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B Maximum Radius for Wall of Death

  1. Jan 18, 2016 #1
    Hi - Can anyone help me out with a question. The answer seems obvious, but maybe so obvious that I'm jumping to wrong conclusions!

    Just been reading Guy Martin's book "When your dead, your dead" (good read by the way) and one of his stories is about breaking the world speed record for a wall of death. I've looked at the math around wall of death mechanics and it all seems pretty straight forward, but here's what's puzzling me.

    1. There's plenty of forces to worry about when riding the wall of death, but the one that appears to limit your speed is the G Force acting on the rider/bike as a result of centripetal force when you run at a constant "speed" - but constant acceleration in velocity terms around the inside of the wall.

    2. The smaller the radius of the wall of death, the higher the G Force acting on the rider and bike, so if you want to go faster, you build bigger.

    3. I've read a number of different sources to try and understand the basics of the forces acting on ride/bike and the diagrams all seem to ignore radius - or rather they remove the complexity of centripetal force due to acceleration around the circle, but look at the bike as the frame of reference using the (fake) centrifugal force. The upshot seems to be that as long as you generate sufficient "centrifugal force" by going fast enough, then you're going to stay on the wall.

    4. There's also a torque factor involved. Because the force of gravity acting downwards on the bike is from the centre of mass, and the force of friction on the wall from the tyres is acting at a distance from the centre of mass, then you need to lean the bike "up" the wall to stop it falling over. A coupe of sites have a graph that show that the slower the speed, the more you have to lean - so the faster you go the more you can ignore this torque effect.

    5. It would therefore seem that in principle there's no maximum limit to the size of a wall of death - as long as you can get sufficient speed, you can ride an ever larger wall. And the larger the diameter the lower the G Force acting on the rider for any given speed. So obvious, but presumably so wrong!!

    6. Case for the prosecution: Before Guy Martin kick starts his bike, they get someone that actually understands the math involved (!!) and determine that the fastest Guy is going to get up to is 80 mph on a wall of a particular radius (around 150 feet) and, that at this speed/radius he will be subject to 6G (which is where he puts his limit of "almost blacking out"!)

    So - my question is what am I missing? Given that Guy is after a world record and has access to bikes that can top 200mph; why build a wall that limits him to 80mph? What is the limiting factor in the radius of wall of death?

    Thanks all!
    Matt
     
  2. jcsd
  3. Jan 18, 2016 #2
    The money to build it?
     
  4. Jan 18, 2016 #3
    I have to say that did occur to me :smile:. The only reason I didn't see the cost as the limiting factor in this case is that the book made quite a point about seeming to work back from a speed and G-Force and coming up with size. Guy Martin is such a straight-talking no-BS kind of guy, I think if it was the other way around he would have said that they built a wall to a budget and then he went balls out around it until he passed out!

    However - I'm happy to be disabused of this idea! Are you actually telling me that there is no limit to the radius, or just suggesting that there may have been a more practical cost limit at play?
     
  5. Jan 21, 2016 #4
    Maybe I've confused the question with the book references :-)

    Can anyone enlighten me on the basic question: Is there a maximum radius for a wall of death** or is it as simple as "the bigger the radius, the faster you need to go"?

    ** that doesn't send some fundamental variable to a level that would invalidate it as being a wall of death - for example, g-forces or a lean angle increasing as radius increases

    Thanks
     
  6. Mar 28, 2016 #5
    hi as seen in the film g is equal to speed squared ( in kph) over 25 x diameter (in meters)
    so guy is doing say 100 kph in a circle thats 100 m diameter so g equals 100 squared (10000) over 25 x 100 (2500) equals 4g
    to do 200 kph and only pull 4 g you need a diameter of 400 meters . so no disrespect to Guy as he pulled more g than most people could stand.
    If you build me a drum of 400 meters diameter, and give me a bike that will do 200 kph i will have a go at breaking the record !! all it takes is money...
     
  7. Mar 28, 2016 #6
    earth has a dia of 12742 kms thats 12742000 meters and rotates at a surface speed at the equator of 1670 kph giving a g force of 0.00875 G so a man that weights 100 kgs on a set of scales at the north pole would weigh 100.875 kgs at the equator on the same set of scales, although his mass would remain unchanged !!!
     
  8. Mar 28, 2016 #7
    sorry i forgot to subtract he would weigh 99.125 kgs at the equator !!!
     
  9. Mar 29, 2016 #8
    I'm so miffed I missed the TV show, having read his book that was one I was looking forward to!!

    Did they talk about the "lean angle" on the show when they were doing their sums on the size of the wall? As noted above:

    4. There's also a torque factor involved. Because the force of gravity acting downwards on the bike is from the centre of mass, and the force of friction on the wall from the tyres is acting at a distance from the centre of mass, then you need to lean the bike "up" the wall to stop it falling over. A coupe of sites have a graph that show that the slower the speed, the more you have to lean - so the faster you go the more you can ignore this torque effect.

    And if they didn't talk about it, did you see anything crazy on the film? I assume Guy had a camera on him or on his bike; from his ride perspective was he riding "on the flat" or did he look like he was leaning into a hard corner?

    Thanks
     
  10. Mar 30, 2016 #9

    mfb

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    The ultimate limit is just given by the size of Earth and the tolerance to g-forces. The size of Earth doesn't matter - even for the fastest aircrafts it is a small effect, and motorbikes are slower than aircrafts[citation needed]. So money, motor bike speed and maybe risk aversion are the limit.
     
  11. Mar 30, 2016 #10
    Yes I also missed half the show, i had promised to let my daughter see something on the other channel and didn't realise it was a two hour show... in the first hour we got the build up to the breaking the existing record, at some 60 kph or so but according to other sources he went on to set a super high record of 78.15 mph that's just over 125 kph !! and pulling over 6.32 G
    Yes of course you have to lean the bike upward slightly as there is still 1 G pulling downwards to overcome, the centre of mass will be down the vertical line of Guy and the bike and the tyre. when it's at any G force , otherwise the bike would be climbing up the wall or falling downward, effectively in a turn, as you would lean a bike to take a turn on the flat!!
    The biggest problem of doing this record is that it's not like a drag-race..it takes time to get up the wall, and start to accelerate to a speed where you are set up to go for the MAX, during this setup you are pulling 3 or 4 G and leaning the bike upward, then by gunning it up to the point where you start to go blind and black out..you have to set in your mind to go past that point for a few seconds and still have the mind control to shut off the gas and really start to slow down..
    The easy part is that a motorcycle is very stable the faster it goes, ( up to a point but well in excess of 300 mph ) but you are also fighting the G force trying to crush you against the tank of the bike..and this is where you can induce wobble and crash! there is also the G forces affecting the components of the bike it's self, compressing the suspension, the massive load on the tyres, the force trying to rip the engine apart and not least the battery acid that already has a high density of 1.8 kg per litre it now weights 11.3 kgs per litre, trying to split open the battery case. Anything that gives way on the bike and suddenly changes the centre of mass or even the weight of the machine if it falls off will upset the balance and start a wobble, not to mention that in a few seconds you will be running it over as you go around again...if all that is done safely you still have to recover from the blackout and not crash. Evil Kenivel jumping a few buses was peanuts compared to Guy's challenge.. but that's GUY MARTIN for you, he's one of the best bikers and sportsmen ever...I would be very humbled to shake his hand...
     
  12. Mar 30, 2016 #11
    There is a limit , the bigger the diameter the "wall of death" the faster you can or have to go to still stick to the wall. As I offered to have a go if someone puts up the cash...i don't think I could even stand 4 g's for any length of time due to my health conditions.

    QUOTE..If you build me a drum of 400 meters diameter, and give me a bike that will do 200 kph i will have a go at breaking the record !! all it takes is money...

    The problem is that with a 1 km drum you could do 400 kph and pull 6.4G but that would be the fastest bike ever, you would need a G suit, and a lot of high G time,,,you would basically be doing a tight turn as in a fighter jet...a 400 meter drum you can do 200 kph at 4 G's that's more realisable, and im sure now the gloves are off that would be the next attempt if guy wanted to go even faster...but then it get's silly, the speed of the bike has to be so high to still stick to the wall and the diameter gets ridiculous...do the maths !!! you just can't stay at 4 g for too long and 6+ g is down to seconds, the actual limit is going to be limited by the time to acquire the g's, the maximum G's, and the top speed of the bike...none of these limits is that far from what Guy has achieved..so it's likely to be a record that will never be broken by much if ever anyone with bigger kahoonies than Guy can be found !!! . Im assuming there is a minimum lean angle for the rules of the record (eg 90deg to the vertical -+ 10 deg) or I could just ride in the desert at 200 kph in a big circle and claim a record !
    AGAIN HAT'S OFF TO GUY MARTIN
     
  13. Mar 30, 2016 #12

    mfb

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    Going faster was the whole point of the thread.
    With higher speeds (which already means we are discussing a new record), the time to accelerate could become an issue, but you can cancel that with an even larger radius (if necessary) to keep the g-forces at an acceptable level. If you have enough money, of course, but we are discussing the physics.
    A vertical wall can be rode at g-levels that trained humans can tolerate for basically infinite time (e.g. 2-3g), even if they would be standing. A motor bike rider is not standing.
     
  14. Mar 31, 2016 #13
    my statement
    quote "but then it get's silly, the speed of the bike has to be so high to still stick to the wall and the diameter gets ridiculous...do the maths !!!"

    you didnt' do the maths did you ...i have just explained why this is why even going to a 1 km dia drum or wall the conditions de-pass human levels and who's standing ??
    the optimum would be somewhere in the 200-300 kph region pulling 4-6 g in a diameter of 400 meters...
    so do the maths !!! take some sample theoretical speed (remember this is for a motorcycle) then take a diameter of you choice, and a human with his ability to withstand high g for the time it would take to get up to that theoretical speed and stop again, and the logistics of time acceleration duration, of the run...
    so do the maths !!
     
  15. Mar 31, 2016 #14

    mfb

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    I did. Every speed is possible, as long as the motorbike allows it and the wall has a suitable radius. There is no optimal speed for a speed record.
     
  16. Mar 31, 2016 #15
    Hi Guys, looking at the math for Guy Martin's wall of death.
    g = kph2 / (25xd) 78.15 mph (125.77 kph)2 / (25x120) = 5.27 g fair enough.
    In the show they showed Patch McGillicutty claim he done 71 mph and then used an equation of rev/s and the diameter of (9m) and said actually done 48mph
    Using g = kph2 / (25xd) 48 mph (77.24 kph)2 / (25x9) = 26.51 g or dead as I'd call it.
    If 6g is a reasonable max Transpose for kph = (The square root of) 6 x 25 x 9 = 36.74 kph or 22.8 mpg is more realistic of Patch's speed.
    Ok 71 mph (cough cough) 71mph (114.263 kph)2 / (25x9) = 58.02 g
    According to the mathematics in show only, are my maths right? and is Patch a liar liar with his pants on fire ?
     
  17. Mar 31, 2016 #16
    YES Patch was a liar and so is someone who says they "have done the maths" !!!! come on
    Do the maths take an example of a 5000 diameter drum or wall of death ( that's a 5 kilometre diameter wall of death..) the fastest bike say 400 kph !!! that's 248 miles per hour, the g force resulting from going around would be 1.29 G yes only 1.29 G so he would fall off the wall, so go faster !!! ok try 500 kph that's 310 miles per hour the G force is 2 G he would be leaning at 45 deg, still not enough
    so go faster still , see where this is going ??? by doing the maths !!

    Lets get really silly, lets say a tripple turbocharged nitro burning motorcycle, and 800 kph !!! that's close to 500 mph and would get 5.2G ok that's about right to have the required angle to be reasonably close to 80deg to the vertical !! But to be travelling at 800 kph you would need a run up of 3 or four minutes and your blood would have drained into your boots well before, and you have to stay concious during the slow down from 800 kph !! and let's face it no one is going to be able to build a 800 kph motorcycle without using rockets, parachutes and the rider would have to be genitally engineered with a bulls heart !! and a non resonating drum that's 5 km's diameter would have to be carved from rock to stop any induced vibration..

    So let's not get silly....The maths actually do tell you that this is not feasible !!! So there is an optimal maximum speed possible !!!! maths tells you that !!!

    I would suggest that my first thought of a diameter of 500 meters and a shot at 200 kph 3.2 G to 250 kph at 5 G would me the maximum humanly possible !!

    But then I did the maths...and im also a bike rider !!! I have ridden a bike like Guys no fairing and at 200 kph, that' s starting to rip you from the handlebars and at 1G try doing 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800 kph each time you double the speed the wind resistance gets quadrupled !!! so that ripping force at 200 kph of 20 kgs ( wind resistance to the exposed body and helmet ) at 800 kph would be like 320 kgs pulling you off the bike backwards.
    Plus you have the downward (relative to the rider) force of 4 or 5 G's and all the forces applied to different parts of the motorcycle !!!t I rest my case !! impossible !!!

    So please use a calculator and come up with some real figures if you think different !!

    I don't wish to annoy or offend but i get annoyed when im told "I did the maths", when very obviously you didn't.
     
    Last edited: Mar 31, 2016
  18. Mar 31, 2016 #17

    mfb

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    Based on a few numbers you made up?
    "It is impossible to walk 1 kilometer. Try to move by 1 kilometer in a single second: it does not work! Try to walk one kilometer in two seconds: it still does not work! Therefore it has to be impossible to walk 1 kilometer."

    You have an 800 km/h rocket motor bike? Fine. Build a 1700 m radius wall of death. The acceleration is now a reasonable 3 g, the leaning angle is about 20 degrees. As the rider is not standing upright on the bike, 3 g is tolerable for minutes, with training probably for at least an hour. The rider does not even have to pass out at all.

    For every motor bike speed you can manage, there is a radius where the acceleration is 3 g and the leaning angle is 20 degrees (those are directly related). There is no fundamental limit below orbital speeds - just motor bike speed, available money, and risk for the rider.
     
  19. Apr 1, 2016 #18
    .
     
    Last edited: Apr 1, 2016
  20. Apr 1, 2016 #19

    mfb

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    I have no idea about the show - does the 9 meter diameter value come from there?
    At that level, the difference between wall radius and center of mass radius of the rider becomes relevant - the person will have the same angular velocity, but a smaller radius, which reduces g-forces. Not sufficient to reduce 27 or even 58 g to a realistic level, but still relevant.
     
  21. Apr 1, 2016 #20
    quote Lets get really silly, lets say a tripple turbocharged nitro burning motorcycle, and 800 kph !!

    as i said lets get silly, i meant in figures not your response !! and i walk a mile every day so does my dog we ain't dead yet
    and you cant have a 800 kph motorcycle!! it would have to be a rocket sled!! perhaps if you had watched the show you would be better informed...
     
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