Car Talk

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jddh1
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Joined: 29 Jan 2007, 05:30
Location: New York City

Car Talk

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I don't know if people outside of the states are familiar with this, but Car Talk is a weekly radio show on NPR radio (broadcasts nationally) and these two brothers host it and try to help (or not) people with their car issues.
In addition, they have a weekly puzzler. Some are very cool. This week's is particularly interesting to the racing community. Here it is as sent by them into my inbox and also read on air.
Slingshot Rail Dragsters


RAY: This was from my “Life Can Be a Drag” series and it was sent in by Kathy Sebastian.

"When I was a kid back in the 60s, my dad fell in love with drag racing. It seemed that every weekend he would drag me to see those things called Slingshot Rail Dragsters tear up the track.

Now, if you remember, these dragsters were weird looking, very long and open-framed cars, I guess, with a humungously powerful engine positioned just in front of the driver, who was seated in a cage in the very back of the vehicle. The front tires, which were way up front, were essentially bicycle tires and they were inflated to something like 80 PSI so as to minimize rolling resistance – nice skinny little tires, high pressure. While the rear tires were just the opposite -- the ones that were driven by the engine were huge racing slicks inflated to 10 PSI so as to maximize rolling resistance or traction. Even at that they would always manage to lose traction and start smoking before they could get out of the gate. It made for great theater and a lot of extra noise.

Well, when these cars started reaching quarter mile-speeds of 200 miles an hour and above, the crews noticed that after each run, the front tires came back with very low air pressure, in fact almost flat. This phenomenon was not observed on the rear tires. The question is, what caused the front tires to deflate? Where was the air going? And why?"
I don't know the answer right now (I just got the question) but I'll think about it this afternoon and try to find a sensible solution. I'll get back on the forum tomorrow to see if anybody found the answer, right or wrong. It would be interesting to read your answers guys.

those cars looked something like this:
Image
Last edited by jddh1 on 05 May 2010, 16:32, edited 1 time in total.

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jon-mullen
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Joined: 10 Sep 2008, 02:56
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Re: Car Talk

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Car Talk is the best program NPR has (not really much of a contest though, tbh). Those guys are friggin geniuses (and Harvard grads, iirc). What they do is like if crime scene investigators did their work over the phone, it's amazing how they can diagnose problems without ever seeing the cars. And they're hilarious to boot, I almost pissed myself listening to that classic "my air conditioner smells awful" one.

As for the puzzler, I don't know. My back-of-the-envelope says the temperature difference alone couldn't do it, maybe a chemical reaction causing a decrease in the number of molecules of gas?
Loud idiot in red since 2010
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roost89
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Re: Car Talk

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Could it be from the air-pressure on the front-tyres? Pressing the air out. Front-tyres just burst? 80psi in a bike-tyres is alot, so possibly that.

Something off-the-wall. Releasing pressure to get more weight over the rear tyres when they're not traction limited. Every little helps.

Something about the question points toward their being a secondary reason for the pressure release...not sure what though.

might point out, I've have almost no idea what these cars are like :lol:
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Jersey Tom
Jersey Tom
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Re: Car Talk

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Centripetal force is so huge on the tires at those speeds that the tire grows radially, sucks in on the sides and starts to de-bead.

Hence the need for beadlock wheels, among other things.

That'd be my guess.
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autogyro
autogyro
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Re: Car Talk

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I never owned a slingshot rail but I did have a 'Heridge' framed rear engined rail with a small block chev on alky about 1970 or there abouts.
That had moped front wheels strengthened (more spokes) and fitted with inner tubes. I cant remember any pressure losses in them. Mind you it only ran just sub 200 mph. We used to test the drag chute on the driveway of the motel next to our garage, that was fun.

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jddh1
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Joined: 29 Jan 2007, 05:30
Location: New York City

Re: Car Talk

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Well, my guess would be that the front tire gets hot, the air cant' go anywhere thus forcing the tire to either blow or air leaking from any gaps between the rim and tire bead.

EDIT: Tom already mentioned that. Cool, so I guess this could be it.

CMSMJ1
CMSMJ1
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Re: Car Talk

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I have to run metal valve caps on my race bike as speed can cause the valves to leak...or so they say!

Do they run Shraeder valves or Presta?
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Jersey Tom
Jersey Tom
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Re: Car Talk

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That's definitely another possibility.. if you have the valve stems mounted "radially" and uncapped, high rotational speeds and weak springs will let them bleed air off.
Grip is a four letter word. All opinions are my own and not those of current or previous employers.

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jddh1
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Joined: 29 Jan 2007, 05:30
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Re: Car Talk

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CMSMJ1 wrote:I have to run metal valve caps on my race bike as speed can cause the valves to leak...or so they say!

Do they run Shraeder valves or Presta?
They didn't mention that. But at least with Presta you can screw the valve so it doesn't move.

Just_a_fan
Just_a_fan
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Re: Car Talk

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autogyro wrote:I never owned a slingshot rail but I did have a 'Heridge' framed rear engined rail with a small block chev on alky about 1970 or there abouts.
That had moped front wheels strengthened (more spokes) and fitted with inner tubes. I cant remember any pressure losses in them. Mind you it only ran just sub 200 mph. We used to test the drag chute on the driveway of the motel next to our garage, that was fun.
I'm betting that was the main reason you didn't get the issues being suggested hereabouts e.g. air loss from de-beading etc.
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jddh1
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Re: Car Talk

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So the answer is...

Centrifugal force is making the Shraeder valve to open up and let out air. Good job to CMSMJ1 for getting the answer right.