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Old 10-09-2007, 11:07 AM   #1
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Questions?? Shock Rebound, WHat Is The Effect?

Can anyone tell me what difference is made to the handling of a car with/without rebound on the shocks (with everything else set exactly the same).

What should I see/feel?

Thanks
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Old 10-09-2007, 12:23 PM   #2
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The car will have less roll with more rebound and sometimes less traction.
Kind of adding or removing the swaybars if you know what I mean....
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Old 10-09-2007, 01:37 PM   #3
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No matter what, you need the shocks to react equally both sides of the car. All my shocks rebound 50% on rebuild and after a few runs they settle at 25% and remain so until one of them goes soft, which is pretty easy to tell especially if you realize that hitting the wall etc will inevitably cause some disruption of the state ofthe shock. Took a few years to discover this stuff as fact of life on my cars.

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Old 10-10-2007, 06:37 AM   #4
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I also meant with regard to the foam 'volume compensators' that can be fitted above the bladder, is there any point?

Thank
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Old 10-10-2007, 06:58 AM   #5
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The foam is in the bladder to accommodate the volume of the shock piston traveling into the shock body when the body is completely filled with oil and doesn't contain any air. (Otherwise the shock would completely lock up.)

Foam will want to return to its natural state when not under load, so it will draw air back into the head of the shock and provide more/more consistent rebound.

I think foam shocks are more tolerant of a bad build but it will also hide a shock needing a rebuild from you, so it's kind of a double edged sword.

HTH,
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Old 10-10-2007, 07:10 AM   #6
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Quote:
Originally Posted by ckearns View Post
The foam is in the bladder to accommodate the volume of the shock piston traveling into the shock body when the body is completely filled with oil and doesn't contain any air. (Otherwise the shock would completely lock up.)

Foam will want to return to its natural state when not under load, so it will draw air back into the head of the shock and provide more/more consistent rebound.

I think foam shocks are more tolerant of a bad build but it will also hide a shock needing a rebuild from you, so it's kind of a double edged sword.

HTH,
Chris
I think he meant the small foams that go above the bladder in shocks such as the TRF's.
Not entirely sure how they work, but I guess they add pack into the shock, by not allowing the bladder to deform as much.

I've found that running with out the foam smooths the car movement of the car out, and that adding them improves reaction. I tend to run with foam in the shocks outdoors, and remove them when running on carpet.

HiH
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Old 10-10-2007, 07:16 AM   #7
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If i am filling all my shocks to an even level (trf) and I am not getting the same rebound, (obviously with the same oil/pistons to make it a fair test) would that indicate something is worn?

I just cant seem to get even consistancy rebound and the shocks are not that old...
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Old 10-10-2007, 11:25 AM   #8
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Quote:
Originally Posted by tc3team View Post
If i am filling all my shocks to an even level (trf) and I am not getting the same rebound, (obviously with the same oil/pistons to make it a fair test) would that indicate something is worn?

I just cant seem to get even consistancy rebound and the shocks are not that old...
I found that differences in the shocks like that are due to very small differences in the quantity of oil in them, it takes awhile to get them all the same.
This is one reason why I like the new Schumacher Mi3 shocks, they have a second hole in the side of the shock cap which 'bleeds' the oil as the cap is put on, out of 8 shocks I've filled only one had to be redone!
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Old 10-10-2007, 02:29 PM   #9
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Quote:
Originally Posted by Skiddins View Post
I found that differences in the shocks like that are due to very small differences in the quantity of oil in them, it takes awhile to get them all the same.
This is one reason why I like the new Schumacher Mi3 shocks, they have a second hole in the side of the shock cap which 'bleeds' the oil as the cap is put on, out of 8 shocks I've filled only one had to be redone!
thanks, i'll have to check more closely when i re oil them, i put it off as much as i can (if the rebound is out then its got to be done in my eyes, otherwise I wont touch them).
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Old 05-27-2011, 02:52 AM   #10
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Sorry for reviving an old thread.
I thought it would be more beneficial to built more information onto existing ones instead of starting a totally new thread

Am i right to say that setting the rebound would actually alter the compression rates throughout the whole suspension travel? It is an attempt to mimic a pregressive spring rate or multi valve damper in a real 1/1scale car?

For example:
If i set my rebound to be 3/4travel (that would be pushing the shock inwards by 1/4travel), the initial 1/4 of stroke travel would have a lower compression rate (softer compression) since the into cavitation of the damping oil is acting against the spring.
Likewise, after full compression, the returning travel would be slower after it reaches 3/4travel.

Need you guys to enlighten me
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Old 05-27-2011, 03:20 AM   #11
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Quote:
Originally Posted by ???man View Post
Sorry for reviving an old thread.
I thought it would be more beneficial to built more information onto existing ones instead of starting a totally new thread

Am i right to say that setting the rebound would actually alter the compression rates throughout the whole suspension travel? It is an attempt to mimic a pregressive spring rate or multi valve damper in a real 1/1scale car?

For example:
If i set my rebound to be 3/4travel (that would be pushing the shock inwards by 1/4travel), the initial 1/4 of stroke travel would have a lower compression rate (softer compression) since the into cavitation of the damping oil is acting against the spring.
Likewise, after full compression, the returning travel would be slower after it reaches 3/4travel.

Need you guys to enlighten me
I think shock bladders give a very slight dual rate. I'm not sure the dual rate is really noticable in most setups as it's a tiny percentage of the spring rate. The two rates you get will depend on the amount of rebound.

With lesser amounts of rebound, the initial rate will be where the bladder is pulling the shaft back into the shock as the bladder has been stretched downwards at full travel. The second rate will begin when the bladder reaches it's normal position and then starts to move upwards. You could make this second stage stiffer by adding o-rings or foam above the bladder. Without o-rings above the bladder it's probably (almost) linear.

With a lot of rebound, the initial rate will be where the bladder is moving upwards normally. At some point the bladder may reach full natural movement and you will get a second (much stiffer) rate where the bladder is actually stretching rather than just changing shape. This is unlikely to happen on TC shocks due to the short shock shafts.

In practice I think a lot of rebound is hard to get consistent between shocks, and results in more oil leaking out due to the pressure. Lots of TC guys now drill the shock caps to give zero rebound and more consistent rebuilds.
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Old 05-27-2011, 01:39 PM   #12
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The more rebound you have the less the initial damping you get when the shock extends or compresses. This is because the shock piston isn't moving through the shock oil when the bladder is expanding or compacting. Fluid, piston the lot is all moving along the cylinder and just the air gap is compressing or decompressing initially when the piston starts to move.

What foam inserts do is to to exaggerate this, as a compressed bit of foam is springier/ denser than a compressed pocket of air.

Effects of rebound are simular to running thinner shock oil or having progressive damping. Quicker direction change, better over little bumps but a little less stable. Good for lower traction but more unpredictable on higher grip tracks.
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Old 05-27-2011, 02:35 PM   #13
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At the risk of getting us into a different conversation here, especially as Dale knows his onions on this stuff...

Rebound is not the action of the shock coming out when the bladder expands. Rebound is the opposite of compression. The rate at which the damper (shock) controls the compression of the spring is compression, and the rate at which it allows the spring to return to its equilibrium position is the rebound rate. On full size cars, bikes, etc., these rates are not equal. Tuning of these rates is key to handling.

What we have here is the compression of the bladder putting load into the system either through the elasticity of the bladder, and/or the compression of the air above the bladder. It slightly raises the pressure inside the damper. Once the energy producing that load is removed, the energy is returned by pushing the piston out.

I agree with Dale, the chances of that making any appreciable difference to the shock performance when compared to the high spring rate is almost negligible.

YZFAndy, your assessment only works if the shock can draw in air. The piston and oil cannot move together unless something comes in behind the piston. The only thing available is air, but if the shock is correctly built, it cannot draw in any air.

If the air above the bladder is trapped, then when the shock is asked to move fast, the piston and rod rises in the cylinder, increasing the pressure in the oil - as the piston rises, more rod comes into the cylinder reducing the space for the oil, and that compresses the oil. Since the oil cannot be compressed, it will increase in pressure preventing the piston moving as fast and stopping the shock damping the spring energy in compression. The trapped air will eventually compress, but that slows the shock down. Effectively, the shock has two rates - one when operating slowly and another when operating fast. This has the effect of making the shock 'pack' when moving fast, which may be OK, but maybe not.

Holes are drilled in the caps to let the air exhaust, and so allowing the shock to compress quickly. The other reason to do this is to ensure that the oil is always doing the work as it is adjustable and constant. Having compressed air do the work is less controllable as it is more variable. For TCs, the best option is to make sure that nothing interferes with the oil doing the work, as this way you have maximum control over the damping, and changes you make will be better reflected in the handling you get.

It's not rebound, it's the return of energy you have put into compressing the bladder and, possible, the air above it. For TCs, to get maximum control over damping changes, it is best to get rid of the foams and the drill a hole to let the air out. As I said earlier, I agree with Dale - the effect you get from this 'de-compression' is so minimal it is almost undetectable by the average driver. HTH
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Old 05-30-2011, 01:07 PM   #14
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One more time for the noobs...

You're saying 0% rebound, and drill the caps to let air out.


Did I understand you correctly?
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Old 05-30-2011, 01:51 PM   #15
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Quote:
Originally Posted by SlowerOne View Post

YZFAndy, your assessment only works if the shock can draw in air. The piston and oil cannot move together unless something comes in behind the piston. The only thing available is air, but if the shock is correctly built, it cannot draw in any air.
No my explanation does work, It doesn't need to draw in air. The shock shaft displaces volume. Not 100% oil volume for air gap volume, just an exaggeration to get whats happening across. Another way to look at it would be that initially the oil is travelling through the shock holes slower while this air gap volume is filled.

Quote:
Originally Posted by Coach-Z View Post
One more time for the noobs...

You're saying 0% rebound, and drill the caps to let air out.


Did I understand you correctly?
If you let the air out of the shock cap then air pressure on the shock shaft and in the air gap at the top of the shock will be equal and there will be almost no rebound. The only rebound that would be left is the diaphragm trying to rebound to its natural shape.
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