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Old 01-28-2011, 01:33 PM   #1
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Default Shock Rebound

I understand what rebound "is" - how much the shock shaft comes back out after pushing it in with no spring on it. 100% rebound means the shaft comes out all of the way. 50% half way, etc. Varys with the amount of shock oil, the bladder, how smooth the shock shafts are (no stiction in the o-rings) etc.

I hear racers setting their rebound for specific handling characteristics - some of which are consistent in their theory, others not.

What I am curious about is what are really trying to achieve here?

If the spring is designed to return the shock to full length or to set the ride height of the chassis, is this extra amount of force due to the "rebound" simply adding to the spring rate; helping the spring out a little?

Does this provide fine tuning to achieve "in-between" spring rates? A progressive feel as the shock is compressed?

Just really wonder what's the overall effect to the shock. Is it the same as adding progressive springs? Progressive dampening?

If you understand this and can answer the above first, then relate that to what that does for handling. Better traction? Better jump handling? Better steering into a corner? Whatever this subtle adjustment is good for.

Keep in mind, I am more interested in the first part - overall effect of rebound on the shock. I say this because of the racers asked, I get inconsistent answers on what rebound does for "their" car. Gets very opinionated with little fact to back it up.
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Old 01-28-2011, 02:06 PM   #2
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Ok, here is my shot at answering your question. For the most part I think your on track with your thoughts. Rebound does act like a dual rate spring during the rebound of the shock. Where is comes into play is when you land off a jump (flat landing, not downhill) and the shocks compress. The rebound setting (1/2 frt & 1/4 rear are how mine are) along with damping stop the buggy from bouncing back and hopping. It also comes into play on bumpy sections of the track allowing the buggy to skate over the bumps instead of dropping into them.

Below is some info I had on the cause and effect of springs and damping for buggies. I hope it helps...


Springs

Stiffer
Stiffer springs make the car feel more responsive, more direct.
They also help the car jump a little better and higher. Stiff springs are suited for high-traction tracks, which aren't too bumpy.

Softer
Softer springs are better for (mildly) bumpy tracks.
They can also make the car feel as if it has a little more traction in low-grip conditions.

Stiffer Front
The car has less front traction, and less steering. It's harder to get the car to turn, the turn radius is bigger and the car has a lot less steering exiting corners. The car will jump better, and maybe a little further. On very high-grip tracks, it's usually beneficial to stiffen the front, even more than the
rear. It just makes the car easier to drive, and faster.

Softer Front
The car has more steering, especially in the middle part and the exit of the corner. Front springs that are too soft can make the car hook and spin, and they can also make it react sluggishly.

Stiffer Rear
The car has more steering, in the middle and exit of the turn. This is especially apparent in long, high-speed corners. But rear traction is reduced.

Softer Rear
The car has generally more rear traction, in turns as well as through bumps and while accellerating.


Damping
Heavier
Thicker oil (heavier damping) makes the car more stable, and makes it handle more smoothly. It also makes the car jump and land better. If damping is too heavy, traction could be lost in bumpy sections.

Softer
Soft damping (and springing) is better for shallow, ripply bumps.
It also makes the car react quicker. Damping should always be adapted to the spring ratio; the suspension should never feel too 'springy' or too slow.

Heavier Front
The turn radius is wider, but smoother. The car doesn't 'hook' suddenly.
The car is easier to drive, and high-speed steering feels very nice.

Softer Front
The steering reacts quicker. More and better low-speed steering.

Heavier Rear
Steering feels quick and responsive, while the rear stays relatively stable.

Softer Rear
Feels very easy to drive, the car can be 'thrown' into turns. More rear traction while accellerating.

If one end of the car has slightly heavier damping than the other, then that end will feel as if it has the most consistent traction and the most stable when turning in and exiting corners.
A car with slightly heavier rear damping, or slightly lighter front damping will feel very stable turning into corners on bumps or whoops sections. It won't feel 'touchy' at all.
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Old 01-28-2011, 03:19 PM   #3
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Quote:
Originally Posted by vwduud View Post

If the spring is designed to return the shock to full length or to set the ride height of the chassis, is this extra amount of force due to the "rebound" simply adding to the spring rate; helping the spring out a little?

Does this provide fine tuning to achieve "in-between" spring rates? A progressive feel as the shock is compressed?

Just really wonder what's the overall effect to the shock. Is it the same as adding progressive springs? Progressive dampening?

If you understand this and can answer the above first, then relate that to what that does for handling. Better traction? Better jump handling? Better steering into a corner? Whatever this subtle adjustment is good for.

Keep in mind, I am more interested in the first part - overall effect of rebound on the shock. I say this because of the racers asked, I get inconsistent answers on what rebound does for "their" car. Gets very opinionated with little fact to back it up.
I don't want to make this too technical, but rebound is basically another spring. Only the rate it pushes back is different than the regular springs we use in our shocks. So you have a combination of the linear spring and the non-linear rebound (which also acts as a spring) when you run rebound in your shocks. This makes the spring rate progressive. What that does for handling.... well that could take some time to explain and everyone has different opinions on that...

http://www.grc.nasa.gov/WWW/K-12/airplane/boyle.html
1. In general a normal spring applies force linearly. ex: 10 inches of compression is 10x the force when compared to 1 inch of compression.
2. In general, when adding "rebound" the force is applied follows the laws of compressed gas. The force that pushes is not linear and is more exponential. The more the the shock compresses the air behind the bladder is compressed and the pressure builds to push back. That pressure is NOT linear.
3. When people vent their caps and add foam, the laws of compressed gas are no longer valid and the second spring is the spring rate of the foam pushing back behind the bladder. For other reasons, on paper it should be more consistent if you pick a good foam.

I did a little testing and imo a dead shock(0 rebound) with a slightly stiffer spring mounted with a little more angle feels pretty close to a shock with rebound, a lighter spring, mounted with a little less angle. (you have to compensate with shock oil also) You're never gonna get the exact same feel, but they are close. What works on paper doesn't always follow what really happens in the dirt. But on paper you could theoretically get the same shock with 0 rebound to feel like a shock with 100 rebound.
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Old 01-28-2011, 08:01 PM   #4
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I decided to run about only 20% rebound for now on. I use to run 100% all the time, but what was happening to me was that in Mains after lots of jumping the bladders would start to deform slightly causing my car to lose ride height. In turn the handling got poor as the main wore on as well as the jumping causing the car to bottom out. With 20% rebound, as the main wears on, you dont feel the negative effect as much as their isnt much rebound to lose. When it boils down to it, you want your shock to be consistent as possible over a long period of time and I was able to acheive this better with less rebound.
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Old 01-29-2011, 10:03 AM   #5
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Originally Posted by mcion View Post
I decided to run about only 20% rebound for now on. I use to run 100% all the time, but what was happening to me was that in Mains after lots of jumping the bladders would start to deform slightly causing my car to lose ride height. In turn the handling got poor as the main wore on as well as the jumping causing the car to bottom out. With 20% rebound, as the main wears on, you dont feel the negative effect as much as their isnt much rebound to lose. When it boils down to it, you want your shock to be consistent as possible over a long period of time and I was able to acheive this better with less rebound.
hence why you see guys using a foam backer and vented caps. only really needed if you run lots of rebound. I know the setup I use can have like 90% rebound and still have it the end of a heat or even two weekends later. the foam-n-vent is a wasted concept if you always run a dead shock.

My thinking on running rebound is that it allows the shock to recover easier rebound dampening wise when deep into its travel. akin to what the full size off road racers do with bypass circuits. I personally feel the pressure added has an affect on dampening also, than as a merely a spring. thats my suspicious opinion.

Last edited by johnny t; 01-29-2011 at 10:16 AM.
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Old 01-29-2011, 10:26 AM   #6
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hence why you see guys using a foam backer and vented caps. only really needed if you run lots of rebound. I know the setup I use can have like 90% rebound and still have it the end of a heat or even two weekends later. the foam-n-vent is a wasted concept if you always run a dead shock.

My thinking on running rebound is that it allows the shock to recover easier rebound dampening wise when deep into its travel. akin to what the full size off road racers do with bypass circuits. I personally feel the pressure added has an affect on dampening also, than as a merely a spring. thats my suspicious opinion.
I use to make it thru the heat no problem, the main is where the problem was until I started using less rebound. Now so far so good, haven't had that issue occur again.
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Old 01-31-2011, 01:44 PM   #7
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hello guys i got question, when i will run less rebound like 3/4 instead off full rebound does it change the overall shock length and ride height of the car?
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Old 01-31-2011, 01:55 PM   #8
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Originally Posted by ELAD .Z. View Post
hello guys i got question, when i will run less rebound like 3/4 instead off full rebound does it change the overall shock length and ride height of the car?
No. Rebound does not change the shock length or ride hight. It only changes how the shock rebounds after it is compressed.
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Old 01-31-2011, 01:59 PM   #9
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aight thanks dude
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Old 01-31-2011, 02:22 PM   #10
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Default shock rebound

try checking out youtube video from adam drake for shock rebound and all the others great info
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Old 01-31-2011, 03:24 PM   #11
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most in a vid from DRAKE
http://www.youtube.com/watch?v=HNJy7kreK90
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