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Comment by jws | Hacker News Reader

That's an average acceleration around 2g. This seems beyond the limits of tires and friction. Do aerodynamic down forces kick in fast enough to make more force usable against the road at these relatively low speeds? Do the tires have more than static friction going for them (say, fine scale interlocking with the pavement)?

Drag racers can do 3gs, but that's at higher average speed.

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Aero might play in a tiny way at the 40-60 MPH space, not meaningfully enough.

As for tires, no actually you can go much further than this. The big detail is wear and heat caused by tire softness and pressure. Road car tires can't enable 0-100KPH much faster than 1.9s or so purely because they can't be so soft as crusing on the highway would cause excess wear(practicality), heat (Danger), and friction(Range).

Drag Racers can accelerate way faster, 0-60MPH in .8s.

The aero packages for these vehicles create a pretty significant amount of downforce at relatively low speeds - for example, our car last season produced ~140 lbs of downforce at 35 mph.

Edit - A lot of that comes from the rear wing though, which the green team didnt run for this particular event.

Although the other comments are correct about the 30mph optimization for the aero package, this car uses active underbody aero (aka, fans) to create additional downforce even when at a standstill. See the recent runs of the Speirling at Goodwood festival of speed to see a purpose built "fan car" in action.

Interestingly, this setup wouldn't be allowed due to rules in Formula SAE in the US but is legal in the german competition cause those teams are just built different.

Edit - Quick plug! If anyone is interested in supporting FSAE and the awesome engineering that goes into these cars, I am a member of the San Jose State team and our aero team is in need of HPC access to run their CFD simulations. My email is in my profile. Cheers.

These formula student/fsae cars have aero packages which tend to be designed around 30mph average track speeds, so they will get contribution from that, especially in the latter half of the run. Even so, the tire friction involves both physical interlocking due to deformation and chemical adhesion.

You can see the tire warmers in the video being removed in addition to their approach burnout which imparts additional heat into the tires, so that the tires more effectively stick to the ground.

For all of the questions about why they don't compete with dragster techniques, the student race series they participate in has very strict regulations regarding vehicle layout which forces them to be more of an autocross car, these kinds of (heavily couched) records being produced are really somewhat incidental.

> That's an average acceleration around 2g. This seems beyond the limits of tires and friction.

Tires don't only rely on friction but also on mechanical grip (the road and the tire are not perfectly smooth surfaces, they interlock) and chemical grip (just like glue). The last two also scale with surface area as opposed to friction. So you get two common misbeliefs about tires from school physics: that you can't go over 1g and it doesn't matter how large the contact patch is. Both false in practice.

That's what I wondered too specially that front wing can't really be helping during the initial acceleration if majority of the power is coming from the real wheels, however, they can be helpful for slowing down under control.

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