Mercedes W13

[TimW]

Will a introduction of side pods mean they have to change the side impact structure and get the chassis re-certified by the FIA?

I'd guess the current location at the mirror support wing will be suboptimal if they go to a sidepod, so probably yes.

[atanatizante]

I would hope not, but as far as I understand it one of the basic underlying principles of creating downforce/lift is the difference in the distance travelled of the air molecules around the object that is moving through them; which does point to more side-pod being beneficial. I know that this is not 100% correct when the object is in very close proximity to the ground, but creating that pressure differential must still be key?

That's correct in general, but it's not really applicable like that for sidepods. Zero-pods leave too much of the rear of the floor exposed (in front of rear tyres) and this means you have no bodywork to support the downforce created by the floor, something I wrote here

***

In wind tunnel, W13 might be the fastest car, but the WT model is still just a model and it can't reflect the actual car 100%. With these rules, this showed with very different model floor stiffness compared to actual floor stiffness.

From this perspective, Merc design has a clear disadvantage with such a big exposed surface. This low stiffness, coupled with potentially smaller ride-height operating window (potential suspension problem, but definitely not as big as it seemed early in the season), leaves the car set-up options compromised. Low stiffness reflects on bouncing sensitivity by providing unstable and unpredictable floor sealing and sudden gain and loss of extra downforce. If bouncing is sorted, bumps on track and roll while cornering affect the predictability of downforce, as floor deflection and vibration increase and decrease it. I'm still not convinced Hamilton and Russell crashed in Austria Q because they pushed too far and not because of mid-corner downforce loss.

To take care of this, the only sensible solution are rod stays, since cable stays don't prevent upward deflection (i.e. floor edges are still prone to vibrations, just limited in downward direction). Rod stays are a big drag penalty, unless concealed within sidepod bodywork, which is what RB was doing from day 1. Somehow Ferrari manages things with cable stays alone, but their floor is significantly less exposed in the critical area (ahead of rear wheel) than Merc. And with wide bodywork, you can fit as many stays as you want, which can also reduce floor weight while increasing stiffness.

***

Merc solved this by raising floor throat height a lot with Barcelona upgrade, reducing overall downforce a lot, and have done a number of other improvements since then to get some of that downforce back. Good pace in Mexico was down to smaller drag penalty than usual, while Brazil race win was also down to top contenders for the win being out of the battle for victory. AD was a reality check, as they had quite a lot of drag to compensate floor downforce loss by running the big wing, while RB and Ferrari could rely on floor downforce more than Merc and use far smaller wings. Even if Merc lowered the car (like Ferrari), evident by experiencing bouncing again after a lot of races.

Hi, Vanja #66!
I might be wrong so feel free to amend my presumptions coz I think that W13 issues were at least three :

1. The first one was that their theoretical DF was maybe the biggest of all cars but they were stuck with that low travel suspension designed just to run the car as low as possible. Unfortunately, the majority of the race tracks have bumps and kerbs and in conjunction with the porpoising effect, they were forced to rise the car with a loss in overall DF. Thus in reality they were having now less DF than both RB18 and F1-75 cars. (Having said that could be possible that with a rake car RB18 now generates less DF (but a constant and predictable one) from the floor/Venturi tunnels but overall, in reality now is bigger than what W13 has due to increased DF generated by the diffuser same manner last year's cars were producing?)

2. Their floor edge "furniture" developed in order to seal the floor or maybe Venturi tunnels were less effective with this raised car, generating now lower and inconstant/unpredictable DF levels.

3. Design a no-pod car they exposed the rear tyres more to the front airflow thus their issue with the drag on the straights. Could this be solved with higher pods or with the little help of the bulges on the Venturi channels entrance slopes in conjunction with the floor furniture?

[hollus]

…Zero-pods leave too much of the rear of the floor exposed (in front of rear tyres) and this means you have no bodywork to support the downforce created by the floor…

…Good pace in Mexico was down to smaller drag penalty than usual…

Wouldn’t if follow that the good pace in Mexico and Brazil could be down to lower downforce (for everybody) due to the lower air density?
Lower drag is part of it, sure, but their floor stiffness stays the same and thus can finally handle the (lesser) downforce as intended?

[Vanja #66]

Hi, Vanja #66!
I might be wrong so feel free to amend my presumptions coz I think that W13 issues were at least three :

1. The first one was that their theoretical DF was maybe the biggest of all cars but they were stuck with that low travel suspension designed just to run the car as low as possible. Unfortunately, the majority of the race tracks have bumps and kerbs and in conjunction with the porpoising effect, they were forced to rise the car with a loss in overall DF. Thus in reality they were having now less DF than both RB18 and F1-75 cars. (Having said that could be possible that with a rake car RB18 now generates less DF (but a constant and predictable one) from the floor/Venturi tunnels but overall, in reality now is bigger than what W13 has due to increased DF generated by the diffuser same manner last year's cars were producing?)

2. Their floor edge "furniture" developed in order to seal the floor or maybe Venturi tunnels were less effective with this raised car, generating now lower and inconstant/unpredictable DF levels.

3. Design a no-pod car they exposed the rear tyres more to the front airflow thus their issue with the drag on the straights. Could this be solved with higher pods or with the little help of the bulges on the Venturi channels entrance slopes in conjunction with the floor furniture?

1 and 2 are basically the part of the same merry-go-round that is the core of the problem - inconsistent downforce. If they have too much, the floor flexes, gets lower, dowforce gets even bigger and bouncing starts. If they have too little, they aren't fast enough. Limitations to suspension design also playes a role in having this problem occur the entire year.

3, rear tyre drag can be solved in several ways I think, we will have to wait and see which approach Merc chooses for W14.

Wouldn’t if follow that the good pace in Mexico and Brazil could be down to lower downforce (for everybody) due to the lower air density?
Lower drag is part of it, sure, but their floor stiffness stays the same and thus can finally handle the (lesser) downforce as intended?

Yeah, I wasn't clear enough. What I meant was - having lower drag penalty than usual meant their usual massive rear wing (compensating for lack of floor downforce) wasn't such a big problem compared to others as it usually is. Lower downforce for the floor was a bonus. RB was in control the whole time though, so Merc was "just" closer than usual.

[maxxer]

So if you design the venturi inlets so that when the car is almost bottoming out they let less air in the problem would be solved?

[Vanja #66]

So if you design the venturi inlets so that when the car is almost bottoming out they let less air in the problem would be solved?

The only time that happens is when the throat is choked - and that's how bouncing starts

[Hoffman900]

So if you design the venturi inlets so that when the car is almost bottoming out they let less air in the problem would be solved?

The only time that happens is when the throat is choked - and that's how bouncing starts

I don’t believe choke flow causes bouncing. That’s never been confirmed anywhere and if anything been said that is what isn’t happening.

Bouncing was caused by vortex shedding / flex / flutter and overpowering the sidewalls of the tires.

[wesley123]

So if you design the venturi inlets so that when the car is almost bottoming out they let less air in the problem would be solved?

The only time that happens is when the throat is choked - and that's how bouncing starts

I don’t believe choke flow causes bouncing. That’s never been confirmed anywhere and if anything been said that is what isn’t happening.

I believe Vanka referred to porpoising, which most definitely starts because of this.

[atanatizante]

Hi, Vanja #66!
I might be wrong so feel free to amend my presumptions coz I think that W13 issues were at least three :

1. The first one was that their theoretical DF was maybe the biggest of all cars but they were stuck with that low travel suspension designed just to run the car as low as possible. Unfortunately, the majority of the race tracks have bumps and kerbs and in conjunction with the porpoising effect, they were forced to rise the car with a loss in overall DF. Thus in reality they were having now less DF than both RB18 and F1-75 cars. (Having said that could be possible that with a rake car RB18 now generates less DF (but a constant and predictable one) from the floor/Venturi tunnels but overall, in reality now is bigger than what W13 has due to increased DF generated by the diffuser same manner last year's cars were producing?)

2. Their floor edge "furniture" developed in order to seal the floor or maybe Venturi tunnels were less effective with this raised car, generating now lower and inconstant/unpredictable DF levels.

3. Design a no-pod car they exposed the rear tyres more to the front airflow thus their issue with the drag on the straights. Could this be solved with higher pods or with the little help of the bulges on the Venturi channels entrance slopes in conjunction with the floor furniture?

1 and 2 are basically the part of the same merry-go-round that is the core of the problem - inconsistent downforce. If they have too much, the floor flexes, gets lower, dowforce gets even bigger and bouncing starts. If they have too little, they aren't fast enough. Limitations to suspension design also playes a role in having this problem occur the entire year.

3, rear tyre drag can be solved in several ways I think, we will have to wait and see which approach Merc chooses for W14.

Wouldn’t if follow that the good pace in Mexico and Brazil could be down to lower downforce (for everybody) due to the lower air density?
Lower drag is part of it, sure, but their floor stiffness stays the same and thus can finally handle the (lesser) downforce as intended?

Yeah, I wasn't clear enough. What I meant was - having lower drag penalty than usual meant their usual massive rear wing (compensating for lack of floor downforce) wasn't such a big problem compared to others as it usually is. Lower downforce for the floor was a bonus. RB was in control the whole time though, so Merc was "just" closer than usual.

I thank you, again, Vanja #66, for your quick and summary replays!

One thing still didn`t give me enough satisfaction, bearing in mind the whole rumours stated on my country forum for F1: with the rake of the RB18, it was an increase in diffuser`s DF and in addition to what the floor/Venturi tunnels were producing, in the end, they had overall the biggest DF from all of the cars?

[Hoffman900]

The only time that happens is when the throat is choked - and that's how bouncing starts

I don’t believe choke flow causes bouncing. That’s never been confirmed anywhere and if anything been said that is what isn’t happening.

I believe Vanka referred to porpoising, which most definitely starts because of this.

Not exactly. To keep the car from bottoming out, you have to stiffen the suspension. You can only make the suspension so stiff before you over power the sidewalls.

This is confirmed by Peter Wright, James Allison when he said “porpoising is not caused by what is being said by reporters” (paraphrasing) (ie: choking), and Lewis Hamilton later saying “we had sidewall stiffness issues” (paraphrasing), as well as

I have made a big post about this before citing several people. The media parroted this choking thing and it’s stuck, but it’s not the cause, and in the sake of correct information, that theory needs to be put to rest because it was by several engineers (plus Lewis parroting what his engineers told him) actually involved in these things months ago.

[Vanja #66]

Bouncing/porpoising is an aerodynamic phenomenon that happens when you stall/choke a portion of the floor, causing it to lose downforce.

Stall/choke happens when the needed expansion of the airflow in diverging section exceeds the capabilities of current air mass flow rate, leading to boundary layer separation and its downstream propagation.

Insufficient mass flow rate happens when the floor gets too close to the ground for its given geometry and downstream-located flow structures (primarily vortices) can't provide sufficient suction - either because they are too weak (broken) or too far away.

Floor getting too close to the ground can happen due to suspension travel, tyre deformation (sidewall stiffness) or floor deformation. Chassis and suspension compliance is well under control in F1, so I think it's safe to rule them out.

[Hoffman900]

Bouncing/porpoising is an aerodynamic phenomenon that happens when you stall/choke a portion of the floor, causing it to lose downforce.

Stall/choke happens when the needed expansion of the airflow in diverging section exceeds the capabilities of current air mass flow rate, leading to boundary layer separation and its downstream propagation.

Insufficient mass flow rate happens when the floor gets too close to the ground for its given geometry and downstream-located flow structures (primarily vortices) can't provide sufficient suction - either because they are too weak (broken) or too far away.

Floor getting too close to the ground can happen due to suspension travel, tyre deformation (sidewall stiffness) or floor deformation. Chassis and suspension compliance is well under control in F1, so I think it's safe to rule them out.

Disagree Vanja, and I base that on talking to a PhD aero friend and hearing Peter Wright, James Allison, and Jean-Claude Migeot’s take. Lewis Hamilton also parroted the sidewall issue which would have come from his engineers. James Allison specifically called out the stall theory as wrong.

If stall caused this, the collapsing rear suspensions of years pass would have also caused this, and they didn’t.

[AR3-GP]

Bouncing/porpoising is an aerodynamic phenomenon that happens when you stall/choke a portion of the floor, causing it to lose downforce.

Stall/choke happens when the needed expansion of the airflow in diverging section exceeds the capabilities of current air mass flow rate, leading to boundary layer separation and its downstream propagation.

Insufficient mass flow rate happens when the floor gets too close to the ground for its given geometry and downstream-located flow structures (primarily vortices) can't provide sufficient suction - either because they are too weak (broken) or too far away.

Floor getting too close to the ground can happen due to suspension travel, tyre deformation (sidewall stiffness) or floor deformation. Chassis and suspension compliance is well under control in F1, so I think it's safe to rule them out.

Disagree Vanja, and I base that on talking to a PhD aero friend and hearing Peter Wright, James Allison, and Jean-Claude Migeot’s take. Lewis Hamilton also parroted the sidewall issue which would have come from his engineers.

If stall caused this, the collapsing rear suspensions of years pass would have also caused this, and they didn’t.

All cars have the same tires and some of them didn't porpoise. Yes, the tire sidewall has deflection under load, but sidewall deflection is simply an additional lowering of the car to be accounted for by increasing the ride height.

Ultimately, increasing the ride height is what prevented porpoising on all of the cars.

[Hoffman900]

Bouncing/porpoising is an aerodynamic phenomenon that happens when you stall/choke a portion of the floor, causing it to lose downforce.

Stall/choke happens when the needed expansion of the airflow in diverging section exceeds the capabilities of current air mass flow rate, leading to boundary layer separation and its downstream propagation.

Insufficient mass flow rate happens when the floor gets too close to the ground for its given geometry and downstream-located flow structures (primarily vortices) can't provide sufficient suction - either because they are too weak (broken) or too far away.

Floor getting too close to the ground can happen due to suspension travel, tyre deformation (sidewall stiffness) or floor deformation. Chassis and suspension compliance is well under control in F1, so I think it's safe to rule them out.

Disagree Vanja, and I base that on talking to a PhD aero friend and hearing Peter Wright, James Allison, and Jean-Claude Migeot’s take. Lewis Hamilton also parroted the sidewall issue which would have come from his engineers.

If stall caused this, the collapsing rear suspensions of years pass would have also caused this, and they didn’t.

All cars have the same tires and some of them didn't porpoise. Yes, the tire sidewall has deflection under load, but sidewall deflection is simply a lowering of the car, triggering the aero induced porpoising, which is a matter of a poor approach to a ground effect floor design.

It has to do with peak downforce. Merc was making so much that a stiffer rear suspension pushed the loads into the sidewall and over powered it. This is a big part of why this didn’t scale to the wind tunnel model and the CFD didn’t pick it up because they don’t have accurate or the ability to use accurate tire models (tires are incredibly hard to model) and why their monster of a downforce making design failed in the real world.

[Vanja #66]

Disagree Vanja, and I base that on talking to a PhD aero friend and hearing Peter Wright, James Allison, and Jean-Claude Migeot’s take. Lewis Hamilton also parroted the sidewall issue which would have come from his engineers. James Allison specifically called out the stall theory as wrong.

If stall caused this, the collapsing rear suspensions of years pass would have also caused this, and they didn’t.

So what's the mechanics of a car bouncing at high speed without oscilating levels of downforce?