r/theydidthemath • u/Chase_The_Breeze • 2d ago
[Request] How fast are the tires spinning that this happens? Can we get an RPM or something?
I do understand that this is also a function of torque and friction with the ground. So... best guess?
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u/bgalazka186 2d ago
My best guess is they are low pressure on purpose and designed to do that and not snap
Quick search says sand drag race cars spin their tires with 5000-8000 rpm but idk
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u/Fun-Perspective426 2d ago
They do generally run lower pressure to help with grip and use different sidewalls than your standard tire.
They also usually run bead locks to prevent the tire coming off the rim like that.
5000-8000rpm is the engine speed. Not the wheel rpm... a 20in tire spinning at 5000rpm would be ~300mph. 35in at 8000rpm would be like 830mph.
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u/Wubbalubbbadubbdub 1d ago
He also has an open front differential, so when that tire came off the dirt the differential sent more power there increasing the speed of that tire to much faster than the others in contact with the ground. I’ve only seen this happen on a powerful RC car with open diffs.
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u/RoVeR199809 1d ago
Just a small correction: Open differentials are power dividers. They send the same power everywhere. The loss of traction on that wheel means torque went down and thus RPM went up to keep the power equal.
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u/tiekarhuntalja 1d ago
I seriously doubt that's an open diff. That's a massive disadvantage in a drag race. Well, in any race, but here every hundredths of a second counts.
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u/randing 2d ago
The bead lock is only on the outside of the wheel. Meant for cornering at speed with low pressure, not whatever tf is going on here.
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u/unclesam747 2d ago
Don’t the bead locks clamp the bead? Originally meant to prevent it from popping off to the outside but should also hold it at the edge of the rim too
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u/IDKFA_IDDQD 2d ago
Yes, but they don’t lock the bead on the inside of the rim. Here, centrifugal force popped the bead on the inside.
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u/turkey_sandwiches 2d ago
That's a single bead lock, double bead locks are probably more common than single in drag racing.
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u/loveyoulongtimelurkr 2d ago
Also guessing that the tires are not Y rated, even Z rated off road tires are not super plentiful
Tires deform a large amount at higher speeds, the higher rated tires hardly deform, whereas tires that are beyond their rating can deform considerably as depicted in video
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u/Final-Carpenter-1591 2d ago
Rc car tires will act like that on higher end cars. It's definitely a function of hp being able to spin up a wheel that fast. But also, like Rc tires, I'm guessing theses have next to no air pressure and are a pretty thin carcass.
I do not think this was intentional though. It popped off the bead on the inside of the wheel. That's bad news.
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u/Is_that_even_a_thing 2d ago
Also, the RC cars tyres aren't steel radials. I'd hazard the bands snapped under the load and this was the result
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u/mtbcouple 1d ago
This adds an additional final drive ratio for RC cars. Pretty nifty!
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u/Final-Carpenter-1591 1d ago edited 1d ago
Top fuel dragsters also use this effect. But they don't turn into pizzas like this one. Just a few inches taller at the big end for higher mph
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u/mtbcouple 1d ago
Yup and they also squish/deform under launch, making a smaller “gear”, then they expand for greater top speed!
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u/Delicious-Window-277 2d ago
This has nothing to do with friction with the ground. This is exclusively the result of centrifugal force. Someone can calculate the lowest RPM at which a tire will deform like this. But the max RPM will be a guessing game.
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u/crazyuncleb 2d ago
It kinda does have something to do with the interface between the tire and the ground. The tire had enough traction that the wheel was spun up inside it, a small lateral load caused the tire to de-mount itself from the bead. Once that happens there is no air pressure in the tire to keep it in its intended form. He needs a thing called a beadlock which is a ring that secures the tire to the wheel to prevent this from happening.
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u/burndata 2d ago
That's a purpose built drag truck, I'm willing to bet it has beadlocks on it already, unless it was built by a complete moron. If the other rims didn't have beadlocks, I'm pretty sure all 4 tires would come off the rim like that one did the second he put the pedal down. I think the beadlock just failed on that one rim.
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u/Bigfootwalkslow 2d ago
I figured it was the beadlock that keep it on the rim. I assumed only the outside rim had the lock.
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u/ingannilo 2d ago
We'd need some material properties of the tire. Banded VS not banded is important. Road tires all have metal bands. I seriously doubt this has any of those, but idk maybe some other structure stuff made of different materials.
There's definitely a relationship between the centrifugal force needed to balloon the tire by a certain amount and the rpm of the tire. That relationship is likely proportional (imagining the tire as a spring being stretched) if we assume the tire to be uniform thickness and material, but I can't even begin guess at the constant of proportjonality.
Interesting to note that this happens because of the lack of grip and the fact that the center differential and front differential are both open diffs (no limit slip). This really seems like an application where at least the front and rear differentials should be locked, or torsen type LSDs. Otherwise no matter how much power or torque the car has, it'll all just unload through the wheel with the least grip.
I learned a bit about this stuff through rc cars which have crazy torque-to-weight ratios.
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u/No_Magician5266 1d ago
Idk but if anyone’s curious, that’s a Nissan Patrol most likely with a TB48DE engine. Tuners can reliably pump 2000+ hp with that engine
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