Air pressure and reaction time

Does anyone had a general rule of thumb as to how much a pound of air pressure in the front tire equates to reaction time increase or decrease,specifically with a 27.5 tire?

Not sure if it affects R/T that much. I thought that the increase in front tire air pressure was to decrease rolling resistance, thus lowering ET.

It does. I've had to vary it between 50 and 26 lbs. Somebody told me the rough average, but I forgot. But the higher you go, there are diminishing returns - i.e.: 30-35 lbs will make more difference than 35-40 lbs.

True.. as the pressure goes up the dia will change
less at some point.. I could get a small amount of
change inn reaction time.. but not much at all...
I changed from 30-50 psi

Well, today wasn't wasted. I learned something new...Thanks..I personally can't see the diameter changing more than an 1/4" at best going from recommended psi to 40 or 50 psi, but evidently I'm not getting it.. Thanks for the info. I run my fronts at 40 psi when racing, 32 on the street.

I can't see any difference. Ive tried numerous times.
Doug

What TP is the manufacturers rec when you pump them up to 40 or 50 PSI?

There's a couple of guy's at the track that play the tire pressure game, one runs one front tire so low the starting line guy has backed him off. He claims he can leave as the 3 yellow comes on . I've watched , he does, he also red lights a lot. Another uses his tire pressure pounds to rpm launch 26 pounds =2600 rpm etc. . Just one mans theory but he wins a lot. Go figure. I just run 35 psi, pull the wheels out of the light on launch and red light.

I used a 2x4 (laid with 4" side down) and a square to figure out how much roll out my tires had with X amount of air. I started at 25 psi and went up to 35 psi, in 5 lb increments. There was a bigger difference between 25-30 than 30-35. I forget the exact measurements but there was 3/32" difference with 5 lbs of air change. Also a "race" tire may change differently than a "street" tire.

I would guess this would make more difference with a bias ply tire than a radial.

I would guess this would make more difference with a bias ply tire than a radial.

rear tire pressure has had more of an affect on our cars. we tried it in .25 lb increments. We run 29.5x10.5 W tires on those cars.

I was gonna try 20psi on my race car but after
lowering it to the 20 I looked at the tires..
I was afraid it would hit the rims on the way
back down from the normal wheel stand.. they
were low.. also on my car I had 3/4" of stagger
on the front end.. the right tire was set back
the 3/4" to give me a littler longer time in the
lights to slow the reaction time.. I used a 24.5"
tall tire and the car reacted very quick.. I was
running a 1.19 60' on a normal basis unless the
tires were getting old(70-70 passes).. the rear
tires were MOST of the time set at 6 7/8 to 7 1/8
psi.. depending on the track.. this pic is with a
lower rpm launch and between the rpm and tire psi
it started rolling/folding the tires

I bought a 10 ft. cloth seemstress tape to check tire circumferences on both my front and rear tires at different tire pressures
I shoot for 1/4 inch vareince on the rear tires, inflate as needed to get that

Im at my limit on front tire size now running 27.5 on my dart.I just put a Gerst tubular front suspension on it and i have the rebound set as soft as i can on the viking shocks to allow for most rise.Im looking to slow things down aprox. .030.I run 28.0 x 10.5 in the back usually at 10 3/4 pressure.I footbrake the car and it reacts very quick.I may try backing off the timing kill it a little off launch.If i tighten the front shocks im assuming that would make it react quicker with less movement up?

I do about the same as I run about 35 on the street and 40 at the track. Does it help any ?? Most likely not much if any. Ron

i also was bulbing a lot and someone told me about slowing my car down reaction time wise with air so i went down to 30 in the front and down 2 in the back and boom i'm at .015. it seems to work. i am on regular bias tires . the rear must wind up the sidewall a little bit more before it rotates the tire and lower in the fromt must make the contact patch a little bit longer.

Exactly! It does make a difference. There are variables that often make it difficult to see. A major variable is a driver's "margin of error". i.e.: If a driver can consistently stay within a .03 spread in reaction times, than a change in air pressure that should make an .01 difference may not be obvious. But keeping very detailed, long-term records of all variables will help. This includes different tracks, day/night, possible lane differences & more.

In my opinion most foot brake guys trying to slow down R/T should look at the driver. Try a pro tree and see where your at. See if you're minus on the full tree and plus on the pro (take out the .100 if its .400 Pro). If so you are anticipating the bottom bulb. You should be within .010 or less difference.
Doug

Thanks to my new Gerst suspension i have staggered the front tires 1/4 inch.Havent had a chance to take it to the track yet to see the results in reaction time.

The way I look at it, Low pressure, the tire bulges out front and back, at rest car would be a tad back farther when touching the beam, like when the old racers staggered the wheels to pick up et. Since the car rolls farther to get out of the beam, reaction time will be slower, the other way with the high pressure, tight, not much bulge, relation time is quicker. I think I read somewhere that measuring from spindle to ground = radius, 3.14 X radius squared gives the true diameter of the tire. Personally I think it's a waste of time, I varied my reaction times by launch rpm. Foot braking, Each 100 rpm was equal to one hundredth. 1900-2100 - .01- .0 -red