when pigs fly

BarryMachine · Jan 30, 2011

Geonerd said:
Greetings, Renjith.

Point #1

Friction is not a problem! There is more friction (raw braking force) available than can be used.

Think of it this way.

11415edrosa_Faceplant.jpg]

Point #2

The green arrows represent the two forces at work; bike (+rider) momentum, and the ground 'pushing' back at the tire's point of contact. Since the bike's center of gravity is above ground level, these two forces will produce a forward torque (red arrow) on the bike. Counteracting this is the weight (blue line) that also acts through the front contact patch to provide a rearward torque. When the forward torque becomes the stronger of the two, the rear wheel will lift off the ground and the bike will start to flip forward. The effect is identical to pushing the base of a tall glass of beer (or anything else that is free to move.) Push gently and the glass remains upright, push harder (higher acceleration) and the glass will topple over.

Since the rider is seated above the bike, his weight will add to the instability of the machine. More weight (or more height) = less stability = less maximum braking force before the bike flips. A lighter rider will produce a lower overall center of gravity, allowing higher braking forces to be applied through the front wheel without ~~smearing Perdrosa's smarmy, smirking face into the cold, hard pavement~~ causing the bike to endo.

You are missing the major forces on the front suspension there ....... the orange arc is just the force that lifts the back wheel off. The ones you should be interested in for your argument are the rotational around the back wheel and the rotational around the CG of the bike/rider combined as well as vector forces that have a downward component. these give a downward force on the front contact patch. Also you have to remember that all forces meet the ground at the contact patches ( more so the front only on heavy braking as the back wheel is either in the air or of an extremely low contact force ).

Geonerd · Jan 30, 2011

Tom said:
If that blue line is ~~bigger or more valuably,~~ further back, the rider can brake harder. Rossi is the tallest guy on the grid, Dani is the shortest. One is particuarly good on the breaks, the other is particularly bad.

As explained, if the rider's center of mass is higher than that of the bike (as viewed from the front contact patch), increasing rider weight decreases maximum deceleration.

Yes, geometry is everything. Rossi's long arms are a big help, they allow him to move further back on the bike. His greater weight is a big hindrance. Personally, I put their braking performance difference - which is not nearly as great as it used to be - down to skill

ptk50 · Jan 30, 2011

Anna said:
Shhhhhhhh. I'm the same height as him!

That's how it should be - otherwise boy friends would have to wear 3" Cuban heels and carry around a box to stand on for saying goodnight. No machoism implied - those boxes are a drag to carry. - once had a taller girlfriend.

Jumkie · Jan 30, 2011

evo9 said:
Yeah! But, you are a female....................

.

And Pedro is not???......

nemesister · Jan 30, 2011

Jumkie said:
And Pedro is not???......

lol

Geonerd · Jan 30, 2011

BarryMachine said:
You are missing the major forces on the front suspension there ....... the orange arc is just the force that lifts the back wheel off. The ones you should be interested in for your argument are the rotational around the back wheel and the rotational around the CG of the bike/rider combined as well as vector forces that have a downward component. these give a downward force on the front contact patch. Also you have to remember that all forces meet the ground at the contact patches ( more so the front only on heavy braking as the back wheel is either in the air or of an extremely low contact force ).

No. There is no mystic voodoo force that will magically press the front wheel into the ground. Neither tire contact patch will carry more than the combined weight of the rider+bike for any sustained period of time. Decelerating the spinning rear (and front) wheel(s) will simply add more forward pitching torque to the rider/bike package. Again, when that forward torque overcomes the gravity induced counter-clockwise torque, the rear wheel will leave the ground.

I don't understand your last sentence. Of course I 'remember.' Load transfer is implicit.

ptk50 · Jan 30, 2011

Geonerd said:
No. There is no mystic voodoo force that will magically press the front wheel into the ground. Neither tire contact patch will carry more than the combined weight of the rider+bike for any sustained period of time. Decelerating the spinning rear (and front) wheel(s) will simply add more forward pitching torque to the rider/bike package. Again, when that forward torque overcomes the gravity induced counter-clockwise torque, the rear wheel will leave the ground.

I don't understand your last sentence. Of course I 'remember.' Load transfer is implicit.

If 12-15 kilos are so important with motogp bikes putting out way over 200 bhp, could we leave the physics class for a few minutes and somebody tell me how that overweight, too tall long armed ape Rossi won 125 races with less than 20% of the power

Mental Anarchist · Jan 30, 2011

ptk50 said:
If 12-15 kilos are so important with motogp bikes putting out way over 200 bhp, could we leave the physics class for a few minutes and somebody tell me how that overweight, too tall long armed ape Rossi won 125 races with less than 20% of the power

I would suggest that as a teenager he was probably shorter and lighter.

In 125 and 250 there were bikes with power and bikes without power. The ones with power ran at the front.

esco · Jan 30, 2011

ptk50 said:
If 12-15 kilos are so important with motogp bikes putting out way over 200 bhp, could we leave the physics class for a few minutes and somebody tell me how that overweight, too tall long armed ape Rossi won 125 races with less than 20% of the power

I'll add simonchelli to that.

Jumkie · Jan 30, 2011

ptk50 said:
If 12-15 kilos are so important with motogp bikes putting out way over 200 bhp, could we leave the physics class for a few minutes and somebody tell me how that overweight, too tall long armed ape Rossi won 125 races with less than 20% of the power

Your logic is Talpesk (that's NOT a compliment.)

Geo isn't saying Pedros weight advantage makes the bike rider combo have some Wonder Twin power dude. Pedro is still one of the top riders in the world. Now add to that fact he weights the same as an obese Chihuahua, and hence the launch advantage, and as Geo proposes, and I agree, a brake advantage (at very least no disadvantage). Not sure if Geo said this, as I skipped over most of the tech talk, but...less momentum, less brake force needed. Think, what's easier to stop, a train or a scooter?

renjith_varma · Jan 30, 2011

Jumkie said:
Your logic is Talpesk (that's NOT a compliment.)

Geo isn't saying Pedros weight advantage makes the bike rider combo have some Wonder Twin power dude. Pedro is still one of the top riders in the world. Now add to that fact he weights the same as an obese Chihuahua, and hence the launch advantage, and as Geo proposes, and I agree, a brake advantage (at very least no disadvantage). Not sure if Geo said this, as I skipped over most of the tech talk, but...less momentum, less brake force needed. Think, what's easier to stop, a train or a scooter?

I am not so sure Jumkie. I did some silly calculations (for which I asked for theshock stiffness and travel in an earlier post) based on high school physics and to me it seems if rider can move 70% or more of body weight to rear during braking it will be more stable for a heavier rider. Add to this, moving all the weight back will be more difficult for pedrosa because he has smaller arms and legs which need to maintain contact with the handlebar/ foot pegs

Tom · Jan 31, 2011

Geonerd said:
As explained, if the rider's center of mass is higher than that of the bike (as viewed from the front contact patch), increasing rider weight decreases maximum deceleration.

The riders mass acts through each of his contact points with the bike, a taller rider will be able to manipulate that weight to his advantage to a much greater extent than a smaller rider, giving him an advantage. In the most basic sense it is easiest to consider the bike and rider as one object with a combined centre of gravity but it is not realistic.

chopperman · Jan 31, 2011

Renjith said:
I am not so sure Jumkie. I did some silly calculations (for which I asked for theshock stiffness and travel in an earlier post) based on high school physics and to me it seems if rider can move 70% or more of body weight to rear during braking it will be more stable for a heavier rider. Add to this, moving all the weight back will be more difficult for pedrosa because he has smaller arms and legs which need to maintain contact with the handlebar/ foot pegs

How far do you think they move back? this aint a road bike with a big duel seat, they only move a few inches.

BarryMachine · Jan 31, 2011

Geonerd said:
No. There is no mystic voodoo force that will magically press the front wheel into the ground. Neither tire contact patch will carry more than the combined weight of the rider+bike for any sustained period of time. Decelerating the spinning rear (and front) wheel(s) will simply add more forward pitching torque to the rider/bike package. Again, when that forward torque overcomes the gravity induced counter-clockwise torque, the rear wheel will leave the ground.

I don't understand your last sentence. Of course I 'remember.' Load transfer is implicit.

Hmmm ...... not sure you are getting what I am saying ......... when you brake hard on the front the force down through the forks to the ground is greater than just the rider/bike force due to their mass and gravity.

The orange arc around the front axle does not show this.

The force to which I refer can be easily seen as compression on the front forks, which equates to the force that exists and is opposed at the contact patch. It is a force that is resultant of the mass of bike and rider attempting to spin around the CoG plus forces due to the front end geometry.

renjith_varma · Jan 31, 2011

chopperman said:
How far do you think they move back? this aint a road bike with a big duel seat, they only move a few inches.

I know. But I think just sitting up while moving back a few inches can change the weight distribution drastically. Anyway, just some silly theory which I feel is better than the current bopper/hater stuff going on in here

Geonerd · Jan 31, 2011

BarryMachine said:
Hmmm ...... not sure you are getting what I am saying ......... when you brake hard on the front the force down through the forks to the ground is greater than just the rider/bike force due to their mass and gravity.

The orange arc around the front axle does not show this.

What you are seeing is simple load transfer. When 99% of the bike+rider's weight (threshold of a stoppie) is being supported by the front wheel, the forks will naturally compress. There is NO WAY the front will see vertical MORE static load than the weight of the bike+rider. As the bike rotates forward a few degrees under braking, there may be some minor inertia-generated downward force, but this is minimal and does not persist once the bike reaches a stable tilt angle.

The force to which I refer can be easily seen as compression on the front forks, which equates to the force that exists and is opposed at the contact patch. It is a force that is resultant of the mass of bike and rider attempting to spin around the CoG plus forces due to the front end geometry.

Yea. I still think you're seeing contact patch forces that don't exist. Depending on available grip, vector to the center of mass, and rake angle, I can see the axial load on the forks exceeding the bike/rider weight. This is not to say that the vertical load on the contact patch is greater than the bike+rider!

Geonerd · Jan 31, 2011

Renjith said:
I am not so sure Jumkie. I did some silly calculations (for which I asked for theshock stiffness and travel in an earlier post) based on high school physics and to me it seems if rider can move 70% or more of body weight to rear during braking it will be more stable for a heavier rider. Add to this, moving all the weight back will be more difficult for pedrosa because he has smaller arms and legs which need to maintain contact with the handlebar/ foot pegs

Looking at various riders, and guestimating their center of mass, I don't think any of the riders can move back far enough to make their weight stabilize the bike. I'm pretty sure the concept in my first picture is correct, that the rider's center of mass needs to move behind that of the bike, as seen from the front contact patch.

Dani's Tyrannosaur sized forelimbs do seem a minor handicap. Honda will have to improve the design of Pedrobot 2.0.

11417:images.jpg]

BarryMachine · Jan 31, 2011

Geonerd said:
This is not to say that the vertical load on the contact patch is greater than the bike+rider![/size]

Are you kidding?

Under braking the front contact load is much greater ......... just work it out by how much the forks compress and the kN/mm rate of the forks

Anyway ......... I'm not going through this crap again ATM so go ahead ........ believe what you wish.

In the end, history has shown that there is no benefit to being "large" riding a motorcycle.

renjith_varma · Feb 1, 2011

My link

Excerpts from interview with Randy Mamola:

"Part of the equation is dependent on the rider’s size. Rossi can move to the back of the seat when he’s braking, the shorter Pedrosa doesn’t have that option, “so when Dani leans off to the left, or to the right, if his head isn’t as far out, or doesn’t have that leverage like Rossi does, 15 lbs. is a lot of weight. So now move that weight around because Rossi’s that tall.”

Seems to be agreeing with my theory that dani is at a disadvantage and it is more because of his height.

Tom · Feb 1, 2011

Renjith said:
Seems to be agreeing with my theory that dani is at a disadvantage and it is more because of his height.

Yes, he says the same as you and I because he is also correct.

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