Norm Peterson

Nope. Rotational inertia is a property of mass, and is not dependent on whether any torque is being passed through it for other purposes (such as accelerating the car forward).

With only one realistic exception you still have to accelerate the front tires rotationally as well as forward with the rest of the car.

As you note, wheelstanding is an exception, but this is at best a brief and very small secondary consequence of having sufficient torque and rear tire bite. This is not something normally considered when simulating acceleration anywhere but at the dragstrip (which OP did not mention). And even there, only for a portion of the run.

I'll stand pat on what I said before unless Andy tells us that wheelstanding is both possible and expected.

Then it gets a bit more complicated, since once the front wheels touch down there becomes a sudden need to bring them from about zero rpm up to "road speed". This will momentarily steal a good bit more from forward acceleration than the average amount lost to front wheel rotational acceleration had the wheelstand not occurred. IOW, there aren't any freebies here, unless you're at the strip and can actually carry the front wheels every inch of the way from launch to the finish line (where anything that happens after the finish "doesn't count" for the race.


Norm

Cruzinaround

Granted you don't carry the front wheels down the entire track. Inertia shifts the effective mass more to the rear of the vehicle by causing a lift of the front. The front wheels become less of a downward force and rolling resistance to overcome. It's what determines the hole shot which the majority of the time dictates the outcome of the race. Also...Granted this is also applied to just drag applications.

Effectively the entire run down the track you use physics and inertia at the fulcrum point where power is applied at the rear to attempt to provide lift in front. If you lift the tires off the Tarmac at holeshot, bonus, thus wheelie bars to prevent flipping .... When they plant again forward momentum will already be in effect to overcome the rolling resistance ... Physics is what it is.

Andy13186

Thanks for the posts ^

guess this guy didnt have to spin the front wheels up at all on his run lol

my fav 1/4 mile vid of all time i think

Norm Peterson

Overcome it, yes. But it still costs you forward thrust when those front wheels have to be brought up to speed once they touch back down. Forward thrust that you'd otherwise be using for a tiny bit more forward acceleration.

You can't claim the benefit of holding the front wheels up without accepting the downside when they come back down. Nor can you claim less rolling resistance up front without realizing that the rolling resistance at the rear has increased (due to the rearward load transfer from acceleration). Physics doesn't work those ways either.


Let's not get into wheelie bars here. It's outside OP's question, but trust me, I can work my way through what's happening with those things.


Norm

Norm Peterson

No problem, Andy

Yeah, that was one amazing run. A 14 second video and more than half of it wasn't even the run.


Norm

Cruzinaround

When you gun your throttle after each shift there is no Mustang I know now or in my past that dives down at the nose to effectively shift the mass of the vehicle towards the front. From a dig or a roll its nose up my friend. Which is why you require splitters and spoilers to direct your nose from going airborne and plant that rear hard on the Tarmac.

Once you're rolling the forces coming down on the front to overcome are minuscule by comparison to the launch since you already have forward momentum. Unless you decided that suddenly braking is going to win the race.

Again it's just physics.

Cruzinaround

I do get your point. And I agree to an extent. Clearly the reason we mod our vehicles to produce massive gobs of torque and HP is to better overcome the effects of physics taking place. We require traction and bite at the rear, less downward forces and rolling resistance at the front and sustained linear power applied to propel us forward down the Tarmac.

Again using Andy's example.... It's a visual text book example of this applied in the real world to perfection.

Norm Peterson

Nose up does not mean that the front tires are clear of the ground. Only that there is less load on them.

As long as they're still rolling, they are still being forced to accelerate rotationally (where their mass moment of inertia matters), and still contributing some amount of rolling drag at something like 10 lbs per thousand lbs at the contact patch.

Yes, these effects are small, but they really are there every moment of your acceleration except for any wheelstand time. Keep in mind that what OP was asking for was to identify a small difference for a small change in a small effect.

Those effects are only zero during those wheelstands. To insist that they are zero when the front tires are not completely clear of the ground is to not understand the physics involved. In practice, they probably do get lost in the noise of driver inconsistency, but even that doesn't mean that they weren't there at all.


FWIW, I was first introduced to the mathematics/physics side of understanding vehicle acceleration about 50 years ago, and I've been refining that mathematical simulation from time to time ever since. By education and by profession I've been an engineer for most of that time (which is a way of thinking more than it is memorization of any specific book learning). Basically, I think I've got a pretty good handle on what's going on here. Below is a list of most of the things I'm considering when I run an acceleration simulation (it's written in Excel).


Redline
LaunchRPM
HookupTime
Grip
CurbWeight
Load
HtCG
Wbase
Track
Unsprf
Unsprr
%WtR
%Antisquat
PosiTorque
SprungWt
%SprungWt
HtSprung
XCgSprung
%XCgSprung
RollDragCoeff
TIREwidth
TIREprofile
RIMDIA
Revspermile
RollingRadius
Tirecircum
Weight
%FrontKRoll
Width
Height
GroundClear
Cd
%FrontalArea
%GroundClearFree
FrontalArea
AirDensity
Aero
Aero100
Transmission
BasicShiftTime- M
BasicShiftTime- A
ShiftTime1-2
ShiftTime2-3
ShiftTime3-4
ShiftTime4-5
Gdiff (axle gears)
xG1 (transmission ratios)
xG2
xG3
xG4
xG5
Bore
Stroke
Cylinders
Displacement
Clutch Dia
BalancerDia
Flywheel Wt
Clutch Wt
Engine RotWt
Balancer Wt
Tire Wt (1)
Wheel Wt(1)
gv
g
a/g
accel
Wheelrev/sec^2
Wheelrad/sec^2
Wheel Treqd


Norm

Cruzinaround

Less load on them is the end game Norm.

I'm also an engineer by profession. I understand where you're coming from.

In the end less load over the front is the answer and you nailed it in your first sentence...

In a real track mindset... Many. Hardcore 1/4 mile aficionados will go as far as to put really skinny small wheels and tires on the front of their stings and modify the upper control arms to enhance the lift and spring upwards and launch and shifts.

Yes power to weight ratios and gearing from the crank to the rear and ultimately the tires all factors. In. But, in the real world for a versatile daily driven vehicle the one thing you can change immediately to match any given driving application is the wheel/tire configuration.

You need to trim weight off. The car back and front then again the easiest variable to tweak is the wheels and tires.

The biggest significant immediate performance tweak you can do to a car is the Wheels and tires.

And no matter how much experience either of us have in engineering, technology or physics....

Real world applied science is what ultimately provides. Your answers. I've busted my knuckles in my family's speedshop for almost 20 years prior to doing my own thing with technology and engineering. So, I'm not in a pissing contest here nor would I want to be. I agree with much of what you're presenting, just not everything, having been there and done that sometimes what we see on paper does not apply when real world physics are introduced.

Again I defer to Andy's example. As it is very elegantly speaks for itself.

Norm Peterson

That's as close as I suspect we'll ever get.

Andy's video is a great illustration of eliminating all front wheel involvement (other than its weight) for an entire quarter mile run, which apparently requires 7-second-ish capability to achieve.

That list of variables was intended to show that I have a reasonable understanding of what goes into vehicle acceleration, of which front tire drag and rotational acceleration are small but finite pieces in all but examples like Andy's video (which I may borrow from time to time for illustration purposes).

This discussion might get me to write in to that simulation a means of dealing with grossly staggered wheel&tire setups, wheelstanding, and maybe a couple other things while I'm at it, none of which has been worth tackling before for street/autocross/road course related uses.


Norm