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In the NASCAR chase race today the winner, car #78, Martin Truex, furniture ROW, the car failed the post race lazer test for the left rear wheel alignment by 10 thousandths of an inch.

A very small infraction by any standard unless you are a nano-technologist. At any rate, it failed but what does that mean? Why do they do that test and what advantage is gained by being what - misaligned?

Design for RELIABILITY, manufacturability, and maintainability

RE: nascar

Peak mu for right side tires is about 1.6. Peak mu for left side tires is usually about 2.5 --> 2.7. Therefore tractive force (forward bite) is strongest from the left rear wheel. Axle shafts and bearings being what they are and different axle stiffness side to side, can put a LOT of dynamic toe change into the rear. Shimming the axle housing to correct for this deflection steer due to tractive forces and moments is a common procedure. Different axle stiffnesses left and right also minimizes power hop during all phases of WOT application. And then there is heavy braking, too. All attempts to produce a more stable (driveable) car in max lateral plus max longitudinal situations. They have to draw the line somewhere.

A lot more going on back there than just two tires pushing the car. Testing with wheel force transducers and playback on a K&C machine shows it all.

RE: nascar

The cars are checked before and after the race using NASCAR's Laser Inspection System (LIS) platform, which is very precise. I think the post-race inspection is pass/fail based on a digital baseline inspection of the car pre-race. From what I can tell the rear wheel toe measurement of Truex's car did not comply with regulations. But what those regulations might be is not made available to the public by NASCAR.

Being penalized for a post-race rear wheel toe measurement being .010" outside the regulation limit might seem a bit excessive. But every NASCAR team knows what the regulations are, and NASCAR has developed an inspection process that gives very precise and consistent results. What seems to be the case with Truex is that his mechanics tried to give him every advantage possible and did not allow quite enough margin in the suspension setup.

RE: nascar

I wonder how they know the mis-alignment is not the result of the wheel making contact with another car during the race?

je suis charlie

RE: nascar

I doubt that the officials care how it happened, otherwise it becomes an unenforceable rule. It wouldn't matter if a tolerance was applied to it either, as all the teams would immediately start crowding the tolerance limit as part of car setup rather than working to the nominal spec.


RE: nascar

Show me a team working to the nominal spec, and not crowding the tolerance, and I'll show you a team at the back of the pack :)


NX 9, Teamcenter 10

RE: nascar

Thank you folks for your replies. I suspected it was something like what you explained but did not know the technicalities. Also I understand there has to be a limit and a fine -- just surprised they can measure it so accurately.

Design for RELIABILITY, manufacturability, and maintainability

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