What happens to your rear shock when you accelerate?

[Degsy]

I had this discussion at the track a few weeks ago with a couple of the racers and we also had a bet on the answer as I remember. I hope those guys who bet with me will pay up when I prove I am right :-)))

SO, what is the consensus of opinion? What does your rear shock do when you accelerate?

Degs

[Honclfibr]

Both. The force of the spinning rear wheel gets to your frame through the swingarm, which is angled up (the axle is lower than the swingarm pivot). This results in an upward force which causes the rear to rise (since the force is behind the center of gravity), and the shock to become unloaded. The tension of the chain pulling the swingarm up at the axle and weight transfer to the rear as the bike accelerates oppose this, and try to compress the shocks. But the shock is not infinitely compliant so they are initially stiff to a sudden acceleration, and do not compress. Eventually the shock should compress, certainly if the front wheel is in the air So as you accelerate, you should get initial unloading of the shock, followed by compression.

At least, that's how it was explained to me by the suspension guys out at Hendrickson when I went on a little field trip (aside, chicago in december sucks my ass), I'm not an M.E. and so I may have screwed a few things up but I hope I got the gist right...

[Stoneman]

Honestly, I could give a fuck what the correct answer is. It's funny just to sit there watchin' you clowns argue about it...

[SEVENSGT]

my gut says unloads(goes up). but the tourque from my motor says compress(goes down) Then again I'm with jay,

WHO GIVES A SHIT

[Honclfibr]

I'm quite interested in suspension right now, since we're involved in a project at work involving electronic control of tractor trailer air suspension...things like this fascinate me. The suspension designers at hendrickson were all motorcycle riders and indulged me in some tech discussion while I was out there, though some of it I'm afraid didn't sink in...

Now, if I'm remembering right, you should be able to win your bet either way If you want to "prove" that the rear suspension compresses, slip your clutch as you hold down the rear brake (can you do this without hurting the bike?). The rear wheel can't move so there's no force on the swingarm except the chain tension, which should pivot the swingarm up and compress the shock.

If you wanna prove the suspension rises, slip the clutch as you hold down the front brake. The rear tire is allowed to move here, so there's a force on the swingarm pushing it forward and upwards, and I believe this force should be greater than the chain tension torque, and cause the rear to rise and unload the rear shock.

[Honclfibr]

Course, if you *really* got something to prove, you just hook up a stringpot to the rear suspension and record the data as you accelerate down a stretch of road. Hehe...I could supply the DAQ and a laptop, but I dunno if work'd take kindly to me borrowing a stringpot, they're about 500 bucks a pop and fragile as all hell, we've broken two already

[PinHead]

The shock compresses.

[beet]

do i win a prize

[stoinkythepig]

Kevin Cameron says: (paraphrased) "In low gears, the tension on the drive chain is strong enough to overcome the natural tendancy for the bike to squat under acceleration. The if the angle between the top chain run and the swing arm were increased, the less likely the bike would squat."

So, based on that, I'd say on a typical race bike, the suspension would extend.

[zx-stewie]

You guys are making this way too complicated...when the bike accelerates, the rear shock accelerates with the bike.



Unless the rear shock falls off, but that's a different story altogether.


Next!!

[RSVMILLE661]

I'll go with the 2 opposing forces cancel each other out and the shock stays static under accelration. (not counting the added sag of the riders wieght.)

[RandyO]

I gotta agree with beet on this one