Tyre Deformation Aerodynamics {Auto Aero #54}

How does tyre deformation affect a car's aerodynamics?
Find out in this video!
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#Aerodynamics #CarWake #CarAerodynamics #AutomotiveAerodynamics #TyreDeformation #TireDeformation #TyreAerodynamics #TireAerodynamics

[@mrdavies09]

Great video as always. It would be interesting to see how they compensate for this in top level motorsport.

[@PremierAerodynamics]

This is an interesting point, and simpler in some ways and more complex in others. In motorsport, they often don't have as much tread, F1 for example, so they don't need to worry about the additional air flow around the shoulder. On the other hand, they also cant their wheels much more, so, one side is squashed into the floor more and the other side not so much. That causes asymmetry in the vortex pattern around the wheel, and makes one side more prone to separation and the other less.

[@c.l.3806]

Hello, Flettner Rotor and Vortex Tongue are related to this topic. For me it makes a lot sense to cage the wheels in. The deforming of the tire is only important to measure the cage size. Just the part of the tire uncaged needs to get in touch with the road. Got to invent a mud cleaner. Cause the cage is going to catch dirt. With a tire mountable cage cleaner brush, the cage might work. With a cage around the front wheels, rubber foiled wheel openings are possible. If the wheel is steered, only the cage is touching and bending the rubber foil. Perhaps it's worth to mention that the moving air around the wheel produces suction, that tries to pull the cage toward the moving tire.

[@PremierAerodynamics]

Covering the wheels are definitely good for reducing drag.

[@rolandotillit2867]

The strategy to minimize tire wake is to stop moving air from getting to the front of the wheel, and cramming as much slow moving air behind the wheel as possible. If the tire jetting vortex is due to air entering the space the tire leaves behind, putting air behind the tire will reduce the favorable pressure gradient that forms from the stagnation pressure at the front. Obviously it's worse at the rear because the air has less kinetic energy left in it as it makes it's way through the car and all it's skin friction. It's also worse at the rear because the turbulent mixing from the tire wake increases the back pressure in the diffuser. The relative flow along the rear wheels is the key to improving diffuser performance. Outwashing bodywork behind the rear wheels, not only directs the tire wake outboard away from the diffuser(reducing the back pressure), but it turns the tire wake into a vortex that entrains airflow outboard as well. Doing this will increase drag from the tire wake, and to make up for it, you have reduced the back pressure in the diffuser. Now you no longer have the tire wake turbulently mixing with your diffuser expansion, flow stays attached longer, your spoiler/wing/gurney becomes more effective at extracting air from the diffuser further reducing back pressure. Just from taking the tire wake out of the equation.

[@PremierAerodynamics]

These are good points. For the flow directly behind the wheel, that is more controlled by the longitudinal grooves that allow air through the contact patch and the shoulders of the tyres that may or may not separate the flow. The jetting vortex usually steers clear of the wake immediately behind the wheel. Sometimes, it can come back into the wake but more downstream. And yep, the rear wheels do face a hard time. And getting cleaner flow from elsewhere is a good idea.

[@rolandotillit2867]

@@PremierAerodynamics I was talking more about the wheel wells themselves. We can do better than just putting a plastic hemisphere around the wheels, and calling it good. The little flaps and strakes around the wheels OEMS place is a good first step, cheap effective and easy to produce. Venting the wheel wells in strategic places is more expensive to produce, but even more effective. Actually sculpting the entire wheel enclosure, is the least practical but most effective method. Least practical because it's by far the most expensive, not just in terms of design, but knock on effects like intrusion into cabin space, packaging requirements. When you look at it from that POV you can see why cars like the Ford GT are so expensive, as the aerodynamics took a front seat, even at the expense of the interior.

[@PremierAerodynamics]

​@@rolandotillit2867 Oh okay. Good point. In that effort, many of the German cars now feature vents coming out of the wheelhouses, generally around where you get high pressure. That serves two purposes. You alleviate that high pressure and you funnel that flow into the flow around the car and in the wake to reduce the drag. Funnily, because they generally look good, some cars have put them on for aesthetics but not connected them to the wheelhouses.😂

[@AwestrikeFearofGods]

If we assume the tire is distortable but incompressible (i.e. solid rubber with constant volume, Poisson's ratio of 0.5), then as it lengthens in the longitudinal dimension, its lateral width would decrease by a greater ratio than its increase in vertical height. In other words, centrifugal force would decrease its frontal area. If instead, we assume the tire is an idealized thin-walled pneumatic balloon (Poisson's ratio 0.5), its frontal area would decrease in a similar manner. Also, it's tread shape would distort towards that of a motorcycle tire, in such a way as to reduce drag coefficient. In reality, a tire is a somewhat-complex, rigidly-constructed pneumatic object, but I'd still hesitate to claim a drag increase without actual measurements (e.g. front-view telephotography). It's worth mentioning that the region near the contact patch is greatly widened and lengthened by normal-load deformation. Without such deformation of the tire and/or road surface, the contact patch would have zero longitudinal length. However, airspeed is low near the contact patch, assuming a zero-slip condition. The top of the tire has an airspeed that is twice groundspeed, and would be have greater aerodynamic effect.

[@PremierAerodynamics]

Good points. For the one about tyre frontal area reduction, perhaps it is more complicated than that because at the front you have rubber being thrown into the ground as well. Also, when the tyres expand radially, they lift up, so you have an increase in height at the front. For the tread deformation, it is in the order of a few millimeters, so it isn't as extreme as a motorcycle tyre. I'm not sure if a tyre can deform that much without popping.😅 There are several papers that show this change. I can link some if you like. For an exposed wheel, definitely, the top is moving twice that of the freestream flow, but in a wheelhouse, the flow the top of the tyre sees is very slow and not close to the freestream speed. The flow also meanders around, so it isn't completely aligned with the freestream direction either.