Why The Increase in Rake?

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Blaze1
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Why The Increase in Rake?

Post by Blaze1 » Sun Aug 30, 2015 4:18 pm

Why has there been an increase in rake recently? I was looking at some images of F1 from the early to mid 90's and back then they ran very little rake e.g:
Image
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Compared to:
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In GP2 and other series that use venturi tunnels, the cars generally run very little rake, so I wonder if the basic aero configuration of the car plays a large role. So if current F1 cars had flat bottoms as used in 1993, would they still run such aggressive levels of rake, using the front wing and other vanes to seal the floor?
Last edited by Blaze1 on Sun Aug 30, 2015 9:45 pm, edited 1 time in total.

Silent Storm
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Re: Why The Increase in Rake?

Post by Silent Storm » Sun Aug 30, 2015 4:27 pm

Correct me If I'm wrong, current cars have higher rake to make the diffuser area larger which was not needed in earlier cars as it had larger diffuser as the rules allowed it.
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Blaze1
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Re: Why The Increase in Rake?

Post by Blaze1 » Sun Aug 30, 2015 4:54 pm

Silent Storm wrote:Correct me If I'm wrong, current cars have higher rake to make the diffuser area larger which was not needed in earlier cars as it had larger diffuser as the rules allowed it.
Even when the size of the diffuser was reduced, it was a number of years before any noticeable rake increase as far as I remember from images.

Engineers are always looking for more downforce, so I would have expected them to pursue this avenue if it was possible.

bhall II
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Re: Why The Increase in Rake?

Post by bhall II » Sun Aug 30, 2015 8:16 pm

Adding rake lowers the front wing, which enhances ground effect. It has an adverse impact on the diffuser, because aerodynamic components are more efficient (to a point) at lower ride heights. However, the resulting increase in diffuser AoA (somewhat) makes up for the loss.

Given a flexible t-tray, the front of the floor also benefits from added rake.

Image
Last edited by bhall II on Sun Aug 30, 2015 8:22 pm, edited 1 time in total.

wesley123
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Re: Why The Increase in Rake?

Post by wesley123 » Sun Aug 30, 2015 8:22 pm

Like said, adding rake would increase the effective area. Simply said; The whole floor becomes a diffuser.

However, the problem then is that this diffuser has the issue of air bleeding in, which reduces it's effectiviness. In the 90s this was a bit of unknown ground, but these days with such complex aero it becomes possible to perfectly shed vortices to create a "wall" that would provide a seal for this. This has made running large amounts of rake possible, where in the 90s this wouldn't have been possible(of course it would be possible, but it wouldn't work).

Another benefit of the rake is, like bhall said, that the front wing can be ran closer to the ground, so that it too can benefit from ground effect(or better said, benefit more).
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Just_a_fan
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Re: Why The Increase in Rake?

Post by Just_a_fan » Sun Aug 30, 2015 10:32 pm

Don't forget that until the high rear wing we have now, the diffuse central section extended further rearwards and was fed by holes in the side s. These linked the beam wing and rear diffuser much more effectively. It's likely that the teams didn't play with rake because they already had all of the rear-end-produced downforce they could use.
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bhall II
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Re: Why The Increase in Rake?

Post by bhall II » Mon Aug 31, 2015 12:08 am

wesley123 wrote:Like said, adding rake would increase the effective area. Simply said; The whole floor becomes a diffuser.
I know that's often said - I've mentioned it a few times myself - but it doesn't make sense to me anymore.

As seen on the velocity gradient below, air flow under the floor is accelerated in two places: the leading edge and the kink line of the diffuser. From Bernoulli, it then follows that those areas are where static pressure is lowest, thus where downforce is highest.

Image

However, air flow slows down between them due to viscous effects, and that causes a decrease in underbody efficiency, because it creates a bottleneck that limits mass flow through the diffuser. (Adding rake further complicates matters, since the resultant expansion slows down air flow even more.) This produces a very messy pressure gradient as freestream flow (high-pressure) is accelerated under the leading edge of the floor (low-pressure), stagnates around the middle of the floor (higher pressure), accelerates again through the kink line of the diffuser (lower pressure), and is finally returned to the freestream (high-pressure). It's true of any flat-bottom car, but it doesn't necessarily have to compromise overall performance.

If you can run an ultra-low ride height, along with a diffuser that has the capacity to handle underbody flow at the same rate in which its introduced into the system, you can reduce the size and scope of the area of stagnation. Combined with regulations that allowed the front wingtips to be as close to the ground as every other part of the car, that's why pre-1995 cars didn't need rake.

The current stepped floors and small diffusers make it very difficult to manage air flow in that fashion, though. So, teams now use barge boards to generate the so-called "sealing" vortices that create a sort of wall across leading edge of the floor. You can see the effect below...

Image

LMP1 cars use more or less the same strategy...

Image

EDIT: It's sorta like how overall downforce potential is dictated by the front wing, and the effect of the front wing is limited by the rear wing. Here, underbody performance is dictated by the floor, and the effect of the floor is limited by the diffuser.

(There's an actual photo of the illustration above that makes the function of the barge boards much more apparent. I just can't find it again.)
Last edited by bhall II on Mon Aug 31, 2015 10:18 am, edited 1 time in total.

Advino116
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Re: Why The Increase in Rake?

Post by Advino116 » Mon Aug 31, 2015 2:12 am

Correct me if I'm wrong. Could it be that with the amount of exposed floor area that modern F1 cars have, the rake turns the entire floor into an inverted wing? So apart from the diffuser effect, the pressure on top of the floor caused by the rake, or camber if we are taking the floor as an inverted wing, creates more downforce than with a flat floor. My inference comes from just one race car aero module in university that touch upon diffusers but not in a scenario where the diffuser is formed using a thin flat structure, so please correct me if this does not make sense.

mertol
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Re: Why The Increase in Rake?

Post by mertol » Mon Aug 31, 2015 9:41 am

In the 90s and 2000s the rules lifted the minimum front wing height a few times. It's only logical to increase rake to compensate for that. At least that is my view on the things.

At first they used loopholes to have a lower central part of the wing like this:
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Image

misterbeam
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Re: Why The Increase in Rake?

Post by misterbeam » Tue Sep 01, 2015 9:05 pm

I think they want to improve turn in by generating oversteer at low speeds (diffuser loss due to ride height), and balance it a bit with sidepods coanda effect to make it less agressive, since the effect won't work at low rake they want the car to have quite high one when stand still and even higher on heavy braking.

Robbobnob
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Re: Why The Increase in Rake?

Post by Robbobnob » Wed Sep 02, 2015 12:43 am

Personally I think it has more to do with the ever increasingly developed front wings. Teams have now got them so finely balanced that a 1mm change in flap height has a huge knock on effect to the vortices being shed.

Back in the day, teams didn't have the computing power to solve dynamic CFD in yaw / variable ride height and the effect this will have on the vortex performance. Now it is safe to assume that this is all standard investigation, with this know how largely exploited by the development of the Coanda exhaust sealing of the diffuser edge, being a transient phenomenon and not a steady state such as straight line aero performance.

Look at the Mercedes, photos showed above and the details in the Car page, the detail to the vortex creation is the primary goal with the front wing, as they only have to balance the front D/F with that generated at the rear for aerodynamic centre stability.

Would they rung a flat plane lower if they could. Somehow I doubt it. You would probably see a similar approach being taken, running angles of rake designed to interact with sealing vorticies and the likes.
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trinidefender
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Re: Why The Increase in Rake?

Post by trinidefender » Wed Sep 02, 2015 4:29 pm

bhall II wrote:
wesley123 wrote:Like said, adding rake would increase the effective area. Simply said; The whole floor becomes a diffuser.
I know that's often said - I've mentioned it a few times myself - but it doesn't make sense to me anymore.

As seen on the velocity gradient below, air flow under the floor is accelerated in two places: the leading edge and the kink line of the diffuser. From Bernoulli, it then follows that those areas are where static pressure is lowest, thus where downforce is highest.

http://i.imgur.com/WXQ6lQP.jpg

However, air flow slows down between them due to viscous effects, and that causes a decrease in underbody efficiency, because it creates a bottleneck that limits mass flow through the diffuser. (Adding rake further complicates matters, since the resultant expansion slows down air flow even more.) This produces a very messy pressure gradient as freestream flow (high-pressure) is accelerated under the leading edge of the floor (low-pressure), stagnates around the middle of the floor (higher pressure), accelerates again through the kink line of the diffuser (lower pressure), and is finally returned to the freestream (high-pressure). It's true of any flat-bottom car, but it doesn't necessarily have to compromise overall performance.

If you can run an ultra-low ride height, along with a diffuser that has the capacity to handle underbody flow at the same rate in which its introduced into the system, you can reduce the size and scope of the area of stagnation. Combined with regulations that allowed the front wingtips to be as close to the ground as every other part of the car, that's why pre-1995 cars didn't need rake.

The current stepped floors and small diffusers make it very difficult to manage air flow in that fashion, though. So, teams now use barge boards to generate the so-called "sealing" vortices that create a sort of wall across leading edge of the floor. You can see the effect below...

http://i.imgur.com/u9Fq2Xd.jpg

LMP1 cars use more or less the same strategy...

http://i.imgur.com/jsTp4t3.jpg

EDIT: It's sorta like how overall downforce potential is dictated by the front wing, and the effect of the front wing is limited by the rear wing. Here, underbody performance is dictated by the floor, and the effect of the floor is limited by the diffuser.

(There's an actual photo of the illustration above that makes the function of the barge boards much more apparent. I just can't find it again.)
While I can kinda (I think) see where your theory is going it does not take into account the rear wheels and a few other factors. Let us imagine a flat floor that is parallel to the road surface (rake = 0). Now let us introduce a flow to the leading edge of the floor. As this stream moves backwards it will encounter the blockage of the rear wheels which will slow the airflow down for the whole forward portion of the floor and increase the pressure. The airflow will then accelerate toward the diffuser dropping its pressure again. That would create a situation that with a perfectly flat floor you will have downforce being generated at the rear through the diffuser and lift created at the front.

From what I can see the floor is operated like an impromptu Venturi. There is a constriction at the front by the lower ride height only allowing in a certain amount of airflow. As the air flows back along the floor it is constantly expanding and therefore has a drop in pressure (assuming a sufficient seal at the edge of the floor) until coming to the restriction of the rear tyres.

By increasing rake you also increase the volume of the diffuser and force a larger expansion of the airflow therefore resulting a loss of pressure before joining again with the higher pressure free stream.

Since 2009 these cars have been known to be largely rear downforce limited. This may sound counterintuitive but IMO that is the exact reason why the front wings have gotten so complicated. The front wings have 3 main jobs now. Create front downforce, the ever more important role of control airflow the floor and thirdly the control front tyre wake (which is also to control airflow around the floor).

Of course the benefit of lowering the front wing to take advantage of running the wing in closer ground effect is always there through an increase in rake however considering these cars are rear downforce limited it seems prudent that a team will do something that may hurt rear downforce (such as you are suggesting) to increase front downforce. (The 'neutral' centre section of the front wing will also start to help contribute to front downforce with rake and you are therefore increasing the useful span of the wing)

Lastly let us not forgot that when you increase rake you also increase the centre of gravity at the rear of the car. With how the cars are now and the amount of rake you see them running then lifting the whole rear of the car that much can raise the centre of gravity centimetres. That seems like a very large deficit just to run the front wing closer to the ground.

bhall II
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Re: Why The Increase in Rake?

Post by bhall II » Thu Sep 03, 2015 3:23 am

Yeah, I understand that's the prevalent line of thought. I just don't agree with it anymore.

For one thing...
As the air flows back along the floor it is constantly expanding and therefore has a drop in pressure (assuming a sufficient seal at the edge of the floor) until coming to the restriction of the rear tyres.
...expansion itself raises static pressure per Bernoulli, because it reduces dynamic pressure. That's the point of a diffuser: to gently return air flow from the area of low pressure under the car to the higher pressure of the freestream.

Image

To add rake is to effectively transform the leading edge of the floor into a diffuser kink line behind which static pressure is constantly increasing. That doesn't sound very efficient, yanno? So, it makes more sense, to me anyway, to assume that adding rake is a compromise solution.

Benefits of lower front wing ride height:
McCabism wrote:The most interesting conclusion of van den Berg's research was that the front-wheel drag is greater at high front-wing ride-heights than it is at low ride-heights.

Image
Figure 1: High ride-height, high wheel drag

Previous research conducted by James McManus (who was snapped up by McLaren before completing his PhD) had identified that the flow field of an isolated rotating wheel contains an arch vortex in the upper region of the near wake (E and F in Figure 1), and a pair of counter-rotating vortices in the lower, ground-level region of the near wake (H and I). There is also a bow wave (D) created by the upstream side of the contact patch.

When an inverted wing equipped with an endplate is placed in front of such a rotating wheel, van den Berg identified three further primary flow features: a vortex from the upper edge of the endplate (A); a vortex from the junction between the trailing edge of the flap and the endplate (B); and a vortex from the lower edge of the endplate (C).

With a 50% scale 580mm front wing-span (relevant to pre-2009 F1 regulations), van den Berg identified that the top edge front-wing vortex passes over the crown of the wheel at high ride-heights (Figure 1), but passes inside the wheel at low ride-heights (Figure 2). At high ride-heights this vortex over the crown keeps the flow attached for longer, increasing the lift of the wheel, and creating a zone of re-circulation (G) behind the wheel, which increases the wheel drag:

"When this vortex...passes over the wheel it starts a strong interaction with the wheel vortex originating from the top of the wheel (feature “F”), the vortex originating from the flap trailing edge (TE) junction (feature “B”) and the lower edge vortex (feature “C”), accumulating in a strong circulation," (Journal of Fluids Engineering, October 2009, Vol. 131).

Figure 2 shows the flow field at a lower front-wing ride-height, where the top-edge vortex goes inside the wheel. In addition, it can be seen that both the bow wave to the inboard side of the wheel, and the inside leg of the counter-rotating vortex pair in the wheel wake, have been replaced by the vortex generated by the bottom-edge of the front-wing, which is strengthened in ground-effect.

Image
Figure 2: Low ride-height, low wheel drag
McCabism wrote: Red Bull, McLaren and Ferrari currently [as of August 2011] appear to be converging on the same aerodynamic solution: a high-rake, nose-down stance to maximise the ground effect component of front-wing downforce, (with the use of exhaust-blown diffusers to retain rear downforce). Front-wing ground effect has always had a role to play, but the current emphasis is perhaps a consequence of the new technical regulations introduced for the 2009 season, which permitted the front-wing to be much closer to the ground.

To understand front-wing ground effect, it's worth revisiting some research performed by Zhang, Zerihan, Ruhrmann and Deviese in the early noughties, Tip Vortices Generated By A Wing In Ground Effect. This examined a single-element wing in isolation from rotating wheels and other downstream appendages, but the results are still very relevant.

The principal point is that front-wing ground-effect depends upon two mechanisms: firstly, as the wing gets closer to the ground, a type of venturi effect occurs, accelerating the air between the ground and the wing to generate greater downforce. But in addition, a vortex forms underneath the end of the wing, close to the junction between the wing and the endplate, and this both produces downforce and keeps the boundary layer of the wing attached at a higher angle-of-attack.

Image
Image

The diagrams above show how this underwing vortex intensifies as the wing gets closer to the ground. In this regime, the downforce increases exponentially as the height of the wing is reduced. Beneath a certain critical height, however, the strength of the vortex reduces. Beneath this height, the downforce will continue to increase due to the venturi effect, but the rate of increase will be more linear. Eventually, at a very low height above the ground, the vortex bursts, the boundary layer separates from the suction surface, and the downforce actually reduces.
Lower drag, higher downforce. Sounds great to me!

What makes this possible is that, in my view, teams have largely abandoned the diffuser as a source of downforce. (Yes, you read that correctly.)

First, and just to keep us on the same page - even if it's the wrong one - if "sealing" the diffuser means excluding air flow from outside, I think it's important to note that teams have never used exhaust gasses for this purpose. Here's why...

Image
Ground Effect Aerodynamics of Race Cars by Zhang, Toet, Zerihan

The vortices mentioned above provide force enhancement identical to that which is created by the vortices found on the front wing. (After all, current front wings are diffusers.)

Image

If the diffuser is "sealed," it cannot produce them.

You can see below how Red Bull initially extended downward the rear wing end plates of the RB5 in order to maintain a minimal diffuser ride height, which maximizes downforce. To compensate for the loss of volume - compared to the Ferrari shown at the top - the team used venturi channels expressly to bring in air flow from outside the diffuser to feed the vortices...

Image

In the same manner, EBDs energized those vortices, and that's what allowed higher rake angles...

Image

(If, in fact, that's what it means to "seal" the diffuser, then that's fair enough. Sometimes it's just difficult for me to understand the reasons behind the nomenclature some folks use.)

I think you're correct to point out that the cars are rear-limited. Check out Red Bull's diffuser...

Image

It's virtually nothing. And without a beam wing to help "drive" it, Red Bull now uses the brake duct fins like little beam wings. You can see it in the FlowVis streaks.

But, if you feed it with air flow at a rate higher than can be vented, you'd just increase static pressure under the car. The solution then is to get rid of the excess air flow, and it's been that way for a while; we just weren't paying attention...

Image
W03

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W06

(Naturally, air flow doesn't lend itself to 2D representation. So, just take the above images to be highly generalized depictions of what I think is happening.)

Since the regulations no longer allow for strong diffusers, why fight it if there are benefits to be had elsewhere? Direct toward it enough air flow to keep it from stalling, because it still has to deal with everything the barge boards don't divert and with any changes that occur in yaw. But, other than that, current diffusers seem to be pretty pointless.

Besides, focusing on the front of the floor allows the center of pressure to be brought forward...

Image
That's downforce, too.

tuj
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Re: Why The Increase in Rake?

Post by tuj » Thu Sep 03, 2015 5:59 am

I think the answer is simpler than the wonderful CFD we have seen so far.

Under the current rules regime, the best *compromise* setup involves a relatively soft rear spring setting, but then that will almost bottom out on the highest down-force parts of the circuit (as you can see the Ti skid sparking in the pic above). Red Bull won 4 championships with a ridiculously rakish car and sure part of that was the blown diffusor and then the coanda, but I think you get several benefits running rakish:

-more rear suspension travel, allowing either higher rear downforce or better slow-speed acceleration.
-more volume for the diffusor to work with.

I think it has been said that some teams 'stall' the diffusor at high speeds. What would make sense to me is a diffusor designed to run in rakish conditions that then cannot flow fast enough when the rear suspension compresses.

bhall II
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Re: Why The Increase in Rake?

Post by bhall II » Thu Sep 03, 2015 10:56 am

I think what really helped Red Bull's cause was a F2007-style flexible t-tray...

Image
Racecar Engineering, Ferrari F2007 Secrets wrote:"The front floor is attached to the chassis via a mechanical hinge system at its most rearward point. The most forward support is a body with one compression spring and one tension spring inside which can be adjusted according to the amount of mass that is fitted to the front floor. There is also a skirt that seals the floor to the chassis, which is made out of rubber and Kevlar to help flexibility and reduce friction in the system."

The system detailed by Stepney allowed the F2007 to ride kerbs harder due to the 14-15mm deflection at the leading edge of the floor, which means the Ferraris could straight line chicanes more than other chassis. Front plank wear would also be reduced, allowing the car to run lower at the front, giving an aerodynamic gain.

Stepney also explains the dynamic behaviour of the car, and the advantages the flexing floor gives: "From around 160-180km/h (100-112mph) the car is about 7-8mm lower at the leading edge of the floor, which multiplies up to nearly 19-20mm lower front wing height. The benefits in terms of ground effects and efficiency would be gained all around, with components like turning vanes [barge boards] and front wings at a reduced height relative to the ground."
Lower is pretty much always better when it comes to downforce, which is why I don't know that designing a diffuser to stall at lower ride heights is viable. You can stall it by lifting it, though. In fact, the FW15 had a push-to-pass button that lifted the car on-demand in order to reduce diffuser efficiency and cut drag. It was sorta like "underbody DRS."

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Image
-more rear suspension travel, allowing either higher rear downforce or better slow-speed acceleration.
That bit makes sense, even if it's not quite for the reason intended.

If the tires compel the use of very compliant suspension settings in order to last longer than a lap or two - lookin' at you, Pirelli - it actually would make sense to run the diffuser at a higher ride height, because downforce at lower ride heights is more sensitive to variation.

With a ride height of 10cm, for instance, a 2cm reduction amounts to a 20% change. On the other hand, a 2cm reduction with a 15cm ride height is only a 13.3% change.