calling suspension engineers

im currently an engineering student in hopes specializing in automotive suspension.

now this isnt related to real cars, but more.. tiny cars. rc cars. it's been a hobby of mine since i was 7, but im having an issue with one and im completely stumped on why it's doing it

now rc shocks work very very similarly to coilovers.
you have a shock body. a shock shaft. a piston. shock oil. a couple o-rings as seals. caps that go over them to keep everything shut. and a spring that goes over it all.






now, the car im working with has a cantilever style suspension







so the suspension connects to the lower arm.

the camber is adjusted using pillow balls(not sure if there is an automotive term for this)




all this is moved by the suspension.

the issue im having is that the truck doesnt have enough rebound to push itself back up.

im using 80wt(about 1000cst) silicone oil inside the shocks.
all the bal joins are loose and move freely.

im using the stiffest available springs for the shocks.
there is no binding within the shocks either. all the bearings are moving freely and have been oiled

my question is, what could be causing this? ive rebuilt the shocks 3 times with different oils(25wt.. around 300cst in thickness and 45wt which is around 550-600cst in thickness)

next step up would be moving the lightweight differential oil at 3000cst.

when i push it down slowly in the video, it should rebound back up to the set height.. but it doesnt...
it only rebounds back when dropped from at least 1ft
others dont have this issue, so im trying to figure out why it's doing this.. and i cant.



any help is greatly appreciated :thumbsup:

I agree.

What I have found is that by running a revo style suspension setup even hard springs get softer. I did a revo setup on my D8 and while I found the steering to be more responsive the suspension was really soft. My only suggestion is try running long travel rocker arms.

Also try rebuilding the rockers and check for any kind of bearing seizing in the rockers. Another reason could be the oil viscosity in the shocks is to thick for the springs. Take the oil out and see if the arms rebound if it does run a thinner oil.

exactly my toughts..

littleazn248 said:

What I have found is that by running a revo style suspension setup even hard springs get softer. I did a revo setup on my D8 and while I found the steering to be more responsive the suspension was really soft. My only suggestion is try running long travel rocker arms.

Also try rebuilding the rockers and check for any kind of bearing seizing in the rockers. Another reason could be the oil viscosity in the shocks is to thick for the springs. Take the oil out and see if the arms rebound if it does run a thinner oil.

Click to expand...

rockers are brand new and are the longer mini summit ones. bearings inside are brand new.
people are running 5k diff oil inside these shocks with softer springs lol.

it was worse with thinner oil. with the stock 30wt, it felt as if there is no oil in the shocks... even now with the 80wt, it feels as if the rear shocks have no oil in them.
the mini revo display model at work doesnt do it... but that one feels like it has no shock oil inside, just bouncing around.

mycrors7 said:

rockers are brand new and are the longer mini summit ones. bearings inside are brand new.
people are running 5k diff oil inside these shocks with softer springs lol.

it was worse with thinner oil. with the stock 30wt, it felt as if there is no oil in the shocks... even now with the 80wt, it feels as if the rear shocks have no oil in them.
the mini revo display model at work doesnt do it... but that one feels like it has no shock oil inside, just bouncing around.

Click to expand...

Hmm you really got me on this one. Adjust preload on the springs? I'd imagine you've already tried that though. Bent shock shafts?

Needs more pics of the overall setup, but a few initial thoughts.

1) you don't seem to have enough rebound force from the spring to over counter the moment on the arm.

2) check your geometry. Your rocker arm might be binding.

3) Check your FOD. Your system is tightly packed within the spring weight. You have some variables you might not have accounted for outside of the dynamic system.

4) again. Check your geometry.

ive checked binding, everything is loose.
the springs are the stiffest possible springs for the car
ive cleaned out everything a couple times, so im pretty sure it's not any type of dirt or debris

this is the only picture i had on hand. ill try to get more detail pictures of it


Untitled by mycrors7, on Flickr

i managed to fix a lot of the issue after 2 more shock rebuilds. didnt use different oils or springs. just cleaned o-rings and seals and put it all back. but it's still there and is noticeable.

Have you tried adding some preload to the springs or using longer springs?

You should be setting ride height with preload, not with spring stiffness.

i have set preload. the collar is adjustable like a coilover.
i have the ride height i want. it's just getting it to rebound back without a 1ft jump..

What's the spring rate? Too much oil weight for the shock to handle maybe?

Did you replace the top cap on the shocks while the shocks were compressed? This might create some vaccum inside the shock when it is extended, effectively lowering the spring rate.

Edit: From the cut-away shock picture it looks like there's a vent hole in to top cap that vents the air above the diaphram. Is this open, or is there casting flash or somthing blocking it?

Also, does it rebound OK when the car is rolling? Side to side wheel scrub while stationary on the carpet might keep it from fully rebounding.

Vancity2Constantinople said:

What's the spring rate? Too much oil weight for the shock to handle maybe?

Click to expand...

nada. spring rates are something around 2.8lbs per square inch

chad1376 said:

Did you replace the top cap on the shocks while the shocks were compressed? This might create some vaccum inside the shock when it is extended, effectively lowering the spring rate.

Edit: From the cut-away shock picture it looks like there's a vent hole in to top cap that vents the air above the diaphram. Is this open, or is there casting flash or somthing blocking it?

Also, does it rebound OK when the car is rolling? Side to side wheel scrub while stationary on the carpet might keep it from fully rebounding.

Click to expand...

top cap is put on wheen the shock is fully extended.
that hole is for excess oil to spill out on initial compression.
because the shocks lay down sideways, it's better to overfill them and have the rest be forced out through a vent. this way, you drastically reduce any air stuck inside the shocks.

it isnt plugged up, and isnt meant to be because of the scale of the vehicle.

it rebounds the same when the car is rolling.
i took into consideration the friction caused my camber and toe changes during compression.

other members on an rc forum helped me figure it out

The lower cap on these shocks have a 3 seal because they are too large.
if the cap is too tight, it causes binding within the shock itself.
loosened the caps 1/8th turn and it's working a lot better

right now, any droop caused is normal, and it rebounds back a little over 3/4 of the way, which is also normal(friction caused my tire movement and unloaded weight)

when driving around, it feels normal as well. :thumbsup: