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Old 12-30-2017, 09:36 PM   #1
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Default increasing shock pack ,what to expect?

I have only one complaint with how the car is handling, not enough pack and scrubbing the ground on large jumps. Should I just expect this as normal? Raise shock oil wt by 5 points and test. Or go to the next smaller hole in pistons? Increasing shock oil or smaller pistons would reduce over all weight transfer correct?

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Old 12-30-2017, 10:01 PM   #2
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If its turf or carpet its gonna make things worse. You want less pack in the rear, 2 x 1.8 or 2 x 1.9
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Old 12-30-2017, 10:08 PM   #3
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If you go from a 2 hole piston to a 3-hole with the same total area of holes added together, and stay with the same oil...
Low speed damping (small bumps, chassis roll) will be pretty close to what you have now. But high speed damping will increase over large jumps due to the increase in pack. This is because the smaller holes cause more turbulent flow as piston speed increases.
If you stay with the same number of holes, but reduce hole size or go heavier on oil, yes, there will be some change in weight transfer.
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Old 12-30-2017, 10:26 PM   #4
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Just remember that there is some math involved if changing the number of holes. Find the AREA of each hole and multiply by the number of holes. If you're pretty happy with the handling other than chassis slap on big jumps, shoot for close to the same area in a 3-hole piston so you don't upset the general handling characteristics of the truck. Smiley from VRP pistons is a regular here. He may be able to help you also. This is for your SC5M on fairly low bite, right?
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Old 12-30-2017, 10:38 PM   #5
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yes that would be correct , sc5m low traction
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Old 12-30-2017, 10:49 PM   #6
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What are you currently running? We ran 2 hole 1.7 with 32.5 in the rear all season and it seemed to work. But we put 3 hole, 1.3 with 32.5 in the new Stock truck for comparison at the Midwest next week. We don't know if Curt will slip a big jump in there for the big race, so we figured we'd cover the bases, then change if necessary
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Old 12-30-2017, 11:09 PM   #7
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I was running the two 2X1.7 but was still bouncing off track in the small bumps so I went to 4x1.3 all the way around with 40wt in front and 30wt in rear

I think I will try the 3 hole, 1.3
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Old 12-30-2017, 11:25 PM   #8
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Ya, 4x1.3 shouldn't give enough low speed damping with those oils, and probably not enough pack off a jump either. Since most of the weight is at the rear, pack in front is not as critical. We've been running 37.5, 2x1.6 up front. Outdoors, we run AE gray truck springs in front and Kyosho gold truck springs in the rear. Not much bounce, and the faster rebound of the VRP pistons seems to let the truck settle faster after a jump, allowing throttle input sooner.
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Old 12-30-2017, 11:33 PM   #9
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Are you running a brass C block or brass weight under that shorty pack?
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Old 12-31-2017, 12:26 AM   #10
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I am running a brass c block , ordered d block , with a square pack. I tried a shorty with the weight and it was totally off . Runs great with the "brick" with the current set up.
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Old 12-31-2017, 10:32 AM   #11
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Quote:
Originally Posted by a1 View Post
I am running a brass c block , ordered d block , with a square pack. I tried a shorty with the weight and it was totally off . Runs great with the "brick" with the current set up.
You're probably sitting about 50-100 grams heavier than us, but with nearly equal F-R weight bias. The shock packages I mentioned should be pretty close to ideal. We came up with our own by trial and error, but I'd still shoot Smiley a message on the Vision Racing Products thread. They make great stuff, right here in the USA, and he seems very helpful to the racers who have questions.
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Old 12-31-2017, 12:23 PM   #12
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If I increased pack that would reduce roll if just going to next smaller size piston. Could i combat that by just lowering roll center all the way around?
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Old 12-31-2017, 12:51 PM   #13
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Quote:
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If I increased pack that would reduce roll if just going to next smaller size piston. Could i combat that by just lowering roll center all the way around?
Pack is a component of high speed damping. So it would have little effect on chassis roll except for situations like a jump just before a corner. If you want to reduce roll, you raise the roll center (typically by removing inner ball stud washers) which will usually make the truck corner flatter. But having too little chassis roll, especially in the rear, can make the rear end wash out on a loose track. So it's a balancing act that must be experimented with at each track you race at.
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Old 12-31-2017, 07:28 PM   #14
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another question on increasing shock pack. Is there a problem with just raising the oil weight instead of replacing the piston to a smaller hole? If I using 40 weight in front and 30 weight in the rear. Should I just increase the oil weight if looking for more pack?
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Old 12-31-2017, 11:00 PM   #15
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Just raising the oil weight will also affect the low speed damping. So it sounds like you want to increase your "pack ratio" which is the amount of pack as compared to your low speed damping. What I have found with my dyno and impact tester is that this ratio increases (more pack given the same low speed damping) with fewer holes in the piston. In other words, two pistons with the same "hole area", one piston with 2 holes, and one piston with 6 holes, the 2 hole piston will have more pack than the 6H piston if they have the same low speed damping properties. This finding is very much not intuitive or in agreement with general public opinion... generally it's believed hole size and total hole area determines the damping.

One piston we developed was the multi-hole piston that was specifically designed to increase the pack ratio. The piston now seems to be making it's way into the market recently. The piston has 2 or 3 holes of larger size, along with 2 or 3, or 6 more holes that are less than 1mm. These smaller holes will allow low speed oil to pass through, but at any kind of higher speed, they pack up quickly, and essentially become a regular 2 or 3 hole piston. Give that a try if you wish... take one of your regular pistons you are using now, and simply add a couple 0.7mm holes to them. Then, you should be able to add a little bit of oil weight to get the pack you desire, and your low speed damping should be what it currently is with your current oil.
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