downshift/braking methods

Discussion in 'The Perfect Line and Other Riding Myths' started by cellige, Oct 6, 2013.

1. ElRodBeen here awhile

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This is a great topic and have enjoyed reading it

I'm way out of my league here but gyroscopic forces (which haven't been mentioned), weight transfer, and counter steering all have an effect that is hard to explain. I'm thinking the laws of physics counteracting.... as in making drifting possible. Steering with the throttle.
2. beendogBanned

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Sorry, I meant as in a corner with respect to the tire only. Of course there was a classroom module that explained the throttle was a suspension/weight control more than a go/stop control.

But the focus wasn't on how does the weight transfer influence traction at the tire. Moreso how can we manipulate the weight to keep the suspension in a sweet spot that maximizes traction as a function of the entire machine.

Edit:

Sorry I was posting from my phone so I didn't get to expound.

When I say in a corner with respect to the tire only, I'm referring to that line in Lee Parks' book that states the additional traction gained from the added weight is less than the increased cornering force on the tire. Now. I am not sure that is exactly what it says because I don't have my book with me, I let a friend borrow it. But I am not sure that is the case. What the book MAY say is "the increased weight while leaned over is not enough to provide traction for both cornering AND the braking forces". And if that is the case, then that statement doesn't apply to coasting, since the weight is added to the front, but you aren't braking, so there is no added braking force.

Now quite possibly, that by coasting, you are removing the front suspension from the "sweet spot", and therefore if you were to hit a bump, you are more likely to now crash than you would have been if you were on the throttle. That is basically what the class teaches, is to keep the bike as upright as possible, and keep the suspension in the most favorable spot, to maximize traction in a corner.

However, on a slippery surface, where the CoF is lower than normal, and presuming that simply adding weight does not decrease cornering traction(but removing suspension range does). Would we rather have no torque at the rear tire to conserve traction back there, but possibly have the front suspension a little more compressed? And how much does the suspension compress on the front by coasting? Consider that you have wind resistance pushing the bike backwards, so it isn't like the weight distribution is the same as if you are just sitting there stationary, the wind resistance is taking weight off the front tire as well since your body and the upper part of the bike is acting as a sail. At least over say 40mph.

So to maximize traction on a less than perfect surface, under 45mph we should be on the throttle, and over 45mph we should coast?
3. BarryJust Beastly

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I believe people FAR more knowledgable than I am suggest even if you go into a turn a little hot, always always be slightly on the throttle, as it helps the bike corner. Never, ever, ever coast through a turn. I believe a number of factors are at work there. Someone smarter than me will expound any time I'm sure.
4. lnewqbanNinjetter

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Think of a leaned wheelie while leaving a turn: how can the bike be balanced on only one tire? Could that be done at extreme lean angles?

The point is that as we transfer weight over one tire (CG moves aft or forward), we also transfer the percentage of the lateral force that the tire is feeling, not only the vertical force.

Going to extreme examples is useful many times:
Being skillful enough to keep the steering and weight balanced, could you go around a turn in stoppie attitude only?
Could you go around the same turn doing only a sustained wheelie?
If so, which way could be a faster coasting and why?
Which way could handle the maximum lean angle and why?
5. beendogBanned

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Wheelie could be done at extreme lean angles if you were close to the balance point already. You couldn't *start* a wheelie with both tires on the ground and a 45 degree lean angle, simply because the acceleration required to loft the front consumes more traction that is available at that time(the rear would wash out). If however you were somehow able to already have front wheel high in the air to the point where there isn't much acceleration required to keep it there, then I would think you could probably do a 45 degree lean angle corner on one tire. At that point you're just a unicycle right?

What do you mean by which way could be a faster coasting?
I would think the rear tire would be handle more lean angle than the front just because of the contact patch size since the weight would be the same. For a given weight and tire/road stickiness, contact patch should affect available traction.(!)

I think you have definitely cleared that little portion of the problem up for me!
So for example, since we add more weight to the front by coasting and not being on the throttle, but the stickiness isn't changing and the contact patch doesn't change very much, it is definitely in a worse position than it was before.

So the crux of my misunderstanding lies here: Both tires gain more traction with more weight on them, but because the contact patches are of a certain size, we have the most traction when the weight is distributed in proportion to the contact patch size on the tires!

Now with that in mind, it gives me another question. Now remember we aren't discussing clearance, we're talking about a slippery surface where clearance isn't going to matter anyway, if you get close to touching a peg down you were already screwed. A wet, dark, freshly paved blacktop. Not a good situation there.

Consider that, when you coast, you are losing speed due to wind resistance, so undoubtedly from the time you enter a corner not on the throttle, you'll be going slower at the apex than you would otherwise, and have less lateral acceleration on the tires. Is that negligible compared to having the weight properly distributed per the contact patch sizes? I'm going to assume yes, going 15mph slower should have a much larger impact on available traction than having the weight distribution correct front to rear. Again I could be wrong. (again this ignores the fact that it is easier to recover from a pucker moment if you're on the throttle)

I'm starting to think the safest bike is one that has the rear tire on the front
6. lnewqbanNinjetter

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Yes, with more or less weight on the contact patch, the area of contact grows or shrinks some, but it cannot change as much as the vertical load or force applied on the patch.
That means that if you double the load, the area does not double.
Furthermore, the area has much less influence on traction than the vertical force.
Reduce area in half and you may reduce traction in 10%; reduce vertical force in half and you reduce traction at least 50%.

The lateral force also deforms each instantaneous contact patch sideways, forcing the next one to land a little off the trajectory (a micro side-slide among successive instantaneous contact patches) and forcing the rider to over-steer (or under-steer if the rear slides more) to compensate.

In real life, the vertical force can be drastically reduced while the lateral force remains the same (bad because traction is reduced but not lateral force) when rolling over road imperfections that the suspension cannot follow impeccably: right after the crests are the killers because the tire and the weight that it supports float for fractions of seconds, due to the inertia with which the weight of the bike and tire fall or rise.

To make things worse, a nervous rider could interfere with the little self-adjustments that the steering geometry does to adapt to the sideways forces that appear when rolling over road imperfections at substantial angles of lean.

In that way, for bad suspension and bad road, the contact patch constantly goes from deformed to symmetrical to deformed.
As a deformed tire is more rigid than a normal one, the inherent suspension of the tire also constantly goes from bad to good to bad (which worsens the overall suspension).
Some tire's manufacturers claim that their tires grow the area of contact patch as those lean (see schematic below); however, there is nothing they can do about the internal stress of deformation and uneven distribution of vertical forces.
The contact patch of a tire leaned 45 degree is feeling and resisting slide from a lateral force as big as the weight that it is carrying (over 200 lbs for a light sport bike with a 50/50 weight distribution).

As I see it, for a constant lateral force (steady turn, radius and speed) and the actual variable vertical force and area and rigidity of the patch, the less weight and inertia the better to stay within the margins of the traction that is necessary to counter-act that force.
While leaned, the closer to a wheelie attitude, the less critical these things that affect front traction become.

...........Very sorry, Beendog, I couldn't understand your last question. :eek1

7. beendogBanned

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This is the best explanation as to why our tendency should be to keep the bike as upright as possible, and everything else is a secondary consideration. Thanks very much lnewqban I understand this to my satisfaction now!

The second question, you are probably overthinking it is all. Is it better to give up the wheelie attitude and be going 10mph slower, or have the wheelie attitude going 10mph faster? Really depends on what speed the corner dictates I would surmise, and would be answered case-by-case, so I doubt any discussion of the preferences of the overall machine would make this clear.

Thanks again. That whole "having a bit of throttle stabilizes the bike" never satisfied my curiosity as to WHY that is the case. But having the bike as vertical as possible + the wheelie attitude affecting the suspension and contact patch is a far more detailed and satisfying explanation.
8. lnewqbanNinjetter

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You are welcome, Beendog

The throttle keeping while cornering is not as far from coasting as many believe.

According to Keith Code in his second book, all it takes is little more acceleration than coasting for the rider to be "fair" to each contact patch.

"Considering that most machines in a static or constant speed situation have a 50/50 weight distribution (+ or - 5.0 percent) front-to-rear, we begin to calculate the guidelines of correct acceleration through a turn.
By the numbers, we want to transfer 10 to 20 percent of the weight rearwards, using the throttle. Technically, this is 0.1 to 0.2 G of acceleration. Simply put, it's the force generated by a smooth fifth-gear roll-on in the 4000 to 6000 rpm range on pretty much anything over 600cc.
That's not much acceleration -but it does the job."

That 0.1 to 0.2 G means a gain of speed of 2.2 to 3.3 mph per each second spent on the turn.
In terms of longitudinal traction of the rear patch that is 10% to 20% of the total weight (bike+rider) pushing rearward on the pavement (48 to 96 lbs in our previous example).

Consequently, your leaving speed will be dictated by the maximum traction and lean angle for those conditions and turn, while your entry speed and initial lean angle will be lower (more for a longer turn).

Coasting is advised by Keith as an OK approach, although not for the totality of the turn:

"When you get to the throttle determines where the bike is actually working. The earlier into a corner you get onto the gas the sooner you have the suspension in-range, weight transferred and so on. The later into a corner you get onto the gas, the more likely you are to be gas ''greedy"
for the exit."

Note that he refers to coasting as zero acceleration rather than deceleration (some minimum gas is supplied).
Wrongly, many riders extend the deceleration (by trail-braking or fully closed throttle) all the way to the apex of the turn, while over-taxing the front patch.
As I see it, they turn slower than they could, simply because, if describing a constant radius turn, apex means nothing and if they didn't slide at the entrance, they wouldn't from there to the apex.
9. beendogBanned

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This leads us to the conclusion that even if ground clearance is not a concern, it will always be better to be on the throttle to transfer that 10-20% of weight rearward.

Going along with what you are saying about people slowing in turns with no apex... If they were going to wash out the front, it would have happened at the moment they were at maximum cornering force early in the turn.

So at some speed for a given corner with a given CoF, there lies a point where you could make it if you were on the throttle, but would wash the front out if you were not. Now it seems to me this is going to be a very narrow range. So up until that point, you could actually get on the brakes, once you get to that point, you would need to be on the throttle, after that point, there's really nothing you can do. How small is that point though? Let's pretend that for given corner X, you could take it at 50mph and have some traction left over for braking with the front tire. But then if you tried to get on the brakes at 55mph you would wash out. If you tried to take it at 55mph on the throttle the whole way, you could take it. At what point after 55mph can we not make it at all? Is it 56mph? Or 65mph? How much "Extra" margin does being on the throttle give you is what I am asking. If it's only 1mph difference, then who gives a crap. If it's a 10-15mph difference, then it's far more critical...
10. Tripped1Bitch called me a feminist.

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There are an amazing about of factors that would have to be included, surface, tire temp, lean angle, speed, acceleration (either positive or negative) The long and the short of it is that max traction actually works out to 1G in any direction,if you exceed that it starts to slip, and dynamic friction is less than static so that tends to be a downward spiral.

That being said I've had issues when I had to chop the throttle and it started pushing the front quickly, you have more time to recover the faster you are going, so at track speeds its sort of "meh, I pushed the front a bit"....on the street it will get your attention.

Its WAY more than 1mph though.
11. lnewqbanNinjetter

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It has been great discussing these things with you, Beendog !!!
I feel, however, that we have hijacked this thread big time, .....and maybe someone will soon come to complain.

Your last question is very interesting, but I don't know the proper response.
Most racers don't calculate these things and find the limits via successive tests that start at a conservative point (plenty of safety margin).

For reasons that I don't understand clearly, I feel the bike much more stable under moderate acceleration.
I have created the habit of extending the straight entry line as much as practical, fully releasing the brakes when entry speed feels safe, then quick-flicking the bike (I feel that I ask a lot from the front patch with that maneuver), then opening the throttle and going for the visible apex trying to keep a constant radius, clamped under-body, distant view and very gentle hold of the handle-bar with my inside arm-hand.
That way I stay away from extreme lean angles and tighter radius as much as possible.

I am not a super-fast rider, but pushing the limits I have tucked the front once and crashed and know that that it is much less fun than sliding the rear tire.
Overwhelming the front tire on a turn is basically a riding mistake from which is almost impossible to recover.

I recommend you reading this recent article posted by a proficient racer and coach:
http://www.motomom.ca/the-limit-exploring-front-tire-traction/