Driving to altitude - temperature impact
 

Took a drive up in the mountains today, and it turned out to be an interesting test of temperature on tire performance. I have a 987S with 18" PS2, about 1/3 from new to done.

I left home, which is about 1200 ft, and within an hour was at 7000 feet, then within another hour back to under 1000 ft. The temperature swing during this drive was 49 to 32 to 53, and it revealed how tires perform across this range of temperatures. We all know that summer tires are not designed to run at cold temperatures, but what happens, and how does it feel? How unsafe is it to push the limits at low temperatures?

I found that the sticky feeling of the tires started to go away significantly in the upper 30s. Certainly in the 40s it was not the same as running on a nice 80 degree afternoon, but the car started pushing more prominently from 40 to freezing. The good news is that nothing surprising happened with the lessening grip. The balance of the car translated into good control even in these conditions, but there was much more understeer and it was harder to hold the same lines as normal. The tires were struggling to maintain solid contact, which left the car feeling somewhat unsettled. At the end of the day, shaving about 10-15% off of "warm speed" kept things well under control, but you still lose some ability to save yourself using extreme braking or turning. Simply less headroom.

As I returned to warmer conditions, I could feel the traction coming back, and the driving became much smoother in and out of the corners, with the tires growing more stable and predictable. I could literally feel my level of confidence change lower and higher moving into the cold and then back out.

Hopefully this info is useful to someone... yes, you can drive summer tires at freezing, on dry roads. And yes, you do need to drive a bit more conservatively to account for the difference in traction.

If anyone else has tried this experiment, I am curious to hear if you found identical results or if you would describe anything differently. Also, what happens well below freezing?

Dot-R's are even worse in cold weather. If your in cold weather for long periods you really should have a second set of cold weather tires if you want to keep the sticky summer tires. Thats what I do.

Good info, I have never pushed my PS2 in the cold,

Ed

I have done a lot of this kind of driving on my PS (first set) and PS2's (current tires), since I try to get the car out early in April, and keep it out into late October. The first couple hours in the early morning (low-low 40's) can be a little tricky! This is when the air temperatures are at their lowest point, for the day.

When the sun comes out and starts warming the road up, it is a very big difference. Or until you can get some heat built-up into the tires.

Another thing to consider is that for every 10 degree temp change, there is a 1 PSI impact on the actual cold-tire pressures. I find that our car/tire combo (especially in 19" wheels) is very sensitive to having a proper cold-tire pressure. So, a 1/2 or 1 PSI change makes a difference in the car's handeling.

In Hops example, the temp he departed at was 49 and when he got to the highest elevation the air temp was 32. If the tires had an opportunity to cool down, I would suspect that you lost at least 1-1.5 PSI. IMHO, this had a major impact on the car's shift in handeling too.

I've driven up to the mountains with my summer tires, during this time of year. I start at 5,000 feet (50°) and drive up to 10,000 feet (30°).
There is a significant difference in handling, from the rubber hardening. But most high performance summer tires, perform very badly (and unsafely) below 40°. It has more to do with the temperature than the altitude.

I have driven in light snow with my summer tires, and my thought was that as long as there isn't snow on the road, the friction from the tires will keep them heated enough to provide sufficient traction, but I wouldn't risk it very long. I only did this until my winter tires arrived. In freezing weather, driving on my summer tires was very scary because there was almost no traction.

I've also found that my car doesn't run as well (overheating to the 200+ mark) at 10,00 feet, and I am guessing it's because the air is so much thinner. :cheers:

In the high 30s
 

around 9AM after driving 3-4 miles at 40 MPH, I totaled a '99 Boxster on very good tread Pirelli summer UHP tires when I tried to brake hard in a straight line and had no traction at around 30MPH.

You can't tell me those tires had warmed up enough to grip the surface of the road (asphalt) effectively.

So be especially cautious if the temperature is rising, the road surface may be cooling those tires, not warming them.

Altitude has nothing to do with it. You can experience the same temperatures at Sea Level. It takes about 10 miles @45 degrees or below to fully warm the tires.

When you go up in altitude, the ambient pressure decreases, increasing the effective tire pressure.

Agreed! But, I took the topic here to be centered mainly on temperatures and their effect on the rubber compounds used in Hi-Per or Summer tires.

There is a pressure differential (the pressure difference between inside the tire and out) which changes with altitude. You're talking about guage pressure which measures pressure above/below atmospheric pressure, and this differs with altitude. So it first matters where you filled your tires.

1ATM (standard atmospheric pressure) is 14.7psi - AT SEA LEVEL. It drops to about 12.2 at 5k' (Denver), and 10.1psi @ 10k'.

So the effect is that your tires become 'over-inflated' as you rise in altitude from where you filled them because the outside air pressure is not acting as strongly on the outside of the tire with increases in altitude. If you filled them in denver, the tires become 'over-inflated' by 2.1psi@ 10k'. But, if you filled them in miami and go to 10k', the tires will be 'over-inflated' by a much more pronouced 4.6psi. If you live in an area with significant changes in elevation, it's a good idea to know the altitude (above sea level) where you live and presumably fill your tires. Solving this issue is easy... just bring a tire gauge along and release the 'excess' air as you gain altitude, just remember to refill once you hit the lowlands again.

Thanks for the good and interesting comments. I didn't know if this would be a very interesting thread, but I think this is an important topic for drivers now that I see everyone's feedback.

The summary I am taking out of this is driving below 45 to 50 degrees = caution, below 40 degrees = warning, below 35 degrees = danger. Again, even below 35 the car handles consistently and predictably, but at any of these temperatures, the margin for error goes away. Any aggressive maneuver in steering or braking may not get the desired results.

BTW, this was focused on temp, not pressure, but good info on effects of pressure is certainly not unwelcome. :cool:

Every tire is different and their characteristics under varying temperatures are different as well. You would need to know how your specific tire reacts to road temps.

Although you may have intended to only address temperature, your experience was no doubt effected by both temperature and the pressure change due to altitude so both should be considered.