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View Full Version : Is racquet flex as simple as the rdc #?


dutch monkey
01-05-2005, 08:03 AM
Does a racquet’s flexibility as measured by a RDC machine give a clear measure of the racquet’s flexibility? In other words, is there more to a racquet’s flexibility than what the RDC rating indicates? Sometimes Tennis Magazine rates racquets as “stiff” even though the racquet may have a low RDC reading. For example:

Dunlop 300G MP – stiffness per Tennis Warehouse 64, “firm” per Tennis Magazine
Head Liquidmetal Radical OS – stiffness per Tennis Warehouse 58, “stiff” per Tennis Magazine
Prince Tour Diablo MP – stiffness per Tennis Warehouse 62, “flexible” per Tennis Magazine

rich s
01-05-2005, 08:10 AM
Ignor what tennis magazine says and just look at the RDC number.

Beware though becuase two racquets with the same RDC number can feel different ---because --- one may have a stiff hoop and flexy throat and the other may have a flexy hoop and stiff throat.

So even though the overall RDC numbers are the same the two frames will feel different.

NoBadMojo
01-05-2005, 08:13 AM
yes what the other main element is at what point(s) on the frame it is stiffer or more flexy. ie, some frames flex more in the head, others more in the shaft, etc

dutch monkey
01-05-2005, 08:53 AM
Also, I've asked the question because of the Dunlop Max 200G in mind. Due to the manufacturing process for that particular racquet, it was stiffer dynamically than statically. As I've read on a Tennis Warehouse reivew: 'Rick Perry, Dunlop's Director of Research & Development offers, "the measure of resistance on the Max 200g is based on the rate pressure is applied. When pressure is applied at a slow rate, such as on a Babolat RA or RDC machine, the frame will seem very flexible. During fast swings, though, the frame deflects less and thus, returns more energy to the ball."'

So, I was wondering if any current models are similar, in which case the RDC# wouldn't mean as much or at least wouldn't be entirely reliable when trying to choose a flexible racquet.

carTW
01-05-2005, 09:09 AM
So how can we tell (for a same RDC number) if a racquet is stiffer in the head or in the shaft, when playing with it? And which one (stiffer hoop or stiffer shaft) is easier on the arm?
What are the advantages/drawbacks of stiffer hoops or stiffer shafts?

NoBadMojo
01-05-2005, 09:21 AM
the rdc machine only measures flex at one point in the frame...others will know where..i am guessing the throat. carTW stifness anywhere in a frame is not your friend as far as comfort goes. the difference betwen stiffer flex in the hoop or shaft is pretty subjective and mostly a matter of preference and feel altho i suspect others may disagree based on some technical stuff.

Gaines Hillix
01-05-2005, 09:44 AM
the rdc machine only measures flex at one point in the frame...others will know where..i am guessing the throat. carTW stifness anywhere in a frame is not your friend as far as comfort goes. the difference betwen stiffer flex in the hoop or shaft is pretty subjective and mostly a matter of preference and feel altho i suspect others may disagree based on some technical stuff.

Ed, you're correct, the RDC measures stiffness at the throat of the frame. As others have already said, there are other points along the frame that can effect the overall feel and response of the racquet. Some frames have two or three tapers in the beam. There are also frame systems like the Head Intelligence series or LM series that will stiffen based on the swing speed. Also the Dunlop as mentioned by Dutch Monkey. It's a good idea to check the makeup of a frame and understand if it's a simple graphite or graphite/kevlar frame without any tricks added or if it's a more complex frame with built-in stiffeners. The TW pages on each racquet are pretty good at including this information. So, the RDC number is just somewhere to start. But, it's the only "standard" that's available to us, AFAIK.

carTW
01-05-2005, 10:41 AM
Thanks! I actually don't have an arm problem, but don't want to start one.....

rich s
01-05-2005, 11:07 AM
Gaines, Mojo-

I actually e-mailed Babolot last year querying them about how the RDC measures stiffness.

their explanation basically said that though the RDC reacts the force that is applied (to the top of the hoop) at the throat and at the handle, the resulting measured stiffness is a composite/overall stiffness for the entire frame not the stiffness of the throat only.

There seems to be a common misconception on the board that the measured RDC stiffiness is only the stiffness of the throat and it's not so.

I thought I'd share this with you in hopes to try to clear up the misconception.

Gaines- I completely agree that this is the only "standard" we have to initially compare frame stiffnesses to each other.

Gaines Hillix
01-05-2005, 11:19 AM
Gaines, Mojo-

I actually e-mailed Babolot last year querying them about how the RDC measures stiffness.

their explanation basically said that though the RDC reacts the force that is applied (to the top of the hoop) at the throat and at the handle, the resulting measured stiffness is a composite/overall stiffness for the entire frame not the stiffness of the throat only.

There seems to be a common misconception on the board that the measured RDC stiffiness is only the stiffness of the throat and it's not so.

I thought I'd share this with you in hopes to try to clear up the misconception.

Gaines- I completely agree that this is the only "standard" we have to initially compare frame stiffnesses to each other.

Rich, you're the engineer. :-) How could the machine do that if it's held stationary at the butt end, the fulcrum is where the frame sits on the machine at the throat and the tip is where the force is applied? It seems like the machine would be measuring the amount of force it took to bend the racquet at the fulcrum point and the frame is not touching the machine any where else?

carTW
01-05-2005, 12:17 PM
If that is the way the machine works, then the RDC index will show the flex number for the most flexible point along the entire racquet, wherever it is.

rich s
01-05-2005, 01:16 PM
CarTW - I have to think about that a bit.

Gaines - it's all relative - and as to how you look at it.

The way the machine currently works is that the machine pulls down on the tip while the middle support pushes up and the support at the handle pulls down.

Babolat could have made the RDC such that the racquet sits on supports at the handle and tip of the hoop and an actuator pushes or pulls down on the middle of the frame --- but, that would be a more complex design to accomplish the exact same thing the machine is doing now.

Whether the machine/actuator(s) pulls down on one/both end(s) and reacts upward in the middle or pulls down in the middle and reacts upward on the ends or the actuator pushes up on the middle and fixed supports pull down on the ends the resulting forces, reactions and deflection of the frame are all exactly the same. Ergomically the current configuration works best for a consumer product, plus the actuator only has to apply half as much force to one end as it would if it applied it to the middle to get the same result.


Look at it this way. Whether you span a 2x4 across two bricks and put 100 lbs in the middle of the span or you balance a 2x4 on one brick and put 50lbs on each end of the 2x4 or you balance the 2x4 on a brick, stick one end of the 2x4 under your car, SUV, mother-in-law, :D etc and put 50 lbs on the other end of the 2x4 the resulting reactions are the same and the deflection of the 2x4 is the same.

This is known as simply supported.

Whether you are reacting at the middle or apply the load to the middle does not matter. You apply a known load, you measure the deflection you divide the applied load by the measured deflection and the result is the effective stiffness of the body in the plane perpendicular to the direction of the applied load.

I'm getting too nerdy.

E-mail me at home and I can give you more detail.

carTW
01-05-2005, 03:35 PM
What I meant to say is that if there is a more flexible spot along the racquet, that point will take more torsion than the rest of the racquet on the Babolat machine. To take an extreme example, if you take one of those straws with the little accordeon section (used for drinks), put it on a pen on its center and press on both ends it will mostly bend at the accordeon.
I would guess that a racquet is a lot more evenly stiff than one of those straws, so maybe my little theory applies only to a very minor degree....

rich s
01-05-2005, 03:38 PM
CarTW - I think you've got it right on the head. A racquet is more uniformly stiff than your straw example but I think you've nailed it.

kinsella
01-05-2005, 04:38 PM
Prior posts have made excellent points that I agree with, but the flex element missing in this thread is dynamic vs. static flex. Some racquets play stiffer than their flex rating, because some materials need more time to flex. The iHeads are designed to react to flexion by stiffening and represent the extreme example. My old 60RDC ROKs played much more firm than that.

The distribution of weight also plays a role in dynamic flexing, but not static. If the weight is concentrated at opposite ends, you will get more flex in actual play than if the weight were concentrated alongside the sweet spot.

The only way to determine the flex in play is to play. Different stroke mechanics are likely to produce subtley different flex.

Swan Song
01-05-2005, 05:57 PM
Prior posts have made excellent points that I agree with, but the flex element missing in this thread is dynamic vs. static flex. Some racquets play stiffer than their flex rating, because some materials need more time to flex. The iHeads are designed to react to flexion by stiffening and represent the extreme example. My old 60RDC ROKs played much more firm than that.

The distribution of weight also plays a role in dynamic flexing, but not static. If the weight is concentrated at opposite ends, you will get more flex in actual play than if the weight were concentrated alongside the sweet spot.

The only way to determine the flex in play is to play. Different stroke mechanics are likely to produce subtley different flex.

The dynamic vs. static flex also relates to how some people says how the i.Prestige Mid feels stiff, despite it's "static" flex rating of 60 on a Babolat RDC.

rich s
01-05-2005, 07:07 PM
I agree with both Kinsella and Swan Song but we really don't have a practical means of measuring dynamic stiffness. We only have RDC static stiffness to use as an initial indicator for frames that may be of interest to us.

Gaines Hillix
01-07-2005, 05:25 AM
CarTW - I think you've got it right on the head. A racquet is more uniformly stiff than your straw example but I think you've nailed it.

Check out this thread on GSS;

http://www.******************.com/cgi-bin/yabb/YaBB.cgi?board=machinestools;action=display;num=11 04953046

I still believe that using a static frame supported at only one point on the machine at the throat is going to give you a flex number relative to the force applied at that point.

NoBadMojo
01-07-2005, 02:50 PM
gaines i agree with that statement....i dont know if that point is adjustable or not on the rdc machine, but i suspect that if it were, you would get varying stiffness ratings based upon which point along the vertical axis the fulcrum was placed, and the racquets composition and layup. perhaps the machine just measures flex at the absolute mid point of a frame? but then again, i'm sure no engineer....i go by feel.

Gaines Hillix
01-07-2005, 03:04 PM
gaines i agree with that statement....i dont know if that point is adjustable or not on the rdc machine, but i suspect that if it were, you would get varying stiffness ratings based upon which point along the vertical axis the fulcrum was placed, and the racquets composition and layup. perhaps the machine just measures flex at the absolute mid point of a frame? but then again, i'm sure no engineer....i go by feel.

Ed, the fulcrum/frame contact point on the RDC for frame flex is fixed, AFAIK. As the post on the GSS board points out the manufacturers have machines that cost BIG $$ that can measure at many points, but mere mortals like us or your local tennis store aren't going to have one of them. :-)

rich s
01-07-2005, 03:08 PM
I still believe that using a static frame supported at only one point on the machine at the throat is going to give you a flex number relative to the force applied at that point.

If you simply support the frame (which is how it is supported in the Bab RDC) the stiffness you are measuring is only applicable to the simply supported loading scenario not the point at which the load is introduced or reacted.

If you cantilever a frame and measure the stiffness that stiffness is applicable only to the cantilevered case, not the point at which the load was introduced or reacted and so on.

This is the issue I have with the RDC stiffnes on the forum .... the way it is interpretted. Because the RDC reacts loading/imparts load near the throat (depending on your frame of reference) the misconception on this forum and at a lot of tennis shops is that the RDC's measurement is the stiffnes of (or at) the throat. Not so. The stiffness is the overall effective stiffness of the racquet when simply supported at the extremes and the load/reaction is applied to/at a point approximately located near the throat.

Does it (the RDC) accurately reflect the way the racquet is loaded when we hold it and strike a ball? No, if it did we would have one hand on the handle and one hand on the tip of the hoop and we would hit the ball with the frame somewhere close to the middle. We wouldn't even need to string it. But right now it is all we have to get an initial indication of how stiff or soft a frame is.

Even if you could move the center support as Mojo suggests the measurements at the different locations only reflect the frame's stiffness based on those loading scenarios. The location of the "fulcrum" as you referred to it does not indicate that that is the stiffness at that point.

Imagine you had a 2x6 steel beam and a wooden 2x6 (but the steel beem was twice as long as the wooden beam) and the two were bolted together. If you loaded it up like the rdc does (simply supported on the ends and load intro/reaction at a point near the middle) the steel portion would rest on the "fulcrum". Most of the deflection in that beam would occur in the wood portion of the beam not the steel portion of the beam - and the steel portion is on the fulcrum. why, because the wood is not as stiff or strong as the steel

In order to get an indication of where and how a racquet flexes, relative to how we hold it and strike a ball, you would need a machine that can hold the racquet in the same basic manner in which we hold it with our hand and then apply a load to the string bed at the approximate location of the sweetspot/center of precussion. Then the machine would have to have deflection gages spaced evenly along the frame to measure the deflection of the frame every few inches.

Know all that data and knowing other basic engineering principles you would be able to determine the stiffness at any point on the frame.

That would be a complex and expensive machine to build nevermind market, so.... Babolat has given us the RDC which simply supports the frame and loads it up. And that is just statically.. dynamic stiffness is a whole nother matter.

We do this all the time at work to calibrate structure and verify/validate analyses.

b.
01-08-2005, 03:27 AM
I think that RDC # is not completely useless. Well - racquet will bend in most flexible part the most, one way or another.

What is missing IMHO is:

1) RDC # for (let's say) 2 different loads - "medium" and "strong" hit. I guess that racquets don't have perfectly linear response, which one can feel during the play.

2) dynamic stiffness instead of static - instead of slowly applied force it should be impuls that mimics hiting the ball. Or something like that.

Gaines Hillix
01-08-2005, 06:37 AM
Rich, o.k. I can understand that the RDC flex rating is an "average," so to speak, of the flex of the frame as a whole when supported at the throat(?). However, the point I think people are trying to make when they say the RDC only measures the flex at the throat is that racquets can vary in stiffness across the length of the frame. Many frames are more flexible in the hoop than the throat or even have flexible throats, stiff hoops and flexible tips like the dual taper beam frames.

rich s
01-08-2005, 07:19 AM
Gaines - I agree with you last statement 100% :D

But there is no need to say "when supported at/near the throat" because that is currently the only way they are measured - Especially when you say the "RDC stiffness is XX".

By saying "When supported at the throat" - It leads those who are not as technically versed/inclined (no slam intended on/toward anyone in any way) to think that the stiffness of the throat is being measured instead of the overall/effective stiffness of the frame.

And by saying "when measured at the throat" couldn't be anymore incorrect.

I just want to get the word out to the forum of this because for the most part if not all part everyone thinks the RDC is measuring the stiffness of the throat.

Fun post....you guys are making me dust cobwebs off of parts of my brain...... :)

NoBadMojo
01-08-2005, 07:33 AM
ok so to summarize, it really isnt important to know the technical nuances of the flex stat from the rdc machine. what is impt to know is that that the flex # is only a guideline, and that all racquets flex more at different points than others...some flex more in the head than in the throat and some are stiiffer in the hoop (like my t10's)..i think that is what some of us have been trying to get across here

rich s
01-08-2005, 07:49 AM
Mojo ... what you said plus "The RDC is the avereage stiffness of the frame not the stiffness of the throat" :)

NoBadMojo
01-08-2005, 08:00 AM
man rich now you are getting technical on us again :) but ok..we are dealing now in semantics and interpretations and i think we get the fulcrum point.....errrrrr point. how about 'the rdc is the average stiffness of the frame as measured at the throat" would that be more precisely correct?

rich s
01-08-2005, 08:13 AM
how about 'the rdc is the average stiffness of the frame as measured at the throat" would that be more precisely correct?

Mojo, No, that is the exact point I am trying (not) to convey because you are not measuring the stiffness at the throat --- And, it is too easily confused with or interpretted as: "The RDC measures the stiffness of the throat"

Just say: "The avereage/overall stiffness of the frame as measured by the RDC is XX".

I think most everyone hear realizes all frames flex differently and some frames flex more in the hoop and some more in the throat and on and on......but I'll say it again to help clear the air:

Just say: "The avereage/overall stiffness of the frame as measured by the RDC is XX".

I hope I'm not gettting too technical, I'm probably getting too anal.... I'm trying to get the forum to understand what the information they are reading actually means.

You and Gaines are the Hall of Famers, so if I can get both of you guys to understand ,with the amount of posting both of you do, you can help spread accurate information. :D

NoBadMojo
01-08-2005, 08:20 AM
rich i cant speak for gaines but i am dumber than a bag of rocks.....hell i kaint even spell nor use caps, nor can i volley well with the rdxMP whatever frame. i've taken to insulting myself now saving other posters the effort :)

rich s
01-08-2005, 08:23 AM
Mojo - Too Funny! :lol:

Gaines Hillix
01-08-2005, 08:35 AM
Rich, thanks for clearing up this misconception. I think Ed and I are long lost cousins! :-) LOL