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Suspension Basics – Compression & Rebound


hey everybody my name is Justin welcome back to another episode of Garage Talk today we’re gonna chat a little bit
about suspension basics let’s go ahead and get started all right so what are we
gonna talk about today now you guys have heard me talk about various suspension
technologies and features and things over the years you’ve seen me install my
race engineering tarmac to suspension on both my FRS and the BRZ back here we now
have even more fancy MCS suspension with the remote reservoirs and both of these
systems were both compression and rebound adjustable meaning you can
adjust the compression damping as well as the rebound damping now for today’s
video I thought we would get real basic and just talk about exactly what is
compression and what is rebound when we talk about a shock absorber so we’ve got
one of my little RC trucks here to do a mini demonstration and kind of
demonstrate what compression looks like in a shock absorber and what rebound
looks like in a shock absorber now we all know or hopefully we all know if you
don’t you’re gonna find out in a second year shock absorbers are installed on
cars in the chassis to absorb shock and dampen shock from bumps in the road or
imperfections in the road when it comes to race cars they become a huge tuning
part of the chassis so we’ll pop the body off of the truck here and I’ll kind
of demonstrate first what compression is and then we’ll demonstrate what rebound
is and then we’ll get a little bit more in detail and again like this is going
to be kind of a high-level video I’m not going to get too in-depth we’ll start
with the basics and then go on from there in the future so we have our RC
truck here it has shock absorbers front and rear with Springs over them just
like a coil over shock on our car this is actually my stock rear shock from the
BRZ obviously it is disassembled and we’ll use that for another demonstration
in a moment here but compression damping and rebound damping so the compression
stroke let’s start off with what is compression now picture your car or your
truck or whatever it is down the road like this one is here when
you hit a bump in the road your tire compresses or the suspension I should
say compresses the shock absorber on the front here as the tire hits the bump it
goes up in the air and we’ll see if we can get a better view here so basically
the action of that shock absorber going up or compressing or basically
compressing the shock together that is the action of compression now inversely
the action of rebound is when the shock is compressed and then it goes down back
into its normal position that is the rebound action of the shock so
compression is a shock absorber compressing upward the rebound action is
the shock re extending downward now we can take another look at this with the
shock absorber off of the truck and demonstrate again the compression stroke
so that is when the shock compresses into itself or the shock shaft
compresses into the shock body the rebound stroke is again the shock shaft
extending out of the shock body all right so hopefully now you understand
the basics of the compression stroke and the rebound stroke when it comes to a
shock absorber just like this RC car shock absorber the stock shock absorber
from your BRZ FRS or 86 or many other cars for that matter behaves exactly the
same now here’s where it gets a little bit more fun when we talk about
compression damping and rebound damping when we have a race dampers like we have
on the car now there is actually levels of dampening force that we can dial into
the shock in other words we can stiffen it or we can soften it or increase or
decrease the damping rate now the increase and decrease in the damping
rate makes the shock itself behave differently now the reason we have the
spring off of the shocks is to demonstrate what damping does for the
compression stroke as well as the rebound stroke because that’s where it
can get a little bit confusing so first up we’ll talk about compression damping
you remember the compression stroke is when the shock shaft gets pushed in to
the shock body disregard Springs that we on the car for now we’re just looking at
the shock itself and its ability to dampen or stiffen or soften the rate at
which that shock shaft goes into the shock body otherwise or truly known as
damping so it will demonstrate what the damping process is on the shock I can
actually take this thing push it into the table and now I have a feeling the
damping force of the compression stroke so how hard that shock is pushing it
back at me when I’m pushing it in to the table now let’s assume that this is an
adjustable-rate shock like my mcs over those tarmac twos I can change the
damping force of that compression stroke so if the damping is set to full soft I
can basically slam this thing to the ground there’s no damping action
happening to the shock absorber when it is set in a full soft state because the
stock shock absorber has a set damping rate and a set rebound rate I can’t
really demonstrate that if I had one of the shocks off the car I could actually
loosen it up to full soft compression damping and I could just push this thing
into the table with virtually no effort at all on the contrary if I dialed in
that compression damping all the way to full stiff or maximum damping for the
compression stroke I would have to really really compress this thing into
the table to try to get it even to move I don’t even know if I could to be
honest with you that’s what you get from increased compression damping now if you
think about that for a moment if you’re driving down the road and you hit a bump
and let’s say for example your compression damping is set to really
really stiff what’s gonna happen well the shock when you get that bump it’s
not going to compress the shock is going to get the absorbed a little bit in the
shock body but most of that force is going to go into the car and give you a
jolt or a jarring feeling basically you’re gonna feel it in the seat of your
pants in the chassis of the car the steering wheel etc now if we go down to
say mid-range compression or halfway between full soft and halfway between
full stiff when we hit that bump the damper is then gonna be able to absorb
that shock in the shock basically taking away some of that hit in the chassis and
the seat of your pants softening the bloat
so to say now if we went to the fully soft on the compression damping side
what do you think is going to happen well there’s going to be very little
damping in the shock absorber itself so like I demonstrated earlier this thing
is just gonna slam in and you’re kind of gonna have a similar situation as to
when the shock absorber is in full stiff mode except what’s gonna happen is this
thing is gonna slam up into the shock body it’s gonna bottom out and then
you’re gonna feel that shock again it come through the chassis come through
the seat of your pants and through the steering wheel so that’s the compression
stroke in a nutshell we won’t go into detail on various compression damping
settings what I do want to show you now is the rebound side of things now with
rebound is very similar to compression damping although it gets a little bit
confusing when we talk about rebound damping rates or rebound damping
stiffness this even confused me for the longest time and a little bit tricky in
your brain to figure out but we’ll try and demonstrate it again here and show
you what it looks like so if we have our shock like this we go
through the compression stroke the compression stroke is done so now we’re
going into the rebound stroke I’m gonna take my hand off this and you’ll see the
shock shaft actually slide out on its own at a certain rate of speed so here
we go all right so that’s the rebound stroke
and the rebound rates or the rebound damping rate is determined by how fast
that shock comes out or how much damping you’re giving it now here’s where it
gets a little bit tricky if you have an adjustable suspension setup again like
with the MCs or the tarmac twos that I’ve had if you go to full soft full
soft rebound damping or no or very little readout damping what’s gonna
happen is this shock chef is gonna shoot out real fast so in other words we’re
not actually dampening the action of the shock coming out of the shock body so
hopefully by now if you can guess what would happen if we go to full stiff or
maximum rebound damping if we crank up the dial full stiff rebound damping
what’s good happen when we go through our
compression stroke and then we let it out this is going to come out super
super slow because we have maximum damping force on that rebound stroke now
it’s a little bit easier to demonstrate with the baby shock absorber here
hopefully you guys can see that but when I push it in it’s actually not even
coming out right now so this is an extreme this wouldn’t happen in a real
car but if we had maximum rebound damping capabilities this shock shock is
gonna just creep out here really really really slow right and again versus very
little rebound compression damping I’m sorry very little rebound to taffy see
it’s confusing me even just doing this video so again if we have full soft
rebound very little rebound damping when that shot goes to extend boom it’s just
gonna come right out alright everybody hopefully that made
sense quick little recap the compression stroke is when the shock shaft goes into
the shock body compression damping is how much resistance there is pushing
back when I’m pushing that shock shaft into the shock body rebound is the
action of the shock shaft coming out of the shock body again and rebound damping
is how much force were applying to that shock shaft as it’s coming out of the
shock body more damping we add to that rebound action the slower that shock
shaft is going to come out of the body and again we have an adjustable
suspension you can get into all kinds of crazy different tuning scenarios and
being able to change the behavior of the car right how much compression dampening
you’re adding to the shocks how much rebound damping you’re adding to the
shocks and so forth all right everybody let’s wrap this one up suspension
compression and rebound basics coming at you from the garage hopefully the video
was helpful and you learned a little bit of suspension information today that you
can apply to my future videos as you remember I talked about suspension and
compression and rebound sway bars all kinds of stuff all the time in a lot of
my race videos so if this helps you understand what I’m talking about a
little bit more and as always don’t forget to leave your comments down
hello I would love to hear from you if you have any thoughts questions or
concerns don’t forget to stay fast out there everybody I am checking out talk
to you all next time

Glenn Chapman

23 Comments

  1. Great video! This reminds me of engineering classes I've taken, and how damping rates increase the faster the compression occurs. This is really cooking my brain now, I'm trying to figure out if that matters to autocross where generally the lots are flat (except for PPIR), or if the rate of steering input would affect things, or if I'm just overthinking things…

  2. Thanks for the video! Really great to have some basics as a lot of us are just getting into more advanced auto tuning, or could simply benefit from a nice refresher course. This was especially relevant to me as I took an into to HPDE events at the Pittsburgh International Raceway this past Saturday, and while we only got to do a couple of parade laps of the course itself (the rest was classroom), I can still see how much benefit you could get from having more advanced suspension settings available to you.

    Makes me wonder what the difference is between the stock BRZ shocks and the SACHS dampers that are in the Performance Pack.

  3. Can you make a part II on how stiffening or softening the damping affects the behavior of the car? pros and cons of a stiff setup vs soft setup.

  4. Just wanted to tell you that your videos are awesome. Keep doing what you are doing and hopefully the views/subscribers start coming in!

  5. could you explain why with a same damper, different spring rates makes the suspension change. Thanks

  6. This is an awesome video, I'm in the process of building my BRZ (just got it a month ago) and I'm learning about what works why, good stuff and I'm going to watch a lot of your old videos!

  7. Hey Justin! We spoke a while back about my interest in the BRZ and I finally bought one! Your videos make even more sense now that I've my own to reference, so kudos for the explanations once again.

    Regarding suspension, would there be any way to lower ride height without jeopardizing ride quality ( for DD purposes)? I feel that if you tried to adjust dampening to prevent bottom out the car, the consequences would be harsh.

  8. Hey Justin, I'm overall content with the stock suspension. Nevertheless I'd want to know it'd make sense to only change the parts that belong to the unsprung mass. If there are some strong light ones that you can recommend. Thanks a lot!

  9. The shock absorber is not a spring, it only pushes with an opposing force when there is velocity on it. No matter where on the stroke you are, being near bottoming out or with rod extended all the way, the damping force is only a result of how fast the rod is being moved.

    That’s how I think of shock absorbers. And rebound is the force when dragging the rod out of the shock body (extending rod) and compression when pulling the rod into the shock body (retract rod).

    The velocity at which the shock rod will extend out by itself (under no loads) after being pushed in really does not mean anything I think? Key is that shocks reacts to velocity and creates apposing force when rod moves, no matter of position of rod. While springs creates opposing force due to position.

    My 2 cents. That how I was told in school. Shocks (velocity) and springs (position).

  10. Hello there, great video. I have a question, I have seen euro setup guys going with softer spring rates and stiffer suspension set up, so how does Spring rates affect suspension along with shocks compression and rebound setting?
    I have a 2013 Mustang which I am setting up for track racing now, what should be my focus points/ concerns for dialing in the suspension? My goal is to have a road drivable car which would perform well on track as well. I know it's counter-intuitive, but this is what I'm working with…please help me out. Thanks.

  11. My motorcycle has compression on front left fork and rebound on right. Both 14 settings. Should they be put to the same settings ?

  12. Sorry, the rebound was what I needed to understand and it didn't help me understand what kind of ride I'll have. Thanks anyway.

  13. I purchased some and I can compress them easy same with the ones I took off. The both rebound at the same speed which I can hold it from doing so with little to no force. I am no weakling by far however it feels like no pressure at all. As far as compressing it I'd say it's about as much force as maybe 40lbs or so which doesn't make sense to me seeing how the springs take so much weight. Well the front anyway the rear springs may be bad or just lighter I can compress them without tool which has me concerned although it is on a little car. Although the shocks on the rear of my jeep feel the same way. I don't not understand how such weak crappie little things can help with suspension. I'd think they'd at least need to support some of the weight to be able to help with suspension. I mean seriously I can hold the bottom of strut and shocks and move them up and down easily are they garbage?

  14. So how will i adjust my shock, soft or hard? Because when driving and there is an uneven road my tires rub against the fender liner at the back.

  15. Two individual valves with witch you can control each. Compression flow and rebound flow.

  16. really it's a confused subject, but you explained it very well. My question, for desert offroading what is more important (or you can say what should be a little faster) compression or rebound ?? cause I'm thinking of rebuild and revalve my 2.5 icon shocks.

  17. On some coilovers, you find fast speed and slow speed rebound and compression settings. What are they ?

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