R Sole
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Posts posted by R Sole
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R-
Not a physics guy, but think of it in terms of load/weight. If you exceed the weight/load the tire can handle, it will push.
CF
So the chopping of the throttle results in the riders weight moving forward onto the front wheel, This additional force downwards on the forks has a downwards component and an outwards component. The acceleration outwards causes the friction from the tire to be overcome, and it to lose grip. Makes sense I guess.
Although an increased weight at the front wheel should also increas the friction from that wheel (as friction is proportional to force downwards)
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Keith - This may or may not be on the same topic, but you explain why "chopping" the throttle mid-turn causes the front end to break away?
After all the same forces apply, but the speed is decreased, so the forces acting outwardly on the bike should decrease, not increase to them to cause them to overcome the friction of the front tyre?
I came up with a few theories:
1) Perhaps the weight shift to the front of the bike causes the forks to dip, and the steering geometry to change, causing a quick change in turn in (i..e a suddenly decreasing turn radius) that increases the cenripetal forces due to the sudden changes in radius? To my mind then, this effect would not be seen in bicycles with no suspension? maybe a cyclist can confirm this?
2) Is it because the sudden deceleration causes the front tyre to start skidding, and the dynamic friction afforded by a slipping front tyre is a lot less than the rolling friction? Although this does not make sense as there si noreason the tyre would not just continure to roll?
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Let me play devil's advocate if I may:
Where you looking into the turn? Had you been, could you have possibly seen this guy coming? Where you searching your surroundings before entering the turn?
How fast where you taking this turn? Where you that leaned over that you couldn't stand it up and break hard enough to pick up your rear tire?
Do you take that or other turns slower now because of this accident?
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It is possible that in the situation you were in there may not have been a better course of action, but was there perhaps a way to have avoided that situation entirely. This may be an even more valuable skill.
-Javier
Aim for his back wheel and use it to slow you down progressively, and make sure that the cyclist fool also walked away with scars.
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I am still not quite sure why sudden deceleration in a corner cause teh wheel to break away though? Angular velocity changes, as does angular acceleration, but why does it make the traction break away?
Is it because the change in trail cause the bike to turn in more suddenly, changing the radius of the turn too quickly and increasing the centripetal acceleration so quickly that the friction of the tire is overwhelemed? This makes sense, as it is only the front wheel that gives way.
Also, you dont get that highsiding effect on bicycles (that do not have a suspension set up, with a changing trail).
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So am I correct in thinking that trail braking (gently so as not to cause sudden deceleration and hence breaking away of the front tyre) into a corner decreases the trail of a bike and helps it turn in, and then rolling on slowly and constantly (again no suddent acceleration/deceleration) to move the suspension to a balanced configuration between front and back wheels settles/stabilisies and optimises the suspension during cornering?
And getting your wight far over allows for the bike to stay as far upright as possible to enable its suspension to be used in the right directions for maximum traction.
I am an engineer, and I love getting technical about these things, because it makes more sense if I can see teh applied mathematics of it. I see the cornering forces as the balanceing of moments (or torque) on the rider and bike (when looking at teh bike from the front) about the wheel traction points (where tire meets road). One monet is the combined inward component of teh weight weight of the bike and rider and countersteering gyroscopic force acting inwardly. The other moment is the centripetal acceleration acting outwardly on the mass of bike and rider at the centre of gravity of bike and rider.
So increasing speed in the corner increases the pentripetal force and hence moment acting outwardly, and needs to be balanced by increased lean angle and/or lower body height. Assuming body height is as low as you can get it anyway, then lean angle must increase (for additional weight component acting inwardly), or the countersteering gyroscopic force must increase by using more countersteering. Does anyone need a force diagram?
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Well yes I am much smoother after doing this exercise! I used to have odd contours on my body that are now more flat and smoothed out thank you Keith Code! You'll be hearing from my attorney in the morning ....just kidding
but seriously this exercise did work, I found myself going faster in the corners than I previously felt comfortable going, but I guess there was a reason that I didn't feel comfortable going faster than i was going?? The road I was on was unfamiliar to me, I think that was my first mistake, I should have picked a road that I had been down 100 times so there would be no surprises! I went though a few corners with little or no brakes and yes it was very mentally intense, it really makes you pay attention to your line and body position, but then I came into a decreasing radius corner and I just didn't have any cushion to correct anything when the corner got tighter, so I ran a little wide, hit some gravel and then ate burm. I must say something about this that nobody has mentioned ....when you come into a corner using the brakes and even trail braking a bit you are in a state of compressed suspension and you are already engaged in the activity of feeling the grip of the tires so when you ease off the brakes and lean the bike over it is one smooth motion and the stresses on the bike remain somewhat constant, where as going from a completely unloaded suspension and free rolling tires it is somewhat of a harsh transition to suddenly load the suspension and tires and try to immediately find the tire feel while in the corner.....while this does not take away from the validity of the exercise, for any kind of fast riding I am always going to give that light touch on the brakes to level the suspension etc. I should also add that I may have picked a much more twisty and steep road than the author had in mind...in retrospect I remember the mention of canyon sweepers, I was on a very steep mountain with sets of 10mph corners. Anyway good luck to you all My advice if you are going to try this is to expect to crash and if you're coming into a corner and you really think you may be going too fast use the effing brakes, that's what they're for!! Take it easy and start by using just a little less brakes and do it several times on the same road so you can GRADUALLY increase the corner speed....but really there is a speed at which you're bike just will not hold the corner so keep that in mind while you try to get as close to that speed as possible! Man I feel like such a retard for not using the brake just because I wasn't *supposed* to, oh well, I'll probably try this exercise again after I heal?
And being inexperienced, you probably could have got through that corner if you had just stayed smooth and not turned to the Survival Reactons (SR's) and chopped/braked and stood up.
I reckon its better to stay smooth and KNOW that you were too fast for the corner than run off (at least off a public road) into God knows what obstacles and die wondering...
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I can say one of the hardest things to overcome with the "no brake" is am I going to fast? what now? Well I've been trying this theory myself not to far away from Talledaga and I noticed as I aproached the wide hairpin turn that my gut was starting to tighten. What to do? I went for it forcing any fear that may arise to stay put. I used counterstearing technic and just concentrated on staying in a drive path which I imagined going through the curve. Next thing I new I was rolling out of the curve free and happy. I hit the road a few more times that day and there was no braking!!! It was great!!!
Mike
Whenever I get that tighteneing in my gut, I find it helps to concentrate on realxing my inside arm,and focus on getting my weight far over and low (to keep teh bike as upright as possible). Relaxing that arm (the one I countersteer with) also makes my turning input easier and smoother, and the bike makes it around no problem. I am quite inexperienced, but what I am noticing more and more is that the physical limits of trning are so bloody high that most problems are caused by the riders mental attitude rather than their bikes capabilities.
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I like to "show intent". If I am riding a particular way, I maintain that way, without changing or being to unpredictable.
I also try and ride like the cars expect bikes to.
They expect bikes to be faster than them and slightly more agressive in riding style. So I ride about 10km/h faster, and overtake aggressively. IfI wishy washed behind tehm, slowing down and speeding up, the get anxious and may do stupid things to try and get you to do what they expect you to do, like slowing down, or pulling over a bit where there is no space to etc.
I ride with intent (from A to B like you say) and try and telegraph this intent by looking where I want to go to change lanes.
I also kep physics in mind. I ride a VTR 1000 firestorm, so I KNOW I can accelerate faster than pretty much any car, but I know that most cars can brake harder than me.
I accelerate into gaps, knowing that at a particular moment things are like X, and it is not physically possible for them to change to Y if I have done things correctly. Then I can just focus on unexpected events.
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I grew up in South Africa and I wrote off a mates bike (DT 125) at 18 on a dirt road when I got on it and thought turning was weight shifting like a bicycle, but harder.
I could not understand why anyone would ride things that they could not control an I could not understand why/how small GP type guys could fling bikes through chicanes the way they did. When I was 28, I started riding a mates bike CBR250RR and finally figured out countersteering (I never learnt anything about it until after). I could have avoided my 2 accidents. Now I am a preacher of c/s. I rode my cousins bike a bit when I was 32 - a BMW R80GS.
I have since moved countries to NZ, and bought a VTR 1000 Firestorm (Night Hawk? in the US) 6 months ago and commute to work on it everyday.
I have not yet attended a track day, but voraciously gobble up everything I can find about riding technique.
I have plans for a track bike and hope to attend the CSBK at Auckland's new Hampton Downs raceway soon.
Holding Your Line
in Articles by Keith
Posted
My understanding is that you can get washout in two ways on the front wheel on corner (assuming decent speed to start off with).
The one is the situation above with overloading of the front wheel by sudden deceleration, and the other is almost the exact opposite - where you accelelrate hard out of the corner while still at a lean, and the resultant speed increases the outward force, and the weight coming off the front wheel reduced the capacity for traction.
Is this correct?