How Suspension Reacts

[The Stranger:Me]

We all know that under acceleration the front of a bike tends to lift and the front suspension unloads to some degree, possibly to the point where it is completely unloaded and the bike wheel stands.

What happens to the rear suspension during acceleration?

[Quasievil]

We all know that under acceleration the front of a bike tends to lift and the front suspension unloads to some degree, possibly to the point where it is completely unloaded and the bike wheel stands.

What happens to the rear suspension during acceleration?

It compresses, yeah ?

[DEATH_INC.]

Actually,no.The chain actually pushes the suspension down (extends it),it tries to shorten itself as much as possible,pulling the top of the sproket down.You do get a certain amount of compression from weight transfer though.

[The Stranger:Me]

Don't know for sure.
On the one hand you tend to think there is now a rearward weight bias and therefore the rear suspension will compress, but does it?

I believe the front tries to lift because the front sprocket is providing lift via tension on the top of the chain which is anchored to the rear sprocket.

If so then the lift is being provided at about the centre of the mass and wouldn't this lift tend unload the rear suspension also?

Sure there is weight transfer front to rear and there is a relative difference in heights front to rear, but that would not necessarily preclude the rear from rising too.

Where am I going with this?

Well I see some prominent riders claiming that a bike gets taller under acceleration (front and rear) and thus you get increased ground clearance say when accelerating around a bend. Some also claim that this a big problem when taking a bend with a trailing throttle, as the opposite happens.

Now they don't state why the bike get taller and I have seen no evidence of this, but I like to know for sure and I was hoping someone may have video footage which could clarify this matter.

[T.W.R]

Actually,no.The chain actually pushes the suspension down (extends it),it tries to shorten itself as much as possible,pulling the top of the sproket down.You do get a certain amount of compression from weight transfer though.

have a wee re-think about that!

Under initial acceleration the chain is drawing (pulling) the rear sprocket towards the drive sprocket & as the swing-arm travels in arc & with suspension length etc & the rigid length of the swing-arm results in the compression.
The swing-arm actually sits slightly below the upward stroke of the arc in normal motion due to the direction of chain rotation, so under acceleration the swing-arm is drawn further up the arc thus compressing the suspension further ( thats why numerous motorcycle tests comment on the tested bikes sitting under acceleration) & why more powerful bikes will loft the front wheel under hard acceleration ( because the rear wheel is being pulled towards the front-end) its only once an equalibrium has been reached that the bike will level out & release the increased load on the suspension.

The only occassions were a bike will rise under acceleration is with the older style shaft drive bikes, this is why BMW designed the PARA-LEVER system to overcome the torque loading on the drive-train.

this is why any vehicle squats under accleration & lifts the front end (e.g dragsters etc)

[TwoSeven]

We all know that under acceleration the front of a bike tends to lift and the front suspension unloads to some degree, possibly to the point where it is completely unloaded and the bike wheel stands.

What happens to the rear suspension during acceleration?

It extends.

[SlowHand]

do a burnout. See what happens - but dont ask me how it works

[Hoon]

this is why any vehicle squats under accleration & lifts the front end (e.g dragsters etc)

But vehicles aren't chain driven are they?

Both are right.

Chain pulling on the sprocket causes the rear to extend.
Weight shifting to the rear causes the rear to compress.

Usually compression is the greater of the two so the other goes unnoticed but rear extension due to chain pull is real, thats what adjustable swingarm pivots are for.....to change the angle between the front and rear sprocket.

[The Stranger:Me]

do a burnout. See what happens - but dont ask me how it works

No good, as so much resistance is lost when the wheel breaks traction.

[dhunt]

Twist of the wrist states

Most riders don't understand this simple fact: The harder they twist the gas, the less compiant the rear suspension is and the more the rear end tries to rise. Most riders believe that the back of the bike goes down they accelerate. It doesn't. (To test this, put the front wheel of your bike up against a wall and begin to engage the clutch with the transmiision in ger. The rear end will come up.)

"Whacking" the throttle on stiffens the rear suspension and reduces traction. That's a problem for most of us. Yet the best riders have figured out a way to completetly reverse the situation and use it to teheir advantage. In the case of a 500cc GP bike, when the power starts to come on and the suspension stiffens, the tire tends to spin because tractions is reduced. What do the best riders do? They let the tire start to spin for the drive out of the turn. The suspension actually becomes more compliant right when the tire begins to spin, because the reduced traction relieves some of the load. ...

[T.W.R]

do a burnout. See what happens - but dont ask me how it works

whats the first thing you do when doing a burn-out ? handful of front brake! so the arse-end is trying to push everything forward thats why the you get the rise in the rear!

[The Stranger:Me]

That may appear to be the end of it, but, does anyone have proof?
Say a video.

Placing your front wheel against a stop may induce a false result as the rear wheel would tend to push swing arm up when it can not push the front wheel forward, so I don't believe that that is a valid comparison to real world riding.

[TwoSeven]

That may appear to be the end of it, but, does anyone have proof?
Say a video.

Placing your front wheel against a stop may induce a false result as the rear wheel would tend to push swing arm up when it can not push the front wheel forward, so I don't believe that that is a valid comparison to real world riding.

Put a data logger on your suspension and look at the pics. I had an example diagram years back, should be some around if you use google.

Generally the bike tries to climb the sprocket which extends the rear on accel.

[Hoon]

That may appear to be the end of it, but, does anyone have proof?
Say a video.

Watch any racing footage when a rider exits a turn and is hard on the gas. The rear is almost topping out.

[NordieBoy]

If it extends under load it's a bad design.

The turning motion of the rear wheel works to push the swingarm down (1) against the effort of the spring - loads the suspension up.
When the spring matches that effort then the rotational motion it translated to forward motion (2) and the spring unloads to an extent as there is less rotational force on it as it has overcome the inertia of the lardarse and bike.

Or something...