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

Reply to
phaeton
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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.

Reply to
John S.

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.

Reply to
Steve

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?

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.

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? ;)

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

Reply to
phaeton

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