Race Car Physics Question: FWD vs. RWD

Race Car Physics Question...

I'm "wordy" instead of articulate, so I apologize in advance. I'll do
my best to make this sound like a valid and intelligent question,
instead of coming off like a blathering idiot. I'll try hard not to
meander.

I live about 6 miles from a 1/2 mile circle track. The track is paved
with asphalt, somewhat to my chagrin. I find NASCAR to be silly and
boring, but I'm all into the local yokel stuff. Most Saturday nights
the missus and I go down to the races and watch the fun. My favorite
is actually the Bandit (pure stock compact cars, safety mods only) and
International Class (compact cars with limited modifications) followed
by Hobby Stocks (60s/70s/80s small-block V8 powered Detroit Iron).

In the International Class there are the expected Accords, Preludes and
Integras, a pile of Dodge/Plymouth Neons, an isolated 80-something VW
Jetta hatchback, a pair of Saab 900s from the 1980s, a Nissan Sentra or
two, a Saturn Coupe(!), some Ford Probes and (the oddballs) about a
dozen or so 85-94 Mustangs. AFAIK, every car mentioned above is Front
Wheel Drive (hereafter referred to as FWD), with the exception of the
Mustangs, which are Rear Wheel Drive (hereafter referred to as RWD).

--(Nevermind that some of the 'Stangs even sound V8-ish, but with race
exhaust I'm not 100% sure. I'm surprised enough that they would allow
FWD and RWD cars to mix on the same track, much less putting cars into
it with twice as many cylinders. Oh, and the Saabs sound like they
might be turbocharged, but this is another discussion).--

So here's the real meat- watching the Neons and Saabs and VWs and
Hondas being flung into that first left turn after the front stretch,
it's very common to see that left rear tire pick up off the ground
about 4 to 8 inches. It's so common that it's every single lap that
the cars are up to speed. I would guestimate that they're hitting that
turn around 50mph or so, and that's pretty tight for that speed. The
suspension on the right front of the car is probably close to
bottomed-out, and since AFAIK all the cars I've mentioned are of
unibody construction, i'll bet chassis flex is a factor too.

So I'm slogging through my $2.50 Leinie Red, watching these guys go
3-wheel for several laps, and I'm thinking to myself "that's got to be
a huge disadvantage for a RWD car. Even if they spool the differential
(and I bet they do!), their rear traction will suffer and they won't be
able to get a good 'bite' out of the turn without spinning the wheel or
spinning the car, etc".

And then out come the Mustangs in round 2 eliminations. To my
surprise, instead of wheels up in the air spinning uselessly they stay
planted to the track. The Mudstains don't fall to the back of the
line- they're competitive with FWD cars, and sometimes they win.

How would you wager that they keep that left rear down? I haven't seen
the rules (the track doesn't post them on the web), but I would imagine
that any suspension modifications that the RWD cars are allowed, the
FWD cars can do too. I'll be they've all been swaybarred and sprung as
stiff as they can get, and there are no traction bars. These are all
production cars (no tubular steel full race chassis, but they do have
rollbars and other safety equipment installed), and none of them have
independent rear suspension.

Do you think:

1) By design, RWD cars are expected to have more torsional load, and
therefore their chassis/body is engineered to resist twisting moreso
than FWD cars?

2) Is the torque from the drivetrain a factor in keeping the left wheel
on the ground? Even during deceleration? If the cars raced clockwise
instead of counterclockwise, would that change everything?

3) What the hell does the blue flag mean?

4) Is it a weight distribution thing? The transmission, driveline and
differential is behind the steering axle on RWD cars, and therefore
'weighs down' the chassis, along with bracing it (with the transmission
mount) to make it futher resist flexing?

Discuss!

-Phaeton

phaeton wrote:
> Race Car Physics Question...
>
> I'm "wordy" instead of articulate, so I apologize in advance. I'll do
> my best to make this sound like a valid and intelligent question,
> instead of coming off like a blathering idiot. I'll try hard not to
> meander.
>
> I live about 6 miles from a 1/2 mile circle track. The track is paved
> with asphalt, somewhat to my chagrin. I find NASCAR to be silly and
> boring, but I'm all into the local yokel stuff. Most Saturday nights
> the missus and I go down to the races and watch the fun. My favorite
> is actually the Bandit (pure stock compact cars, safety mods only) and
> International Class (compact cars with limited modifications) followed
> by Hobby Stocks (60s/70s/80s small-block V8 powered Detroit Iron).
>
> In the International Class there are the expected Accords, Preludes and
> Integras, a pile of Dodge/Plymouth Neons, an isolated 80-something VW
> Jetta hatchback, a pair of Saab 900s from the 1980s, a Nissan Sentra or
> two, a Saturn Coupe(!), some Ford Probes and (the oddballs) about a
> dozen or so 85-94 Mustangs. AFAIK, every car mentioned above is Front
> Wheel Drive (hereafter referred to as FWD), with the exception of the
> Mustangs, which are Rear Wheel Drive (hereafter referred to as RWD).
>
> --(Nevermind that some of the 'Stangs even sound V8-ish, but with race
> exhaust I'm not 100% sure. I'm surprised enough that they would allow
> FWD and RWD cars to mix on the same track, much less putting cars into
> it with twice as many cylinders. Oh, and the Saabs sound like they
> might be turbocharged, but this is another discussion).--
>
> So here's the real meat- watching the Neons and Saabs and VWs and
> Hondas being flung into that first left turn after the front stretch,
> it's very common to see that left rear tire pick up off the ground
> about 4 to 8 inches. It's so common that it's every single lap that
> the cars are up to speed. I would guestimate that they're hitting that
> turn around 50mph or so, and that's pretty tight for that speed. The
> suspension on the right front of the car is probably close to
> bottomed-out, and since AFAIK all the cars I've mentioned are of
> unibody construction, i'll bet chassis flex is a factor too.
>
> So I'm slogging through my $2.50 Leinie Red, watching these guys go
> 3-wheel for several laps, and I'm thinking to myself "that's got to be
> a huge disadvantage for a RWD car. Even if they spool the differential
> (and I bet they do!), their rear traction will suffer and they won't be
> able to get a good 'bite' out of the turn without spinning the wheel or
> spinning the car, etc".
>
> And then out come the Mustangs in round 2 eliminations. To my
> surprise, instead of wheels up in the air spinning uselessly they stay
> planted to the track. The Mudstains don't fall to the back of the
> line- they're competitive with FWD cars, and sometimes they win.
>
> How would you wager that they keep that left rear down? I haven't seen
> the rules (the track doesn't post them on the web), but I would imagine
> that any suspension modifications that the RWD cars are allowed, the
> FWD cars can do too. I'll be they've all been swaybarred and sprung as
> stiff as they can get, and there are no traction bars. These are all
> production cars (no tubular steel full race chassis, but they do have
> rollbars and other safety equipment installed), and none of them have
> independent rear suspension.
>
> Do you think:
>
> 1) By design, RWD cars are expected to have more torsional load, and
> therefore their chassis/body is engineered to resist twisting moreso
> than FWD cars?
>
> 2) Is the torque from the drivetrain a factor in keeping the left wheel
> on the ground? Even during deceleration? If the cars raced clockwise
> instead of counterclockwise, would that change everything?
>
> 3) What the hell does the blue flag mean?
>
> 4) Is it a weight distribution thing? The transmission, driveline and
> differential is behind the steering axle on RWD cars, and therefore
> 'weighs down' the chassis, along with bracing it (with the transmission
> mount) to make it futher resist flexing?
>


Yes, I can conclusively state that some of your statements may be
partially correct and others may be completely correct. And there is
the possibility that at least one may be partially or completely
incorrect.

phaeton wrote:


> And then out come the Mustangs in round 2 eliminations. To my
> surprise, instead of wheels up in the air spinning uselessly they stay
> planted to the track. The Mudstains don't fall to the back of the
> line- they're competitive with FWD cars, and sometimes they win.
>
> How would you wager that they keep that left rear down? I haven't seen
> the rules (the track doesn't post them on the web), but I would imagine
> that any suspension modifications that the RWD cars are allowed, the
> FWD cars can do too. I'll be they've all been swaybarred and sprung as
> stiff as they can get, and there are no traction bars. These are all
> production cars (no tubular steel full race chassis, but they do have
> rollbars and other safety equipment installed), and none of them have
> independent rear suspension.
>


Its all about the setup- initial front/rear weight bias (much more
weight on the rear of a RWD to start with), spring rates, shock rates,
suspension travel limiting devices, and how the driver WANTS the car to
behave. The FWD cars are deliberately configured to transfer as much
weight to the front as possible during cornering to overcome their HUGE
inherent disadvantage in circle-track racing- the fact that they will
naturally try to understeer like dumptrucks when cornering under power.
Rear-drive cars like the Moosefangs you saw are set up to keep the
rear wheels planted better, and they typically are more neutral in
handling or else are a little loose (prone to OVERsteer rather than
understeer like FWDs want to do naturally). A little bit of oversteer
in a rear-drive car is the quickest way around a corner because the
thrust axis will help push the car into the corner, PLUS it lets you
keep the power on. The typical setup is to have the car neutral or
slightly prone to understeer when rolling (not powering) through the
corner, and then use the power applied to the rear wheels to induce
neutralize the understeer (pavement) or induce oversteer (dirt track).
None of that applies with FWD.

I'm surprised that the FWD cars ever win in a class like that- the
Mustangs MUST be 4-bangers and thus are penalized by a terrible power to
weight ratio compared to the lighter FWD cars. Why anyone would
circle-track race a front-drive car is beyond me. Autocross- sure.
Rally- heck yeah. But straight circle-track? And especially in a class
where rear-drives can whoop up on them? Nutso.

Steve wrote:
> bunch of useful stuff

Thanks for the pointers. Do you suppose then that the right front
spring on the FWD cars has been lessened or weakened somewhat to allow
the bottoming out?


>
> I'm surprised that the FWD cars ever win in a class like that- the
> Mustangs MUST be 4-bangers and thus are penalized by a terrible power to
> weight ratio compared to the lighter FWD cars.

When I first saw them come out with the imports I thought the same "Oh,
this isn't fair at all.". Surprisingly, it seems pretty even. They
have to work just as hard for the win, and when they get beat it's fair
and square. Weight-wise, i suppose that once you strip everything down
to race-ready status, they should all be pretty similar, or at least in
the ballpark. Whether the track imposes a penalty on the mushbangs, i
don't know. One of the reasons it's hard to tell what they have under
the hood is because they're so much quieter than the rest of them.
Especially the Neons. Ye gawds are those things loud when they let off
at the straight.

> Why anyone would circle-track race a front-drive car is beyond me.

Because they're cheap and available? It's still some pretty fast
racing, believe it or not. Sure the FWD cars won't handle the same as
RWD cars (that huge built-in push you were talking about) but as long
as the field is level, who cares?

>And especially in a class
> where rear-drives can whoop up on them? Nutso.

True. I don't especially agree with that mix either, but they do it
and it seems to work. FWIW all the Bandit-class cars are FWD. And
since we're talking the most inexperienced drivers behind the wheels of
junk cars that are in no way-shape-or-form set up to do this, it's a
complete riot. If I had a spare $2000 to blow on a car, i'd probably
try to get into it myself.

-phaeton

phaeton > wrote in article
.com>...
>SNIP<

>
> Do you think:
>
> 1) By design, RWD cars are expected to have more torsional load, and
> therefore their chassis/body is engineered to resist twisting moreso
> than FWD cars?
>

There are NO OEM cars with enough torsional rigidity for racing. Roll cages
are designed not only for protection of the driver, but also for the
stiffening of the chassis.

Properly designed oval-track race cars run off the cage - not off the
chassis. Some tracks limit roll cage structure in a way that precludes
building "race" cars, and keeping speeds down while still protecting the
driver.

One example might be your favorite "road-runner" division which might be
limited to what is referred to as a "four-post cage" with no structural
tubing extending forward or rearward to the suspension points.

By design, FWD cars have 100 percent of their steering, acceleration,
deceleration inputs and 90 percent of the braking inputs applied to the
front wheels - which makes them prone to understeer - or "pushing".

Were the front wheels expected to also handle most of the body roll
resistance, these cars would not make the first turn - possibly ending up
in your lap.

With the rear axle in a FWD required to handle a larger opercentage of the
roll resistance than on a RWD, it is quite stiff, and the Left rear wheel
comes up as the axle pivots off the relatively stiff Right rear.

RWD cars are more "balanced". The front suspension can handle more of the
body roll resistance, and it is asked to.

The rear of a RWD car is often run quite soft for forward traction, and the
inside wheel generally stays on the track.

On a RWD car, the RF/LR wheel combination usually carries a larger
percentage of the car's weight than the other two wheels. This can be
called, "wedge" "Crossweight" and/or "diagonal.

If the weight measurements on the RF and LR of the car add up to 55 percent
of the car's total weight, it is said to have "55 percent crossweight."

Put enough crossweight into a RWD race car, and the left rear tire will
actually be hotter than the right rear.....but, that doesn't necessarily
make it the best setup.

On a FWD, the "fast" way is to carry a larger percentage of weight on the
LF/RR combination than the RF/LR combination....sometimes called "reverse
crossweight" etc.....which takes some of the overload off the RF.

On a FWD, the left front is often the hottest tire on the car following a
race.

Were you to use "regular" RWD-type RF/LR crossweight on a FWD, you would
exacerbate the natural tendency of a FWD to understeer by placing even more
stress on the RF tire when less is needed.

While it is true that the fastest way around a turn is on FOUR wheels, FWDs
often do it better planting the LF with reverse cross, traversing the turns
resembling an over-hydrated dog..

> 2) Is the torque from the drivetrain a factor in keeping the left wheel
> on the ground? Even during deceleration? If the cars raced clockwise
> instead of counterclockwise, would that change everything?
>

Engine torque effect on the rear wheels is minimal when compared to the
cornering forces.

Drag racers like to load the right rear a bit to offset the tendency of the
torque that is applied to the rear axle, but the right-left differences are
only a few hundred pounds - at most.

Some of the early NASCAR guys such as Smokey Yunick actually built reverse
rotation engines which caused the engine torque to really plant the left
*front* in a turn - contributing to better handling.

Lifting the left-front down the straightaway was common through the
late-'60s when more sophisticated front suspensions started to come into
play.

In early attempts to make a car "handle", the use of excessive "wedge" - an
actual metal wedge driven between the old Ford transverse leaf spring and
its mount - also contributed to this phenomenon.

Unequal spring rates, individual wheel weights, and the above-mentioned
crossweight help to plant the inside rear tire on a RWD car.

To oversimplify things, the LR is set up to carry more static weight, while
the car - theoretically - will balance off to both rears carrying equal
dynamic weight while the car is turning and left-side weight is transfering
to the right-side..

Rule-of-thumb ideal tire temps will find tire temps pretty much equal -
again an over-simplification due to various chassis and suspension designs.



> 3) What the hell does the blue flag mean?
>

Blue flag with Orange stripe.....Maintain position - you are being
overtaken. Usually applied to a car that is being lapped by the leaders.

Some people think it means to pull over, but, in most associations, it
simply means a car is trying to pass you, and you need to stay in one
groove.


> 4) Is it a weight distribution thing? The transmission, driveline and
> differential is behind the steering axle on RWD cars, and therefore
> 'weighs down' the chassis, along with bracing it (with the transmission
> mount) to make it futher resist flexing?
>

Most racing organizations have rules about engine setback. Some allow it -
some require absolute OEM engine placement.

I will take every inch I can get in engine setback. It allows softer front
susopension, and contributes to better weight transfer and forward bite.

Same with side-to-side offset.

I got burned by a "one-inch engine offset" rule interpretation once.

I interpreted it to mean I could mount the engine so the center of the
crankshaft was one-inch off the centerline of the chassis.

The tech inspector - whose interpretation mattered much, MUCH more than
mine - insisted that it meant one-inch difference between the measurements
taken from the right frame rail and left frame rail to the crankshaft
centerline - IOW - one-half-inch off center.

There are often many different interpretations of the same rules. Many
times rulebooks are written with intentional vagueness to allow for
situational application and interpretation of the rules.

Racers have to build their cars NOT according to the rulebook, but
according to the rulebook interpretations of those who will be enforcing
the rules.


HTH

* wrote:
>
> --snip--
>

> HTH



Dear gawds that was a beautiful post. Thankyou thankyou thankyou. It
inadvertently answers a few more questions, no less. ;-)


What do you race?

-phaeton

phaeton > wrote in article
.com>...
>
> What do you race?
>
>


I run a one-man chassis and fabrication shop.

I used to hate spending money with a race shop only to have them come out
with their own car and beat me - using the very same money I gave them,
also causing me to suspect that they hadn't given me 100 percent in order
to retain a slight advantage for themselves.........

.......so, I do not have a car to race against my customers, and I do not
support/sponsor a "house car" owned and driven by somebody else.

My customers seem to understand that I am giving them everything I know,
and not holding anything back.

I build oval-track race cars - mostly on stock chassis such as the 108" GM
'Metric" and the 116" Impala.

Most of my customers are in the Canadian Maritimes.

I built a car for a cousin who lives up there, and he's managed to do well
with it......which often increases my incoming telephone traffic on Monday
mornings.

I always know how well he did before he calls me himself on Monday night.

After 44 years of racing, I'm not quite as fast or as durable as I once
was.

I go to the race track ONLY to service my specific customers - at their
request.

I spend more time at mid-week, all-day test sessions than I do at weekend
races. We can learn and change a lot more at a day-long test session than
we can in a couple of short practices while under the gun to prepare for a
race in a few hours.

I'm branching out into street rod chassis a little - specializing in
late-model chassis swaps and incorporating some oval-track stuff - such as
raising roll centers in high-sitting cars to provide a street car that
handles better than most.

It is a little less demanding.

Street and Hot Rod owners seldom wreck their cars on the weekend, drop them
off in front of my shop door on Sunday night, expecting them back by
Wednesday morning.....LOL!!

* wrote:
> phaeton > wrote in article
> .com>...
> >
> > What do you race?
> >
> >
>
>
> I run a one-man chassis and fabrication shop.
>

Excellent. Aside from money, that's essentially my barrier to entry.
I can probably work on cars in my driveway, but I don't have a proper
garage. I sold all my tools going on 10 years ago, and I'd have to
re-acquire them all over again. I highly doubt that any of these guys
racing Bandits farm any work out. I bet they all do it themselves, and
I bet there's stuff constantly breaking on these cars. It's all so
'small-time' there's no way you can afford to pay someone $80/hr to
replace your struts, brakes and drive axles on a monthly basis. A
single, minor crash can cost a few grand in parts, i bet. I haven't
seen it happen yet, but I'm sure they're blowing up engines and
transmissions regularly too. What do you do then?

Then there's the stuff I can't do myself- I can't weld. I can't work
metal (except sheet metal). I can remove and replace body panels but I
can't do a 'pro level' paint job. My car would have to be flat-black
krylon wet-sanded to a low sheen. My best friend used to do all that
stuff (very well, too) so I never bothered to learn. How common are
chassis/fab shops? I wouldn't know how to pick a shop out of the book
that would install a 4-post rollcage and walk around the car welding
the doors shut and stuff. That would probably cost as much as the car
itself did.

There's all the 'odds and ends' stuff that will stack up if you have
someone else do it, but would be total cake if you have the skills.

Oh yeah, and I forgot to mention-

I'd have to get a different truck to tow the car with ;-) Ye Olde 4cyl
Ranger won't cut it.

I live so close to the track that I could conceivably strap lights and
turn signals onto the car to make it street legal and drive it back and
forth, but that all goes to **** when you wreck the car in the feature
and can't drive it home 0_o

-p

phaeton wrote:
> * wrote:
>
>>phaeton > wrote in article
ps.com>...
>>
>>>What do you race?
>>>
>>>
>>
>>
>>I run a one-man chassis and fabrication shop.
>>
>
>
> Excellent. Aside from money, that's essentially my barrier to entry.
> I can probably work on cars in my driveway, but I don't have a proper
> garage. I sold all my tools going on 10 years ago, and I'd have to
> re-acquire them all over again. I highly doubt that any of these guys
> racing Bandits farm any work out. I bet they all do it themselves, and
> I bet there's stuff constantly breaking on these cars. It's all so
> 'small-time' there's no way you can afford to pay someone $80/hr to
> replace your struts, brakes and drive axles on a monthly basis. A
> single, minor crash can cost a few grand in parts, i bet. I haven't
> seen it happen yet, but I'm sure they're blowing up engines and
> transmissions regularly too. What do you do then?
>
> Then there's the stuff I can't do myself- I can't weld. I can't work
> metal (except sheet metal). I can remove and replace body panels but I
> can't do a 'pro level' paint job. My car would have to be flat-black
> krylon wet-sanded to a low sheen. My best friend used to do all that
> stuff (very well, too) so I never bothered to learn. How common are
> chassis/fab shops? I wouldn't know how to pick a shop out of the book
> that would install a 4-post rollcage and walk around the car welding
> the doors shut and stuff. That would probably cost as much as the car
> itself did.
>
> There's all the 'odds and ends' stuff that will stack up if you have
> someone else do it, but would be total cake if you have the skills.
>

If you really want to get into it, just burn your money.

Seriously... I run a 76 Camaro at the local dirt track, and it's a hell
of a lot of fun. And a lot of work. And a lot of money.

Buy someone else's used car that way you don't have to start from
scratch. There's always one or two for sale. Barter system works well
- I have a friend who's a welder, so I feed him and fix his computer
when I need welding done. Be nice to your crew. Ply them with
beverages and snacks.
Yes, you will wreck lots, so get spare parts. You will blow stuff up,
so decide how you want to proceed... some guys run junk and swap it when
it blows, some guys re-ring their engines basically every race.

Don't worry about making it pretty... worry about making it fast and safe.

fwiw, in three years I've gone through two front bumpers, 4 engines, one
axle, one transmission, a dozen tires, two rads, two balljoints, and
more. But I can rebuild front wheel bearings in my sleep, rivet on
fairly straight sheet metal, paint it to a decent finish, swap a cam,
time a small block by ear, etc etc...

Ray
Ray