Keith Code: Twist of the Wrist 2

[dwilson]

Sportbikes are still like black magic to me so I went to amazon.com to get some books...  One of them was Twist of the Wrist 2, a good book but slightly confusing in some spots for a newer racer.  If any of you guys & gals with a little experience can clear up a few things I'd appreciate it...
I'm half way through the book and the following things stick in my mind.

1. I know this differs by prefference, but coming into a low speed (tight) turn, where are you
 A: off the brakes
 B: finished down shifting
 C: on the gas

2. Keith preaches over and over that bike will not handle properly when you are off the gas.  He says that 60% of the weight needs to be on the rear wheel & 40% on the front and that this ratio can only be accomplished while on the throttle.  He goes further saying that for the machine to handle properly you must be on the gas (at least a little) in the apex of any turn.  Does all this sound correct?

3.  Still on the topic of handling while on the gas, Keith says that the action of accelarating pushes the rear suspension up not down, increasing ground clearance.  I always felt that the rear compressed under accelaration.  What really happens to the rear suspension under accelaration?

I like the book and will probably finish it today, next I also got the "Sportbike Performance Handbook" by Kevin Cameron.  If you have any books you think a newer ambitious racer should read, I'm open to your suggestions.

[tigerblade]

I'm by no means an expert but I'll give you my two pennies:

1.  I let off the brakes just before my turn-in point MOST of the time, unless I still need to scrub a little speed in which case I may trail just a tiny bit.  I get my downshifts done as quickly as I can as soon as I'm off the gas and on the brakes.  I use engine braking to my advantage which is what I was taught at STAR school (others will disagree on this).  The second I let off the brake, I crack the throttle just a little bit.  I don't open it up yet but it's not completely closed.  It depends on the corner as to when I'm feeding in throttle.  

2. It feels like the bike settles better to me when I have the throttle cracked as mentioned above.

3.  Acceleration pushes the rear up.  I've read that a good way to demonstrate this is to put your front wheel up against a wall and try to go forward.  The rear will rise, not squat.  I'm guessing what makes the rear squat is the weight transer and not acceleration itself.  I'm sure someone like Super Dave will chime in to clear this up.

 8)

[ecumike]

My .02

1.  Pretty much the same as Kris.. off gas, then brake hard while downshifting at the same time. Off brakes turn in and crack, slowly open till apex, then really open it up.

2.  Yes, correct.  Think about it.. too much weight on that front while turning = front end tuck.

3.  As Kris said, yes it pushes up, it's the lbs xfer and force of accel. that makes it squat

[ecumike]

Forgot.... The other one that's good is Keith's "Soft Soft Science of Roadracing"

[lfg929]

For question #1 it depends. I try to be on the gas most of the time. Sometimes it isn't always possible though. If I come in too hot I may still have a small amount of front brake applied. Sometimes I may not have any brakes on but will use the turn itself to scrub off that last little bit of speed going into the turn. By the apex I am always on the gas. Theoretically you *should* be on the gas at least a little at the entrance to insure you aren't loading up the front and risking a low side.

For question #2 the bike definitely handles best when under acceleration. If you are coasting then it will handle ok but you have to watch loading the front. If you hit any bumps then you need to be careful about letting the bike do its thing.

For question #3 you can do as tigerblade says. You can also stick it on your rear stand (securely please), pop it into 2nd gear or so and give it a bit of throttle. Look closely at your seat and you should see it go up.

[raptorduck952]

1. It is true that the bike handles best in a turn when about 60% of the weight is on the rear tire. One reason for this is simply that the rear tire has a larger contact patch and so with more weight back there you have better grip.

2. It is also true that the bike's rear will rise when you accelerate. One reason is the geometry between the chain angle and rear sprocket.  This geometry will actually make the rear suspension expand when you apply force to the chain, since the swing arm will move.

[Roach]

For question #1 it depends. I try to be on the gas most of the time. Sometimes it isn't always possible though. If I come in too hot I may still have a small amount of front brake applied.

Just a quick comment ... if you're able to make the corner (without going wide / missing your line) while increasing load on the front tire by trailing the brakes ... you certainly could have made it without the brakes and on neutral (or even slight positive) throttle.

When off the brakes and on the throttle, weight is transfered onto the rear wheel, and less weight is on the front. Even at neutral throttle (not accelerating or decelerating) you have reduced the weight being carried on the front.

The coefficient of friction is a constant in this equation. Weight and corner speed are combined forces (load) that act to overcome it. The more weight you have on the front tire, the less corner speed it will take to overcome the coefficient of friction and cause the front to lose grip and slide.  The inverse is then also true - less weight allows for higher corner speed.

- Roach
(who agrees 100% with the physics as described by Code)

[StuartV666]

What they all said.

For # 3, it actually depends. Some bikes will rise. Some will squat. The determing factor is whether the rear axle is above or below the swingarm pivot.

When you are on the gas, the motor is pulling on the chain to turn the rear wheel. It would pull the rear wheel forward if the swingarm wasn't holding it in place. Since it can't move straight forward, it moves in an arc to get closer to the counter sprocket.

If the counter sprocket and swingarm pivot were at the same point, this would not happen.

Anyway, when the chain pulls on the sprocket, if the rear axle is below the swingarm pivot, then it will pull the rear wheel "down" (i.e. raise the rear of the bike).

If the rear wheel is above the swingarm pivot, then accelerating will pull the rear wheel up (i.e. compress the rear suspension).

As has been noted, all of this is also affected by weight transfer which tries to make the rear of the bike squat.

Modern sportbikes generally have the rear axle a bit below the swingarm pivot, so generally, the rear of the bike will rise when you get on the gas.

- Stu

[lfg929]

Just a quick comment ... if you're able to make the corner (without going wide / missing your line) while increasing load on the front tire by trailing the brakes ... you certainly could have made it without the brakes and on neutral (or even slight positive) throttle.

Being able to make the corner and having the balls to do it are 2 separate things. ;) I believe what you are saying, but my eyes and mind sometimes don't. It is all part of the learning experience. This is just my first year racing (hell, I only have a single endurance race under my belt so far) so I have a long way to go in figuring out what I can and can't do with the bike. My corner speeds have shot way up just from the one race I have done. Plus my lap times aren't too far off (3-5 seconds) of some of the expert class racers I know who are competitive so I am happy with what I can do so far.

[Roach]

Being able to make the corner and having the balls to do it are 2 separate things. ;)  ... so I have a long way to go in figuring out what I can and can't do with the bike.  

That's the bad part ... often the first indicator that you've exceeded the coefficient of friction is the sharp pain you feel from hitting the tarmac  :o

- Roach