Piston Slap: Land Ark's Radial Tire Pressure Bias?
TTAC Commentator Land Ark writes:
Hello again, Sajeev!
I’ve been wondering about tire pressure for a while. I changed the wheels and tires on my car which originally came with 14-inch wheels and high profile bias-ply tires. The new tires are 255/40/18.
How do I account for the drastically different sizes when it comes to determining the correct air pressure I should run in the new tires?
I have the original sticker which says to run 24 psi up front and 28 in back. That seems low so I usually keep it around 33 in the front and 38 in the back. But I have no basis for these numbers other than what it says for my other cars. Is there any sort of guide to figure it out?
Keep up the great work!
Sajeev answers:
ZOMG.
That is a fantastic restomod, proving once again that the 1967 Impala is a seriously under-appreciated Bowtie.
No matter, your owner’s manual or body sticker is intended for application to bias ply tires, so it’s now irrelevant. Those shoes behave completely different than radial tires, reflected in their pressures. From my first car’s (’65 Galaxie) owners manual (24 psi all around, up to 36 psi rear on overstressed wagons with 8-ply tires) and some Googling, bias ply tires are rarely inflated over 30psi. But how many radial tires are inflated under 30psi, without pitfalls? But I digress…
Because tires are an integral part of suspension tuning, your bespoke restomod has no correct PSI found via Internet searching.
I know nothing of a ’67 Impala’s bias ply specific tuning (much less a suspension not 100 percent reconditioned?). So play suspension engineer with your new wheels: start at 32 psi all around. Drive on bumpy, twisty and wet roads. Then bump it up to 35 psi or down to 30 psi. Feel the difference in ride, in how quickly it breaks into under or oversteer, and how the steering feels at low and high speeds. If the higher or lower number feels better overall, try adding (or removing) 1 psi and see if it improves further. Spend a few days trying this out, perhaps let the tires cool overnight before changing the pressures.
The sweet spot for the vast majority of lower profile radial tires is between 30-35 psi. You can adjust front/rear pressures individually to fine tune under/oversteer, preferably off of public roads and on a road course, please!
Depending on road quality in your neck of the woods, I reckon your restomod is happiest at 32 psi all around. What say you, Best and Brightest?
[Images: LandArk]
Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.
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Sajeev didn't cover tire wear in the article. Longer term if the vehicle is aligned correctly you will see how the pressure affects how your tread wears. If it is eating the edges, you'll probably need to bump it up some more. If you find it wearing mostly in the middle then the pressure is set too high (which is a trick used by hyper-milers to increase fuel economy by lowering rolling resistance at the detriment to ride and handling). I'm really surprised that was left out. Manufactures find a compromise between ride, wear and handling following the tire manufacturer load and inflation ratings as mentioned in the comments above. For example the ratings on my Fiesta vary greatly between different markets depending on roads, expected carrying load etc. The European model even carries different ratings for how many people are in the car. In North America they only give one rating in the manual that falls in between both euro specs. That is definitely a nice restoration. Cheers.
Sorry for not replying yesterday, I'm in Las Vegas for a wedding and not staring at my work computer as usual. I will give the first suggestion a try. My main concern is for tire wear since I don't drive it in any real spirited manner. Unfortunately at this point I rarely drive it in general. Thanks for all the comments. I realize it isn't to everyone's tastes but few cars are. the cliffs notes version is it's been in the family sine the early 70s and I got it in 93 as seen above. I wanted to restore it instead of buying a new car as my first car. I drove to through out high school and it then sat until I bought my house in 09. Since then I've sloooooowly been doing more to it including the suspension, brakes, and trim parts. I might write about it in the forums but no one will ever see it.