quit following my lines and tighten up the loose nut behind the wheel

I will try that, thanks

Happy Friday everyone. Wanted to share a bit of setup info from my B4 and maybe get a civilized discussion on pistons, oil and pack going. First, I run on a good sized, gnarly, clay/hard packed black dirt track in my back yard. The jumps aren't huge but I've intentionally left some sections VERY bumpy. I had a heck of a time finding a good setup to handle the fast, hard edged bumps and the mid sized jumps on the track. I ended up ordering a set of RC Shox 2 stage pistons and just stuck them in with 25 weight front and back (no mods to the hole diameters at all) and proceeded to rip off my fastest lap time. No intended sales pitch here. The car just handled pretty dog gone good.

This track usually is what I would consider a low bite track. Its been a huge challenge getting the rear end of the car locked down. I've got what I think is good spring balance at this point (Durango light green in front and Durango black in back). I've tried lots of different tires. The problem has typically been in the rear end. I've moved the roll center up and down and adjusted from the longest camber length to the shortest, mainly to educate myself on how the changes feel as you are driving. What has eventually turned out to feel the best and turn the best lap times was a long camber link and a high roll center. The Kyosho Triumph I have been working on alongside this B4 has basically headed in the same direction with the long camber link and high roll center. Interested in any thoughts on this. I'd flesh out the setup more but I'm eating pizza right now and that's slightly more important to me.

I'd also like some thoughts on what folks think is actually occurring that causes a shock to "pack" up. Is this a function of the speed of the fluid moving through the hole and a sudden transition from laminar to turbulent fluid flow? Is there really a difference between simply changing oil weight vs. changing hole diameter?

i am walking out the door to eat dinner with the wifey so i will try and chime in later. i am glad the pistons are working for you. there are a few things going on with the shocks that you must take into consideration. one is the fact that the "pack" you get from my pistons is partially from the pistons "clapping" together under impact and the actual hole size. its a pretty deep discussion as there are many different theories and i can only account for my own experience and observations. probably need to start a new thread!

http://home.scarlet.be/~be067749/58/ You've probably seen that site already, it's mirrored at a different address, and available for download also. I thought it explained it rather well, scroll down to the Dampening section. To me it seemed like packing was all a matter of the speed the oil travels through the piston holes. If two pistons are moving at the same speed, the one with the smaller holes forces more oil through, basically the oil moves faster, since there is less room for the oil to pass through. This causes more packing, especially with thinner oil, since it can flow faster too. As to changing oil vs pistons, I would think they are very different. That you can't make a lighter oil feel thicker or vice versa. Well you could to a point, but you are also changing the packing characteristics, which will have unintended consequences. Sounds kind of like not trying to increase the spring rate with preload, it does increase the spring rate, but there are unintended side effects of doing it that way.

You are learning what the car wants. The results shouldn't surprise you unless you believe that being different from everyone else means you did it wrong. A 2wd buggy has lots more mass at the rear than the front and is going to require a higher amount of rear roll stiffness in back to control it. No surprise to me. If you were tuning a 4wd you'd get very different results. Nice work on learning what the car wants rather than copying someone else and hoping they know.

I read this entire thread to see if this information was in here but it isn't so I have a few things too add.

First, because the question was just asked again, and it was never explained, I'll start by explaining what shock pack really is. In the real world if you very slowly compress an rc shock there is a small amount of resistance (damping) provided by the oil going through the piston holes. If you compress the shock very fast there is a huge amount of resistance (damping). The thing to note is the relationship between the amount of resistance is not linear. In layman terms if you move the piston through the oil twice as fast there isn't twice the resistance there is much more than twice the resistance.

If you don't care about the mechanics you can skip the next couple paragraphs. So bear with me for a minute here. I have worked in the civil engineering profession for 20+ years. I tried multiple times to calculate what was going on based on the turbulent vs. laminar flow theory on the elvo web site. If you read a fluid dynamics book you can convince yourself the turbulent/laminar thing is legitimate, but bottom line is the numbers don't prove out. The difference between the resistance of turbulent vs. laminar flow in a pipe .04" in diameter 1/16th of an inch long doesn't increase enough to even begin to explain what is going on. In fact the hole is acting like an orifice and not a pipe. I ran across a guy a few years back that worked designing nozzles for rocket motors. I asked him why I couldn't make that calculation work out. After he finished laughing at me for my ignorance he explained what is really going on.

This is how he put it. Fluids don't like going around square corners. It's kind of like a wind break by a picnic table. The air doesn't come to the edge of the wind break, make a 90 degree turn and continue blowing the same speed on the opposite side of the wind break. What the air does is kind of blow around in a little mess behind the wind break. What is going on in the shock is, as the piston speed is increased the oil coming from the sides of the hole can't make the 90 degree turn through the hole so they force a little mess around the outside of the hole in the piston like a wind break effectively reducing the area of the hole allowing free oil flow. The result is a dramatic increase in resistance at high shock piston speeds.

The obvious next questions are, what does that mean on the track, and how do you control it.

What this means on the track is it's possible to have a real soft damped shock that will soak up the little bumps, but still not chassis slap the face of jumps or bottom out and bounce off the chassis on a big landing. You control it with hole size and number of holes. One result of this pack phenomenon is the smaller the initial hole in the piston, the more the resistance increases for a given increase in speed.

The following numbers are bogus and just for explanation. Let's say one piston has one .04" hole and another has one .02" hole. If you double the speed of each piston through the oil lets say the .04" piston triples the resistance, the .02" hole would quadruple the resistance. Because of this if you have a three hole piston with the same total hole area as a 2 hole piston the low speed damping will be the same but the three hole piston will have a lot more pack because of the greater resistance from the smaller holes.

Here's a real world example because it's what I ran a few years ago. A B4 with a #1 (two #54 holes) shock piston and 30 weight oil in the rear works great on fairly rough track; however if you hit a steep jump face at high speed the chassis will slap the face of the jump and kick the rear over, and it'll bottom hard landing from a big jump. If you replace the #1 piston with a Losi #57 piston you can drop the oil weight to 20 wt and have the same or better little bump performance but a dramatic increase of pack to where it won't bottom on jump faces or bigger landings (within reason). IMO the sweet spot for a B4 is a three hole 7-7-6 piston. Thats two #57 holes and one #56 hole.

This will most likely get me beat up, but here it is anyway. I've been racing for 23+ years and I tried the dual stage thing back in the day when RPM came out with them. IMO they're just overly complicated and unnecessary for 1/10th if you understand shock pack. I don't have an opinion on them relative to 1/8th. I only raced 1/8th for a few months so I didn't get around to messing with pistons.

Also as far as rebound goes. First you have to be sure everybody is talking about the same rebound. Sometimes when people say rebound they are talking about how fast the spring on the shock extends the shaft, and other times they're talking about the internal air pressure when they build the shock.

As far as the internal pressure rebound is concerned I build my shocks with no rebound. The reason is not only is it hard to get it equal but after 5 minutes on the track the rebound will be gone anyway so unless your refilling your shocks for each run it isn't doing anything except making the shocks less consistent. IMO the only time the shock would compress far enough for that rebound to have an effect would be bottoming off a jump landing. Much better drivers disagree with me on this but no-one has ever been able to explain why 1/4" of rebound at the top of a shock stroke would change the way my car handles.

Enough for tonight.

Interesting. Go ahead and voice your opinions. If we can all do that without being petty we will learn more. I'm an engineer as well. By nature, I tinker with stuff. Can't help myself. I've been racing on and off for decades as well. My brother had some of the Kyosho Platinum shocks that had 2 stage pistons that were externally adjustable for float with an outer shock shaft that you could turn. I've always thought that two stage pistons made a great deal of sense. The RC Shox worked better in my car than the #1's with 25 weight. And that was without doing anything further with hole diameters.

Is there something to consider with hole shape? Given the right amount of control, you could add different edge breaks (angles or depth) to the hole on top and bottom. In theory, this would change the effective piston hole diameter depending on whether your shock was compressing or rebounding. Anyone with a decent drill press should be able to do this without a problem.

I'm not necessarily suggesting that any of this needs to be tried out or that it would make anyone's car turn quicker laps. But I am into RC as much for tuning and practicing as I am for the racing. Chiseling away at lap times on your home track is almost as satisfying to me (and way cheaper) than actually driving to a real track and racing. That's the great thing about this thread. Its a thread about understanding and learning.

So I have been reading all these posts about camber, roll center and shocks/car balance. I was wondering why do the shocks have to be so similar according to the posts i read the shocks should feel balanced or feel the same front and rear..I dont understand why this will help one out since a lot of the time my front is a tad softer just because i run on a tight track, the rears are a little lighter oil but a smaller piston holes...
front-#1 pistons 30 weight oil and 3 limiters.
rear-#2 pistons 25 weight oil and 2 limiters..
dark yellow kyosho springs all around which are a bit stiffer than stock b4.1 ft setup..

o yea b4.1 factory team is what I am running..

a lot of help full stuff on here thanks!

The idea is that each component has its function and you tune that component to maximize its effectiveness in performing its function. Its easy to get more steering by running a softer spring in the front. But what do you compromise by doing that? That's the idea. Not everyone is going to appreciate this method and some folks will be plenty quick without doing any of this. I think all of us on the thread would agree that practice is probably the most important element of all.

Following through the ideas in the thread gives you steps that you can take to maximize each components effectiveness and minimize the number of compromises you need to accept. Finding balanced springs (with no oil in the shocks) puts that cars chassis at balance. Putting a stake in the ground for springs and damping means that you can focus on making the shocks handle the bumps and jumps on the track. You can then use adjustments to the roll center/camber gain/toe-in, etc to help get you through turns. It crystalizes the process of tuning a chassis. Make more sense now?

It crystalizes the process of tuning a chassis. Make more sense now?[/QUOTE]

alright thanks, that is most likely why most b4.1 setups are very very similar(not the 8mm chassis but still that is similar also.) the rear shocks are always inside on the arm, in on tower, and for the front it is inside shock tower, outside on the arm... now they are trying middle shock inside arm for more steering for the 8mm...

i also have to remember these sponsors race on high traction well groomed track, MABYBE medium bite....no bumps no loose layer and soft surface. something we should remember before we just copy a setup. I only copy a setup if i run on the surface described, on the indoor clay track the caveleiri setup for indoor worked amazing. I guess we ran on a very similar tracks...
but outdoors there is no way i can find a setup for a loose, surface that has high traction to low traction around every turn... ik somebody who i race with who always says stop messing with your car just drive..he is also slow.

when my track turns wet to dry i need stiffer rear springs,why? it pushed more and i need to break loose easier. When is turns to a dark blue groove i lower my roll center(add washer to rear) to make me slide more. But it is based on the track for the most part...on a tight track you dont want a pushing car you want a car that slides around corners. it will saves lots of time..

this is why i like this thread, a lot of helpfull things.

I know several people that have tried tapering and rounding the edges of the holes on one side to reduce the dampening on rebound. Some said it worked others said they didn't feel any difference. In the early 90's someone was making pistons with tapered holes but I can't remember who it was for sure (I think it was either Tamiya or Kyosho). For me it seemed like just another variable added to an already extremely complicated system.

(This is just a educated guess) My personal opinion is that even though the theory is sound. Tapering or rounding one side of the hole wont make a significant difference because the hole is so small relative to the shock body diameter rsultig in a relatively very sharp corner even if rounded perfectly from one side. (As I said just a guess)

I'm not going to say anything else about 2 stage pistons they certainly will work. Some 2 stage piston oil combos will certainly work better than some standard piston oil combinations. I just have enough trouble working the current variables in the shock out without having a dramatically different rebound and compression rate beyond what I already get from the pack. After I put my thoughts in on the spring balancing thing it should be easy to understand why I try to keep it as simple as possible.

I've been reading your posts about setup and have reached a point where I need to ask a question concerning roll center & link adjustment. My SCRT 10 sct has the piviot ball suspension conversion on the front. To adjust RC in the front would changing the inserts that hold the upper and lower A arm hinge pins be the same as adjusting a link length? Could the upper & lower be adjusted seperately or do they need to be adjuted the same amount in the same direction? Or, what would be the effect of changing the pin position using the different inserts?

Raising and lowering will only move the roll center higher or lower. Changing the length changes how far it will 'roll'. If you want one end of the chassis to roll more lengthen the camber link. For less roll shorten it. Unfortunately you cannot do that. Only thing you can change is roll center (at the front end).

Having said that find something you like up front. Then go to the rear of the car to change things. This is what I do with my Mugen MBX5 buggy. It also has a front similar to your truck.

That's a pretty good guess. I have a similar chart to this one on my wall at work for reference.

http://gasho.org/rotron%20pages/orif...alculation.htm