Tire growth

[jacksoni]

We have to keep the tire on the wheel and so adequate tire pressure is necessary. Exactly what that is I recognize as perhaps controversial. True LSR tires run very high pressures. There are MFG recommendations for load and tire ratings which I suspect are freely ignored (without being "excessive". My question is, how is tire growth affected by pressure? Do they grow less with higher pressure or is it not really affected much. I know my older MT 25/6-15's grow 1.5" diameter at 75-100lbs pressure at 250mph. Don't go that fast these days but that "overdrive effect" is nice.  If I want my tires to grow more, do I lower the pressure?

Thoughts on the issue?

Thanks

[colinski6]

Please keep in mind that I am a newbie as you read this, but here are my thoughts:

I would hypothesize that lower air pressure would allow the tire to "grow" more at speeds, because there would be less force pushing the sidewalls outward, holding the tire in its original shape.

I guess this would depend on how much of the change in size is caused by the tire material itself stretching, and how much is the tire changing shape from being relatively wide and smaller diameter, vs narrow and larger diameter.

[tauruck]

There are member here that can answer that question down to within an 1/8" I'm sure.
Ask Tom Burkland. I know he knows more than anyone on the subject.

[Stainless1]

Wow Jack, that is way more than I would have thought.  I know our tires grow with pressure, so their size can be adjusted to be equal.  But you are telling us your tires grow more than 15% with speed.  Our data tells our 21 inch Goodyear tires probably grow 1/2 inch past the size they measure, but that data is tempered with an unknown tire slip value. 
Please explain the method you used to determine your speed growth...
Thanks and 

[jacksoni]

Wow Jack, that is way more than I would have thought.  I know our tires grow with pressure, so their size can be adjusted to be equal.  But you are telling us your tires grow more than 15% with speed.  Our data tells our 21 inch Goodyear tires probably grow 1/2 inch past the size they measure, but that data is tempered with an unknown tire slip value. 
Please explain the method you used to determine your speed growth...
Thanks and 

These were the MT Bonneville tires they were making in the early 1980's (they had two versions, 260 rated and 350 rated. These were the latter). When I built the liner there were parts of the  belly pan that came within about 3/4" of the tire( it might have been less closer to half an inch I suppose). I do not remember what year it was as first year with the liner I went 145 running I/GS and then G/GS at 193, both short course in 1984 (it was wet and only had short course). The next year I think best pass was 239. Somewhere in there the tires rubbed a nice shave in the middle of the tread on one side. I don't remember which year. Tin snips to the body work fixed the problem. I ran various tire pressures in these tires, but never less than 75. I still have the tires and could send a photo if you want.

[SteveM]

I am pretty sure that centrifugal force (centripetal) has a much higher effect on tire diameter as compared to air pressure.

Like a previous poster stated, if you get ahold of Tom Burkland, he is the mastermind of such topics.

Steve.

[jacksoni]

Actually, Tom responded to me in a PM at the time of the original post. I don't think he will mind me repeating it here (at least hope not, my apologies if so)

Jack,
All of the M/T land speed tires are tested at 90psi inflation pressure and should be run at this pressure as well. We have had the tire pull back from the wheel bead flange enough to let the inflation pressure out up near the top of the tested speed ranges (depending on which tire size this can be over 9000rpm at the axle). Once the inflation pressure goes down the tire very quickly becomes a "pup tent" stretched way up in the center of the tread face and tears itself apart. One of these was actually violent enough to fail the steel race wheel as it disassembled itself!
Even if you are running lower speeds where tire failure may not be a concern I would strongly recommend not letting the tire pressure down for purposes of growing an overdrive as the casing load distribution is all out of whack and the resulting tread growth will quickly wear only the center portion of the tread rubber thickness.
Our testing experience is mostly M/T, but we have also run a few of the Goodyears (both drag race and land speed eagles), a few Hoosiers, some Michelin Pilot Sports, Dunlop vintage oval tires, and even a couple BFG motorcycle tires. This manufacture recommended inflation pressure is crucial, along with proper wheel selection and very careful inspection program to maintaining the safety of your racing operation.
Good Luck and be safe.
Tom Burkland 

[631]

I went through the spinning historical record file and found 2 - 25x6x15 MT tires that Gene and Betty spun in 1994.  From static unspun  circumference to + 500 mph spin test- the at rest circumference grew: a). 1 7/8" and b). 1 1/2".  RADIUS growth at 300 mph was a and b). 1/2". RADIUS growth at 370 mph was a). 7/8" and b). 3/4".  RADIUS growth at 500 mph was: a and b). 1 1/4".  Begin of test pressure was 90 psig and end of test pressure was 82 psig.  A couple items to consider:  land speed tires can take a 'set' after running at speed and will be larger in diameter affecting speed calculations.  The gas laws are at work here so as the tire grows in volume it drops in atmospheric air pressure assuming a constant temperature which we do not have in a racing application. When building tire clearance into an LSR vehicle know the diameter / radius at your desired speeds and leave plenty of extra clearance for the tire.  As Tom says have a careful tire monitoring and maintenance program and stay at the mfg recommended tire pressures.   the above info is for entertainment purposes and no recommendation is made.  your mileage and emission readings may vary.. thanks  Rex
 

[WOODY@DDLLC]

I got this a few years back.

At 400 MPH, I estimate you will see the following growth factors, based on the radius:            
            
22.5x5.5-15:      .675”            
            
24.5x7.5-16:      .75”            
            
26.5x9.0-18:      .88”            
            
30.0x9.0-18       .95”            
            
At speeds, other than 400 MPH, the above growth factors can be linearly interpolated            
            
We do speed-rate the above tires for 350, 375+, 475+, and 590+, MPH, respectively.              
            
You will need to generate your own static radius data.            
After the tire/wheel assembly is mounted on your vehicle, measure the center of the wheel hub with respect to the surface of the ground.            
That’s your static radius.  For a start, you can estimate a deflection of 10% of the sidewall height ((OD - rim dia.)/2).            
            
Keep in mind that as the speed increases, the tire starts to act increasingly as a solid.            
I.e. the deflection % approaches “0”, so the tire growth becomes the overriding issue.            
            
Unfortunately, we have no other information to offer you, other than the above and our Bonneville spec sheet.            
            
Lester Garbicz            
Product Engineer\            
Mickey Thompson              

[tauruck]

Thanks guys. This is really valuable info that will help eliminate clearance problems
for many of us.