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Over The Bars... but why...?

downy_ball

Chimp
May 2, 2019
6
0
Cali/SF East Bay
Long time road cyclist putting in 80-100 miles each week in the Oakland / Berkeley hills mostly. I rode a mountain bike way back in 1983/4. It was a chrome Mongoose with a Marzocchi fork. The the trails where I lived at the time sucked so rather than doing premeditated damage to my head, body and limbs I stuck with being a roadie. Full circle and now I have an Ibis Mojo 3 and ride all over the amazing SF Bay Area and spend the majority of my time mtb'ing.

Most recently I've had a horrible sinus infection for the whole month of April, so no riding really. Which finally brings me to the title of this post. In my down time I've entertained myself by watching mtb videos of all sorts. Some good, some bad, some super bad. Under the heading 'Fails' there is much to ponder, most notably why do so many riders go over a jump or drop or ramp only to find their front tire making a beeline for the surface of the planet and their body doing the inevitable? I see it in so many videos and can't for the life of me figure out the physics behind it all.

I'm sure it's all body positioning or weighting and unweighting but I would really love for someone to explain it to me so I can start saying "Oh man... they should have... "

Thanks.

:busted:
 

Jm_

sled dog's bollocks
Jan 14, 2002
13,567
5,209
AK
Speed, confidence, was taking with my AZ buddy here last night about upper body strength and holding yourself rigid when going downhill, rather than just “collapsing”, that also goes for keeping the front wheel straight, if it goes sideways, you’ll endo.
 

OGRipper

back alley ripper
Feb 3, 2004
10,248
689
NORCAL is the hizzle
Lots of reasons this can happen. If you post a specific video you'll probably get plenty of opinions. It's often a lack of technique, or applying the wrong technique to a particular situation. The jump matters - steep, short lips can be a problem if you're a novice. (Sometime the front wheel can be already heading down before the rear is off the ground.) Suspension can often play a role as well - a soft front end coupled with a fast rebounding rear shock can make a bad kick even worse. Also, in some of those videos you also see people grabbing the front brake in a panic, which makes a bad situation worse.
 

HardtailHack

used an iron once
Jan 20, 2009
3,240
963
It's never the rider, I'm guessing they all had Fox CTD shocks with no damping* left in them.

*Is it dampening in 'murica?
 

rockofullr

confused
Jun 11, 2009
7,345
922
East Bay, Cali
Most OTB on jumps come from a combination of two things.

1. Lack of jumping technique (for the particular jump). If you can't do a proper bunny hop/j-hop or at least a good wheel lift you shouldn't be launching yourself off of jumps. Most beginners ride off a jump and allow the jump to set their rotational momentum in the air. This sometimes works for long wedge type jumps but fails miserably with steep dirt jump type jumps. If the jump sets your angular momentum too far forward or back you're gonna eat shit.

You should be preloading as you approach the jump, slightly bending your elbows and knees to get your chest and hips closer to the bike then when you are leaving the lip you will actually be "popping" to set your angular momentum to essentially 0. If you watch a talented rider hit a jump they will perform a j-hop/bunny hop as they leave the lip. They will be pulling up on the bars while extending their legs. This allows them to set a fairly level flight path despite the varying shapes of the jumps.

2. poor suspension setup. Too much rebound in the rear or two little in the front can unbalance you in the air. The proper settings will depend on the speed and shape of the jump as well as the bike. If you've been jumping your bike long enough to have good technique you've probably figured out how you like your suspension set up.

I've been trying to teach my girlfriend to jump and bunny hop so all the things I used to just do I've been trying to put into words. It's not easy since endless hours on the bike are the true key. There are some YouTube videos that I've found helpful. Follow Phil's progression of videos from bunny hop to jumping.



 

downy_ball

Chimp
May 2, 2019
6
0
Cali/SF East Bay
Thank you for the descriptions and videos. I know it's hard to put into words but I understand a lot more than I used to. And Phil's vids are the only ones I'll watch for any real advice or help. I did find this when I was poking around and thought it did a good job of putting all of this into words and pictures:

https://medium.com/@Duffsam/mountain-biking-the-dead-sailor-a-physics-perspective-8ed023d7a85a

The drawings were especially helpful.
fyi - I'm in the East Bay too, and a long time ago grad from UCSC ;-)


Most OTB on jumps come from a combination of two things.

1. Lack of jumping technique (for the particular jump). If you can't do a proper bunny hop/j-hop or at least a good wheel lift you shouldn't be launching yourself off of jumps. Most beginners ride off a jump and allow the jump to set their rotational momentum in the air. This sometimes works for long wedge type jumps but fails miserably with steep dirt jump type jumps. If the jump sets your angular momentum too far forward or back you're gonna eat shit.

You should be preloading as you approach the jump, slightly bending your elbows and knees to get your chest and hips closer to the bike then when you are leaving the lip you will actually be "popping" to set your angular momentum to essentially 0. If you watch a talented rider hit a jump they will perform a j-hop/bunny hop as they leave the lip. They will be pulling up on the bars while extending their legs. This allows them to set a fairly level flight path despite the varying shapes of the jumps.

2. poor suspension setup. Too much rebound in the rear or two little in the front can unbalance you in the air. The proper settings will depend on the speed and shape of the jump as well as the bike. If you've been jumping your bike long enough to have good technique you've probably figured out how you like your suspension set up.

I've been trying to teach my girlfriend to jump and bunny hop so all the things I used to just do I've been trying to put into words. It's not easy since endless hours on the bike are the true key. There are some YouTube videos that I've found helpful. Follow Phil's progression of videos from bunny hop to jumping.



 

rockofullr

confused
Jun 11, 2009
7,345
922
East Bay, Cali
https://medium.com/@Duffsam/mountain-biking-the-dead-sailor-a-physics-perspective-8ed023d7a85a

The drawings were especially helpful.
fyi - I'm in the East Bay too, and a long time ago grad from UCSC ;-)
That seems like a lot of over thinking, but I do like the effort that went into it from a physics perspective. You're not going to be doing free body diagrams in your head as you hit a jump. Add in the fact that the way you hit a steep/lippy jump is very different from how you hit a long wedge shaped jump and there's just too many variables.

I'm currently recovering from a broken collarbone but I'll hit you up for a ride once I'm back in the saddle. I generally ride JMP, Pleasanton Ridge, Lime Ridge, and the Pleasanton BMX Track during the week then do some trips to SC or up to the mountains on the weekends.