Tire Pressure Question

[Denney Bridges]

Last weekend, myself and two fellow NM racers participated in the HSR-West event ar PIR in Phoenix. The organizations we normally race with do not race on a wet track. It rained all weekend--we were given conflicting advice regarding tire pressure adjustment for wet (and cold) track conditions. We were also advised to disconnect the anti-roll bars. Please shed some light on this to help the ignorant!

[Keith Robinson]

Living in the rain forest of British Columbia, we have some experience in this area!
[size=2]In the wet I run 12psi in the front, 14psi in the back. Removing or softening the sway/roll bars is also accepted practice. I believe the theory is that the weight will transfer onto the outside tires.By 'rolling' the weight onto them, you have reduced the lateral forces which are trying to slide the car sideways.[/size]
[size=2]We are still racing on the lake up here (ice racing is done in sedans with the heater on!) but 'soft' suspension is the rule on ice too!! [/size]

[EYERACE]

in the Crossle 32F on GY 160s under dry conditions i run 14 front/ 16 rear, the rain set up is three turns of the brake bias to favor the rear, disconnect front and rear sway bars and mount the rain tires that i run at 20 all corners. i'm sure there's could be a better idea out there but that's what i run...........by the way, i've probably used up all my racing luck ......done 100+ mph backwards in the rain and didn't hit anything or mess up myself or the car and then drove away - to the paddock to check out what the heck was wrong with the car - i had no front brakes and the rears grabbed unevenly so the car flipped end to end - suddenly i had front brakes, i was simply looking in the wrong direction!

[Don Denomme]

What you're looking for in the rain is a car with the handling of a '57 Buick. Its more than just tire pressures. First, you should run approx. 1 to 2 lbs more pressure in each tire. This is due to the fact that a cold or wet track will not allow the tires to come up to normal dry temps. You're going slower and not generating the required heat in the tires to get the desired tire profile. Heat expands air and changes the footprint of the tire.

As to sway bars, I generally set the the front bar at full soft and disconnect only the rear. This allows for better weight transfer onto the tires in the corners for better grip. Leaving the front bar connected at full soft means that it will still have some effect by giving the car a slight understeer in the corners. This is safer in the rain than a car that snaps around on you. If you disconnect both, you may end up with oversteer. I would have to know more about your particular car to give you more precise info.

Two more items to be done. If you have adjustible shocks, set them at full soft in both bump and rebound (remember the '57 Buick). This will again allow for better weight transfer and give a softer, more controllable feel to the car. Also adjust your brake bias to the rear. Can't say how much since this is a "driver feel" situation. The reason is that, since you don't have as much weight transfer to the front tires under braking in the rain, you will tend to lock up the fronts. Weight transfer helps the front tires to stick better under braking in the dry. Less weight transfer under rain or cold conditions means less front tire grip.

These are very simple "general" guidelines with limited details. Every driver and car combination is going to have some variation in these settings. If you keep good notes on what you do to your car at each rain race or cold event and what gives you a better feel and lower lap times, you know what to do next time. Doubt that you will have that many rain races or that you can get in a day of "rain testing". But, every bit of info helps. Also, if you had a practice session, qualifying and a race in the rain, you could try slight changes for each session and see what works.

Don

[Gary Payne]

What Don said plus consider softer springs front/rear if easily changed and youve got em available.

[Charles Warner]

Depends on a couple of things. How old are your rain tires? If more than a year old they are getting hard and will not respond as well regarding temperature increases. If over two years old they may be only good for touring laps. Most will advise setting the wets a pound or two below the pressures you see when your slicks are hot. For these cars around 18 psi should work. If the track is merely moist (but just wet enough for wets - i.e. just barely see spray from cars) then drop the pressure down close to where you would set your slicks as the wets will heat up quickly. If it is flood conditions then set the pressures at full hot pressures.

Also depends on how hard you are going to race. The harder you race the lower you can set your starting pressures. You'll have to develope your own settings based on your own rain racing tendencies.

[R. Pare]

Some bad info being put forth here:

Softening or disconnecting sway bars does not change the amount of weight transferred for any given lateral g load - it will only change where the transfer goes to ( and therefore the final balance) and how fast it gets there (reactiveness). The effect that the driver percieves in slippery conditions is a more "forgiving" car because of the increase in timespan over which the car reacts.

The same for shocks - they don't change the amount of transfer, but how it gets transferred during the transitional phase. Going softer gives a more benign and controlable feel to the driver.

Starting tire pressures desired will quite often be the same as for the dry - since the tire is not being stressed as much at maximum cornering load (since that load is less), the extra pressure is not necessary to "support" the tire profile, so normal starting pressures is most often the way to go. The softer tire ( because of the lower, cooler, pressures), will tend to grip better because of its lower spring rate - the input to the contact patch is "softer" and slows the reactions that tend to prematurely break the bond. Don't forget, also, that rain days tend to be cooler than dry days, so the starting temp at ambient air temperature will always be slightly lower than on hotter days so as to get the same pressure at the same tire temp when on the track.

In general, at the pro levels, there is almost no change between dry and wet settings.

You can also quite often lower the car a bit. Because you are going slower, you won't have as much tendancy to bottom the car in the rough stuff. Lowering the car will decrease the total amount of weight transferred for any given g load, and increase total traction potential.

[gib gibson]

One of the reasons you start with a higher tire pressure is to get the rain tire tread to open up and throw the water out and off of the tire,and also you want to be careful of lowering the car and getting into a lot of water on the track,like deep puddles,etc,if you know your track and know that by lowering the car you are not going to get into deep water etc,thats ok but if you get into deep water and you pick up a lot of water and the nose puts a lot of water right into the cockpit,thats no fun,been there. Gib Gibson

[Jim Garry]

More tire pressure to "open up the tread" is something that gets said alot. In my experience, going with higher starting pressures in the rain always results in going slower. As Richard said, less tire stress due to less max cornering means less pressure needed. And the softer tire will grip better due to the lower spring rate. I've experienced this benefit both in autocross with stock and formula cars and on the track.

Jim

[Don Denomme]

If you notice in my response, I wrote "better weight transfer" not more. These adjustments will simply change the rate at which the weight is transferred and where in the corner it peaks. By "better", my intent was to make the car feel more stable and safer for the driver with limited experience. You will also notice that I closed with the statement that each driver will have differing settings depending on car and experience. This allows for the driver who changes nothing.

In a forum such as this, you can only give limited answers. This is why I've suggested that more people need to attend full seminars. There are very few simple answers in racing or life.

[R. Pare]

Sorry, Don! I should have specified that it was not your post that I was referring to!

One thing for sure about racing is that "your results may vary", as they say in the commercials! All a racer can do is learn the "whys" behind car behavior so that when he/she makes changes, the reasons behind the results will make sense!

[Don Denomme]

Amen brother!

[EACIII]

I would be carefull about lowering the car. I run Hooiser tires and the rains are a little shorter then the drys. That combined with the low right height I run had my belly pan sliding accross the puddles on Lime Rock's front straight the last time I ran the car in the rain. Not fun. Next time it pours I will raise the right height.

[EYERACE]

sebring on sunday saw about an inch of rain......i lowered my tire pressures about 3 lbs for the second rainy session. don't know any lap times..will wait to see at mylaps.com. didn't loop it in the second session so maybe that helped, first session slow loops done by just a teeny tiny little too much rolling on power at track out - that set of tires will never be run again if i can help it and they shouldn't have been used even a year ago much less yesterday

[Jonathan Hirst]

Denney –

As this is in the Vintage thread and the tire generally used in the wet and dry is a treaded Dunlop/Avon -
Can I assume you were using one of these on your car? What works with the treaded vintage rubber may not coincide with a soft block "rain" tire as used in more contemporary events and the pressure adjustment reccomenadations may not be the same.

I am not referring to anyone on this thread specifically, please no one take offense. Nor am I referring to the vehicle dynamics discussion, as they still applicable regardless of the tire used. There is gold posted in this thread if you read through it.

As stated above, your results may vary, however, here is my experience.

First if all – in the wet, your best-case scenario is that the Dunlops are new and were properly stored or “saved” for just such wet conditions. Using your dry session or worn Dunnys is feasible but not ideal. The outside shoulders of each tread cut wear away quickly and they need to be square for maximizing a low grip track condition. Depending on your home track weather patterns, keeping a spare set of rain tires may be unrealistic and in that case try the recommended suspension softening and do your best.

With my Dunlops on the Vintage FFord, I have always run a higher pressure in the very wet (standing water) conditions in order to decrease the contact patch size and thus increase the psi of the remaining contact area between the road and the tire to take advantage of whatever weight transfer I can induce. It’s a theory – I have not done back-to-back tests to actually prove this out as a performance advantage over running lower pressures. I never did determine if the decreased contact patch size (and crowning of the tread) was a poor trade off to lowering the pressures and relying on the larger resulting patch for grip.

It just doesn’t rain often enough for me to try everything I would like.

With the vintage cars, the suspensions were generally on the soft side anyway but I did soften things up regardless in the wet. (backed off the Spax , discoed the rear ARB, full soft on the front).

The lowering of the chassis sounds like something I will try next time I have the opportunity.

My best advice is to make a plan and have the right people around to help you between (and during) sessions to minimize “pit in” time and try some things. Vintage weekends being what they are with the variety of cars on the track at the same time, it can be hard to string together enough consistent laps to be conclusive.

The nice thing about a wet session though is that there are a lot of people that tend to “sit out” wet sessions if it looks like the race might be dry. Dare I say more so on a Vintage weekend? This leaves you with a lot less traffic to deal with when evaluating the set ups that work for you.

Good Luck.

[Brian]

To agree with what Jon said...

My experience of the Dunlop FFord tire is that the performance stays very constant with age in the dry - I actually have consistently been faster on very old tires than on new ones in the dry. But in the wet, new tires have a really big advantage over old. I have gone out on new Dunlops in heavy rain at Mont Tremblant and had a very drivable, consistent, quick car. I went out on old Dunlops at Robeling Road in similar conditions and went from outside pole to dead last in one lap! I gave up after I was outbraked in a really embarrassing fashion by a Spitfire going into turn one - he passed me on the outside...

I was in a Merlyn Mk6 sports racer. Basically a Mk11 with fenders.

Brian

[trout roberts]

Richard:

Just a question about lowering the ride height. Does that also necessitate realignment? As I recall, when you alter the ride height, the camber and toe settings are also affected. I would think that you would want to retain the base line suspension settings regardless of the rain/dry status of the track.

[R. Pare]

Whether or not to re-align will really depend on what the ideal settings are for those rain tires. Unfortunately (thankfully?) most of us rarely run in the rain, so testing for different setups is damn near impossible - it becomes knowledge garnered over many years with the same car and tires - IF the danged things haven't been changed!

Sorry!

[EYERACE]

the car was faster with the slightly lower pressures 17 vs 20

[Mark H]

Don't lower it to much at a place like Road Atlanta, if it rains hard that river that runs across the track down in the esses will float the car!!!!
Than you will need a paddel to steer your cannoe across it? But the spray looks real cool kinda like a snow plow. Hmm like I know about snow, but I did see a pic of snow once.

Hey EYE we have the same # of poasts! Now I'm a big mouth I hope that dosen't reflect bad on you.

[Bud Byrnes]

Well Denny,

And you thought you got conflicting advice at Phoenix? Well I think there is a fair amount of consensus and conflict in the responses posted above. I think everyone assumed you were running "Spec" Dunlop tires, however Eyerace with the Crossle 32 would probably be on Goodyear slicks or maybe American Racers.

Are you guys in New Mexico still planning a race in October to tie it in with the Balloon Festival? Try to talk it up on this website and I'll post it on the VARA and HSR sites. It was nice meeting you guys at Phoenix.

Bud Byrnes

[Denney Bridges]

Bud--Please email me at fspilot1 "at" msn.com-- I'll send you some info on the October race-it is in conjunction with the Balloon Fiesta. We've got at least one FF from VARA coming. The 3 of us are going to the VARA race in Vegas and then on to Laguna Seca in Oct. I think I've sorted the best info on my question here. I appreciate everyone's response.