4WD and regenerative braking

[gcdugas]

Admittedly the rules need, in the words of the FIA and the TWG, a "major revamp" to accomodate regenerative braking. Though still a season or two off, my question is this... wouldn't it be timely to get rid od the ban on 4WD in order to best exploit regenerative braking since such a system would invariably employ "drive shafts" to the front wheels in order to capture braking energy. I see this as a necessary corollary to regenerative braking. And while we have that, why not have intelligent diffs with torque steer?

What say ye all ye engineers?

[Ciro Pabón]

... wouldn't it be timely to get rid od the ban on 4WD in order to best exploit regenerative braking since such a system would invariably employ "drive shafts" to the front wheels in order to capture braking energy. I see this as a necessary corollary to regenerative braking. And while we have that, why not have intelligent diffs with torque steer?

I am not an expert, so all this is pure autodidactism. What I understand:

Not necessarily. For example, the Prius or the Insight have the alternator/electric engine on the drivetrain. For F1 it has the advantage that it can be used as alternator/brake/starter and that you do not increase the unsprung weight. The Insight design has another twist: the asymetric winding of the poles on the ultra-thin, brushless DC motor has the advantage of requiring less space.



The hybrid vehicle "standard" braking by electric motors and battery has a disadvantange: in DC motors, the voltage has to be controlled (you cannot use full brake force at all speeds) and in AC motors you have to control the frequency.

The idea of using supercapacitors instead of batteries seem rational, as the peak power you can recover from them could be used on a push-to-pass button. Peak energy available with supercapacitors would be high, while the peak power you would get from batteries would be meager.

Hydraulic regenerative braking is used on trucks (recent design, Ford F350 Tonka, for example) but the weight of the tanks and fluid makes this a difficult design for racing.

Electric brakes have a curious problem (again, this is what I understand): you cannot achieve "full braking", that is, you cannot fully stop the car and you have an increasing action at the beginning of braking, which requires modulation of the "normal" brake.

The Abbey Ales Brewerie, located at the top of a hill near London, proposes a unique challenge: you carry on your vehicle a full barrel of beer downhill. Then you have to use the energy stored in your regenerative braking system to carry an empty barrel uphill. This implies you have to drink the barrel at the pubs located downhill before you attempt to go back to the brewerie. I can help with that one.

[gcdugas]

Well that is all very nice but unless you are willing to forfeit all the braking energy that would be captured by the front wheels, then you will have "drive shafts" at the front axle. And once having them, why not use them for propulsion since whatever means is used to capture said energy would then be ready to use at the front axle. No matter what methods of energy storage/capture is used, it is all the same. In fact you might want to forfeit the energy that the rear brakes "capture/generate" since it would further complicate the rear drivetrain, plus the rear brakes only ammount to about 20% brake work.

[Ciro Pabón]

Well that is all very nice but unless you are willing to forfeit all the braking energy that would be captured by the front wheels, then you will have "drive shafts" at the front axle.

Again, not necessarily. You can mount the electric generators on the front wheels themselves. I repeat, the unsprung weight would be a headache in any case, generators-on-wheels or drive shafts, whatever you chose (probably the shafts are a better option). The drive train and the cardan would be another "dead weight". Of course, anything is possible, but the question is if it is a winner design. As your poll suggests, rules are made to be changed, but racing constrains are hard to circumvent.

I find hard to believe 80% of the braking is done in the front wheels, but if you say so you must have a reason. I thought that the geometry of the suspension prevented some of the weight transfer (you can ask DaveKillens, I never "got the thread" about it very well).

Finally, the amount of recovered energy is abismally low. An optimized Prius recovers 30% of the braking energy. When it is converted to electricity and converted again to mechanical energy you get another 30% slash. This gives you 10% energy back. According to Prius site, at this car, optimized for energy recovery, it is equivalent to 1 liter of gas per 100 km of normal driving. The reason some people (like me) hopes for F1 to concentrate on this matter is because we also hope they can improve the efficiency of the system. Anyway, putting a lot of weight on the wheels for such a small improvement in energy (thinking about F1 cars as energy hogs) doesn't seem to be the best racing option. Another problem here (hey, I am playing "devil's advocate , people have seen me arguing for the system) is that the 4WD introduces its own energy losses.

Of course, 10% recovered energy is better than nothing. However, you have to take in account that the proposed rules for 2008 (and the energy density and weight of most storage systems) restrict the amount of energy you can store (without converting the car into a truck weighing over 600 kg), so I believe that even if you use only the rear brakes to get it, you do not need a lot of input to fill the storage system very quickly.

I only hope your smart question (and my incomplete answer) will encourage the members of the forum to think about a thing that seems fantastic on theory and hard to engineer.

If you are right (why not?) and the 4WD is the logic answer, perhaps Audi would be interested in an F1 team. This would be nice. Perhaps they can take Williams position, once the last of the privateers is squeezed out...

Finally, you mentioned a controlled differential. In my ignorance I agree: this actually seems a winner design. Anyway, let me play the devil's part again (after all, we seem to be discussing this between the two of us, slow post day, I guess): do you really think pilots can take more lateral g's?

[bhall]

Aside from everything else, what about the fact that 4WD cars have poorer handling when compared to rear-wheel drive cars?

[flynfrog]

Aside from everything else, what about the fact that 4WD cars have poorer handling when compared to rear-wheel drive cars?

not with proper control

espcialy if they are electric motors powering the front wheels

it would be almost scary to see how fast an eletric 4wd f1 car could turn

[NickT]

Ciro as always an excellent insight, thankyou

I find hard to believe 80% of the braking is done in the front wheels, but if you say so you must have a reason. I thought that the geometry of the suspension prevented some of the weight transfer (you can ask DaveKillens, I never "got the thread" about it very well).

Your right Ciro with softly sprung road cars it is possible, which is why many still use drum brakes at the rear of the car. But with a modern F1 car it is much closer to 55:45 front to rear. In the rain it gets closer to 50:50 as the front tyres clear the water the rears have more grip, hence why the drivers have a remote control for adjusting brake bias.

Brake balance aside, this is a facinating subject and one where Honda and in particular Toyota potentially have an advantage as they already have production cars on the road and understand the pros and cons of the technology. Also given what you said about the operating ranges of the motor/generator braking, maybe we should also allow contineousely variable transmisions to be attached to these units to?

[flynfrog]

there would realy be no need for a cvt on an electric motor

they tend to be much less peaky than an IC engine

[gcdugas]

Your right Ciro with softly sprung road cars it is possible, which is why many still use drum brakes at the rear of the car. But with a modern F1 car it is much closer to 55:45 front to rear. In the rain it gets closer to 50:50 as the front tyres clear the water the rears have more grip, hence why the drivers have a remote control for adjusting brake bias.

No there is weight transfer that does make it 80/20 or thereabouts. It matters not if the suspension is hard as a rock or soft. If it was hard as a rock it is still possible to lift the rear wheels right off the ground were it not for the rear wing. If the CG is 6 inches above the front axle and 80 inches behind it (very arbitrary numbers), then under 5G braking you have a you have unweighting of the rear wheels of 330 lbs.

(600 Kg = 1320 Lbs) X 5G = 6600 pounds at 6in [1/2 ft] or 3300 ft lbs torque which equals 3300/6.6666= 495 lbs lift at location of CG which is approx 80 inches (6.666 ft) aft of front wheels. With a wheelbase of approx 10 ft, this would have an effect of 6.6666/10 X 495 = 330 lbs at the rear axle. This is almost 25% of the weight of the car. So the axle loads would be at rest F/R 33%/67% or 440/880 Lbs. Under the braking weight transfer it would be 440 + 330 or 770 front and 880 - 330 = 550 rear, which is close to 60%/40% F/R front weight bias.

These figures are very arbitrary, but they just go to show that the weight shift effect is independent of the softness of the suspension since the equation has mostly to do with the location of the CG relative to the front axle under load.

Mind you all this is independent of aero which mitigates against this considerably. Though the aero downforce "CG" of the front wing is ahead of the front wheels causing a weak leverage "moment" of lift at the rear wheels which the rear wing has always had to counteract, the effective overall aero "CG" of the car is around 45%/55% F/R, it is a declining constant as the car slows. But regardless of the aero loads, it is safe to say that there is considerable weight shift during braking and it is certain that the front brakes do the lion's share of the stopping. And that is something that certainly shouldn't be forfeited in the quest to "capture" braking energy.

[Ciro Pabón]

No there is weight transfer that does make it 80/20 or thereabouts...

gcdugas, I only hope you do not "hold" this thread against me in the future... it seems I've been contradicting you every step of the way. I am sorry...

What you says is exactly what I thought at the beginning of my incursions in this forum. However, our friends at the suspension design department have changed the rules, not by changing the physics that you describe, but the suspension mechanism.

You can read this thread on controlling dive. You can arrange the level arm of the braking force to lift the car.

This is what I was telling you I do not "grasp intuitively", as I did with the concept of normal suspension that shifts weight around (the way they explain to you at the racing pilots introductory course and that you describe). As we say in spanish: you can kill me if I understand it... You can check in the thread I point you to, that half the explanations given are a "little" confused.

The idea (as I half-get it) is that you have the two A-arms on a line that it is not vertical, and the points where the A-arms are fixed to the wheel are not in an horizontal plane. This way, when the car tries to dive, it "twists" the arms instead of diving the nose. This makes a very hard suspension, but from what I read, this is another compromise for the tuning engineer.

You can achieve "perfect" 50-50 braking with any load if you "transfer" all diving to twisting the arms, but the car becomes unmanageable (or at least, if I get it, hard on your kidneys; besides, you do not "feel" the car with your *ss, as you normally do). This antidiving geometry is used not only for "frontal" diving, but also for lateral roll.

DaveKillens, flynfrog, RacingManiac, NickT, RH1300S and probably mep (if he finally understood) can explain it better, I guess. Help, you guys!

[gcdugas]

No there is weight transfer that does make it 80/20 or thereabouts...

gcdugas, I only hope you do not "hold" this thread against me in the future... it seems I've been contradicting you every step of the way. I am sorry...

What you says is exactly what I thought at the beginning of my incursions in this forum. However, our friends at the suspension design department have changed the rules, not by changing the physics that you describe, but the suspension mechanism.

You can read this thread on controlling dive. You can arrange the level arm of the braking force to lift the car.

This is what I was telling you I do not "grasp intuitively", as I did with the concept of normal suspension that shifts weight around (the way they explain to you at the racing pilots introductory course and that you describe). As we say in spanish: you can kill me if I understand it... You can check in the thread I point you to, that half the explanations given are a "little" confused.

The idea (as I half-get it) is that you have the two A-arms on a line that it is not vertical, and the points where the A-arms are fixed to the wheel are not in an horizontal plane. This way, when the car tries to dive, it "twists" the arms instead of diving the nose. This makes a very hard suspension, but from what I read, this is another compromise for the tuning engineer.

You can achieve "perfect" 50-50 braking with any load if you "transfer" all diving to twisting the arms, but the car becomes unmanageable (or at least, if I get it, hard on your kidneys; besides, you do not "feel" the car with your *ss, as you normally do). This antidiving geometry is used not only for "frontal" diving, but also for lateral roll.

DaveKillens, flynfrog, RacingManiac, NickT, RH1300S and probably mep (if he finally understood) can explain it better, I guess. Help, you guys!

First off I certainly hold nothing against you for discussing this. I believe that I could persuade you easily if you can grasp even the most basic engineering principles, and I certainly give you that much credit.

You are confusing dive and weight transfer. Ask all those whom you named and they will explain the difference. But this illustration might make it easy for you to understand... Imagine that 99.9999% of the weight of an F1 car was in the camera above the airbox. When the driver hits the brakes, the inertia from that mass wants to keep going forward. If you are braking at 5G that is 6600 lbs of force. The car would tip over like a bicycle with a stick jammed in the spokes. It would happed so fast that it would make your head spin. Now if the car is normally 33/67 F/R weight biased (at rest), the rear springs have to support 1320lbs X 67% = 880lbs and the front 440lbs. Now under the weight transfer to say 80/20 F/R the front springs suddenly have to support 1056lbs. This would cause the front to lower considerably whily the rear springs would be relieved of some load and actually rise. This effect is called "nose dive" or just plain old "dive". Your friends are talking about counteracting dive.

As a matter of fact, it is because of the very real weight transfer that these anti-dive geometries are employed to counteract. It is not nullified, it is counteracted.

There will still be weight transfer if the CG is above the front axle height. But by mounting the caliper so that a portion of its forces are "channeled" into the push rod there will be no "dive". This affects the attitude of the car longitudinally but the actual loads where "the rubber meets the road" in the contact patch still increases dramatically under braking. For that matter, were it possible to place the CG below the fromt axle height, there would be weight transfer to the rear under braking. And in an ideal world if the CG is at exactly (or within a cm or so) of the front axle height, there would be no weight transfer. It is simple geometry and leverage principles. Motorcycles wheelie and do "stoppies" because the CG is signifigantly above the axle height and this accentuates the effect.

[Carlos]

gcdugas--dive is the result of weight transfer-- we need antidive geometry because of weight transfer. I understand this. Ciro understands this. I believe Ciro's grasp of basic engineering principles is well developed. He has experience with this subject--- the rest of the F1tech forum can confirm this for you gcdugas. His remark concerning "hold" was a gesture of courtesy. I felt someone must note this for you. Social grace is unusual today---isn't it gcdugas?

RegardsCarlos

[gcdugas]

gcdugas--dive is the result of weight transfer-- we need antidive geometry because of weight transfer. I understand this. Ciro understands this. I believe Ciro's grasp of basic engineering principles is well developed. He has experience with this subject--- the rest of the F1tech forum can confirm this for you gcdugas. His remark concerning "hold" was a gesture of courtesy. I felt someone must note this for you. Social grace is unusual today---isn't it gcdugas?

RegardsCarlos

I have noticed the welcomed absence of "flaming" on this forum and I hope to treat all with respect and deference. If my reply was out of line or in the wrong tone, please forgive me. I was just getting back to the main point, weight transfer, and by extension of that... to the ammount of energy that is available for "capture" at the front axle/"drive shafts".

[Ciro Pabón]

You are confusing dive and weight transfer. Ask all those whom you named and they will explain the difference. But this illustration might make it easy for you to understand...

gcdugas: I am not corteous, simply I am not sure of what I am saying (I know about roads, I am learning here about cars). You can flame to your heart's content in this forum, but is hard to flame about facts.

I have to reckon you are right: the weight transfer occurs (I told you I did not grasp it very well) even if the car do not dive. I also understand vaguely that F1 cars can "dial" the brake force. I think these (and a low CG) are the reasons for NickT arguing about a less asymetric braking force than the one you estimate.

Anyway, let me remark again that you can only store around 20 Kwatts (this is like 30 Hp) in the regenerative system. You can get this, from the 20% braking force you estimate the rear axle delivers, in just one deceleration from an F1 car (a car at 300 kph needs like 300 Kwatts to stop so even 20% gives you 60 KWatts).

Finally, I have no idea why 4WD is banned. It should promote stability and a better handling. Do you know why?

[Mikey_s]

I need to tread very carefully here - I am a simple chemist at heart, but very interested in engineering!

I an not sure of the facts, but I do lean towards gcdugas' explanation. It is very clear on road cars that the majority of braking effort comes from the front wheels; very often (even on quite modern cars) you will see large disk brakes on the front wheels and much smaller ones on the rear wheels, in fact many cars (although not sports cars) are still fitted with drum brakes on the rear. I can certainly be led to believe that the inertia of the vehicle puts more load onto the front wheels and unloads the rears, therefore more effort can be applied to the fronts than the rears; what the percentage is i have no clue. However I also heard another (perhaps unrelated) explanation for more braking force at the front:

One explanation given to me was that locking the rear wheels before the front wheels is extremely hazardous as the car will simply swap ends (in fact you can very simply demonstrate this with a toy car and some "blue tac" (the stuff you used at college to stick posters to the walls!); block the front wheels and slide the car down a slope and it carries on in a straght line... block the back wheels and it rapidly changes ends... so from a safety perspective manufacturers typically ensure that the fronts will lock first by putting larger brakes on the front! It si a graphic demostration of the fact that a skidding wheel has less friction than a rolling wheel, try it yourselves, it is fun!