I'm running Hotchkis v.2 sway bars with full stiff up front and full soft in the back. Azenis RT-615 (245/40/18 & 255/40/18) on stock Track rims. On dry courses I've been happy with 42lbs in the front and 39lbs in the back... with the weather being sunny and 80-90 degrees.

The forecast for this weekend is wet and 65 degrees Fahrenheit. This will be my first wet event ever. I'm not going to mess with the sways, but figure I should at least drop the tire pressures a few pounds... any suggestions?

Thanks,

Joe

 

A few pounds less should help.

Going softer on the bars would be a good idea IMO...

 

By a few... say 3lbs? So 39/36 should be pretty close?

 

Why are you running a lower pressure in the rear tires ?

And why full stiff in the front ?

This is a setup to counter massive oversteer problems!

Yeah, start with 3 psi and go by feel.

 

During rain conditions try changing your sways to full soft all the way around.

And remember that the wet line is different than the racing line!

 

If it's raining and pudding on the track, it's a MUST to go full soft on the swaybar settings. If it's really raining and you have too much weight transfer, consider disconnecting the end links on one side of your swaybar to take all the bar off the car. You didn't mention what kind of tread depth are on your Falkens, but beware of hydroplanning above 55 MPH if they've been shaved

Stay off any concrete that is wet!

David Muramoto
Senior Editor
Nissan Sport Magazine

 

Thanks for all the tips... I appreciate the input.

I went full stiff in the front and full soft in the rear with the sways per Hotchkis installation recommendations. That has seemed to fix the understeer (I know, that's counter intuitive).

Thankfully, there's no concrete on the track.

I won't be able to get at the sways, but will drop the tire pressures (maybe balance them for more oversteer) and let you know how it goes.

Thanks again.

 

I can't believe I'm the only one saying this... but in the wet, you should really bump up your pressures to a lightly over-inflated state... this makes the contact patch more... 'round' which will cut through the water better. Reducing pressures will increase the size of the contact patch which will increase the chances of hydroplaning. Maybe this is more for slick-type tires, but I would think the theories still apply even to a treaded tire.

Honestly... I'd leave your pressures where they are at... they sound high to me. I think you're losing time in the dry with the set up you've described.

 

agree with stack.
raise tire pressure in the wet.
that's what two of the racing schools i attended recommend( for the reasons stack mentioned)
regards

 

Interesting Stacy.

I’m not sure I agree though. Tire science is pretty hard to grasp and I’m nowhere near knowledgeable in the field. Let’s have a discussion and see what good we can get out of it.

If I remember my readings correctly, hydroplaning is a factor of tire width. Given the inertia and viscosity of water, it will take a certain time for it to be displaced to the sides so the thread can make contact with the pavement . Hydroplaning will set in when the car is traveling too fast for the water to be displaced outside of the tire or channeled into a groove. A grooved tire will effectively act as a narrower tire compared to a slick.

Within reasonable limits, tire pressure doesn’t affect the tire’s contact patch width. Only its length will change. So, for a given tire, hydroplaning will set in at the same speed whatever the tire pressure your running.

Running less tire pressure in the rain will allow for a larger contact patch, maximizing the grip. Overheating the tire won’t be an issue either, as the water will cool it.

I can see one advantage at running a higher pressure thought. The tire will give better feed back and might help the driver sense how much grip is available and be more comfortable at speed.

 

Actually, hydroplaning is a function of both tire width and tire pressure, so a higher air pressure -- all other things remaining the same -- will help counter hydroplaning.

Also, don't forget that part of the issue isn't how much water is on the ground, it is how much builds up in front of the tire. By lowering the tire pressure, you actually reduce the amount of support that the center of the tread receives, causing the tire to start to accumulate water in the center of the tire. Once that happens, hydroplaning is an inevitable conclusion if everything remains the same.

"Common wisdom" in this case is wrong. Increasing tire pressure in the wet actually helps the tire channel water by lengthening (and narrowing) the contact patch of the tire.

 

Interesting idea - it makes sense, I will try going to a higher pressure when it rains at a track event.

 

Lerxst, what camber are you running?

In the dry I run my 255/40/18 Azenis 36psi front, 35psi rear (I have four 8.5" Track rims). Camber was -1 front and -2 rear.

For the wet, I'd drop another 2-3 psi.

I agree with the softer rear though. The 350z seems to respond much better to a softer rear. Having said that I find the Hotchkis full stiff up front causes too much steering kick-back (bump steer?)

 

Often its better to drive less then the ideal line in the wet to avoid the larger puddles. If you can stay on a damp surface instead of going through standing water your sticky tires will work better then wet rain tires.

 

Good stuff :-)

 

In wet conditions you should aways drive off the race line unless there is a large puddle. This is due to the cars wearing down the surface on the racing line, and off line will have a rougher surface which will handle water a lot better!

So my question would be, on dry conditions my target hot pressure is 38 psi so I start with 32 to 33 psi cold - in wet conditions what pressure should I start with to achieve a higher hot pressure since I am sure the tires won't heat up as much on wet pavement?

 

Remember you won't have as much heat generated in the wet, so your cold and hot temperatures -- and the resulting pressure rise -- will be smaller.