Ah yes, the DeltaWing...

[Andres125sx]

He's not talking about straight line braking capacity. Any car will spin if you overload the rear tires braking capacity while trail braking. The delta wing guys claimed their car would not, which defies physics.

Any usual car, but you can´t do that claim when talking about a whole new design

According to Wikipedia the mass distribution is 27.5% on the front and 72.5% on the rear. Also 76% of the downforce is at the rear.

I don´t know the real numbers when braking, but anycase the mass distribution when braking will be the most balanced out there for sure, so to me that assertion doesn´t look crazy

Any usual car will spin if you overload the rear tires braking capacity because any usual car has a very far forward CG when braking (too much weight at the front, and too little at the rear), but if the car keep the balance when braking or can even keep more weight at the rear, then there´s no reason to think the rear tires will lock or slide before the front ones

Now I´d like to know the min cornering speed at constant speed with just 27.5% of the weigth at the front, but that´s a different debate.... and it only weight 475 kg

[Pierce89]

You can still lock the rear tires. It happened in the race. Now,locked rear tires + turning= spinning . Even on the delta wing.

[Lycoming]

He's not talking about straight line braking capacity. Any car will spin if you overload the rear tires braking capacity while trail braking. The delta wing guys claimed their car would not, which defies physics.

Any usual car, but you can´t do that claim when talking about a whole new design

what part of any car did you not get there? draw a free body diagram and you will see this to be the case for any car.

I don´t know the real numbers when braking, but anycase the mass distribution when braking will be the most balanced out there for sure, so to me that assertion doesn´t look crazy

Sure, but how does that prevent you from locking the rear wheels? Do you know what brake bias is? it's the split between braking force, typically front to rear. Let's say you have a car which, under braking, has a 60/40 F/R weight distribution; Thus, assuming all else equal, the front wheels do 60% of the work, and the rear wheels do 40%. Thus, you want to split your brake line pressure so that the front brakes provide 60% of the torque, and the rear brakes do 40%. Anything other than that will mean that you're not distributing brake torque to make 100% use of the tires.

Now, let's take the DeltaWing. let's say it has the other way around; 60% of the weight is on the rear under braking, and 40% is on the front. You would do the same thing with the brake bias, but the other way around. Do you understand what this means?

Although the rear has more weight on it, and is therefore harder to make it slide, You're applying more brake torque to it; you're able to apply more force to make it slide.

Let's say we have the ultimate deltawing; let's say it has 99% weight on the rear wheels and 1% weight on the front, and it's CG is so low that weight transfer is negligible under braking. What will happen if I set my brake bias to 100% rearward? this way, the front CANNOT SLIDE; thus, if you apply enough force to the brake pedal and assume the brakes are capable of locking the wheels, you will force the rear to slide before the front.

And guess what happens then?

The Rearward weight distribution does not mean that the rears will never slide before the fronts. That assumption only holds if you assume that you're applying the same braking torque to both the front and rear. Guess what: nobody does that; Indeed, if you have a look, you will notice the deltawing has larger rear brakes than front, which would imply a rearward brake bias.

[Andres125sx]

Ok, agree about that, but that´s not the question I think.

If you want you can cause a rear wheel lock with brake bias, obviously, but why would you want to do that?

If you set brake bias 80% forward on a standard car, you´ll have front wheel locks for sure, but that does not change the oversteer tendency of the car when braking (if you don´t lock the wheels) because the problem remains the same, the car is too light at the rear when braking.

With brake bias you can set the braking behaviour, but the car tendency will remain the same, if it´s too light at the rear you have more grip at the front so the car will tend to oversteer, and you can´t solve the problem or find more grip for the rear wheels with brake bias. You´ll set it up to do it as driveable as possible, but the car tendency is what it is and brake bias is only a partial solution, a patch

But the DW mass distribution is so far back even on braking the rear is not lighter than the front wheels, so even when braking there´s more or less the same grip at both ends, so the tendency of the car is not to oversteer when braking anymore. Then you can adjust brake bias to your preference and induce some oversteer or understeer tendency, but with a balanced car with no bad habits when braking as a start point.

Since a bit of oversteer when braking is good for cornering, I guess they set the brakes to induce some oversteer, wich may be the reason they´ve locked rear wheels on race. But even if they spin because of a too far back brake bias, just releasing the brakes the rear wheels will have more grip than the front ones again (remember with brakes off 72% of the weight is at the rear wheels), so even if the driver causes a spin with the brakes, at the moment he release the pedal the rear wheels will bit again, so controlling the rear end of the car when braking must be a world easier than with any other car, wich must be the reason for their statement about some inherent braking stability

[Jersey Tom]

But the DW mass distribution is so far back even on braking the rear is not lighter than the front wheels, so even when braking there´s more or less the same grip at both ends, so the tendency of the car is not to oversteer when braking anymore.

Not quite correct.

It's not just the mass and dynamic load distribution that makes a car's handling balance what it is. It's that in relation to the springs, bars, aero, etc. And - for what it's worth - all things being equal moving CG (as in true mass, not dynamic load transfer) rearward will tend to make for more oversteer. That's Handling 101.

Anyway. For any given mass distribution you're going to set the rest of the car up (mechanical + aero) to make for a neutral cornering balance - or whatever amount of understeer you desire. Let's say your driver likes 0.1 deg/G understeer. Regardless of what the car is (DeltaWing or not) you'll adjust the setup to get to that point of nearly neutral handling. That means that when you go to trailbrake, you'll still move toward oversteer regardless!

The only things that are going to affect how much balance moves with forward load transfer on brakes are (a) brake bias, (b) cg height and wheelbase, (c) tire properties. Doesn't matter whether your starting mass distribution is 55/45, 50/50, or 35/65.

[Cold Fussion]

I recall when this concept was first announced and they had the complete BS assertion that it would be "inherently stable on braking."

First hour of Daytona 24.. locks up brakes and spins out

It's all a question of hats, and he isn't obviously going to be wearing his engineering hat when talking to the media.

[Andres125sx]

But the DW mass distribution is so far back even on braking the rear is not lighter than the front wheels, so even when braking there´s more or less the same grip at both ends, so the tendency of the car is not to oversteer when braking anymore.

Not quite correct.

It's not just the mass and dynamic load distribution that makes a car's handling balance what it is. It's that in relation to the springs, bars, aero, etc.

Yes, but that´s the same as with the brake bias, those are final adjustments, but the chassis is the base of the car, what will make it behave one way or another. Then the rest of the parts you put on it can help solving some problems or improve something, but the part wich mainly dictates how the car behaves is the chassis.

You can use the parts you want on a DW, but it will never behave like a 458 because the chassis are night and day. So a rule of thumb for a standard design like a 458, may be not applicable for the DW.

And - for what it's worth - all things being equal moving CG (as in true mass, not dynamic load transfer) rearward will tend to make for more oversteer. That's Handling 101.

I´m not sure if I get this correcly or it´s my poor english, but if you mean the further back you move the CG (with no mass change) the more oversteer you will suffer, sorry but I only can disagree

As a motocross driver I can tell you when you need grip for a wheel, you move your weight to that axis, that´s basic driving technic and it´s valid for any vehicle. Move your body backwards when braking to avoid the rear wheel jumping but also to add grip (you also do it on slipery terrains when there´s no risk for the rear wheel to jump), take the corner balanced to have good grip on both wheels, and then move backwards again to hit the throttle and improve the traction, or if you´ve slided more than expected you have to move backwards as much as you can.

For a given mass, more weight on a wheel (or moving the CG) means more grip.

The DW is like a MX driver who is always tilted backwards, he will strugle when cornering but on braking he´ll be the fastest because is placed at the perfect position for braking, while a standard chassis design is like an MX driver who is always centered, he will be the fastest cornering but will lack tons of rear grip when braking or accelerating, but since most cars are designed this way we consider that as the standard and don´t think standard cars lack rear grip, but DW should be way better in this aspect. And should strugle when cornering at constant speed, but that´s another debate, here we were discussing the statement about the inherent stability when braking

Another example are 911´s. DW is an extreme 911, everything is loable on a 911 (traction, braking perfomance) will be even better on a DW, and any weak point of a 911 (understeer at constant speed) will be even worse on the DW..... but they maybe solved this with a really light design (something Porsche can´t say), only 475kg, but again, that´s a different discussion

Anyway. For any given mass distribution you're going to set the rest of the car up (mechanical + aero) to make for a neutral cornering balance - or whatever amount of understeer you desire. Let's say your driver likes 0.1 deg/G understeer. Regardless of what the car is (DeltaWing or not) you'll adjust the setup to get to that point of nearly neutral handling. That means that when you go to trailbrake, you'll still move toward oversteer regardless!

I agree that´s the usual aproachment, but not for the DW. They´ve decided to go a different route. With 72% of the mass on the rear wheels you can´t expect neutral handling at constant speed, so obviously that´s not what they had in mind when in the drawing board.

My speculation is they´ve accepted the car will understeer at constant speed, but they live with it because they´re so light even with that tendency to understeer their min cornering speed must be quite good, so the weak point of that design is not so weak if they´re so light. If they don´t have any huge weak point because of this, and for braking and accelerating their design is the best (on most situations), then I can understand DW engineers and even applaud them =D>

The only things that are going to affect how much balance moves with forward load transfer on brakes are (a) brake bias, (b) cg height and wheelbase, (c) tire properties. Doesn't matter whether your starting mass distribution is 55/45, 50/50, or 35/65.

Again disagree, your starting mass distribution is a KEY factor that change it all, weight distribution does change the grip, and therefore the behaviour

[Jersey Tom]

For a given mass, more weight on a wheel (or moving the CG) means more grip.

Guess we'll have to leave it at agreeing to disagree then. But I would definitely encourage reading up on some authoritative vehicle dynamics texts (i.e. don't take my word for it), going through some tire data, or playing around with a car at track. I think you'll find that moving CG forward (aka more forward weight distribution or more nose weight at setup) will increase cornering understeer in just about any situation I can think of.

[autogyro]

I think you are arguing at crossed purposes.
I used to lean forward onto the outside front wheel during cornering when Karting.
You had to get the timing right though.
To early at corner entry and you went strait on in slippy conditions.

[Lycoming]

For a given mass, more weight on a wheel (or moving the CG) means more grip.

Sure, but we typically think of oversteer/understeer in terms of the moments caused by forces, and not the forces in isolation. If you take into account load sensitivty of the tires and assume that the yaw center is about the CG...

Anyway. For any given mass distribution you're going to set the rest of the car up (mechanical + aero) to make for a neutral cornering balance - or whatever amount of understeer you desire. Let's say your driver likes 0.1 deg/G understeer. Regardless of what the car is (DeltaWing or not) you'll adjust the setup to get to that point of nearly neutral handling. That means that when you go to trailbrake, you'll still move toward oversteer regardless!

I agree that´s the usual aproachment, but not for the DW. They´ve decided to go a different route. With 72% of the mass on the rear wheels you can´t expect neutral handling at constant speed, so obviously that´s not what they had in mind when in the drawing board.

Sure you can. Why do you think the front tires are so small?

My speculation is they´ve accepted the car will understeer at constant speed, but they live with it because they´re so light even with that tendency to understeer their min cornering speed must be quite good, so the weak point of that design is not so weak if they´re so light. If they don´t have any huge weak point because of this, and for braking and accelerating their design is the best (on most situations), then I can understand DW engineers and even applaud them =D>

Well... it would be natural to set the car to understeer. You tend to do that with any car. But It seems likely to me that if it understeered more than a little bit, you would have heard it from one of the drivers from now. It's not like you go into a corner, turn the wheel full crank, the car barely turns, but it still maintains high cornering speed... you can see from the onboard videos that it does not understeer anywhere near that much.

[DaveW]

It's not just the mass and dynamic load distribution that makes a car's handling balance what it is. It's that in relation to the springs, bars, aero, etc. And - for what it's worth - all things being equal moving CG (as in true mass, not dynamic load transfer) rearward will tend to make for more oversteer. That's Handling 101.

Forgive me, people & JT in particular, but the Milliken's state in RCVD (equation 5.47) the the static margin (SM) is given by:



where is the Neutral Steer Point, is the distance of the centre of gravity behind the front axle, and is the wheelbase.

On that basis, the JT's "Handling 101" is correct, because increasing (moving the centre of gravity rearwards) will reduce the and move the balance towards oversteer with no other changes.

On the other hand, the Milliken's also state (equation 5.46) that:



where describes the proportion of the roll moment carried by the rear axle (in their simple model).

Moving the centre of gravity without also changing the roll moment distribution would seem to be a fairly special case...

[machin]

One thing I have learnt as an engineer is that it doesn't matter how well you explain something, or how many pertinant numbers or equations you use, there will always be someone who brings up some anecdote to "prove" you are wrong.... so how about these videos:-

A toy car with rubber tyres is loaded up with weight over the rear so that the CG is just in front of the rear wheels (i.e. within the wheelbase). All four tyres are covered with cellotape (I think it is called skotch-tape in the US?) with the smooth side down to reduce the coefficient of friction. The car is placed at the top of a smooth sloping surface to use gravity to replicate a lateral force due to cornering.
[youtube]http://www.youtube.com/watch?v=1rvPcvTOyVk[/youtube]
The rear of the car slides the most indicating that the rear-mass bias car would naturally oversteer and therefore that a front-mass bias car will naturally understeer.

Now we take the same car, but remove the cellotape from the rear tyres, giving them a higher coefficient of friction than the front and replicating a situation in which the front wheels are locked:-
[youtube]http://www.youtube.com/watch?v=xwe23nSu-RI[/youtube]
The front of the car slides the most indicating that the rear-mass bias car, with locked front wheels will understeer

Now we take the same car, and fit cellotape to the rear tyres only, giving them a lower coefficient of friction than the front and therefore replicating a situation in which the rear wheels are locked:-
[youtube]http://www.youtube.com/watch?v=o0ogXKuqSrI[/youtube]

The car VERY quickly snaps into oversteer (rear end of the car slides very quickly).

As mentioned above by various people; the Deltawing team have to some degree, re-dressed the balance by fitting the car with very small front tyres, and probably by fitting a (stiffer) anti-roll bar at the rear.

Please forgive "Dragons Den" being on in the background of the videos!

[Lycoming]

As mentioned above by various people; the Deltawing team have to some degree, re-dressed the balance by fitting the car with very small front tyres, and probably by fitting a (stiffer) anti-roll bar at the rear.

That was actually really interesting, thanks for the demonstration.

Just wanted to note that the deltawing does not have a front ARB at all. Between that and the narrow front track, pretty much all the roll stiffness is at the rear. In a sense I guess you could say the rear ARB is stiffer.

[Jersey Tom]

Moving the centre of gravity without also changing the roll moment distribution would seem to be a fairly special case...

I would make the argument that F/R mass distribution change without touching the force elements is a fairly typical racecar adjustment, because of the way it affects handling.

I don't have RCVD handy at the moment but the example I think of here is (I believe) somewhere in a section about "alternate expressions for understeer gradient." There's one relating CG-to-axle distances a and b, and front and rear cornering stiffness (or cornering stiffness coefficient). Knowing that cornering stiffness coefficient decreases with normal load (other than in extreme circumstances), sliding ballast forward then makes for an increasing USG.

[Andres125sx]

For a given mass, more weight on a wheel (or moving the CG) means more grip.

Guess we'll have to leave it at agreeing to disagree then. But I would definitely encourage reading up on some authoritative vehicle dynamics texts (i.e. don't take my word for it), going through some tire data, or playing around with a car at track. I think you'll find that moving CG forward (aka more forward weight distribution or more nose weight at setup) will increase cornering understeer in just about any situation I can think of.

Then might you explain me the reason 911´s are usually acused of understeer when the engine is so far back?


On a car is dificult to test this, but even on karting you use your weight to add some grip as Autgyro said, moving forward at the enter of a corner to add some grip to the front wheel and turn as fast as possible, and then moving back to add some grip to the rear wheel and accelerate as hard as you can. On bikes this is a must and anyone with some experience will tell you the same, you move your weight towards the wheel you need grip, maybe on tarmac it´s not as necessary because a wheelie is more of a problem (more grip than necessary), but on dirt tracks, specially when they´re slippery, you need to play with your weight constantly to avoid uncontrolled slides.


BTW, it´s more than 10 years testing this on a bike, and almost 20 on a car, I don´t need more real world practice

Another real world example, the only way you can induce an oversteer on a car with an acused tendency to understeer, is while braking, when the CG is moving forward so the rear wheels get lighter and the front ones support more weight than usual (so have more grip than usual). More weight, more grip.... while the tires can handle it obviously

The grip is dependant of the pressure aplied between the two parts, don´t know the formula but I can provide you a hundred more real world examples if you want