How Suspension Reacts

[TwoSeven]

Try examining it from the front sprocket in relation to the seat.

[T.W.R]

When a motorcycle is in motion and accelerated hard the initial reaction of the of the drive-train is compression then expansion (load transfer), the initial power delivered by the drive shaft upon then sprockets makes them contract towards each other, then as the power is transfered into momentum the expansion happens (the only reason that it looks as if theres just extension of the suspension is that it happens so fast, only the result is truely noticeable).
Its simply a pull push effect, the engine pulls the wheel towards it and the energy is transfered to the tyre which is pushing upon the ground & when in motion that push effect rises the whole motorcycle.

Check the attachment (even though the bike in it is a turbo powered V-MAX) watch closely at the first couple of meters, the bike is squating down until it gains full traction then once that happens it appears to rise)

[T.W.R]

[QUOTE=Hoon]But vehicles aren't chain driven are they?

factoid: maybe not in this day and age, but all the first cars and other assorted forms of transport were chain drive!

it doesn't matter whether its chain driven or shaft driven the transfer of energy has the same effect

[TwoSeven]

What if I was to say that the swingarm piviot point is above the axle would that make a difference. Would it be that the wheel base would shorten and the rear ride height increase, buggering up rake and trail and cornering ability.

You'd need to keep messing with the chain angle and pivot points and rebound in order to ensure the rear doesnt climb to quickly. Almost like an anti-squat system if one existed.

Have a look at the history of the swingarm on the R1 and see what I mean.

[Brian d marge]

look would you lot stop posting interesting threads ...I am at work at must at least look like i AM WORKING ...

Actually ,,This one messess my head up ...I know what it does cos all the books tell me it does ,,,but my head goes all in circles and me brain gives up the ghost on this one ....

I have a computer prrgram that does all this sort of stuff when I gwet home Ill see if I can post anything worth while


Stephen

[HDTboy]

Look at your bike from the right hand side. As the bike accelerates with traction the wheel will turn clockwise in relation to the swingarm. The more torque the wheel can apply to the swingarm, the more the swingarm will try to rotate anticlockwise; Lifting the bike

[erik]

Interesting thread indeed.

I tried thinking about it, couldn't figure it out. It's bloody complicated.

the tension in the chain will try to make the swingarm rotate up towards the rear seat.
But this might be balanced by the force due to acceleration if it passes below the swingarm pivot. But it's hard to tell which force would be stronger, and it all varies with how fast the bike accelerates.

[TwoSeven]

The basic principle is that the chain pulls the wheel forward. Because the swingarm piviot is above the wheel axle height it pushes the swingarm piviot up and forwards which forces the suspension to extend. Think if it as trying to break the bike apart

[Draco]

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)

No,you're wrong.The top of the rear sprocket is above the top of the front sprocket generally so it pulls the arm down,not up.Plus the bottom of the rear wheel where the drive force is transmitted is well below the swingarm pivot so it'll try to drive underneath,again causing extension.
As a side note,a well setup dragger will also extend under acceleration as do any well setup 4 bar circuit cars etc.Ever watched a sprinter sit up and down under acceleration/decceleration?
Death.(on draco's pooter.)

[erik]

The basic principle is that the chain pulls the wheel forward. Because the swingarm piviot is above the wheel axle height it pushes the swingarm piviot up and forwards which forces the suspension to extend. Think if it as trying to break the bike apart

But the chain passes above the swingarm pivot (at least it does on my bike) so it will pull the swingarm up, toward the mudguard etc.
But because the distance of the chain from the swingarm pivot is small, the torque created is small.
By the sounds of what Death's said, I'd guess the force from acceleration passing below the wingarm pivot makes the bike stand up.

But it would depend on the location of the swingarm pivot and the bike's centre of mass (and how hard the bike was accelerating).

I can't see any way other than the acceleration force passing below the swingarm pivot that would make the rear of the bike raise.

[TwoSeven]

But the chain passes above the swingarm pivot (at least it does on my bike) so it will pull the swingarm up, toward the mudguard etc.
But because the distance of the chain from the swingarm pivot is small, the torque created is small.
By the sounds of what Death's said, I'd guess the force from acceleration passing below the wingarm pivot makes the bike stand up.

But it would depend on the location of the swingarm pivot and the bike's centre of mass (and how hard the bike was accelerating).

I can't see any way other than the acceleration force passing below the swingarm pivot that would make the rear of the bike raise.

The Chain Run passes above the swingarm piviot but the force [well most of it] travels along the swingarm.

The chain run angle is slightly different from the swingarm angle which must be positive. The force if neutral would run from a line drawn from the rear axle to the front axle, but because the piviot is higher than this plane, it gets pushed up as the back wheel gets pulled towards the front.

[erik]

The Chain Run passes above the swingarm piviot but the force [well most of it] travels along the swingarm.

The chain run angle is slightly different from the swingarm angle which must be positive. The force if neutral would run from a line drawn from the rear axle to the front axle, but because the piviot is higher than this plane, it gets pushed up as the back wheel gets pulled towards the front.

In other words:
The force to accelerate the bike is all transmitted through the swingarm.
Because the swingarm is pivoted at both ends, it can only transmit force along it's axis, it can't transmit torque at the pivot (ignoring suspension here).
Because the innertia force (reaction resisting acceleration) of the bike and rider acts below the swingarm pivot, it tends to rotate the body of the bike downwards about the swingarm pivot (clockwise if bike is viewed from right side).
And since the weight has all transferred to the rear wheel, the front of the bike rises as the forks don't have so much weight on them.
In effect this makes the rear of the bike raise up and the swingarm rotate anticlockwise if viewed from the right.

The above can be more easily visualised if you imagine the swingarm at an exaggerated angle.
In the attached pic (sorry for the drawing quality )
If the bike had the blue swingarm, it's fairly easy to imagine that the wheel would try to ride under the bike and the bike would kind of fold like scissors about the swingarm pivot, making the swingarm pivot raise.

But with the red swingarm, the innertia force acting from the bike's centre of mass passes way above the pivot, and it's reasonably easy to see the pivot would move down, the bike would fold in the opposite direction to before.

However, in both cases, if the front sprocket was moved with the swingarm pivot so that the chain passed above and below the pivot, the effect of the tension in the chain would try to rotate the swingarm around clockwise (relative to the body of the bike). This is easier to visualise if you imagine the rear sprocket fixed to the swingarm (ie, can't rotate around the rear axle). See pic 2.

[TwoSeven]

If I read that correctly, then yep, thats about right as I see it


Edit:
I take it your really good at math then. Having never done math, I have to draw lots of pictures and then spend days thinking about it. Would have never picked up the front end rotating bit.

[GeorgeR]

You can't assume that the sprocket is fixed to the swingarm - that allows torque to be transmitted into the swingarm which as you said is impossible.

Google makes things much easier to explain - no need to draw my own diagrams...
http://www.tootechracing.com/Engine%...Suspension.htm

[NordieBoy]

You can't assume that the sprocket is fixed to the swingarm - that allows torque to be transmitted into the swingarm which as you said is impossible.

Google makes things much easier to explain - no need to draw my own diagrams...
http://www.tootechracing.com/Engine%...Suspension.htm

On the MotoCycz the front sprocket is on the swingarm pivot.

Taken to an extreme is my Cannondale V3000.
The front sprocket is so far below the swingarm pivot that any engine/leg motion is first translated to rear suspension extension and then to forward motion as less energy is needed to extend the rear than propel forward.
Every time you push the pedals you first go up - then forward.