Computrainer Question

[sub55]

If you don't add the weight of the bike to your weight when entering that value - what is the end result? Are watts under reported or over reported? My sense is that time and speed will remain the same but watts will be under reported if you just report/enter your body weight.

 

[rmur]

Originally posted by sub55
If you don't add the weight of the bike to your weight when entering that value - what is the end result? Are watts under reported or over reported? My sense is that time and speed will remain the same but watts will be under reported if you just report/enter your body weight.

Hi there. I'm not an expert but, based on my use of the CT for the past year, I believe you will find Watts to be correctly reported but your speed will be higher and time lower than one would expect when adding bike weight.

I add 20 lbs or 9kg all the time and slog up the hills as a result!

Regards,
Rick

 

[sub55]

do you mean your speed would be higher and time lower if you DON"T add the weight of the bike?

 

[rmur]

Originally posted by sub55
do you mean your speed would be higher and time lower if you DON"T add the weight of the bike?

Yes - that's correct. My understanding is that power is calculated from rear wheel speed and the resistance applied to the load generator - thus Watts are always "correct" (within the limitations of the CT). If one decides to "cheat" by not adding a bike weight or entering a lower body weight then the software assumes you are more powerful per net kg. This will make you faster on the uphills for sure. I'm not sure of the algorithm for the flats so I can't say if you will be faster there. The drag calculation (CdA) may based on height alone or it may be a combination of height and weight.

If you'd like to get an anwer straight from CT, I would suggest contacting: [email protected]

As I mentioned before, I just add 9kg all the time. I do find the CT speed to be a tad slower than I expect outdoors but really what does that matter?

Regards,
Rick

 

[ART]

The CT applies a load to the power generator that is determined based on an algorithm that takes into account wind resistance, grade/slope of terain and weight. If you keep all other variables the same but input a lower weight the load generator will apply less drag on your rear wheel and at the same time will indicate the appropriate lower wattage. This makes sense because a lighter rider needs less wattage output to maintain the same speed uphill as a heavier rider. What this also means is that a lower weight input will allow the CT user to go up a hill faster than he could if he input the correct (ie. heavier) weight than the lower weight because the CT is putting less drag on the load generator because it thinks the rider/bike weight is lighter than it really is. (Lighter riders need less power to climb hills.)

Art

 

[larrynipon]

Sorry Rick, but you're incorrect. If you want to test the theory, do a ride with the weight not including the bike weight. Save that ride as a performance file. Then do the same ride, this time with the bike weight added to your body weight. Use the performance file as your pacer. The pacer will SMOKE you! Each pound of weight requires about 3 additional watts of power to accomplish the same result. So, if your bike weighs 18 lbs., you would have to pump out an additional 54 watts just to keep up with your "18 lb. lighterself" pacer...

Originally posted by rmur
Hi there. I'm not an expert but, based on my use of the CT for the past year, I believe you will find Watts to be correctly reported but your speed will be higher and time lower than one would expect when adding bike weight.

I add 20 lbs or 9kg all the time and slog up the hills as a result!

Regards,
Rick

 

[sub55]

Originally posted by larrynipon
Sorry Rick, but you're incorrect. If you want to test the theory, do a ride with the weight not including the bike weight. Save that ride as a performance file. Then do the same ride, this time with the bike weight added to your body weight. Use the performance file as your pacer. The pacer will SMOKE you! Each pound of weight requires about 3 additional watts of power to accomplish the same result. So, if your bike weighs 18 lbs., you would have to pump out an additional 54 watts just to keep up with your "18 lb. lighterself" pacer...

I was in touch with Computrainer and asked them teh question and what they said was that the default value was for a 160 pound rider. so ... if you weight 140 pounds and unless you are into a lot of pain there is no sense in adding bike weight - the default value ahs you there already.

I used the Analytic Bicycling site to calculate speed under a given power (watts) and rider+bike weight and the amount of effect that rider weight has on speed under a set wattage isn't as great as you think.

 

[rmur]

Originally posted by larrynipon
Sorry Rick, but you're incorrect. If you want to test the theory, do a ride with the weight not including the bike weight. Save that ride as a performance file. Then do the same ride, this time with the bike weight added to your body weight. Use the performance file as your pacer. The pacer will SMOKE you! Each pound of weight requires about 3 additional watts of power to accomplish the same result. So, if your bike weighs 18 lbs., you would have to pump out an additional 54 watts just to keep up with your "18 lb. lighterself" pacer...

Uh Larry,
Perhaps my wording was somewhat obtuse but I basically agree with what you're saying : adding the bike weight slows you down ... as least on the uphills!

I won't quibble about the magnitude of the power increase per lb to maintain the same speed but surely it's very dependent on gradient.


Regards,
Rick