Rear suspension under load

I've been reading Twist of the Wrist 2 and it mentions that when hard on
the throttle the rear of the bike *rises*. And when braking the rear of
the bike will drop.

I'm curious as to why this is. Surely under acceleration there is
additional weight on the rear of the bike which would make the
suspension compress. So why does the rear of the bike rise?

 

I would agree, and can't help but think that you've misread the book.
I've not got TotW2 (only the first one), so can't compare, I'm afraid.

 

Stoneskin says...

Shaft drive bikes do this as the pinion tries to climb up the gear drive
during acceleration and so extends the suspension. I'm buggered if I can
think of a reason why a chain drive bike would do it, the Fatblade
certainly seems to squat at the back when giving it some welly.

 

Champ left a note on my windscreen which said:

From memory;

"Most riders don't understand this simple fact. The harder they are on
the gas the more the rear of the bike *rises*. To test this put the
front wheel of your bike against a wall and, with it in gear, slowly
engage the clutch - the rear of the bike will rise"

The only thing I can think of is that this is amongst other information
regarding cornering. But I still fail to see why a bike would act in
this manner during cornering and not in a straight line. The experiment
which the book suggests doesn't seem to be anything specific to
cornering either.

 

I would think it would rise if the front can't go anywhere. The front would
compress as the back would try and move forward, be unable to, and rise up.
If that wall suddenly disappeared the front would lift and the back would
compress, surely.

 

Ah, I remember that bit now.

But isn't it followed by some discussion on weight transfer, that
counteracts this effect.

But in this case, they're taking about a static bike.

It's a while since I read it - I lent it to someone and haven't got it
back - but at some point in the book they mention about how you should
be on the power through the corner to transfer weight to the back wheel,
blah-de-blah-de-blah.

 

Porl left a note on my windscreen which said:

I've managed to find this on the subject;

http://tinyurl.com/3kheq

"Let's take an example where you're going through a corner and you find
that some part of the motorcycle is just barely touching down, at
maximum lean point at the apex. You determine that you're not really
applying the steady and continuous throttle roll on throughtout the
turn, and may actually be at a "neutral" throttle at the apex of the
corner, right at the point where the bike is bottoming.

Now approach the same turn, at the same speed, and insure that you're
applying some throttle at this same leaned over point, and see what
effect it has on the bike's tendency to drag parts. You'll find that
the bike will (if the throttle control is done properly, and the bike
was just barely touching parts down previously), now make it through the
corner without any interference with the ground. This effect is in
agreement with what Keith Code implies in his reference to the "rear
rises" ..... being that the chassis "rises", as the swingarm is brought
downward away from the frame.

The reason for this additional clearance, is that the application of
throttle will cause the rear swingarm to be pulled downward by the chain
torque action trying to pull these two points (the countershaft sprocket
and the center of the rear wheel) closer together, resulting in the rear
suspension actually rising up and giving more ground clearance between
the suspended portion of the chassis and the ground. This is true, as
long as the swingarm position is at a point in it's travel where the
line between the countershaft sprocket center, and the center of the
rear axle, fall below that of the swingarm pivot. As long as this
condition exists, any torque on the chain will pull the wheel
downward, resulting in this suspension extending effect."

 

I got lost at the part where it says "Let's"

 

As you apply more power the chain will get tighter (on its top edge,
as it were) and try and shorten. The centre of rotation of the
swinging arm is not the same as that of the front sprocket, so there
will be a tendency to pull the rear towards the front, resulting in a
downward moment between the centre of rotation ond the rear sprocket.

No, it's not.

 

Porl left a note on my windscreen which said:

More gas... chain pulls rear wheel toward centre of bike... swingarm
goes down... rear of bike goes up.

 

I think it's something to do with the chain acting on the swing arm.

You get a similar thing on bicycles with rear suspension. The pulling
along the top of the chain causes the swingarm to pull down, acting
against the shock.

This fact is actually desirable on some pedal bikes to lock the
suspension out when pedaling hard to give better power transfer.

 

torque reaction around the front sprocket.

err...not sure if I agree at all with this statement, unless you just
mean the rear suspension isn't as extended as under acceleration.

 

Assuming you're talking about chain drives, it depends on the rear
suspension, & which brake you use. Using the rear brake only will make
the back of the bike sink, as the force on the rear wheel will try to
drag the swingarm pivot down (assuming the swingarm pivot is higher than
the rear spindle). Using the front brake only will make the rear rise,
for obvious reasons.

With acceleration, the rear wheel will try to push the swingarm
forwards, making the rear of the bike rise as the front of the swingarm
tries to climb over its pivot point. However if you've got a soft
spring, the back might squat instead, especially if it's set with very
little rear sag. For example big trailies & off-road bikes will
invariably squat at the rear when you accelerate hard.

 

darsy left a note on my windscreen which said:

I haven't got the book with me here but Code states that both the front
and rear of the bike will drop when decelerating.[1]

If the back does, indeed, rise when accelerating then the reverse should
also be true.

I imagine he isn't taking about braking power which produces monster
stoppies - just suspension travel.

[1]I don't think he was specifically talking about Bandits here either.