Stretched Shock Cord & Time

[bloomp]

Slidewinder,

Perhaps you should look at what causes the resistance on the handle. The shock cord provides none of the resistance, all the resistance comes from the chain which is attached to the flywheel. The shock cord only acts to pull the handle (and chain) back to the initial position against the cage.

[Carl Watts]

The force required is that of the cord plus the resistance of the flywheel amoung other forces such as those to overcome friction in the bearings etc but lets keep it simple, the flywheel is by far the greatest user of energy and it's a cubic law in terms of power so the faster the pace the smaller the other losses become in relative terms.

There are far more important things to worry about than the "Losses" in the cord, I have found that as long as it retracts the handle with enough force to have it stick to the cage without it falling to the bottom under its own weight then your sweet as far as the cord is concerned !

[bloomp]

Actually, I feel the reverse of what was suggested is to be true. As the cord is stretched, the resistance comes more from the flywheel alone and therefore it becomes easier to pull a lower split. And no energy is wasted on stretching a tight shock cord.

[Slidewinder]

Bloomp wrote, "All of the resistance comes from the chain which is attached to the flywheel..."

Yes, the chain goes around the sprocket of the flywheel and then it is attached to the elastic cord, adding the resistance of the cord to that of the flywheel. When you pull on the handle, approximately seven pounds of force is used to stretch the cord. This adds to the tiring effect of the workout, but it has nothing to do with flywheel inertial/air resistance.

Why is this important? It's important because elastic cord tension can vary, and as a variable, it affects ergometer times. In a competition, winning times are tainted by the possibility that those times might have been posted on a machine with reduced elastic cord tension. Assertions by the C2 faithful to the contrary, from machine to machine it is not a level playing field. Elastic cord tension needn't vary much to give an advantage to a competitor. For equally matched athletes, a slightly reduced cord tension is all that would be needed for victory. Let's face it, indoor rowing competitions are fun, but essentially, they are a farce.

[hjs]

Why is this important? It's important because elastic cord tension can vary, and as a variable, it affects ergometer times. In a competition, winning times are tainted by the possibility that those times might have been posted on a machine with reduced elastic cord tension. Assertions by the C2 faithful to the contrary, from machine to machine it is not a level playing field. Elastic cord tension needn't vary much to give an advantage to a competitor. For equally matched athletes, a slightly reduced cord tension is all that would be needed for victory. Let's face it, indoor rowing competitions are fun, but essentially, they are a farce.

At most raced the machines are new, so the shockcordstension are the same and are not an influence of any importance on the results.

[Slidewinder]

hjs,
Re: New machines have identical shock cord tension and no influence on results
Even if those new machines had absolutely identical shock cord tension (which is unlikely), there is still a problem with all the erg times users post on-line. Those times could be posted on machines with the shock cord loosened or even replaced with a lighter weight cord.
Indoor rowing competitions will never be recognized as serious athletic contests. They will never, for example, be part of the Olympics, because elastic cord tension is a variable that can and does affect results.

[hjs]

hjs,
Re: New machines have identical shock cord tension and no influence on results
Even if those new machines had absolutely identical shock cord tension (which is unlikely), there is still a problem with all the erg times users post on-line. Those times could be posted on machines with the shock cord loosened or even replaced with a lighter weight cord.
Indoor rowing competitions will never be recognized as serious athletic contests. They will never, for example, be part of the Olympics, because elastic cord tension is a variable that can and does affect results.

Of course it will will never become an olympic event, but not due to it being not fair, there are lot's of other sports with lot's of other variables that make does sports unfair in a way. Weather, referees, drawings, lanes to name a few.

You are making to big a deal from that shock cord, I think a loose one will even slow you down. The recovery will become very sloppy, that does not help. Records can only be set during races, so online results don't mean that much.

And If some one set's a top effort on a erg, you can find that not very impressive, but people who do are very fit/trained and talented. To row a 5.40 2k on an erg matches every sport in top performance.

[Slidewinder]

hjs,
A variable, such as weather, affects all participants equally, but shock cord tension is not like the weather. Variations in shock cord tension from machine to machine will result in some competitors having an advantage over others.
I don't question the fitness level of those who can pull a six minute 2K. Its very impressive, but if two of those athletes were racing head to head and one beat the other by just a few seconds, I think that win is meaningless. The slightest difference in shock cord tension would have affected that result.
The only way ergometer competitions and recorded times could be fair and accurate would be to redesign the machine with a chain return that maintains a precise, untamperable pull and return force - possibly some kind of electro-mechanical device.
Robert
PS: Thanks for the fitness website reference. I'll have a look.

[hjs]

hjs,
A variable, such as weather, affects all participants equally, but shock cord tension is not like the weather. Variations in shock cord tension from machine to machine will result in some competitors having an advantage over others.
I don't question the fitness level of those who can pull a six minute 2K. Its very impressive, but if two of those athletes were racing head to head and one beat the other by just a few seconds, I think that win is meaningless. The slightest difference in shock cord tension would have affected that result.
The only way ergometer competitions and recorded times could be fair and accurate would be to redesign the machine with a chain return that maintains a precise, untamperable pull and return force - possibly some kind of electro-mechanical device.
Robert
PS: Thanks for the fitness website reference. I'll have a look.

Weather, no it does not

examples, skyjumping, tailwind/headwind, yoo don,t jump at the same time.

Rowing, sidewind, the lanes close to the wall has less waves

Track and field, the inner line for 200 400 meter is always slower

Trowing event track and field, wind can help or not with javalin and discus.

Longjump / tripple jump / pole vault. head or tail wind does accur, you don,t jump at the same time

Drawing, when you draw a very strong opponent in the early stayes..... even the nmr 2 can loose then



I can go on and on, I do agree that there is a certain variation due to the shock cord, but it really is just a very small portion of the total performance.



Cheers about the site.

[gregsmith01748]

Major edit, changes conclusion completely...

This is a really interesting topic to me. I think that Slidewinder is correct in that part of the energy that goes into the erg is going into stretching the shock cord. The devil is in the details however.

The website on the physics of ergometers states that the rowperfect performance monitor includes a term for the force used to pull the shock cord that returns the chain. This term is subtracted from the force applied to the flywheel.

Looking at some papers on the web (http://www.biorow.org), it appears that the average force during the drive in competition is in the neighborhood of 400 Newtons

400 Newtons is about the same force as lifting 40 Kg (on earth at least)

So if the shock cord is providing 3Kg (6.6 pounds), then the force required to stretch it is 30 newtons, or in effect a 7.5% loss between the force applied to the handle and the force applied to the flywheel.

But.... and this is important, we are not talking about the difference between no return force and full return force, but rather the variation between machines. If the range of normal return force is ~1Kg (say 5 to 7 lbs), then the uncertainty is 10 newtons or 2.5%.

Elsewhere on the forum, c2jonw let us know that flywheels are calibrated to +/- 0.33%. It seems to me that the uncertainty of the shock cord return force is a much bigger error factor.

I'm sure there are people out there who understand this far better than I do, and this is my second attempt at the analysis, so please correct me if I'm wrong.

[Slidewinder]

Thankyou Greg for your useful contribution to this topic.
Interesting that the RowPerfect PM subtracts the non-inertial (shock cord) component from the inertial (flywheel) component. As you point out though, it is the difference in shock cord force between machines that is the issue.

In any athletic competition in which equipment is used, there are always regulations to minimize or eliminate equipment differences that could result in an unfair advantage. Weather induced advantages and disadvantages as catalogued by hjs are beyond human control, but every effort is made to level the playing field as it relates to equipment.

If the variation of shock cord return force between machines can be as high as 2.5 % as you wrote, then my skepticism about the fairness of ergometer competitions, and whether some wins are deserved, is well founded.

[bloomp]

Whether it is 'easier' to stretch the shock cord will not influence your split. If anything, rowers despise a loose shock cord even though according to your argument, it would be 'easier' to put more force directly into the flywheel without wasting energy on the bungee. Yet that's not the case at all, it feels very weird and frustrating to row with a loose shock cord, and you do not produce better numbers.

As for what RP does, I would like to see how they calculate the force generated by the shock cord. Since it changes as the shock cord ages, do they have some algorithm that adjusts for a less taut shock cord?

Honestly, the forces related to various tightnesses of the shock cord are insignificant. How can I tell you this? Because an athlete that pulls a 7:00 2k on a newer model D can also pull a 7:00 2k on a much older model B. Shocking, right? According to your hypothesis there would be a difference one way or the other.

[Slidewinder]

bloomp,
I agree that a shock cord so slack that the chain return becomes sloppy will provide no advantage, but I suspect a cord with five pounds of return force will not produce this sloppiness. It is nevertheless 2-3 pounds lighter than a fully tensioned cord.
For the competitor on the machine with the fully tensioned cord, two to three pounds of extra force required on every stroke to obtain the same PM reading doesn't sound like much, but over the course of a 2K race it is the equivalent of lifting a few hundred pounds. This cannot be dismissed as having no effect on times.

[hjs]

Whether it is 'easier' to stretch the shock cord will not influence your split. If anything, rowers despise a loose shock cord even though according to your argument, it would be 'easier' to put more force directly into the flywheel without wasting energy on the bungee. Yet that's not the case at all, it feels very weird and frustrating to row with a loose shock cord, and you do not produce better numbers.

As for what RP does, I would like to see how they calculate the force generated by the shock cord. Since it changes as the shock cord ages, do they have some algorithm that adjusts for a less taut shock cord?

Honestly, the forces related to various tightnesses of the shock cord are insignificant. How can I tell you this? Because an athlete that pulls a 7:00 2k on a newer model D can also pull a 7:00 2k on a much older model B. Shocking, right? According to your hypothesis there would be a difference one way or the other.

You are indeed right, on sprint events a sloppy cord even slows you down, the chain wil fold to slowly hindering the stroke.

It may seem a disadvantage, but there are more factors then the tension alone. From experience, fresh, new machines with a tight cord are the fastest. It could very well be a pro and not a con

[gregsmith01748]

Hi,

I did some more thinking about this and there are two factors that would make the 2.5% difference smaller. Both have to do with the nature of force and displacement on a shock cord. The more you stretch it, the harder it pulls back.

1. The force that the shock cord will be less at the beginning of the drive than at the end. So, the average force through the drive might be a bit less than the 7-8 pounds.

2. You are putting potential energy into the shock cord with the drive, which it will give back to you on recovery, essentially pulling you back up the slide. This won't make your quads burn less, but the average aerobic load will be reduced by the assist back up the slide.

If each of these cut the 2.5% in half, then you end up with each machine with a percent of each other.