speed bag platform "sweetspot"

[kenk]

Hello: I'm a new member...age 60. I had a homemade speedbag platform in my home when I was 20yrs old. It worked OK, but was loud, noisy and vibrated. My parents were not too happy when I would hit the bag, because you could hear it throughout the house. My son (19yrs) has been asking me to get a speed bag platform and set up for several years. Finally decided to get one, but did not want to relive the past, I needed a quiet, vibration free platform. I decided to get a valor C-53 adjustable platform. I installed it in a finished basement. I mounted 2x6 pine horizontally on three studs. The unit is designed to mount to the studs directly. Instead of mounting directly, I inserted four rubber shock mounts (furniture coasters about 3/8" thick) and just tightened the bolts enough to slightly compress the rubber. The result...a surprisingly quiet and vibration free install. I can barely hear this in any other room. I have a valor chain link swivel that was making some noise, so I inserted thin nylon washers to take up any metal to metal slop...it worked very well and cut out all metal to metal noise. The rubber shock mounts seem to be doing a nice job....they seem to isolate the platform movement from the wall...and keep the wall from acting like a "drum".
I also looked into the concept of "center of percussion"...AKA "sweet spot" of the platform. The sweet spot is not the center of gravity, but having the bag hit the sweet spot on the platform should result in the highest rebound speed of the bag, and the least amount of vibration to the platform. If you ever used a baseball or tennis racket, you'll get the best rebound and least shock to your hands if you hit the sweet spot. I looked up the formula for sweet spot and it is a function of the mass distribution. Ideally, I would like to have a platform that has a large sweet spot, to accommodate different size bags. I tested my platform, but tapping lightly with a small hammer in different places. You can hear the change in tone while tapping. It appeared to be most consistent for 2/3 (radius)of the way out from the swivel. The last 1/3 sounded somewhat dead. I believe I can enlarge the sweet spot by adding added weight around the out side edge of the platform, or just make a larger, thicker, heavier platform.
It think you can tune a platform to perform to your bag. I've heard of sandbags, but I think there must be a better method. Any ideas?

[Speedbag]

Interesting concept. If what you say IS true than you would want to use bags of the size where the belly corresponds to the sweet spot of your board.

However, the sweet spot of every bag is the belly, and that's what hits the board.

[kenk]

I think you're right...I think that if you had bags of different sizes, they would not all rebound well if your platform had a small sweet spot. Try the "tap/rap" test on your platform, and if you don't hear a sound difference from the center to the outer edge, then I think your board will rebound well with any bag size. To check this, you can download a vibration app for an everyday "smartphone", since smart phones have three axis accelerometers in them. You lay your phone on top of the platform and hit the bag. If you're getting higher vibrations toward the edge of the platform, I think that means you're off the sweet spot. To get the platform to vibrate, the bag has to impart energy to the platform that could have been used to accelerate the bag...vibrating platform=slow bag speed.
Tennis rackets are now sold with the sweetspot identified for each brand. (see attachment) Over the last 40 years, tennis racket sweetspots have increased from about 1" in diameter to about 4" in diameter, due to design changes, without increasing the racket weight. The result is that you don't have to be as good player to hit a decent shot...
The key is to design the platform to have a large sweetspot. My son is in his junior year of mechanical engineering at UMDCP, and I have suggested that he does a senior engineering project on characterizing the engineering aspects of speed bag platforms. It would involve modeling the platform mathematically. Could be very interesting...

[Zaza]

...I'm going to go out on a limb here and guess...you're an engineer?

Tuned platforms. What will they thing of next!

(I've been toying with the idea of making a platform from elastic cord. I've seen the design in some old advertising. Kind of like a tennis racket. Might be nice as a quite-puncher.)

[Bag-A-Holic]

[QUOTE=kenk;36751] Finally decided to get one, but did not want to relive the past, I needed a quiet, vibration free platform. I decided to get a valor C-53 adjustable platform. I installed it in a finished basement. I mounted 2x6 pine horizontally on three studs. The unit is designed to mount to the studs directly. Instead of mounting directly, I inserted four rubber shock mounts (furniture coasters about 3/8" thick) and just tightened the bolts enough to slightly compress the rubber. The result...a surprisingly quiet and vibration free install. I can barely hear this in any other room.

Can you please elaborate on the rubber coasters. Maybe a brand or link to the ones you used? Did you need to drill holes in them and use washers etc? I'm having a similar issue where I'm in a location with people living above me and need to find a platform that does not vibrate. Recently ordered a high end platform, and it still vibrated alot and had to return it. Would you be able to possible show a video of the platform with the rubber coasters in action? If this works, I may have a platform back on the wall sooner than I thought.

[Speedbag]

...I'm going to go out on a limb here and guess...you're an engineer?

Tuned platforms. What will they thing of next!

(I've been toying with the idea of making a platform from elastic cord. I've seen the design in some old advertising. Kind of like a tennis racket. Might be nice as a quite-puncher.)

Put Piezo triggers inside or attach to the bottom and tune them to drum sounds. Different areas of the board could make different drum sounds.

rough attempts at this find it works very well, for about 90% of the normal rebounds. Unfortunately some of the 3rd or 4th unpunched rebound gets dropped and doesn't fire the trigger, so it changes the expected rebound beat occasionally. But this would be an incredible "drum set" in the hands of a competent Punch Drummer (tm).

[kenk]

rubber shock mounts

Antny72...I will find get a picture/brand of the coaster. I'll also try to take some video that shows the platform slightly vibrating at the coaster...but the wall mounts don't have vibration (based on "feel with hands" test)...
Also, I drilled a hole into the middle of the coaster, to allow the bolt to go thru...connected the bolt to 2x6 boards with a blind nut and machine screw, instead of lag bolts (didn't want the bolts pulling out of the board)...tightened just enough to slightly compress mounts...
I also noticed that a noise fro the swivel was annoying too...I have chain link style and just used thin nylon washers to take up side play...the result...no noise from metal to metal contact...quiet swivel!

Alan...yep, I'm an engineer, mechanical...I've been thinking of how to design a better speed bag platform that is quiet, vibration free, and doesn't weigh a ton!...I work on unmanned DOD helicopters and vibration is always an issue...there are many methods in use to isolate vibration from aviation equipment..."rap" tests are used to find the natural frequency of installed components to ensure that it doesn't coincide with the rotor generated frequency...or else the component can vibrate wildly...

[paranday]

Hi kenk, how about while you're at a silent, low friction, omni-directional swivel that allows you to easily change bags? And is lubricant free. And can be used in cold temperatures.

[Dutchman]

You don't want much, do you?

[Zaza]

...and it can only cost $5.00!

[paranday]

...and it can only cost $5.00!

Might cost less than that. I think the vintage plain rope idea was correct, but the rope tends to twist which messes with rebound. Maybe there's a simple way to let rope spin freely, and some attachment means for the rope or cord that permits a quick change. I'm not an engineer and am stumped. That's why I asked.

Put it this way, as a car racer, would you seek the most viscous fluid to put in your transmission? The heaviest wheels for your tires?

The steel ball hook swivel fulfills the omni-directional and quick change requirements, but it adds some mass and friction between the bag and the drum. Reducing those aspects may be judged desirable. Seems to me there are subtle factors which alter the feel of the bag quite a bit. Where and how the bag pivots is another one of them. And so forth.

[Speedbag]

Might cost less than that. I think the vintage plain rope idea was correct, but the rope tends to twist which messes with rebound. Maybe a slippery Delrin thrust washer to let the rope spin freely, and some attachment means for the rope that permits a quick change. I'm not an engineer and am stumped. That's why I asked.

Put it this way, as a car racer, would you seek the most viscous fluid to put in your transmission? The heaviest wheels for your tires? A cast iron chassis?

The steel ball hook swivel fulfills the omni-directional and quick change requirements, but it adds some mass and friction between the bag and the drum. Reducing those aspects may be judged desirable. Seems to me there are subtle factors which alter the feel of the bag quite a bit. Where and how the bag pivots is another one of them. And so forth.

That's true. The feel and flight of the bag on a light plastic ball made me feel like the bag would "float" and spin a bit more than a steel metal ball. Maybe it was due to weight or less friction sitting in the housing. I'm not sure but my sense is that the bags rebound better with a bit heavier ball inside.

[kenk]

I never really thought about a replacement for a ball swivel. Perhaps a rope made of Kevlar or similar high strength line might be a candidate. The line would to be highly fatigue resistant, considering the constant bending. I don't really understand the pros and cons of the different swivels. I used to have a ball swivel 40 yrs ago, and it was loud and had friction...I hated it. The chain link that I'm using is quiet and virtually friction free, but I see that the pivot point changes when going from front to back, and then side to side until the unit rotates 90degs.

Also, did anyone ever consider installing a plasma lightening plate near their platform? They are sound activated, probably would flash to the beat of the bag...
check out this video in action...

http://www.youtube.com/watch?v=MroEMBDUtIs

[kenk]

I like the idea of a natural wood drum, but would probably still use steel for the support system...carbon fiber products offer superior stiffness, but tend to be brittle, can fail catastrophically, and cost way too much...putting a ring of steel around the drum, much like an old time wagon wheel would help to distribute the weight out from the center, increasing the mass moment of inertia, and increasing the center of percussion to give better rebound and less vibration...good example of mass moment of inertia is a ladder. Pick up the ladder and try to rotate it fast...you'll feel lots of resistance while starting...pick up a bag of concrete of the same weight and try to rotate, and it will be very easy...

[Scootas Priest]

Hi Kenk. I was happy to discover your posts (not to mention this forum). I just bought a house with a basement and have decided to purchase the Valor CA-53 and mount it exactly as you have described. Can you please tell me what size blind nut and (therefore) machine screw you used? Do you feel that is the right size or would you please recommend a certain size?

Thanking you or anyone else who can respond in advance,
Scootas