| Author |
Message |
    
Bomber
| | Posted on Tuesday, November 21, 2006 - 10:19 am: | 
|
force and velocity RE different, hence low and high speed damping adjustments on better suspension units . . . . .
now THIS is FUN! |
Diablobrian
| | Posted on Tuesday, November 21, 2006 - 10:33 am: |
|
on an extremely bumpy stretch of pavement, with too much rebound damping dialed in,
your suspension can "pack down" because it does not rebound fully before the next
bump hits it. Even in that case though it would not end up at the stop because of
the incredible amount of energy that would be stored in the spring.
However that is not a good argument for running equal compression and rebound damping.
As Shawn discusses in the article settings depend on the rider, the riding
style, and
the surface you're riding on.
Take the time to feel what the settings on your suspension do. You will understand it
much better if you do.
You'll also be better able to understand what you need to adjust to get the feel you
want on the roads you ride. |
Jlnance
| | Posted on Tuesday, November 21, 2006 - 12:29 pm: |
|
Thanks for taking the time to answer Blake (and others.) I'm going to keep arguing with you. You've already convinced me I'm wrong, but it's a good way to learn. Please take it in that spirit.
The spring prevents that.
Yes it does. I realized that on the way home last night.
Also, the force imparted to the suspension resulting from the mass of the motorcycle/rider when hitting a bump can be orders of magnitude greater than the force the spring excerts in rebounding. Thus, the compression damper must be able to handle much greater energy input compared to the rebound damper.
Here is what bothers me. You ride down the road and surface features cause compression and rebound forces to be applied to the suspension. You turn around and ride back down the road in the other direction and you get the same forces, except the ones that used to be compression are now rebound and vice versa. You would think it would be good to have similar handling no matter which direction you were riding. 
Thats not a perfect example because the forces come in reverse order when you're riding backwards, and that matters. But if you could invert the road surface then it would swap the forces.
I'm focusing on the energy side of the issue, how much energy a compression versus rebound damper must be able to absorb. Maybe that is not what you are talking about?
I'm coming at it from a little different angle. If you didn't have the damper there at all and you hit a bump, all the energy would be absorbed by the springs. That would be fine during the brief moment you were rolling onto the bump, but it would cause the bike to bounce after that. I'm assuming the purpose of the dampers is to prevent bouncing. I'm also assuming that you want the system as close to critically damped as you can get.
Critical damping is a function of the mass, spring constant, and damping factor. It is not a function of the magnitude of the force applied to the bike, at least as long as the front wheel remains in contact with the ground.
I think my mistake revolves around that "front wheel remains in contact with the ground" part. As you point out, you can get large compression forces from hitting a bump in the road. My thinking is that if you turn the bump into a dip, and expect the wheel to stay on the road, the rebound forces required for that are equal to the compression forces in the original case. I realize that in extream cases the tire will not stay in contact with the road, but assumed this was rare and could be ignored. It appears that it is not as rare as I thought.
Now, if the front is not in contact with the road, the damping of the system changes dramatically. The spring constant is the same, but the weight of the system has been reduced from that of the rider & most of the bike, to that of the wheel. This means we have to substantially reduce the damping to remain critically damped. I can see that having different values for compression and rebound damping accomplishes this.
Another thought on energy. I'm thinking that the energy absorbed by the damper when you hit a bump is constant regardless of the amount of damping. The difference is just how long it takes to absorb it all. I'm not sure about that though. |
Bud
| | Posted on Tuesday, November 21, 2006 - 12:43 pm: |
|
your all talking about the old battle2win artikel ?
or did mr. Higbee wrote a new one ? |
Djkaplan
| | Posted on Tuesday, November 21, 2006 - 12:45 pm: |
|
Cycle World magazine used measure damping forces provided by forks and shocks on their test bikes. The rebound damping forces were always much higher than compression damping forces. Sometimes they'd measure very little compression damping at all, but there was always significant rebound forces pressent in both forks and shocks. |
M1combat
| | Posted on Tuesday, November 21, 2006 - 01:15 pm: |
|
Just to clarify...
Units with high and low speed damping have a shim stack that's spring loaded yes? When the energy moving the oil is great enough, the spring loaded shim stack opens allowing more flow, but when it's not great enough, it only allows the low speed circuit to be effective. Is that correct? |
Bomber
| | Posted on Tuesday, November 21, 2006 - 01:25 pm: |
|
M1 -- that IS one way to do it -- I'm sure there are others |
Djkaplan
| | Posted on Tuesday, November 21, 2006 - 01:48 pm: |
|
"Thus, the compression damper must be able to handle much greater energy input compared to the rebound damper."
Not sure what you mean by this. Rebound damping forces are greater than compression damping forces in just about every suspension damper I've ever encountered or read about. You can physically feel the difference in just about any damper (shock absorber) you care to try it with... they compress much easier than they extend. |
Macbuell
| | Posted on Tuesday, November 21, 2006 - 02:06 pm: |
|
This is why I love this board. I have no idea what you guys are saying but it sure sounds interesting. Now I'm going to go and see if I can truly screw up my suspension settings.  |
Jlnance
| | Posted on Tuesday, November 21, 2006 - 02:27 pm: |
|
they compress much easier than they extend.
OK, thats the opposite of what I would have expected. I'm learning a lot. |
M1combat
| | Posted on Tuesday, November 21, 2006 - 03:58 pm: |
|
Let's define the forces and figure out why then...
Compression...
Force exerted by the bump.
Inertia builds in the unsprung mass.
Spring is always pushing down.
The air compresses as well (another spring).
Rebound...
Stored energy in springs.
Gravity.
Is it the gravity that requires heavier damping for rebound? Is it just due to what a human needs to feel (I'm assuming that if the bump damping was as heavy as the rebound then we'd all have broken wrists? Is it due in part to making a bike more ridable (corner exit wobble if there's too little rebound damping up front)?
That's an interesting question. I suppose I always just took it for granted. |
Djkaplan
| | Posted on Wednesday, November 22, 2006 - 08:53 am: |
|
"Rebound damping forces are greater than compression damping forces in just about every suspension damper I've ever encountered or read about. "
I need to qualify that statement. For automobile drag racing, you want front dampers which allow the front end to rise easily and then lower slowly. Except for this one very special application, just about all dampers have more resistance to rebound than compression. |
Blake
| | Posted on Wednesday, November 22, 2006 - 07:56 pm: |
|
DJ,
You're not wrong about the relative damping forces. It is a real brain twister for sure and I may have mis-spoke/mislead in my prior posts. Not intuitive stuff at all. Fun stuff though.
Recall that...
Energy = Force * Distance
For a damper: Force increases with increasing velocity. |
|
