In rain with standing water and such, what's the rule of thumb? Do you go up in tire pressure, or down? I'm asking b/c I came across two differing opinons today at laguna; one school of thought advocted increasing tire pressures to inflate the center tread, and therby promote evacuation of the water from the inside out. The other school of thought was to lower pressures, and therby increasing rolling resistance and generating more heat within the tire. Thoughts?

 

Rain pressures = generally speaking, go higher.

 

I've always heard "go lower". Tires won't stick in the corners without the heat in the tires.

 

So... which is it? Up? Down?

 

Both ?? whichever works for the car

 

Typically, lower pressures work better in the wet. And softer springs and shock settings...

In standing water on tires with little tread, I've heard that higher pressures can help by crowning the tire such that it rides on the center section of the tread.

Brian

 

When I ran a wet autocross in Atlanta on Falken Azenis they liked lower tire pressure better. I normally run about 40psi front and 29-30psi rear in the dry. Ended up going to around 34psi front and 27psi rear. Since tires won't have as much traction in the wet, it would follow that they aren't rolling over as hard since the maximum lateral loads are lower - so you can go with lower tire pressures to achieve best grip and have less drastic slip when they lose traction. BFG KDW's are the same way too (lower tire pressure when wet), but a much better wet traction tire than Azenis.

 

I am by no means an expert opinion but I would *think* that it would take some huge variances in tire pressure to crown the tire enough to change the way it forces water out/through the tread pattern. I would be thinking of it as softening the springs and having lower pressure to increase heat more easily as no matter how hard you drive in the wet, the tires never seem to get "hot." But most of my rain races have been on cooler tracks so I may be looking at it more as a temp/wet condition vs. pure wet. I am also limited to most of my experiences being on intermediate tires (never ran a dirt stocker) and only a few races on drys in the wet (and if I knew it'd be that wet, I'd probably have had different tires on so the setup wouldn't have been changed to begin with).

 

I increase the tire pressures when in the rain to compensate for the lack of heat that will get into the tire compared to a dry setup. The more heat into a tire will increase its pressure (when dry) and you will not be able to generate the heat (tire pressure) in the rain so you need to compensate.

I will generally soften the shocks and sway bar a bit also to take the edge of the setup to minimize the oversteer in the car.

 

This is a curious thread. So, are we speaking of cold pressures or hot pressures?

Since I'm looking for about 38-40 psi hot (Toyo), I'll start accordingly in the dry to allow for the increase. In the wet, the tire won't heat up like it does in the dry, so you have to start higher from cold if you're still looking for 38-40 psi.

IMO, I like the tire a little softer in the rain, so I'll shoot for about 3-4 psi (or so) lower when it's raining. This means I start about the same (or 1 psi higher) than I do when it's dry. Make sense? I'm also not an expert, but living in Seattle, I do drive in the rain on occation and this works for me.

$.02

 

that's what my "general" thinking was too, but what's got me dumbfounded is that michelin actually reccommends you increase the pressure (cold) when using ther pilot sport cup tires in the rain.


Modified by bb6h22a at 11:06 AM 1/5/2004

 

For as much rain as we get here in the Northwest, you would think that we would know how to answer this question. Funny thing is we rarely have just a full out rain race. One where it is raining just as hard while setting the car up prior to heading to pregird and all the way through the race. Typically it starts raining as you leave pregrid or stops half through the session and the line starts to dry.

So, as I see it, it is another point of guessing and comprimising. 95% of the time, we are on a dry set up, 4% on an intermediate, and 1% full rain.

If it is wet to start, I will usually start with a slightly higher pressure to obtain the hot pressure that I want. If I am on intermediates and it dries, I will drive off line on the straights to try and cool them.

I have run the Dirt Stockers in torrential dourpours and they are the chit. Back to Back comparison between a "slicked" Proxes RA-1 and a Hoosier Dirt Stocker at Pacific Raceways: 2:23-2:25's on the RA-1's and 1:51's on the Dirt Stocker. Dry track with the Proxes: 1:42 flat. Problem with the Dirt Stocker is if the track dries they will chunk in 2-3 laps. I have never had the opportunity to race on them in the last 12 years. I love them but they actually make for very expensive canopy weights.

Even here in the great NorthWet we don't get the needed time in the rain to make a truly good comparison between differences in wet tire presssures. With all the car changes, updates, etc trying to pin down the proper pressure from month to month and year to year is nearly impossible.

Rick

 

I don't have too many data points from autocrossing in the rain... but I will say that

lowering the pressures worked well in rain on:
DS WRX w/Hoosiers
DS WRX w/Ecsta MX
ITR w/Azenis
STS Integra w/712s

increasing the pressures really DID NOT WORK on:
bonestock Integra w/XGTV4's

That was the only car I tried bumping the pressures up on on the "crown" theory... and it really really didn't work.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by WRXRacer111 &raquo;</TD></TR><TR><TD CLASS="quote">I don't have too many data points from autocrossing in the rain... but I will say that

lowering the pressures worked well in rain on:
DS WRX w/Hoosiers
DS WRX w/Ecsta MX
ITR w/Azenis
STS Integra w/712s

increasing the pressures really DID NOT WORK on:
bonestock Integra w/XGTV4's

That was the only car I tried bumping the pressures up on on the "crown" theory... and it really really didn't work.
</TD></TR></TABLE>

But are you actually ever going fast enough in an autocross to extrapolate the data to a track situation? Not knocking your contribution, but maybe the inverse proportionality of speed had an effect; maybe, slow track/auto = lower pressures to build/maintain heat; faster track, big track = higher pressure to aid in evacuation. Hmm...