My Dear Old Departed Tire Tread
You’ve registered for your first performance drivers school and you have this technical inspection form to complete; one item is ‘tire condition’. So what to look for and what to do about it?
- Are they all round and black? Meaning do you see any bulges in the sidewall or areas of the tread surface where cords are showing?
- Are they the correct size for your car? Maybe you’ve never really looked at your tires before
- Are they the same brand and model in all four positions?
- How old are your tires?
- Dents in the wheel rims? Hit those railroad tracks pretty hard, right?
- What’s the tread depth?
- Check pressures please, ideally after the car has cooled for a while, overnight is ideal, then the tires are cold (vs hot immediately after driving)
Now for the advice and guidance:
- Bulges/ blisters in the sidewall are broken cords internally, see your tire dealer for some new tires
- I’ve seen mismatched tire sizes on cars, especially used cars, it happens
- If you have tires from different manufacturers it doesn’t mean you need to go buy a new full set; but what it does mean is you must point this fact out to your driving coach as it may explain some funky handling characteristic he discovered while out in that last on-track session. Tires from different manufacturers will handle and ride differently, especially notable as you approach the limit of adhesion (which may happen during our school). Worst case scenario is your mismatched set does become a safety concern. Call us for advice first (847-380-9508), well in advance of your school!
- Age of the tire is a safety concern; industry guidelines recommend replacement about every five years. If you see cracks in the sidewall by all means check their “born on date” molded into the sidewall. Look for a four digit number that’s the year and week the tire was manufactured. A tire made this past week would show “2511”. Here’s a great tutorial from the Tire Rack.
- Large dents in the rim where the tire seats is a definite safety concern, you will stress the tire more than you will on an average day (though not dangerously so)
- 3/32nds of an inch is the prudent minimum (legal) tread depth, but know that a tire this worn will not handle water well at all, perhaps taking 300% longer to stop in the wet. Dry weather performance may actually improve though due to less “tread squirm” (the carcass of the tire moving in the direction the wheel is pointed but the rubber in contact with the road surface is shearing molecules perhaps 5° difference in heading (slip angle))
- Proper tire pressures are critical for proper tire performance. Air in the tire is what’s supporting the tire, all the internal cords, belts, wire and rubber are experiencing forces in tension—not compression as one would think—and if air pressure isn’t right bad things will happen. Worst case is complete tire failure, best case with improper pressures is decreased fuel economy and/ or increased or abnormal tire tread wear. Check and set your tire pressure early in the morning after the car’s cooled overnight. Adjust to “door jamb” pressure (the placard typically affixed to the drivers door jamb area), then go drive. Reason to check after a nights cooling is that’s same conditions your vehicle manufacturer uses when they specify those placard pressures; it gives you a known baseline.
Why your tire pressures increase when driving
As your tire happily rolls along the road surface, whether driving to work or on a racetrack, it’s heating up due to friction, both with the pavement and internally. In fact internal friction is a larger contributor to temperature increases. Since the volume of the tire’s not changing (significantly) if the temperature is going up the pressure must also increase. Racers are always concerned with their tires and speak of cold and hot tire pressures. They write them down in a logbook and track trends in the data; they then win or lose based on this data and how they utilize the information.
Now that you’ve properly set your cold tire pressures and driven for a while, check them again, they will be higher (even in winter). Note the conditions you just drove (to work, or a 20 minute session on the track) and note the pressure gain. This is a hot tire pressure.
So now that you see the routine, what to do with this information? Modern production cars typically come with pretty sophisticated tires in low profile sizes (an article in itself). A hot tire pressure that will allow your car to perform well will be around 38 – 44 psi.
After your session on-track check your tire pressures, if they’re above 44 psi then lower to 42 psi now while still hot from your session. Check again post-track session; consult with your driving coach for pressure advice.
A powerful tool to balance your car’s handling on track is tire pressure changes. Most cars respond well to adding perhaps 3 psi to the front tires to counter some of the built-in understeer. When making tire pressure changes to alter handling, first make note of where they’re set now and then the adjustment (“down 3 psi front”) so you don’t forget while preparing for your next track event or performance drivers school.
Dear Departed Tire TreadWhere has my tire tread gone?
With all this heat and pressure build up going on, the heat must go somewhere, right? It must be dissipated via the surface of the tire, either transferred into the air or conducted into the wheel. If you’re running full tread depth tires (8/32” – 10/32”) there’s the distinct risk of entire chunks of rubber shearing off while you drive due to the fact that a full tread depth tire:
- Builds more heat as the tread blocks have more mass (and thickness) hence build more heat through molecular shearing
- Are less efficient at heat rejection (rubber is an insulator)
You can run a new tire at one of our schools (Hooked On Driving), just let your coach know what’s going on and ask that he help you manage tire conditions, reminding you to run at a slightly reduced pace (which you will anyway if you’re a beginner), two quick laps followed by a “cool the tire” lap at reduced pace (but still hitting your turn-in, apex and track-out). Using this technique to manage tire temperatures will ensure a fun day and no damage to your American Express card.
So what are your questions regarding tires, tire technology and how tires affect the way your car rides and handles? Post your questions here.