Hot Rods The Underslung Chassis, Dead Forever ?

https://www.jalopyjournal.com/forum/threads/underslung-rods-thread.565759/

https://www.jalopyjournal.com/forum/threads/underslung-front-suspension.1286405/

 

Super low profile tires and giant rims ? Looks like you could drive them around on their enormous rims and just scuff off some plastic. I have seen plenty blow a tire and make it back to the pits. Mike

 

@AccurateMike Yeah, I've seen them limp back too. I'd say it's a matter of three things.
Very smooth pavement without 'street' bumps and dips
Sacrificial trays, skirts or undersides, not chassis that low although they are still very close
tires that rarely come off the bead, although I have seen that too.
So I'd say that they do (barely) meet scrub even if they don't look like they do
I'll also mention that the safety factor of tracks is different than public roads. Senna was tragic, but this article doesn't mention scrub being one of the changes.
https://www.biography.com/athletes/a63001762/ayrton-senna-death

 

Hey Art, thought I’d share some pics of my T bucket on an underslung ‘28 Chevy truck frame. It works fine and is a blast to drive. We shaped the frame rails to follow the shape of the cowl and then customized a straight tube front axle to have the spring pads aligned with the frame rail width. 15 inch wheels front and rear. Quarter elliptics in rear serving as locating members on the bottom and radius rods on top with a panhard bar.

 

Play nice little boys. Lol

 

That is a really cool looking car!
However, it doesn't take much thought to look at the frame height in relation to the tires/wheel height from the ground to understand what would happen if one of those tires went flat. On the dirt, like the picture shows, the frame digs into the ground and the guy at best is in for a heck of a ride, or he takes up flying with out an airplane. At worst the rear end of the car comes over top of him. the car frame digging into the ground will likely stop it faster then any automotive brakes can, that is where the biggest danger is on the dirt, too much fast braking on the very front of the car.

If the car was on pavement, its a very different story. The frame drops down on the pavement and starts sliding. Sparks fly as the pavement grinds the frame away. The driver has no control because the weight is mostly resisting on the frame, so even if the other tire is still holding weight, it usually won't help control the vehicle direction. Since the front tires are not contributing to the steering, they also are not contributing to stopping the car. The only braking there is is what ever the rear brakes can provide, and the friction of the grinding metal on the pavement. The car is simply skating on the pavement without control. The driver and any riders in the vehicle are along for a long ride, going anyplace the car wants to go, until its ground the frame away enough to stop the car, or the car hits something to stop it, or the car slides off the pavement and digs into the ground.
That is what scrub line is all about, and that is why almost every state has laws concerning it.

The questions regarding low race cars is interesting. Most race cars that sit low use thin sheet metal, plastic, carbon fiber, or fiberglass to control the amount of air that can get under the car body. Those materials grind away, or break off very quickly when the make contact with the pavement. Its a pretty good bet very few frames or structural things are below the scrub line. Most of the race cars you see sliding on the pavement after a crash have broken suspension or missing wheels. I can assure you there are not many race car crew chiefs that want to see their cars suspension or frame dragging on the ground if a tire goes flat. They want to replace the tire and go racing, knowing 1/2 the suspension has not been ground away by sliding on the pavement.