rear suspension dynamics/geometry question

so i've done some research on this but i'm not sure i have this quite right. Anyway, here is my assumption on how the rear suspension works.

1 - rider opens throttle
2 - rear shock extends upwards
3- simultaneously the swingarm pivots downward on its pivot point
4 - as the swingarm pivots down, the rear of the swingarm raises

The result of this is the rear tire is "lifted" up thus reducing rear traction. Conversely, the front of the bike ends up w. an increase in trail if the rear squats too much. Am i correct so far?

If so, how does stiffening the rear shock (via compression or spring rate) affect the squatting action of the swingarm?

BTW -i barely passed highschool geometry.....like 20 years ago.

 

What you talking about is squat and anti squat. Using compression damping is more for fine tuning. Once you in the ball par a clich or two for compression can reduce or increase the amount the shock compresses.

 

You really should try and not use preload to adjust for squat or anti-squat. depending how firm your shock is set will have an direct effect on how much the swing arm moves. (They are bolted together. Other factors come into play like Wheel bases rear ride height tire diameter which affect swing arm angle. Your gearing will also effect squat and anti squat. So if you change gearing for different tracks you will need to make adjustments to your suspension to have a bike that reacts the same. You are on the right track though so keep reading. Understanding how all of this stuff works together will give you a leg up on your competitors.

 

The shock compresses, not extends, on throttle opening. Start there.

 

rear suspension dynamics

i used to think the same thing i.e. the shock compresses when opening the throttle. From what i've read and seen, it actually extends.

That being said, I can can see how a stiffer spring will help w/ anti-squat - stiffer makes the shock slower to extend under hard acceleration thus preventing the swingarm from dropping at the pivot point. I just can't see how compression comes into the equation to help w/ anti-squat.

 

i plan to use the ride height adjuster on my ohlins ttx shock to adjust the swingarm angle to roughly 13 degrees. This seems to be the recommended angle for the swingarm on an 07 GSXR 750. Of course then you have to manage the trail number in the front. From what i've read the 07 Gixxers like to ride tall, so i am planning to drop the forks to flush or slightly below flush w/ the triple trees, then manage the behavior of the bike via dampning and correct sag settings.

Is that a good plan, or am i missing anything here?

thanks

 

+1

 

It's not simple. Opening the throttle pulls the chain. Because the angle of front sprocket to rear is positive ( that is, there is approx a 10-12 degree angle down), the power of the bike pulls the swing arm down. It has to. So it extends. Simultaneously, the bike unweights the front and puts weight on the rear. This pushes the chassis down. Most bikes extend on accel, but the DEGREE of extension is what governs push on corner exit. The big HP bikes are notorious for "squat and push" exiting corners. But it's all relative. They are extending, just not much.

 

Not sure if I did that clear. The chain pull extends the swingarm. the weight transfer reduces the extension. But it does extend.

 

I'm looking at data traces right now. All of them show rear suspension compression upon throttle opening. I'd like to see the data you guys have because it's not jiving.

If I knew how to do a screen capture with this application, I'd post it up.

 

Think of what the bike does on the dyno when you make a pull....

 

http://www.techsmith.com/download/jing/

Or, just ctrl + printscreen, then open up MS Paint and ctrl + v. Save.

On a dyno there is no weight transfer, there is only the tension on the chain.

 

Exactly my point...

 

I suppose I was interpreting the question as opening the throttle, leaned over, exiting a turn. weight transfer overcomes chain pull, and the shock compresses. Data confirms this. This seemed to be what the OP was asking about, maybe I'm wrong. On a dyno, or shifting gears down the straight, yes, extension.

 

I was trying to show theory vs real world. Yes, the forces of the chain will extend the shock, but in the real world on the track, there are other forces that counter that action...unless you were to run a parallogram suspension or like setup to isolate tq input from suspension.

 

if a line drawn from the top of the c/s sprocket to the top of the rear sprocket puts the swingarm pivot above this line, then, yes, you could extend the shock as the torque tries to pull the rear wheel towards the engine and the swingarm "folds" at the pivot.
i don't see any bikes set up like that...you'd saw thru the swingarm with the chain.

puttin' it on the dyno is false proof. you're not allowing for the bike being held back. again, the wheel is just trying to drive forward. it ends up wanting to get under the bike giving the impression that the suspension extends when giving it gas. no.
keith code made this same mistake when he put a bike against a wall, gave it gas and observed the rear of the bike rising. he induced that giving a bike gas extends the suspension. he allowed for his not being an engineer when he drew that conclusion.

the farther the swingarm compresses and the more torque applied, the farther the swingarm will be driven to compress. if the shock is crappy enough, you could drive the swingarm straight into full compression, bottomed out.

 

Again, it's not a simple issue. There are any number of factors. Heat of shock, throttle opening, spring rate, lean angle, fork rebound setting, etc etc

 

There have been bikes built with concentric swingarm pivot/countershaft (Bimota, and there's a current dirtbike, can't recall which one) and this would eliminate torque jacking.

 

no.
with the swingarm being the component in question and the chain pulling completely on one side of the swingarm, the swingarm will go in the direction of the tension.
so,
gas = compression. you're pulling the swingarm towards the tension side of the chain, viewed as chain on top of swingarm.
engine braking? = extension. you're pulling the swingarm towards the tension side of the chain, from viewed as chain on bottom of swingarm.

there's no mystery math, linkage set-up, shock parameters, nothing. it's straight physics. an object goes in the direction it is being pushed or pulled. you pull the chain with throttle, the axle rises. pull it with engine braking and the axle drops.

none of that other stuff matters until you try to control the movement.
none of that other stuff can alter the direction of that movement.

the concentric swingarm-c/s pivot is in attempt to limit the chain's interference with the suspension workings. constant chain tension means no tight spots or stretching due to overly tight chain adjustments and zero effect on chain induced torque biases due to varying swingarm angles. doesn't matter what size sprockets you run, they don't affect the geometry.

 

rear suspension dynamics

man my head hurts -

so let's assume the bikes singarm angle is at 13 degrees based on ride height, tire size, and gearing- i'm valentino rossi chasing jorge lorenzo at the last turn of the last race, winner claims the moto gp championship - i aggressively roll on throttle coming out of the final turn - WTF is happening to the geomtery and suspension of my bike- in general terms.

BTW, Ben Spies, runs up on both of them and drafts them to the line for the win.