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-   -   [ANSWERED] fishtail to the RIGHT (https://www.corvetteforum.com/forums/ask-tadge/4257336-answered-fishtail-to-the-right.html)

jvp 03-29-2019 04:18 PM

[ANSWERED] fishtail to the RIGHT
 
The original question is here.


ZMMMMM asked:
I have a perception that Corvettes (and all cars) nearly always fishtail to the RIGHT. Iím wondering if Corvettes are engineered to do this (different shaft sizes, (e)LSDs, alignments, etcetera), OR, is there some physics explanation (road crowning, differences in weight, etcetera), to explain my perception.

Perhaps, it is a safety feature designed to prevent cars from fishtailing into on-coming traffic. Thx!


Tadge answered:
Interesting question, ZMMMMM.

I can tell that by design, we try to make the car handle the same whether turning left or right. I am assuming by "fishtailing" you are referring to oversteer, or the cars tendency to rotate, or yaw, more that the driver's request based on steering inputs. We design the car as symmetrically as possible: The suspension geometry is mirrored left to right and of course the tires are as perfectly symmetrical as Michelin can make them. There are limits however. Some manufacturers have gone to such extremes as placing the driver at the center line of the car, but that makes entry/egress a real chore. With the driver positioned on the left side, we are forced to place instrumentation steering, brake and throttle controls on the left as well. When added to the driver's mass, it makes for a pretty significant left-bias in the weight distribution. We can do things about that too, such as placing the battery and other heavy components on the right side. The fourth gen Corvette even went so far as to move the engine and trans to the right of center, but that resulted in a very small passenger footwell. There are other asymmetries too such as the gyroscopic forces generated by a spinning engine, but I believe they are too small to explain a general tendency to oversteer to the right (i.e. when turning left).

I think you guessed the major factor in your question: Road crown. Although barely noticeable on many roads, virtually all have some degree of crown. Road engineers put 1-2 degrees of crown in roads to shed water efficiently and avoid hydroplaning. That may not sound like much or even be perceivable from the driver's set, but 2 degrees gives a side load of over 120lbs on a 3500lb car. That means even when driving straight, dynamically the car is behaving like it is cornering about 0.035G's. The probably seems trivial, but when you are approaching the vehicle's limits, say 1.0G, that can make the difference between gripping and slipping. Compounding the asymmetry, when you are rounding a right-hand turn, the road crown acts like a banked turn and you have the same side load as a starting advantage making the car feel more secure.

I have to be honest, we do not have data validating the premise of your question, and I certainly can not speak for other manufacturers, but this is the most plausible explanation I can think of. Thanks for making me think about it
.

cdngolfer 03-30-2019 05:22 PM


Originally Posted by jvp (Post 1599132874)
The original question is here.

What about drive shaft rotation and the ring gear location that causes the one side to become heavy and the other side to lift?

Kozzzz 03-31-2019 01:47 PM

So is it generally accepted that cars tend to fishtail to the right? This is not something I've noticed (not saying it doesn't happen, just that I haven't noticed it).

robertfa 03-31-2019 02:54 PM


Originally Posted by cdngolfer (Post 1599138461)
What about drive shaft rotation and the ring gear location that causes the one side to become heavy and the other side to lift?

I think I've experienced the same thing. When accelerating heavily on a wet road, driving straight, getting some rear wheel spin, I would experience a yaw to the left or, the rear end shifting to the right (fish-tailing, assuming this is the same thing?). I found that I could repeat this again under same conditions, getting the same movement. I had to steer right to compensate. Other than the road crown, I reasoned that perhaps the torque of the engine, where the crank is rotating counter-clockwise (from the driver's perspective), and creating a torque toward rotating the vehicle clockwise, therefore would transfer a small amount of the vehicle's mass to the right side. With a little more mass on the right side, I would expect the right rear wheel to have a little more traction than the left. If both rear wheels are spinning at the moment (traction control hasn't quite taken over yet), then more traction on the right would yaw the car to the left (rear end then moving to the right).

Bowtie52 04-02-2019 09:04 AM

Road Crown is a coverall excuse......the car can and does break loose causing the rear end to swing to the right.....yes it can and does this on wet pavement, dry pavement and it even happens if acceleration occurs as the tires make contact with road pavement striping. Outside ambient temps above 60 with full sun on the streets/road and the car will break free......something that might be worth looking into is the rubber compound used by Michelin as my tires are the original ones that came on the car with just over 17500 and it is a 2017 Z06 3LZ M7.

I'm not a rocket scientist, brain surgeon, or race car driver I'm just an average guy who does not track the car but the breaking loose of the rear does happen pure and simple maybe not on all Z06's but it does happen.

K-Spaz 04-02-2019 04:19 PM


Originally Posted by Bowtie52 (Post 1599153377)
Road Crown is a coverall excuse......the car can and does break loose causing the rear end to swing to the right.....yes it can and does this on wet pavement, dry pavement and it even happens if acceleration occurs as the tires make contact with road pavement striping. Outside ambient temps above 60 with full sun on the streets/road and the car will break free......something that might be worth looking into is the rubber compound used by Michelin as my tires are the original ones that came on the car with just over 17500 and it is a 2017 Z06 3LZ M7.

I'm not a rocket scientist, brain surgeon, or race car driver I'm just an average guy who does not track the car but the breaking loose of the rear does happen pure and simple maybe not on all Z06's but it does happen.

The original question was about a difference in traction in a right turn vs a left turn. NOT that the car can break traction. If you're looking for a car that cannot break traction, you bought the wrong car.

CorvetteBrent 04-06-2019 01:51 AM


Originally Posted by robertfa (Post 1599143069)
I think I've experienced the same thing. When accelerating heavily on a wet road, driving straight, getting some rear wheel spin, I would experience a yaw to the left or, the rear end shifting to the right (fish-tailing, assuming this is the same thing?). I found that I could repeat this again under same conditions, getting the same movement. I had to steer right to compensate. Other than the road crown, I reasoned that perhaps the torque of the engine, where the crank is rotating counter-clockwise (from the driver's perspective), and creating a torque toward rotating the vehicle clockwise, therefore would transfer a small amount of the vehicle's mass to the right side. With a little more mass on the right side, I would expect the right rear wheel to have a little more traction than the left. If both rear wheels are spinning at the moment (traction control hasn't quite taken over yet), then more traction on the right would yaw the car to the left (rear end then moving to the right).

Well said. I have experienced this with the C4, the C5s, and other sports cars I have owned.

CorvetteBrent 04-06-2019 01:52 AM


Originally Posted by Bowtie52 (Post 1599153377)
Road Crown is a coverall excuse......the car can and does break loose causing the rear end to swing to the right.....yes it can and does this on wet pavement, dry pavement and it even happens if acceleration occurs as the tires make contact with road pavement striping. Outside ambient temps above 60 with full sun on the streets/road and the car will break free......something that might be worth looking into is the rubber compound used by Michelin as my tires are the original ones that came on the car with just over 17500 and it is a 2017 Z06 3LZ M7.

I'm not a rocket scientist, brain surgeon, or race car driver I'm just an average guy who does not track the car but the breaking loose of the rear does happen pure and simple maybe not on all Z06's but it does happen.

Agreed. I know from personal experience that it also happens with the C4 and C5 Corvettes.

slickstick 04-07-2019 02:51 AM

I was thinking this could be related to a recent Ask Tadge response:


We traditionally use half shafts of different stiffness’s on either side of the car to help with power hop. Power hop occurs when spinning the rear tires on surfaces with friction characteristics that create a stick/slip condition. The stick/slip phenomena alternately winds and unwinds the driveline like a big spring preventing smooth acceleration and creating quite a disturbance in the car. Having a very stiff half shaft on one side will break that tire loose sooner than the one on the other side, creating an asynchrony that helps dampen the resonance of the system.
https://www.corvetteforum.com/forums...post1598712923

Z-man 04-09-2019 01:29 AM

It's the rotation and curvature of the Earth. In Australia, cars fishtail to the left...

Tom400CFI 04-09-2019 07:06 PM


Originally Posted by cdngolfer (Post 1599138461)
What about drive shaft rotation and the ring gear location that causes the one side to become heavy and the other side to lift?

No. If there were a solid axle car, the tq imparted on the diff housing by the input would do what you're saying. But the entire drive train is bolted together; one unit from crank damper to the diff cover. In other words, the tq fed into the diff is resolved in the trans housing, tq tube, and bell housing. The net axial tq on the drive train (any one part of all of the drive train wanting to "rotate lengthwise" under the car), should effectively be zero. It should behave like a FWD engine/transaxle w/regard to reaction tq since both are all one unit, all bolted together.

Mikec7z 04-10-2019 01:34 AM

has anyone asked Tadge about why the rear caster setting is backward from what most race car alignment shops (and myself) suggest are the proper settings? (i wont say all shops, because i have not talked to all shops, but all shops i have talked to say to put it the opposite way as what GM suggests for the c7 rear caster)

I believe this applies to this question by the OP, since I have driven multiple C7 that pull to one side or the other severely, which have factory alignments, and then once we reverse the rear caster angle, the car suddenly handles fantastic under hard acceleration, and does not pull or favor to pull to either side in particular (and then road crown does become the determining factor... if the road is sloped left, rear will fade out left, and visa versa for right slope/crown.

Where did GM come up with its rear caster setting, and why?

Tom400CFI 04-10-2019 09:41 AM

I don't "get it", what caster has to do with straight line stability (as it relates to the REAR suspension). Can you elaborate on that?

Mikec7z 04-10-2019 10:23 AM

its essentially a known fact at this point that the caster settings on the rear wheels is backwards of what 6 of 6 race alignment shops i have asked, recommend it be.

DSC Sport Mike L. was the first to bring this to my attention on this forum that the rear caster setting IS adjustable on the c7. you see, in the past, a person had to get a coil over suspension on a c5 and c6 to be able to properly adjust rear caster. I could never figure out why the coil overs caused a vette to handle better than the leaf springs, as i see the physics advantages of leaf springs. Hind sight, it was the rear caster alignment that my race shop would do to the rear caster angles with the coil overs and custom mount points for the control arms.

In the "DSC SPORT ALIGNMENT or DSC CONTROLLER OBSERVATIONS" Thread in the c7 and c7z sections of this forum, and several other parallel threads about DSC, you will see a unanimous agreement that the rear caster, when set to the inverse angle as GM recommends, allows the car to handle much better and less over-steer snappy in the rear.

It has also been observed on many peoples cars, including my own, that from the factory, not only were the rear caster angles inverse of what they should be, those AND the Camber were different on one side of the car vs the other.

So what this does is, in WOT acceleration, the car has the majority of its weight on the rear 2 wheels/tires... and this means IF those 2 wheels and tires are not aligned properly, and more importantly perhaps, not aligned SYMMETRICALLY, then the car pulls hard on the rear in 1 single lateral direction every time the rear tires break loose from WOT.

once this is remedied with the DSC Sport recommended rear alignment specs, the cars handle very well and are comfortable and predictable to drive, and forgiving when the rear steps out. To get to the proper rear alignment, washer/spacer/shims have to be removed from the GM alignment settings... it is that LARGE of a change required to make these cars straight.

Most top racers on this forum can be seen on these DSC threads chiming in about how their lap times improved, and from what I can gather, they ALL had to remove spacer/washer/shims to get their rear casters right. In fact, an aftermarket bar sold from Granetelli has to be installed on the car to reach the rear caster settings recommended by DSC and by most race alignment shops, for a safe and balanced automobile, as the GM stock rear links do not even allow for the proper adjustments due to their original lengths and geometries.

I could keep typing, but there is a magnitude of information right here on this forum alone, about this exact topic, and how berserk so many people's rear alignments are, on their new c7 vettes, off the truck, straight from the BG factory.

illmac77 04-11-2019 01:59 PM

Once I used the DSC street/track alignment the Z stopped breaking to the right. It just squatted and went.

sstonebreaker 04-11-2019 02:01 PM

The reason the car has a tendency to fishtail to the right is primarily due to engine torque. In a car as stiff as the vette, you can't really see the weight transfer onto the right rear wheel. But if you look at a car with poor suspension setup like this old boy here, you can better visualize the weight transfer that's happening:

https://cimg8.ibsrv.net/gimg/www.cor...2096e55e88.jpg

So what happens is the left rear tire, having less weight on it, loses traction first resulting in a net force pushing the car counter clockwise around the cg of the car.

Tom400CFI 04-11-2019 05:01 PM

No. Already addressed....

Originally Posted by Tom400CFI (Post 1599196467)
No. If there were a solid axle car, the tq imparted on the diff housing by the input would do what you're saying. But the entire drive train is bolted together; one unit from crank damper to the diff cover. In other words, the tq fed into the diff is resolved in the trans housing, tq tube, and bell housing. The net axial tq on the drive train (any one part of all of the drive train wanting to "rotate lengthwise" under the car), should effectively be zero. It should behave like a FWD engine/transaxle w/regard to reaction tq since both are all one unit, all bolted together.

The car you posted a pic of is a solid axle car. In a solid axle car, the engine/trans package reaction tq, acts on the frame/body of the car and causes it to tip to one side as a result. As already stated above, In a Corvette (or any IRS vehicle), engine tq is resolved in the structure that ties the rear diff and the engine, together. In a C2-C4, that would be the car's frame rails. In a C5-C7, it is the tq tube. IRS 'Vettes launch level, with IRS.

https://cimg6.ibsrv.net/gimg/www.cor...1e12e237da.jpg



https://cimg7.ibsrv.net/gimg/www.cor...7acbb1fc13.jpg

Mikec7z 04-11-2019 09:33 PM

Not true on our cars, they have torque tubes, if the car was twisted, the torque tube would have been cracked like a roll of muffins. Thats the reason our cars dont do this... torque tube connects trans, rear diff to the engine. Its all 1 piece, or its broken if its 2 pieces.

has little to nothing to do with the type of rear axle... it is the torque tube that unites the engine to the trans and the rear diff as one piece.


Originally Posted by sstonebreaker (Post 1599207445)
The reason the car has a tendency to fishtail to the right is primarily due to engine torque. In a car as stiff as the vette, you can't really see the weight transfer onto the right rear wheel. But if you look at a car with poor suspension setup like this old boy here, you can better visualize the weight transfer that's happening:

https://cimg8.ibsrv.net/gimg/www.cor...2096e55e88.jpg

So what happens is the left rear tire, having less weight on it, loses traction first resulting in a net force pushing the car counter clockwise around the cg of the car.


sstonebreaker 04-12-2019 09:55 AM


Originally Posted by Tom400CFI (Post 1599208467)
No. Already addressed....


The car you posted a pic of is a solid axle car. In a solid axle car, the engine/trans package reaction tq, acts on the frame/body of the car and causes it to tip to one side as a result. As already stated above, In a Corvette (or any IRS vehicle), engine tq is resolved in the structure that ties the rear diff and the engine, together. In a C2-C4, that would be the car's frame rails. In a C5-C7, it is the tq tube. IRS 'Vettes launch level, with IRS.

It's not about launching level. It's a question of basic physics. Newton's 3rd law, to be precise (for every action, there is an equal and opposite reaction). The motor twists one way, it's going to twist the car the opposite direction. No matter how well the car is set up, that right rear tire is going to initially have more traction due to weight transfer from engine torque. Solid rear, IRS, torque tube... doesn't matter. You still have two big old engine mounts up front that are going to throw a certain amount of the engine's torque directly into the body of the car without going through the suspension. And that is going to initially transfer more weight to the right rear tire than the left, causing the car to fishtail to the right.

The top fuel guys tried to solve this back in the day by running a "sidewinder" engine. It was a good idea in theory but there were some practical obstacles that never got solved adequately.
https://cimg5.ibsrv.net/gimg/www.cor...7231801d6d.jpg

Tom400CFI 04-12-2019 11:32 AM


Originally Posted by sstonebreaker (Post 1599212589)
It's not about launching level. It's a question of basic physics. Newton's 3rd law, to be precise (for every action, there is an equal and opposite reaction). The motor twists one way, it's going to twist the car the opposite direction. No matter how well the car is set up, that right rear tire is going to initially have more traction due to weight transfer from engine torque. Solid rear, IRS, torque tube... doesn't matter.

You're not "getting it". It does matter. Here is an opportunity for you to learn a thing, so pay attention....
Look, take your example in the pic above. Lets say that engine makes 500 lb-ft. The trans is bolted to the engine; they're one unit. The trans multiplies engine tq by about 3:1 in first gear, so we're sending ~1500 lbs off tq down the drive shaft, right? Right. You are right in this example, the 1500 lbs of REACTION tq (the opposite and equal reaction) is going to be send into the frame/body of the car, causing it to tilt. But the car stops tilting at some point, right? It doesn't barrel roll down the track from reaction tq of the engine/trans, does it? Where does that tq resolve...and stop the car body from listing to one side? Through the rear springs (and fronts, if it's still on the ground).

Now look at an IRS car. ANY IRS car. Let's use the C2/3/4. Same 500 lb-ft engine, same gearing, so engine/trans "unit" sends 1500 lbs down the driver shaft. At launch, you're putting 1500 lbs of twist into the rear diff housing...right? Right. If that Diff housing weren't bolted to the frame of the car, what would happen to it? It would twist clockwise like a mofo! It would spin uncontrollably, in the same direction as the drive shaft it turning. Since it IS bolted to the frame of the car, that force is sent into the frame of the car. SO, you have the engine/trans unit at then front of the car applying 1500 lbs of reactive tq to the frame, in a CCW direction (opposite crank rotation as view from front of car). You have the diff housing, applying 1500 lbs of reactive tq to the frame, in the CW direction. That tq is resolved through the frame of the car...or the frame twists up like a candy cain if it's a real shit box. :lol:

Now, in the C5,6,7, the whole thing is simplified b/c those opposing, reaction tqs between the front and rear are resolve w/in the assembly itself; the engine/bellhousing/Tq tube/trans/diff....b/c they're all bolted together in one piece. The only meaningful tq exerted on the body is the tendency to lift the front end; the reaction tq of the drivetrain trying to rotate the rear tires. That's precisely why my pics show IRS 'Vettes launching level...and you're SRA car launching all "crossed up".





Originally Posted by sstonebreaker (Post 1599212589)
The top fuel guys tried to solve this back in the day by running a "sidewinder" engine. It was a good idea in theory but there were some practical obstacles that never got solved adequately.

What kind of "obstacles"? I think that nearly every FWD car, motorcycle, and snowmobile would beg to differ with you on that. :yesnod:


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