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Old 10-23-2009, 11:34 AM   #17716
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Originally Posted by Korey Harbke View Post
This is something from my point of view... Not every will agree with me on this, but this is just what I have found on my car in how it effects handling.

Typically less rebound will give more overall grip, but the car reacts a lot slower. Its good for when it's really hot outside, or on very long sweeping tracks. It doesn't seem to work quite as well in bumpy tracks because the shock won't extend as quick after compression.

More rebound tends to make the car a little more reactive, but maybe a little bit less grip overall. I find it good when it's colder, because the tires seem to heat up more. I use more rebound on technical track layouts, or when the track is bumpy. The quick extension after the bumps keeps the tires on contact with the ground... I think. It's just speculation at this point because it seems the car is more stable in bumpy sections of tracks.

Hope that helps some. You might have to try it for yourself and see what you think. I know my opinion on how rebound works is different than some others. It's worked for me though

-Korey
Korey, I would ask Pete Tozser about this. The track that we race on has lots of bumps.

This is my opinion in regards to your theory about rebound.. I think of it this way. If you ride a bicycle that are using springs as the only form of suspension. When you go over a bump at a fast speed, what will happen. 1 - will you bounce once and return to the previous position or 2 - will you continue to bounce several times after passing the bump. The answer is affected by the type of spring you are using. If you are using soft springs, you are likely going to multiple times. To control the rate of bounce, you use damping or what we call "rebound".

Basically, to get optimal grip, you want to control the amount of bounce or suspension movement. Rebound will help extend the shock back to its normal position, but it is better to choose a spring for this. Rebound is the speed at which it happens.

Temperature plays a role with rebound. If it is hotter, use thicker oil. Colder use thinner.

Pete, if you read this please let me know your thoughts..
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Old 10-23-2009, 12:04 PM   #17717
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I'm not sure how much rebound has an effect on the car. The up and down movement of the shock is controlled by the spring. The speed of the movement is controlled by the Oil and pistons.

I know the rebound has some effect but it really can't be that much. The spring has a greater effect on the rebound than the oil pressure.

I guess that if you are in between spring rates then you can use rebound as an adjustment.

I'm no expert so I might be way off. I do think this is a great thing to discuss. I would also like to know how you can get 0 rebound.
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Old 10-23-2009, 01:01 PM   #17718
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I guess that if you are in between spring rates then you can use rebound as an adjustment.
On motorcycle front forks we use oil level as an adjustment when the spring is good but there is that one spot where the fork bottoms. raising the oil level would be the same as increasing rebound.

On a bike it has no other effect to damping but creates a rising rate air spring as the fork compresses. We set oil level without the springs installed, I have never thought of measuring rebound but im sure it would work the same.

I can see where rebound would make very fine adjustments in spring rate and work the tires more like Korey said.

Last edited by balistic; 10-23-2009 at 01:43 PM.
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Old 10-23-2009, 01:30 PM   #17719
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I've always found rebound to be a pretty fine adjustment. I usually always run full or no rebound at all... because I can barely tell the difference. After running into some situations where adjusting it has helped, what I explained before are the situations on where the adjustments were successful. Hiro is busy racing this weekend, but maybe he can pop on here and say what he thinks and Peter as well.

-Korey
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Old 10-23-2009, 02:06 PM   #17720
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Awsome. I'm glad that I'm not way off with my thought.
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Old 10-23-2009, 02:51 PM   #17721
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Changing rebound affects the damping on the up-stroke vs. the down-stroke. With 0 rebound, the static pressures above and below the piston are equal, so you get equal damping... ie: 40W up and down.

With more rebound there is more pressure above the piston (which is what pushes the shaft back out). This extra pressure increases the damping on the up-stroke and reduces damping on the down-stroke. It is a subtle adjustment but it is noticable in how the shock feels on the bench and how it affects the car on the track. If I had to estimate the effect, with 40W oil and 100% rebound, I'd put it somewhere around a 1/2 weight difference... around 42W on the up-stroke and 38W on the down stroke.

For on-track performance, my findings line up with Korey's... with more rebound, the car returns to center a little quicker off corners and in chicanes. On longer more sweeping tracks, I prefer less rebound as it smoothens out the car's transitions.

In the bumps.. well, it depends on what you want the suspension to do. The two schools of thought regarding this are:
1.) Increase traction by keeping the tire on the ground. To accomplish that, you'd want to have your suspension trace every bump more closely. You'd run either more rebound and/or less "pack" to bring the tire back down quicker. The issue with this theory is that your wheels will be riding lower in the bumps, so at each bump the wheel will take a harder hit, which reduces stability.

2.) Increase stability by skipping over the bumps. In this scenario, you're going with either less rebound and/or more pack, which allows the wheels to ride the bumps higher, so the car doesn't get upset as much. The trade-off is that you will have less traction since the wheel is in the air more.

If you talk to any off-road guys, they'll go on and on about which theory is correct and why. Personally, I think it depends on where the bumps are, how they affect the car, and what you need the car to do at that point.

Yes, Gordon, our track has a few bumpy sections. Being an accomplished fence sitter, I run 1/2 rebound all around here. It would return to center better in the chicane and the tight sections with full rebound, and it would be nice to run no rebound in the approach to the bumpy 180 for more traction... it skips along the top of that washboard section more than I'd like.
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Old 10-23-2009, 03:51 PM   #17722
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Coming from an Off-road background I completely understand how you explained it. Now I feel like I can tell all my locals, even the novices and have them understand.
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Old 10-23-2009, 04:56 PM   #17723
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Quote:
Originally Posted by Korey Harbke View Post
I've always found rebound to be a pretty fine adjustment. I usually always run full or no rebound at all... because I can barely tell the difference. After running into some situations where adjusting it has helped, what I explained before are the situations on where the adjustments were successful. Hiro is busy racing this weekend, but maybe he can pop on here and say what he thinks and Peter as well.

-Korey
I agree with Korey. I think the rebound to be a very fine adjustment too.
But we really can't say "more rebound" means more or less traction because it just changes how linear the shocks move up and down. I got to give prop to Peter for his great explanation for this part. it is well explained.

With more rebound (i.e 100%), the shock shaft comes back out faster by the pressure above the piston. That means the car comes back a little faster when it rolls. Also, more rebound generates a bit more tire pressure to the ground so sometimes more rebound makes more traction overall. As Korey mentioned earlier, this pressure on the tires also generates more tire heat as well.

In the bumps, I really don't think that the rebound adjustment should be used for this as we basically tune/adjust it by anti/pro-squat in the rear or kick up in the front. In fact, in off-road category they almost never use 100% rebound even if they have to deal with the track that is way more bumpier. May be in some situation, more rebound such as 100% might works but i believe that we can get better result with proper setup on the suspension angle.

Basically, the shock absorber(without spring) should work for damping effect only since the down stroke force will be generated by the spring. Just like Ray (aka: Pound It Racing) said previously, the shock absorber controls the shock moving speed and the up/down stroke movement should be controlled by the shock spring.

However, with using the adjustment of the shock rebound percentage, we can also fine tune the shock movement. But again, this will change how linear the shock moves for both up and down stroke only so i don't think we can say that easy which is more traction or something like that.

I hope it helps a bit...
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Old 10-23-2009, 04:58 PM   #17724
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Team HB is like Voltron... Together we make a good explanation!!!
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Old 10-23-2009, 05:14 PM   #17725
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Korey, were does STLNLST fit in Team HB's Voltron, the BACKSIDE???
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Old 10-23-2009, 07:31 PM   #17726
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Quote:
Originally Posted by rc pete View Post
Changing rebound affects the damping on the up-stroke vs. the down-stroke. With 0 rebound, the static pressures above and below the piston are equal, so you get equal damping... ie: 40W up and down.
The air in the shock that is compressed to compensate for the volume of the shock shaft creates an air spring that will push the shaft out if enough oil is present IE rebound.

The damping of the shock is determined by the piston and oil, it's measure will be the same no matter the spring used. a particular piston and oils damping has a range of spring it will work with, a soft spring needs less damping than a stiff spring.

I disagree that rebound any effect on damping itself and only serves as a progressive air spring in addition to the coil spring, and in that capacity changing the total spring rate, effectively raises the spring rate compared to the damping rate. It's a very minor point and doesn't change any of the observations presented about rebounds effect on the car.
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Old 10-23-2009, 07:58 PM   #17727
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Go team HB. What little part I donated. It's good to know that we can ask a really tricky question and have a few people give the same answer using different examples and word usage. I feel like I went to a physics class today. I love this stuff.
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Old 10-23-2009, 11:48 PM   #17728
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Korey, were does STLNLST fit in Team HB's Voltron, the BACKSIDE???
LMAO not sure man. I think we should ask him though
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Old 10-24-2009, 08:07 AM   #17729
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Originally Posted by Korey Harbke View Post
Team HB is like Voltron... Together we make a good explanation!!!

While on the subject of shocks, does using a vacuum pump make a difference when building the shocks ? I know what it does, I watch a fellow racer at my track use one and he swears by it. I have built shocks and rebuilt them a month later and found no air bubbles inside without using a pump or even letting them sit for for any length of time before putting the cap on.
Thanks.
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Old 10-24-2009, 08:55 AM   #17730
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Enough speculation

What you need is some way to read the suspension movement while the car is going around the track ie shock potentiometers, maybe a IR or sonar setup so it doesnt interfere with the shocks action then hook them up to a data logger to see what is happening between rebound and no rebound.

Also, make a shock dyno for rc shocks to test the difference between rebound and no rebound. I dont know if its possible to get load cells that will measure loads that small, Im sure there is something out there.

Food for thought...
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