Looking for anyone that owns Babolat RDC machine..

jmnk

Hall of Fame
I'm looking to find out the details on how RDC machine works to measure racket flex, and string stringbed stiffness. I understand the concept, I've seen the videos. But I want to know if :
Does the machine try to flex the racket/strings certain distance and it measures how much force it takes to do it? Or
Does the machine flexes the racket /strings with certain force and measures the distance the object flexed?
If one owns a machine there's an easy test to confirm : just use any racket and then a long wooden ruler (or anything significantly more flexible than a racket) , and see if the distance both objects move is the same or not.
 
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Irvin

Talk Tennis Guru
Off target but Wilson used to hang a weight (5 lbs I think) on the tip of the frame and measure the deflection. The less the deflection the stiffer the racket. So a 2.8 racket would deflect 1/2 as much as a 5.6 racket.
 
I'm trying to answer your question, although I'm not sure what you want to know for.
Since I have an RDC, I can confirm that there are two wheatonian bridges on the left side of the RDC. Guess one of them is for the total weight of the racquet.
The detachable hook is electromechanically closed after entering the right buttons and thus fixed. Inside is a fine thread bolt which is guided in a housing which is connected to the second bridge and works as follows.
The measuring instrument for the flex recognizes in any case (due to the immersion depth) whether it is a rigid frame or a wooden racquet. The depth of immersion of the hook can be seen with the naked eye that it goes deeper with the softer part, which then shows the measured result on the display.
The same happens with the stringbed. The softer the braid is, the deeper it is drawn in and this measures the stiffness.
I hope you are satisfied with my explanation.
 

jmnk

Hall of Fame
I'm trying to answer your question, although I'm not sure what you want to know for.
Since I have an RDC, I can confirm that there are two wheatonian bridges on the left side of the RDC. Guess one of them is for the total weight of the racquet.
The detachable hook is electromechanically closed after entering the right buttons and thus fixed. Inside is a fine thread bolt which is guided in a housing which is connected to the second bridge and works as follows.
The measuring instrument for the flex recognizes in any case (due to the immersion depth) whether it is a rigid frame or a wooden racquet. The depth of immersion of the hook can be seen with the naked eye that it goes deeper with the softer part, which then shows the measured result on the display.
The same happens with the stringbed. The softer the braid is, the deeper it is drawn in and this measures the stiffness.
I hope you are satisfied with my explanation.
many, many thanks. Yes that is exactly what I needed to know. Since, as you stated, "The depth of immersion of the hook can be seen with the naked eye that it goes deeper with the softer part" that means that the machine uses a set force (whatever that force is does not matter that much) to bend the frame. As a result a softer frames bend more (i.e. the immersion of the hook goes deeper) than more rigid frames.

I was wondering since essentially the same measurements could be done using the logic where the machine _always_ bends a given frame a set distance (like let's say 1cm), and internally measures how much force was needed to do that. But from your response it is not the case since depending on the racket the immersion of the hook varies.

All good, many thanks. Out of curiosity - on a normal modern racket, how much is the tip of the racket bent? I mean as we established it will vary depending on the racket - but ballpark estimation wise are we talking about 1cm? 5cm?
 
Good question, just found out with a probe.
On a 67 RA frame, it was 4 mm deflection.
On an old wooden racquet I measured 8 mm and the reading was 27 RA.
I don't want to chisel this in stone and derive a formula from it - I did not measure that exactly.
 

jmnk

Hall of Fame
Good question, just found out with a probe.
On a 67 RA frame, it was 4 mm deflection.
On an old wooden racquet I measured 8 mm and the reading was 27 RA.
I don't want to chisel this in stone and derive a formula from it - I did not measure that exactly.
this is most awesome data! Many thanks! I recall some older posts where folks were stating, based on memory and not actual measurements, that the tip of the racket is displaced about 3cm. That seemed like a lot to me. if you are saying that it is only about at most 10mm for most flexible frames (1cm) that makes way more sense.
 
If you are so interested, I will make a series of measurements with a micrometer, because I am interested myself in where the journey is going.
 
I just checked it with a digital indicator.
Had to improvise on the assembly.
I think these are now correct numbers, which I also measured several times.

 
By the way - a good 4 years ago in stringbedstiffness mode I had determined the suit weight with a "McGuyver" construction - that was almost 2.5 Kilos.

 

jmnk

Hall of Fame
@fritzhimself - you sir are a first class engineer! Many, many thanks. So:
when the racket tip deflection is 11.84mm -> the machine shows stiffness value 25
when the racket tip deflection is 4.98mm -> the machine shows stiffness value 71
for the purpose of measuring stringbed stiffness RDC machine appears to be using the force ~2.48kg
now, since you are so, so kind and in good co-operating mood - would you be able to find out what is the force RDC machine is using when measuring teh racket stiffness? I would imagine the same contraption you have used for stringbed stiffness would work as well....

p.s. I think I have the exact same micrometer....... :)
 
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I have now read again - this measurement was already for the racket stiffness - sorry my mistake, I remembered it differently.
Btw - it is "only" 4,58 mm - the display is mirrored and you can not see it exactly.
 

jmnk

Hall of Fame
I have now read again - this measurement was already for the racket stiffness - sorry my mistake, I remembered it differently.
Btw - it is "only" 4,58 mm - the display is mirrored and you can not see it exactly.
Oh, ok, that's fine as well.
So I was always wondering if Babolat RA value is a linear function of the flex, or perhaps something else (logarithmic maybe?). There's no way to tell with just two data points. But if you could do at least one more measurement (to see another flex value versus the amount of racket tip displacement) , perhaps with the frame that would be in between the two data points you provided - then with three data points we could attempt to plot the function and see what that function is?
 

afeller

New User
Hmm it seems that the factor is around 0.1579 -> 100-(mm/0.1579)=RA Value
Unfortunately i sold my RDC, so i cannot check this.
To "rebuild" the RDC, the distance from hook to the fix point in the middle is important.
 
I am a happy man - just today a Wilson Profile 95 has arrived for stringing - RA 86. It fits exactly into our measurement series.
 

jmnk

Hall of Fame
Hmm it seems that the factor is around 0.1579 -> 100-(mm/0.1579)=RA Value
Unfortunately i sold my RDC, so i cannot check this.
To "rebuild" the RDC, the distance from hook to the fix point in the middle is important.
Of course. But that i already know.
 
Ok - I have now measured 13 different frames. RA 86 and immersion depth 2.26mm, RA 63 at 63mm and RA25 at 118.4mm on the scale mean that these data are linear. The measurements were transferred to a drawing program to 10th of a mm to determine the progression.
More questions?

 

Irvin

Talk Tennis Guru
Ok - I have now measured 13 different frames. RA 86 and immersion depth 2.26mm, RA 63 at 63mm and RA25 at 118.4mm on the scale mean that these data are linear. The measurements were transferred to a drawing program to 10th of a mm to determine the progression.
More questions?

@MathieuR werent you asking about this at one time?
 

jmnk

Hall of Fame
Ok - I have now measured 13 different frames. RA 86 and immersion depth 2.26mm, RA 63 at 63mm and RA25 at 118.4mm on the scale mean that these data are linear. The measurements were transferred to a drawing program to 10th of a mm to determine the progression.
More questions?

I'm speechless. this is absolutely above and beyond. I owe you big time, let me now when you are ever on this side of the ocean.....
 

MathieuR

Professional
(@Technatic )
@Irvin , yes, I did ask this. My complaint was, that the flex measured by a RDC was a "magic number", without the option to measure it yourself.

the StringLab II measures flex in a different way: fixed distance, measuring the force. This gives a flex in kg/cm.
 

Irvin

Talk Tennis Guru
… yes, I did ask this. My complaint was, that the flex measured by a RDC was a "magic number", without the option to measure it yourself.

the StringLab II measures flex in a different way: fixed distance, measuring the force. This gives a flex in kg/cm.
Sorry I thought you were really interested in how it worked and what there reading meant.
 

bfroxen

Rookie
@fritzhimself, thanks! To replicate these measurements, we need two distances: from the tip of the racquet to the center balance bar and from the center balance bar to the clamp bar.
 

jmnk

Hall of Fame
I was very happy to do so, as this topic has been with me for years - your insistence on asking was the initial spark.:D
ok, great. If so - one more question. When stringbed stiffness is measured - what is the size of that 'thingy' that pulls the string downward? It seems to be a circle shape - what is the radius? (since clearly the bigger the size the less the strings would deflect for a given force - so knowing that size is important)
 

jmnk

Hall of Fame
(@Technatic )
@Irvin , yes, I did ask this. My complaint was, that the flex measured by a RDC was a "magic number", without the option to measure it yourself.

the StringLab II measures flex in a different way: fixed distance, measuring the force. This gives a flex in kg/cm.
@Technatic , @MathieuR - Would you happen to know what is that fixed distance for StringLab II device? And is that distance the same when measuring racket flex and when measuring stringbed stiffness?
 

MathieuR

Professional
Sorry I thought you were really interested in how it worked and what there reading meant.
I am really interested! @Technatic / Stringway made a graph showing the relation between the kg/cm flex measured by the SL2, and the "magic number" of the RDC.

The kg/cm value "sounds" more exact, but this also is a arbitrary value. Cause that value is 100% dependant on the cm flex you chose.

This holds same for stringbed-tension! If you measure the kg/cm it will increase to infinity (or string breakage) when increasing the depth of impression
 

jmnk

Hall of Fame
I have now read again - this measurement was already for the racket stiffness - sorry my mistake, I remembered it differently.
Btw - it is "only" 4,58 mm - the display is mirrored and you can not see it exactly.
Now that I'm re-reading those posts. So when racket flex is being measured RDC machine utilizes the force of 2.48kg to bend the racket. It's the same force used when the stringbed stiffness is measured?
 
I can't tell - haven't measured it yet.
Maybe you'll tell us exactly what you're up to and why this data is so interesting for you.
 

jmnk

Hall of Fame
I can't tell - haven't measured it yet.
Maybe you'll tell us exactly what you're up to and why this data is so interesting for you.
Like I said - I want to make a home-built RDC machine, although recreating swingweight measurement function will be a bit difficult (fortunately we have an app for that already). So I want to replicate any and all parameters of the actual machine. there's nothing more to it. It's a combination of tennis and engineering interests.
again, thanks for all the help.
p.s. I've sent you IM too.
 
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coachrick

Hall of Fame
I've got folks in the Pickleball realm who wonder if a swingweight measuring device would be useful with Pickleball paddles.
With a much shorter lever arm(max 17") and total weight(usually under 9oz), I'm not so sure the racket diagnostic machines would be useful.
(Not much interest in total flex or "bed" stiffness with a paddle...other than meeting USAPA regs.).
 
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