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Compu trainer drag factor
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Would anyone know a good drag factor to use for an aggressive position on a tri bike? Assuming everything is a high end setup for best in aero.
This of course would be in place of trying a TT on a know course and trying to figure the corresponding drag number to match.
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Re: Compu trainer drag factor [rp87ebw] [ In reply to ]
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I could never get consistent results with Drag Factor.
At one point, 75% was working for me. Recently I tried 80% and it made no difference, as if the DF was not working.Not sure what changed

One thing you can do is import the results file into Golden Cheetah and using Aerolab you can actually get a CDA. With a 80% DF I was getting a .32 cda. At least you can do a few tests and see if you can get realistic numbers.

I wonder if it's any more consistent in Racermate 1
Last edited by: marcag: Aug 5, 14 4:20
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Re: Compu trainer drag factor [rp87ebw] [ In reply to ]
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I use 80% in the older software. It's kind of close but then "close" is good enough for me on the trainer.

jaretj
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Re: Compu trainer drag factor [marcag] [ In reply to ]
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Thanks for that info. I never heard of the Cheetah page. I have racermate 1 so hope its working the way things are supposed to work!
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Re: Compu trainer drag factor [rp87ebw] [ In reply to ]
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Everyone should know how Aerolab works :-)

It basically takes your ride file : power, speed and weight and it calculates what a theoretical profile(elevation) would look like with a given cda.
You then compare that profile to the actual profile and adjust the cda to make them match and bingo you have your cda

Since the computrainer file (pwx I think) has a) the actual profile b) your power and speed you can do the same thing.

I'll probably try it with RM1 this week, as soon as we get a rain day :-)
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Re: Compu trainer drag factor [marcag] [ In reply to ]
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I can't find the original reference (IIRC, it was on a Computrainer forum), but one of the Racermate engineers had noted that the drag factor was determined by the following formula:

Drag Factor = CdA * 310

A 100% drag factor corresponds to a CdA of 0.33, which was agrees with an earlier post in the thread as being the Computrainer default. That's much higher than you would expect for anybody using aero bars. An extremely aggressive aero position for a smaller rider could result in a CdA as low as 0.19, or a drag factor of 59. Years ago, Charles Howe had a document floating around with CdA estimates for TT riders that had a 'typical' 72Kg male racer at a CdA of 0.265, and that might be a good guess for a reasonable drag factor of 0.265 * 310 ~= 82 for a medium sized male rider with a good position on the bike. I would guess more aggressive positions would end up in the 70-80 Computrainer drag factor range.

For whatever reason, drag factor default doesn't scale with rider size in the Computrainer software (everybody gets 100%), so the drag factor throws numbers off even more for smaller riders (who naturally have a smaller CdA).

Hope this is of use to somebody else.
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Re: Compu trainer drag factor [bjbest] [ In reply to ]
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DragFactor is 100 by default regardless of rider weight. Rider weight is accounted for in other parts of the physics model (acceleration and rider inertia), but the frontal area was previously fixed. The method suggested to aid in its adjustment was to use heart rate as recorded on the road (ride a very flat 1-mile, same speed, do an out/back, average your HR out/back to negate head/tail wind) and then setup a 2-mile flat course on the trainer and adjust DF until your HR stabilized the same indoors and out. Though we were adjusting wind-load, heart rate is a very good determining factor of work.

I've had no one report it is not adjusting correctly, so let us know. On the old software you adjusted it prior to starting a ride timer. You could also pre-set a value and try it.

The original load curve was pretty stiff (male rider in the drops, but not aero at all) and so we didn't allow the setting much upper range, but assumed it would need more lower range.


Roger Moore
Software Development Mgr.
RacerMate Inc. http://www.racermateinc.com
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