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  1. #1
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    G Forces in Downhill and/or Dual Slalom Runs

    So I recently watched a video recently and while I've known professional drag racers pull 3/-3 to 5.3/-5.3 G Forces from launch/60 foot to the shut down area at the 1320 foot mark, I am wondering how many G Forces is in Downhill and Dual Slalom. Cornering, braking and acceleration. Would I be correct in saying it's roughly between 1 to 1.5 Gs, -1 to -1.5Gs and 1 to 2Gs depending on course/track and rider ability? Basing these numbers on just the MTB video I watched.

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    This device will track G forces for you, maybe email them and ask them what a typical range is?

    https://www.shredmate.co.uk/

    Normal riding I wouldn't expect to be very much, but crashes probably get some good numbers.

  3. #3
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    It would appear that the Redbull Rampage riders hit some serious g-forces on the landings. I don't know what the actual numbers may be though.
    Riding Washington State singletrack since 1986

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    Yeah, I'd wager landings and high speed g-outs see the highest force. There probably already is good numbers on this in DH racing.
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    Quote Originally Posted by Curveball View Post
    It would appear that the Redbull Rampage riders hit some serious g-forces on the landings.
    But until the landing it's zero

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    I doubt you are going to reach 1 G on acceleration or cornering. A new Porsche Boxster will just top 1G on a skidpad for comparison.
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    Quote Originally Posted by Darth Lefty View Post
    But until the landing it's zero
    Do you know where g comes from in g force?

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    Quote Originally Posted by jeremy3220 View Post
    Do you know where g comes from in g force?
    From Wikipedia: The letter 'G' was introduced in the Old Latin period as a variant of 'C' to distinguish voiced /ɡ/ from voiceless /k/. The recorded originator of 'G' is freedman Spurius Carvilius Ruga, the first Roman to open a fee-paying school, who taught around 230 BCE.

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    Quote Originally Posted by 93EXCivic View Post
    I doubt you are going to reach 1 G on acceleration or cornering. A new Porsche Boxster will just top 1G on a skidpad for comparison.
    True for a flat surface. If the corner is banked then larger g's can be attained depending on speed and radius of the corner.

    Turning, G-Forces and Banked Tracks. : The Building Speed Blog

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    Quote Originally Posted by jeremy3220 View Post
    Do you know where g comes from in g force?
    Same as G-bong?

    I wander how many negative G's I can pull with like a 3 liter bottle, man.
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  11. #11
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    Quote Originally Posted by jeremy3220 View Post
    Do you know where g comes from in g force?
    g is a unit of acceleration. g x mass = force (weight)

  12. #12
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    Quote Originally Posted by dragracer88 View Post
    So I recently watched a video recently and while I've known professional drag racers pull 3/-3 to 5.3/-5.3 G Forces from launch/60 foot to the shut down area at the 1320 foot mark, I am wondering how many G Forces is in Downhill and Dual Slalom. Cornering, braking and acceleration. Would I be correct in saying it's roughly between 1 to 1.5 Gs, -1 to -1.5Gs and 1 to 2Gs depending on course/track and rider ability? Basing these numbers on just the MTB video I watched.
    Sort of. There are no negative g forces in drag racing. Really, no g forces in the way most people understand them. When you talk about "pulling Gs", most people think fighter jets and the g forces work in a vertical axis, pulling blood from the brain to pool in the horizontal areas of the body (legs, feet, lower arms).

    A dragster will have acceleration forces, but they are horizontal to the driver; that is, they push backwards or forwards and have no impact on the blood flow to the brain. Though strong horizontal forces can rattle your eyeballs. And you can certainly put them in terms of G force (where 1 G = the normal pull of gravity we all feel every day just walking around).

    I would question your negative 5.3 Gs, too. I've pulled -2 and it hurts. -5? I think your head would damn near explode from all the blood rushing up into it!

    As for +/- 5Gs laterally, on acceleration or deceleration, meh. No big deal.

    Riding a bike and leaning, you will get some G forces coming out of a bermed turn, but I'd guess no more than maybe 1.5. I'm just guessing, though, from my own experiences.

    Oh, for the record, those experiences include pulling 7.5Gs and -2.1Gs. And accelerating from 0 to ~175 mph in 2 seconds (in a 54,000# aircraft) and slowing from ~150mph to 0 in a few hundred feet flying on and off an aircraft carrier.

    Quote Originally Posted by Darth Lefty View Post
    But until the landing it's zero
    They'd get a bit more than one on a ramped launch, if it's at the bottom of an acceleration section. They'd get slightly less than one as they climbed up and zero in the transition to "falling". When falling, they're experiencing 1 G, as we all do when we're jumping something.

  13. #13
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    It might be more intense on some of the DS or 4X tracks where there is a good berm coming off the tranny of a good jump. The turns are tighter in DS and a DH bike has to mitigate a lot more of those forces with suspension damping, right? There's a lot of that happening on big bike park trails, but that's no place to hold a proper DH race.


    I have no clue as to what those numbers might be.
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    Quote Originally Posted by 93EXCivic View Post
    I doubt you are going to reach 1 G on acceleration or cornering. A new Porsche Boxster will just top 1G on a skidpad for comparison.
    That's 1 G lateral acceleration which must be added vectorially to other sources of acceleration to get the total. Usually in corning tests it's 1 G of gravity, so with 1 G lateral that'd be 1.41 G total.
    Do the math.

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    We can make some nice simplifications and use physics to get a number instead of just guessing. Let's assume a rider and bike that weighs 200lbs (91kg) is going 25 MPH (11.2m/s) around a berm with a 25ft (7.6m) radius.

    The formula for Centripetal Force is F= m*v*v/r
    https://en.wikipedia.org/wiki/Centripetal_force
    Force F = 91*11.2*11.2/7.6 is 1502 N or 337.6 lbf

    So if the rider/bike is 200lb and the centripital force is 337.6lbf then the G force would be ~1.7G

    If rider is going faster or bermed turn is tighter then they'll feel more G's

    https://sciencing.com/convert-newton...e-8720337.html
    https://www.youtube.com/watch?v=GPa59XcS6pQ

  16. #16
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    Quote Originally Posted by Spindelatron View Post
    We can make some nice simplifications and use physics to get a number instead of just guessing. Let's assume a rider and bike that weighs 200lbs (91kg) is going 25 MPH (11.2m/s) around a berm with a 25ft (7.6m) radius.

    The formula for Centripetal Force is F= m*v*v/r
    https://en.wikipedia.org/wiki/Centripetal_force
    Force F = 91*11.2*11.2/7.6 is 1502 N or 337.6 lbf

    So if the rider/bike is 200lb and the centripital force is 337.6lbf then the G force would be ~1.7G

    If rider is going faster or bermed turn is tighter then they'll feel more G's

    https://sciencing.com/convert-newton...e-8720337.html
    https://www.youtube.com/watch?v=GPa59XcS6pQ
    Honest question... Intuition tells me that a 175lb rider on a 25lb bike might perceive the force differently than a 155lb rider on a 45lb downhill bike. Is that assumption wrong?
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  17. #17
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    Quote Originally Posted by MOJO K View Post
    Honest question... Intuition tells me that a 175lb rider on a 25lb bike might perceive the force differently than a 155lb rider on a 45lb downhill bike. Is that assumption wrong?
    I don't think so unless you start looking at the short periods of time when the suspension is loading and unloading as the centripetal forces change, what I wrote and assumed above is for steady state, where the rider is in the berm and traveling along a circular path.

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    Except that how we feel the G force is basically to multiply the Gs by rider weight. So a 200# rider experiencing 2Gs would feel like 400# while the 150# rider would feel like 300#.

    So does that make them experience the forces differently? Seems to me they would feel a difference.

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    Quote Originally Posted by MOJO K View Post
    Honest question... Intuition tells me that a 175lb rider on a 25lb bike might perceive the force differently than a 155lb rider on a 45lb downhill bike. Is that assumption wrong?
    It depends on what perception you are talking about. The force the rider would feel on his butt (lets assume hard tail bike) would be higher because the increased g acceleration (which is constant between the two riders) would be applied to the larger mass of the 175# rider. (that's a complicated way of saying what skiahh said ) Now the force applied to the berm would be the same as the mass of bike+rider in both scenarios is the same as well as acceleration.

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    Quote Originally Posted by BlueCheesehead View Post
    It depends on what perception you are talking about. The force the rider would feel on his butt (lets assume hard tail bike) would be higher because the increased g acceleration (which is constant between the two riders) would be applied to the larger mass of the 175# rider. (that's a complicated way of saying what skiahh said ) Now the force applied to the berm would be the same as the mass of bike+rider in both scenarios is the same as well as acceleration.
    Nope.

    where is the extra force that the berm sees due to the extra weight of the bike going? Draw a picture...

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  21. #21
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    I think my assumption was based on the thought that, as the speed approaches zero, the big guy feels like a big guy and the small guy not so much, and the mass of the bikes would have no bearing on that.
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  23. #23
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    Much like the Veyron, top fuel dragsters are built entirely out of factoids, many of which have never been checked by anyone.

  24. #24
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    Quote Originally Posted by dragracer88 View Post
    Skirrrrt

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    Quote Originally Posted by Spindelatron View Post
    Nope.

    where is the extra force that the berm sees due to the extra weight of the bike going? Draw a picture...

    Sent from my SCH-I545 using Tapatalk
    Yep.

    I think you mis-read my post. I said the BERM would see the SAME force as both the mass and acceleration in both scenarios would be the same. F=ma --> F=(175+25)*a =(155+45)*a. The same 200# is acting on the berm. It does not care what % is in the rider and what % is in the bike.

    You might want to do your own force diagrams.

  26. #26
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    Someone should mount an accelerometer on Aaron Gwin's bike and end this conjecture.
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    Quote Originally Posted by J.B. Weld View Post
    Someone should mount an accelerometer on Aaron Gwin's bike and end this conjecture.
    Notorious for suspension set very stiff, if you see him bottom (rare, his bike control usually sends it over the g-out) you know its a rough ride.
    oops I wasn't clipped in

  28. #28
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    Quote Originally Posted by J.B. Weld View Post
    Someone should mount an accelerometer on Aaron Gwin's bike and end this conjecture.
    That shouldn't be too hard, he does travel with his own mechanic.
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    I will leave it to you guys, but I think if you called up Ray's Indoor MTB Park or hit 'em up on social media, they could come up with a guy on a bike and a g-force meter - even if it's just a phone. They have some awesome wood berms, that would allow for multiple runs in identical conditions. And because they are smooth, the G-force meter will see much less noise.

    Course map shows plenty of 180* high-speed high-G turns:

    G Forces in Downhill and/or Dual Slalom Runs-coursemap.png


    The Red Bull berm:

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    The pump track turns are really high-G, and there's a double 180* s-berm on the XC loop that has a fast roll-in from a bridge. Those are the ones I can hit pretty hard, but I'm def. not the fastest guy there.

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  30. #30
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    Quote Originally Posted by Fleas View Post
    I will leave it to you guys, but I think if you called up Ray's Indoor MTB Park or hit 'em up on social media, they could come up with a guy on a bike and a g-force meter - even if it's just a phone. They have some awesome wood berms, that would allow for multiple runs in identical conditions. And because they are smooth, the G-force meter will see much less noise.

    Course map shows plenty of 180* high-speed high-G turns:

    Click image for larger version. 

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    The Red Bull berm:

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    The pump track turns are really high-G, and there's a double 180* s-berm on the XC loop that has a fast roll-in from a bridge. Those are the ones I can hit pretty hard, but I'm def. not the fastest guy there.

    -F
    But that wouldn't answer the question of what happens on a DH/ DS/ 4X track.
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    Quote Originally Posted by J.B. Weld View Post
    Someone should mount an accelerometer on Aaron Gwin's bike and end this conjecture.
    I'm going to assume that's been done. All the top guys that are collecting sus data are using accelerometers to collect that or am mixing up devices?

    More conjecture; I'd wager the pro street rider dropping 25' to flat on an uber rigid bike with 60psi tires sees far more G's than any DH racer.
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  32. #32
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    Quote Originally Posted by MOJO K View Post
    But that wouldn't answer the question of what happens on a DH/ DS/ 4X track.
    At Ray's, they had a competition where guys were dropping out of the rafters onto tiny little landing areas. Those G-forces were enough to break the floor. So, while not DH/DS/4X, maybe still a fair approximation. ...or more like a worst case scenario.

    And I can hardly believe that one berm can "generate" more/less G's because it's on a mountain instead of in an old building. The limit is on the rider, not on the track.

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    Quote Originally Posted by BlueCheesehead View Post
    Yep.

    I think you mis-read my post. I said the BERM would see the SAME force as both the mass and acceleration in both scenarios would be the same. F=ma --> F=(175+25)*a =(155+45)*a. The same 200# is acting on the berm. It does not care what % is in the rider and what % is in the bike.

    You might want to do your own force diagrams.
    Name:  8d1846a277c0489ded7b7013836585a2.jpg
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    if the berm is seeing 200lbf then the bike is seeing -200lbf which also means the rider is seeing 200lbf unless there is some other force acting on the rider.

    "It does not care what % is in the rider and what % is in the bike." that is what I said originally...but then you said it depends on perception blah blah blah. don't blame me for your confusing word soup.

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    Quote Originally Posted by dragracer88 View Post
    Well, OK then. A 45 second video with some words on it. Must be gospel then... it's on the interwebz, after all, and my physiology training and actual experiences are all wrong. Go figure.

    Quote Originally Posted by Fleas View Post
    At Ray's, they had a competition where guys were dropping out of the rafters onto tiny little landing areas. Those G-forces were enough to break the floor. So, while not DH/DS/4X, maybe still a fair approximation. ...or more like a worst case scenario.

    And I can hardly believe that one berm can "generate" more/less G's because it's on a mountain instead of in an old building. The limit is on the rider, not on the track.

    -F
    Those aren't "G forces", just force (mass x acceleration). Same as if you drop a rock onto the ground. Sure, you can put it in G force terms, but that's not what is usually considered G forces.

  35. #35
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    Quote Originally Posted by Spindelatron View Post
    Name:  8d1846a277c0489ded7b7013836585a2.jpg
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    if the berm is seeing 200lbf then the bike is seeing -200lbf which also means the rider is seeing 200lbf unless there is some other force acting on the rider.

    "It does not care what % is in the rider and what % is in the bike." that is what I said originally...
    That all makes sense to me now. Thanks
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    Watching the Shredmate review video on Youtube, a mild jump line gave their rider a peak G force reading of 3.6g. Meanwhile their demo shows just over 4. How accurate is that? Beats me.
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    Quote Originally Posted by Spindelatron View Post
    Name:  8d1846a277c0489ded7b7013836585a2.jpg
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    if the berm is seeing 200lbf then the bike is seeing -200lbf which also means the rider is seeing 200lbf unless there is some other force acting on the rider.

    "It does not care what % is in the rider and what % is in the bike." that is what I said originally...but then you said it depends on perception blah blah blah. don't blame me for your confusing word soup.
    Sorry, at 1g the rider does not experiencing 200# of force. You are forgetting to subtract the other force in the equation, the weight of the bike.

    Riders A and B do not suddenly feel like they are 200# just because they sat on a bike, which is exactly what your statement that the rider would see 200# of force would imply.

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    Quote Originally Posted by skiahh View Post
    ...



    Those aren't "G forces", just force (mass x acceleration). Same as if you drop a rock onto the ground. Sure, you can put it in G force terms, but that's not what is usually considered G forces.
    I'm not going to argue, but when someone says "G-forces" the meaning is how many times the normal force of gravity the body is subjected to (The berm doesn't care. The bike doesn't care.). Any change in direction will cause what we colloqiually call "G-forces" (e.g. vertical to horizontal, North to East, etc). A change in speed will do the same. The only thing it has to do with gravity is to know how much 1G is in the frame of reference in which the math is being done (in this case, on Earth).

    -F
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    Quote Originally Posted by dragracer88 View Post
    This is your most successful thread yet, don't screw it up now with more dragster nonsense!

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    Quote Originally Posted by BlueCheesehead View Post
    Sorry, at 1g the rider does not experiencing 200# of force. You are forgetting to subtract the other force in the equation, the weight of the bike.

    Riders A and B do not suddenly feel like they are 200# just because they sat on a bike, which is exactly what your statement that the rider would see 200# of force would imply.
    Here is another way to think about it G Forces in Downhill and/or Dual Slalom Runs-tallest-merry-go-round-world-carousel-tivoli-gardens-copenhagen-b681f2.jpg

    If the merry-go-round chair is 20% of the combined weight of a person plus the chair or 80% of the combined weight does the tension in the wire rope that goes to the top change? remember that tension is the only thing keeping the rider and the chair traveling in a circle instead of in a straight line.

    The berm in the above example is only thing keeping the rider and the bike traveling in a circle instead of in a straight line.

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    Quote Originally Posted by Spindelatron View Post
    Here is another way to think about it Click image for larger version. 

Name:	the-tallest-merry-go-round-in-the-world-carousel-tivoli-gardens-copenhagen-B681F2.jpg 
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    If the merry-go-round chair is 20% of the combined weight of a person plus the chair or 80% of the combined weight does the tension in the wire rope that goes to the top change? remember that tension is the only thing keeping the rider and the chair traveling in a circle instead of in a straight line.

    The berm in the above example is only thing keeping the rider and the bike traveling in a circle instead of in a straight line.
    Yes, the cable and the berm are the same in that they resist the force of BOTH the rider and the bike(or seat) and if the sum of the bike (seat) + rider are the same the reactive force in the cable or berm is the same. On that we agree.


    In the above scenario the rider only accounts for 80% of the tension in the cable and the chair accounts for the other 20%. Don't believe me, take the rider out of the chair and the tension will be reduced to 20% (do the math). Make the chair weightless and the tension would go down to 80%. That should make it plain to see what the force the rider experiences is only his/her mass (and any mass they are supporting) times the acceleration. In neither example, berm or merry go round, is the rider supporting the bike or chair.

    Look at it this way, the rider weighs 174# and has a 1# helmet just sitting on his head. The ground provides a 175# normal force upward to the rider with the helmet. Are you saying that the helmet is experiencing the full 175# normal force wall only exerting a 1# downward force (weight)? That is exactly the same as you saying the 175# rider (and rider only) on a 25# bike will experience a 200# normal force.

    If you do not get how the sum of forces work, I am sorry. I am an engineer, not a teacher. This dead horse has been beat enough.

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    Quote Originally Posted by BlueCheesehead View Post
    Yes, the cable and the berm are the same in that they resist the force of BOTH the rider and the bike(or seat) and if the sum of the bike (seat) + rider are the same the reactive force in the cable or berm is the same. On that we agree.


    In the above scenario the rider only accounts for 80% of the tension in the cable and the chair accounts for the other 20%. Don't believe me, take the rider out of the chair and the tension will be reduced to 20% (do the math). Make the chair weightless and the tension would go down to 80%. That should make it plain to see what the force the rider experiences is only his/her mass (and any mass they are supporting) times the acceleration. In neither example, berm or merry go round, is the rider supporting the bike or chair.

    Look at it this way, the rider weighs 174# and has a 1# helmet just sitting on his head. The ground provides a 175# normal force upward to the rider with the helmet. Are you saying that the helmet is experiencing the full 175# normal force wall only exerting a 1# downward force (weight)? That is exactly the same as you saying the 175# rider (and rider only) on a 25# bike will experience a 200# normal force.

    If you do not get how the sum of forces work, I am sorry. I am an engineer, not a teacher. This dead horse has been beat enough.
    Thanks for the discussion...I think you are right, my mistake.

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    Quote Originally Posted by TheDwayyo View Post
    This is your most successful thread yet, don't screw it up now with more dragster nonsense!
    Things I have learned from, or been reminded of by, this thread:
    • Centripetal / centrifugal force will always trip up students no matter how you explain it
    • Reference frame is everything in a FBD
    • Engineers like thinking they are teachers but aren't always (we were mansplainers before it was a thing)
    • You can brag about having landed on an aircraft carrier (third wire every time, surely) at the same time you forget that dragsters have parachutes.
    • There are Youtube videos that consist entirely of text from an email from fifteen years ago
    • Top fuel dragsters race on 1000 feet, not a quarter mile, and a guy pretending to be named "dragracer88" didn't know that
    • Top fuel dragsters really are pretty amazing
    • Aaron Gwin likes a stiff suspension but that assertion came along with a reason he wouldn't need it

  44. #44
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    Quote Originally Posted by dragracer88 View Post
    Quote Originally Posted by Fleas View Post
    At Ray's, they had a competition where guys were dropping out of the rafters onto tiny little landing areas. Those G-forces were enough to break the floor. So, while not DH/DS/4X, maybe still a fair approximation. ...or more like a worst case scenario.

    And I can hardly believe that one berm can "generate" more/less G's because it's on a mountain instead of in an old building. The limit is on the rider, not on the track.

    -F
    Quote Originally Posted by Darth Lefty View Post
    Things I have learned from, or been reminded of by, this thread:
    • You can brag about having landed on an aircraft carrier (third wire every time, surely) at the same time you forget that dragsters have parachutes.
    Didn't forget the parachute on dragsters and not the 3-wire every time. And not really bragging; I didn't land the plane since I was an NFO... just adding frame of reference, in that I've accelerated and decelerated with quite a bit of (horizontal) force.

  45. #45
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    Quote Originally Posted by Darth Lefty View Post
    Things I have learned from, or been reminded of by, this thread:
    • Centripetal / centrifugal force will always trip up students no matter how you explain it
    • Reference frame is everything in a FBD
    • Engineers like thinking they are teachers but aren't always (we were mansplainers before it was a thing)
    • You can brag about having landed on an aircraft carrier (third wire every time, surely) at the same time you forget that dragsters have parachutes.
    • There are Youtube videos that consist entirely of text from an email from fifteen years ago
    • Top fuel dragsters race on 1000 feet, not a quarter mile, and a guy pretending to be named "dragracer88" didn't know that
    • Top fuel dragsters really are pretty amazing
    • Aaron Gwin likes a stiff suspension but that assertion came along with a reason he wouldn't need it
    Ok. I know you aren't ridiculing my choice of a screen/user name. It's irrelevant. So I like drag racing, big deal. There's no pretending anything, thanks for being a smartass. It's a username. Didn't think I had to bring this up, but funny someone with a Sith prefix for their username is trying to ridicule me about my name choice on a forum. Yes, nitro classes run 1000 foot now for safety reasons , following the death years ago of John Forces teammate but that's beside the point of this thread. 98 percent of smartass, irrelevant or asshole posts is why I rarely visit this forum anymore.

    9 out of ten times, I just look for help or a challenge such as my race challenges in past threads or search for a riding partner and instead, I just get mostly excuses, bullshit or something I'm not looking for. While I appreciate the handful of direct, on topic, serious and helpful replies and posts, most need to stop.

  46. #46
    Thinking about riding.
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    Quote Originally Posted by dragracer88 View Post
    Ok. I know you aren't ridiculing my choice of a screen/user name. It's irrelevant. So I like drag racing, big deal. There's no pretending anything, thanks for being a smartass. It's a username. Didn't think I had to bring this up, but funny someone with a Sith prefix for their username is trying to ridicule me about my name choice on a forum. Yes, nitro classes run 1000 foot now for safety reasons , following the death years ago of John Forces teammate but that's beside the point of this thread. 98 percent of smartass, irrelevant or asshole posts is why I rarely visit this forum anymore.

    9 out of ten times, I just look for help or a challenge such as my race challenges in past threads or search for a riding partner and instead, I just get mostly excuses, bullshit or something I'm not looking for. While I appreciate the handful of direct, on topic, serious and helpful replies and posts, most need to stop.
    Here's a perfect example of why that's your experience; you just skipped through ~40 posts that are actually seriously discussing the topic you created and only responded to one smart ass.

    The internet just doesn't suit you very well. You have to assume that there will be smart ass posts, that's a given on the internet, so just ignore those and respond to the good ones. This is a productive discussion, no need to whine about it like you do in all your threads.

  47. #47
    rth009
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    Quote Originally Posted by WHALENARD View Post
    Same as G-bong?

    I wander how many negative G's I can pull with like a 3 liter bottle, man.
    thanks for the flashback, man!

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