Tire Pressure explained by a tire engineer

[1939mgtb]

Hello all, This is to introduce my friend Mike (Riley guy.....well, actually ANY pre-war British car..). He is the head of a department at a large multi-national tire manufacturer with a LOT of years and experience with tire design and manufacture. In fact, pretty much anything to do with tires. He is currently involved in the F1 tire development program, as well as road car tires. I now turn the note over to him! Best, Ray

----- Original Message ----- From: "Long, Mike" To: "1939mgtb" 1939mgtb@comcast.net> Sent: Tuesday, June 14, 2005 9:37 AM Subject: RE: [mg-tabc] Tire Pressure Ray, The trouble with fitting today's tires to old cars is probably that they have too much info on the sidewall. DOT regulations in the 60's made "maximum" load carrying capacity and inflation mandatory verbiage on the sidewalls of tires. That alone was a bit confusing with USA tires since they were converting from numeric sizing (like 5.60-15, 8.25-14 etc)to alpha-numeric (A78-13, F70-14 etc) while metric radial tires were being imported that followed European standards. Once all of the mandatory information went on the sidewalls, people began to match and mismatch tires based upon the sidewall data that seemed to be useful.....and forgot to use their most important information which is the data for the original tire size and the corresponding manufacturer's pressure recommendation. That info is usually in the owner's manual and for newer cars also on the vehicle placard (glovebox or door jamb sticker). Always inflating a tire to the sidewall maximum is pretty safe for the tire as it means less flexing in service so it sees lower stress. However, if the maximum on the sidewall is too high for the specific application, the tire will tend to ride hard, loose traction, and wear in the center of the tread as the high pressure rounds the tread countour and the tire is working primarily at its center. Each vehicle manufacturer determines what tire size and type is best for their vehicle and through handling and wear testing they set pressure recommendations to target their opinion of the best compromise for wear, handling, and ride. For instance, I believe the MGTC had front and rear pressure recommendations of 24 and 27psi on the 4.50-19 which had a 4 ply rating and a max inflation of 32psi (not stamped on the sidewall in those days). If the car maker specifies a front-to-rear pressure difference, it is probably to compensate for loading and/or oversteer/understeer circumstances so maintaining the pressure differential is a good place to start even if you want to bump pressures up at each end or if you've applied different sized tires. Dropping to lower inflations than recommended is never a good idea unless the tire size is larger than the original. To clarify the maximum pressure differences seen on sidewalls, remember that older (pre 1965ish) 4 ply rated-and-or "B" load range bias tires and metric radials all had 32psi maximums. In the 70's, metric radials went to lower aspect ratios (70 series, 65 series etc) and the European standards for them moved to 35/36 psi max, while USA sizing moved to the current sizing type with a "P" before the metric nomenclature and they are called "standard load" with their 35psi max. Along with all of these normal passenger tires from both sides of the ocean, there are also some stronger versions (reinforced, 6 ply rating, extra load, C load range)that show higher max inflations for their correspondingly higher load capacities required for utility/light truck type applications. For cars newer than MGTC's there are many speed rated and high performance passenger tires that may be replacement candidates. Following the lead of European experience, most of these tires carry a higher maximum inflation (greater than the 35/36psi for which maximum loads are calculated) on the sidewalls that is intended to stabilize the tires and reduce flexing at the correspondingly higher speeds the tires can attain. These 44psi, 52psi, etc. recommendations are not primarily for normal driving and they are not accompanied by higher load capabilities (except for aerodynamic downforce, cars don't get heavier as they go faster!). There are many other tire selection issues like radial vs bias on older cars, speed ratings, and even the load indices now on new tires....but those topics weren't issues in the correspondence you referenced so I'll just stop here! Let me know if this helps. Mike -----Original Message----- From: 1939mgtb [mailto:1939mgtb@comcast.net] Sent: Monday, June 13, 2005 5:18 PM To: Long, Mike Subject: Fw: [mg-tabc] Tire Pressure Mike would you comment on this and give some of your background. Your employer need not be mentioned. Some of this crap sorta scares the snot outta me! Best, Ray "Much can be achieved with a smile. However, much more can be achieved with a smile and a gun." ----- Original Message ----- From: "Leinen, Robert" rleinen@ci.dearborn.mi.us> To: "Paroor" paroor@t-online.de>; DougPulver@aol.com>; mg-tabc@yahoogroups.com> Sent: Monday, June 13, 2005 9:16 AM Subject: RE: [mg-tabc] Tire Pressure Madhu: Although well intended, your advice is incorrect. The tire pressure listed on the sidewall of any tire is the maximum pressure for that particular tire, as determined by the manufacturer, regardless of what vehicle it is mounted on. As you are aware, one tire can fit many different vehicles. Each vehicle will likely require a different tire pressure due to many factors, weight, suspension, shocks, load, etc. The maximum pressure on the sidewall is typically somewhere above 40 pounds. That much pressure would cause serious ride, handling and braking problems in most cars. Bob Leinen Deputy Director of Public Works City of Dearborn 2951 Greenfield Dearborn, MI 48120 Ph: (313) 943-2073 Fax: (313) 943-2067 E-mail: rleinen@ci.dearborn.mi.us -----Original Message----- From: mg-tabc@yahoogroups.com [mailto:mg-tabc@yahoogroups.com] On Behalf Of Paroor Sent: Monday, June 13, 2005 3:13 AM To: DougPulver@aol.com; mg-tabc@yahoogroups.com Subject: AW: [mg-tabc] Tire Pressure Dear Dog Pulver, Most of the tyres do have a recommendation of air pressure for the normal load conditions casted on the side wall where you can also read the production date of the tyre. Madhu -----Urspr ngliche Nachricht----- Von: mg-tabc@yahoogroups.com [mailto:mg-tabc@yahoogroups.com]Im Auftrag von DougPulver@aol.com Gesendet: Montag, 13. Juni 2005 01:37 An: mg-tabc@yahoogroups.com Betreff: [mg-tabc] Tire Pressure A customer came into my friends shop yesterday with his TC. He has 16" wheels and the Dunlop tires looked "low". I checked the tire pressure and it was 16 pounds on all four tires. I know we have had this discussion several times in the past and the general consensus seemed to be that tire pressure between 26 - 30 pounds was OK. Is that still a good rule of thumb for the tires on 16" wheels? Doug Pulver TC 5850 (with 6 new studs in the RH bearing carrier - one of which is now spinning as I tighten up the brake drum holding nut - aaaaaarrrrrrgggghghhhhhhh!) San Diego, CA Yahoo! Groups Links Yahoo! Groups Links