Re: Wheelspin

["Jeremy R. King" <jrking@ces.clemson.edu>]

STEADIRIC@aol.com wrote:
> 
> >No hard feelings, just hard facts.  I don't disagree that you get better
> >acceleration in a roadcar by spinning the tires a little off the line -
> >I'd be an idiot to argue with this fact.  But to say that traction is
> >increased with "wheelspin" is not right.  "Slip", I will agree,
> >increases traction.  "Slip" is not equal to "wheelspin".
> 
> Go back and take a look at the Delta V number....  It's less than 10% for
> a Street compound So for max acceleration from a stand still the tire has
> to be spinning 10% faster than Road speed....  Race tires which operate a
> smaller slip angles (on the order of 2-4/deg or % depending on
> acceleration direction) spin even less for max traction.  I think that
> the problem that your having with this is your equating optimum Wheelspin
> with tire smoking wheelspin.  The other proof of this is that max braking
> occurs when the tire is working at optimum slip which means that the tire
> is rotating 10% slower than road speed.  Try to kick that delta V up to
> 11% you get lock up. And BTW Dragsters spin their wheels off the line....
> 
> Later!
> 
> Eric Fletcher
> '87 5KCSTQIA2RSR2B
> St. Louis, MO
> 
> STEADIRIC@aol.com

What your trying to say is right, eric.  You're just using the wrong
terminology.  In case you don't believe me (which is obviously the case), I'll
quote directly from the Milliken and Milliken book, "Race Car Vehicle Dynamics".

"Tractive force FT and braking force FB are a function of slip ratio.  As the
slip ratio increases (numerically)from zero, the forces rise rapidly to a
maximum which usually occurs in the range of 0.10 to 0.15 slip ratio, after
which the forces fall off."

This is what you're saying.  And quite honestly, I'm impressed with your
accuracy on the 10% slip ratio.  But in the next paragraph:

"For the traction case, note that the force falls off rapidly after the onset of
spinning.", and "Once a tire exceeds the slip ratio for peak force in either
traction or braking, it becomes unstable and the wheel tends to either spin-up
(traction) or lock (braking)."

Thus my point - "slip" IS NOT THE SAME AS "wheelspin".  It should be clear from
these quotes (from an 890 page SAE publication no less) that maximum tractive
force occurs at ~0.10 to 0.15 "slip ratio", but falls off dramatically with the
onset of "wheelspin."

---------------------------------------------------
Jeremy R. King  Clemson University, S Carolina, USA
///////////     '86 VW Quantum GL5     \\\\\\\\\\\\
 Rear Dynomax Glasspak, Hollow cat, Drilled Airbox
   Boge / KYB struts, Round 100W Driving Lights
\\\\\\\\\\\  Still Slower'n Christmas  ////////////
---------------------------------------------------