08 February 2009

sidewall vs. fuel filler door

i've always followed the vehicle manufacturer's suggestion for tire pressure, and used the sidewall max pressure value only to ensure it's not lower than what's inside the fuel filler door (or door jamb).

now i'm reading many suggestions @ fred's tdi forum about ignoring what's written on the car and going with 80-85% of the max printed on the sidewall. seem some are even exceeding that.

sounds crazy to me, but i'm willing to be edumacated.

fwiw, my current (winter) tires are Dunlop Winter Sport 3D (max pressure 50 psi) and my other set (all-season) are Goodyear Eagle RSA (max pressure 51 psi). i usually run 30-32 psi, based on what's written on the car.

am i being too conservative? is there a world of better handling and mileage awaiting me?

7 comments:

pyker said...

Wouldn't higher pressure be counterindicated on ice and snow?

zim said...

the adherents are claiming everything is better with higher pressure: mileage, handling (in all conditions), wear.

the car guru @ work sets his 4 under the sidewall pressure.

i visited the sites of Goodyear, Dunlop and Pirelli, they all say to use the manufacturer's numbers. noticed that the Goodyear and Dunlop FAQ item were exactly the same.

i'm dying for Joe to weigh in here; if anyone knows the scoop it should be him.

JustJoeP said...

Romba Kalaippu a irrukku
(I am really tired - in Tamil) It is 1:24am here, and I ahve another hour of waiting in the Bagalore airport before my Lufthansa flight leaves. Too many automatic weapons in the Bangalore airport guards' hands....

At the Michelin tire technology school that I attended, they talked about how it is important to never exceed the min and max sidewall ratings. The Auto's MFG rating is ONLY relevant IF you have the exact same tires that the vehicle shipped with when it left the factory, and then, the Auto's MFG rating is slanted towards what the MFG wants you to feel and experience, not what the tire manufacturer intended. Rarely are tires made for only one vehicle specifically - yes, there's lots of exceptions, with Dodge Viper mono-directional position specific tires, UPS delivery van tires with thicker sidewalls for curbs, BMW run flats etc.. but the VAST majority of tires manufactured are intended to be placed on more than one vehicle.

From the 6 years of tire mfg exposure I had, and the "tire disease" I acquired that is incurable (looking at truck tires here in Bangalore, when we pull up next to them at the ubiquitous speed bumps) I personally adhere to the sidewall rating. The -4 psi below the USL (upper spec limit) is a good rule of thumb for all weather driving. For extremely hot desert driving, -4 might be fine when standing still, but rolling resistance, pavement temp, and hysterisis will send the internal tire temp soaring, and the ideal gas law still applies. On the car we drive once a week, I keep it around -5 from the USL. On Traci's car I keep it mid-range of the sidewall pressure (running Michelin MXV4s) and on cooler months, do a visual check as well for what looks like inordinate bulge at the 6o'clock tire position.

Michelin, who invented the steel belted radial tire, and who - in my opinion - makes the best touring and all season and performance tires (best = longest lasting, fewest defects, best rolling resistance, most reliable, but also [no surprise] most expensive, followed closely by Japanese MFG Bridgestone) really wants everyone to inflate their tires in the middle of the range on the side wall, for optimum performance, handling, economy, least under-steer or over-steer. At the Greenville SC Sales Training School they sent all us engineers to for a day, they had a little model car that they let roll down an inclined 3 foot ramp with a curve at the end. They over-inflated, 5 psi above the USL, and rolled it, and it crashed in an understeer, and then 5 psi below the LSL, and it crashed in an oversteer - or vice versa.. my brain is really tired - then they inflated to precisely midrange, and the car followed the prescribed dashed white line without crashing at the bottom of the curved ramp. It made a lasting impression on me... 20 years ago.

To Ron's point, higher pressure DOES decrease the foot print (and thus, one would think, the amount of available frictional surface) but it ALSO stiffens the side wall. You have to look at the whole tire assembly as a spring-damper system, with different spring (resistor) and dash-pot (capacitor) constants in the sidewalls, shoulders, tire beads, treads, shock absorbers, sway bars or steering linkages, and suspension springs, all trying to reach equilibrium.

I'll take anothe rlook at this this coming weekend, once I am back inthe GMT- time zones.

Mike Sankowski said...

I wish I had seen this post just 5 days ago as I just bought new Michelins.

I tend to think that a higher pressure will equal more MPG. Following the links at hypermilers.org leads to the claim that higher tire pressures -> better mpg

http://hypermilers.org/index.php?title=Main_Page

zim said...

@joe -- holy cow! what a ton of great info!

how do i find the LSL for my tires? neither manufacturer (Dunlop or Goodyear) posts those number on their site.

my Dunlops have a USL of 51 psi. so tonight i raised the pressure from 30 to 37 (measured at 40 degrees ambient after 2 miles of driving).

i do like the ride better, feels more comfortable and a bit more responsive. i'll try this for a bit, then probably try a bit higher.

joe, what was/were your role(s) at michelin?

JustJoeP said...

The USL is typically in the 20s. Visually, when you see the shoulder (transition where tread ends and sidewall begins) starting to lay flat against the pavement, that is "too low". I've seen contact images that show the center of the tread actually starts to bow up slightly when the shoulders are in full contact, since the center of the tread is very thin (and flexible) and the shoulders are more than 2X as thick.

Mike Slam, you can't make a bad purchase with Michelins - they just cost your first born. They will out-last and out-perform every other tire on the market, statistically, and in my personal experience (and several of my friends who drive high performance cars, like Saleens), realistically.

Zim, I was a mechanical engineer there who designed and built tire making process equipment. Back in the late 80s, Michelin introduced tire treads with high silicon content, replacing the high carbon content (since sand is cheaper than carbon black). Handling these high Si treads, positioning them, cutting them, joining them, was a serious challenge.

Working at Michelin was tremendously fun and interesting, they just stopped giving raises for 3 years across the board during the Euro-recession of the early 90s, and my rent and bills didn't remain frozen, so I was forced to seek employment elsewhere, to get a raise.

Michelin was (and I have heard still IS) obsessed with data. Every week, in our departmental staff meeting, we'd have charts, graphs, polar plots, of mounds of data, on Good Years, Dunlops, Pirellis, Continentals, Korean tires, Chinese tires, and our closest competitor, Bridgestone (who bought Firestone while I was working at Michelin). They'd show skid pad data, wet & dry comparisons, lateral Gs, rolling resistance, etc, all gathered from test driver's calibrated behinds, test track data (Laurens SC has a large Michelin test track), destructive testing, and R&D reports. This was pre-internet, so it was all hard copied. Stacks and stacks of the stuff, every week.

My Si tread process work kept me in touch with the R&D guys, which exposed me to even more data and insights. We did fun field tests, like taking one of the prototype 2 tires we were able to make with the experimental Si tread, putting on the flat bed trailer with a whole forklift worth of weight above the axle, and then intentionally locking up the trailer brake to skid the tire down through the entire tread joint cross section as the Semi tractor dragged the trailer forward, destructively testing the joint, using the asphalt as a giant grinder.

Michelin's culture fully embraced training. My first 6 months there, I rotated through every maintenance shop, from the wire winding, through extrusion, tire building, curing, and ending in the testing area. I was pre-designated to end up in the extrusion shop, and spent another 6 months being their green behind the ears engineer, designing and building racks, rolling mills, water cooling baths, fork lift interfaces, etc. During the maintenance rotation, I was sent to trade classes at the Michelin school, adjacent to Greenville Tech, where I took months of training on tire building, tolerances, lathe work, milling, welding, tire technology, etc. It was a holistic approach that most corporations today do not understand.

I do sort of miss working there... but I'm getting paid alot more now, and Michelin never let me travel anywhere, except twice to their plant in Halifax Canada where they had installed German designed, Italian made, French managed experimental tire equipment equipment.

Be careful, tire disease has no cure.

zim said...

well, i think i'm fairly convinced that after my Eagles wear out, i'll be getting a set of Michelins.