So indeed, how serious is this?

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Analysis

Using the OEM tires for the Lusso as an example, the load index for the Potenza RE050A 205/45 R17 tires is 84, meaning that @ maximum pressure of 51 psi*, the tires are each rated for a load of 1,102 pounds.   ⇒ The Lusso curb weight with automatic tranny (50 pounds heavier than manual) and a full tank of gasoline is stated to be 2,476 pounds, which leaves a theoretical maximum tire loading capacity for 1,932 more pounds, with no safety margin.

However, unlike trucks or trailers, tires pressures in cars are usually not spec’d for heavy weight loading, but are instead recommended for the best blend of performance, fuel economy and driving comfort – which is why the recommended pressure is stated as 29 psi, and not 51.  So in trying to determine what the weight capacity of these tires are at 29 psi, I found a Toyo tire document “How to apply the apply the load inflation tables“, and used a similar tire with a load rating of 83 as a favorable example…

to come up with a realistic number of 893 pounds weight capacity for each tire @ 29 psi.   ⇒ This results in a maximum cargo and occupant capacity of 1,096 pounds (with almost no margin of safety)**, or 548 pounds with a 50% margin of safety.

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Conclusion

So, in my opinion, assuming the Chinese manufactured OEM wheel rims aren’t the weak link in all of this, 500 pounds is still a realistic number, but what we learn from this is that if there are two typical Americans 😉 , along with a trunk filled with all their stuff, heading on a road trip on some windy roads (more stressful on the tires), it sure wouldn’t hurt to insure that all four tires are equally inflated at 32 psi (or perhaps even 34 or 36), but for sure – not lower than 29 psi!

I’m thinking the extra safety factor is spec’d because the manufacturer knows that some owners will allow tire pressures to vary from 29 psi by ±5 psi, for a variety of reasons including laziness, inaccurate tire gauges, or because they mistakenly assume that the TPMS will let them know if the pressure goes below the spec. (it most likely won’t sound off for a 3 – 5 psi drop, atleast in the Classica and Lusso).  ⇑ I ran the numbers for an Abarth with automatic @ 26 psi (only 3 pounds lower), and the capacity with a 50% margin of safety came out to 396 pounds, which is pretty darn close.

Even though 29 psi is recommended, I’ve discovered today that 32 psi will afford me a greater safety margin and accordingly, will be my new norm, as noted on my placard with a Sharpie, as a permanent reminder.   ⇒ So, in regards to tire loading**, with a 50% margin of safety, my cargo and occupant capacity increases to 702 pounds!

(GVWR** however restricts that capacity to 456 pounds)

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P.S.  My research today has also revealed what some Europeans perhaps have known all along – for our V (Classica) and W (Lusso + Abarth) rated tires, an additional 4.35 – 7.25 psi respectively in the tires, is recommended for those travelling at high speeds (over 99 mph, as on the Autobahn), so that as the tire becomes firmer, deflection / flexing of the sidewalls resulting from the higher RPMs is reduced, producing less stress and heat.

Our owner’s manual words it differently:

High speed driving with your vehicle under maximum load is dangerous. The added strain on your tires could cause them to fail. You could have a serious collision. Do not drive a vehicle loaded to the maximum capacity at continuous speeds above 75 mph (120 km/h).

but is essentially concurring that 29 psi might not be sufficient for highway driving, if cargo and occupants weigh close to (or more than) 340 pounds.  Wish this had been made clearer somehow, before we took all of those long weekend trips, but fortunately, we haven’t experienced any problems.

I used to have a single-axle travel trailer that pushed the tire’s weight capacity limits to the max, and I learned the hard way, that if you went on one trip with tire pressures lower than 50 psi, you were destined to have an eventual blowout from the over-stressed sidewalls, even if it didn’t occur during that particular trip.

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*   The listed max. pressure (51 psi), is also the (“factory”) pressure that is pumped into tubeless tires after installing them onto the wheel rims, so that:

• the beads will seat properly into the double hump profile of the rim, to minimize the chance of bead seat dislodgement, even when the inflation pressure is lower than the operating pressure
• tire disfigurement (flat spots) is minimized during the long boat ride overseas, while strapped tightly to the deck of the ship, and while parked in storage lots for long durations
• any faults in the rim castings, tires and/or tire valves might be revealed, should the pressure not be retained during the build and shipping process

As many have discovered, because their dealer failed to reduce the pressure to 29 psi as part of their assigned new-car vehicle prep, the ride with tires @ 51 psi is rather harsh.  One wonders how many sales have been lost, because the dealership let the car go out for test drives with tires @ 51 psi.

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**   Just because the tires can accommodate an additional 702 pounds, doesn’t mean that the other structural components (like cast suspension arms, coils, etc.) or mechanical components (like wheel rims, brakes, shocks, transmission gears, etc.) in the vehicle are able to, and that is all reflected in Fiat’s calculated gross vehicle weight rating (GVWR) stated on the other B-pillar sticker (VIN # placard), which is the overall total weight limit of that automobile.

2,932 pounds is the GVWR for my Classica (I imagine it applies to the Lusso and Abarth as well), so with a full tank of gas (that 95 pounds is reflected in the curb weight), regardless of higher tire weight capabilities, 456 pounds is my maximum occupants and cargo capacity.

In theory, if occupants and cargo on a particular trip weigh 500 pounds, 44 pounds needs to be reduced from curb weight, which can only be accomplished by not pumping the last 5.5 gallons of gasoline into your tank (half-tank max.).