2022 cars 'porpoising' at high speed

[Vyssion]

There's a couple of places where chat about the porpoising is taking place, but I just made a post in the aero sub-forum about it, so here's a link if you're interested to read it (rather than repost the whole thing here?)

click here!

[vorticism]

The frequency of porpoising reminds me more of stone skipping/skimming. Therefor I motion to rename porpoising skipping or skimming, depending on which side of the pond you are on.

[godlameroso]

The highest speed of airflow on the car is NOT in the tunnels, it's the air under the plank. That's the fastest air in the whole car, the air that goes under the plank is sealed in by the tunnels. The sealing is done by the strake generated vortices, mostly.

Like you said with your video, the plank skids across the track. You ever seen a boat or water craft with a sharp leading edge on the bow? What makes a good skipping stone? Round flat oval shapes.

That video I posted shows oval shapes aligned with the flow showed the least oscillations. So in order to create the fewest disturbances, the least oscillation you have to use elliptical, or bell shapes.

No surprise then that the best nozzles, from a thrust efficiency perspective all use bell or parabolic shapes. The parabolic, or Rao nozzle for example is very efficient for high back pressure environments.

[vorticism]

Your post makes me think of the air cushion effects of flat surfaces in close proximity. If you tip over a finely machined surface, a block of metal or plastic, onto a smooth surface, like a granite plate or something, or if you allow a sheet of plywood to fall to a floor, it never slams. Likewise lifting it back you up you will notice a vacuum resistance.

On one hand the plank may be drawn down by low pressure and the downward force of gravity and the downforce applied by the other bodywork. Conversely, when it raises back up it will require an exaggerated force to overcome the vacuum beneath it. Like when trying to separate two mirror smooth surfaces. If that upward force needed to detach the plank from the track comes from undamped forces like the tires or bodywork flexibility, that might set up porpoising/skipping.

Running zero rake might make this more of a risk. Some rake would keep the bib riding on the track, while the plank stays elevated. Think that before now the teams never combined a low-rake plank with ground effect tunnels. For a while it was flat floors plus rake, and for RB, usually a lot of rake.

[godlameroso]

Your post makes me think of the air cushion effects of flat surfaces in close proximity. If you tip over a finely machined surface, a block of metal or plastic, onto a smooth surface, like a granite plate or something, or if you allow a sheet of plywood to fall to a floor, it never slams. Likewise lifting it back you up you will notice a vacuum resistance.

On one hand the plank may be drawn down by low pressure and the downward force of gravity and the downforce applied by the other bodywork. Conversely, when it raises back up it will require an exaggerated force to overcome the vacuum beneath it. Like when trying to separate two mirror smooth surfaces. If that upward force needed to detach the plank from the track comes from undamped forces like the tires or bodywork flexibility, that might set up porpoising/skipping.

Running zero rake might make this more of a risk. Some rake would keep the bib riding on the track, while the plank stays elevated. Think that before now the teams never combined a low-rake plank with ground effect tunnels. For a while it was flat floors plus rake, and for RB, usually a lot of rake.

You could do like McLaren and use the channel built in to create some lift at the leading edge. Or like RB did and make a smooth shaped bow. From what I can tell Mercedes went full axe head on their bib, very sharp angles. A smoother shape would have more of a hover craft effect that would keep the plank from bottoming out, or at least dampen the downward motion.

The waves that travel from the floor are connected to the suspension and reflect back, so there could also be constructive interference if the suspension and floor interact in a specific way. Say the airflow separates at the leading edge of the floor because of a very sharp surface. Now add yaw in the mix, would the flow pattern change more if you had a smooth leading edge or a sharp narrow one if the upstream flow changed from dead ahead to a few degrees? If the flow detaches you get buffeting, which alters the flow field and slows it down due to turbulence.

[Big Tea]

It would take someone with far better understanding than me, but is it possible that at a particular RPM there could be a resonant frequency from the new tyre? This would have to beat with the car, I know. Is there a possibility of this or am I 'off on one' again?

[NL_Fer]

It would take someone with far better understanding than me, but is it possible that at a particular RPM there could be a resonant frequency from the new tyre? This would have to beat with the car, I know. Is there a possibility of this or am I 'off on one' again?

I think they would balance the wheels, fat more precise than the average tyre mechanic on our street cars.

[Big Tea]

It would take someone with far better understanding than me, but is it possible that at a particular RPM there could be a resonant frequency from the new tyre? This would have to beat with the car, I know. Is there a possibility of this or am I 'off on one' again?

I think they would balance the wheels, fat more precise than the average tyre mechanic on our street cars.

I was thinking back to last years slo-mo of the tyres almost having standing waves under some acceleration and curb hopping. I did not mean them being out of balance as that would only 'shake' that corner of the car, not move the whole 'body'

[godlameroso]

It would take someone with far better understanding than me, but is it possible that at a particular RPM there could be a resonant frequency from the new tyre? This would have to beat with the car, I know. Is there a possibility of this or am I 'off on one' again?

I think they would balance the wheels, fat more precise than the average tyre mechanic on our street cars.

I was thinking back to last years slo-mo of the tyres almost having standing waves under some acceleration and curb hopping. I did not mean them being out of balance as that would only 'shake' that corner of the car, not move the whole 'body'

Probably just the hydraulic inerters reflecting damping back into the tire under high speed oscillation. Instead of being transmitted to the body the vibrations stayed in the tire. All waves are traveling waves, standing waves are traveling waves that have both destructive and constructive interference, and produce a standing pattern. The standing is only in appearance.



These are traveling sound waves interacting with a transonic shock, notice how the frequency of the standing waves affects buffeting downstream more than the shock itself. With 1 or 2 standing waves we have a lot of separation and buffeting, but when we have 6-10 standing waves, there's little buffeting at all.

This agrees with this study where traveling waves were used on an aerofoil to delay flow separation.



Low amplitude traveling waves in the 6-10hz region effectively suppressed stall.

Standing waves worked as well, but traveling waves were more efficient.

What this tells us is that if the natural harmonic frequency of an airfoil is between 6-10hz it will be prone to vortex shedding, so destructive interference of the 6-10hz range will reduce the probability of flow detaching.

[vorticism]

I was thinking back to last years slo-mo of the tyres almost having standing waves under some acceleration and curb hopping. I did not mean them being out of balance as that would only 'shake' that corner of the car, not move the whole 'body'

I don't think they're standing waves, the sidewall ripples rotate with the sidewall. They arise from the buckling of the sidewall which is result of the tensile strength of the sidewall and the constant vertical and torsional loads delivered through the wheel. At least not any more than a leaf spring deflecting under a constant load is considered to be a standing wave. The porpoising frequency looks quite slow (maybe 1-4 hz), vibrations from sidewall buckling if felt by the car would be much higher (number of ripples times wheel RPM), hundreds or thousands of Hz.

[Big Tea]

I was thinking back to last years slo-mo of the tyres almost having standing waves under some acceleration and curb hopping. I did not mean them being out of balance as that would only 'shake' that corner of the car, not move the whole 'body'

I don't think they're standing waves, the sidewall ripples rotate with the sidewall. They arise from the buckling of the sidewall which is result of the tensile strength of the sidewall and the constant vertical and torsional loads delivered through the wheel. At least not any more than a leaf spring deflecting under a constant load is considered to be a standing wave. The porpoising frequency looks quite slow (maybe 1-4 hz), vibrations from sidewall buckling if felt by the car would be much higher (number of ripples times wheel RPM), hundreds or thousands of Hz.

(not doubting your explanation or the others, but) I have a 'boiler' with a fan that runs at (guess) 5k RPM and the side panel resonates at maybe 200-250 Hz. I don't know this 'shift' in frequency is transferred, I was thinking airflow through the flu, but probably mounts( I put a magnetic knife rack on the cabinet side and it silences it )

Anyway, I have gone far enough off topic now, so thanks everyone for your reply's.

[NL_Fer]

I really doubt it is the tyres. The have less rubber and air this year, much of that function is moved over to the suspension. Maybe this is part of the problem.

The suspension needs so much redesigning because of the new wheels, but also the new aero and all the suspension tricks they already had, like the Mercedes drop at high speed. Maybe they got it wrong there. I mean, Mercedes suspension as always been the weaker one, since the start in 2010.

We have seen them with bad traction, heat up problems, tyre fatigue.

[Holm86]

Isn't it possible to use bump stops to prevent the car's bottoming out?? So the floor doesn't choke on the straights??

[djos]

Isn't it possible to use bump stops to prevent the car's bottoming out?? So the floor doesn't choke on the straights??

I got shot down for suggesting that idea a few pages ago.

viewtopic.php?t=30265&start=195#p1042279

[Holm86]

Isn't it possible to use bump stops to prevent the car's bottoming out?? So the floor doesn't choke on the straights??

I got shot down for suggesting that idea a few pages ago.

viewtopic.php?t=30265&start=195#p1042279

I would argue that it's still better to bottom out on the bump stops, than on the floor. You don't have any suspension when riding on the floor either