I'd equate a shock with some inherent pack vs one that has no pack that is relying on a bump stop to a face hitting an airbag vs one smashing into a steering wheel.

Care to elaborate? From what I've come to know over the years, hydrolock in ANYTHING is a bad thing and very hard on parts (most of the time it's worse than metal on metal) So i'm trying to get a grip on why we build our shocks expecting them to hydrolock. Only motorsport I can find any info where they actually try to make the shock hydrolock is in rock crawling. Everything else uses valving to bleed off pressure so it doesn't hydrolock. In most forms of racing a progressive spring is used or a bump stop on a linear spring. I know 1:1 stuff doesn't translate to scale stuff most of the time, but the principles should still be the same.

And yes I realize "pack" occurs anywhere during the shocks stroke, which seems like an even worse idea to me.

Now that I'm thinking about it...I think we do use a "bump stop"....it's the chassis. I've noticed a lot of cars by design, have the chassis bottoming out before the shocks do. (maybe thats where the face into the steering wheel analogy comes from...)

Pack isn't just a hydrolock or no hydrolock situation. It is progressive. It varies and will taper into that condition. The key is to do it controllably. A bump stop is not progressive. It either is or is not. All of that energy is released into the chassis at one moment and this could lead to rebound shock. I know a hydrolock situation is bad on a real vehicle. We have the advantage why our small, light rc cars to utilize this phenomenon and control it to do what we want. Forces are relatively small and shocks are completely over built for the job.

I would also add that in real cars, bump stops are used to gain a bit of comfort, as well as to keep some very expensive suspension/frame parts from slamming together.

If you've ever been in a lowered vehicle that has had the bump stops removed you will know how terrible it feels when you go over a change in the road that is sufficient enough to fully compress the suspension. It is not pleasant.

Rock crawlers will also use bump stops/travel limiters/axle straps and hydrolock to limit articulation.

Bump stops are progressive (at least the ones on all my race cars were) tapered diameters, progressively thicker material, accordion like designs (collapsible) etc etc. If they didn't have some sort of progressive nature to them...why even use them?!

I just dislike "pack" in the way its used in our cars because it is completely un-controllable. you can set it any which way you see fit, but depending on a multitude of variables (o-ring health, shaft health, shock body health, contamination, air, etc etc) the "pack" you built your shocks with, can/will vary from the moment your car hits the dirt.

Not starting an argument here, just trying to think outside the box....I dislike not being able to quantify an aspect of a race vehicle, so I'm trying to stimulate the who/what/why/why/why of shock tuning. Because shock tuning directly effects every other aspect of the car...which we document, quantify, and correlate to other aspects of the car.....but shocks, we just experiment until it "feels" good. seems counter-intuitive to me.

I still play by the rules that are in motion...but also looking thinking about alternatives.

Question everything!

Fred thank you for another gem.
When I was testing setups on the 1/8th, there was a 2" bump I could hit full throttle on the straight. On some occasions the rear end would kick and the buggy would do a front flip. Screws loosening themselves cut my after season testing session short and now the track is underwater on some places.
So the question for today is:
How can I achieve less pack in the rear?I'm on 1.4 x 6 pistons and 35wt.
-Thiner oil?
-Larger piston holes?
-More piston holes?

I've started doing the drop test even after oil is installed. My final setups are fully balanced now and not just the springs. The front and rear both move at the same frequency. Since this is easy to see, especially once it is slowed down by oil, I will choose a piston and then adjust oil weight until I get balance. One particularly extreme setup I tried in my 210 used 6 X 1.2 pistons up front with 25W oil but custom drilled 6 X 1.7 pistons at the rear. It took 60W oil to balance it out as a result. The rear had too little pack though. The next step to get a little pack back was to try a 6 X 1.6 piston and then go down in oil weight until it balanced again. You get the idea.

I personally run equal holes all around and vary only the size of the holes. The reason being that if I have a 6 hole piston and a 4 hole piston and they have the exact same hole area, they don't have the same pack characteristics since the total edge area of the holes is different. I want the same relative characteristics front and rear and keeping the same number of holes but only varying their size and oil weight to compensate keeps things simple and straight forwards.

Update on my change from 8x1.3 pistons to 8x1.3 & 8x1.4 pistons.

With all 1.3's in and 37.5 & 32.5 oils the rear was noticeably slower with a bench drop, even with 30 in the rear.
Changed the rear to 1.4 it was about as equal as the naked eye can detect.
I want to try 35 rear oil next but I was happy with how it calmed down the rear end on small obstacles and with general rear traction in a turn.
I have gone from 35 to 30 in the rear with the 1.3's and was never satisfied.
More time at the track next time will allow me to mess around with oil weights.

Thanks for your input. I'm going to start by drilling the rears to 1.5mm or so

Hi Everyone,

what an awesome read!!!

I have a bit of a question relating to shock absorbers.

Rebound.

What does it affect on the car/driving wise, should it be changed for different track conditions, should it be used at all???

any help in clearing up some confusion i have about it would be great

Luke

Fred, do you have any experience with the AE tapered pistons? Specifically the 3x1.4? I've been running them all around but the rear end has always been a little off. Basically, the garden hose effect that was mentioned earlier in the thread.

Pack is controllable even with standard shocks. Plenty of alternative systems have come and gone over the years but right now for example you have MIP which use a valve shim stack just as real motorcycle and automobile shocks have. The variables you mentioned are beyond tuning and more like simple maintenance. The biggest effect all that would have together is simply being worn and sloppy it has less stiction and will allow a slightly faster shock speed.
I have an old book here somewhere that goes into "pack" quite heavily. It doesn't use that term because it is about the oil only and not about the shock. So maybe think outside the shock box and more about the oil box. btw pack is not hydroloc, far from it.
Hey your heat is coming up on the cam gotta go.

You mean building rebound into the shock so the shaft extends on its own after being compressed (sans spring of course) right?

From my experience, you want little to no rebound on tracks that don't have 40' triples or quad jumps and is not rutted up (indoor) as the car will stick to the ground a little better and feel more settled.

But if you have large landings to deal with a little rebound in the shock will give you better bottom out resistance and the wheels will extend back down to help absorb the next rut (or crater on a blown out track) in the track (outdoor).

Building with rebound leaves more oil volume in the shock so it is naturally trying to push the piston down compared to no rebound where you push the shaft in and it stays in (no spring)... since you screwed the shock cap down with the shaft all the way in purging more fluid than building with rebound holding the shaft some amount out of the body leaving more fluid in than a no rebound build.

Maybe Fred or others have something to add to that or even a correction but that's what I've got for ya

I only run bladder shocks. I don't believe what anyone says about emulsion. I don't want it.

But that didn't really answer the question about rebound... unless you did not intend to. Not sure where emulsion came in.

On emulsion, I build shocks both ways depending on what I'm driving on and the kit. Emulsion build has been around forever, there's got to be some advantage to it on some surface on some kit or it wouldn't be built that way any more. Just sayin.