Tire Traction in Wet Weather

[plus 3 golfer]

The attached study on recent wet tire tests confirms what I have always experienced as tire wear especially approaching the wear bars and is why I always replace tires well before 2/32” tread depth usually just below the 4/32” depth. Just look at finding 2 below. The wet stopping distance and lateral acceleration have significantly worsened from new at 4/32”. Dry traction and handling generally improve as tires wear but not so under wet conditions. The data is best case data as the tires were worn down using Michelin test equipment but the tires were not aged (which would further degrade the test results of worn tires.

 

https://www.aaa.com/AAA/common/AAR/files/Tire-Study-Report.pdf#page6

 

“Key Findings:

1. On average, new high-priced tires did not perform significantly better than new low-priced tires in terms of stopping distance on a wet road surface, maximum lateral acceleration on a wet road surface and NVH characteristics.

2. Compared to new tires, tires worn to a tread depth of 4/32” exhibit:

a. An increased stopping distance of 42 percent for the Toyota Camry and 44 percent for the

Ford F-150. When decelerating from 60 mph, worn tires are still traveling at 39 mph and 37 mph at the average stopping point of new tires for the Toyota Camry and F-150, respectively.

b. A decreased maximum lateral acceleration on a wet road surface of 33 percent for the Toyota Camry and 28 percent for the Ford F-150.

3. A set of four higher priced tires cost, on average, $247.52 more for the Toyota Camry and $203.80 for the Ford F-150.”

[ptjones]

Seems reasonable and I think raising tire pressure to 50 psi also improves wet performance by decreasing contact spot some and increasing psi on spot. The deeper the tread the better wet performance.  Just put new Michelin's E/S on front and almost new on the back and with 50 psi and they don't handle as good as the old E/S in the dry and hurt the gas mileage a little too.

 

Paul

[plus 3 golfer]

Seems reasonable and I think raising tire pressure to 50 psi also improves wet performance by decreasing contact spot some and increasing psi on spot. The deeper the tread the better wet performance. Just put new Michelin's E/S on front and almost new on the back and with 50 psi and they don't handle as good as the old E/S in the dry and hurt the gas mileage a little too.

 

Paul

See attachment and link (although I doubt most will read the 699 pages) about the highlighted red text.

 

Basically, as tire pressure is increased, the smaller tire contact path when cornering can result in tire deformation because the sidewall stiffening is overcome by the increased lateral forces due to the smaller contact path. So, once this pressure is reached, the tire is more likely to slide because the lateral force (cornering) + longitudinal force (straight line) exceeds the traction force. One thus would want to keep inflation pressure at or below this condition. Add wet roads and the tire would likely slide sooner than on dry roads. This sliding is not the same as lateral slip. "The lateral slip of a tire is the angle between the direction it is moving and the direction it is pointing. This can occur, for instance, in cornering, and is enabled by deformation in the tire carcass and tread. Despite the name, no actual sliding is necessary for small slip angles. Sliding may occur, starting at the rear of the contact patch, as slip angle increases."

 

We don't know what this pressure might be for the Michelin but their wet track performance is not as good as others. I would not advocate increasing pressure to improve wet performance as I want the “best” pressure for emergency situations. I ran 44 psi in my E/S as that seemed to give the best all around performance for me.

 

I believe the above is why my General Altimax RT43 (98) tires seem to keep the tires on the road better than the Michelin E/S (94) when cornering. “Sliding” is very noticeable under wet and dry conditions in the Michelin’s. I'm currently running the Altimax tires at 50 PSI. I feel a lot safer in the Altimax tires than the Michelin E/S. :)

 

My points are: the AAA study indicates tread depth affects wet tire performance and NHTSA (and others) indicate that increasing inflation pressure too high may hurt tire performance. As I've said before there is more to tire performance than MPG. :)

 

http://www.safetyresearch.net/Library/NHTSA_Pneu_Tire.pdf

Edited June 19, 2019 by Plus 3 Golfer

[fbov]

Good stuff, Plus, but I'd like to comment on the approach of this report rather than it's predictable and expected results. Two passages are telling. 

"AAA has traditionally recommended that motorists replace their tires at a tread depth of 4/32”."

and

"For wet stopping tests, an average water depth of 1.0 mm is an appropriate approximation for typical rainfall ...."

 

It's important to know an organization's motivation, it's point of view, when examining "scholarly" works such as this. The first quote appears early in the 4/32" test methodology, and it's key to knowing why they spent the money. AAA thinks 2/32" is an unacceptable legal limit, and wants the law changed. 

 

It's equally important to know how the test was conducted, as the details matter. Water depth is the critical detail here, given the focused nature of the study. These results only apply while you're driving in heavy rain. Their use of "wet" to describe their test road is misleading. The road was flooded. 

 

It would have been very interesting to see a range of tread depths tested in this manner, to see how "worst-case wet" performance changes with tire age. That would have made even more obvious that there's a huge variation among the tires at any price. Certain tires (Michelin Premier) were much better than others. Only by artificially grouping them by price, and not performance, can these conclusions be supported. More evidence of political, vs. scientific, motivation...

 

Where I live, it rains all the time... light soaking rain that barely fills the space between the aggregate on an asphalt roadway. 

Plus, I'll bet AZ doesn't get much rain... although I understand it can rain very heavily when it does. 

Now Paul is another story, as AAA's choice of the southeast as a model region was a good choice for lots of rain. 

 

Bottom line is that a required driving skill is the ability to judge "road feel" when it rains, because the car will tell you when you're driving too fast, puddles not withstanding. Every tire and every wet road will hydroplane/lose traction if you drive fast enough. 

 

And thanks for the NHTSA report; dated but comprehensive in the extreme! Kind of like Hucho's book on aerodynamics; basic science doesn't change. 

 

HAve fun,

Frank

Edited June 19, 2019 by fbov

[plus 3 golfer]

I do a lot of high speed driving on interstates / freeways outside of Phoenix and the biggest issue is pooling / draining of water across the lanes and in the "ruts" worn in the pavement by vehicles. I do not like to start a 4+k mile round trip with less than 4/32" of tread. There's a noticeable difference when running newer tires and worn tires when driving through these areas of water on the pavement with respect to hydroplaning. In Phoenix, I probably drive in a "haboob" more days a year than in the rain. :) We have very heavy localized summer storms (during the "Monsoon" season) that might dump at the rate of 2-4 inches an hour and are generally fast moving and tend to pop up and die out and account for most of the 8" average annual rainfall measured at the airport.

 

I believe the worst rain I ever drove through was between Atlanta and Birmingham taking the southern route back from Pittsburgh due to snow / ice storms in KY and further north and west. I don't recall what our average speed was but we'd hit bands of torrential rain every so often for maybe 20 miles at a time. Even trucks were pulling off the road as one could only see the taillights of vehicles from maybe 75 feet (it was mid-day).

 

Attached is some NHTSA 2005-2007 data on tread depth vs % of tire-related vehicle crashes vs other vehicle crashes. It clear that cars with more tread wear have a higher likelihood of being in a crash (95% statistical significance). You can find lots of reports on NHTSA web site.

 

The way I look at it: if I drive with 3/32 to 4/32" tread, I am about 1.25 X more likely to be in a tire related crash than driving with more than 4/32" tread. But, what's my odds of being in a crash? I was in one crash in 1967 when I pulled out at night unto a 35 mph street and was rear ended by someone just as I shifted to second gear. I wonder what his tread depth was. I had to instruct the police officer to measure his skid marks of 133 feet before the impact that totaled my car. The driver said he was following someone. Just before I pulled out, a car went by me likely doing over 70 mph.

 

So, I've driven well over 1 million miles with one crash. Maybe I'll run my tires down to 2/32" as the 1.25 X factor hardly changes my miles per crash - nah I'll stick with the around 4/32". :)

 

Bottom line: people will always construct tests to justify their beliefs and interpret data to suit themselves including AAA. We had people saying the C-Max is a 47 mpg car because they can get 47 mpg and it is for them but not for the vast majority. Everything has a price. NHTSA established the 2/32" (we trust the government to protect our interests / lives ;)). I am always amused on the Energi sight when people who purchased Energis new / used believe Ford should have told them how to operate their Energi to preserve the HVB. Don't rely on others to protect your interest (tire safety, HVB, and so forth).

 

Edited June 20, 2019 by Plus 3 Golfer

[ptjones]

In the OM it tells you to look at door jam to get the minimum recommended and don't exceed the tire Manufactures max pressure on the sidewall of tire. This would imply that you can use any pressure from 35 to 51psi. The tire experts at Michelin, Tire Rack and Discount Tire all agree with this statement.

 

 

 

 

 This is my Michelin Energy Saver AS using 50 psi since new with 80k miles on it and now has 85k miles going for 90K miles.  Actually  got  95k miles out of  tires on the rear that were rotated to the front when I replaced fronts that had a hole in the sidewall. As you can see it has even tire ware, definitely not Over Inflated.  From the information we have I still go with I said before Properly Inflated will have 44 psi to 50 psi. to get even tire ware.  BTW my tires squealed while cornering hard and they didn't at 50 psi.

 

Paul

[fbov]

Bottom line: people will always construct tests to justify their beliefs and interpret data to suit themselves...

Thus the use of peer review in technical, scholarly and research publications, to separate the wheat from this sort of chaff. 

 

My comments were intended to add perspective, that driving conditions vary, as do driving styles, so that not everyone needs to use conservative criteria. In my case, I want to stretch the original tires for one more summer, before trading-in the car.  Rain just means taking the slower roads, or slowing down a little in the downpour. Of course, we have are lucky enough to have a second car, with new tires, for long trips. No disagreement there!

 

Have fun,

Frank