Forum: Something I have read about but not experienced.
Is the dreaded Trailing Throttle Oversteer.
This is on my list of things that I do not need to experience!

I started doing some reading about C3 aeros. I remember in the past reading that a C3 w/o PC kit had lift on the rear wheels.
The PC aero kit was supposed to improve CD aeros by 15%.
As I remember it, the PC kit added #50 lbs of downforce?

Now I'm wondering if TTO is caused by lack of downforce and traction?
Or is it something engineered into our cars that we are stuck with?

TIA

R

 

This is part of my homework/research for my car.
If there is one thing in common on this forum. It is people have wasted a lot of time and money on the way to getting what they want! lol
One aero expert mentioned 4% of wheelbase for rear spoiler, another said 6%-8%.

Not being an expert, I am a serious reader and try to be observant.
Most local circle track use a generic body similar to our C3s.
I've noticed that the F/R spoilers seem to be at about the same angle in reverse?

If the front spoiler is pointing down at 7:00-7:30 on a clock, the rear seems to be at an upward opposite of 2:00-2:30? Is this making any sense?

The spoiler on the PC kit barely catches the smoke in the pics I saw. It looked to be about 4" tall. Now what would happen w/8" spoiler laid down at less angle?
I'm hoping to get either more downforce w/same drag or same downforce w/less drag.
I see a few cars w/adjust F/R spoilers, are they posing or does it work?

Most of us do not have a wind tunnel for testing.
But there are a few of us who have done some high speed testing!
But only on a closed course, Right!

TIA

R




It seemed

 

maybe others have a clearer picture of what you are discussing, I know TTO means the rear swings out on you as you back off the gas. You lost me on PC aero kit, used google couldnt find it. What kind of speeds are you dealing with that downforce is becoming a factor?
Have you ever looked at the early versions of the Chevrolet Power Book where they have a whole chapter on vehicle dynamics, chapter 7?

 

Pace Car aero kit ????

 

I personally have not experienced tto in a Vette. But I have to add I've been road racing for years and you never chop the throttle in a corner unless someone is sideways in front of you of course. Smoothness is the KEY to driving fast. TTO is notorious on Porsches but not so much on a Vette unless you have a poorly balanced car. If you find you've overdriven a corner then scrub off as much speed as possible on entry and GENTLY feed the throttle in on exit. If you are on the ball you can catch a slide using throttle and counter steer. That's just how I do it, your experience and reflexes may vary.
As far as spoiler angles go , you have to experiment with them. If you get a spoiler too steep (or a wing in my case-formula car) it will stall and produce nothing but drag. Your observations sound like a good starting point and probably where I will start with my soon to be completed Vintage Vette racer .

 

The smoke wand test I saw pics of on a C3 showed the smoke passing an inch or two over the rear spoiler. Using PC kit.
That's where I got the idea of taller at a more laid back angle.
In reality I guess you chop off the excess till we get what we want?

Or do the expensive adjustable spoilers?
Dropping the nose would raise the rear spoiler a fraction.

BTW: In everything I've ever done:
Smooth is fast, fast is smooth!

Thanks

R

 

Smoke over the rear spoiler I believe is called laminar separation. If the smoke hits the spoiler then it is too steep and disrupting airflow/ creating drag. There is a trade off to increasing downforce and that would be increasing drag. Can't get one without the other that I know of. Downforce is a wonderful thing IF you have the HP to overcome the drag. In my high downforce formula car I am constantly tinkering with the wings, gurney flaps/endplates, winglets and tunnels. And I am rule limited to about 250 HP. It's truly a balancing act and wish I knew more and had access to a wind tunnel.
Are you building a track car and or are you rule limited? If it's a track car I would add 2" to the rear then compare lap times, then add 4" and compare again. Also get the front as low as possible without compromising geometry and watch the rake. Think CG.

 

Ah, trailing throttle oversteer (aka TTO). This is oversteer induced by lifting off of the loud pedal. More likely in vehicles with a rear weight bias, but every bit as possible with 50/50 or even front biased vehicles (yep, BTDT in a C3). And, the car in question needn't necessarily have a poor setup, as many a decently balanced one may still exhibit TTO if one lifts too suddenly, causing excessive weight transfer off of the rear end sufficient to upset its overall balance (mechanical and/or aero). This is why manufacturers rightly tend to engineer road cars with understeer, tho typically a good bit more than us enthusiasts would wish to suffer. In any event, on the street you just never know when some idiot might do something stupid at the precise moment you've committed to a corner, which is why I strongly urge driving enthusiasts to set up for no less than at least mild understeer on any road going vehicle.

Aero wise, from what few baby steps I've begun taking into understanding CFD, apparently those cool pics and vids of smoke streams passing over bodies in wind tunnels only illustrate the very tip of the iceberg. That said, smoke can definitely point to areas of turbulence, which is where you'll typically find the most significant drag. Smooth flow several inches above a body doesn't really tell you all that much, other than a lack of significant turbulence there. Well arranged tufts attached to the body itself will tell you a bit more than can smoke, especially of what's more likely going on at the boundary layer, but it takes somewhat sophisticated CFD modeling and/or a modern wind tunnel to put any meaningful numbers to Cd and downforce/lift on a given body. ...neither of which come very cheap.

As for achieving aero nirvana, much depends on the corner of the performance envelop in which one is operating. For AX & RR type purposes (including spirited open road stints), every course has its own unique combination of turns and straights, so unfortunately there isn't a one-size-fits-all aero package that will yield the best lap ETs in every situation. General rule of thumb here is the more time is spent braking, turning and accelerating off of low to medium-speed corners relative to time spent on the straights, the more downforce you'll want and the less you should worry about drag. Regardless of whatever downforce/drag compromise best suits a given course, one must always strike a good balance (front to rear), which would ideally result in mild aero understeer in high speed bends. Obviously, aero oversteer is to be avoided like the plague.

For drag racing, tho we'd probably want to maintain sufficient steering, the more aero understeer bias can be tolerated. Certainly, we want to limit drag. However, if encountering wheel spin and/or dangerously light steering nearer the big end, the priority should first be placed on adding enough downforce sufficient to counter either problem. If a car isn't suffering with such issues, then drag reduction should take the priority. It follows that closed-course top speed runs would have similar considerations, with drag reduction definitely taking the spot light.

The PC aero package does appear to be a fairly well balanced one, and IMOE definitely makes a real difference at and above the century mark. But, the center section of the front dam/splitter desperately needs additional support. I've both seen and experienced this section oscillating and even folding under at high speed (still working on the problem myself). If MPG is the game here, I'd recommend starting with the '80-'82 body, which is said to have the lowest Cd of all C3s, tho IMHO it most likely generates less downforce than the '78-'79 body with PC kit.

Of DIY mods possible for improving downforce beyond the stock options we have at our disposal lowering ride heights, rake, air dams, spoilers, wings, splitters, under-trays, diffusers, fender vents, hood extractors, dive plates, fender cutouts, cut bumpers... And, a number of these same options can also be utilized to reduce drag, depending on execution. In any event, bearing in mind that many a successful race car (particularly those with ample power) actually have some pretty unimpressive Cd numbers, I highly suggest the primary purpose for the vehicle in question be the overriding factor in determining on which end of the aerodynamic puzzle to place one's priorities. FWIW, when these discussions arise I always think of the F-4 Phantom, aka America's proof to the world that given enough power even a brick can fly.


TSW

 

I have not experienced TTO either with my 78 as Pop chevy states. TTO with rear wheel drive vehicles often happens when the driver lifts off the gas abruptly from an on the gas posture...any RWD car will exhibit such behavior with increasing seriousness resulting the more the weight balance is to the rear....TTO is much more difficult to induce with a FWD car, as an example, where 60-65% of the weight distribution is over the front wheels. The other end of the spectrum is a rear engine Porsche with older Porsches known for snap oversteer dialed out of the newer ones with a massive front sway bar to induce more easily controlled understeer. The SBC C3's are actually slightly biased to the rear with a weight distribution of 52%R:48% front so the tendency to TTO can be more prevalent IF the driver does not exhibit some semblance of balance and smoothness. Balanced weight distribution can be a blessing or a curse, depending on what the driver is doing and knows.

 

Thank you TSW: I knew those w/experience would eventually share.
The problem for many of us is the time/effort to test. Much easier to follow the crowd.

R

 

Aero is the poorest on the sugar scoop cars, 68 to 76. The 80 is one of the best in areo - from previous research.

The rear steering is usually a result of bad / worn rear suspension components.

The biggest problem is front end lift with the SBC vs BBC at high speed due to the engines lower weight.

Just to play with your mind - the navy did work years ago and found a sandpaper bottom on a PT boat was better than a mirror surface. Water on water is easier to shear then water on a smooth surface. That is why most racing scoops have a space above the hood before the opening / intake, Air on air is better flowing then ....

 

Not in the Corvette but I TTO'ed once.

WOT blast onto the Freeway. Came into the turn TOO hot and lifted the throttle. Scared the crap out me as I faced the fence.

Pushed in the clutch and it came right back in line.

 

I've worked on different closed wheel race cars and they all have canards. I always thought about incorporating them on my Vette. I went with my light weight front end fiber glass and got rid of all the heavy front end metal. I really think the best way to go is to try achieve 40% on the front tires and 60% on the rear.

I made an extended chin spoiler back when I had a stock front end and under braking it would scrape the ground with 550# front springs. That is why I continued to go up in spring rates with QA-1 coil overs. With big brakes and 295 width R1 slicks on the front you can really generate weight transfer. Or even body roll in a turn.

I made my front end light enough that 700 or 750 pound was all I ever needed. Some of the BBC GT1 vettes were in excess of 850#

You attempt to do all heavy braking in a straight line, but a high compression big ci V-8 and doing an early down shift will instantly cause TTO You can really go through sets of tires when you are trail braking and 4 wheel drifting through turns. I would advise everybody to attend a drifter school and then go on to one of the competition driving schools for powerful cars.

When I went for a job at a Las Vegas exotic car driving schools, one of the first questions during the interview is what drifter car school did I attend.

http://www.rapid-racer.com/aerodynam...mic%20Upgrades.

 

I have attended many Skip Barber racing/high performance schools over the last 25 years and have raced in the skip barber formula racing series as well...

I literally just went to the BMW High Performance Driving School over the Xmas break...M2/M3/M4's on the track..totally 100% factory stock cars racing on a road course. The 2017 M3/M4's 100% stock with carbon ceramic brakes are phenomenal on the track! These cars are expensive but there is a real good reason why.....................Most people will never use the ability of these cars on the street.

Always great to compare mechanical ability/theories on my street cars with real world experience driving on the track. Also, nice to see folks who have real world track experience versus just theorists....

 

Yes sir! Beyond the entry level racing school (which no driver should think is beneath them), one of the best things any driving enthusiast or hopeful racer will ever do for themselves is to take Skip Barber's Car Control Clinic (or whatever it's called nowadays). The wet/dry skid pad time is not only a blast, but simply invaluable training for anyone serious about improving their skill set. Managed 8 laps with the tail hanging out, BTW.

 

 

For those forum members that are still interested.

If the Pace Car Aero kit is a good start.
Would boxing in the rear spoiler increase down-force, Cd or both?

I don't have the resources to test.
Reinventing the wheel is too much for me.

I'm sure someone has done some research on this.

R

 

If I follow what you intend by "boxing in the rear spoiler" as adding leading fences running down the rear fender lines similar to as was done on the "CanAm" and "GTO" bodies, then I would expect the pressure differential at the rear to increase further in favor of downforce as well as for overall drag to increase.