Jackknifing Your Bike Hurts... Stop Doing It | How NOT To Bike with Ben Cathro

There we go. Hi, picture this. You're cruising down the trail. It's all flowing like it does in your dreams. Not a care in the world. Traction for days. And then, bam, wheels turn. Jackknife, throws you off. Next thing you know, you're on the ground and you're questioning reality. This isn't a super common incident, but when it does happen, it can leave you pretty clueless as to why it happened. That's where I come in. Who is this Jack? Why did he knife you? Let's go get that stabby prick. Jackknife. What is jackknifing? I was hoping this term would hold more mystery, history, or even a little bit of intrigue. Maybe there was a hitchhiker named Jack who knifed a truck driver and the resulting crash and folding of the articulated truck into the now familiar position was named after the incident. Imagine my crippling dissatisfaction when the top search result spelled out in simple words that a jackknife is a large folding pocket knife. My disappointment is immeasurable and my days ruined. I shall bravely endeavor to battle through my rapid onset depression to provide you with the contextual information to smoothly transition into the why does it happen section. The original use of jackknifing is a reference to articulated trucks that have a trailer and a separate tractor in front which holds the engine and steering mechanisms for pulling the aforementioned trailer. When the trailer is loaded in a way that there is not enough weight over the driving wheels of the tractor or the road conditions are slippy, it is possible for the rear wheels of the tractor to slide under hard braking. If this happens, the weight of the trailer can push the tractor into a yaw that if not caught in time, can fold the tractor into the side of the trailer in a motion that is akin to a folding knife closing most of the way, the jackknife. But that's on an articulated truck with a pivot point between the two wheeled objects. If that is the definition, then bikes cannot jackknife. So if bikes can't jackknife, we need to find out what is actually happening and how it happens and why. Well, class, despite this slightly mundane history lesson and the fact that a bike isn't actually capable of jackknifing, the type of crash we've borrowed that name to represent involves a very interesting combination of terrain and physics. Yes, Randall. Well, unfortunately, the reason I got into teaching was for the long holidays and the fact that I get to be abusive towards children. Sit down. Let's set the conditions of what a mountain bike jackknife actually is. We can do this by saying that the front wheel is the tractor and the rest of the bike is the trailer. If the front wheel for reasons currently unknown is turned and there is a deceleration force acting upon that wheel and the contact point of the front wheel with the ground is off-centre towards the direction of the turn and the contact point of the tyre is in front of the steering axis, then the mass of the bike and a large proportion of the rider acts in the direction of travel and then these two forces act like levers that just twist the wheel round. Jackknife. Kinda. Well, this is not a true jackknife. It is a close fit for this juicy little mishap. You may have heard it called just like a knife or even a tuck. The term tucking the front is actually the correct name for this, but I mean, jackknifing is a better video title, so there we go. Bicycle uses a clever mix of head angle, which is the angle of the steering axis measured from horizontal and offset, which is the perpendicular distance that the front axle is mounted from the steering axis. This combination of these two geometries creates another called trail, which is the horizontal distance from the tyre's contact patch with the ground to where the steering axis intersects the ground. This trail measurement is our topic of interest today. Another. So what things can decrease the trail measurement? Well, increasing the offset would decrease the trail, but that's not exactly easy to do. And everyone's actually moving to a shorter offset for forks at the moment for different reasons. If the head angle is set to perpendicular to the ground, so it has effectively a 90 degree head angle and the offset was zero, we'd have zero trail. That means we'd actually need a negative head angle to move the tyre's contact point in front of the steering axis. And that, children, isn't really possible. Quelle surprise. We know that steepening the head angle can increase the chance of this happening because it reduces the trail. So if your forks are too soft and compressed into their travel, it steepens the head angle, which reduces the trail and could increase the chance of Jack sneaking in there. Oh, nice. Well, now we need to look at the terrain. We only need to make one little change to our diagrams here and little Jacky boy can knife away to his heart's content. Bikes aren't actually always parallel to the ground. Sometimes they come in, say, nose heavy from a drop or a jump, slightly off balance with the wheel turned just a little bit. Which means the contact point is definitely in front of the steering axis. And that could cause a jackknife. I mean, there's the contact point, the force coming in. Or sometimes the front wheel hits an object or an upslope. Leave a brown crayon for this one. And big object. And look at that. There's the contact point. There it is right there. And where is it, ladies and gentlemen? That's right. It's in front of the steering axis. Jackknife. Yeah. Is that convincing? Can you just, can you do that in post? There is one other factor that is tricky to conceptualise. And it's that when the bike is leant over and the wheel is turned, the contact point of the tyre actually rolls forward along the edge of the tyre. That means steering while leant over can increase the chances of a jackknife. Then if you factor in a slight front wheel washout, that increase in lean angle encourages more turning input to try and compensate. And then an object catches the tyre and it grips. Now we have the perfect recipe for a knife jack. Rendall, what's up? Oh, pest yourself. Well, good. Get up here, clean my board so I can do some viewer reviews. That's right, come on. Come on, get in there. Wet cloth, wet cloth for the wet boy. Come on. You. That's piss. You absolutely. It smells like piss. You're telling me. So we've got Addison Everett here. We've got two angles in this one. Oh no, that was sudden. The double angle though. So that front angle is the one. That's what's really showing us a good clear image of what's going on. So that was a beautiful jack knife. So he comes down up to this rock and that sucker just turns on him. Absolutely horrific. So the issue here, now this is something that you have to be professional to figure out, is he rode into this big rock instead of going round it. That's the problem. Okay, next thing. Yeah, like usually, usually most common thing for getting offline and riding into stuff is looking in the wrong place. He's gone, oh yeah, look at that huge rock and ridden into it because he's looking at it. So you just go, like you'll balance for wherever you're looking. Or maybe it looks like it's a race and there's loads of people watching. He just tried to go too fast and just rode straight into it. And then the front edge of the tire hit it as he was turning and it's just folded up on him. This next one, I think it was uploaded by DarkDustin. I don't know if that's the right username or if that's the rider, but let's have a watch. Whoa, my boy Steve. Apparently it's his boy Steve and his boy Steve absolutely stoved his front wheel into that turn. That was a jack knife. Absolutely, I loved it. I love the energy. You got to have friends that give you the hype. So he comes into the corner. You can see, it's kind of tricky to see because of the way it's been filmed, but he definitely comes in quite tight into the corner, which means he has to do like a sudden tight turn. And it looks like he gets kind of unweighted coming into the turn as well. So the front wheel's in the air and then it hits the corner at an angle. And that just causes front wheel to kind of like hit that edge and just turn on him. It's not able to kind of like see and get a good drawing. You're just going to have to trust me. That is what happened. My boys. But I'll get a pause and we'll get a zoom in because you can see his wheel has turned and the wheel turning sharp like that caused a deceleration and actually put him over the bars. So that is an OTB by JK jackknife. So your boy Steve may come into the corner tight. It's absolutely fine. Coming in at that angle is not a problem. You just have to expect those forces that are going to try and pull your bar. So as he was coming into the corner, he either needed to try and keep the wheels on the ground so he could carve into it, or he needs to, with his core and his upper body, hold strong and keep the bike straight. If you can let, if you let the bike fold up on itself, that is something that can be easily resisted. It can be resisted with prep, with strength and with experience. So yeah, use those muscles. Keep that wheel pointing where you want it. Don't let the ground box you about and bicycle geometry and physics. Tell physics, I'm going that way. So now we've got Yannick Dominic, who actually did the crash I didn't think I would find. I had it drawn on the blackboard before. I was like, I don't think this actually happens. So he does a jump, comes in nose heavy. Oh, lands nose heavy and the bar turns. I can draw on this one because it's filmed in portrait. If anyone's going to crash in the future, can you do it? Not in portrait, profile, a profile view so we can get a good look of it. All right, come on, let's see it. Right, he comes in, that bike is small. The man is big, he touches down. Let's draw. If we look at the angle of the steering axis that's coming down like that to where the tyre kind of is touching the ground and you can see the tyre is touching the ground just in front of that steering axis. So that means that at the contact point where he's coming down, he's hitting at an angle. It's creating a deceleration force on the front edge of the wheel and all the momentum from his body is wanting to go that way and it's just going to twist that wheel round. Unbelievable, Yannick, thank you. You have illustrated what I drew earlier in a way that just cannot be bettered. Beautiful, beautiful. Ideally, work on jumping techniques so you don't know it's dive. And it is hard to tell from the profile angle that maybe he was a little squint and that's what caused his wheel to get turned. But in that situation, it is possible to save it. I mean, he was going to be tensed. He was going to be tense for landing. So I feel like he could hold it straight. I feel like he actually would have given the way he was coming in. He would have been bracing for landing. So yeah, I don't think there's much he could really do there other than get the jump takeoff. Actually better work on that jumping technique so he doesn't get pitched forward. So now we got a user, burn one up. Oh, this is a rocky bit. Oh my God, this is techie, techie trail. Oh God, his weight's back a bit. He rolled out of it good. Good bail. As so many people say in their situations. So for me, as a mountain bike coach watching this, just seeing him riding in, I'm waiting for the crash. So that's a big sign of what's been going wrong here. His body position is super, super defensive and you can see every hole the front wheel drops into, it's like the bike's pulling him into it. So if we get a little bit closer, there we go, he's right on the edge of a drop here. So you can see his front wheel is about to go off a step and his arms are already pretty straight. They're pretty straight. And when you're riding down kind of bumpy kind of steps and trails like this, you need to have enough movement that you can push the front wheel fully down these obstacles. If you don't have the amount of room available, then you need to be doing like a dropping technique. So he's got his weight back and then he straightens his arms the little bit of movement he's got left, but it's not enough. And then the wheel drops and drags him into each of these steps. And what that does is it causes a forward rotation and a weight shift onto the front wheel. So he drops into this hole, forks bottom out, head angle steepens, trail decreases. And you can see it's this front edge of the tire that is kind of made contact with the kind of upslope in the rock. Where that front edge is, it's in front of the steering axis. So that is pushing back with a lot of force, lot of force in there. That's a forceful bit of energy going on in there. And then his mass, the inertia of it moving forward and being dragged forward as he went off of that step is going in this direction. And that poor little wheel is just like, oh God, please no, no more, as it gets wrenched round. And then if you look closely, it's kind of cool. He like turns the wheel, it folds the tire over, burps some air out and it just fires up a little dramatic cloud of dust. It's like someone's gone in there in After Effects and done dust explosion one. And so yeah, his wheel was forcefully turned to an angle not conducive with going along on a bicycle and he crashes. Mainly because he was being defensive and it's so frustrating that the people or just in any situation when you're like trying to be careful and defensive and ooh, actually causes more problems, which makes you more defensive the next time. So it's kind of frustrating, but a lot of time the answer is just be more confident or ride trails that are within your comfort level. And if you really don't feel like it, you don't have to do it. Even if your mate gets out your camera and then goes, go on, yeah, it'll be fine. Yeah, it'll be fine. Go on. Softie. Yeah, you big softie. Hey, that one wasn't too bad either, was it? No, no big crashes, some gentle examples. And you got to do a low tire pressure shroud, didn't you? You did. Naughty thing. Well, we figured out who the stabby burger was and it wasn't Jack, it was you. Don't beat yourself up about it. We all get a bit knifey sometimes, but now you know how to stop it. Good weight distribution, corner with his grip, look and aim early, then don't let the bike get away from you. Nail all these steps and I guarantee there will be a 99% reduction in jackknifing because remember, you can't actually jackknife a bike. Hi-ho. In. Jackknife. Graceful flop. Much grace. Sorry, Pike. Oh, it's a corner turn. My bike made a noise. And you're not too injured either. Not yet, still got more to do. Yeah, I think we got it. We've got it. Oops. He said it. Get a wee sleep now. All right.