We have a 1200 mile trip coming up (Portland to Lake Tahoe and back) and plan to carry two 17' kayaks on the Yakima roof rack. I also want to fit my roof pod, a long slim Thule pod, to carry light but bulky things like sleeping bags. I think I will be able to fit it all, canting the kayaks over on their sides (they will sit cockpit-to-cockpit) and pushing the mounts all the way to the outside of the bars. Total weight on the roof will be about 200 lbs, excluding the rack system itself (kayaks 65 lbs x 2, plus pod 35lbs, plus pod contents <30lbs). In addition, the car will be carrying two adults and two children and luggage. From prior roadtrips, I know the car will be riding low in the rear. Anyone carry this much in their Prius? Do you think it is unwise? Are there any easy-to-install helper springs or air bags to raise the rear end? My OEM Integrity tires have 6/32" of tread depth. I'm thinking about replacing them before this trip, just to be safe - would order some Nokian WRs. At minimum I'm thinking about lowering the pressure from 42/40psi to high 30s psi. Maybe I'm being unnecessarily cautious?
First thing, adding load does not mean lowering tire pressures. If anything it means the opposite. Consider being at the sidewall max of whatever tires you use. Also you can measure the air pressures 'hot' and compare them to the 'cold' values. If hot is more than 10% increase then your tires are inherently overloaded. Doesn't mean you're about to die, but drive carefully and pack less on the next trip (or buy tires with higher load rating). Anything on the roof will decrease mpg and this increases steeply with speed. So, go slow or buy more fuel. As far as ground clearance I don't think there is a simple increaser available, so just taker 'er easy. You probably won't scrape off anything important
Yes, the tire's max load capacity is at the max rated cold inflation pressure. RV and Truck tire manufacturers (e.g. Michelin) publish inflation guides to give you the correct inflation pressure for a given load, but I haven't seen a guide for passenger car tires. But if you look at the sidewall, it will usually give you the max load capacity at the max cold inflation pressure.
