How to calc road gap speed [Archive] - North Shore Mountain Biking Forums

synchro

03-13-2003, 06:32 PM

Hey for those of you in on the How much risk thread, I've just posted the physics need to calculate your take-off speed for a road gap. I'm simply making a new thread to direct y'll to the solution in case you haven't checked the thread or weren't planning on going back.

road gap calculation (http://bb.nsmb.com/newforum/showthread.php?s=&postid=179025#post179025)

:thepimp: :thepimp: :thepimp: :thepimp: :thepimp: :thepimp: :thepimp:

Rexington420

03-13-2003, 06:45 PM

wow...
looks like you know your math!
i wish i did:rolleyes:

125

03-13-2003, 06:54 PM

holy shit your a :nerd:

synchro

03-13-2003, 06:58 PM

Actually no, more of a jock. I can easily understand all of that crap but I found it kinda booring most of the time so I never bothered to study or take it very far. I actually work as a personal trainer. Check out http://www.bodymasterspt.com

.glib

03-13-2003, 07:11 PM

Nice coverage of the first chapter in Phys12 :).

You didn't need to use trial and error. You could have just solved for Vx, but hey, I doubt I would have put the time to write it all down and explain it. Good on ya.

The only problem is you haven't factored in wind resistance, which becomes more and more important as the gap/drop (really the time in the air...so the angle of the launch also matters) increases. Then the acceleration of gravity isn't quite 9.8, and the horizontal velocity isn't constant. But that'll give you a general idea yes.

Where are you taking phyiscs? I did this in Grade12 physics in grade 10. :???:

synchro

03-13-2003, 07:25 PM

Originally posted by Gandalf
Nice coverage of the first chapter in Phys12 :).

You didn't need to use trial and error. You could have just solved for Vx, but hey, I doubt I would have put the time to write it all down and explain it. Good on ya.

The only problem is you haven't factored in wind resistance, which becomes more and more important as the gap/drop (really the time in the air...so the angle of the launch also matters) increases. Then the acceleration of gravity isn't quite 9.8, and the horizontal velocity isn't constant. But that'll give you a general idea yes.

Where are you taking phyiscs? I did this in Grade12 physics in grade 10. :???:

Sorry, but you can't solve for Vx on an angled take-off unless you know the amount of time you spend in the air first or the initial velocity. The whole point of this exercise is to find out what your initial velocity needs to be, so you have to do a bit of guesswork. Air resistance is a negligible factor, but wind resistance only needs to be taken into account if it's anything over 2 km/h.

I'm not taking physics, but this is a good example of a Gr 12 exam or 1st year college QUIZ question. I had to do some physics for my exercise science program.

Neo

03-13-2003, 07:26 PM

oh christ just hit it and if you come up short use more speed next time and if you over shoot, then slow down. christ, i don't bring my calculator or sliderule on fucking ride :rolleyes:

synchro

03-13-2003, 07:30 PM

Originally posted by Dante Alighieri
oh christ just hit it and if you come up short use more speed next time and if you over shoot, then slow down. christ, i don't bring my calculator or sliderule on fucking ride :rolleyes:

So if you come up short, case the rear, plant your face into your stem or a rock or something and are laying there with your brain oozing down the side of your skull are you still gonna 'just hit it' again?

Neo

03-13-2003, 07:33 PM

lol, its all about judging the risk. i know my physics believe me, but to sit there and do all these calcs and measure height and distance is retarded. you either have the confidence to do it or you don't. sure i've bailed hard before but thats how we learn. my point was that its kinda lame to be going through all that trouble to hit a gap. if you have any doubts in your skills to do it, then why try it?

synchro

03-13-2003, 07:40 PM

Yeah I know waht you're saying. When I posted that monster I knew I was settin myself up a bit. But I don't think it's necessary to do a whole buch of calcs on the side of the trail. I've never done or seen someone do a big gap like the 30/40 example, but at least now I've got a really good idea of the speed needed to do it right.

Scarface

03-13-2003, 07:41 PM

Originally posted by Dante Alighieri
lol, its all about judging the risk. i know my physics believe me, but to sit there and do all these calcs and measure height and distance is retarded. you either have the confidence to do it or you don't. sure i've bailed hard before but thats how we learn. my point was that its kinda lame to be going through all that trouble to hit a gap. if you have any doubts in your skills to do it, then why try it?

if i really wanted to do it and had the balls i would use this calculation to help me in gaining more confidence. i think knowing that you will make it will help a ton. but you are also right were not gonna be sittin there with a calculator calculating

.glib

03-13-2003, 07:43 PM

Originally posted by synchro
Sorry, but you can't solve for Vx on an angled take-off unless you know the amount of time you spend in the air first or the initial velocity. The whole point of this exercise is to find out what your initial velocity needs to be, so you have to do a bit of guesswork. Air resistance is a negligible factor, but wind resistance only needs to be taken into account if it's anything over 2 km/h.

I'm not taking physics, but this is a good example of a Gr 12 exam or 1st year college QUIZ question. I had to do some physics for my exercise science program.
Vx = VcosØ
Vy = VsinØ

h = -½gt² + VsinØ
x = VcosØt

Solve the h formula for t, then plug that into the x formula. You then have a formula that relates x (the gap), h (the height), Ø (the angle) and V (your velocity, and your only unknown).

Solve for V.

Anything I'm missing?

Action Hero

03-13-2003, 07:50 PM

Why not just ride as fast as you can off and hope you don't break you legs?

Shane you know I :love: you.

synchro

03-13-2003, 08:04 PM

That only works if the launch and landing are on a level plane. In my example, there is a step down gap. Subbing in the solved equation for time will not take into account the inital velocity the rider will have after reaching his max height and and then returning to the level of the launch. This initial velocity will result in the rider falling for a shorter timeframe, read falling faster, than if he simply fell from the specified height. That why the problem must be broken into two parts. Since we don't know the initial velocity, we do not know the vertical distance to the apex of the riders path. We must first find the distance and time to the apex, and then we can find the distance and time for the return to earth. It is the landing at a different height than the launch that makes this problem tricky.

.glib

03-13-2003, 08:10 PM

Ah right. Forgot about that. It's been a while since I've done this.

Okay then:
Vy² = V0y² + 2ad
d = (VsinØ)²/(2g)

So now your system is:
(VsinØ)²/(2g) + h = -½gt² + VsinØ
x = VcosØt

Solve for t in the first one, plug into the second, solve for V.

Neo

03-13-2003, 08:21 PM

so whats the point of all that anyways? you gonna hit that road gap gandi?? :lol:

.glib

03-13-2003, 08:28 PM

Originally posted by Dante Alighieri
so whats the point of all that anyways? you gonna hit that road gap gandi?? :lol:
Oh hells yeah. Would you pack the landing over there a bit more? I'll try to time it so that if I need to bail, I'll be able to drop into the water near the ferry for easy rescue.

dudski

03-13-2003, 08:30 PM

Originally posted by tweek
Why not just ride as fast as you can off and hope you don't break you legs?

sounds like he's the only one that knows his shit:P '

thats my kinda person . sounds liek you're not the sharpest tool in the shed when it comes to math(like me:o )

shorebiker

03-13-2003, 08:44 PM

Originally posted by Dante Alighieri
oh christ just hit it and if you come up short use more speed next time and if you over shoot, then slow down. christ, i don't bring my calculator or sliderule on fucking ride :rolleyes:

hehe ive gotta say im with you on that. i seriously think its rad how you can calculate that but i suck at that kinda thing, if my physics teacher wasnt so nice i woulda failed physics 11:P trial and error is always a way to go, but it could go very bad. last road gap i tried on my bmx i clipped the rail and ate it :S but now i know what to do if i want to hit it again.. i was hesitant coming into it, so next time i have to put in more pedal strokes, try a wider line so i dont slip out on the turn.. that kinda stuff :) first try i just went up to it bit my lip and just hoped to make it across the rail and onto the tranny

Action Hero

03-13-2003, 09:12 PM

Originally posted by dudski
sounds like he's the only one that knows his shit:P '

thats my kinda person . sounds liek you're not the sharpest tool in the shed when it comes to math(like me:o )

This crap bothers me, you're taking away from the ride.

If you want to do somthing, use you're skills and progress....If you're a math wiz with no skills you probably won't stick the landing.

Props for figuring that out, but dude....just ride.

synchro

03-13-2003, 09:21 PM

I do just ride, I'm only looking at the math for somethin extreme. It's not everyday that someone will do a 30/40 stepdown or Rosehill or something. This is just a little extra info to help you nail it if you so desire. When I ride I use my skills and my experience to figure out what to do. I don't think I'm taking away from the ride at all. In the case of going extreme it's just an examlple of one extra thing you could do to increase the safety margin. It's not like I'm riding with a laptop to calculate my every move.

How

03-13-2003, 09:23 PM

this is my safety sir

Action Hero

03-13-2003, 09:24 PM

:kiss:

Perhaps, I'm just a little ticked right now.....

Bonecracker

03-13-2003, 09:25 PM

1 thing we dont exactly have a speedometer on r bikes do we so even know this is really cool u did this its useless in a trail cause like no one uses speedometers on big shit:P
sorry to rain on ur parade but ya

snow_homie

03-13-2003, 09:26 PM

(VsinØ)²/(2g) + h = -½gt² + VsinØ
x = VcosØt

and I thouhg programming was complicated :eek:

synchro

03-13-2003, 09:40 PM

Originally posted by Bonecracker
1 thing we dont exactly have a speedometer on r bikes do we so even know this is really cool u did this its useless in a trail cause like no one uses speedometers on big shit:P
sorry to rain on ur parade but ya

You actually don't need a speedo, just measure off a distance like maybe 10m, and have a buddy time you from the start to the finish. Then you can figure out the speed. :P

It's not all super dialed, but at least you won't overshoot by like 20ft or come up 10ft short. I like my legs.

k-dawg

03-13-2003, 10:39 PM

i tried to do that last year, but the whole speed/time thing was getting anoying :P

i'll check it out :)

Loopie

03-13-2003, 11:06 PM

First of all guys......relax on dissin the syncro....he's just illustrating for the sake of illustrating. Not saying the math is something you guys should be doing for every 10'er you come across:rolleyes:

I believe almost ALL the BIG-ASS new drops/gaps and such that are attempted are figured out a bit mathematically before they are hit.
That Gillard road gap......they calculated the speed needed(that guy that cased just didn't reach that speed is all)...they didn't just hit it blind. Steve said he had to be goin' 57km/hr to clear it....I didn't ask him if he had a speedo...but he might have.

But I also think there are guys that can hit BIG stuff by eyeballin' it......if you take a guy that has hit a LOT of 30,40, 50 whatever's...he can probably just "go for it" on stepping it up one notch.

Then there's BIG stuff that's been hit before.....and you have the luxury of watching someone hit it.....for most good riders, just seeing someone do a hit is enough to give you the knowledge you need to get in the ball park at least.......and MOST big-ass stuff has a reasonable allowance for overshooting a bit too I find....most good riders will nail it or overshoot a bit.....not usually case the first time. Casing is from being lazy or stubborn. You guys know what I'm talking about! :lol:

But anyways......without a speedo on the bike.....the exact calculations would be hard to put into practice.....Cool:cool: that they can be calculated though:)...and I'm sure they are for the latest and greatest stunts out there.

TREV

03-13-2003, 11:08 PM

point and shoot:P

synchro

03-13-2003, 11:22 PM

Originally posted by Loopie
First of all guys......relax on dissin the syncro....he's just illustrating for the sake of illustrating. Not saying the math is something you guys should be doing for every 10'er you come across:rolleyes:

Thanks dude, someone finally understands me! I think I'm going to cry.

peppergut

03-14-2003, 08:30 PM

Synchro,
Actually you can calculate the speed with out having to guess! To actually solve the equation by hand you need to be an algebra expert, otherwise you can use a solver program like Mathematica.
It can be done because you can reduce the problem to 1 equation with 1 unknown (Vo). heres how: we want the answer to the eqn: 14m = Vo*cos15*t1 + Vo*cos15*t2, where t1 is the time during the initial parabolic trajectory and t2 is the time is takes to drop the 10 meters. t1 is easy to calculate in terms of Vo, but you were very perceptive in the fact that you "fall faster" during t2. But, you can also calculate t2 in terms of Vo with the eqn 10m = 1/2*g*t2^2 + Vosin15*t2 , where the second term is gotten from the fact that, excluding air resistance, your initial y velocity will be the same at that point as when you took off. The above equation can obviously be solved quadratically, and now you have t2 in terms of Vo. So, plugging the answer back into the first equation has, which now only one unknown, but it looks intimidating, but you can use a solver to do the rest (as I did) to get an answer.

synchro

03-15-2003, 12:50 AM

Yeah you're right. One thing though, you have to make sure that the 10m drop is entered into the eqn as -10m, otherwise you will end up solving the quadratic trying to take the square root of a negative number, and then everything will fall to shit. By the way, did Gandalf get you to solve this and post it up here.? Just seems kinda odd for someone's very first post.

Ari

03-15-2003, 03:59 AM

I find it usually comes down to instincts, when you are hammering for that big gap.

But sometimes for example if the run-in is short or otherwise sh*tty, it might be a good idea to calculate an
estimate for the speed you need to clear that gap.
Then you can figure it out if it is somewhat possible to achieve that speed.

In the real-life situation it is still all about your instincts.

Oh, btw.. as an attachment i have provided my solution to the "roadgap speed" problem. Plug in the dimensions and the possible take-off angle, and you're all set.

Oh yeah, it works for normal gap jumps too (height difference = 0)
:thepimp:

peppergut

03-15-2003, 11:27 AM

Synchro,
I dont know anyone on this board, I'm just a long-time lurker, but I like this kind of stuff too (physics major). Im surprised that people on this board dont find this kind of stuff interesting. Riding, of course, is why we came to this site in the first place, but understanding how/why stuff works the way it does just seems to go hand in hand with it. BTW, Isnt Dangerous a physics teacher?

.glib

03-15-2003, 11:37 AM

Originally posted by peppergut
BTW, Isnt Dangerous a physics teacher?
WAS a physics teacher. I believe he also was a hobby carpenter.

A few of my teachers used to know him before he really got into "extreme" riding. They've talked to him recently and said he's a much different person. Like personality...everything. That many knocks on the head will do some damage...

K Towner

03-15-2003, 11:53 AM

I got 50 percent in math 11..
But I do know my horizontal speed effects flight time fo sho.
not only does my arc go longer, I pop up higher, and drop farther.
So I dont know how right that equation would be for a road gap??

.glib

03-15-2003, 12:14 PM

It's right K-town. I won't bother explaining. Take our word for it.

The Vision

03-15-2003, 12:21 PM

There is a time when calculations are just plain stupid. In the case of bike gaps, calculations are stupid.

How the hell are you supposed to know what 32.6843 km/h is anyways?

The Vision

03-15-2003, 12:22 PM

Originally posted by K Towner
I got 50 percent in math 11..
But I do know my horizontal speed effects flight time fo sho.
not only does my arc go longer, I pop up higher, and drop farther.
So I dont know how right that equation would be for a road gap??

But he is right about bunny hopping from the lip. Calculations don't take that into account. There are just tooooo many factors.

counterpoint

03-15-2003, 12:30 PM

Originally posted by The Vision
There is a time when calculations are just plain stupid. In the case of bike gaps, calculations are stupid.

How the hell are you supposed to know what 32.6843 km/h is anyways?

because you will at least know 32 km/h is not 10 or 15 or even 20km/h. They are not stupid.

K Towner

03-15-2003, 12:31 PM

hehe.. no kidding. just round up. Im shure its closer then what you can judge pedaling ;)

synchro

03-15-2003, 08:41 PM

Originally posted by peppergut
Synchro,
I dont know anyone on this board, I'm just a long-time lurker, but I like this kind of stuff too (physics major). Im surprised that people on this board dont find this kind of stuff interesting. Riding, of course, is why we came to this site in the first place, but understanding how/why stuff works the way it does just seems to go hand in hand with it. BTW, Isnt Dangerous a physics teacher?

Yeah, I hear ya. I not big into physics and stuff, just didn't have the patience for it in college unless I could directly relate to something important in my life. But is is kinda cool to use physics to see how it affects mtb riding. BTW, I was only looking at this with a pen and piece of paper at the time, so I had to work from memory. Because of my approach to solving the problem (splitting into the the upward displacemnt and downward displacement) the idea of solving for one variable didn't even occour to me at the time even though it was basically sitting under my nose.