4WD and regenerative braking

[pRo]

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.

It's not that simple. Many rear/mid-engined sports cars with more weight on the rear have bigger discs on the rear. The fronts however have bigger piston(s), so in the end the smaller diameter discs are doing most work. The fronts are also always(?) cooled, while the rears may not be, which also tells that the fronts are doing most of the work.

Some sports cars have a brake balance controller, which makes it easy to experiment. Set it to full front and you don't notice much difference in "emergency" braking, but set it to full rear and it feels like it takes forever to stop. And that's with a relatively hard suspension (but soft compared to F1 I'm sure). I believe that the weight transfer matters more than the angle of the chassis during braking. Even if the car had rock solid suspension and it kept level during braking, the fronts would still do most of the work.

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

That's true. But ABS is a simple cure for that in road cars.

[NickT]

Sorry guys I am up to my eyes in aligators so time is short, so this will be very basic

As I said before in normal road cars, motor bikes and full suspension mountain bikes for that matter the brake bias can be as suggested, 80:20, but F1 cars are far far removed from this:

A very low centre of gravity, does indeed have a significant effect. Lets go back:

Imagine that 99.9999% of the weight of an F1 car was in the camera above the airbox

Now switch it around so that the weight of the car is below ground and put a channel under the car what happens when it brakes now you still have some weight transfer but where which wheels load up and which wheels unload A modern F1 car's CoG is well below the axle line.

Anti dive geometry, stiff suspension and third springs, all help maintain the attitude of the car and contribute to limit weight transfer caused by excessive pitching or front end dive, but in F1 these are more about maintaining the correct aerodynamic ride height and dealing with the aerodynamic loads.

Which leads us nicely into aerodynamic down force, the tyres see this as more weight (from their perspective) and it is almost always present but it has no mass to transfer. Even at a relatively slow 80 miles an hour it is still a very significant load, but lets say its just 600 kilos and evenly distributed. So even if the weight transfer is as high as 70:30 or 420 kilos front and 180 rear, the tyres see 720 kilos front and 480 kilos rear or 60:40 and as the speed rises so does the downforce moving the brake bias more and more towards the rear of the car.

I hope this helps

PS gcdugas keep up the good work and keep asking the questions it is the only way we all learn

[Ciro Pabón]

PS gcdugas keep up the good work and keep asking the questions it is the only way we all learn

Thanks, NickT for the explanation.

BTW, gcdugas, we do not talk about "group hugging" or "flame wars" anymore. Nowadays, we use a new term that encompases both: "group pinnacle"...

Hey, good verb! What about "I pinnacle, you pinnacle"... Isn't "I had pinnacled before but you pinnacled me more than ever" a kinky phrase or what? It would work even on a Murray comment: "The Ferrari is about to pinnacle that Renault, but wait...".

Or, like I should say: "I finally pinnacled the dive-geometry thing".

[NickT]

You are welcome Ciro but what have you been drinking

Maybe not, but what time is it over there man

[Ciro Pabón]

It's 6 am, NickT. I've been working since 4 am, FYI. The proposal has to be ready by 3 pm...

[Mikey_s]

I think Ciro has been drinking the biofuel again... oh sorry, that's another thread! looks like I pinnacled too soon!

[Ciro Pabón]

Well, I had a few caipirinhas last night. I'm in Campinhas, Brazil. This must be it.

[Carlos]

JCdugas My apology for my rudeness-- I have also disrupted the thread--
apologies to you all. I was misguided by my social perception

Sincerely Carlos

[Carlos]

These links are related to this thread:

http://www.teslamotors.com Electric Telsa Supercar

-------------------------------------------------------------------------------------

This link concerns a 4WD GM concept car using regenerative braking. I had an article with details I wanted to post. Unfortunately it was "recycle day" and I may have disposed of it. Perhaps a search will uncover it. From The National Post-- details by memory---1100 to 1500 LB torque at 2 RPM
utilizing all 3 elcectric motors--CVT Transmission--lithium-ion batteries.

USA Government directed about 300M to Lithium Ion battery developement
funding 2 companies research with the intertion that LI batteries will be a viable power source--- just last week--some features of the Sequel
could contribute to the overall design philosophy being discussed:

http://www.greencarcongress.com/2006/08 ... es_dr.html

If I should find the article I will post more details.

Regards Carlos

[NickT]

Carlos, some excellent links. Thankyou

Ciro you are obviously a morning person, I am not, I just can't get out of bed, but I often get to work the same hours from the night before

Just posted this on another thread, relevent here to though http://www.autocarmag.com/News_Article.asp?NA_ID=222519

[gcdugas]

A modern F1 car's CoG is well below the axle line.


Anti dive geometry, stiff suspension and third springs, all help maintain the attitude of the car and contribute to limit weight transfer caused by excessive pitching or front end dive, but in F1 these are more about maintaining the correct aerodynamic ride height and dealing with the aerodynamic loads.

These two things cannot be. If the CG is below the axle line there would be no dive, even the opposite (rise) as you alluded to....

Now switch it around so that the weight of the car is below ground and put a channel under the car what happens when it brakes now Question you still have some weight transfer but where Wink which wheels load up and which wheels unload Question

I mentioned this before...

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.

Additionally, we know that F1 cars use anti-roll bars. If the CG wasn't above the axle line, then there would be no need for anti-roll bars or anti-dive geometry. Moreover, were it possible for F1 designers to get the CG that low, they would refrain from doing so as the ideal is to have it at the same height as the axle line. If they were able to place it at the axle height, we would see intricate fuel tank systems that would use two tanks, one mounted above the axle line and another below it with scavenging systems that "balanced" the tanks like we see on aircraft, or ships.

Even with 80+ Kg ballast mounted as low as possible, there is not one F1 car that has its CG below the axle line as you claim. They would never want that were it achieveable.

However your points on aero loadings are well taken.

[gcdugas]

gcdugas' replies placed in context and in red.

[ I think these (and a low CG) are the reasons for NickT arguing about a less asymetric braking force than the one you estimate.

I agree that it is less than 75/25 by far, but the front will do more than 50% in any car that has a CG above the axle height, period.

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).

Point well taken, but why the 20Kw limit? Is that another example of Max's arbitrary rules? If so, then his attempt to make F1 "green" will surely go over as well as his goofy "fuel burn" quali.

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

Because it was inherited from previous generations in an unquestioned manner as was 4WS (4 wheel steering). Both of these prohibitions should be lifted. But Max is stubborn, and without real vision.

Ask yourself this... what will the average car look like in 50 yrs time? 4WS? Certainly. 4WD? Of course, most likely it will be "electromotive" (like trains) with a motor at each wheel and instantaneous electronic torque management. Regenerative braking will be the inverse of this with brake torque anti-skid and torque steer. Energy storage, and light weight motors will be the chief frontiers for technological gain. The IC engine will remain (possibly a hydrogen variant) connected directly to a generator. Road cars will be equipt with crash avoidance technologies that won't allow tailgating, detect electronically when vehicles in front are decellerating and navagation systems etc.

This is not "pie in the sky" dreaming. It is "manifest destiny" (pardon the term). It is a concensus among all in the automotive industry as well. Now, if Max wants F1 to retain its standing as the "pinnacle of motorsport", then he had better do some serious rethinking and allow these technologies to be developed in F1 or that will happen elsewhere and F1 will suffer diminished status, even as push-rod carburated NASCAR is scoffed at technologically. The technology will likely find a home in LMP cars. F1's "crown" as the "pinnacle of motorsports", even the self-congratulatory "World Driving Championship" are largely determined by public perception. You can call it the "World Driving Championship" and the "pinnacle of motorsports" all you want, but if it is reduced to just another spec racer series, then the public will look elsewhere for the "pinnacle". And LMP is the chief rival for that crown. Just look at the huge PR splash Audi has gotten with its "hi-tech" diesels? F1 does not have a monopoly as the home of "hi-technology", it enjoys what reputation it has only to the degree that it is the home of "hi-technology". When that changes (in the public mind), then F1 will be just another series.

[Carlos]

Nick T. I think your onto something with " Amodern F1 cars Cof G is well below the axle line''--I think you are hinting at " polar moment" and "roll centre"---you definitely have something there. What is often thought of as the most important factor---roll centre----

" Setting aside the question of weight, the chief factor influencig roadholdinng through the suspension geometry is the rool centre, which must be considered a not merely as a single point-like the centre of gravity-bbut as two separate roll centres,front and rear. Ove the past few years racing practice has seen a trend towards lowering roll centres at both front and rear, but there is still some divergence of opinion on the subject and thereis certainly no specific rule to follow when deciding roll centres. The chief question in this case is to decide between the benefits of low roll centres-which reduce lateral deflection of the wheels to a minimum at the expense of very considerable lateral wheel deflection on bump. High roll centres may also be impractical due to the shortness and angularity of the linkages involved. Whichever layout is choosen, it is desirable to restrict roll centre movement to a minimum to maintain relatively constant weight transfer.

As suspension linkages are the product of roll centres, this is obviously one of the first basic design features which must be decided before a car can be made. In the extreme, very low roll centres lead to large roll angles and suspension being used up by roll. This leads to difficulties when suspension movement is needed to cope with uneven road surfaces. High roll centres, with their reduced roll angles, bring the attendant disadvantage of considerable lateral disturbances of the tyre tread contact patches,which leads to marked instability in cornering. Both extremes obviously have major disadvantages, and it is thus necessary for the designer to compromise in order to obtain the best of both features. As can be seen in current advanced designs, the best way of achieving the optimum is to use fairly low roll centres in conjunction with anti-roll barsThe latter howerver, require considerable study, or prolonged practical experiment before the optimum anti-roll stiffness can be achieved."

Regards Carlos

[Reca]

Here the speed vs time of Massa’s first chicane braking at Monza during the qualifying lap. Black is what I obtained from this lap (http://www.youtube.com/watch?v=VcUGC_GoVnc) and green is a 3rd order polynomial approximation I used for the calculation :
[IMG:150:128]http://img56.imageshack.us/img56/4339/s ... or6.th.jpg[/img]
And that’s consequently the acceleration vs speed :
[IMG:150:128]http://img146.imageshack.us/img146/4884 ... tq5.th.jpg[/img]
Now a few educated guesses about the parameters.

Mass 625 kg (Massa stopped on lap 19 hence at that point in qual had probably 8-9 laps of fuel, 25 kg sounds about right as fuel load)

Wheelbase 3 m.

Static weight distribution F/R 0.43/0.57

CG height 0.23 m. (that’s what I heard from a reliable source about 3 years ago)

Density = 1.15 kg/m3 (according to data I have temp was about 28-30°C and pressure 1003 mbar)

SCd = 0.8 m2
(meaning top speed 346 km/h needs about 580 hp only for aero, adding the roughly 100-120 for rolling resistance and the transmission efficiency we should be more or less in the ballpark of current F1 engine power)

Efficiency = 3

Static aero distribution F/R 0.4/0.6 (I’ll consider it constant during the braking)

Using these parameters and the velocity vs time I calculated a first order approximation of vertical load distribution on the wheels, from the moments of weight, inertial force and downforce. I didn’t consider the moment generated by aero drag because honestly I have no idea where an equivalent centre of pressure would be in term of height. Nevertheless consider that that moment goes in the opposite direction compared with the moment of inertial force so if anything it would reduce load at the front.

Here the result of vertical load percentage at the front as function of speed :
[IMG:150:128]http://img56.imageshack.us/img56/8292/s ... nc8.th.jpg[/img]

Obviously that’s a first order approximation but shouldn’t be off by much, I tried different simulations varying the parameters I’m less sure of and keeping them in reasonable range the result didn’t change by much, peak varying by 2-3% at max.
Even using “crazy” data for the parameters the result is most sensible to, as for example a very high 0.3 m for cg height and efficiency as low as 1.5 (hence half the downforce estimated in the above example) the maximum load at the front was always under 65%.


At the end an interesting thing I noticed when I went to the track the Friday of the Gp, watching GP2 cars at the first chicane it was quite usual to see the blockage of the rear wheel :
[IMG:150:128]http://img269.imageshack.us/img269/3591 ... qy2.th.jpg[/img]
(something that in F1 you hardly see since the throttle is carefully controlled to use the engine like sort of ABS for rear wheels).
In fact it’s impressive the difference in the “strategy” to attack the chicane between F1 and GP2 cars, it’s difficult to explain (particularly in English...), but GP2 included lot more sliding, it was like a smooth continuous action with the impression that the driver was about to lose it any moment, while for F1 cars it was more “stop & go” and with the rear end constantly glued to the track. The impression of “stop & go” is then enhanced by the fact that in the central part of the chicane F1 cars have tremendous traction in spite of the low speed, the car gains km/h in very few meters while GP2 cars don’t have a similarly brutal acceleration so between the right and left turn there’s no the evident change of speed and that contributes to make the whole action smoother.

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.

Last time I checked the braking forces were applied at the tyres contact patch, hence at the ground, not at wheel axle.

This mean that what you say above is true when the reference is the ground, not the wheel axle.
There will be always weight transfer unless the cg is at ground level, and there will be weight transfer to rear only if the CG is below the ground. Both these things not really easy to achieve...
Obviously that’s the weight transfer related only to cg position and car wheelbase. Once these parameters are set that weight transfer for a given acceleration is set.

The dive/squat is another matter altogether and is conceptually similar to roll. In presence of body movement, being it roll or pitch, you have a further weight transfer.
In braking the inertial force applied to the cg will make the body rotate around a certain point that we can call “pitch centre”, the position of this point related suspension geometry. If that point is below the CG, in braking the front of the car will point down, if the “pitch centre” is above the CG in braking the nose will point up, if the pitch centre coincides with CG the car will remain flat.
Same concept applies to acceleration and same concept applies to roll.

The whole antidive/antisquat design of suspensions is related to put the pivot of the body rotation as close as possible to CG so to minimize body movement and minimize that further weight transfer that is related to body movements.
Anyway all of that has no influence on the basic weight transfer related to car’s longitudinal/lateral acceleration, that one is absolutely independent by suspension geometry and by body movement. To reduce it there are only two things you can do, reduce CG height (and there’s no “too low” for it) or increase wheelbase/wheeltrack.

[NickT]

WOW

Thank you Reca, an excellent explination as always Even better considering your 1st language is not English

Your coments on the relative cornering styles of F1 and GP2 were quite interesting. I wonder if it comes from the ratio of aero grip relative to mechanical grip. Hmmm... will have to ponder some more

gcdugas your enthusiasm and persistance is comendable but very rarely is anything as black and white as it first seams in F1, sometimes you have to look behond the obvious to understand what is really going on.