Weight The Front To Turn In?

[botte]

At the last VIR sessions, both Cobie and Tim coached me to feather the throttle just before initiating a turn. Part of their explanation was that this would steepen the front forks a bit, thereby reducing the trail and increasing the "quickness" of the steering, making it easier to "quick-turn". I buy this part of their explanation.

 

If I understood them correctly, they also said that this puts more weight on the front tire, giving it more "bite" for the turn in. I'm having trouble with this part of their explanation, and would like some discussion/help here.

 

The second part of their explanation seems to contradict something else that we are taught at the school, and something that I believe I experience on the track.

 

Once we are leaned over in a turn, we are taught at CSS to add throttle to stabilize the bike. This raises both the front and the back of the bike, per TOTW-II. Even if the front and back lift the same amount (which would NOT increase the fork angle!), the trail will still increase as the bike rises, thereby slowing the steering a bit and stabilizing the front end. So far, so good. We are also taught that stabilizing throttle requires 0.1 - 0.2 "g" (where "g" is the acceleration a falling rock experiences), which shifts the weight balance toward the back tire. TOTW-II states that the ideal weight transfer would make the ratio of the weights on the front and rear tires equal to the ratio of the front and rear tire contact patch areas; that this amount of weight transfer provides the best cornering traction that a matched set of tires can achieve.

 

When I roll on a "good" amount of stabilizing throttle (not charge-out-of-the-turn throttle) in a fast sweeper, I perceive that the bike tends a bit toward oversteer (i.e., the bike turns a tighter radius for a given lean angle) than if I roll off the throttle, whereas rolling off the throttle seems to actually make the bike understeer (widen the turn radius for a given lean angle) a bit. This seems to imply that moving weight from the front toward the rear (with stabilizing throttle) reduces the sideslip on the front tire, or increases the sideslip on the rear tire, or maybe does a combination of both. This seems to be in complete agreement with TOTW-II's teachings on throttle control in turns, and I know I am a LOT more comfortable when the bike seems to be willing to turn tighter than when it seems to be turning wider!

 

Do y'all experience some oversteer in corners under "stabilizing throttle"? Do you experience understeer when you back out of the throttle in a turn?

 

So here's where I'm having trouble: If the front tire is side-slipping less in a corner when it is weighted less, why would it be good to weight the front in order to "get more bite for the turn-in"?

 

I'd guess there is some middle ground here. Turning the handlebars with the front end really light (say, while doing a 2" wheelie) would obviously have no steering effect. On the other hand, braking hard enough to transfer 100% of the weight to the front would leave very little traction for any lateral force.

 

More weight on a tire creates more adhesion to the road-- but braking forces "use up" that adhesion. Any of us can lock up the front end with two fingers, even with the rear tire just off the ground! Could it be that the best turn-in adhesion is with zero braking, which would place only the "normal" split of bike and rider weight on the front end, without robbing any traction for braking?

 

If this is true, then a rider might be tempted to shift his weight forward for turn-in, except that this would move the bike-rider center of gravity forward too, raising the amount of front tire traction required to turn the bike. Is there a "best" rider position (front/rear) at turn-in?

 

I do believe that steepening the fork angle at turn-in lessens the steering force required to quick-turn. That requires some braking, or at least a roll-off of the throttle, to compress the forks and lift the rear. However, I'm not sure whether to believe in the "improved bite" part of the story. Any thoughts?

 

-Eric

[GregGorman]

At the last VIR sessions, both Cobie and Tim coached me to feather the throttle just before initiating a turn. Part of their explanation was that this would steepen the front forks a bit, thereby reducing the trail and increasing the "quickness" of the steering, making it easier to "quick-turn". I buy this part of their explanation.

 

If I understood them correctly, they also said that this puts more weight on the front tire, giving it more "bite" for the turn in. I'm having trouble with this part of their explanation, and would like some discussion/help here.

 

Please define "feather the throttle" for me. My definition is along the lines of: lightly touch or modulate off and on gently to find the right amount; to control with great feel. If that's basically what your definition is then this is something only Cobie or Tim could clear up.

 

Oh, were they talking about a fast turn like maybe the front "straight" turn at VIR, or some other turn you felt you didn't have to roll off for?

[Cobie Fair]

Hi Eric,

 

If there is more weight on the front (combined bike and rider weight), say by rolling off the throttle, will the tire have more grip at that point? Lets look at the opposite--what if the front was just skimming the surface, not in a wheelie, but very light? Would it have less grip/traction/bite?

 

CF

[botte]

Greg:

 

By "feather", I took the instruction to mean a very slight roll-off. We were talking about achieving a quick transition between turns 4 and 5 at VIR, and neither Cobie nor Tim wanted a big roll-off that would do any radical unsettling of the bike.

 

Interesting-- on the school's 600's I didn't feel a need to roll off the throttle at all in the kink in the long straight near the start/finish line. Seemed like a WFO situation, but I think we had a ~10MPH headwind on the straight those days. Bet it's more entertaining on an open bike!

 

Cobie:

 

I'm interpreting your response below as a [deserved] dope-slap. A throttle roll-off (even a chopped throttle) produces weight transfer WITHOUT allocating any front tire traction to breaking forces. So at least that amount of front-end weighting and associated trail reduction should come for "free".

 

I confess I don't even know if my rear brakes work, but if they do then I could transfer even more weight to the front without sacrificing front end traction, huh? Touching the front brake would be a different matter entirely, though.

 

I will try experimenting with throttle roll-offs from mild to severe, and at different speeds, on a straight, to see what kind of front-end dive can be achieved without use of brakes. Wish I had a suspension and throttle data logger!

 

Do the pros generally roll off when coming OUT of turns? If so, is the correct order of events (1) Lift the bike; (2) Some throttle roll-off; (3) Quick-steer toward the turn center to exit the turn?

 

Best Regards,

 

Eric (occasional Curley Howard emulator)

[harnois]

You are bring up 2 scenarios here regarding traction: the traction you need for quick-turning and the traction you need for actually going around the turn....

 

1. When you are countersteering /quick-turning, the tires need traction for the purpose making your bike rotate into a lean. As long as they have *enough* traction to make this happen then you are fine, you don't need to find a way to get more, as long as you've got *enough*. In most cases you'll have way more than enough to make this happen, whether you are off-throttle, mildly on-throttle, or even mildly braking. Perahps *idealy* you wold be off-throttle, but you don't have to be, like between turn 4 and 5 at VIR North. I'm very familiar with this track, turn 5 can be taken slightly faster than turn 4 so it's natural to be accelerating during that transition from left to right, just like after turn 7 up tot turn 10, you can accelerate the whole way while simultaneously transitioning back and forth. Nobody goes off-throttle coming out of turns, unless slowing for the next sharper turn. The quick-turn is more for the turn entry, the exit is often not so quick and the lean angle change at the end of the turns often happens more slowly and when you are already well on throttle. The only time you really don't have *enough* traction to make a lean angle change is at maximum throttle in low gears when the front is skimming. And you'll see in MotoGP often the front wheel turning severly out of turns even though the bike keeps going straight and not changing lean angle. This is the due to the riders trying to countersteer but not having enough front traction to make the bike change lean angle (due to very heavy throttle).

 

2. When you are in the corner, the tires need traction for the purpose of actually making the bike travel in a curved path, to fight the inertia of the bike that makes it want to go straight. Totally different thing here. And in this case, there's no such thing as *enough*! Learning how to get more and use more is always a good thing! And the recommended throttle roll-on here you say is contradicting because it would reduce front traction... but you need 2 tires to get you around a turn! The point of the throttle roll-on is the balance the traction demand between the two tires for maximum cornering speed and stability. If you went through the turn off-throttle, you're likely to push the front, if you went through the turn with heavy throttle, you're more likely to slide the rear, but if you go through with that recommended gradual roll-on, you can get a better lean angle without either tire slipping. Rolling on the throttle might reduce front traction, but it also reduced the *demand* for front tire traction (this is like your term of how much traction do you "use up"), because now the rear tire is sharing more the work of getting you around the turn.

[harnois]

Do y'all experience some oversteer in corners under "stabilizing throttle"? Do you experience understeer when you back out of the throttle in a turn?

 

My impression is this:

 

Rolling off the throttle in a turn doesn't really cause understeer or oversteer, but it does make the bike slow down, and if you are slowing down and maintaining your lean angle, then obviously you're going to end up turning sharper! The sensation that rolling off causes understeer, I think is because when riders roll-off they are panicking, and so they are simultaneously looking at the outside of the turn and tightening their grip on the bars and those are the things that cause the sensation of understeer (running wide). If you roll-off but stay loose and keep looking through the turn it's not so bad really.

 

And my impression is that the "stabilizing throttle" does have a tendency to cause mild oversteer, but not in a negative way, just that the acceleration from the rear tire helps the bike around the turn, and not only because it stabilizes the bike and balances the front to rear traction demand, but also because it causes *torque steer*. When leaned around a right hand turn, all your weight is to the right of the point where the tires touch the ground (which is where the acceleration comes from), and therefore the acceleration from that rear tire *torque steers* the bike around the turn. It's like if you accelerated with only the left wheel in a car, it would veer to the right. This is just another reason why accelerating transfers some of the traction demand to the rear tire, or how the acceleration ends up transferring some of the cornering work to the rear tire - the rear tire pushing forward rotates the bike around in the direction of the turn, relieving the traction demand on the front tire. It's just the same as how accelerating rotates the bike up into a wheelie. This is just my own personal wacky theory, I don't know that I've ever seen it written or discussed anywhere, but doesn't it makes sense?

[Cobie Fair]

Do the pros generally roll off when coming OUT of turns? If so, is the correct order of events (1) Lift the bike; (2) Some throttle roll-off; (3) Quick-steer toward the turn center to exit the turn?

 

Best Regards,

 

Eric (occasional Curley Howard emulator)

 

Eric,

 

Not 100% sure I follow you on this--to be honest, I think a quick phone call would be more to the point--I'll be in the office on Friday if you get this in time.

 

Best,

Cobie

[hubbard_28]

Great posts Harnois. I enjoyed them.