Rally Tire Pressures

[A1337STI]

What tire pressures are people using for stage rally?

Are you changing it for rougher events?

Also what's your front/rear weight bias and how much lower are you setting your rears to?

I'm using Silverstone S505 and it seems that about 32 Front provides the most grip , 30 may be better but i haven't tested much. I'm 2800lbs with driver+navi 59% weight front and i've been setting my rears only 2 psi lower .. i'm starting to strongly suspect that's not enough of a difference as the rear just doesn't feel as hooked up as it should

Sometimes i run 36 up front if i'm worried about bending rims , i can tell i lose some grip , but i've nailed some big rocks setup like that, and didn't bend a rim.

guess i should experiment in the upcoming rallyX ...

[Carl S]

28psi warmed up f/r on a 3000lbs open class sti with hankooks. Only flat ever was due to the tire shop ripping the bead when they mounted them and not telling us.

[A1337STI]

did your sidewall bulge at all? Mine look perfectly straight until i bleed down to about 30 ..

Also if you're 28 front, what do you set your rears to? (and are you trying to max out rear grip or are you dialing some out for handling) my car is very neutral if not a bit loose right now.

[UP2MTNS]

I run mine at 27psi on all 4 corners. my car turns in just fine, so I don't lower the rears at all...one less thing to worry about 'adjusting'.


26-27psi is what's recommended for the Lassa's though. even at 25psi there's no noticeable bulge in the sidewall.


BTW, are you going to Plan B? Gaylor Van Brocklin (red VW) will be on a fresh set of the Rally 1s...you should see if he likes them. (yes, always selling :) )

[Carl S]

Sidewalls might have buldged, but not enough to make me think twice or make the tires look flat. Rears set at the same pressure as the fronts.

Have you asked the Tabors what their recommendations are?

[A1337STI]

I asked Kristen Tabor @ Idaho last year, but i forgot :oops: exactly what she told me

I remember telling her i was running 36 and she was like "wow , that's way to high, i think Matt was running them at 30 then at 28" .. i think but i saw some slight bulging at 30 so i never actually tried it out .

not like flat tire bulging but at 34 it has the exact same shape as when its not on the car and laying on its side.

[thewizard]

Pressures will vary with conditions. I can tell you 36 is way too high! Most will not run more than 30 pounds in any circumstance. The range will be 26 to 30 for most events. I would say 29 front and 28 ish rear is safe for most cars

[WRR]

2950 heafty pounds of car...

I like to be 27-28 front and 30-31 rear just to help keep the rear a little looser (or more loose?) but I have very little science behind my thoughts... 35 is my normal rear at a rally-x or I can't seem to turn

[thewizard]

I was crewing for a friend at Sandblast and we lowered pressures down to 26 pounds and they had better grip after we lowered it. Conditions were wet sand (very wet)

[Stockline]

Another thing to think about: Try counting the ration of right to left handed turns, if more to one than another, go another pound lighter on that side. It will help keep the tires on the most used side down into the more loose stuff on the apex.

ex:
FL FR
29 28
RL RR
27 26

This is would obviously be set up for a majority of rightys, and I would think it may only have its advantage in a higher right hand ratio stage (I'm thinking 65-70%), of course you have to transit and do multiple stages too so that comes into effect.

Probably more helpful to the rally-x guys as its a less daunting task to count corners on a track you can see fully.

[thewizard]

Factoring in ground temperature will also help if you dont have multiple compounds

[WAM]

Any chance you guys are overthinking this? Seat of the pants data is often whatever you want it to be.

[Stockline]

Any chance you guys are overthinking this? Seat of the pants data is often whatever you want it to be.

Possibly... my data is from my high performance steering and suspension class, stuff is just fresh in my mind. Never know, little stuff like that makes good racers into great racers. :rally-driver:

[31motors]

Be sure to check your pressures hot. It is very easy for your pressures to rise 3 to 6 pounds with some heat. with a cold of 36# it could easily be over 40#s on a stage.
You can run nitrogen and only gain maybe 1#. Nitrogen has very little humidity so the tires gain much less pressure when hot. Rally tires seem to have very stiff sidewalls so I would think 25# would be safe cold.

[Stockline]

Be sure to check your pressures hot. It is very easy for your pressures to rise 3 to 6 pounds with some heat. with a cold of 36# it could easily be over 40#s on a stage.
You can run nitrogen and only gain maybe 1#. Nitrogen has very little humidity so the tires gain much less pressure when hot. Rally tires seem to have very stiff sidewalls so I would think 25# would be safe cold.

Im not saying what your saying isn't true, but Nitrogen is almost 80% of earths Atmosphere right now... I think it was just a big marketing ploy for Tire companies and dealers to get more money.... Still no hard evidence as of late so I'm sticking with this dude> :bs: (not to you, just to big tire manus :D)

[UP2MTNS]

Any chance you guys are overthinking this? Seat of the pants data is often whatever you want it to be.

F*CK NO!

I also take soil samples approx 12" deep and cross reference that with my starting position, how many times we're running a particular stage, and approx how many spectators and volunteers will also be driving those roads so I know exactly what the road conditions will be precisely as I drive over them.

I also have an RSS feed to the US Geological Society dbase for info on any recent and local tectonic plate movement, which could significantly alter the soil structure and stability (i.e. liquifaction, etc).

Lastly, I like to snuggle with my tires the night before a race...they seem to like it and I love the smell of rubber in the morning :rally-driver:

[31motors]

I forgot the big tire companies were pushing nitrogen. The set up is cheap to get if you make it up yourself.

I use nitrogen instead of an air compressor in service area. It is very useful for many things. The fact that there is little moister and the tire pressures are more stable is just a plus to me.

I made a mount in the trailer where I keep the bottle and piped the air hose where it was easy to get to from out side. I barely run my noisy generator any more. I dont think it will change the 12" dirt sample to speed factor but I do like having it!

[UP2MTNS]

Im not saying what your saying isn't true, but Nitrogen is almost 80% of earths Atmosphere right now... I think it was just a big marketing ploy for Tire companies and dealers to get more money.... Still no hard evidence as of late so I'm sticking with this dude> :bs: (not to you, just to big tire manus :D)

here's some extensive testing:

http://powertank.com/truth.or.hype/


I use the power tank system (liquid CO2), a little pricey, but VERY convenient....if I set my tire pressure at 25psi, by the end of the first stage, it'll be around 30psi (on a warm to 'hot' day).

[31motors]

That is a good test of the 3. Thats funny the real world race car test of over 15 years shows nitrogen to only gain maybe 2#s . but the test shows diff results. cant argue that the test is wrong or can I prove the other is write. If it comes down to cost Nitrogen is about $18.00 for a 250# cylinder it last me a full season. I would guess I could fill maybe 30 Rally tires on 1 bottle.

CO2 seems like it would be a good alternative but I have no idea of what it cost.
I never new Tire places are getting that good of money to fill a tire. I would never pay 5 bucks per tire to have them filled. Thats crazy

[WAM]

Yeah, it turns out for all practical purposes, gas is gas. I've never found one test that didn't just show it just doesn't much matter. IF you had liquid water in your tires, and IF you got the tire above 212F, sure then it would boil and raise pressure. Real world, not so much.

I use a CO2 system from "The Source" which is similar to Jon's Powertank but a little cheaper. And I picked up an additional but much smaller 5-lb tank that's good for a single weekend. But it's just a convenient way to carry compressed "air", not a better fill gas. I get the tanks refilled by my local welding gear shop.

[A1337STI]

Well oddly a sealed container with liquid water at the bottom doesn't need to reach 212 for all of the water to change from liquid form to gas. nor does a temp of 212 ensure that all of it will be gas either.

Relative humidity , dew point, temp, pressure all factor in.

but basically braking heats up your brakes. flexing your tires sidewalls heat those up.
hot brakes heat up your rims, now you have Hot rims and hot tire sidewalls (not to mention the actual surface as well) this heats up the Gases and liquids that are with in your tire.

heating up ANY gas in a fixed area will raise the pressure PV=NRT Ideal gas law (http://www.google.com/url?sa=t&source=web&ct=res&cd=1&ved=0CAYQFjAA&url=http%3A%2F%2Fen.wikipedia.org%2Fwiki%2FIdeal_g as_law&ei=jVHfS5fDEITysgPiz4z0BA&usg=AFQjCNH3gOYNJPm1ArsPlMYS7C1CKP9Dig)
Changing Temperature & Pressure will change humidity , and how much water is liquid Versus Gas.

so filling your tires with any Gas (even 100% Dry) will allow them to change temps with heat.

having water in there makes it worse.

also you can't Purge your tires of all of the H20 in there, short of mounting the tire in a complete vacuum and then beading with with a 100% dry gas.

So while having less water in your tires is greatly desired, since your tire pressure is gonna change anyways , forcing you to check before each stage anyways , it becomes the definition of "moot point"

:) one could get those pressure warning caps and set them to your ideals settings, then checking pressure is just a glance ;)

[WAM]

Actually race houses like Pegasus offer self-bleeding rim fittings so pressure build-up never occurs. But if one of them ever stuck open...

[REDrum]

FWIW....@ TON rally this past weekend, on Hankook 203s w/soft compound, I started with 27psi cold all around. In the afternoon, after the rain made the gravel pretty greasy, I dropped down to 23psi. Thoughout the day the gain from cold to hot was only 3psi. At NEFR, which is very rough and ladden w/ baby-heads I ran 25psi in Hankok 202s w/soft compound cold on dry gravel. No flats, no punctures. Perhaps Murphy helped since I hauled two spares in the trunk...some very rough sections on Middle dam stage which was run 3 times....