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Drag Racing Tires

Drag Racing Tires

Drag Racing Tires

Top Fuel drag race cars currently accelerate at over 5 g's in the first hundred feet or so.  The cars weight about 2175 at the start of the 1320' run, typically 4.5 to 4.7 seconds.  This is about 12,000 pounds of force pushing the car, only by the tires.  There is a force procuced by the exhaust thrust pushing the car down and back also but it is probably less than 500 # in each direction.  How can these tires develop this kind of stick?  Do they behave like a typical friction model or more like duct tape?  The tires are soft and sticky and the track is also treated to be very sticky.

RE: Drag Racing Tires

Pretty good since Physics Professors in the 60's said that
Top Fuel dragrace cars would never go faster than 9 seconds ..then later to 7 seconds because of the current thinking of the tire friction coefficient model

are you sure about the 5 gforce for 100 Feet ?
more like 5 g in first few feet then 4.7 to 4.5 tapering
to 4 gs at 660 feet

heres a pretty good run
60 Ft = .855 seconds
660 Ft = 3.053 at 272.17 mph
1320 Ft = 4.527 at 326.40 mph

or a more complete run of;
60 Ft= .850
330 Ft = 2.138
660 Ft = 3.081 - 265.64
1000 Ft = 3.880
1320 Ft = 4.570 - 321.77

Larry Meaux  (meauxrace2@aol.com)
Meaux Racing Heads
MaxRace Software
ET_Analyst for DragRacers

RE: Drag Racing Tires

did you consider the Force "hitting" the tire ??
distorting it enough to reduce markedly the tire's radius ?
if you do your calculations from the static tire radius
the Torque will be different than the dynamic radius

Larry Meaux  (meauxrace2@aol.com)
Meaux Racing Heads
MaxRace Software
ET_Analyst for DragRacers

RE: Drag Racing Tires

Duct tape, they glue the tyre to the track with a strip of soft adhesive rubber they lay down in the burnout.  Get the burnout right, get into stage to launch just as the tackiness reaches it's peak and mu can exceed 5 so 5G is possible.  By the end of the strip where there is no rubber laid down the coefficient of friction is down to about 2.5/1.7 - dependant pretty much on temperature.

RE: Drag Racing Tires

     Some friends and I tried to do a rough calculation to determine the effect of the exhaust gas thrust/downforce and came to a solid conclusion that this is not the cause for the enhanced acceleration capabilities.
     It is certainly not classical Coulomb friction propelling the car @ the contact patch, there is a glue-like effect that allows the tires to get more grip than Coulomb alone would provide, but even the glue-like effect is not widely accepted as being the "reason" for the awesome G's attainable @ launch.
     There is an SAE paper written by Chuck Hallum entitled the "The Magic of the Drag Tire" in which he tries to explain the phenomenon in terms of a momentum change of the tread as it contacts the pavement (i.e., the tread is travelling downward as it comes around the front of the tire, and then it's vertical momentum is reduced to zero as it enters the flat contact patch.)  I don't believe that this explanation alone is responsible, b/c you would think you would have a cancelling effect as the tread particles are pulled up on the back side of the contact patch (essentially being given back the momentum that they lost coming in), however, as you can see by watching a pass of a top fueler/funny car, there are some pretty extreme deformations going on in the tire that may make the momemntum effect asymmetric.  I would get your hands on the Hallum paper for some interesting reading, unfortunately, to me he seems to be back solving the problem w/ the way he presents the numbers.  It is commendable anyhow to even attempt any such calculations in such a radical situation.

RE: Drag Racing Tires

The specific paper that bhart mentioned is the following:

942484 The Magic of the Drag Tire

Mr. Hallum also wrote a number of other papers on the topic of tires.  Check out the following 2 SAE papers:

983028 Understanding Race Tires

2002-01-3302 Dynamic Traction Characteristics of Tires

RE: Drag Racing Tires

The rubber on these tires is very soft and will grab the texture of the road. Coefficient of friction is not really relavent. If you want to understand how so much traction is possible you need to studdy the properties of rubber.

RE: Drag Racing Tires

Actually, the distortion of the tire has to be an outcome of the increased stiction, not the other way round.  

Without the stiction of the tire,  the torque would simply cause the tire to spin loose and burn rubber, which is what happens when it's done on purpose.


RE: Drag Racing Tires

     In the tire world when people say coefficient of friction it is usually understood as the "effective" coefficient of friction, or simply normalized traction (traction over vertical load).  So yes, coefficient of friction is not exactly valid, but it is also not exactly what is being referenced.
     Even among the "experts" in the tire world, it is a mystery how the tires do what they do.  I assume they understand the properties of rubber, as well as having access to very complete information concerning its traction capabilities.    

RE: Drag Racing Tires

Rubber on glass has a coefficient of friction of 2, about as good as it gets.  Drag tires get additional normal force on the contact patch through the deformation of the tire.  Has to do with the momentum change in the tread as it enters the contact patch region.  Another good book is Paul Haney's book on High Performance and Racing Tires.


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