DE/TT tire pressures

[DonCostello]

I plan to try Time Trials next season and am unsure about tire pressures. For AX, I drop my cold presssures 2 lbs front and 3 lbs rear and it works well (2008 911 4S, factory pressures 34-39, dropped to 32-36). For time trials should the same basic pressures apply, or should I do somethimg totally different? Thanks for any advice.

[Mmagus]

I can't speak to your tires specifically as I run a very different car and tire. I'm sure you have thought of this, but you can expect your pressures to go up more during a TT due to the time on track, so you may have to run your initial pressure a little lower. Personally I try to hit the same max pressure as I do in AX. For me that's 34psi hot.

We will probably end up in the same run group. I look forward to playin' with ya.

Mark

[DonCostello]

Thanks Mark.

[rshon]

Don -
Cold pressures are only a rough starting point, as it's the hot pressures you need to dial in. Having good data is everything. If you haven't started already, you should get into the habit of taking your hot tire pressures right after an AX or DE/TT run session (and write them down). Might as well also jot down the ambient temperature (all the new cars display it right on the dash), as well as your impressions of the tire behavior.

You can also scan the internet for what others use as hot tire pressure targets for your particular tire model, but try to use info from those with similar cars, as optimal tire pressures are not only dependent upon tire model, but also vehicle weight, suspension camber, track temperature, and even track layout. (what kind of tires are you using?)

You also need to use the "seat of your pants" to develop an opinion of what hot pressures are too low or too high for your car and driving style. This will actually be easier at a DE/TT because of the continuous lapping, as the tires will essentially start out at ambient temperature, and as you continue to drive around the circuit, they will heat up and the pressures will rise, until at some point they will stabilize. If the tires on one end of the car or the other become "greasy" well before the end of a session, then your pressures are probably too high at that end. Conversely, if your tires feel "flabby" and soft on one end or the other for the whole session, your pressures are probably too low. Regardless, you will most likely end up starting with lower pressures than at an AX, as the tires run a lot longer at a DE/TT, and gain a lot more heat.

Another technique is to "chalk" the corners of your tires. Use tire marking crayons, or chalk or a light colored grease pencil, and draw some radial hash marks on the shoulders of your tires where the tread meets the sidewalls, for at least an inch onto the tread and in inch onto the sidewalls. Once you come back from a session, you will be able to see how much of the tread has been in contact with the pavement, where the chalk has been scrubbed off. If the chalk remains well into the tread past the corner of the tire, your tire pressures are too high. If the chalk is rubbed off well onto the sidewall, your tires pressures could be too low, (or you may be turning in too hard). Unfortunately, it also could be not enough camber in your suspension setup, but that is a topic for another day.

When you really get serious, you may one day resort to taking tire temperatures across the tread with a contact pyrometer, but that's also a whole 'nother can of worms (data).

Hope this helps. Have fun.

[Mmagus]

Great informative answer!

And yes, Don. We really do many or all of the things he mentioned . My wife even made me a nice excell sheet for my tablet that I can input pressures in and it averages the change for each session. In addition on the side of your tires are little wear indicator arrows that point toward the tred. They are where the sidewall "should" be rolling over to, according to the manufacturer. I don't use the chalk much, but I do take a peek at my little arrows.

Mark