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TT5-6 Track Width Rule


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I have tried to search to confirm track width and the way that it is measured and I have ran into a mixture of how to measure track width and what impacts track width.  This is with regards to the TT5-6 / ST5-6 rule that says:


Increase in OEM track width greater than four (4) inches, which may be due to non-OEM axles, control arms, brake rotors/hats, wheel spacers, hubs, wheel offset, and/or camber adjustment shall be assessed a Modification Factor.


I am confirming what track width both is and is measured.  Track width per the OEM description (OEM user manual) is center of tire to center of tire.  I have left out the tire on my modeling because I can’t draw a good looking tire…. And since the tire centers on the wheel the tire should have no impact on track width.  A wider or narrower tire will still be centered on the wheel and thus have the same center.







For my very specific application I am looking at a 2000-2004 Ford Focus.  The OEM user manual for my 2001 Focus shows a range of track widths vs my 2004 Focus showing a single track width across trim/door configurations.  My thought on why the difference is was there were trims that had different wheels with different offsets due to trim levels (steel wheels /w hub caps vs 15” wheels vs the 16” premium).  There is no BTM requirement and being that all 2000-2004 Ford Focus fit in the ST6 category (even special allowance of the SVT Focus) I can use the largest OEM specified offset.  That off set is 58.8”


I am looking at wheels and tire that will keep me in the ST6 classing with out taking a negative modification factor.  An increase of track width greater than 4 inches over OEM is a -0.7 modification factor so getting this right and understanding how NASA interprets and measures this rule is important on my build.


Below is the drawing I have created to show the impact to track width (assuming we are agreeing to the OEM’s definition and drawing of track width.)




This shows the factory wheels (16x6) and the offset they have (+52.5 mm/2.07 in) on top and the wheels combination I am looking at on bottom (17x9) and the offset they have is (+23 mm / 0.91 in).  Additional measurements were backed in to show the hub face spacing for the wheel mounting and outside to outside measurements.


By changing the wheels to my planned wheels I am seeing a track width grow from  58.8” to 61.11”  This is a 2.31” increase of track width over OEM.  1.69” of margin of the 4” maximum.  My over all outside wheel to outside wheel has grown from 64.8” to 70.11” for an increase of that measurement 5.31” (but this is NOT track width).


This is assuming all things are equal between the two set ups with regards to brake rotors, camber, axles, wheel spacers, control arms, etc.


Now I understand that measuring from the center of the tire to the center of the tire is difficult.  It is easier to perform this measurement one of two ways:

  1.  Outside to outside and then subtract the measured tire width.  In this example we see that this would measure 70.11” and then subtract 9” for the 6.11” track width.

  2. Inside to outside with no subtraction of tire width.  As shown by the added measurement a inside to outside measurement also is the same as the track width.


    Some vehicles run a certain amount of toe or other alignment conditions and this measurement may be taken in the front and the back of the tire and then either use the max or average the two.  (don’t think any one for NASA TT/ST would be running large toe #’s to have the front and back measurement be off by more than a 1/8”.)


    So is there any way someone from NASA can provide a thumbs up on this definition, measurement methodology, and that the largest OEM specified for a model/trim/year range can be used?


    Thank you for your time in reading this and also responding.  I hope this is not the ramblings of a newbie.  I have also posted the same thing in the ST/SU page since its the same rule in question I hope this is ok.



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