Changing stem-Is a complete re-cal needed?

[GWPOS]

I am getting ready to put on a longer stem, same angle as the shorter one. Question is do I have to do a complete re-cal to get wind scaling and tilt or is there another way to do this without going through the whole re-cal process?

[coachboyd]

Make sure that the iBike is mounted in the same position relative to the faceplate of the stem and you should be good enough to where you won't need to do a different calibration.

You may need to if the front of the stem is in a shape that would influence the "air wake" going into the windport, but if your iBike is slightly forward of the front of the stem then you should be all right.

[dbrouse]

Boyd...if it was a different stem angle would we need to recalibrate?

[coachboyd]

The wind scaling could change.

But what you could do is a cal ride and determine what your new wind scaling is and then update it on your previous profile, making sure to keep the cda the same (or right around the same depending on the new angle of the stem).

[GWPOS]

Ok I just changed my stem from a 90 to a 110. I then did a short cal ride and the tilt stayed the same -0.3, and the wind scaling did change from .690 to .680. Should I change it in my profile, and if so what do I put a hold on, the cda or the aero? Thanks for your help many times over.

Clint

[Velocomp]

CdA is a characteristic of you and the bike. Changing your stem does not materially change your CdA.

So, to modify your profile with the updated value of WSF, hold CdA fixed and change WSF to 0.68. You'll see your aero number change a bit, in line with the updated value of WSF.

[GWPOS]

Thanks Velocomp/John

[redherring10]

"CdA is a characteristic of you and the bike. Changing your stem does not materially change your CdA."

Probably nitpicking here, but increasing stem length on a stem that has significant rise angle will result in a more upright position, no? And slight increase in CdA?

[lorduintah]

If you mount the iBike in the approximate same distance from the handlebar, you are not changing the angle of attack to the air incoming. If you change the stem angle - that's a different story as you expose a different cross-section of the air port to the incoming flow.

You have, admittedly, changed the cross-section of you and the bike ever so slightly by this modification, but the bike and you are a much more significant potion of the overall exposure to air than the addition of a small extra stem length.

FWIW

Tom

[redherring10]

The post had to do with increasing stem length only, not about stem angle, or altering the position of the iBike relative to the end of the stem or to the bars, nor about angle of attack, etc. It was about stem length.

My point had simply to do with altering the angle of the torso relative to the bike's top tube resulting from the longer stem, the slight increase in exposure to oncoming wind and a resulting increase in CdA. I understand that the totality of the change and its relation to the exposure imposed by the bike, the rider's legs, arms, torso, head, helmet, etc., etc. is very small. I previously made that point and thanks for acknowledging it.

[lorduintah]

You can check that with coast downs - which is what is used to derive that and Crr. The cal ride examines the wind scaling and tilt. The wind scaling should be very little unchanged and the aero will be influenced by any CdA adjustment.

Tom

[GWPOS]

I am now experiencing higher power numbers than I am actually doing on downhills. Should I do the cal check in the coast down screen to fix this, and try and get it as close to zero as possible. If not then I am posting my out and back ride I did today if someone can look at it that would be great. Thanks

Clint

iBike_04_09_2010_1726_13_Miles.csv

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