I tried to research this on my own but couldn’t find anything…
In motorcycle circuit track racing, why do the riders put their “inside” knee down towards the pavement?
I ride a motorcycle and I love to take the tight turns, but I don’t usually put my knee out or down. In fact, my balance instincts are to shift my body away from the turn, so that the motorcycle leans underneath me.
I haven’t thought much about all the forces going on in a turn. I would guess that angular momentum tends to move the motorcycle away from the center of the turn (centrifugal apparent force) and that leaning into the turn is simply a way of having the wheels resist angular momentum.
At any decent speed, you’re not likely to drop sideways into the turn. To the contrary, the centrifugal force is pushing you away from the center of the turn. So why not just lean the whole motorcycle over with your inside knee against the tank?
Furthermore, I would think that leaning your knee out only invites you to either tear it off or smash in your kneecap if you do fall.
I’m hoping the physicists/motorcycler racers/masters of minutiae will be able to enlighten me!
Here is a good, medium-res photo of a motorcycle racer in a tight turn. You can clearly see that not only is his knee shifted towards the center of the turn, but his whole body is, as well. My WAG is that the rider wants to shift his weight in the direction of the turn in order to lean the bike as sharply as possible, which in turn allows the bike to turn as sharply as possible. Tight turns = faster lap time.
You hang off the bike (possibly getting your knee down as well) in order to reduce the amount that the bike is leaned over. Any motorcycle can lean over only so much before hard parts begin to drag and before you run out of sidewall on the tire. Neither of those are good things to have happen. I’ve got this book that has a better description (with diagrams!) of the physics behind the whole thing, but sadly I don’t have it here at work. So for now, you’ll have to take my word for it, hanging off is done to make the bike lean less.
A common rookie mistake that most new riders make is to lean out of the turn (they try to keep their upper body vertical) which causes the bike to run out of clearance much sooner than hanging off or keeping your body in line with the bike.
Great photo, demonstrating exactly what I’m talking about. I take PR’s word that it’s done to make the bike lean less; the physics of accelerating bodies is not intuitive! I’d still like to see the diagrams.
Ditto. Thinking about it, leaning your body toward the turn would shift the overall center of gravity, as a result the bike would have to lean less, since the whole system (bike + rider) is what is leaning to make the turn tighter. Makes sense to me.
Only posting to note that my father raced motorcycles for 25 years. I will call him and ask him and then come back to post the answer.
I will point out that, back in the day (when my dad raced – up until the mid 1970s), he had this steel shoe that went over his regular racing boots/shoes just on the left foot. It still hangs in his garage to this day. Because I was raised next to the race track, watching him lean way the hell over like that… I have asked for that steel shoe as part of my inheritance.
I’ll let you know what he says.
Oh, and Dorjan: I’m from the town Mid-Ohio is in. Nice form. Did you win that one? (You’re not from there, are you? Do I know you?)
Thanks! I’m from the Cleveland area, about 2 hrs away from Lexington. I was merely participating in a trackday there, not racing. Hopefully, I’ll make it in a WERA race next year. Most of my (hopeful!) racing season will be at Nelson Ledges.
Did your dad do flat-track racing? That’s usually where I see the steel overboots used. Flat track racing is insane!
Umm… yes, I think. Might have been called something different in the 60s and 70s. I don’t recall hearing that term before, but then again, I was 10 last time I was immersed in motorcycle racing culture/jargon.
He used to “run sprints” and race “short track” and sometimes “half mile” if any of those things mean anything to you. It was mostly a whole bunch of really crazy dudes chasing each other in very small circles on usually dirt (rarely pavement) with relatively low-powered bikes (say 275-400cc). This was waaaayyy before Superbikes (Which might be the coolest thing I’ve ever seen since Ice Racing at the now-gone Richfield Coliseum. You haven’t seen anything until you’ve seen a pile up on ice with 1/2" spikes in the tire treads!) He also said he tried his hand at something called “hare scrambles” in his younger days, but it beat him up too much so he didn’t do much with that.
Let’s see, when he quit (when I was old enough to remember) he was running a Bultaco… I wanna say 350cc, but I have no idea how close that is to accurate. I better call my dad if I’m going to continue this conversation intelligently!
I’ll also say, because I was weaned and raised at the track, I do not possess the ability to remain awake while watching races on TV. Mom put us down for naps in the back of the van, so when I hear the monotonous revving of engines for any length of time… yawn… it’s like a lullaby…
It’s a combination of factors. You can lean your body into the turn without completely sticking out your knee just as well. It doesn’t happen as naturally as professional racers make it seem. The main benefit is that it helps the rider better control rear wheel power sliding to turn the bike into the curve. Other benefits include: it helps the rider gauge the lean angle better, it can sometimes save a crash when the front end washes out, it’s another form of sensory input and it looks cool. Some will even defend that the “sail” effect is useful, although at the speeds most curves are taken the effects from air resistance on a small surface like the leg are surely minimal to negligible.
But dragging your knee should be a racing technique only. On the street it’s dangerous and the only reason to do it is to show off.
It’s interesting that any child can figure out the basic dynamics of cars on his or her own but motorcycles on the other hand can be quite complicated to understand.
I did used to drag my left foot on my 10-speed, when I took a turn too fast, because I’d seen my dad do it on his “bike”. Wore the soles off my sneakers.
I’m a doofus, I know.
Knee-dragging (only on road-course tracks) was pioneered in the era of Kenny Roberts. He was already hanging his body off the inside of the turn when he found, by accident, that he could use his knee as a skid to extend the absolute edge of disaster. Things changed. Racing leathers were made with a plastic, and sometimes metal overlay on the knees. Dunlop began making a rear racing tire with a more-or-less triangular profile. On the straights, it ran on a narrow stripe, and in the turns, it had a much wider footprint.
Later on, riders developed the two-wheel slide, but that’s another story…
Round these parts the motorcycle riders like wearing knee pads that produce showers of pretty sparks when in contact with the road.
Interestingly, it’s a technique I use to great effect when mountain biking, and though the speeds and forces involved are much less than with a motorbike, the effect is very pronounced as you’re still cornering on the limits of grip, and the rider: bike mass ratio is much larger on a bicycle, so any shifting about of the rider’s centre of gravity will significantly shift the C of G of the whole setup.
For a left-hand corner, say, shifting as much body weight as possible to the left and downwards will make the bike more upright, and the handling much more stable. It will be much less prone to squirrelling, and easier to recover from a slide. Motorcylists do this with arse end and knee, with maybe some knee-road contact at the corner apex. On a mountain bike I do it with torso and some knee, but kept well clear of the ground and knee not so bent that I can’t still pedal hard.
Shifting your weight around is a useful way of gaining extra stability in any vehicle that isn’t significantly heavier than your body. Forwards/backwards and up/down shifts of C of G also have their uses under certain circumstances.
A friend of mine has raced most of his life, his Dad put him on a bike when he was three. He raced the Pro Thunder series nationally for a few years until the AMA eliminated it*.
He explained to me thet you shift your butt over so your crack is at the edge of the seat, one cheek is always on the seat. (Not an elegant way of phrasing that, but hard to misunderstand.) As said the bike only has so much clearance at a lean that the pegs or muffler will scrape the ground and maybe dig in and scuttle the bike.
More often than not, the knee doesn’t touch the ground at all. Try this sometime on a bike on a centerstand, slide your butt half off the seat. Where is your leg? Keeping your knee tucked in is an akward position.
One of the most memorable times I had at Mid-Ohio was a Saturday night track walk during Superbike weekend. I was with this friend and another rider after dinner and we walked the whole track. They were compairing what lines they take into each corner and what brake markers they use where. It was facinating to me who loves my bike, but can’t bring myself to try out a track.
*rotten lousy no good AMA, Mid-Ohio was dull dull dull this year, they keep eliminating classes and there are fewer and fewer races grumble grumble.
Brilliant Google ads regarding “Gravity” and “Knee Replacements”
My Dad recounted that the most terrifying time he had on a bike was when he offered his land lady a ride on the back of his motorbike and she kept upright through every corner with Dad trying to lean in.
The most terrifying time for his friends and family was when he came off after hitting (and killing) a dog with the front wheel and coming off over the handlebars. Dad was either unconcious at the time of the accident or on morphine in the hospital afterwards so it was all a bit of a pleasant daze for him
This isn’t some huge sin either, leaning away from the apex on the street. It will not give you the best times on the track but it’s a faster way to lean the bike in an emergency and it reduces your lateral profile on left (right for Brits) turns so it allows you to take better trajectories without invading the opposite lane with your body. This is particularly important on narrow roads with blind turns. It’s always wise to enter a left turn away from the opposite lane (but not too much obviously) if you don’t have good visibility into the turn. For right blind turns, it’s also advantageous because it allows you to see farther into the turn. This is a lot more important than maximum corner speed.
I think that Jarno Saarinen was one of the earlier exponents of this, and its a technique that Kenny Roberts took much further by ensuring his machines were constructed with knee sliding in mind.
You also need to bear in mind that modern machines have very much wider tyres and part of the reason for this is to allow the rider to make use of hanging from the bike without running out of tread and going onto the sidewall.
Oh, wow, it sure did. Thank you! I remember going to Millersburg – it was an old quarry I think, because I remember playing in a big hole full of rocks about 2 feet from the track with a couple dozen bikes whizzing past my head. (I bet my mom hated that.) Seems like we also went to Dover, Newark, Mt. Vernon I think… and New Philly. All those little bitty towns scattered all over the state. I’ve probably been to Bike Week at Daytona but was too young to remember.
Dad said:
And there you have it.
Now he rides a Honda Goldwing with a big fairing (sp?) and saddle bags and intercom system built into the helmets. I tell him he must be getting pretty old to be riding a living room around on two wheels. He thinks that’s pretty funny.
I was talking with a motorcycling friend (with a good grasp of physics) on this very subject last night, and he made the same point as Pedro about visibility through a corner on public roads.
Also, something I didn’t realise is that hanging a knee down on high speed corners adds instant extra aerodynamic drag on that side of the bike, enough to pull a rider even tighter into the corner. I had no idea about that - I thought it was just a weight distribution thing - but upon reflection it makes perfect sense. Two corner-enhancing effects in one knee movement.
Actually, three. Motorcycle racers will sometimes use a slight knee contact on the track surface as an aid to gauging the angle of a lean.
