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Weight Transfers Under Acceleration...


jax
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Hello!

 

I don't know if this is the right place to ask this question, but still... :unsure:

And if this problem was already discused, sorry... I used the search button and failed to find it... :ph34r::)

 

I was reading Twist of the wrist 2 and I have a problem understanding something. I always like to understand how things work so that I know why a certain reaction is apropriate at a certain moment.

 

In the third chapter of throttle control, it is writen that the rear end wants to got up under acceleration. I don't understand that. Is it meant that it wants to go up, but the torque forces off acceleration still push it down?

 

The example which should explain this, says that we should put a bike next to a wall with the front tire facing the wall and try to accelerate. Than the rear end really goes up but ain't that becouse of the torque forces? To explain... If the bike had the center of gravity in the samo hight as the contact of the front tyre and the wall, and if the center of gravity wouldn't rise when the front forks lowered, then the rear end wouldn't rise, right? So in this case, the elevation of the rear end happens due to the forces of the wall on the bike and the torque forces...

 

When the bike is moving, these torque forces work in the other way becouse of the acceleration. The weight distributes to the rear, right? So how can the rear end rise then? :blink:

 

And anohter thing. A few chapters earlier it was explained that weight distribution under acceleration tends to distribute traction on the tires... It states that more gas (acceleration) transferes more weight to the rear tire...

 

So where does the rear end tendency to elevate come in? :huh:

 

There is an explanation that it has a tendency to rise, becouse of the direction of the rear tire spin, but if we compare that to the forces wich are a result od the weight transfer... Seems kind of irelevant :unsure::)

 

PS: sorry for the crappy english... I'm not from an english talking state :)

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The entire bike lifts. The important part of geometry that has the affect is the relationship between the rake angle and swingarm angle. The rear wheel moves in an arc, the front in a straight line. As you get on the gas you want the wheelbase to stay the same. You need what they call "anti-squat" on the rear.

 

 

The axle and swingarm pivot are the important points (along with the action of the chain). If the axle is lower than the swingarm pivot point, then the force applied in a forward direction will attempt to push the swingarm under the motorcycle. Due to the fact that it's fixed at the front, it will rotate. It rotates downward. This lifts the rear of the bike. The key is to try to keep the wheelbase as static as possible so the steering geometry doesn't change as you apply power. Two things change when one end goes up/down more than the other... Wheelbase and rake. The less these change, the easier it is to keep the bike near/at it's limit. It's why I hate peaky IL4's :). I'll take an engine with linear power over one that's peaky any day of the week.

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I think you actually stated it in your post. I'm pretty sure that if there were no weight transfer at all front to rear the rear would actually want to rise under acceleration however if I remember right the whole idea is to counter that rising motion by using proper throttle control to get the proper weight shift front to back. This counters this "rising" and ensures good grip as well as putting the suspension of the bike within proper range. I think when it is said that "the rear wants to rise" is only true when the application of the throttle has not met the load requirments of the suspension to "sit the tire down".

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Weight transfer under acceleration is different from the rear-end lifting as the motorcycle's drive train applies force to the rear tire. However, they are not mutually exclusive, but can both occur simultaneously. You can literally feel the weight transfer from acceleration in your gut and butt. The rear-end rise, is not as apparent, hence the illustration of placing the front wheel against a wall and easing out the clutch.

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  • 3 weeks later...

1. Transferring weight rearward is talking in terms of weight on the rear TIRE for traction. Not in terms of pushing down on the suspension>

 

2. Rear end rise from the suspension extending is caused by drivetrain and the angle of the swingarm, both dependant on tourque.

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This is a case of mythical forces that people make up to explain something. From a Physics's viewpoint, you have the force from the chain pulling the rear sproket forward, you have angular momentum of the wheel (and sproket), the friction of the wheel AND the friction from the road. The suspension is not going to pull the rear wheel up, if anything it is pushing it down so let's ignore it. The fact that the rear is on a pivot arm is also a distraction. If there is a force pulling up, it will act regardless of the type of suspension you are using.

 

If you've ever loaded a suspension in a car to drag (or do a 0-60 run), the combination of brake and gas drive the rear wheels (assuming rear wheeel drive) up into the suspension. It works best in an auto transmission. In that case the brake is applying the same type of force - a resisting force - as the road. Now a brake can provide more resistance than a road, which is how you can induce the effect to an extreme. The road can never give you that much friction (and the level will vary.)

 

In a cycle, take the load transfer extreme to get it out of your head - braking after along fast straight - you get front end dive, the rear suspension should be extending. If anything you are looking at the rear wheel leaving the road at some point. BUT you can still load the suspension by adding gear brake and gas. The rear will drive up into the suspension. This has nothing to do with weight transfer - it has to do with balancing the forces on the wheel - the chain pulling up and forward against the resisting wheel. According to someone somehwere the Honda MotoGP camp in Japan (I assume it is a real thing) teaches loading the rear to quiet the suspension in a turn. In a turn and out of it, weight transfer is going to compress the rear suspension (along with the drive doing some minor compressing). To keep the rear suspension from going to extremes, in theory, you can load the rear suspension under braking so that when you get off the front brake the rear is already on its way to a compressed state.

 

In theory of course since getting the front braking down is probably more of an issue than trying to help the rear suspension by pre-compressing.

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  • 2 weeks later...

This is a case of mythical forces that people make up to explain something. From a Physics's viewpoint, you have the force from the chain pulling the rear sproket forward, you have angular momentum of the wheel (and sproket), the friction of the wheel AND the friction from the road. The suspension is not going to pull the rear wheel up, if anything it is pushing it down so let's ignore it. The fact that the rear is on a pivot arm is also a distraction. If there is a force pulling up, it will act regardless of the type of suspension you are using.

 

If you've ever loaded a suspension in a car to drag (or do a 0-60 run), the combination of brake and gas drive the rear wheels (assuming rear wheeel drive) up into the suspension. It works best in an auto transmission. In that case the brake is applying the same type of force - a resisting force - as the road. Now a brake can provide more resistance than a road, which is how you can induce the effect to an extreme. The road can never give you that much friction (and the level will vary.)

 

In a cycle, take the load transfer extreme to get it out of your head - braking after along fast straight - you get front end dive, the rear suspension should be extending. If anything you are looking at the rear wheel leaving the road at some point. BUT you can still load the suspension by adding gear brake and gas. The rear will drive up into the suspension. This has nothing to do with weight transfer - it has to do with balancing the forces on the wheel - the chain pulling up and forward against the resisting wheel. According to someone somehwere the Honda MotoGP camp in Japan (I assume it is a real thing) teaches loading the rear to quiet the suspension in a turn. In a turn and out of it, weight transfer is going to compress the rear suspension (along with the drive doing some minor compressing). To keep the rear suspension from going to extremes, in theory, you can load the rear suspension under braking so that when you get off the front brake the rear is already on its way to a compressed state.

 

In theory of course since getting the front braking down is probably more of an issue than trying to help the rear suspension by pre-compressing.

 

 

From the riders perspective what happens is the suspension stiffens becauase of the torque force being applied that is, from the sideview perspective, twisting the back of the bike upwards. What we are looking for in the middle of the corner is just enough throttle to maintain good suspension compliance and traction.

 

Once we start to bring the bike up then the stiffness of the rear end actually becomes a benefit in that it allows the bike to spin the wheel easier (because the suspension is stiffer and the tire can't follow the road surgface as well) this is a benefit becuase it cleans the tire and makes it ready for the next corner with clean rubber.

 

If you start the drive too early or spike the gas the rear stiffens up and that is a primary reason for an in-turn rear end slide. Not just the amount of gas but the stiffenng rear suspension not allowing the tire to grip evenly is what creates it.

 

All anyone has to do is look at race telemetry graphs and they will see what is happening at the front and rear end. On a fast lap with a good rider you see mid stroke suspension action in mid corner and you see a very definite rise in the rear and front at the point where the bike is being brought up and the hard acceleration begins.

 

This is an intersting point because years ago there was a thing called the ATK system that was two rollers set near the swingarm pivot. The chain ran over the two rollers so the chain going back and comig foreward to the engine were parallel instead of angled down to the countershaft sprocket.

 

It eliminated a huge portion of the torque reaction and stiffening of the rear suspension.

 

I set up a test day for Eddie Lawson and Team Kawasaki to test the device. Eddie went 1/2 second faster with it but didn't like the feel, he missed that stiffening at the rear and took it off and never tried it again.

 

Keith

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